Stellar Populations Of Early-type Galaxies Research Articles

The formation of early-type galaxies through monolithic collapse of gas clouds in Milgromian gravity

ABSTRACT Studies of stellar populations in early-type galaxies (ETGs) show that the more massive galaxies form earlier and have a shorter star formation history. In this study, we investigate the initial conditions of ETG formation. The study begins with the collapse of non-rotating post-Big-Bang gas clouds in Milgromian (MOND) gravitation. These produce ETGs with star-forming time-scales (SFT) comparable to those observed in the real Universe. Comparing these collapse models with observations, we set constraints on the initial size and density of the post-Big-Bang gas clouds in order to form ETGs. The effective-radius–mass relation of the model galaxies falls short of the observed relation. Possible mechanisms for later radius expansion are discussed. Using hydrodynamic MOND simulations, this work thus for the first time shows that the SFTs observed for ETGs may be a natural occurrence in the MOND paradigm. We show that different feedback algorithms change the evolution of the galaxies only to a very minor degree in MOND. The first stars have, however, formed more rapidly in the real Universe than possible just from the here studied gravitational collapse mechanism. Dark-matter-based cosmological structure formation simulations disagree with the observed SFTs at more than 5σ confidence.

Stellar population properties of ETGs in compact groups of galaxies

ABSTRACT We present results on the study of the stellar population in early-type galaxies (ETGs) belonging to 151 compact groups (CGs). We also selected a field sample composed of 846 ETGs to investigate environmental effects on galaxy evolution. We find that the dependences of mean stellar ages, [Z/H] and [α/Fe] on central stellar velocity dispersion are similar, regardless where the ETG resides, CGs or field. When compared to the sample of centrals and satellites from the literature, we find that ETGs in CGs behave similarly to centrals, especially those embedded in low-mass haloes ($M_{\mathrm{ h}} \lt 10^ {12.5}\, \mathrm{M}_{\odot }$). Except for the low-mass limit, where field galaxies present a star-forming signature, not seen in CGs, the ionization agent of the gas in CG and field galaxies seem to be similar due to hot, evolved low-mass stars. However, field ETGs present an excess of H α emission relative to ETGs in CGs. Additionally, we performed a dynamical analysis, which shows that CGs present a bimodality in the group velocity dispersion distribution – a high- and low-σ mode. Our results indicate that high-σ groups have a smaller fraction of spirals, shorter crossing times, and a more luminous population of galaxies than the low-σ groups. It is important to emphasize that our findings point to a small environmental impact on galaxies located in CGs. The only evidence we find is the change in gas content, suggesting environmentally driven gas loss.

The MASSIVE Survey. XII. Connecting Stellar Populations of Early-type Galaxies to Kinematics and Environment

Abstract We measure the stellar populations as a function of the radius for 90 early-type galaxies (ETGs) in the MASSIVE survey, a volume-limited integral-field spectroscopic (IFS) galaxy survey targeting all northern-sky ETGs with an absolute K-band magnitude of M K < −25.3 mag or a stellar mass of , within 108 Mpc. We are able to measure reliable stellar population parameters for individual galaxies out to 10–20 kpc (1–3 R e ) depending on the galaxy. Focusing on ∼R e (∼10 kpc), we find significant correlations between the abundance ratios, σ, and at a large radius, but we also find that the abundance ratios saturate in the highest-mass bin. We see a strong correlation between the kurtosis of the line-of-sight velocity distribution (h4) and the stellar population parameters beyond R e . Galaxies with higher radial anisotropy appear to be older, with metal-poorer stars and enhanced [α/Fe]. We suggest that the higher radial anisotropy may derive from more accretion of small satellites. Finally, we see some evidence for correlations between environmental metrics (measured locally and on >5 Mpc scales) and the stellar populations, as expected if satellites are quenched earlier in denser environments.

Integral-field kinematics and stellar populations of early-type galaxies out to three half-light radii

We observed twelve nearby HI -detected early-type galaxies (ETGs) of stellar mass $\sim 10^{10}M\odot \leq M_* \leq \sim 10^{11}M\odot$ with the Mitchell Integral-Field Spectrograph, reaching approximately three half-light radii in most cases. We extracted line-of-sight velocity distributions for the stellar and gaseous components. We find little evidence of transitions in the stellar kinematics of the galaxies in our sample beyond the central effective radius, with centrally fast-rotating galaxies remaining fast-rotating and centrally slow-rotating galaxies likewise remaining slow-rotating. This is consistent with these galaxies having not experienced late dry major mergers; however, several of our objects have ionised gas that is misaligned with respect to their stars, suggesting some kind of past interaction. We extract Lick index measurements of the commonly-used H$\beta$, Fe5015, Mg\, b, Fe5270 and Fe5335 absorption features, and we find most galaxies to have flat H$\beta$ gradients and negative Mg, b gradients. We measure gradients of age, metallicity and abundance ratio for our galaxies using spectral fitting, and for the majority of our galaxies find negative age and metallicity gradients. We also find the stellar mass-to-light ratios to decrease with radius for most of the galaxies in our sample. Our results are consistent with a view in which intermediate-mass ETGs experience mostly quiet evolutionary histories, but in which many have experienced some kind of gaseous interaction in recent times.

The hELENa project – I. Stellar populations of early-type galaxies linked with local environment and galaxy mass

We present the first in a series of papers in T$h$e role of $E$nvironment in shaping $L$ow-mass $E$arly-type $N$earby g$a$laxies (hELENa) project. In this paper we combine our sample of 20 low-mass early types (dEs) with 258 massive early types (ETGs) from the ATLAS$^{\mathrm{3D}}$ survey - all observed with the SAURON integral field unit (IFU) - to investigate early-type galaxies' stellar population scaling relations and the dependence of the population properties on local environment, extended to the low-{\sigma} regime of dEs. The ages in our sample show more scatter at lower {\sigma} values, indicative of less massive galaxies being affected by the environment to a higher degree. The shape of the age-{\sigma} relation for cluster vs. non-cluster galaxies suggests that cluster environment speeds up the placing of galaxies on the red sequence. While the scaling relations are tighter for cluster than for the field/group objects, we find no evidence for a difference in average population characteristics of the two samples. We investigate the properties of our sample in the Virgo cluster as a function of number density (rather than simple clustrocentric distance) and find that dE ages negatively correlate with the local density, likely because galaxies in regions of lower density are later arrivals to the cluster or have experienced less pre-processing in groups, and consequently used up their gas reservoir more recently. Overall, dE properties correlate more strongly with density than those of massive ETGs, which was expected as less massive galaxies are more susceptible to external influences.

UV emission lines in passively evolving galaxies can reveal the progenitors of type Ia supernovae

The question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, the two leading models being the so-called single and double degenerate scenarios. The former implies a large population of accreting, nuclear-burning white dwarfs with photospheric temperatures T∼105–106 K during some part of their accretion history. Recently, we demonstrated that a population of accreting white dwarfs large enough to reproduce the observed SN Ia rate would contribute significantly to the ionizing radiation expected from the stellar population in early-type galaxies, now commonly observed to host spatially extended regions of neutral and ionized gas. From our photoionization calculations, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in early-type galaxies from upper limits on the luminosity of a number of emission lines characteristic of ionization by high-temperature sources. Detection (or strong upper limits on) He II 1640 A and [C II] 1335 A, expected to be overluminous in these galaxies if the single-degenerate channel holds true, can strongly constrain the total luminosity of nuclear-burning white dwarfs in these populations. In the near-UV, our photoionization calculations demonstrate that the EW of the [O II] 3727 doublet and the [Ne III] 3869/[O II] 3727 ratio can also provide a powerful diagnostic, particularly in post-starburst galaxies. Together with the He II 3203 A (5 → 3) recombination line, these lines present an excellent opportunity for strongly constraining the population of accreting, nuclear-burning white dwarfs, and in general the available ionizing continuum, at relatively short delay-times (time from initial starburst).

The connection between shape and stellar population in early-type galaxies

AbstractWe comment on the observed connection between the shape of the photometric profiles and the stellar population content in early-type galaxies.

The ATLAS3D project – XXI. Correlations between gradients of local escape velocity and stellar populations in early-type galaxies

We explore the connection between the local escape velocity, Vesc, and the stellar population properties in the ATLAS3D survey, a complete, volume-limited sample of nearby early-type galaxies. We make use of ugriz photometry to construct Multi-Gaussian Expansion models of the surface brightnesses of our galaxies. We are able to fit the full range of surface brightness profiles found in our sample, and in addition we reproduce the results of state-of-the-art photometry in the literature with residuals of 0.04 mag. We utilize these photometric models and SAURON integral-field spectroscopy, combined with Jeans dynamical modelling, to determine the local Vesc derived from the surface brightness. We find that the local Vesc is tightly correlated with the Mg b and Fe5015 line strengths and optical colours, and anti-correlated with the Hβ line strength. In the case of the Mg b and colour–Vesc relations we find that the relation within individual galaxies follows the global relation between different galaxies. We intentionally ignored any uncertain contribution due to dark matter since we are seeking an empirical description of stellar population gradients in early-type galaxies that is ideal for quantitative comparison with model predictions. We also make use of single stellar population (SSP) modelling to transform our line strength index measurements into the SSP-equivalent parameters age (t), metallicity ([Z/H]) and α-enhancement [α/Fe]. The residuals from the relation are correlated with age, [α/Fe], molecular gas mass and local environmental density. We identify a population of galaxies that occur only at low Vesc that exhibit negative gradients in the Mg b– and Colour–Vesc relations. These galaxies typically have young central stellar populations and contain significant amounts of molecular gas and dust. Combining these results with N-body simulations of binary mergers we use the Mg b–Vesc relation to constrain the possible number of dry mergers experienced by the local early-type galaxy population – a typical massive early-type galaxy can have experienced only ∼1.5 major mergers before becoming a significant outlier in the Mg b–Vesc relation.

A HYBRID MODEL FOR THE EVOLUTION OF GALAXIES AND ACTIVE GALACTIC NUCLEI IN THE INFRARED

[Abridged] We present a comprehensive investigation of the cosmological evolution of the luminosity function (LF) of galaxies and active galactic nuclei (AGN) in the infrared (IR). Based on the observed dichotomy in the ages of stellar populations of early-type galaxies on one side and late-type galaxies on the other, the model interprets the epoch-dependent LFs at z \geq 1.5 using a physical model for the evolution of proto-spheroidal galaxies and of the associated AGNs, while IR galaxies at z<1.5 are interpreted as being mostly late-type 'cold' (normal) and 'warm' (starburst) galaxies. As for proto-spheroids, in addition to the epoch-dependent LFs of stellar and AGN components separately, we have worked out the evolving LFs of these objects as a whole (stellar plus AGN component). The model provides a physical explanation for the observed positive evolution of both galaxies and AGNs up to z \simeq 2.5 and for the negative evolution at higher redshifts, for the sharp transition from Euclidean to extremely steep counts at (sub-)mm wavelengths, as well as the (sub-)mm counts of strongly lensed galaxies, that are hard to account for by alternative, physical or phenomenological, approaches. The evolution of late-type galaxies and of z<1.5 AGNs is described using a parametric phenomenological approach. The modeled AGN contributions to the counts and to the cosmic infrared background (CIB) are always subdominant with maximal at mid-IR wavelengths. The model provides a good fit to the multi-wavelength (from the mid-IR to millimeter waves) data on LFs at different redshifts and on number counts (both global and per redshift slices). A prediction of the present model is a systematic variation with wavelength of the populations dominating the counts and the contributions to the CIB intensity. The implied specific trend for cross-wavelength CIB power spectra is found to be in good agreement with the data.

THE EFFECT OF HELIUM-ENHANCED STELLAR POPULATIONS ON THE ULTRAVIOLET-UPTURN PHENOMENON OF EARLY-TYPE GALAXIES

Recent observations and modeling of globular clusters with multiple populations strongly indicate the presence of super helium-rich subpopulations in old stellar systems. Motivated by this, we have constructed new population synthesis models with and without helium-enhanced subpopulations to investigate their impact on the UV-upturn phenomenon of quiescent early-type galaxies. We find that our models with helium- enhanced subpopulations can naturally reproduce the strong UV-upturns observed in giant elliptical galaxies assuming an age similar to that of old globular clusters in the Milky Way. The major source of far-UV (FUV) flux, in this model, is relatively metal- poor and helium-enhanced hot horizontal branch stars and their progeny. The Burstein et al. (1988) relation of the F U V - V color with metallicity is also explained either by the variation of the fraction of helium-enhanced subpopulations or by the spread in mean age of stellar populations in early-type galaxies.

The star-formation histories of early-type galaxies from ATLAS3D

AbstractWe present an exploration of the integrated stellar populations of early-type galaxies (ETGs) from the ATLAS3D survey. We use two approaches: firstly the application of line-indices interpreted through single stellar population (SSP) models, which provide a single value of age, metallicity and abundance ratio. And secondly, by fitting a linear combination of SSP spectra to our data, smoothly weighted in the free parameters of age and metallicity, thereby inferring a star-formation history of these galaxies. Despite the significant differences in these approaches, we obtain generally consistent results, such that galaxies that are more massive appear older with enhanced abundance ratios using line indices, and have shorter star-formation histories weighted to early times. We highlight two limitations of the index-SSP approach. Firstly the SSP-equivalent ages belie the fact that ETGs are overwhelmingly composed of ancient stars. Secondly, the young stellar contributions implied in our star formation histories are required to obtain realistic UV-optical colours. We remark that, even fitting solar-abundance models, we can recover a star-formation duration that correlates with the measured alpha-enhancement, in agreement with other recent work.

The stellar populations of early-type galaxies - II. The effects of environment and mass

The degree of influence that environment and mass have on the stellar populations of early-type galaxies is uncertain. In this paper we present the results of a spectroscopic analysis of the stellar populations of early-type galaxies aimed at addressing this question. The sample of galaxies is drawn from four clusters, with <z>=0.04, and their surrounding structure extending to ~10R_{vir}. We find that the distributions of the absorption-line strengths and the stellar population parameters age, metallicity and alpha-element abundance ratio do not differ significantly between the clusters and their outskirts, but the tight correlations found between these quantities and velocity dispersion within the clusters are weaker in their outskirts. All three stellar population parameters of cluster galaxies are positively correlated with velocity dispersion. Galaxies in clusters form a homogeneous class of objects that have similar distributions of line-strengths and stellar population parameters, and follow similar scaling relations regardless of cluster richness or morphology. We estimate the intrinsic scatter of the Gaussian distribution of metallicities to be 0.3 dex, while that of the alpha-element abundance ratio is 0.07 dex. The e-folding time of the exponential distribution of galaxy ages is estimated to be 900 Myr. The intrinsic scatters of the metallicity and alpha-element abundance ratio distributions can almost entirely be accounted for by the correlations with velocity dispersion and the intrinsic scatter about these relations. This implies that a galaxies mass plays the major role in determining its stellar population.

The stellar populations of early-type galaxies - I. Observations, line strengths and stellar population parameters

The influence of environment on the formation and evolution of early-type galaxies is, as yet, an unresolved issue. Constraints can be placed on models of early-type galaxy formation and evolution by examining their stellar populations as a function of environment. We present a catalogue of galaxies well suited to such an investigation. The magnitude-limited (b_J<19.45) sample was drawn from four clusters (Coma, A1139, A3558, and A930 at <z>=0.04) and their surrounds. The catalogue contains luminosities, redshifts, velocity dispersions and Lick line strengths for 416 galaxies, of which 245 are classified as early-types. Luminosity-weighted ages, metallicities, and alpha-element abundance ratios have been estimated for 219 of these early-types. We also outline the steps necessary for measuring fully-calibrated Lick indices and estimating the associated stellar population parameters using up-to-date methods and stellar population models. In a subsequent paper we perform a detailed study of the stellar populations of early-type galaxies in clusters and investigate the effects of environment.

EVIDENCE FOR INTERMEDIATE-AGE STELLAR POPULATIONS IN EARLY-TYPE GALAXIES FROM K -BAND SPECTROSCOPY

The study of stellar populations in early-type galaxies in different environments is a powerful tool for constraining their star formation histories. This study has been traditionally restricted to the optical range, where dwarfs around the turn-off and stars at the base of the RGB dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, AGB stars are the main contributors. In this letter, we measure the near-infrared indices NaI and D$_{\rm CO}$ for a sample of 12 early-type galaxies in low density environments and compare them with the Fornax galaxy sample presented by Silva et al. (2008). The analysis of these indices in combination with Lick/IDS indices in the optical range reveals i) the NaI index is a metallicity indicator as good as C4668 in the optical range, and ii) D$_{\rm CO}$ is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low density environments show higher NaI and D$_{\rm CO}$ than those located in Fornax cluster, which points towards a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.

GALICS. II: the [ α/Fe] -mass relation in elliptical galaxies

We aim at reproducing the mass- and sigma-[alpha/Fe] relations in the stellar populations of early-type galaxies by means of a cosmologically motivated assembly history for the spheroids. We implement a detailed treatment for the chemical evolution of H, He, O and Fe in GalICS, a semi-analytical model for galaxy formation which successfully reproduces basic low- and high-redshift galaxy properties. The contribution of supernovae (both type Ia and II) as well as low- and intermediate-mass stars to chemical feedback are taken into account. We find that this chemically improved GalICS does not produce the observed mass- and sigma-[alpha/Fe] relations. The slope is too shallow and scatter too large, in particular in the low and intermediate mass range. The model shows significant improvement at the highest masses and velocity dispersions, where the predicted [alpha/Fe] ratios are now marginally consistent with observed values. We show that this result comes from the implementation of AGN (plus halo) quenching of the star formation in massive haloes. A thorough exploration of the parameter space shows that the failure of reproducing the mass- and sigma-[alpha/Fe] relations can partly be attributed to the way in which star formation and feedback are currently modelled. The merger process is responsible for a part of the scatter. We suggest that the next generation of semi-analytical model should feature feedback (either stellar of from AGN) mechanisms linked to single galaxies and not only to the halo, especially in the low and intermediate mass range. The integral star formation history of a single galaxy determines its final stellar [alpha/Fe] as it might be expected from the results of closed box chemical evolution models. (abridged)

Ages and metallicities of early-type galaxies

AbstractThe study of stellar populations in early-type galaxies give us clues on how they form and evolve. We calculate age, [Z/H], and [α/Fe] ratio for 162 early-type galaxies using the SSP models from Thomas, Maraston, &amp; Bender (2003) applied to Lick indices measurements, such as Hβ, Mgb, Fe5270 and Fe5335. Those were obtained from longslit spectra observed in the ESO 1.52m telescope as described in Ogando et al. (2008). We study the relations between the SSP parameters and velocity dispersion, as well as the influence of environment on these relations. We find that age, [Z/H], and [α/Fe] correlate well with velocity dispersion, so that more massive galaxies, have on average, higher metallicities, ages and abundance ratios than that of the low-mass ones. Galaxies in high density regions are older and more metal-rich than those in regions with low number of neighbors. These results are not consistent with standard predictions of hierarchical clustering. In the last decade, this “anti-hierarchical” behavior has also been generally tagged as downsizing and has challenged the current theoretical framework of galaxy formation, calling for new ways of star formation regulation in early-type galaxies.

Universal X-ray emissivity of the stellar population in early-type galaxies: unresolved X-ray sources in NGC 3379

We use deep Chandra observations to measure the emissivity of the unresolved X-ray emission in the elliptical galaxy NGC 3379. After elimination of bright, low-mass X-ray binaries with luminosities 1036 erg s-1, we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass 1027 erg s-1 in the 0.5-2 keV energy band. This value is in good agreement with those previosuly determined for the dwarf elliptical galaxy M 32, the bulge of the spiral galaxy M 31 and the Milky Way, as well as with the integrated X-ray emissivity of cataclysmic variables and coronally active binaries in the Solar neighborhood. This strongly suggests that i) the bulk of the unresolved X-ray emission in NGC 3379 is produced by its old stellar population; and ii) the old stellar populations in all galaxies can be characterized by a universal value of X-ray emissivity per unit stellar mass or per unit K band luminosity.

Internal kinematics and stellar populations of early-type galaxies in the Fornax cluster

AbstractWe present a study of the internal kinematics and stellar populations of early-type galaxies in the Fornax cluster. 10 galaxies in a luminosity range of −21.8≤MB≤−17.4 were observed with the integral field units (IFU) of Gemini South GMOS and VLT-VIMOS. Velocity maps and age-metallicity diagrams are presented for NGC 1404 and NGC 1419.

Young stellar populations in early-type galaxies in the SDSS

AbstractWe use a purely data-driven rectified factor analysis to identify early-type galaxies with young (≲ 4 Gyr) stellar populations in the Sloan Digital Sky Survey Spectroscopic Catalogue. We call these galaxies E+F galaxies, analogous to E+A galaxies. These galaxies lie in the ‘Green Valley’, between the blue cloud and the red sequence on the colour-magnitude diagram. As such, these galaxies may represent an important transient stage in the evolution of galaxies from blue and star-forming to red and passive. We investigate the distribution in projected local galaxy surface density of the E+F galaxies, and compare it with the environment of early-type and E+A galaxies. We find that i) the E+A distribution peaks strongly in projected local galaxy surface density, Σ5, at ~ 0.1−0.2 Mpc−2, ii) early-types have a flatter peak at ~ 0.06−0.2 Mpc−2, iii) the E+Fs lie somewhere in between, and iv) the distributions of the models do not agree well with the data, peaking at higher densities, and under-predicting the number of E+As at low (Σ5 &lt; 0.3 Mpc−2) densities. The dearth of E+A and E+F galaxies in dense environments confirms that E+A and E+F galaxies are most likely the products of galaxy-galaxy merging or interactions, rather than star-forming galaxies whose star formation has been quenched by processes unique to dense environments, such as ram-pressure stripping or galaxy harassment. The similarity of the environments in which the E+F population and the E+A galaxy sample are found, together with the spectral evidence, suggests that E+F galaxies are E+A galaxies, which have evolved by a further ~ one to a few Gyr.

Fast and slow rotators: the build-up of the red sequence

AbstractUsing the unique dataset obtained within the course of theSAURONproject, a radically new view of the structure, dynamics and stellar populations of early-type galaxies has emerged. We show that galaxies come in two broad flavours (slow and fast rotators), depending on whether or not they exhibit clear large-scale rotation, as indicated via a robust measure of the specific angular momentum of baryons. This property is also linked with other physical characteristics of early-type galaxies, such as: the presence of dynamically decoupled cores, orbital structure and anisotropy, stellar populations and dark matter content. I here report on the observed link between this baryonic angular momentum and a mass sequence, and how this uniquely relates to the building of the red sequence via dissipative/dissipationless mergers and secular evolution.