Study of three distinct populations of stellar clusters in the spiral galaxy M81

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Study of three distinct

populations of stellar clusters in

the spiral galaxy M81

by

Mayra Santiago Cort´es

Thesis submitted as partial fulfillment of the

requirement for the degree of

Doctor of Philosophy in Astrophysics

at the

Instituto Nacional de Astrof´ısica, ´

Optica y

Electr´onica

August 2017

Tonantzintla, Puebla

Advisors:

Dr. Yalia Divakara Mayya

INAOE

Dr. Daniel Rosa Gonz´alez

INAOE

c

⃝

INAOE 2017

The author gives permission to INAOE to

reproduce and to distribute copies of this thesis

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Figure 1: Imagine yourself on a world where the sky is always ablaze with the sight of a thousand stars as bright as the full Moon. Such would be the view in the crowded confines at the heart of a globular star cluster.

Brian W. Murphy, Butler University.

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Resumen

Los cúmulos estelares son considerados bloques fundamentales del Universo, as´ı como laboratorios únicos para el estudio de procesos relacionados con la historia de formación estelar, actual y pasada, de las galaxias que los albergan.

En esta tesis se reporta una muestra completa de cúmulos estelares en M81 que cubren completamente el disco óptico de la galaxia, as´ı como el análisis de sus propiedades, utilizando imágenes de alta resolución del Telescopio Espacial Hubble obtenidos con

la c´amara ACS (HST/Advanced Camera for Surveys en ingl´es) en los filtros F435W,

F606W y F814W. Esta muestra esta compuesta por 435 c´umulos estelares compactos

(F W HM < 10pixeles; 9 pc), la cual incluye 263 cúmulos azules y 172 cúmulos glob-ulares. Además reportamos la detección de 30 cúmulos estelares de bajo brillo y difusos, llamados faint fuzzy, por primera vez en M81.

Del análisis de los cúmulos compactos azules encontramos que están distribuidos a lo largo de los brazos espirales de la galaxia y su función de luminosidad sigue una ley de potencia con ´ındice 2.0, el cual es un valor t´ıpico reportado en el estudio de poblaciones de cúmulos compactos jóvenes masivos encontrados en otras galaxias. Por otro lado los cúmulos globulares tienen masas fotométricas entre105_y₂_×₁₀7_M

⊙y est´an distribuidos uniformemente sobre M81.

De la muestra de c´umulos globulares analizamos los perfiles de brillo superficial de 128

objetos en las bandasB eI a trav´es del ajuste de un perfil de King (King 1962). De este

an´alisis, obtuvimos sus par´ametros estructurales tales como radio de core(rc), radio que

contiene la mitad de luz (rh), par´ametro de concentraci´on (c) y brillo superficial central

(µ0). La distribuci´on de estos par´ametros en las 2 bandas analizadas puden ser representada

muy bien con una funci´on log-normal con valores medias de rc = [0.6,0.7] pc, rh =

[3.8,4.2] pc , c = [76,73] y µ0 = [17.7,15.5] mag/arcseg2 en B e I, respectivamente.

En general, los valores obtenidos en las 2 bandas correlacionan entre si. Identificamos 6

c´umulos en su fase depost core collapse y 3 c´umulos con coresmuy azules, incluyendo

el cúmulo más brillante de galaxia, llamado M81-GC1. Al comparar las propiedades de la población de cúmulos globulares en M81 y la V´ıa Láctea, encontramos que su número total as´ı como su función de luminosidad es similar en ambas galaxias. Por otro lado, del análisis de los parámetros estructurales resulta que los cúmulos globulares en M81 tienen

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n´ucleos mas compactos (< rc >= 0.7 vs 1 pc) y halos mas extendidos (< rh >= 4

vs 3 pc) que aquellos en la V´ıa Láctea. La mediana de la distribución del parámetro de concentración en M81 es más de 2.5 veces mayor que su valor en la V´ıa Láctea (< c >= 80vs 30).

De los 30 c´umulos faint fuzzy, 24 tienen colores rojos como los c´umulos globulares de

M81, pero con luminosidades en el extremo d´ebil del rango de magnitudes (MB >−8mag)

y tamaños sistemáticamente más grandes (reff >7pc), con caracter´ısticas similares a los

c´umulosfaint fuzzyencontrados en las galaxias NGC 1023 y M51a. Analizamos sus

per-files usando la misma metodolog´ıa aplicada para los cúmulos globulares. Esto nos permitió hacer una comparación de los parámetros estructurales de estos dos tipos de cúmulos en una misma galaxia, por primera vez, además de explorar si ambas muestras pertenecen a

la misma poblaci´on. Espacialmente los c´umulosfaint fuzzyse encuentran en 2 anillos, uno

a 6 kpc y otro a 9 kpc del centro, probablemente ubicados en el disco de la galaxia. En

el caso de los c´umulosfaint fuzzyrojos el modelo de King tiene un muy buen ajuste a lo

largo de todo el perfil de brillo superficial, con< rc >= 5pc y< c >= 12. Encontramos

que los c´umulos globulares yfaint fuzzyocupan diferentes intervalos de valores en la dis-tribuci´on deµ0yrc, lo cual sugiere que pertenecen a diferentes poblaciones. Sin embargo,

el origen de estos objetos es a´un incierto y estudios m´as profundos son necesarios para concluir sobre su origen.

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Abstract

Star clusters are considered the fundamental building blocks of the Universe and they provide a unique laboratory for studying the ongoing and past star formation in their host galaxies.

In this thesis we report a complete sample of stellar clusters in M81 along with the analysis of their properties. This work was done using high-resolution observations carried out with

Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) images in the filters

F435W,F606W andF814W. These images cover the entire optical extent of the galaxy.

The sample contains 435 compact star clusters (F W HM <10pixels; 9 pc) which include

263 blue clusters and 172 globular clusters. Additionally, we report the detection of 30 low surface brightness and fuzzy stellar clusters, named “faint fuzzy”, for the first time in M81.

From the analysis of the blue compact star clusters we found that they are distributed along the spiral arms on M81 and its luminosity function follows a power-law distribution with an index of 2.0, which is the typical value found for young massive compact star clusters in other galaxies. On the other hand, the globular star clusters have photometric masses

between105_and₂_×₁₀7_M

⊙and they are distributed uniformly across the face of M81.

We analyze the surface brightness profiles of 128 globular clusters in theB andI bands

and fit them with a King model (King, 1962). From these analyses we obtained their structural parameters core radius (rc), half-light radius (rh), concentration parameter (c)

and central surface brightness (µ0). The distributions of these parameters in the two

bands can be represented by log-normals function with median valuesrc = [0.6,0.7]pc,

rh = [3.8,4.2]pc ,c = [76,73] andµ0 = [17.7,15.5]mag/arcsec2 in theB andI bands,

respectively. In general, there is a good correlation between the structural parameter val-ues in both bands. We found that 6 stellar clusters are in their post core-collapse phase and 3 clusters have blue cores, including the brightest cluster in M81, named M81-GC1. Through the comparison of the structural parameters of the globular cluster population in M81 with those in the Milky Way we found that they have almost the same total number of globular clusters, as well as similar luminosity distribution. On the other hand, a detailed analysis of their structural properties shows that the cores of M81 globular clusters are on an average more compact (< rc >= 0.7 vs 1 pc) than those in the Milky Way and also

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median value of the concentration distribution of clusters in M81 is 2.5 times larger than the median value on the Milky Way (< c >= 80vs 30).

Of the 30 faint fuzzy star clusters in the sample, 24 have colors similar to those of globular clusters, but in the fainter end of the magnitude range with (MB > −8mag). Their sizes

are systematically much larger than for ordinary GCs (reff > 7 pc), with characteristics

to those of the faint fuzzy cluster samples found in the galaxies NGC 1023 and M51a. We analyzed their surface brightness profiles using the same methodology used for the globular clusters. This allowed us to compare the structural parameters of two types of stellar clusters in the same galaxy, for the first time, and specially to explore if these two samples belong to the same population. The faint fuzzy population is distributed in two rings at 6 kpc and 9 kpc from the center of the M81 galaxy and likely located in its disk. In the case of the red faint fuzzy clusters, a King function fits very well the entire profile, with< rc >= 5pc and< c >= 12. We found that the globular and faint fuzzy clusters

cover different intervals of values in theirµ0 andrcdistributions, which suggests that they

belong to different populations. Never the less, the origin of the faint fuzzy clusters is still unknown and new analyses are necessary to elucidate their origin.

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Acknowledgments

I would like to thank the Consejo Nacional de Ciencia y Tecnolog´ıa (CONACyT), M´exico, for the economical support for the development of this thesis.

Thanks to my thesis supervisors, Dr. Yalia Divakara Mayya and Dr. Daniel Rosa Gonz´alez, for guiding me for the succesful completion of this thesis.

Thanks to the members of examiner committe for reading the thesis and giving me useful coments to improve it:

• Dr. Luis Carrasco Baza (INAOE).

• Dr. Ivanio Puerari (INAOE).

• Dr. Emanuele Bertone (INAOE).

• Dr. Lino Hctor Rodrguez Merino (INAOE).

• Dra. Rosa Amelia Gonzlez Lpez Lira (IRyA-UNAM).

Thanks to all the people at the Astrophysics department and at INAOE who gave me their support during my stay in this place.

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Dedication

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Publications

The following articles have been prepared and published based on the work carried out in this Ph.D. thesis.

Refereed papers

• On the nature of the brightest globular cluster in M81. Mayya Y. D., Rosa-Gonz´alez

D., Santiago-Cort´es M., Rodr´ıguez-Merino, L. H., Vega, O., Torres-Papaqui, J. P., Bressan, A., Carrasco, L., 2013, MNRAS, 436, 2763M.

• Wide-field HST/ACS images of M81: the population of compact star clusters.

Santiago-Cort´es M., Mayya Y. D., Rosa-Gonz´alez D., 2010, MNRAS, 405, 1293S.

Proceedings

• Structural parameters of M81 globular clusters: analysis of their intensity profile.

Santiago-Cort´es M., Mayya Y. D., Rosa-Gonz´alez D., 2014, mysc.conf, 133S.

• Metallicity of the globular clusters in M81. Rodr´ıguez-Merino, L. H., Mayya, Y. D.,

Vega, O., Rosa-Gonz´alez, D., Santiago-Cort´es, M., 2014, mysc.conf, 151R.

• On the nature of faint fuzzies in M81. Mayya Y. D., Santiago-Cort´es M.,

Rosa-González D., Gómez-Rosa-González, M., Rodr´ıguez, L., Carrasco, L. 2014, mysc.conf, 147M.

• Compact star clusters-environment connection in M81. G´omez-Gonz´alez, V. M. A.,

Mayya, Y.D., Rosa-Gonz´alez, D. and Rodr´ıguez-Merino L. H., Santiago-Cort´es M., 2014, mysc.conf, 245G.

• GTC long-slit spectroscopy of compact stellar clusters in M81. Mayya Y. D.,

Rosa-González D., Santiago-Cortés M., Arellano-Córdova, K., Rodr´ıguez, M., 2013, RMxAC, 42, 22M.

• GTC long-slit spectroscopy of compact stellar clusters in M81. Mayya Y. D.,

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• Structural parameters of M81 compact star clusters. Santiago-Cort´es M., Mayya Y. D., Rosa-Gonz´alez D., 2011, RMxAC, 40, 94S.

• Study of the young and old compact stellar cluster population in M81.

Santiago-Cort´es M., Mayya Y. D., Rosa-Gonz´alez D., 2010, IAUS, 266, 532S.

• Searching for compact star clusters in M81 using HST/ACS images.

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Chapter 1

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1.1 Star clusters

Star clusters provide a unique laboratory for studying the ongoing and past star formation in spiral and irregular galaxies. A stellar cluster is generally defined as any stellar group whose members are physically bound.

Classically the star clusters have been classified as open and globular clusters (GCs). The

majority of open clusters have ages < 109 _{years and masses} _∼ ₅_× ₁₀3_M

⊙. They are

found in the disc of galaxies and interact gravitationally with it causing their disruption and their asymmetric morphology (Lynga & Lundstrom, 1980; Janes & Adler, 1982; Moitinho,

2010). On the other hand the GCs are older (∽ 1010 _{years) and more massive than open}

clusters, with typical masses between∼104₋₁₀6_M

⊙(Harris, 1996). The GCs are found

in the halo of galaxies and have spherical shape. However, it is clear that, besides the “classical” globular and the open clusters there exist other types.

In the last two decades, new technological advances and extended surveys have allowed to

look beyond the Milky Way (MW) galaxy with theHubble Space Telescope (HST). Thanks

to these new surveys a new, different, type of clusters has been discovered; they are known as compact star clusters (CSCs), also designated super star clusters (SSCs). The CSCs are gravitationally bound systems with sizes and shapes similar to GCs but with ages that resemble the open clusters in the MW (Meurer, 1995; Zhang & Fall, 1999; de Grijs et al., 2001; Portegies Zwart et al., 2010). The young compact star clusters (YCSCs) have been found in different types of galaxies, with the most massive of them predominantly found in interacting and starburst galaxies (Whitmore et al., 1999; Ma´ız-Apell´aniz, 2001; Scheepmaker et al., 2007). These objects could represent the early stages of present GCs, and their study can give us clues about the process of globular cluster formation. Another new type of star clusters that has been discovered in some galaxies, in recent time, are the faint fuzzy star clusters (Larsen et al., 2001; Scheepmaker et al., 2007). These star clusters have low surface brightness and colors similar to the MW globular clusters; however, they are much larger than ordinary GCs (reff > 7pc). All these cluster populations are

found in the M81 galaxy and a general overview about their properties and astrophysical importance is presented next.

1.2 Young compact star clusters

As already mentioned, noticeable characteristics are that YCSCs have the same shape and

size as the GCs (reff < 6 pc) but with ages that resemble the open clusters in the MW

(∼10Myr).

The young compact star clusters have been found in different type of galaxies, including the MW, but predominantly in interacting galaxies and starburst regions (Whitmore et al., 1999). In the MW the most massive young cluster known is Westerlund 1 (Wd 1) with

a mass∼ 6×104_M

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Tacconi-Garman, 2007; Brandner et al., 2008). A little farther, in the 30 Dorados region in the Large Magellanic Cloud (LMC, 50 kpc), a prominent example of YCSCs is R136. This

YCSC has∼ 105 _{young stars within its 10 pc diameter and is important for the study of}

OB associations and Wolf-Rayet (WR) stars (Weigelt & Baier, 1985; Massey & Hunter, 1998; Andersen et al., 2009). In starburst galaxies the best example is the galaxy M82. The star cluster population in M82 has been studied over the years by different authors (de

Grijs et al., 2001; Melo et al., 2005; Mayya et al., 2008). UsingHST/Advanced Camera

for Surveys (ACS) images, Mayya et al. (2008) found a disk cluster population of YCSCs with estimated age 100-500 Myr, which corresponds to the epoch of the interaction with

M81. For comparison, the M82 clusters have estimated masses in the range104₋₁₀5_M

⊙.

YCSCs have been observed in other starburst galaxies, like NGC 1569, NGC 253, NGC 1705, NGC 4449, NGC 1313, NGC 5236 and NGC 7793 (O’Connell et al., 1994; Watson et al., 1996; Billett et al., 2002; Annibali et al., 2011; Larsen et al., 2011). One of the best-known cases that include merger galaxies is the Antennae (NGC 4038/4039) system (Holtzman et al., 1992; Miller et al., 1996; Schweizer et al., 1996; Cao & Wu, 2007). The Antennae is the merger galaxy prototype and has giant HII regions with intense star

formation. Whitmore & Schweizer (1995), usingHST images in theU,B,V andIbands,

found a population of over 700 blue point-like objects within the disks of these merger galaxies. Whitmore et al. (1999) estimated that the YCSCs in the Antenna have a median effective radii similar to those of GCs in the MW (reff = 4±1pc), masses of∼ 105M⊙,

and ages around∼107 _years.

As has been explained, the YCSCs found in different galaxies have properties (mass, shape, size) that resemble those of the GCs. But while the GCs are as old as the age

of the Universe, the CSCs are younger, with ages < 1Gyr. This means that, while the

studies of GCs reveal the early formation history of their host galaxy, including the MW (Harris, 1996; Barmby, 2003), studies of YCSCs delineate the recent star formation his-tory, that in some cases is related to interactions with neighboring galaxies (Holtzman et al., 1992; Whitmore et al., 1999; Mayya et al., 2008). It has been speculated that YCSCs could be young analogues of GCs. It is an intriguing idea that GCs, as those we see in the halo of our own and other galaxies, could still be formed today. Most important is that the study of YCSCs would provide not only a direct insight into the conditions that were present in the early days of GC formation, but also about the disruption processes that the clusters experience over their lifetime.

Luminosity function

Related to the disruption process, a variety of methods have been employed to address the issue of the long-term survival of massive star clusters. The most promising and popular approach to study whether a significant fraction of an entire population of YCSCs might survive for any significant length of time (e.g.×109_{yr) is to analyze the cluster luminosity}

function, or its equivalent mass function (LF, MF).

The luminosity function for the young star clusters in M31 galaxy is described by a power-law fitN(L)dL ∝ L−α_dL _{with index}_α _≈ ₂_{(Elson & Fall, 1985). Similar results have}

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been found in different galaxies, that cover a wide range of environments, such as in the Antennae (Zhang & Fall, 1999), LMC and SMC (Hunter et al., 2003), NGC 628, NGC 1313, NGC 3184 and NGC 5236 (Larsen, 2002), NGC 1033 and NGC 6745 (de Grijs et al., 2003). Nevertheless Whitmore et al. (1999) found that the LF in the Antennae galaxies is much better approximated by a double power-law distribution. They found that the bright side (MV ≲ −10) of the LF has a steeper slope (∼ −2.7) than the faint

side (∼ −2). This tendency also was found by Gieles et al. (2006a) in M51 and they

interpreted it as a turn-over in the cluster initial mass function (CIMF). They affirm that the LF consists of clusters with different ages and due to the age dependent light-to-mass ratio, the LF does not necessarily have the same shape as the mass function. If the CIMF would be a continuous power law with the same index at all ages, then the LF will be a power law with the exact same index. Age dependent extinction or new star forming bursts would not cause a difference between the CIMF and the LF. They affirm that the bright-end of the LF is slightly steeper as a indication that the CIMF is not a continuous power-law function. Gieles et al. (2006a) reproduced the curvature in the LF considering

that the cluster initial mass function is a Schechter type withM⋆ ≈ 2×105M⊙ (where

M⋆, is the truncation mass). This CIMF can be obtained if the cluster disruption is due

to the tidal field of the host galaxy. Encounters with giant molecular clouds flattens the low-mass end of the mass function.

The luminosity function of the YSCs, power-law or Schechter type, are markedly different from the peaked log-normal distribution found for old GCs. The LF of GCs in our Galaxy

is well represented by a Gaussian function with a characteristic luminosityM0 which is

the mean or peak (turnover) magnitude of the distribution. In the Local Universe, it has

been found that the LF of GCs have aMV,0 = −7.3magnitudes (m∗ ≈ 1.2×105M⊙)

andσ = 1.2mag (Harris, 1991). Similar result has been found from the analysis of the

LF on farther galaxies independent of their morphological type (Whitmore et al., 1995, 2002). Although the process leading to the formation of GCs remains as a subject of study (Harris & Pudritz, 1994; Elmegreen & Efremov, 1997; Adamo & Bastian, 2015; Bastian, 2016). Theoretical investigations have shown that evolutionary and dynamical processes significantly altered the initial properties of star clusters which can explain the difference between the luminosity distributions found for YSCs and GCs (Gnedin & Ostriker, 1997; Fall & Zhang, 2001). Some authors suggested that the GCs born with a more wide distribu-tion of masses that later is modified by dynamical and evoludistribu-tionary effects that selectively destroy the low mass clusters (e.g. gravitational shocks and dynamical friction; Fall & Zhang (2001); de Grijs & Parmentier (2007)). In this case the initial mass distribution of GCs could be a power-law mass function, like the young star cluster, that might evolve into a log-normal mass function (Ostriker & Gnedin, 1997; Fall & Zhang, 2001). This idea is promising because the masses and sizes of the brightest young clusters found recently in merging galaxies are similar to those of the GCs supporting the idea that YSCs could evolve and form GCs.

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1.3 Globular clusters

Globular clusters are among the oldest objects known (∽ 1010 _{years) and their internal}

structure and kinematics gives us important information about their formation conditions and dynamical evolution processes and how these are influenced by the host galaxy en-vironment. Consequently, the derivation and study of the structural parameters of GCs is fundamental to understand all these processes.

The GCs can be found in almost all galaxies, irrespective of their morphological type, gi-ant elliptical (Madrid et al., 2009; Harris, 2009; Webb et al., 2012), dwarf elliptical (Da Costa & Mould, 1988; Sharina et al., 2006; van den Bergh, 2009), irregulars (Blecha, 1988; Hodge et al., 1999; Huxor et al., 2013) and spirals (Larsen et al., 2002; Barmby et al., 2007; Simanton et al., 2015). The high number of GCs detected in galaxies be-yond the Local Group has been possible because they are bright and massive objects (∼MV=−5 to −10magnitudes and masses of∼ 104₋₁₀6_M

⊙; van den Bergh et al.

(1991)). Additionally, its number has increased, in the last two decades, thanks to the

new surveys carried out by telescopes such as theHST. Due to their proximity, the

glob-ular cluster systems of the MW and M31 represent two of the best studied and they have remained as the standard sample to compare the properties of new groups of star clusters.

Globular cluster structure and dynamics

A GC can be described like an object composed of a group of stars which were formed over a very short period of time and in a very small volume of space, it has a nearly spherical geometry with an observed ellipticity less than 0.1 and in principle it formed from a chemically homogeneous material (Harris, 1996). This description allows us to think that GCs can be considered as simple systems. However the development of complex dynamical models was necessary to study and understand their structure and the dynamical processes that modify them (Michie, 1961; King, 1962; H´enon, 1971).

Currently we know that the structure of GCs, in general terms, can be described by a rather dense core and diffuse halo. In the halo the number of stars gradually declines out to an external boundary called the cluster tidal radius (King, 1962; Binney & Merrifield, 1998). The entire GC structure is modified by some dynamical process during their lives. These processes are related to their internal evolution such as mass loss by stellar evolution, mass segregation and low-mass star evaporation, and others which are related with external forces such as tidal stress and dynamical friction originated by the host galaxy (Elson et al., 1987a; Meylan & Heggie, 1997). In order to note the importance and give a general idea of these processes we describe them briefly:

Stellar evolution: During the late stages of their evolution, the stars can lose a large frac-tion of their mass either by stellar winds or by supernova explosions. This material is blown out of the cluster and this loss of mass causes that the radius of the star cluster increases.

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same mass, however in a real cluster the most massive stars will segregate to the center of the cluster. In GCs due to their old age, massive stars have already evolved to compact objects such as neutron stars and white dwarfs. Even though a cluster may have only 1 to 2 percent of its mass in neutron stars, most of them will rapidly move to the inner regions of the cluster. Observational evidence comes from the distribution of X-ray sources and blue stragglers in clusters which are found more centrally concentrated than normal stars (Sarajedini, 1993).

Evaporation: The stars in a GC undergo encounters between themselves (two-body en-counters). The stars are not physically colliding just deflecting each other gravitationally. These close encounters between stars transfer energy from one star to another and forces stars in the core of the cluster to migrate to the halo. If the star gains enough energy, its velocity can exceed the escape velocity resulting in evaporation of this star. In this process the core loses energy, contracts a little due to energy conservation, and the remaining stars have to move a bit faster (increasing the velocity dispersion) to compensate for the increase of the gravitational binding energy of a denser core. If the energy absorbed by the halo is not compensated, the contraction of the core accelerates toward an infinite central density which is termed core collapse (or gravothermal catastrophe; H´enon (1971); Djorgovski & King (1986); Elson et al. (1987a)). In ”normal” uncollapsed clusters the central surface density tends to a a constant value displaying flat cores, but the central surface brightness of cuspy clusters continues to rise all the way to its center showing a steeply rising core (Lugger et al., 1995; Barmby et al., 2002; Wang & Ma, 2013).

Tidal friction with the galaxy environment: The rate of evaporation of star from a cluster is enhanced by tidal shocking of the cluster as it passes through the Galactic bulge, disk and giant molecular clouds on the galactic plane (Ostriker et al., 1972; Gnedin & Ostriker, 1997). Evaporation due to tidal shocking can be as important as evaporation due to two-body relaxation in determining the evolution of GCs, and it can cause the concentration to either increase or decrease (King, 1966; Ostriker et al., 1972).

King model

As I mentioned before, the dynamical processes can change the structure of the cluster which in turn changes the spatial distribution of light (Gnedin et al., 1999). The surface brightness profile (SBP) traces the spatial distribution of the brightness, usually measured in annulus around the cluster center (e.g. Harris et al. (2002); Bonatto & Bica (2008)). The traditional method to derive structural parameters of the clusters is based, precisely, by fitting a theoretical model such as King profile (King, 1962) or EFF profile (Elson et al., 1987b) to their observed SBPs. The difference between these two models are at larger radius of the SBP, where the EFF profile is extended and fitted by a power law while the King model is tidally truncated.

The King model is based on the internal stellar dynamics of the cluster. This model as-sumes positions and motions of the cluster stars and predicts their changes due to the cluster evolution process and the interactions with their environment. The King model assumes that the cluster is composed by equal-mass stars of an isothermal sphere in

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equi-librium. Actually, this model referred as “King” represents a family of curves (associated with the SBP) described by three parameters (I0,r0andrt). I0is the central surface

bright-ness,r0 is a scale factor that depends on the central surface density called King radius and

rtis the tidal radius at which the surface brightness drops to zero. The King radius is

re-lated to the tidal radius though the concentration parameter (c=rt/r0). The concentration

parameter gives the curvature of the brightness profile at intermediate radii, which is more easily observed than the tidal radius. It turns out that King models become unstable to the gravothermal catastrophe for values oflog c≥2.5(Wiyanto et al., 1985).

Surface brightness profile and structural parameters

The King model profile properly fits the SBPs of majority of the GCs in the MW, but

also the SBPs of GCs found in external galaxies where surveys withHST have been done

(King, 1962; Larsen et al., 2002; McLaughlin & van der Marel, 2005; McLaughlin et al., 2008; Harris et al., 2009, 2010). Specially, the King model very well describes how the intensity falls in the outer part of the GCs at the tidal radius. However, there are a few clusters that show SBPs that deviate from the King profile in their external part having over-densities of stars outside the tidal radius (tidal tails) (Grillmair et al., 1995; Leon et al., 2000; Federici et al., 2007). This type of profiles can be explained by the presence of escaped stars beyond the tidal radius originated from the tidal friction with the galactic field (Meylan et al., 2001; K¨upper et al., 2010). Others show a systematic excess, as a power-law cusp, in its central parts. These cuspy clusters are manifestation of the core-collapsed dynamical effect (Djorgovski & King, 1986; Noyola & Gebhardt, 2006). Observations indicate that roughly 20% of all GCs in the MW have undergone core collapse (Harris, 1996). Trager et al. (1995) presented a catalog of surface brightness profiles of 125 Galactic GCs, and classified 16% of their sample as core-collapsed clusters and 6% as core-collapsed candidates. Some authors have indicated that possibly the cuspy clusters are located close to the center of their host galaxy (Chernoff & Djorgovski, 1989; Barmby et al., 2002; Noyola & Gebhardt, 2006). A generic expectation is that clusters closer to the galactic center will be more dynamically evolved, as tidal shocks accelerate their internal evolution towards the core collapse or dissolution (Lynden-Bell & Eggleton, 1980; Goodman, 1989). These clusters can be fit by a power-law model or even with

a high-concentration (c > 250) King model which can be successfully adjusted to their

central SBPs (Barmby et al., 2002; Wang & Ma, 2013). Grabhorn et al. (1992) reported that a concentration parameterc= 3(c= logrt/rc) fitted the surface brightness profile of

M15, the prototype of the core-collapsed globular clusters in MW. In general any globular cluster with a concentration parameter valuec > 2.0−2.5is considered a core-collapsed candidate.

Much of our understanding regarding the influence of dynamical effects on the SBPs of star clusters has come by comparing the observed SBPs of star clusters to theoretical mod-els. Additionally, the structural parameters are used to explore a multitude of scaling relations and interdependence between the different properties of the clusters as size, con-centration, metallicity, galactocentric distance (van den Bergh et al., 1991; Djorgovski & Meylan, 1994; Larsen & Brodie, 2003; Brodie & Strader, 2006; Barmby et al., 2007).

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Con-sequently, the derivation of reliable structural parameters of star clusters and their study is fundamental to get a deeper understanding of the formation and evolution processes of the star clusters themselves and their host galaxy. This implies that we need to know and take into account all the factors related with obtaining the surface brightness profile of the star clusters and measuring of their structural parameters.

1.4 Faint fuzzy clusters

During a search for GCs in NGC 1023, a lenticular galaxy at approximately 9 Mpc dis-tance, Larsen & Brodie (2000) found a peculiar star cluster population. Faint objects (MV>−8mag) that have GC colors but size (reff >7pc) much larger than ordinary GCs

that have effective radii of∼3pc (Harris, 1996). After a detailed analysis of these objects the authors concluded that they have found a new class of star clusters, which they called “faint fuzzies” (FFs).

Using the Low Resolution Imaging Spectrograph/Keck Telescope Larsen & Brodie (2002) derived the mean metallicity and radial velocities of NGC 1023 FFs. They found that the

FF clusters in NGC 1023 are moderately metal-rich, with a mean metallicity of ([F e/H] =

−0.58±0.24dex) and areα-element–enriched with [α/F e] = 0.3−0.6dex. Brodie &

Larsen (2002) compared the values of [F e/H] and Hβ to stellar evolutionary models

from Maraston & Thomas (2000) and they found that the age of these clusters is≥ 7−

8Gyr. They also found that the FFs are associated with disks of their host galaxies, rather than its halo, showing a systematic rotation similar to the rotation of the disk. Further observations have discovered FF clusters in other lenticular galaxies (Larsen & Brodie, 2000; Brodie & Larsen, 2002; Lee et al., 2005; Hwang & Lee, 2006; Chies-Santos et al., 2007)), some spirals (Scheepmaker et al., 2007, 2008; Simanton et al., 2015), and giant elliptical galaxies (G´omez et al., 2006).

It is not surprising that in the numerous studies of extragalactic GCs carried out before the

HST, they have not been detected or discarded as possible background galaxies because of

their faint magnitude (MV>−8mag; Larsen & Brodie (2000)).

At first glance, the size and luminosity distributions of the FF clusters are reminiscent of the faint “Palomar” type GCs in the outer halo of the MW. Brodie & Larsen (2002) compared the magnitudes versus the metallicity (color) of the the FFs on NGC 1023 and the GCs on MW. They found that although the are similar in size and luminosity, the

group of Palomar-like globulars are bluer objects ([F e/H] ≈ −1.4dex) compared to the

FF clusters in NGC 1023 ([F e/H] =−0.58±0.24dex). Additionally, according to Harris (1996), all clusters withrh >7pc in the MW are located more than 4 kpc from the plane

of the Galactic disk. The extended clusters in the MW are clearly not associated with its disk but are rather halo objects, presumably formed in the low density environment in the outskirts of the proto-galaxy (McLaughlin, 2000). So they are different in nature to the FF clusters in NGC 1023 (Brodie & Larsen, 2002).

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After the FFCs where found by Larsen & Brodie (2000), different theories about the origin of faint fuzzy cluster has been proposed until now.

Brodie & Larsen (2002) established that the faint fuzzy clusters in NGC 1023 are old metal-rich and have a rotation that is similar to the rotation curve of the host galaxy, a lenticular disk. Brodie et al. (2004) based on kinematic analysis of these data suggest that the faint fuzzy clusters could be formed in the disk of NGC 1023 when sufficient gas was still available, possible triggered by interaction process between galaxies.

Fellhauer & Kroupa (2002) noted that in interacting galaxies, like the Antennae, young stars are found in small star clusters that are part of larger groups. Using numerical sim-ulations, they proposed that if these groups are gravitationally bound, successive mergers of their constituent sub-clusters would lead to new, larger clusters, which they called su-per clusters or cluster complexes. After taking in account dynamical evolution of such superclusters, Fellhauer & Kroupa (2002) reported that the resulting merger objects have properties similar to the faint fuzzy clusters reported in NGC 1023. Then, the faint fuzzy origin may be related to the hierarchical nature of star formation in disk (Bastian et al., 2005). Burkert et al. (2005) after making a kinematic analysis of NGC 1023 FF clusters suggested that they formed in a dense ring of metal-enriched gas, similar to the dense en-vironments where young massive clusters form as a result of galaxy-galaxy interactions. Using simulations Br¨uns et al. (2009, 2011) found that most of the individual compact clusters into a cluster complex, residing within an area where the self-gravity of the clus-ters dominates over the tidal forces of the parent galaxy, merge and form systems which are stable for periods of more than 5 Gyr. Accordingly, they proposed that objects like the young massive cluster complexes in M51 are likely progenitors of the FF clusters observed in NGC 1023.

Another idea was exposed by Peng et al. (2006) who suggested that the best Galactic ana-log to the faint fuzzy clusters, which they called Diffuse Star Clusters (DSC), were the old open clusters. Supporting this idea Chies-Santos et al. (2013) compared the kinematics of FF in NGC1023 with those of planetary nebulae and the HI gas finding disc-like kine-matics and concluded that the red FFs are not associated with an ongoing galaxy merger but are simply long-lived open disc clusters. They argued that the reason faint fuzzies are observed in lenticulars but not in spirals is that the faint clusters are quite difficult to pick out against the strong structure and variable luminosity caused by star formation in spiral disk.

1.5 This thesis

The M81 galaxy have a P.A.= 157◦

, an inclination angle i = 58◦

and a distance of 3.63 Mpc (de Vaucouleurs et al., 1991; Freedman et al., 1994)). These characteristics along with available high resolution images that cover the entire extension of the galaxy, gave us the opportunity to make for first time homogeneous, extensives and detailed study

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of three star cluster populations in this galaxy. This study expands previous works and develops new ones.

In a previous work, Chandar et al. (2001a,b) found a sample of 114 CSCs usingHST/Wide

Field Planetary Camera 2 (WFPC2) images of M81. The frames covered an area of

40 arcmin2 over the galaxy. They found a population of 55 young star clusters with

pho-tometric ages<600Myr, which are probably related to the interaction between M81 and

M82. Until recently, this was the only existing work about the YCSCs in M81. Hence,

new HST/ACS observations with high spatial resolution and extending the covered area

to∼340arcmin2over this galaxy motivated us to detect a complete sample of YCSCs in

M81 and analyze them as fundamental part of this thesis.

In the case of GCs in M81, they have been studied in the past by several groups (Brodie & Huchra, 1991; Perelmuter & Racine, 1995; Perelmuter et al., 1995; Schroder et al., 2002). However only some of the structural parameters of the M81 GCs were previously studied (Chandar et al., 2001a; Nantais et al., 2011). Now, thanks to the high resolution observations of this galaxy is possible not only to obtain the detections of new GCs, but also to realize a detailed study of their surface brightness profile and structural parameters. This work offers an opportunity to extend the studies carried out in the MW to external galaxies.

With respect to FFCs, there is a small number of galaxies where they have been reported. M81 is a spiral galaxy that contains a large population of star clusters and now we have the opportunity to report the detection and analysis of a larger faint fuzzy cluster population, for the first time, in this galaxy.

The main aim of this thesis is to analyze how the photometric and structural parameters of the star clusters change along with their different evolutionary stages and with the char-acteristics of their host galaxy in order to understand how the dynamical and evolution processes affect them. The results obtained of this analysis are reported in this document through the following structure:

In Chapter 1 (this chapter) we present a brief and general overview about the star clusters, centering our attention on the new types of clusters that have been found, in recent times.

Chapter 2 is focused on the compact star clusters population in M81. In this Chapter we describe the observational material used in this work. We give a summary of the compact cluster detection and selection method, and present the resulting catalog with their photometrical properties. This chapter also includes the analysis of the compact star cluster properties.

In Chapter 3, we obtained the surface brightness profiles and structural parameters of the GCs in M81. We study the reliability of the method to obtain accurate structural parameter values. Also, we compared the structural properties of the GCs in M81 with the MW and M31 cluster systems.

In Chapter 4 we present a new catalog of fuzzy clusters detected in M81 that includes their photometrical and structural properties along with their surface brightness profiles.

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A general analysis of their properties and their comparison with the faint fuzzy cluster properties found in NGC 1023 and NGC 5195 is carried out. We discuss some theories about their origin and present our conclusions.

In Chapter 5, we present the general conclusions of this thesis.

Finally, to complement this work we include some relevant data in the Appendix section. In Appendix A is reported the complete catalog of the compact stellar clusters in M81 along with their photometric data(Chapter 2). In Appendix B, C and D, are reported the resulting data related with the analysis of the structural parameters for the GCs in M81 (Chapter 3). These data are the derived structural parameters, the observed surface bright-ness profiles with their respective best-fitting King model profiles and the RGB images,

composed using theHST/ACS images onI -band,V -band andB -band, for the globular

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Chapter 2 The population of compact star clusters in

M81

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Mon. Not. R. Astron. Soc.405,1293–1302 (2010) doi:10.1111/j.1365-2966.2010.16531.x

Wide-field

HST

_{/ACS images of M81: the population of compact}

star clusters

M. Santiago-Cort´es,

⋆

Y. D. Mayya

⋆

and D. Rosa-Gonz´alez

⋆

Instituto Nacional de Astrof´ısica ´Optica y Electr´onica, Luis Enrique Erro 1, Tonantzintla 72840, Puebla, Mexico

Accepted 2010 February 15. Received 2010 February 9; in original form 2009 September 11

A B S T R A C T

We study the population of compact star clusters (CSCs) in M81, using theHubble Space Telescope/Advanced Camera for Surveys (ACS) images in the filtersF435W,F606Wand

F814W covering, for the first time, the entire optical extent of the galaxy. Our sample contains 435 clusters of full width at half-maximum less than 10 ACS pixels (9 pc). The sample shows the presence of two cluster populations, a blue group of 263 objects brighter thanB₌22 mag, and a red group of 172 objects, brighter thanB₌24 mag. On the basis of analysis of colour–magnitude diagrams and making use of simple stellar population models, we find the blue clusters are younger than 300 Myr with some clusters as young as few Myr, and the red clusters are as old as globular clusters (GCs). The luminosity function of the blue group follows a power-law distribution with an index of 2.0, typical value for young CSCs in other galaxies. The power law shows unmistakable signs of truncation atI ₌ 18.0 mag (MI = −9.8 mag), which would correspond to a mass limit of 4×104M_⊙if the brightest

clusters are younger than 10 Myr. The red clusters have photometric masses between 105_and 2_×107_M

⊙for the adopted age of 5 Gyr and their luminosity function resembles very much

the GC luminosity function in the Milky Way. The brightest GC in M81 hasMB0= −10.3 mag,

which is_∼0.9 mag brighter thanωCen, the most massive GC in the Milky Way.

Key words: catalogues – galaxies: individual: M81 – galaxies: spiral – galaxies: star clusters.

1 I N T R O D U C T I O N

With the advent of theHubble Space Telescope(HST) a new class of stellar clusters have been identified: the compact star clusters (CSCs) with typical masses of_∼104_{to 10}6_M

⊙and sizes between

1 and 6 pc (Meurer 1995). CSCs have been found in several envi-ronments, including violent star-forming regions within interacting galaxies (Whitmore et al. 1999). The similarity between the com-pactness and mass of the CSCs and that of the globular clusters (GCs) is a reason to think of an evolutionary connection between them. Moreover, the compact stellar clusters are unique laborato-ries for studying diverse star formation processes related to the star formation history of the host galaxy. The detailed studies of GCs – with ages comparable to the age of the Universe – have revealed the early formation history of nearby galaxies and the Milky Way (MW; Harris 1996; Barmby 2003), whereas the studies of younger CSCs – ages<1 Gyr – delineate the recent star formation history, that in some cases are related to interactions with neighbouring

⋆_{E-mail: [email protected] (MS-C); [email protected] (YDM); danrosa@}

inaoep.mx (DR-G)

galaxies (Holtzman et al. 1992; Whitmore et al. 1999; Mayya et al. 2008).

M81 (NGC 3031) is a large Sab spiral galaxy, very similar to M31 in appearance and roughly as massive as the MW. M81 at a distance of 3.63 Mpc (m₋M ₌27.8_±0.2; Freedman et al. 1994) is the biggest member of the M81 group, which includes the prototype starburst galaxy M82. An interaction∼100–500 Myr ago between different members of this group has been discussed by several authors (Brouillet, Combes & Baudry 1991; Yun 1999). Recent observations of M82 show that it has a large population of CSCs, with young clusters (age<10 Myr) concentrated towards the centre, and relatively older clusters (∼100 Myr) homogeneously distributed across the disc, the latter population having formed as a part of the disc-wide burst following the interaction (Mayya et al. 2006, 2008; Konstantopoulos et al. 2009). It is of interest to in-vestigate whether the interaction also triggered CSC formation in M81.

The population of GCs in M81 has been studied in the past by several groups. Perelmuter & Racine (1995) used an extensive data base to find∼70 objects classified as cluster candidates in the in-ner 11-kpc radius of M81. After completeness corrections for the unobserved area, they estimated the total GC population would be 210±30. Perelmuter, Brodie & Huchra (1995) obtained spectra

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1294 M. Santiago-Cort´es, Y. D. Mayya and D. Rosa-Gonz´alez

of 82 cluster candidates and confirmed 25 as bona fide GCs. The derived mean metallicity of the GCs was [Fe/H]= −1.48±0.19 confirming previous results from Brodie & Huchra (1991). Schroder et al. (2002) obtained spectra of 16 additional GC candidates se-lected from an extended list of Perelmuter & Racine (1995) cata-logue and confirmed all of them to be GCs. Hence, in total there are 41 objects that are confirmed as GCs using spectroscopic data. The metallicity distribution of these GCs is similar to that in M31 and the MW, two galaxies that are morphologically very similar to M81. Hence, it of interest to determine whether M81 also contains a similar number of total GCs as in the MW.

Chandar, Ford & Tsvetanov (2001a) and Chandar, Tsvetanov & Ford (2001b) carried out a search for compact objects in M81 based on observations with theHST/Wide-Field Planetary Cam-era 2 (WFPC2) camCam-era. They discovered 114 CSCs in an area of 40 arcmin2_{. The analysis found, for the first time, two different}

clus-ter populations, 59 red clusclus-ters [(B−I)0≥0.85 mag] which are

candidate for GCs and 55 young clusters with photometric ages <600 Myr. The authors related the latter population with the inter-action between M81 and M82.

In the present work, we carried out a search for CSCs in 29 adjacentHST/Advanced Camera for Surveys (ACS) fields centred on the nucleus of M81. The present data set offers not only an improved spatial resolution, but also covers a field of view that is 8.5 times larger than that covered by Chandar et al. (2001b). Results obtained from a subset of 12 adjacent central fields were presented in Santiago-Cortes, Mayya & Rosa-Gonz´alez (2009).

This paper is organized as follows: Section 2 presents the ob-servational material used in this work; Section 3 gives a summary of the cluster detection and selection method; Section 4 describes the analysis of colour–magnitude diagrams (CMDs) and luminosity functions (LFs) for the selected clusters; the discussion and conclu-sions of these studies are presented in Section 5.

2 O B S E RVAT I O N S

The observations used in this work were carried out with the ACS Wide Field Channel on board theHST. They were part of the projects with proposal IDs 10250 (PI: John Huchra) and 10584 (PI: Andreas Zezas). Table 1 lists the details of the observations. The HSTdata base contains 29 adjacent fields covering a field of view of∼340 arcmin2_{(see Fig. 1), with a sampling of 0.05 arcsec pixel}−1

(0.88 pc pixel−1). For each field, observations were carried out in the F435W,F606WandF814Wfilters, which for the sake of brevity we will refer to asB,VandIfilters, respectively, throughout this pa-per. The standard pipeline process (CALACS) provided by the Hubble Heritage Team were used for bias, dark and flat-field corrections. The pipeline uses theIRAF/STSDAS MULTIDRIZZLEtask to combine the images of a single field and produces weight maps related with the background and instrumental noise. Also this task corrects bad pixels, rejects cosmic rays and eliminates artefacts (Mutchler et al. 2007). However, images taken with different programs have slightly different astrometric coordinates. We used common stars in the ad-jacent images to tie all the images to a single coordinate system.

3 C L U S T E R D E T E C T I O N A N D S E L E C T I O N

We used the automatic detection code SEXTRACTORto create an unbiased sample of cluster candidates. SEXTRACTORautomatically detects sources on fits images, makes photometry and generates a data catalogue (Bertin & Arnouts 1996). SEXTRACTORfirst generates

Table 1. Filters and exposure times.

Field ID Filter Proposal ID Exp. time (s)

F1 F435W 10584 1×900

F1 F606W 10584 1×880

F1 F814W 10584 1_×895

F2 F435W 10584 3×1565

F2 F606W 10584 3×1580

F2 F814W 10584 3×1595

F3–F14 F435W 10584 2×1200

F3–F14 F606W 10584 2×1200

F3–F10 F814W 10250 3×1650

F11 F814W 10250 2×1100

F12–F14 F814W 10250 3×1650

F15–F16 F435W 10584 3×1565

F15–F16 F606W 10584 3×1580

F15–F16 F814W 10584 3_×1595

F17 F435W 10584 2×665

F17 F606W 10584 1×350

F17 F814W 10584 1×350

R2–R13 F435W 10584 2×1200

R2–R13 F606W 10584 2×1200

R2–R13 F814W 10250 3×1650

Figure 1.The footprints of 29HST/ACS pointings superposed on a 23×

28 arcmin2_GALEX_{image of M81. Identification number of each field is}

indicated. These 29 pointings cover the entire optical/UV extent of the galaxy.

a background map by computing the mean and the standard devia-tion of every secdevia-tion of the image with a user-defined grid size for which we choose 64×64 pixels. The local background is clipped iteratively until the values in every remaining pixel is within±3σof the median value. The mean of the clipped histogram is then taken as the local background. Every area of at least five adjacent pixels that exceeded the background by at least 3σ was called a source candidate.

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The population of compact star clusters in M81 1295

TheB band was used for the detection of candidates, and we carried out aperture photometry of all the detected sources in each of theB,VandIimages. The process was repeated for each of the 29 fields, resulting in a preliminary list of 565 438 sources. This list contains both unresolved (stellar-like) and resolved (extended) ob-jects. The distribution of sizes peaks at full width at half-maximum (FWHM)₌2.1 pixel, which corresponds to the typical point spread function (PSF) of the ACS images. We chose FWHM=2.4 pixel as the dividing line to separate cluster candidates from point sources. Our aim is to create a catalogue of compact sized clusters and hence we restricted our catalogue to sources with FWHM < 10 pixel. Thus our preliminary list of CSC candidates includes all sources with 2.4<FWHM<10 pixels.

Among the resolved objects selected using the above-mentioned criterion, we have two kinds of sources that contaminate the gen-uine CSCs. The first of these contaminating sources is formed by the unevenness of the local background due to the presence of dust and complex small-scale disc structures. The second type of contami-nating sources is caused due to the blending of several point sources due to stellar crowding. These contaminating sources are rejected by using the AREA parameter (defined as the number of contiguous pixels above the 3σ detection limit) of SEXTRACTOR. By visual

in-spection of the images, we found that the fraction of contaminating sources is highest for sources having area less than 50 pixels. Hence, we rejected all sources if they have an AREA<50 pixels. By nu-merical calculations, we found that even the most compact objects have AREA > 50 pixels if they are brighter thanB = 23 mag, which effectively sets the completeness limit of our selection pro-cess.

While a great majority of blended stars are eliminated from the list by the imposed AREA criteria, some of them still sneak through. In order to eliminate such sources, we analysed the ellipticities of the sources. All genuine clusters are expected to be round with ellip-ticityǫ <0.1. However, because SEXTRACTORmeasures ellipticities at the limiting (3σ) isophote level, we found that some genuine clus-ters haveǫ >0.1. This happens when a cluster is surrounded by a diffuse background or is immersed within a stellar group. This kind of source is characterized by a prominent peak, with the aperture photometry saturating at a small radius. On the other hand, aper-ture magnitude of a source formed by an elongated chain of stars would continue to rise with increasing radius. This property allows us to separate clusters from the blended stars even when the mea-sured ellipticities are>0.1. We found that if the difference between the aperture magnitudes of diameters 2 and 4 pixels is less than 1.5 mag, then they are true clusters. Hence, among the elongated sources, we retained only those sources if the difference in mag-nitudes between apertures of diameters 2 and 4 pixels is less than 1.5 mag. In summary, all sources with 2.4<FWHM<10 pixels, AREA>50 pixels and ellipticity<0.1 are retained, whereas among the elongated sources, only those showing evidence of a compact core are retained.

Note that at the distance of M81, 2.4 pixels correspond to a physi-cal sphysi-cale of 2.1 pc. Given that the PSF of ACS images is 2.1 pixels, all clusters smaller than 1 pc of FWHM will have a measured FWHM of 2.4 pixels, and hence our method cannot recover clusters more com-pact than this, if present. A Gaussian FWHM of 1 pc corresponds to a core radius of 0.5 pc for a King profile withc =rt/rc=30

(Chandar et al. 2001a), which is almost the limiting size for the compact clusters, and very few such clusters are known to exist (Ashman & Zepf 2001; Barmby et al. 2006; Scheepmaker et al. 2007). Hence, we are not missing many clusters because of this criterion. The measured FWHM>5 pixel, the smearing due to the

PSF is only marginal. The upper cut-off of FWHM=10 pixel used in this work, corresponds to a PSF-corrected physical size of 8.6 pc (core radius 4.3 pc).

We carried out a visual inspection of the images to make a list of objects that have diffraction spikes or are saturated in any one of theB-,V- orI-band images, with majority of them saturated only in theIband. A total of 83 such objects are found and there is no published information about the nature of these objects from spectroscopic surveys such as the one carried out by Sandage (1984), or several other follow-up studies. We hence analysed the colours of these objects in order to investigate whether some of these could be compact young star clusters. To avoid the use of theHSTcolours that may be erroneous due to saturation, we carried out photometry of these objects using the Sloan Digital Sky Survey (SDSS) images and constructed au−gversusg−rdiagram for the selected objects, which is shown in Fig. 2. The main-sequence colours are obtained using the Girardi et al. (2002) calculations for the grid ofTeffand

logg that define each spectral type (Mas-Hesse & Kunth 1991). Notice that in this diagram the reddening vector is almost orthogonal to the track defined by the spectral types for stars earlier than A0. The colours of all except three objects are consistent with them being stars of spectral types later thanF. These saturated objects are extremely bright to be stars of M81, and hence, are likely to be foreground Galactic stars. However, some of these objects could be GCs. Young clusters are expected to have colours of stars earlier than spectral-type B5, and there are only three candidates (shown by filled circles) that can be interpreted as dusty young clusters. None of these is brighter than the brightest selected cluster. Thus, our selection criteria have not eliminated possible bright clusters from our cluster sample.

The selection criteria described above resulted in a catalogue of 1123 compact stellar cluster candidates. Artificial sources due to stellar blending are still present in our catalogue, with their fraction increasing systematically at fainter magnitudes. These contaminat-ing sources are relatively bluer in colour, limitcontaminat-ing principally our capability of detection of blue clusters. Hence, we restrict most of our analysis toB = 22 mag for the blue clusters. For relatively

Figure 2.The SDSSu−gversusg−rdiagram for all the objects that

are saturated in at least one of theHST/ACS bands. The locus of the

main-sequence stars as well the direction of the reddening vector forAv=1 mag

(Cardelli, Clayton & Mathis 1989) are shown. Only three objects could be interpreted as reddened young clusters (colours bluer than B5 stars; shown by filled circles), with the rest following the track defined by the

main-sequence stars of spectral types later thanF.

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1296 M. Santiago-Cort´es, Y. D. Mayya and D. Rosa-Gonz´alez

redder clusters, contamination by stellar blending is not a serious limitation allowing us to retain them all the way toB =24 mag. After applying this colour-based selection criterion, which will be discussed again in Section 4.2, we ended up with a list of 435 clus-ters. The catalogue of Chandar et al. (2001a) contains 114 clusters up to a limiting magnitude ofV =22. Thus, our wide-field search has more than tripled the number of clusters in M81.

TheBmagnitude used in this work is ISOMAG parameter cal-culated in SEXTRACTOR. This parameter measures the magnitude by

integrating the background-subtracted counts in all the pixels that define the source. Colours were obtained by subtracting the mag-nitudes calculated within a fixed aperture of diameter₌20 pixels. Aperture corrections as suggested by Sirianni et al. (2005) were ap-plied to the magnitudes in each filter. The method adopted by us to calculate colours ensures that the internal errors on the colours are minimum. We estimated the errors on the magnitudes and colours using the multiple observations of the same star as described below. The 29 ACS fields used in this study offered around 10 per cent area overlap between the adjacent fields (see Fig. 1). The overlap region contained around 30 000 stars. We used the two independent photometric measurements for the common stars in each of theB, VandIbands to estimate typical photometric errors on the magni-tudes. As expected, errors are found to be the least for bright stars (0.10 mag forB <20 mag), increasing systematically for fainter stars (0.20 mag forB = 24 mag). Similar errors were estimated in all the three filters. Errors on any two bands are found to be uncorrelated, and hence errors on the colours were calculated by quadratically adding the errors on the magnitudes. All the mag-nitudes and colours quoted in this work are on the standard Vega system of magnitudes.

Our list contains 20 of the 41 spectroscopically confirmed GCs (Perelmuter et al. 1995; Schroder et al. 2002). Among the missing objects, eight are outside our field of view, and another eight have stellar appearance (most are saturated) on the ACS images. These objects could be very compact GCs. However, given that the galactic halo stars share the metallicities and radial velocities of the M81 GCs, these eight objects are most likely to be galactic stars, rather than very compact GCs. These objects are listed in Table 2 along with their observational properties. The remaining eight objects classified as GCs do not satisfy one or the other of our selection criteria. We also recover 53 of the 114 objects reported by Chandar et al. (2001b). The principal reasons for the absence of the rest of the Chandar et al. (2001b) clusters are either they are blended stars (ellipticity>0.1) or that they are foreground or M81 field stars wrongly classified as clusters due to the relatively poorer spatial resolution of WFPC2 images as compared to our ACS images. Thus, our catalogue of CSCs supersedes Chandar et al. (2001a)

Figure 3.TheF435−F814 (B−I for brevity) colour histogram for

the CSC population is plotted separately for the bright (B <22 mag) and

relatively faint (B=22–23 mag) clusters. Our CSC sample clearly divides

into blue and red groups, with the dividing colour beingB−I=1.7, which

is shown by the downward pointing arrow.

catalogue, both in its robustness of selection and in the spatial coverage.

4 A N A LY S I S

4.1 Colour histogram

In Fig. 3, we present theB−I colour histograms for the clus-ter candidates separately for bright (B <22 mag; solid line) and faint (22.0 < B <23.0 mag; dotted line) members of our cata-logue. It can be easily noticed that the distribution is bimodal in nature, especially for the brighter sample. Bimodality is also seen in the distribution of theB−Vcolours for our sample objects. This bimodality has been noticed previously by Chandar et al. (2001b), who used this property to separate GC candidates from the relatively younger clusters. In the next section, we use CMDs to firmly estab-lish this interpretation. On the basis of bimodality, we separated the cluster sample into two groups: a blue group withB−I <1.7 and a red group withB−I >1.7. The colours of the red group mem-bers compare well with the colours of GCs in the MW and M31 (Harris 1996), and the colours of the blue clusters are similar to those of young- and intermediate-age clusters found in the Mag-ellanic Clouds and M33 (Chandar, Bianchi & Ford 1999). In Table 3, we include the physical properties of the brightest compact clusters.

Table 2. HST/ACS point sources erroneously classified as GCs in previous studies.

ID RA (2000) Dec. (2000) B(mag) B−I FWHM (pixel) IDa _Va

rad(km s−1) [Fe/H]a

45861F1 09:55:03.823 69:15:38.10 19.09 1.33 2.27b _Is40165 ₆

−1.57

9514F3 09:55:44.079 69:14:12.00 20.11 2.14 1.74b _Is40181 ₄₆ ₀_.₆₄

113F10 09:55:06.265 68:56:24.78 18.60 1.00 2.47b _Is50037

−18 −2.34

8041F11 09:56:40.582 68:59:52.44 19.68 1.95 2.61b _Is50225

−7 −0.04

3740F6 09:54:19.980 69:09:11.57 19.97 1.45 2.02b _Is51027 ₃₀₀

−2.47

901F17 09:55:56.866 68:52:13.42 19.63 1.60 1.85b Is60045 −28 −1.03

10870R8 09:54:58.754 69:00:58.21 20.97 1.72 2.19 Is50286 −9 −0.04

10600F9 09:56:31.774 69:02:38.47 21.38 2.20 2.15 Id50401 −283 −0.04

a_{Last three columns data were taken from Perelmuter et al. (1995).}

b_{Mildly saturated stars.}

C

Study of three distinct populations of stellar clusters in the spiral galaxy M81