Properties of giant extragalactic HII regions and the Hubble constant

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Properties of giant

extragalactic HII regions and

the Hubble constant

by

David Fern´andez Arenas

Thesis submitted in partial fulfillment of the

requirements for the degree of

DOCTOR OF PHILOSOPHY IN ASTROPHYSICS

at the

Instituto Nacional de Astrof´ısica, ´

Optica y

Electr ´onica

May 2018

Tonantzintla, Puebla

Under the supervision of:

Dr Roberto Terlevich

Dr Elena Terlevich

Dr Manolis Plionis

©INAOE 2018

The author hereby grants to INAOE permission to

reproduce and to distribute publicly paper and

electronic copies of this thesis document in whole or

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and especially to my wife Andrea, for being there always supporting me.

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Ainulindal¨e

...The human mind, endowed with the powers of generalization and abstraction, sees not only green-grass, discriminating it from other things (and finding it fair to look upon), but sees that it is green as well as being grass.... -The mind that thought of light, heavy, grey, yellow, still, swift, also conceived of magic that would make heavy things light and able to fly, turn grey lead into yellow gold, and the still rock into a swift water...

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Acknowledgements

It is necessary the same compromise to write a thesis that to select the best words to thank all people that have contributed to this process.

Firstly, I would like to express my sincere acknowledgements to my advisors Dr. Roberto Terlevich, Dr. Elena Terlevich, for their knowledge, patience and advice offered over the last years, to my teachers in special to Dr. Manolis Plionis.

I would like to thank the rest of my thesis committee: Dr. Daniel Rosa Gonz´alez, Dr. M´onica Rodr´ıguez and Dr. Divakara Mayya. I am also very grateful to all the staff of the OAGH and OAN-SPM for made my observing runs easier and for all their help.

I also thank my thesis evaluation committee: Dr. Casiana Mu ñoz-Tu ñón, Dr. Anna Lia Longinotti, Dr. Sébastien Fromenteau, Dr. Vahram Chavushyan, for taking the time to review this manuscript.

I would like to thank the CONACyT without their scholarship (No. 360396), this thesis would not have been possible.

Last, but no least, I would like to thank my friends Diego, Zavala, Alejandro, Ricardo L., Izbeth and Ricardo C., with who I was sharing amazing adventures during all this time in Mexico. Thanks to my family for their love and their help in finding my way through life.

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Declaration

I hereby declare that the present thesis entitled: Properties of giant extragalactic Hii regions and the Hubble constant was composed by myself, that the work con-tained herein is my own, except for that jointly- authored publications included, and that it has not been submitted for any other degree or professional qualification, at any other Research Institute or University.

Parts of this work have already been published in refereed journals as follows:

• The work presented in Chapter 3 and Chapter 4 was accepted to be published in Monthly Notices of the Royal Astronomical Society as An independent

de-termination of the local Hubble constant. (Fern´andez-Arenas et al., 2018)

• The section 2.5 is presented in Melnick et al. (2017). Including D. Fern´andez Arenas was published in Astronomy & Astrophysics as TheL₋σ relation for HII galaxies in green

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Abstract

The relationship between the integrated Hβ line luminosity and the velocity dis-persion of the ionized gas of Hii galaxies (HIIGs) and giant Hii regions (GHIIRs) represents an exciting standard candle that with present instrumentation can be used up to redshiftsz _∼3.5. Locally it is used to obtain precise measurements of the Hub-ble constant by combining the slope of the relation obtained from nearby (z _≤ 0.2) HIIGs with the zero point determined from GHIIRs. This thesis aims to contribute with a new calibration of the zero point of the L− σ relation in order to obtain a determination of the local Hubble constant through a detailed analysis of the possible systematic effects associated with theL(Hβ)₋σrelation and their impact in the value of the Hubble constant.

We have tested the possibility of using the sizes of HIIGs as a second parameter in order to reduce the scatter of this correlation. We present a morphological classifi-cation according to the emission line profiles of these objects. We also estimated the dynamical and photometric masses and compared them to test the gravity scenario as the origin of the supersonic motions in these massive star forming regions.

Finally, we compared the existing relation between absolute blue magnitude and velocity dispersion (MB−σ) for HIIGs and GHIIRs, with that followed by old stellar

systems as globular clusters, elliptical galaxies and bulges of spiral galaxies. To this end we have evolved the relationMB−σ during 12 Gyrs and we found that after

dy-namic and stellar evolution is taken into account, all systems follow the same relation suggesting a possible parentage line.

In general, we analysed a possible scenario of the origin of the L(Hβ)₋σ relation followed by HIIGs and GHIIRs and its use to determine locally the value ofH0.

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summarizes the methods normally used for determining distances, and for calculating the Hubble constant; B pertains to the data used for this work and shows the spectra obtained for my sample of objects and their images in Hα taken from NASA/IPAC Extragalactic Database.

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Resumen

La relaci ón entre la luminosidad integrada de la l´ınea de Hβ y la dispersi ón de ve-locidades del gas ionizado de galaxias Hii (GsHII) y regiones Hii gigantes (RsHIIG), representa una exitante candela estándar que puede ser usada con la instrumentaci ón actual hasta corrimientos al rojoz _∼3.5. Localmente es usada para obtener medidas precisas de la constante de Hubble combinando la pendiente de la relaci ón obtenida de GsHII cercanas (z ≤0.2) y el punto cero determinado de las RsHIIG. Esta tesis pre-tende contribuir con una nueva calibraci ón del punto zero de la relaci ón L(Hβ)−σ

con el fin de obtener una determinaci ón local de la constante de Hubble a través de un análisis detallado de los posibles efectos sistemáticos asociados con la relaci ón

L(Hβ)₋σ y su impacto en el valor deH0.

Hemos probado la validez de los tama ños de las galaxias Hii como un segundo parámetro que reduce la dispersi ón de esta correlaci ón. Presentamos también una clasificaci ón morfol ógica y de acuerdo a los perfiles en las l´ıneas de emisi ón de es-tos objees-tos. También estimamos las masas dinámicas y fotométricas y las compara-mos para probar el escenario gravitacional como el origen de los movimientos su-pers ónicos en estas regiones masivas de formaci ón estelar.

Finalmente, comparamos la relaci ón existente entre la magnitud absoluta en el azul y la dispersi ón de velocidades (MB−σ) para galaxias Hii y regiones Hii gigantes con aquella seguida por sistemas estelares viejos como c úmulos globulares, galaxias el´ıpticas y bulbos de galaxias espirales. Hemos evolucionado la relaci ón MB −σ

durante 12 Giga-a ños y encontramos que después de la evoluci ón dinámica y estelar todos los sistemas siguen igual relaci ón sugiriendo una posible conexi ón evolutiva.

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constante de Hubble.

Al final de la Tesis, he agregado dos apéndices; A: donde resumo los métodos normalmente utilizados para determinar la constante de Hubble. B: se refiere a los datos utilizados para este trabajo donde muestro los espectros obtenidos para mi muestra y sus imágenes en Hαtomadas de NED.

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1 Introduction

The mere formulation of a problem is far more essential than it’s solution, which may be merely a matter of mathematical or experimental skills. To raise new questions, new possibilities, to regard old problems from a new angle requires creative imagination and marks real advances in science.

Albert Einstein

I

n 1929, astronomer Edwin Hubble discovered that the universe is expanding, a discovery that still stands as one of the most profound in the twentieth century cosmology. He made this discovery by noticing a relation between the distance of a galaxy d and the speed at which it moves away from us v, and wrote this relation as his famous Hubble law,v =H0d, wereH0 is the “Hubble constant”.

In the past decade a combination of several different distance indicators (e.g. Cepheids, SNe Ia, surface brightness fluctuations, etc.; seeFreedman & Madore 2010) has been used to improve the determination ofH0 = 72±2(random)±7(systematic)

km s−1 _Mpc−1 _{by the Hubble Space Telescope Key Project (i.e. with a 10% uncertainty,}

Freedman et al. 2001). One of the most recent analysis (Riess et al., 2016) uses new HST optical and infrared observations of Cepheids to determine the distance to 8 lo-cal galaxies that hosted recent SNe Ia. Their best estimate for the Hubble constant isH0 = 73.24±1.74 km s−1 Mpc−1 (including random and systematic errors), i.e. the

uncertainty is 3.3%. Pushing the uncertainty toward 1% appears feasible, and can lead to constraints on the mass-energy content of the universe Suyu et al. (2012) that would be unbiased by model-dependent predictions (in contraposition to those that

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can be derived from WMAP,Komatsu et al. 2011). Therefore, the direct determination of an ever increasingly accurate value forH0, obtained by measuring cosmic distances

and mapping the local expansion of the Universe, remains one of the most important measurements in cosmology.

As an alternative strategy to measure the Hubble relation a long term program was started using Hii galaxies (HIIGs), compact objects experiencing a strong burst of star formation and with a high luminosity per unit mass, as a standard candle that can, in principle, be applied out to z ∼ 4 (Melnick et al., 2000; Plionis et al., 2011; Siegel et al., 2005). The potential of HIIGs as distance indicators stems from the existence of a correlation between the luminosity of a hydrogen recombination line, e.g. L(Hβ) (proportional to the number of ionizing photons) and the velocity dispersion of the ionized gas of Hii galaxies and giant Hii regions (GHIIRs) known and investigated for a long time (Bordalo & Telles, 2011;Melnick et al., 1988;Terlevich & Melnick, 1981). It represents an extremely interesting standard candle that, in principle, can be used up to redshiftsz _∼4. Locally it can be used to obtain high precision measurements of the local Hubble parameter. This can be done using a sample of nearby (z _≤ 0.1) Hii galaxies and, crucially, an anchor sample of giant Hii regions in nearby galaxies for which distance moduli independent of the Hubble constant (e.g. Cepheids, Tip of the red giant branch, etc.) are available.

Ch´avez et al. (2012) used a sample of 69 Hii galaxies (z < 0.16), observed with high dispersion spectroscopy at Subaru and the VLT and low dispersion spectropho-tometry at the Observatorio Astron ´omico Nacional San Pedro Martir (OAN-SPM) and the Observatorio Astrof´ısico Guillermo Haro (OAGH) both in Mexico, together with a local calibration of theL(Hβ)−σ relation based on 23 giant Hii regions (GHIIRs) in 9 nearby galaxies, whose distances are known from primary distance indicators, as an alternative “geometric” approach for estimating the Hubble constant. They ob-tained a value of the Hubble constant of H0 = 74.3±3.1 (random) ±2.9 (systematic)

km s−1 _Mpc−1_{, in excellent agreement with, and independently confirming, the most}

recent SNe Ia-based results (the overall uncertainty is 4%). Efstathiou (2014) using as a central calibrator NGC 4258 and the SNIa data base used byRiess et al. (2011) and obtained a lower value ofH0 consistent with the PLANCK results, but the most recent

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combining as anchors the Large Magellanic Cloud, the Milky Way and the mega-maser galaxy NGC 4528, and reduces the systematic errors to a final uncertainty of 2.4%. This value is 3.1 sigma higher than the value obtained byPlanck Collaboration et al. (2016a) ofH0 = 67.8±0.9 km s−1 Mpc−1, predicted by observations of the Cosmic

Microwave Background combined with a flatΛCDM cosmology.

This highlights the importance of using independent methods for theH0

determi-nation. It is possible to reach a better understanding of the systematic uncertainties, effects over every one of those methods, by having alternative determinations.

Several articles indicated that much of the scatter in theL(Hβ)₋σ relation of Hii galaxies is correlated with its size (Bordalo & Telles, 2011; Ch´avez et al., 2014; Telles & Terlevich, 1993). Using the size as a second parameter Ch´avez et al. (2014) show that the scatter in theL(Hβ)−σ relation is radically reduced from an rms∼0.35to an rms∼0.20. This is expected in a scenario where these systems are in virial equilibrium or close to it.

The weakness in Ch´avez et al. (2014) result stems from their use of the sizes de-termined by SDSS and the lack of such data for the GHIIRs. This has prompted a project to homogeneously determine the size of a sample of HIIGs and GHIIRs using as measurement the Petrosian radius in the uand rbands from images of Sloan Dig-ital Sky Survey. The data collected is not only useful to study the second parameter but also to explore different scaling relations between the physical properties of this stellar systems as this provides important clues for understanding their formation and evolution (e.g.Djorgovski, 1993; Kormendy, 1985).

The study of the properties of HIIGs and GHIIRs has generated many questions about the nature of such objects, they have been proposed as a population of proto-globular clusters, or the predecessors of the nuclei of nucleated dwarf elliptical (dE, N), or the aggregate remains of massive young star clusters that merge following their formation in gas-rich mergers, or the end products of small-scale primordial density fluctuations that collapsed in dense environments a the tidally stripped nuclei of otherwise normal dE, N galaxies, (see Has¸egan et al., 2005, and the references therein).

Understanding the processes of formation and evolution of stellar clusters is a key issue and in order to address it, different scaling relations for these objects have

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been investigated and compared with objects better understood; as globular cluster, elliptical galaxies, bulges in spiral galaxies, among others.

I investigated the relationship between the magnitude in blue and the velocity dis-persion for HIIGs and GHIIRs and I compared it with a similar relation followed by other star systems like globular clusters, bulges of spiral galaxies and elliptical galax-ies. However, it is a delicate issue because the direct comparison of their properties with those of old stellar systems is inadequate and necessary corrections for evolution of different parameters like mass, velocity dispersion, size, etc must be carried out. The fundamental assumption is that the massive bursts of star formation (HII galaxies and giant HII regions) are virialized and evolve as closed systems.

In Chapter 6 I compare the relationship between luminosity and velocity disper-sion of GHIIRs and HIIGs with bound systems as elliptical galaxies, bulges of spiral galaxies and globular clusters, but to do this I have used dynamic and photomet-ric evolution models because HIIGs and GHIIRs are younger systems than elliptical galaxies or globular cluster or bulges of spiral galaxies. This comparison is very im-portant because it allows us to test whether the HIIGs and GHIIRs are consistent with virialized systems, which gives a strong support to the gravitational origin of the velocity dispersion being self-gravitating systems where the emission-line profiles reflect the motions in the gravitational potential and the posible gravitational origin for theL(Hβ)₋σ relation.

Nowadays the origin of the turbulent motions (supersonic turbulence) in Hii galax-ies and giant Hiiregions is an open question that had led to propose different mech-anisms; stellar winds of massive stars, gradient pressure produced by the radiation of the stars and gravity where discrete gas clouds are moving in the gravitational potential generated by star and gas mass, and the coefficients found in the empirical relation for Hii galaxies and giant Hii regions are favourable to this last scenario with observed L∝ σ5 _and _R

∝σ2.5 _{close to the coefficients as expected for virialized}

systems (Melnick et al., 1988; Terlevich & Melnick, 1981) and the possible departures in the exponents of sigma are due to inhomogeneities in theM/Lrelation or that the surface brightness is not constant.

Therefore it is important to investigate the possible second parameters (e.g. the radius) and to study the evolution of these systems not only to obtain a precise

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dis-1.1. Motivations and aims of this work

tance indicator but also to understand the physics associated to this empirical relation between the luminosity and the velocity dispersion.

1.1 Motivations and aims of this work

The L(Hβ)−σ relation is very useful as a distance indicator and covers a wide range in redshift. In Ch´avez et al. (2012) the zero-point value, provided by the local calibrator of the GHIIRs, represents the weak link towards a more accurate value of

H0 from theL(Hβ)−σ relation. This is due to the fact that the high-dispersion

spec-troscopic data, that provided the σ measurements, and the spectrophotometry, that provided the L(Hβ) values, date back mostly to the 1980’s, and better measurements can be carried out with modern instrumentation.

In this work the zero point of theL(Hβ)−σ relation was calibrated anew using a sample of 36 GHIIRs in 13 galaxies having accurate Cepheid distances for which their integratedHβ fluxes were measured from observations at the 2m Mexican telescopes of San Pedro Martir and OAGH-Cananea, and using the same telescopes and high resolution spectrographs, the profiles of the emission lines (Hα) were observed to measure the velocity dispersion. The sample includes two GHIIRs in NGC 4258, the “maser galaxy” for which very precise “geometric” distance measurements are available.

In this work the Petrosian radius was estimated from images of Sloan Digital Sky Survey (SDSS) using circular apertures in the bands u and r of the SDSS “ugriz” system. This procedure has a double purpose: firstly, to reduce the dispersion in the

L(Hβ)₋σ relation and secondly, to use accurate velocity dispersion and sizes, in order to obtain better estimates of the dynamical masses. This will allow us to compare these dynamical masses with stellar or photometric masses, obtained through stellar population synthesis models, in order to test the gravity scenario as the origin of the supersonic motions in these massive regions of star formation.

In addition and as the third part of this thesis, the relationship of some param-eters of the massive young star-forming bursts, such as the magnitude in blue and the velocity dispersion, was explored in order to compare it with that followed by older stellar systems like globular clusters, bulges of spiral galaxies and elliptical

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galaxies. However, the direct comparison of these properties of young stellar systems with older systems has to be carefully performed and corrections for evolution are needed. Therefore in this work I have compiled a sample of giant Hii regions and Hii galaxies complementary to the “anchor sample”, and corrections for photometric and dynamical evolution have been applied to this sample using models ofstarburst99

and the dynamical evolution models, presented in Lamers et al. (2010).

In summary the aims of this work are:

• To obtain an extended “anchor sample” of giant HII regions in nearby galaxies for which the distance are determined using primary calibrators as Cepheids or the Tip of the red giant branch (TRGB).

• To determine the Hubble constant using the L(Hβ)₋σ relation as a distance indicator, with the new calibration of the zero point, and analyzing the possible systematic effects associated with the method and the calibrators GHIIRs and HIIGs.

• To estimate the sizes of GHIIRs and HIIGs in order to obtain a second parameter in the L(Hβ)₋σ relation, and to explore the connection between photometric and dynamical masses.

• To analyze the dynamical and photometric evolution of GHIIRs and HIIGs with the purpose of comparing their scaling relations with those followed by older stellar systems, as elliptical galaxies, bulges of spiral galaxies and globular clus-ters.

1.2 Structure of this Work

Through the next chapters, I describe the fundamentals over which this thesis is developed:

• Chapter 2 describes the fundamental physical properties of Hii galaxies and

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1.2. Structure of this Work

• Chapter 3 presents a complete description and data analysis of the “Anchor

Sample” used to calibrate the zero point of theL(Hβ)−σ relation.

• Chapter 4 explores the determination of the Hubble constant via HIIGs and

GHIIRs and the sensitivity of the method to changes in different parameters including, photometry, extinction laws, etc.

• Chapter 5 shows the complex task of the size determination of massive stellar

clusters and their classification according to the surface brightness profiles using images from Sloan Digital Sky Survey in the band u and r . I also derive the fundamental plane for HIIGs and GHIIRs including a comparison between the stellar and dynamical masses.

• Chapter6 presents the dynamic and photometric evolution of GHIIRs and

HI-IGs, and their comparison with the fundamental scaling relations followed by older stellar systems.

• Finally,Chapter7presents the general conclusions of this thesis and outline the future work in order to improve our understanding of the underlying physics of the empirical relation between the luminosity of the recombination lines and velocity dispersion and its use as an independent distance indicator.

• In addition, this Thesis presents two appendices: A: where I present from the literature the problems and derivation of the distance ladder and the Hubble constant, and B: the high resolution profiles and the low resolution spectra for the GHIIRs.

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2 Properties of H

ii

Galaxies and Giant

H

ii

Regions

Science is not only a disciple of reason but, also, one of romance and passion.

Stephen Hawking

2.1 H

ii

Galaxies, Blue Compact Dwarfs and Green Peas

N

owadays the Hii galaxies are also known as BCDs, however the term Hii galaxy is used when the objects have been selected for their strong and narrow emission lines (Terlevich et al., 1991a) while BCD galaxies are selected for their blue colours and compactness. Although these selection criteria provide samples of galax-ies with a young population, in contrast with Hii galaxies, the underlying galaxy is clearly visible in the images and the spectrum.

The sample ofCh´avez et al. (2014) shows a zoo of colors for the Hii galaxies, many of them look green or blue in the images from Sloan (Figure2.1), and the question is: are Hii galaxies Blue Compact Dwarfs (BCDs) or the recently discovered Green Peas galaxies (GPs) ?

BCDs are galaxies with low surface brightness MB > −18 mag, characterized by

their compact morphology in the optical, sizes less than 1 kpc and strong emission lines superposed on a blue continuum (Thuan & Martin, 1981). The blue colors and

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Figure 2.1: A selection of colour images of HIIGs. Taken from (Ch´avez et al., 2014)

strong emission lines indicate intense current star formation activity. Population syn-thesis models yield typical star formation rates between 1 and 20 M yr−1, for

exam-ple in works asCair ´os et al. (2001);Kong & Cheng (2002);Mas-Hesse & Kunth (1999). Their current star formation rate and neutral gas imply gas consumption timescales of 109_{yr, much shorter than the age of the Universe. This fact, combined with the}

low metal abundances (1/50 < Z < 1/3 Z; Gil de Paz et al., 2003) prompted some questions as: are these objects young galaxies undergoing their first starbursts or have they experienced an episodic star formation history (Thuan et al., 1999)? Recent observations have however confirmed that most BCDs have underlying older stellar populations at least109_{yr old (e.g}_{Amor´ın et al., 2007}_;_{Cair ´os et al., 2001}_).

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2.2. Giant extragalactic Hii regions and Hii galaxies

continuum in BCDs: we need to increase the continuum of Hii galaxies by a factor at least of 50 times in order to observe similar features as for the continuum of BCDs, while not all BCDs are dominated by Hii regions in their spectra. Therefore only those BCD that satisfy the criterion EW(Hα)>200_{A are HII galaxies.}˚

In Hii galaxies the luminosity is completely dominated by the burst and as a consequence they show the spectrum of an Hii region. They are very compact and are discovered mainly in spectroscopic surveys due to their strong narrow emission lines, and in principle, the information for these massive stellar clusters HIIGs, is not contaminated by the parent galaxy.

On the other hand, what does happen with the green colour in Hii galaxies. Are Hii galaxies the Green Pea galaxies? The GPs are a class of very compact, extreme star formation galaxies at low redshift (0.11 < z < 0.36) that have been recently discovered in the “Galaxy Zoo Project”, first reported and analyzed by Cardamone et al. (2009) using the Sloan Digital Sky Survey DR7.

The main properties of the Green Peas can be summarized as follows: very faint continuum emission and strong optical emission lines, mainly [OIII]λ5007 with very high equivalent widths up to 2000 ˚A.

The GPs are extremely compact objects with typical sizes smaller than 5 kpcs, with large star formation rates up to 30 M yr−1, low stellar masses between 108 and

1010.5_{, properties that represent the largest specific star formation rates (SFRs) seen in}

the local universe. Those objects are preferably located in low-density environments and are rare systems, less than 2 objects per square degree. The Green Peas are a genuine population of metal-poor galaxies with metallicity around 0.2 Z (Amor´ın et al., 2010).

In figure 2.2 we can see the position of different types of star-forming galaxies in the diagnostic diagram [NII]/Hαvs [OIII]/Hβ (BPT diagram; Baldwin et al., 1981).

2.2 Giant extragalactic H

ii

regions and H

ii

galaxies

Sargent & Searle (1970) identified a class of compact galaxies whose spectra are very similar to those of giant Hii regions in spiral galaxies. They called themisolated

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Figure 2.2: BPT diagram for different star-forming galaxies. The Green Peas (green crosses) were taken from the sample of Cardamone et al. (2009) using SDSS DR7. The circles (yellow and blue) were taken from the spectrophotometric catalogue ( Ter-levich et al., 1991a), orange circles are objects with EW(Hβ)> 50_{A and the blue cir-}˚ cles EW(Hβ)<50_{A. The blue squares are objects from the catalogue of}˚ _{Moustakas &} Kennicutt (2006) the objects used have EW(Hβ)< 50A, the yellow stars are the H˚ ii galaxies used in this thesis taken from Ch´avez et al. (2014).

extragalactic Hii regions. After analysing their spectra they conclude that the galaxies

are ionised by massive clusters of OB stars and are metal poor systems (Searle & Sargent 1972, Lequeux et al. 1979, French 1980,Kunth & Sargent 1983).

The giant extragalactic Hii regions (GHIIRs) are zones with intense star formation observed in irregular galaxies, disks of spiral galaxies. These regions are due to the presence of a large number of young and massive stars whose ultraviolet flux ionize

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the gas around them indicating the occurrence of active star formation (e.g. Chu & Kennicutt, 1994).

The typical sizes are in the range of a hundred parsecs although there are cases in the literature of one order of magnitude greater sizes, however, they are referred to as multiple regions, which can be resolved in several components only distinguishable with high spatial resolution for nearby galaxies (Bosch et al., 2002). These regions have a luminosity in Hαof the order of1039_{erg s}−1 _{with a rate of ultraviolet photons}

between1051₋₁₀52_s−1 _{, ionizing large amounts of gas}₁₀5₋₁₀6 _M

with low density

Ne ≈ 1−100 cm−3 (Kennicutt 1984, Shields 1990, Fuentes-Masip et al. 2000a,

Garc´ıa-Ben´ıtez 2009). Some examples of GHIIRs are 30 Dor in the Large Magellanic Cloud (LMC) and NGC 604 in the spiral galaxy M33.

GHIIRs are the best scenarios to study star formation in the local universe occurring at a rate of the order of 10−2 _M

yr−1 Mpc−3 (Madau et al., 1996), the light is almost

completely dominated by large numbers of massive stars recently formed ionizing the gas. GHIIRs can be classified in a scale intermediate between star forming regions of lower scale, i.e. Orion in the Milky Way, and extreme star formation with intense bursts (“starburst”;Searle & Sargent, 1972).

The characteristic definition of galaxy starburst is that its spectrum is dominated by a population of young stars or by an Hii region with a very young burst, this type of galaxies come from early observations in the late seventies and early eighties of star-forming regions, obscured by dust, in the centres of nearby galaxies although the concept as such dates back to much earlier (Searle et al., 1973).

The intensity level of a starburst is highly variable, according to Terlevich (1997). In a starburst galaxy the luminosity produced in the burst(LSB)is much larger than

the light provided by the rest of the galaxy, i.e.,LSB LG, in a galaxy with one burst

of star formation LSB ∼ LG and in a normal galaxy we have LSB LG. Terlevich

(1997) proposes different phases of the starburst,nebular phasecharacterized by the presence of strong emission lines from gas photoionized by young massive stars, with an age less than some 10Myr it corresponds to Hii galaxies;Early continuum phase with emission lines relatively weak with an age between 10 and 100 Myr, e.g. BCDs, and finally; Late continuum phase where the continuum is blue and dominated by Balmer absorptions, typical age is several 100Myr up to ~1Gyr.

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The Hii galaxies are massive bursts of star formation at dwarf irregular galaxies, almost completely dominating the total luminosity output (up to 1043 _{erg s}−1 _{in H}_α

line luminosity), with total masses less than1011_M

and radius of less than2kpc with

a surface brightness µV > 19mag arsec−2. They have a high star formation rate and

some studies have revealed that the recent star formation is concentrated in super star clusters (SSC) with sizes ~20pc (Garc´ıa-Ben´ıtez, 2009; Searle & Sargent, 1972; Telles, 2003).

Hence both GHIIRs and HIIGs offer an important opportunity to study violent and intense episodes of star formation. In figure2.3we can see spectra and images of local Hiiregions and of one Hii galaxy.

2.2.1 Morphology and structure

HIIGs are compact and isolated objects, as was suggested by Melnick et al. (1987). HIIGs present a variety of morphologies and the study of their morphological prop-erties can be difficult, because although all Hii galaxies have at least one giant Hii region of star formation this may or may not be in the center. Telles et al. (1997) classified HIIGs in two classes Type I: which have irregular morphology and high luminosity, andType II:compact objects with regular outer structure.

Studying the surface brightness profiles in HIIGs, I found three main types: 1: a single exponential fit represents the whole range of radii of the profile; 2: double profile with a plateau due to a double morphology and an exponential fit to the outer regions and 3: a steep bright central region and disk-like component. In general, the central part of Hii galaxies is dominated by one or more knots of star formation giving rise in most cases to excess surface brightness, and the outer parts of the luminosity profiles of Hii galaxies are well represented by an exponential scaling law. In figure2.4we can see the light profiles proposed by Telles et al. (1997).

2.2.2 Age of H

ii

galaxies

In general, the ages of Hii galaxies and starbursts are estimated from the equiva-lent width of Hβas was initially suggested byDottori & Bica (1981a) who investigated

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(a)

(b)

(c)

Figure 2.3: (a) Spectra and images of one HII galaxy ( IZw18); (b) Hii region in M33 (NGC 604) and (c) 30 Doradus, the spectra were taken from Sloan Digital Sky Survey (SDSS), (D´ıaz et al., 1987) and Terlevich et al. (1991b), credit for the images Hubble Space Telescope (HST).

the variation of the equivalent width of Hβas a function of the evolution of the ioniz-ing stars in Hii regions. Usually, two models of time evolution are used: An

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instan-Figure 2.4: Light profiles proposed by Telles et al. (1997) d: A single exponential fit.

dd: Double profile. bd: A steep bright central region and outer disc-like component.

taneous starburst model, which assumes that all stars are formed simultaneously in a short starburst episode and this generally is applied to an individual low mass star cluster. Alternatively, a model with a continuous starburst, which assumes active star formation is constant in time, that could also be understood as a sequence of small bursts separated by a small interval of time. In any case both models are simply limit cases of the possible star formation evolution. In figure 2.5 the two models describe the evolution of EW(Hβ) as a function of time. These models of stellar population syn-thesis, used to estimate the age of young galaxies can be obtained using starburst99

(Leitherer et al., 1999) available in http : //www.stsci.edu/science/starburst99. Ter-levich et al. (2003, 2004) have showed that the second model (continuous starbursts) fits better the observations of Hii galaxies, indicating that while the observed emis-sion lines trace the present burst, the underlying continuum contains the star forma-tion history of the Hiigalaxy, which indicates that these are not truly young systems and that they have probably experienced previous star formation episodes.

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(a) (b)

Figure 2.5: Hβ equivalent width vs. time,αis the IMF exponent. Solid line: α = 2.35,

Mup = 100M Long-dashed line: α = 3.30, Mup = 100M Short-dashed line: α = 3.30, Mup= 30M (a): instantaneous, (b):continuous mode.

2.2.3 Chemical composition of H

ii

galaxies

Hii galaxies are metal poor systems, the abundances of metals in these systems range between 1/50Z-1/2Z. The first metallicity analyses in these systems were

made bySearle & Sargent (1972) and they showed that the abundances of oxygen and neon of IZw18 and IIZw40 are lower than the abundances found in the interstellar gas in the solar neighbourhood. Works with different samples have shown the same results and concluded that Hii galaxies are metal poor systems (Searle & Sargent 1972, Lequeux et al. 1979, French 1980, Kunth & Sargent 1983, Terlevich et al. 1991b, Pagel et al. 1992, Holovatyy & Melekh 2002, P´erez-Montero & D´ıaz 2003,Izotov et al. 2006).

The metallicity of Hiigalaxies is an important parameter to characterise its state of evolution and to link them to other objects that present similar properties, e.g. dwarf Irregulars (dI) or Low Surface Brightness Galaxies (LSBG). The metallic content is also the ground of global relations for the luminosity, the fraction of the mass of gas and

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the width of the emission lines (Hunter & Hoffman, 1999; Pagel, 1997; Terlevich & Melnick, 1981).

Particularly interesting is the fact that since Hiigalaxies are chemically unevolved systems, the analysis of helium abundances in these systems is a good method for determining primordial helium abundances.

In the Hii regions, the oxygen is the most abundant of the metals that they con-tain. Therefore the oxygen abundance is normally considered as representative of the metallicity of Hii galaxies. The range of the abundance of oxygen in Hii galaxies is

7.1_≤12+log(O/H)≤ 8.3, obtained for more than 100 Hii galaxies with good quality data (P´erez-Montero & D´ıaz, 2003). Recent calculations made byCh´avez et al. (2014), using a sample of 100 Hii galaxies between redshift 0.02 and 0.2, found a median value of 12+log(O/H) =8.08 whose distribution of the oxygen abundance is shown in the figure2.6.

Figure 2.6: Distribution of oxygen abundances in Ch´avez et al. (2014). The dashed line shows the median.

2.2.4 H

ii

regions kinematics

Studying the kinematics of Hiiregions has proved not to be an easy task to achieve. The gas movements must be observed in the velocity profiles of the emission lines

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of the gas in the Hii regions, which show either a broad supersonic component or several components. Previous studies have shown evidence of low-intensity broad components in the integrated spectrum of Hii regions, not only in galactic regions but also in external galaxies.

Arsenault & Roy (1986) found that a significant fraction of their sample of Hii re-gions showed integrated profiles that are better fitted by a Voigt profile than a Gaus-sian profile, whereas Chu & Kennicutt (1994) found the same broad component for nearby extragalactic Hii regions. So far the interpretation of extended components has not been studied for reasons such as the low intensity of the broad component that makes it difficult to confirm its existence (Rela ˜no & Beckman, 2005a).

The kinematics of extragalactic Hiiregions, notably 30 Doradus in the Large Mag-ellanic Cloud, and NGC 604 in the spiral galaxy M33 has been carefully studied and rapid motions of the their gas have been found (Chu & Kennicutt, 1994; Mu ñoz-Tu ñ ón et al., 1996;Yang et al., 1996). Studying gas kinematics in Hiiregions through the emission line profiles, low-intensity components with high velocities were found. The interpretation however of these kinematic components is complicated due to the high velocities, low densities and large sizes of the emission region (Rozas et al., 2006, and references therein).

Attempts to explain these issues have suggested a variety of mechanisms, typically, the kinematic components have been associated with expanding shells, as in NGC 604 (Sabalisck et al., 1995) or 30 Doradus (Yang et al., 1996). Several works related to the kinematics of Hii regions have shown broad components with supersonic velocities in the emission line profiles, where a simple Gaussian is not enough to fit the profiles; such results have been confirmed by recent works on kinematics of the gas and stars in circumnuclear Hii regions (H¨agele et al., 2013).

There are open question n the filed since some authors are in favor for the existence of a broad component that could explain the wings of the integrated profile of the emission lines (D´ıaz et al. 1987,Mu ñoz-Tu ñ ón et al. 1996,Terlevich et al. 1996,Melnick et al. 1999, Hägele et al. 2007, Hägele et al. 2009). While others support the idea of a profile with two wings, one blue and one red surrounding the main component (Chu & Kennicutt, 1994;Rela ño & Beckman, 2005b; Rozas et al., 2006).

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2.2.5 L

(H

β

)

₋

σ

relation of H

ii

galaxies

Melnick (1977, 1978) found that the turbulent width of the nebular emission lines is correlated with the GHIIR diameters. In the follow up work, Terlevich & Melnick (1981) found a tight correlation between the turbulent emission lines velocity disper-sion and their integrated luminosity: the L(Hβ)₋σ relation. This correlation, valid for HIIGs and GHIIRs roughly follows the relations:

luminosity ∝(linewidth)4

size_∝(linewidth)2

which are valid for pressure supported stellar systems (elliptical galaxies, bulges of spiral galaxies and globular clusters). Therefore they concluded that Hiigalaxies and giant Hii regions are self-gravitating systems in which the width of the profile in the emission line represents the velocity dispersion in the gas clouds in a complex gravitational potential of the gas and stars. They also claimed that the scatter in the

L(Hβ)₋σ relation is correlated to the metallicity.

In particular,Melnick et al. (1987), from the integrated properties of GHIIRs, found that the turbulent motions of their gaseous component are supersonic. Furthermore, they obtain correlations well represented by power laws of the form:

Rc∼σ2.5±0.5

L(Hβ)∼σ5.0±0.5

They also suggest that the scatter in these correlations is due to the effects of the metallicity and that observations are better explained by a model in which the giant Hiiregions are assumed as virialized systems of discrete gas fragments that are being ionized by a central star cluster. Therefore the global properties of giant Hii regions must be modelled in term of masses, ages and chemical composition of the ionizing cluster. However, they recognize the possibility that stellar winds could have some, then unknown, effect on the velocity dispersion of the nebular gas. (e.g.Rozas et al., 2006).

Melnick et al. (1988) studying the L(Hβ)₋σ relation for Hii galaxies in a sam-ple of objects that are included in the Spectrophotometric Catalogue of Hii galaxies Terlevich et al. (1991b) , found a relationship of the form:

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logL(Hβ) = (4.70±0.30) logσ+ 33.61±0.50, δlogL(Hβ) = 0.29 (2.1)

which is represented by a dashed line in figure2.7with H0 =100 km s−1 Mpc−1

Figure 2.7: L(Hβ)₋σ relation relation for GHIIRs and HIIGs. The solid line shows a least squares fit to GHIIRs and the dashed line, the corresponding fit for HIIGs (from Melnick et al., 1988).

They also inferred that metallicity (O/H) is a major component of the scatter in the previous relation, and they propose as a distance indicator:

MZ =

σ5

(O/H) (2.2)

obtaining a new relation:

logL(Hβ) = (1.0_±0.04) logMZ+ (41.32±0.08), δlogL(Hβ) = 0.271 (2.3)

It is necessary to note that this last relation uses the distance scale ofAaronson et al. (1986) withH0 = 89±10 km s−1 Mpc−1.

More recentlyCh´avez et al. (2014) analyzed a sample of 128 Hii galaxies selected from the Sloan Digital Sky Survey with high equivalent widths of their Balmer emis-sion lines (e.g. EW(Hβ>50 ˚A)) to test the validity of the L(Hβ)−σ relation and its

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use as an accurate distance indicator. Ch´avez et al. (2014) obtained high S/N high-dispersion ESO VLT and Subaru Echelle spectroscopy to accurately measure the ion-ized gas velocity dispersion and low dispersion wide aperture spectrophotometry at Cananea and San Pedro M´artir (Mexico) to measure integrated Hβ fluxes. They found that the L(Hβ)₋σ relation, for the best quality data only including systems with logσ < 1.8, is:

logL(Hβ) = (33.71±0.2) + (4.65±0.14) logσ, δlogL(Hβ) = 0.332 (2.4)

We can see this relation in the figure 2.8.

Figure 2.8: L(Hβ)−σ relation fromCh´avez et al. (2014). The inset shows the distribu-tion of the residuals of the fit.

2.3 The physics of the

L

(H

β

)

₋

σ

relation

There is in the literature a big debate about the interpretation of the width in the profile of the emission lines in Hii regions and Hii galaxies. Terlevich & Melnick (1981) proposed a model in which the motions of the Hiigalaxies gaseous component

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2.3. The physics of the L(Hβ)₋σ relation

Figure 2.9: Correlation between blue magnitude and velocity dispersion for elliptical galaxies, bulges of spiral galaxies, globular clusters and Hiiregions. The dashed line represents the linear fit to all data. The solid line is the fit for elliptical galaxies, and the dotted line shows the fit to Hiiregions. Taken fromTerlevich & Melnick (1981).

are of gravitational origin. The base of this argument is explained by the correlation

L(Hβ)∝σ4 _and _R _∝_σ2 _{observed in H}_ii _{galaxies. These correlations are expected for}

virialized systems as they are observed in elliptical galaxies, bulges of spiral galaxies and globular clusters.

An additional fact that can be contributing to the origin of the supersonic turbulent motions in the gaseous component of Hii galaxies is the stellar wind generated by massive evolved stars; although this effect is dominant in the case of evolved Hii regions (Melnick et al., 1999) it does not seem to be very important for Hii galaxies.

Figure 2.9 shows the comparison between the luminosity and velocity dispersion for system gravitationally bound; elliptical galaxies, bulges of spiral galaxies and globular clusters and GHIIRs. For producing this plot the ionising stellar clusters were evolved as closed systems (i.e at constant mass) until their M/L ratios became similar to M/L ratios of gravitationally bound systems. The results give strong support to the gravitational origin of the velocity dispersion in GHIIRs.

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to the fundamental plane defined by elliptical galaxies (figure 2.10). However the scatter observed in the L(Hβ)−σ relation may be due to the presence of a second parameter, perhaps possible variation in the initial mass function (IMF), rotation or the duration of the burst of star formation that powers the emission lines (Melnick et al., 2000). This scatter in theL(Hβ)₋σ correlation can be reduced rejecting objects with σ >65 km s−1 ₍_{Melnick et al. 1988}_,_{Koo et al. 1995}_).

Figure 2.10: Fundamental plane for HIIGs and normal elliptical galaxies form Telles (1995). The radii and magnitudes of H II galaxies are measured from continuum images. The velocity dispersions are the widths of the emission lines. The upper panel is a continuation of the lower panel, for HIIGs the luminosity of the continuum have been evolved Hubble time.

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2.3. The physics of the L(Hβ)₋σ relation

2.3.1 Age effects

In order to reduce the systematic effect associated to fast evolution of the ionizing stars, it is necessary to constrain the sample to objects with the largest EW(Hβ), to ensure choosing the youngest starburst and minimize the effect of the underlying population and stellar winds. The evolution of the starburst has different stages: after the first 3Myr the emission fluxes decay and then remain constant up to around 6 Myr. Therefore in this range of ages the attenuation inL(Hβ) can be estimated directly from the changes in the equivalents width (Copetti et al., 1986;Terlevich & Melnick, 1981).

2.3.2 Extinction effects

When the light of celestial objects traverses the interstellar medium it suffers from a combination of absorption and dispersion in dusty zones, depending on the wave-length, mainly affecting visible and ultraviolet light; this effect is called extinction. Therefore the extinction has a systematic effect on the L(Hβ)₋σ relation because it attenuates the Hβ line. Two possible sources of extinction must be considered: dust in our Galaxy and dust in the Hii galaxies themselves. This effect combines with the underlying population of intermediate and old age, observed in absorption in the Balmer series as the emission lines overlap the absorption stellar lines and this effect is more important for the higher order Balmer lines. The correction by extinction can be determined from the Balmer decrement (Melnick et al., 1987,1988;Osterbrock, 1989a) and the correction for the underlying absorption will be discussed in section3.2.1.

2.3.3 Metallicity effects

The metallicity has an important effect on the L(Hβ)₋σ relation as was argued by Terlevich & Melnick (1981) who found that the scatter in the relation is correlated with the metallicity. This result however has not been confirmed in the recent work ofCh´avez et al. (2014).

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2.3.4 Size effect

If theL(Hβ)₋σrelation is a consequence of young massive clusters being in virial equilibrium, then the strongest candidate as the second parameter is the size of the star-forming region (Melnick et al., 1987; Terlevich & Melnick, 1981). Ch´avez et al. (2014) explored this possibility using the SDSS measured size at u,g,r,i,z bands. They used measurements of the Petrosian radius of SDSS corrected by seeing. In particular, using multiparametric fits with oxygen abundance O/H (stimated using empirical methods N2 or R23), EW or continuum colour they found that the scatter in the

L(Hβ)₋σ relation is only reduced significantly when they use the size as determined in the u-band.

2.4 H

ii

galaxies as Distance Indicators and cosmological

probes

The relationship between the integrated Hβ line luminosity and the velocity dis-persion of the ionized gas of Hiigalaxies and giant Hiiregions represents an exciting standard candle that presently can be used up to redshifts z∼3.5. Locally it is used to obtain a measurement of the Hubble constant by combining the slope of the relation obtained from nearby Hiigalaxies (z ≤ 0.2) with the zero point determined from gi-ant Hii regions located in nearby galaxies for which distances can be estimated from primary methods, Cepheids, RR Lyrae and Eclipsing Binaries.

Hii galaxies have been proposed as such an alternative for the SNIa to measure the Hubble constant, and a first attempt to estimate H0, using giant Hii regions as

local calibrators, was presented in Melnick et al. (1988) and recently by Ch´avez et al. (2012). Furthermore, the group investigated the viability of using Hii galaxies to constrain the dark energy equation of state (Ch´avez et al., 2016;Terlevich et al., 2015), accounting also for the effects of gravitational lensing (Plionis et al., 2011; Terlevich et al., 2016).

The potential use of HII galaxies as distance indicators, as an alternative to the traditionally used SNIa, is based on the following facts:

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2.4. Hii galaxies as Distance Indicators and cosmological probes

• Local and HIIGs at high redshifts define a relationship between Hβ line lumi-nosity and velocity dispersion which remains valid at cosmological distances (z

∼3.5). Thus, high redshift HIIGs can be used as alternative tracers of the Hubble expansion.

• HIIGs can be observed at higher redshifts than those currently probed by SNeIa samples.

• The differences between cosmological models is more important at redshiftz >

2, as is showed in figure 2.11.

Figure 2.11: Left panel: The expected distance modulus difference between the dark energy models shown and the referenceΛ-model. Right panel: The expected distance modulus differences once theΩm−w(z)degeneracy is broken (imposing a uniqueΩm

to all models). Taken fromPlionis et al. (2011)

From figure2.11we can see the difference between some cosmological models with regards to one used as reference:

∆µ=µΛ−µmodel, (2.5)

where∆µis the difference between distance moduli of models,µmodel, and the

refer-ence model µΛ. The relative deviations of the magnitude between different models

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very accurate photometry of the objects used as distance indicators in this range of redshift. The largest deviations in the distance modulus occur at redshiftz ≥1.5and therefore high-z tracers are needed to effectively constrain the values of the dark en-ergy equation of state, in fact at redshifts higher than those currently probed by SNIa. The above arguments show the necessity of at least two independent cosmological probes in order to break the degeneracies.

Therefore the potential of using HIIGs as distance indicators is based on the fact that we can see HIIGs at redshifts higher than SNIa. We know that the Hubble func-tion depends on the cosmological parameters following the relafunc-tion:

H(z) =H0E(z) (2.6)

where

E2₍_z_{) =}

Ωm(1 +z)3+ Ωk(1 +z)2+ ΩQexp

3

Z z

0

1 +w(x) 1 +x dx

(2.7)

derived from the Freedman equations with: Ωm, Ωk, ΩQ(≡ 1−Ωm −Ωk), the matter,

curvature, dark energy densities, respectively normalized to the present epoch. In practice, the distance modulus can be related to the luminosity distance (in Mpc) in which the cosmological parameters enter:

µ=m₋M = 5 logDL+ 25 (2.8)

with

DL=

c(1 +z)

H0

√ Ωk

sinh

p

Ωk

Z z

0

dx E(z)

. (2.9)

For a flat universe (Ωk= 0) we have:

DL=

c(1 +z)

H0

Z z

0

dx

E(z) (2.10)

Taking the concordance ΛCDM cosmology as the reference model, Terlevich et al. (2015) calculated the distances and hence the luminosities for local and high redshift HIIGs. The result is a notoriously tight correlation that justifies the use of theL(Hβ)₋

σ relation as a distance estimator over a wide redshift range. It is remarkable that the Hubble diagram covers a huge dynamical range with a single distance estimator connecting galaxies in the local group (LMC, SMC, NGC 6822, M 33) and up to at

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2.4. Hii galaxies as Distance Indicators and cosmological probes

Figure 2.12: Hubble diagram for giant H II regions and H II galaxies, the residuals are in the bottom panel. The lines are different cosmological models. Taken from Terlevich et al. (2015)

least z ∼ 2.3, a range of almost 30 magnitudes in distance modulus or more than 5 dex in redshift (see figure2.12).

TheL(Hβ)−σrelation found, for the joint local HIIGs (107 objects) and GHIIRs (24 objects) samples, is

logL(Hβ) = 5.05_±0.097 logσHα+ 33.11±0.145 (2.11)

and

µobs = 2.5 logL(Hβ)σ−2.5 logF(Hβ)−100.2 (2.12)

withlogL(Hβ)σ estimated from the equation 2.11

So, to restrict the set of cosmological parameters they minimised theχ2 _function:

χ2(p) =

n

X

1

µobs

i (σi, fi)−µthi (p, zi)

σ_µobs i

(2.13)

whereµobs

i (σi, fi)are the observed distance moduli obtained from equation2.12; σi is

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from equation 2.8, obtained from the redshift and using a particular set of cosmo-logical parameters; and σµobs

i are the errors in observed distance moduli propagated from the uncertainties in the velocity dispersion, fluxes, slope and the zero point of the distance estimator given by equation2.11.

Figure 2.13 shows the comparison of restrictions on the plane (Ωm, w0) obtained

for 25 high-z HIIGs and the local sample (131 HIIGs and GHIIRs). The level contours 1 and 2σ are shown and the recent results for 580 SNIa, CMB and BAO (Suzuki et al., 2012). From the comparison of panels a and b it can be seen that there are no systematic shifts between the HIIGs and SNIa solutions.

Figure 2.13: Panel (a): comparison of restrictions on the plane (Ωm, w0) obtained for

25 high-z HIIGs and the local sample (131 HIIGs and GHIIRs. Panel (b): the recent results for 580 SNIa, CMB and BAO, The level contours 1 and 2σ are shown. Taken fromTerlevich et al. (2015).

2.5 L

₋

σ

relation in green and the Hubble constant

The velocity dispersion measured using a Gaussian fit to the Hβ lines is slightly larger than that measured from the [OIII] 5007 ˚A profiles. The cause of this difference

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2.5. L₋σrelation in green and the Hubble constant

which has already been noticed by other authors Hippelein (1986) and Bordalo & Telles (2011) is not clear. In general, the Balmer recombination lines for Hii galaxies are systematically wider than the forbidden oxygen lines, perhaps due to the ion-ization structure where more excited ions are closer to the ionizing source and in denser regions. Caution is therefore needed when using the forbidden oxygen lines to calibrate theL₋σ relation.

This issue and its consequences for the distance estimator are thoroughly discussed inMelnick et al. (2017) where we find the mixedL(Hβ)−σ[OIII]relation to be at least

as powerful as the canonicalL₋σ relation as a distance estimator, and we show that the evolutionary corrections do not change the slope and the scatter of the correlation, and therefore, do not bias theL₋σ distance indicator at high redshifts.

Locally, however, the luminosities of the GHIIRs that provide the zero-point cali-brators are sensitive to evolutionary corrections and may bias the Hubble constant if their mean ages, as measured by the equivalent widths of Hβ, are significantly differ-ent from the mean age of the Hii galaxies. Using a small sample of 16 ad-hoc zero point calibrators we obtain a value of H0= 68.4₋+33..42 km s−1 Mpc−1which is consistent

with the CMB based modern determinations, and that is not biased by evolutionary corrections. In figure 2.14 we present the the L−σ relation in green corrected by evolution and the fits using standard least-squares (LSQ) ignoring the observational errors and maximum-likelihood (ML).

The scatter of theL−σrelation using only [OIII] 5007 is substantially larger than for L(Hβ) mostly because the [OIII] luminosities of the starburst that power Hii galaxies evolve faster than the Hβ luminosities as the ionising stars age. Thus, while the advantages of using [OIII] 5007 for measuring σ are strong, the same is not the case for the luminosities. The green L₋σ relation is not as good as the canonical L₋ σ relation as a distance indicator; however combining Hβ luminosities with [OIII] velocity dispersions provides the best compromise; the scatter of this “mixed” L−

σ relation is somewhat smaller than that of the “canonical” relation using Hβ for both parameters, with the additional advantage that in Hii galaxies [OIII] 5007 is significantly stronger than Hβ and much less affected by thermal broadening.

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Figure 2.14: The “mixed” L₋σ relation corrected for evolution using at a fiducial “age” of EW(Hβ)∼133 ˚A corresponding to about 3.5Myr for the Geneva isochrones. Here the mean value of EW(Hβ) is shown for each sub-sample and for the complete sample of Hii galaxies. The lines show the LSQ and ML fits, The green dots cor-respond to the 16 GHIIRs used to calibrate the zero point of the relation and thus establish the value of H0. The inset shows the χ2 curve (actually χ2 −χ2min) and the

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3 The determination of the Hubble

constant: The anchor sample

If I have seen further than others, it is by standing upon the shoulders of giants.

Isaac Newton

T

heuse of theL(Hβ)−σrelation as a distance indicator and as a tool to derive the Hubble constant, requires accurate determination of both the luminosity and the FWHM or velocity dispersion of the emission lines in GHIIRs and HIIGs.

To determine the value of the local Hubble constant, the L(Hβ) ₋σ relation for HIIGs is anchored to a sample of GHIIRs in nearby galaxies having accurate distances determined using primary distance indicators. Although the scatter of theL(Hβ)−σ

distance indicator is about a factor of two to three larger than the one based on SNIa (Ch´avez et al., 2014), this is partially compensated by the larger number of local cali-brators available for theL(Hβ)₋σ method, i.e. galaxies with distance determination independent of redshift, compared to those available for SNIa, plus the fact that the number of GHIIRs per galaxy is usually more than one, thus reducing the uncertainty per anchor galaxy.

A fundamental problem with the determination of the Hubble constant using SNIa is related to the low expected rate of SNIa inside the 30 Mpc reach of the HST for accurate Cepheid studies (Riess et al., 2016). The present sample of SNIa in galaxies with accurate distance estimates is 19 and it would not substantially increase over the

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remaining lifetime of the HST given that their average rate is only about one SNIa per year (Riess et al., 2016). On the other hand the number of anchor galaxies with GHIIRs and accurate Cepheid distances is presently 73 in the primary sample, with a total of 130 GHIIRs. Moreover, GHIIRs in special galaxies like the LMC, the SMC, and NGC 4258 with very accurate redshift-independent distance determinations are also included in the sample of anchor galaxies.

To determine H0 Ch´avez et al. (2012) relies on the slope of theL(Hβ)−σ relation

obtained from the sample of 69 Hiigalaxies and on a zero-point provided by the local calibrator of the 23 GHIIRs located in 9 different galaxies, whose distances are known from primary distance indicators. However the zero-point value, provided by the lo-cal lo-calibrator of the GHIIRs, represents the weak link towards a more accurate value ofH0 from theL(Hβ)−σ relation. This is due to the fact that the high-dispersion

spec-troscopic data, that provided the σ measurements, and the spectrophotometry, that provided the L(Hβ) values, date back mostly to the 1980’s, and better measurements can be carried out with modern instrumentation.

In this chapter, I present new data for 36 giant Hii regions in 13 galaxies of the “anchor sample” that includes the megamaser galaxy NGC 4258, in order to calibrate the zero-point of theL(Hβ)−σ relation. The data are the result of the first four years of observation of the primary sample of 130 giant Hii regions in 73 galaxies with Cepheid determined distances. This chapter follows closely the first part of the paper “An independent determination of the local Hubble ” (Fern´andez-Arenas et al., 2018), its layout is as follows: in §3.1 I present a description of the “Anchor Sample” and the Hii galxies used in this work. In §3.2 I describe the different observations and the data reduction and I show the data analysis of the anchor sample and compare it with data from the literature and finally the results of the L(Hβ)₋σ relation for the “anchor sample” are presented in §3.2.4.

3.1 The actual sample

In this section I discuss the observations and the quality of the obtained data in the new sample of GHIIRs in nearby galaxies.

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3.1. The actual sample

started in 2012 a long term project to acquire integrated Hβ fluxes and velocity dis-persions of a new sample of 130 GHIIRs in 73 galaxies for which accurate distances have been determined using primary distance indicators. Here I present the results of the observations of 36 GHIIRs hosted by 13 such nearby galaxies representing about

1/4th of the primary sample of GHIIRs.

Much of the variance in the value of H0 is related to the choice of distance to

the galaxies in the anchor sample which in turn is intimately linked to the choice of calibration of the Cepheids period-luminosity (PL) relation. A thorough discussion of this aspect can be found inRiess et al. (2016).

3.1.1 The GHIIR sample

In this section I present the results of the observations of 36 GHIIRs hosted by 13 nearby galaxies with redshift-independent distances.

3.1.1.1 Adopted distances

The Cepheid distances to the sample galaxies were obtained from NASA/IPAC Extragalactic Database1 _{. The adopted distance for each galaxy is the average value} provided by the references in Table 3.1, weighted by the reciprocal of the quoted distance modulus error. I only considered distances based on CCD photometry, that have been obtained, almost entirely, from determinations published more recently than the year 2000. Where necessary the published distance moduli were adapted using as reference an LMC value of(m₋M)LM C=18.50 (Riess et al., 2016).

In addition to the references in Table3.1, I provide the following specific comments:

1) From the Hubble Space Telescope Key Project team papers I only used the re-sult published byFreedman et al. (2001), adopting their metallicity-corrected distance values.

2) FromKanbur et al. (2003) I adopted the metallicity-corrected distances obtained from the LMC Cepheid PL relation.

1_{This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated}

by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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3) In the case of Paturel et al. (2002), where they used the period-luminosity rela-tion for Galactic Cepheids with HIPPARCOS distances, I used their adopted distance moduli, given in their Table 4 (Column 8).

I note that only one Cepheid distance was available for each of the galaxies IC 10, NGC 2366 and NGC 4395; for these three galaxies the adopted distance is the average of the Cepheid value and the mean of the Tip of the Red Giant Branch (TRGB) values. For MRK116 (I Zw 18), the distance values reported in the literature rely on theoretical models, because of the very low metallicity of this system (1/40th Z), which prevents the use of empirical Period-Luminosity relations.

The distances obtained from the TRGB provided an important sanity check. The good agreement between the two sources of distance is shown in figure3.1.

The targets are listed in Table 3.2 and a journal of observations is given in Table 3.3. Table 3.4 presents the relevant data for the new sample that I use in this chapter to determine the zero-point of the L₋σ relation and thus to derive the value of the Hubble constant.

Properties of giant extragalactic HII regions and the Hubble constant