Stellar Population Parameters Research Articles

Evidence for temporal evolution in the M33 disc as traced by its star clusters

We present precision radial velocities and stellar population parameters for 77 star clusters in the Local Group galaxy M33. Our GTC and WHT observations sample both young, massive clusters and known/candidate globular clusters, spanning ages � 10 6 10 10 yr, and metallicities, [M/H] � 1.7 to solar. The cluster system exhibits an age-metallicity relation; the youngest clusters are the most metal-rich. When compared to HI data, clusters with [M/H]� 1.0 and younger than � 4 Gyr are clearly identified as a disc population. The clusters show evidence for strong time evolution in the disc radial metallicity gradient (d[M/H]dt / dR = 0.03 dex/kpc/Gyr). The oldest clusters have stronger, more negative gradients than the youngest clusters in M33. The clusters also show a clear age-velocity dispersion relation. The line of sight velocity dispersions of the clusters increases with age similar to Milky Way open clusters and stars. The general shape of the relation is reproduced by disc heating simulations, and the similarity between the relations in M33 and the Milky Way suggests that heating by substructure, and cooling of the ISM both play a role in shaping this relation. We identify 12 “classical” GCs, six of which are newly identified GC candidates. The GCs are more metal-rich than Milky Way halo clusters, and show weak rotation. The inner (R < 4.5 kpc) GCs exhibit a steep radial metallicity gradient (d[M/H]/dR = 0.29 ± 0.11 dex/kpc) and an exponential-like surface density profile. We argue that these inner GCs are thick disc rather than halo objects.

THE NEXT GENERATION VIRGO CLUSTER SURVEY. XII. STELLAR POPULATIONS AND KINEMATICS OF COMPACT, LOW-MASS EARLY-TYPE GALAXIES FROM GEMINI GMOS-IFU SPECTROSCOPY

We present Gemini GMOS-IFU data of eight compact low-mass early-type galaxies (ETGs) in the Virgo cluster. We analyse their stellar kinematics, stellar population, and present two-dimensional maps of these properties covering the central 5"x 7" region. We find a large variety of kinematics: from non- to highly-rotating objects, often associated with underlying disky isophotes revealed by deep images from the Next Generation Virgo Cluster Survey. In half of our objects, we find a centrally-concentrated younger and more metal-rich stellar population. We analyze the specific stellar angular momentum through the lambdaR parameter and find six fast-rotators and two slow-rotators, one having a thin counter-rotating disk. We compare the local galaxy density and stellar populations of our objects with those of 39 more extended low-mass Virgo ETGs from the SMAKCED survey and 260 massive ($M>10^{10}$\Msun) ETGs from the A3D sample. The compact low-mass ETGs in our sample are located in high density regions, often close to a massive galaxy and have, on average, older and more metal-rich stellar populations than less compact low-mass galaxies. We find that the stellar population parameters follow lines of constant velocity dispersion in the mass-size plane, smoothly extending the comparable trends found for massive ETGs. Our study supports a scenario where low-mass compact ETGs have experienced long-lived interactions with their environment, including ram-pressure stripping and gravitational tidal forces, that may be responsible for their compact nature.

The stellar populations in the low-luminosity, early-type galaxy NGC 59

Low-luminosity galaxies may be the building blocks of more luminous systems. Southern African Large Telescope observations of the low-luminosity, early-type galaxy NGC 59 are obtained and analysed. These data are used to measure the stellar population parameters in the centre and off-centre regions of this galaxy, in order to uncover its likely star formation history. We find evidence of older stars, in addition to young stars in the emission-line regions. The metallicity of the stellar population is constrained to be [Z/H] ∼ −1.1 to −1.6, which is extremely low, even for this low-luminosity galaxy, since it is not classed as a dwarf spheroidal galaxy. The measured [α/Fe] ratio is subsolar, which indicates an extended star formation history in NGC 59. If such objects formed the building blocks of more massive, early-type galaxies, then they must have been gaseous mergers, rather than dry mergers, in order to increase the metals to observed levels in luminous, early-type galaxies.

STELLAR MASS FUNCTIONS OF GALAXIES AT 4 &lt;z&lt; 7 FROM ANIRAC-SELECTED SAMPLE IN COSMOS/ULTRAVISTA: LIMITS ON THE ABUNDANCE OF VERY MASSIVE GALAXIES

We build a Spitzer IRAC complete catalog of objects, obtained by complementing the $K_\mathrm{s}$-band selected UltraVISTA catalog with objects detected in IRAC only. With the aim of identifying massive (i.e., $\log(M_*/M_\odot)>11$) galaxies at $4<z<7$, we consider the systematic effects on the measured photometric redshifts from the introduction of an old and dusty SED template and from the introduction of a bayesian prior taking into account the brightness of the objects, as well as the systematic effects from different star formation histories (SFHs) and from nebular emission lines in the recovery of stellar population parameters. We show that our results are most affected by the bayesian luminosity prior, while nebular emission lines and SFHs only introduce a small dispersion in the measurements. Specifically, the number of $4<z<7$ galaxies ranges from 52 to 382 depending on the adopted configuration. Using these results we investigate, for the first time, the evolution of the massive end of the stellar mass functions (SMFs) at $4<z<7$. Given the rarity of very massive galaxies in the early universe, major contributions to the total error budget come from cosmic variance and poisson noise. The SMF obtained without the introduction of the bayesian luminosity prior does not show any evolution from $z\sim6.5$ to $z\sim 3.5$, implying that massive galaxies could already be present when the Universe was $\sim0.9$~Gyr old. However, the introduction of the bayesian luminosity prior reduces the number of $z>4$ galaxies with best fit masses $\log(M_*/M_\odot)>11$ by 83%, implying a rapid growth of very massive galaxies in the first 1.5 Gyr of cosmic history. From the stellar-mass complete sample, we identify one candidate of a very massive ($\log(M_*/M_\odot)\sim11.5$), quiescent galaxy at $z\sim5.4$, with MIPS $24\mu$m detection suggesting the presence of a powerful obscured AGN.

How elevated is the dynamical-to-stellar mass ratio of the ultracompact dwarf S999?

Here we present new Keck ESI high-resolution spectroscopy and deep archival HST/ACS imaging for S999, an ultra-compact dwarf in the vicinity of M87, which was claimed to have an extremely high dynamical-to-stellar mass ratio. Our data increase the total integration times by a factor of 5 and 60 for spectroscopy and imaging, respectively. This allows us to constrain the stellar population parameters for the first time (simple stellar population equivalent age $=7.6^{+2.0}_{-1.6}$ Gyr; $[Z/\textrm{H}]=-0.95^{+0.12}_{-0.10}$; $[\alpha/\textrm{Fe}]=0.34^{+0.10}_{-0.12}$). Assuming a Kroupa stellar initial mass function, the stellar population parameters and luminosity ($M_{F814W}=-12.13\pm0.06$ mag) yield a stellar mass of $M_*=3.9^{+0.9}_{-0.6}\times10^6 M_{\odot}$, which we also find to be consistent with near-infrared data. Via mass modelling, with our new measurements of velocity dispersion ($\sigma_{ap}=27\pm2$ km s$^{-1}$) and size ($R_e=20.9\pm1.0$ pc), we obtain an elevated dynamical-to-stellar mass ratio $M_{dyn}/M_*=8.2$ (with a range $5.6\le M_{dyn}/M_* \le 11.2$). Furthermore, we analyse the surface brightness profile of S999, finding only a small excess of light in the outer parts with respect to the fitted S\'ersic profile, and a positive colour gradient. Taken together these observations suggest that S999 is the remnant of a much larger galaxy that has been tidally stripped. If so, the observed elevated mass ratio may be caused by mechanisms related to the stripping process: the existence of an massive central black hole or internal kinematics that are out of equilibrium due to the stripping event. Given the observed dynamical-to-stellar mass ratio we suggest that S999 is an ideal candidate to search for the presence of an overly massive central black hole.

The ATLAS3D Project – XXX. Star formation histories and stellar population scaling relations of early-type galaxies

We present the stellar population content of early-type galaxies from the Atlas3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive non-parametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced than their larger counterparts. From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of today's local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.

STELLAR POPULATIONS FROM SPECTROSCOPY OF A LARGE SAMPLE OF QUIESCENT GALAXIES ATZ&gt; 1: MEASURING THE CONTRIBUTION OF PROGENITOR BIAS TO EARLY SIZE GROWTH

We analyze the stellar populations of a sample of 62 massive (log Mstar/Msun > 10.7) galaxies in the redshift range 1 < z < 1.6, with the main goal of investigating the role of recent quenching in the size growth of quiescent galaxies. We demonstrate that our sample is not biased toward bright, compact, or young galaxies, and thus is representative of the overall quiescent population. Our high signal-to-noise ratio Keck LRIS spectra probe the rest-frame Balmer break region which contains important absorption line diagnostics of recent star formation activity. We obtain improved measures of the various stellar population parameters, including the star-formation timescale tau, age and dust extinction, by fitting templates jointly to both our spectroscopic and broad-band photometric data. We identify which quiescent galaxies were recently quenched and backtrack their individual evolving trajectories on the UVJ color-color plane finding evidence for two distinct quenching routes. By using sizes measured in the previous paper of this series, we confirm that the largest galaxies are indeed among the youngest at a given redshift. This is consistent with some contribution to the apparent growth from recent arrivals, an effect often called progenitor bias. However, we calculate that recently-quenched objects can only be responsible for about half the increase in average size of quiescent galaxies over a 1.5 Gyr period, corresponding to the redshift interval 1.25 < z < 2. The remainder of the observed size evolution arises from a genuine growth of long-standing quiescent galaxies.

A TALE OF A RICH CLUSTER ATz∼ 0.8 AS SEEN BY THE STAR FORMATION HISTORIES OF ITS EARLY-TYPE GALAXIES

We present a detailed stellar population analysis for a sample of 24 early-type galaxies belonging to the rich cluster RXJ0152.7-1357 at z=0.83. We have derived the age, metallicity, abundance pattern and star formation history for each galaxy individually, to further characterize this intermediate-z reference cluster. We then study how these stellar population parameters depend on local environment. This provides a better understanding on the formation timescales and subsequent evolution of the substructures in this cluster. We have also explored the evolutionary link between z$\sim$0.8 ETGs and those in the local Universe by comparing the trends that the stellar population parameters follow with galaxy velocity dispersion at each epoch. We find that the ETGs in Coma are consistent with being the (passively-evolving) descendants of the ETG population in RXJ10152.7-1357. Furthermore, our results favor a downsizing picture, where the subclumps centers were formed first. This central parts contain the most massive galaxies, which formed the bulk of their stars in a short, burst-like event at high-z. On the contrary, the cluster outskirts are populated with less massive, smaller galaxies, which show a wider variety of Star Formation Histories. In general, they present extended star formation episodes over cosmic time, which seems to be related to their posterior incorporation into the cluster, around 4Gyr later after the initial event of formation.

Modelling the ages and metallicities of early-type galaxies in Fundamental Plane space

Recent observations have probed the formation histories of nearby elliptical galaxies by tracking correlations between the stellar population parameters, age and metallicity, and the structural parameters that enter the Fundamental Plane, radius and velocity dispersion sigma. These studies have found intriguing correlations between these four parameters. In this work, we make use of a semi-analytic model, based on halo merger trees extracted from the Bolshoi cosmological simulation, that predicts the structural properties of spheroid-dominated galaxies based on an analytic model that has been tested and calibrated against an extensive suite of hydrodynamic+N-body binary merger simulations. We predict the radius, sigma, luminosity, age, and metallicity of spheroid-dominated galaxies, enabling us to compare directly to observations. Our model predicts a strong correlation between age and sigma for early-type galaxies, and no significant correlation between age and radius, in agreement with observations. In addition we predict a strong correlation between metallicity and sigma, and a weak correlation between metallicity and radius, in qualitative agreement with observations. We find that the correlations with sigma arise as a result of the strong link between sigma and the galaxy assembly time. Minor mergers produce a large change in radius while leaving sigma nearly the same, which explains the weaker trends with radius.

3D-HST WFC3-SELECTED PHOTOMETRIC CATALOGS IN THE FIVE CANDELS/3D-HST FIELDS: PHOTOMETRY, PHOTOMETRIC REDSHIFTS, AND STELLAR MASSES

The 3D-HST and CANDELS programs have provided WFC3 and ACS spectroscopy and photometry over ~900 square arcminutes in five fields: AEGIS, COSMOS, GOODS-North, GOODS-South, and the UKIDSS UDS field. All these fields have a wealth of publicly available imaging datasets in addition to the HST data, which makes it possible to construct the spectral energy distributions (SEDs) of objects over a wide wavelength range. In this paper we describe a photometric analysis of the CANDELS and 3D-HST HST imaging and the ancillary imaging data at wavelengths 0.3um to 8um. Objects were selected in the WFC3 near-IR bands, and their SEDs were determined by carefully taking the effects of the point spread function in each observation into account. A total of 147 distinct imaging datasets were used in the analysis. The photometry is made available in the form of six catalogs: one for each field, as well as a master catalog containing all objects in the entire survey. We also provide derived data products: photometric redshifts, determined with the EAZY code, and stellar population parameters determined with the FAST code. We make all the imaging data that were used in the analysis available, including our reductions of the WFC3 imaging in all five fields. 3D-HST is a spectroscopic survey with the WFC3 and ACS grisms, and the photometric catalogs presented here constitute a necessary first step in the analysis of these grism data. All the data presented in this paper are available through the 3D-HST website.

Radial gradients in the SLUGGS survey

AbstractThe SAGES Legacy Unifying Galaxies and GlobularS (SLUGGS) survey probes the outskirts of early-type galaxies (ETGs). SLUGGS uses the DEIMOS spectrograph on Keck to simultaneously obtain globular cluster and spatially resolved galaxy spectra. The galaxy spectra allow for the reconstruction of kinematic maps out to ∼3 effective radii (Re), while globular clusters (GCs) push the galactocentric limit out to ∼8–10Re. Stellar population parameters (abundances) can also be recovered. This provides a unique dataset for exploring the outer gradients (abundance and/or kinematic transition). We briefly present recent and salient results of the SLUGGS survey so far.

Connection between dynamically derived IMF normalisation and stellar populations

AbstractIn this contributed talk I present recent results on the connection between stellar population properties and the normalisation of the stellar initial mass function (IMF) measured using stellar dynamics, based on a large sample of 260 early-type galaxies observed as part of the ATLAS3D project. This measure of the IMF normalisation is found to vary non-uniformly with age- and metallicity-sensitive absorption line strengths. Applying single stellar population models, there are weak but measurable trends of the IMF with age and abundance ratio. Accounting for the dependence of stellar population parameters on velocity dispersion effectively removes these trends, but subsequently introduces a trend with metallicity, such that ‘heavy’ IMFs favour lower metallicities. The correlations are weaker than those found from previous studies directly detecting low-mass stars, suggesting some degree of tension between the different approaches of measuring the IMF. Resolving these discrepancies will be the focus of future work.

Spatially resolved stellar population parameters in the BCGs of two fossil groups

We report the results of Gemini/GMOS long-slit spectroscopic observations along the major and minor axes of the central galaxies in two fossil groups, SDSS J073422.21+265133.9 and SDSS J075828.11+374711.8 (the NGC 2484 group). Spatially resolved kinematics and stellar population parameters (ages, metallicities and $\alpha$-element abundance ratios) derived using ~20 Lick indices are presented. Despite remarkable similarities in their morphologies, photometric properties (luminosity and colour) and kinematics, the two galaxies exhibit significantly different stellar population parameters. SDSS J073422.21+265133.9 exhibits a strong metallicity gradient (Delta[Z/H]/Delta R ~ -0.4) all the way into the centre of the galaxy. It also exhibits an age profile that suggest a relatively recent, centrally concentrated burst of star formation superimposed on an older, more spatially extended population. NGC 2484, a well known X-ray AGN, exhibits a flat core-like structure in its metallicity gradient, but no detectable age gradient. The alpha-element abundance ratio ([E/Fe]) profiles of the two galaxies are also significantly different. SDSS J073422.21+265133.9 exhibits a slightly positive gradient (Delta [E/H]/Delta R ~ 0.1), perhaps again suggesting a more recent central burst of star formation, while NGC 2484 shows a negative gradient (Delta [E/H]/Delta R ~-0.1), indicating that star formation may have happened "inside out". Our analysis of these two galaxies of similar mass, morphology and kinematics therefore suggests two different mechanisms to have been in action during their formation. Consequently, we conclude that the central galaxies of fossil groups can not be considered a homogeneous group with regard to their formation processes or star formation histories.

SN Ia host galaxy properties from Sloan Digital Sky Survey-II spectroscopy

We study the stellar populations of SNe Ia host galaxies using SDSS-II spectroscopy. We focus on the relationships of SNe Ia properties with stellar velocity dispersion and the stellar population parameters age, metallicity and element abundance ratios derived by fitting absorption line indices to stellar population models. We concentrate on a sub-sample of 84 SNe Ia from the SDSS-II Supernova Survey. In agreement with previous findings, we find that SALT2 stretch factor values show the strongest dependence on stellar population age. Hence, SNe Ia peak-luminosity is closely related to the age of the stellar progenitor systems, where more luminous SNe Ia appear in younger stellar populations. We find no statistically significant trends in the Hubble residual with any of the stellar population parameters studied, including age and metallicity contrary to the literature, as well as with stellar velocity dispersion. Moreover, we find that the method of stellar mass derivation is affecting the Hubble residual-mass relationship when lower number statistics are used. We extend the sample to also include SNe Ia with available SDSS host galaxy photometry only. For this larger sample (247 objects) the reported Hubble residual-mass relation is strongly dependent on the stellar mass range studied and behaves as a step function. In the high mass regime, probed by our host spectroscopy sample, the relation between Hubble residual and stellar mass is flat. Below a stellar mass of ~2x10^10 Msun, i.e. close to the evolutionary transition mass of low-redshift galaxies reported in the literature, the trend changes dramatically such that lower mass galaxies possess lower luminosity SNe Ia after light-curve corrections. This non-linear behaviour of the Hubble residual-mass relationship should be accounted for when using stellar mass as a further parameter for minimising the Hubble residuals.

A PUBLIC K s -SELECTED CATALOG IN THE COSMOS/ULTRAVISTA FIELD: PHOTOMETRY, PHOTOMETRIC REDSHIFTS, AND STELLAR POPULATION PARAMETERS ,

We present a catalog covering 1.62 deg^2 of the COSMOS/UltraVISTA field with PSF-matched photometry in 30 photometric bands. The catalog covers the wavelength range 0.15um - 24um including the available GALEX, Subaru, CFHT, VISTA and Spitzer data. Catalog sources have been selected from the DR1 UltraVISTA Ks band imaging that reaches a depth of K_{s,tot} = 23.4 AB (90% completeness). The PSF-matched catalog is generated using position-dependent PSFs ensuring accurate colors across the entire field. Also included is a catalog of photometric redshifts (z_phot) for all galaxies computed with the EAZY code. Comparison with spectroscopy from the zCOSMOS 10k bright sample shows that up to z ~ 1.5 the z_phot are accurate to dz/(1 + z) = 0.013, with a catastrophic outlier fraction of only 1.6%. The z_phot also show good agreement with the z_phot from the NEWFIRM Medium Band Survey (NMBS) out to z ~ 3. A catalog of stellar masses and stellar population parameters for galaxies determined using the FAST spectral energy distribution fitting code is provided for all galaxies. Also included are rest-frame U-V and V-J colors, L_2800 and L_IR. The UVJ color-color diagram confirms that the galaxy bi-modality is well-established out to z ~ 2. Star-forming galaxies also obey a star forming "main sequence" out to z ~ 2.5, and this sequence evolves in a manner consistent with previous measurements. The COSMOS/UltraVISTA Ks-selected catalog covers a unique parameter space in both depth, area, and multi-wavelength coverage and promises to be a useful tool for studying the growth of the galaxy population out to z ~ 3 - 4.

THE VIRUS-P EXPLORATION OF NEARBY GALAXIES (VENGA): SURVEY DESIGN, DATA PROCESSING, AND SPECTRAL ANALYSIS METHODS

We present the survey design, data reduction, and spectral fitting pipeline for the VIRUS-P Exploration of Nearby Galaxies (VENGA). VENGA is an integral field spectroscopic survey, which maps the disks of 30 nearby spiral galaxies. Targets span a wide range in Hubble type, star formation activity, morphology, and inclination. The VENGA data-cubes have 5.6'' FWHM spatial resolution, ~5A FWHM spectral resolution, sample the 3600A-6800A range, and cover large areas typically sampling galaxies out to ~0.7 R_25. These data-cubes can be used to produce 2D maps of the star formation rate, dust extinction, electron density, stellar population parameters, the kinematics and chemical abundances of both stars and ionized gas, and other physical quantities derived from the fitting of the stellar spectrum and the measurement of nebular emission lines. To exemplify our methods and the quality of the data, we present the VENGA data-cube on the face-on Sc galaxy NGC 628 (a.k.a. M 74). The VENGA observations of NGC 628 are described, as well as the construction of the data-cube, our spectral fitting method, and the fitting of the stellar and ionized gas velocity fields. We also propose a new method to measure the inclination of nearly face-on systems based on the matching of the stellar and gas rotation curves using asymmetric drift corrections. VENGA will measure relevant physical parameters across different environments within these galaxies, allowing a series of studies on star formation, structure assembly, stellar populations, chemical evolution, galactic feedback, nuclear activity, and the properties of the interstellar medium in massive disk galaxies.

The impact of a non-universal Initial Mass Function on the star formation histories of early-type galaxies

Recent results on the non-universality of the Initial Mass Function (IMF) have shown strong evidence of IMF variations with galaxy velocity dispersion, with a corresponding impact on other stellar population parameters, line indices and colours. Using a set of stellar population models with varying IMF slopes, we provide additional caveats on the assumption of a universal IMF. The present study shows that the derived star formation histories of early-type galaxies vary significantly with the IMF slope. For instance, a steepening in the slope of a single power-law IMF decreases substantially, by a factor of up to four, the contribution of the old stellar populations to the total light/mass. This trend is milder for a segmented-like IMF shape, where the contribution of the very low mass stars is decreased. It is also shown that, by tuning each IMF slope to its prescribed value according to each galaxy velocity dispersion, a sample of early-type galaxies covering a range of masses yield comparable star formation histories. On the one hand, a small contribution from relatively young stellar populations appear in the star formation histories of most massive elliptical galaxies when adopting a steep IMF. On the other hand, we find that low mass early-type galaxies that look like genuinely young objects with a standard IMF (i.e. ``baby elliptical galaxies``), turn out to be older when a slightly flatter IMF is employed. In summary, the use of a non-universal IMF, tuned according to the velocity dispersion of the galaxy, seems to provide more consistent results.

Evolution of the most massive galaxies to z ∼ 0.6 – II. The link between radio AGN activity and star formation

We analyze the optical spectra of massive (log M*/Msun > 11.4) radio-loud galaxies at z~0.2 and z~0.6. By comparing stellar population parameters of these radio-loud samples with radio-quiet control samples, we investigate how the presence of a radio-emitting jet relates to the recent star formation history of the host galaxy. We also investigate how the emission-line properties of the radio galaxies evolve with redshift by stacking their spectra. Our main results are the following. (1) Both at low and at high redshift, half as many radio-loud as radio-quiet galaxies have experienced significant star formation in the past Gyr. (2) The Balmer absorption line properties of massive galaxies that have experienced recent star formation show that star formation occurred as a burst in many of these systems. (3) Both the radio and the emission-line luminosity of radio AGN evolve significantly with redshift. However, radio galaxies with similar stellar population parameters, have similar emission-line properties both at high- and at low-redshift. These results suggest that massive galaxies experience cyclical episodes of gas accretion, star formation and black hole growth, followed by the production of a radio jet that shuts down further activity. The behaviour of galaxies with log M*/Msun > 11.4 is the same at z = 0.6 as it is at z = 0.2, except that higher redshift galaxies experience more star formation and black hole growth and produce more luminous radio jets during each accretion cycle.

Full spectral fitting of Milky Way and M 31 globular clusters: ages and metallicities

Context: The formation and evolution of disk galaxies are long standing questions in Astronomy. Understanding the properties of globular cluster systems can lead to important insights on the evolution of its host galaxy. Aims: We aim to obtain the stellar population parameters - age and metallicity - of a sample of M31 and Galactic globular clusters. Studying their globular cluster systems is an important step towards understanding their formation and evolution in a complete way. Methods: Our analysis employs a modern pixel-to-pixel spectral fitting technique to fit observed integrated spectra to updated stellar population models. By comparing observations to models we obtain the ages and metallicities of their stellar populations. We apply this technique to a sample of 38 globular clusters in M31 and to 41 Galactic globular clusters, used as a control sample. Results: Our sample of M31 globular clusters spans ages from 150 Myr to the age of the Universe. Metallicities [Fe/H] range from -2.2 dex to the solar value. The age-metallicity relation obtained can be described as having two components: an old population with a flat age-[Fe/H] relation, possibly associated with the halo and/or bulge, and a second one with a roughly linear relation between age and metallicity, higher metallicities corresponding to younger ages, possibly associated with the M31 disk. While we recover the very well known Galactic GC metallicity bimodality, our own analysis of M31's metallicity distribution function (MDF) suggests that both GC systems cover basically the same [Fe/H] range yet M31's MDF is not clearly bimodal. These results suggest that both galaxies experienced different star formation and accretion histories.

Virgo cluster and field dwarf ellipticals in 3D – I. On the variety of stellar kinematic and line-strength properties

We present the first large-scale stellar kinematic and line-strength maps for dwarf elliptical galaxies (9 in the Virgo Cluster and 3 in the field environment) obtained with the SAURON integral-field unit. No two galaxies in our sample are alike: we see that the level of rotation is not tied to flattening (we have, e.g. round rotators and flattened nonrotators); we observe kinematic twists in 1 Virgo and 1 field object; we discover large-scale kinematically-decoupled components in 2 field galaxies; we see varying gradients in line-strength maps, from nearly flat to strongly peaked in the center. The great variety of morphological, kinematic, and stellar population parameters seen in our data points to a formation scenario in which properties are shaped stochastically. A combined effect of ram-pressure stripping and galaxy harassment is the most probable explanation. We show the need for a comprehensive analysis of kinematic, dynamical, and stellar population properties which will enable us to place dwarf ellipticals and processes that govern their evolution in the wider context of galaxy formation.