This thesis aims at improving our understanding of the evolution of quiescent galaxies since z ∼ 1, with the ultimate goal of providing a general picture for the formation and evolution of these objects along the history of the Universe. Making use of data from the Advanced Large Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) survey, this Ph. D. thesis is novel at facing for the first time an extensive, observational study that comprises the time evolution of the number density of quiescent galaxies, as well as their masses, stellar populations (ages, metallicities and extinctions) and sizes, to ultimately build up a phenomenological evolutionary model based on the merger, "frosting" and "progenitor bias" scenarios that tries to reconcile the observed trends in the above parameters.
There are a lot of objectives that should be properly accounted for in order to achieve our aims. These ones are listed below:
• Reliable determination of stellar population parameters via SED-fitting and only using data
from large scale multi-filter surveys. In particular, the code MUFFIT, developed as part of this
work, is devoted for determining the stellar population parameters using any photometric dataset. Although we exclusively use the ALHAMBRA data throughout this work for determining stellar populations of galaxies, we planned to span all the methodologies to other multi-filter surveys. It
1.4. Goals of this work 15
is noteworthy that the ALHAMBRA dataset is an excellent test bench for the performing of these techniques. Once the large-scale surveys J-PLUS (Cenarro et al., in prep.) and J-PAS (Benítez et al., 2014) start the observations, all the methodologies performed under the ALHAMBRA dataset will be directly applied on these fruitful surveys.
• Extraction of a pure sample of quiescent galaxies. There exist several diagnostic diagrams to select samples of quiescent galaxies (see Sect. 1.1), some of them with interesting advantages with respect to others. The combination of optimal diagnostic diagrams with the stellar population determina- tions for each galaxy in the parent dataset can provide a reliable sample of quiescent galaxies, where typical sources of contamination (e. g. reddened-by-dust star forming galaxies) may be successfully mitigated.
• Evolution of the stellar population parameters of quiescent galaxies with redshift. In particu- lar, we aim to determine the evolution of ages, metallicities, and extinctions of quiescent galaxies via SED-fitting, through their distributions of values at different redshifts and stellar masses. The evolution of these parameters will be key to trace how these galaxies have evolved up to present days. • Quantifying and discerning the likely mechanisms affecting the evolution of quiescent galaxies. Once galaxies quench star formation, its evolution is suitable to other mechanisms less representative than starbursts or efficient star formation. Other effects, such as the proper ones of a hierarchical merging, are suitable candidates to drive the evolution of galaxies that do not experiment a significant star formation.
• Correlations between size and the stellar content of quiescent galaxies and implications for their
growth in size. During the past and present decade, many efforts were performed for quoting the
responsible mechanisms of the increase in size of galaxies. Owing to the large set of quiescent galaxies that we can retrieve from ALHAMBRA and it is wide range in redshift, we are able to explore the mechanisms driving the increase in size of galaxies through the study of the stellar content of quiescent galaxies.
On the basis of all the above pieces of information, in many case unique and unprecedented due to the amount and type of data, as well as to the analysis techniques, this work is ultimately expected to shed light on a global view on the formation and evolution of red sequence galaxies, being this overall consistent with all the observables in play.
Notice the both Chapter 2 and Chapter 6 were actually published in referred journals under references Díaz-García et al. 2015, A&A, 582A, 14D and Díaz-García et al. 2013, MNRAS, 433, 60D respectively.
2
MUFFIT: a MUlti-Filter FITting code for stellar population
diagnostics
A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty.
Winston Churchill
This chapter has been published as Díaz-García et al. 2015, A&A, 582A, 14D
Introducción al artículo
A lo largo de este artículo se presenta una de las partes más largas y difíciles de desarrollar, y al mismo tiempo indispensables, para alcanzar los objetivos de esta tesis doctoral: el código genérico MUFFIT. Éste ha sido cuidadosamente y específicamente desarrollado con el objetivo de extraer los parámetros de las poblaciones estelares de galaxias mediantes datos fotométricos de cartografiados multifiltro (fotoespectros). Al mismo tiempo, se comprueba su fiabilidad y viabilidad con galaxias reales procedentes del cartografiado ALHAMBRA y con simulaciones, con un resultado satisfactorio en todos los casos. A pesar de existir múltiples herramientas disponibles para la comunidad científica para llevar a cabo ajustes de fotoespectros, SED-fitting, éstos carecían de los detalles técnicos para cubrir todas las necesidades para llevar a cabo el análisis de poblaciones estelares con el nivel de detalle necesario para este tipo de estudios. Entre las características más llamativas de MUFFIT podemos destacar:
i) La libertad de incluir diferentes modelos de poblaciones simples (SSP) para llevar a cabo el SED- fitting de galaxias. Modelos que han sido cuidadosamente elaborados con múltiples restricciones físicas para predecir de la forma más fiable la evolución de una población estelar, determinando pará- metros que pueden ser interpretados físicamente como edad y metalicidad. Esto marca una diferencia respecto otros códigos que simplemente incluyen conjuntos de modelos que, o bien, han sido calcula- dos empíricamente y que por tanto sus parámetros de poblaciones estelares son de difícil calibración o interpretación; o bien, modelos que han sido seleccionados con el único objetivo de determinar otros parámetros que difieren de las edades y metalicidades que describen una población estelar (por ejemplo, desplazamientos al rojo o masas estelares).
ii) MUFFIT ha sido diseñado para lidiar con todo tipo de cartografiados multi-filtro, incluyendo J-PAS y J-PLUS que incluyen multitud de filtros lo suficientemente estrechos (FWHM ∼ 125 Å) como para ser sensibles a líneas de emisión. Esto supone un reto para otro tipo de códigos debido a la presencia 17
18 CHAPTER2. MUFFIT: a MUlti-Filter FITting code for stellar population diagnostics
de líneas emisión, las cuales pueden perjudicar drásticamente los ajustes a modelos, como los SSP (que habitualmente no las incluyen y cuya contribución es de difícil estimación).
iii) MUFFIT ha sido analizado y calibrado detalladamente para lidiar con datos fotométricos de la forma más precisa posible, comparando con otros estudios de la literatura (incluyendo estudios espec- troscópicos).
MUFFIT está basado en un test de χ2pesado con errores, donde se comparan los flujos de las diferentes
bandas de la galaxia con la fotometría sintética de modelos mezcla de dos poblaciones estelares simples, a diferentes desplazamientos al rojo y con diferentes extinciones, para obtener el rango probable de sus parámetros de poblaciones estelares (mayormente edad y metalicidad), extinción, desplazamiento al rojo y masa estelar. Para mejorar la fiabilidad del análisis, MUFFIT identifica y descarta del proceso de análisis aquellas bandas con indicios de contener lineas de emisión de intensidad significativa. Los parámetros finales junto con sus incertidumbres son derivados a partir de una metodología Monte Carlo, usando las incertidumbres de la fotometría de cada banda.
A lo largo de este trabajo se concluye que MUFFIT es un código preciso y fiable para derivar los paráme- tros de las poblaciones estelares de las galaxias de ALHAMBRA. Es más, hacemos un análisis exhaustivo y detallado de todos los problemas que pueden acarrear este tipo de análisis, detallando en todo momento la capacidad para determinar particularmente edad, metalicidad y extinción, y de forma complementaria, masa estelar y desplazamientos al rojo fotométricos. Parte de estas conclusiones son obtenidas a partir de simu- laciones que incluyen diferentes valores de señal-ruido, las cuales también demuestran que estos tipos de análisis basados en los colores del continuo son lo suficientemente sensibles a este conjunto de parámetros estelares. Al mismo tiempo, podemos cuantificar los tipos de incertidumbres intrínsecos a esta metodología, así como los tipos de degeneraciones que podemos esperar entre los parámetros involucrados. Por otro lado, complementamos estas pruebas para comprobar la fiabilidad de MUFFIT utilizando galaxias reales de ALHAMBRA. Utilizando predicciones de desplazamientos al rojo fotométricos como datos de entrada, MUFFIT es capaz de mejorar la precisión de éstos en un ∼ 10–20 %. Además, MUFFIT es capaz de detectar emisiones nebulares en galaxias y suministrar predicciones físicas de su intensidad. Las medidas de masa estelar calculadas por MUFFIT muestran un acuerdo excelente con los valores dados por COSMOS y SDSS para casos en común. También obtenemos que los mapas de edad–metalicidad para una muestra de galaxias de tipo temprano a z ≤ 0.22 están en acuerdo con las obtenidas en diagnósticos espectroscópicos de SDSS. La comparación uno a uno entre desplazamientos al rojo, edades, metalicidades y masas estelares, derivadas con espectroscopía en SDSS y por MUFFIT en ALHAMBRA, muestran un buen acuerdo cualitativo entre todas ellas, reforzando así el potencial de los cartografiados multifiltro cuando son analizados con las téc- nicas de análisis apropiadas, como MUFFIT, para llevar a cabo estudios de poblaciones estelares de forma correcta.
Lamentablemente, las técnicas y metologías de este tipo han sido pobremente explotadas en cartografia- dos multifiltro. Habitualmente no van más allá de tareas como determinar desplazamientos al rojo fotométri- cos o masas estelares, mientras que para edades y metalicidades son injustamente despreciados porque en muchos casos no han sido calibrados debidamente. En ciertos casos también han sido utilizadas como un so- porte para corroborar los propios resultados espectroscópicos, centrados en zonas del espectro más sensibles a ciertos parámetros poblacionales como edad (e. g. Fagioli et al., 2016; Gargiulo et al., 2016). Es más, los resultados demuestran que las incertidumbres típicas con las que se recuperan los valores de metalicidad en cartografiados tipo ALHAMBRA pueden llegar a ser más fiables que ciertas estimaciones de metalicidades espectroscópicas, y que además reforzamos mediante la estadística del gran número de galaxias con los que cuentan los cartografiados multifiltro.
2.1. Introduction 19
Stellar populations of galaxies in the ALHAMBRA survey up toz ∼ 1:
I. MUFFIT a MUlti-Filter FITting code for stellar population diagnostics
Díaz-García et al. 2015, A&A, 582A, 14D
ABSTRACT:We present MUFFIT, a new generic code optimized to retrieve the main stellar population parameters of galaxies in photometric multi-filter surveys, and check its reliability and feasibility with real galaxy data from the ALHAMBRA survey.
Making use of an error-weighted χ2-test, we compare the multi-filter fluxes of galaxies with the
synthetic photometry of mixtures of two single stellar populations at different redshifts and extinctions, to provide the most likely range of stellar population parameters (mainly ages and metallicities), extinc- tions, redshifts, and stellar masses. To improve the diagnostic reliability, MUFFIT identifies and removes from the analysis those bands that are significantly affected by emission lines. The final parameters and their uncertainties are derived by a Monte Carlo method, using the individual photometric uncertainties in each band. Finally, we discuss the accuracies, degeneracies, and reliability of MUFFIT using both simulated and real galaxies from ALHAMBRA, comparing with results from the literature.
MUFFIT is a precise and reliable code to derive stellar population parameters of galaxies in AL- HAMBRA. Using the results from photometric-redshift codes as input, MUFFIT improves the photometric- redshift accuracy by ∼ 10–20 %. MUFFIT also detects nebular emissions in galaxies, providing physical information about their strengths. The stellar masses derived from MUFFIT show excellent agreement with the COSMOS and SDSS values. In addition, the retrieved age–metallicity locus for a sample of z ≤ 0.22 early-type galaxies in ALHAMBRA at different stellar mass bins are in very good agreement with the ones from SDSS spectroscopic diagnostics. Moreover, a one-to-one comparison between the redshifts, ages, metallicities, and stellar masses derived spectroscopically for SDSS and by MUFFIT for ALHAMBRA reveals good qualitative agreements in all the parameters, hence reinforcing the strengths of multi-filter galaxy data and optimized analysis techniques, like MUFFIT, to conduct reliable stellar population studies.
2.1
Introduction
Studying the stellar content of galaxies is crucial to understanding their star formation histories (SFH), which in turn provides us with valuable information about the possible evolutive paths from their formation at high redshift down to the present time. Despite the strong efforts and advances achieved in this topic during the past decades, it still remains as one of the most challenging and promising ways to understand galaxy evolution.
Early attempts to study the stellar content of early-type galaxies were based on colours from wide and narrow band photometry (Baum, 1959; Tifft, 1963; Wood, 1966; McClure & van den Bergh, 1968; Faber, 1973) and on empirical synthesis of the populations using the observed colours of nearby early-types as basis. These early methods can be considered as the pioneers of the current photo-spectral fitting techniques, which are the main topic of the present paper. The above methods were gradually displaced by techniques based in more specific features (Faber, 1973; Pritchet, 1977) that were defined in narrow spectral ranges.
The arrival of absorption line-strength indices to study the stellar content of galaxies (Burstein et al., 1984; Faber et al., 1985) brought a significant breakthrough in the field. On this front, it is worth noting
20 CHAPTER2. MUFFIT: a MUlti-Filter FITting code for stellar population diagnostics
the Lick system of indices (Gorgas et al., 1993; Worthey et al., 1994b), which for the past decades has been the standard for most spectroscopic studies in stellar populations in the optical (e. g. Trager et al., 1998; Jørgensen, 1999; Kuntschner et al., 2001; Thomas et al., 2005; Bernardi et al., 2006; Sánchez-Blázquez et al., 2006b; Gorgas et al., 2007). The combination of a certain number of absorption lines mainly sensitive to age, such as the Balmer lines, or to the metallicity, as traced by certain elements such as Fe , Mg , Ti ,
C , Ca , and NaWere proven to be an efficient way to break the well known degeneracy between these two
parameters, at least to some extent (Worthey, 1994a). The way to measure these features is delicately chosen to be very sensitive to a parameter of interest, focusing its study on narrow spectral ranges. By construction, line-strength indices are quite insensitive to the influence of extinction, and by fine-tuning their definition or combining the sensitivities of different indices, some of them may end up being almost independent of other parameters, such as metallicity (Vazdekis & Arimoto, 1999b; Cervantes & Vazdekis, 2009) and α-element overabundances (Thomas et al., 2003).
In the past fifteen years, the development of stellar libraries in spectral ranges other than the optical has driven the definition of new indices that allowed this kind of study to be extended to other regions with unexplored sensitivities (Cenarro et al., 2002; Mármol-Queraltó et al., 2008). In addition, the index system of reference in the optical spectral range has been revisited and improved (see e. g. Vazdekis et al., 2010) thanks to the availability of much better stellar libraries at much better spectral resolution.
It was with the arrival of improved stellar libraries, such as CaT (Cenarro et al., 2001a,b), ELODIE (Prugniel & Soubiran, 2001), STELIB (Le Borgne et al., 2003), INDO-US (Valdes et al., 2004), Martins et al. (2005), and MILES (Sánchez-Blázquez et al., 2006c; Cenarro et al., 2007), and the consequent evolu- tionary stellar population synthesis models (e. g. Bruzual & Charlot, 2003; Vazdekis et al., 2003; González Delgado et al., 2005; Maraston et al., 2009; Vazdekis et al., 2010; Conroy & van Dokkum, 2012; Vazdekis et al., 2012), that fitting techniques over the full spectral energy distribution of galaxies appeared as an alter- native to line-strength indices. SED-fitting can also be used to derive several physical properties of galaxies (Mathis et al., 2006; Koleva et al., 2008; Coelho et al., 2009; Walcher et al., 2011; Liu et al., 2013). In fact, there is a growing number of public codes specifically devoted to carrying out SED-fitting with different
procedures, such as hyperz (Bolzonella et al., 2000), LEPHARE (Arnouts et al., 2002; Ilbert et al., 2006),
STARLIGHT (Cid Fernandes et al., 2005), STECKMAP (Ocvirk et al., 2006), VESPA (Tojeiro et al., 2007),
ULYSS (Koleva et al., 2009), FAST (Kriek et al., 2009), and SEDfit (Sawicki, 2012).
Nowadays, there is an increasing number of present and future multi-filter surveys, including COMBO- 17 (Wolf et al., 2003), MUSYC (Gawiser et al., 2006), COSMOS (Scoville et al., 2007), ALHAMBRA (Moles et al., 2008), CLASH (Postman et al., 2012), SHARDS (Pérez-González et al., 2013), J-PAS (Benítez et al., 2014), and J-PLUS (Cenarro et al., in prep.), each of them with a vast volume of high-quality multi- filter data. These kinds of surveys pursue diverse goals with a common feature: sampling the SEDs of galaxies using top-hat and/or broad-band filters that mainly cover the optical range. Owing to this configu- ration, the retrieved SEDs are half-way between classical photometry and spectroscopy, because in practice they are like a low-resolution spectrum whose resolution depends on the filter system (e. g. R ∼ 20 for ALHAMBRA; R ∼ 50 for J-PAS).
Although multi-filter observing techniques suffer from the lack of high spectral resolution, their advan- tages over standard spectroscopy are worth noting: (i) the galaxy samples of multi-filter surveys do not suffer from selection criteria other than the photometric depth in the detection band of the survey, because all the objects in the field of view are observed. For a fixed observational time and similar telescopes, this leads to much larger galaxy samples than in multi-object spectroscopy, where achieving multiplexities greater than ∼ 1000 is a challenge at present. (ii) Unlike standard spectroscopy, the SED of galaxies ob- served in multi-filter surveys does not suffer from the typical uncertainties in the flux calibration that lead to systematic colour terms, since the photometric calibration of each individual band is independent of the rest. This advantage is crucial, because it is the overall continuum of the stellar population that in most cases dominates the diagnostic with SED-fitting techniques. (iii) With similar telescopes, the depth of multi-filter surveys is usually much greater than for spectroscopic survey, since direct imaging is much more efficient than spectroscopy. (iv) Multi-filter surveys provide spatially resolved photo-spectra, similar to an integral