Galaxies In Low-density Environments Research Articles

Molecular gas content and SFR in Hickson compact groups: enhanced or deficient?

We study the effect of the extreme environment in Hickson Compact groups (HCGs) on the molecular gas mass, \mhtwo, and the star formation rate (SFR) of galaxies as a function of atomic hydrogen (HI) content and evolutionary phase of the group. We have selected a redshift limited (D$<$100 Mpc) sample of 88 galaxies in 20 HCGs with available atomic hydrogen (HI) VLA maps, covering a wide range of HI deficiencies and evolutionary phases of the groups, and containing at least one spiral galaxy. We derived the far-infrared (FIR) luminosity (\lfir) from IRAS data and used it as a tracer of the SFR. We calculated the HI mass, \lfir and \mhtwo deficiencies. The mean deficiencies of \lfir and \mhtwo of spiral galaxies in HCGs are close to 0, indicating that their average SFR and molecular gas content are similar to those of isolated galaxies. However, there are indications of an excess in \mhtwo\($\sim$ 50%) in spiral galaxies in HCGs which can be interpreted as either an enhanced molecular gas content or as a higher concentration of \mhtwo towards the center in comparison to galaxies in lower density environments. In contrast, the mean \mhi of spiral galaxies in HCGs is only 12% of the expected value. The specific star formation rate (sSFR= SFR/stellar mass) tends to be lower for galaxies with a higher \mhtwo\ or \mhi\ deficiency. This trend is not seen for the star formation efficiency (SFE=SFR/\mhtwo). We found tentative indications for an enhancement of \mhtwo\ in spiral galaxies in HCGs in an early evolutionary phase and a decrease in later phases. We suggest that this might be due to an enhancement of the conversion from atomic to molecular gas due to on-going tidal interactions in an early evolutionary phase, followed by HI stripping and a decrease of the molecular gas content because of lack of replenishment.

Tests of modified gravity with dwarf galaxies

In modified gravity theories that seek to explain cosmic acceleration, dwarf galaxies in low density environments can be subject to enhanced forces. The class of scalar-tensor theories, which includes f(R) gravity, predict such a force enhancement (massive galaxies like the Milky Way can evade it through a screening mechanism that protects the interior of the galaxy from this ``fifth'' force). We study observable deviations from GR in the disks of late-type dwarf galaxies moving under gravity. The fifth-force acts on the dark matter and HI gas disk, butnot on the stellar disk owing to the self-screening of main sequence stars. We find four distinct observable effects in such disk galaxies: 1. A displacement of the stellar disk from the HI disk. 2. Warping of the stellar disk along the direction of the external force. 3. Enhancement of the rotation curve measured from the HI gas compared to that of the stellar disk. 4. Asymmetry in the rotation curve of the stellar disk. We estimate that the spatial effects can be up to 1 kpc and the rotationvelocity effects about 10 km/s in infalling dwarf galaxies. Such deviations are measurable: we expect thatwith a careful analysis of a sample of nearby dwarf galaxies one can improve astrophysical constraints on gravity theories by over three orders of magnitude, and even solar system constraints by one order of magnitude. Thus effective tests of gravity along the lines suggested by Hui, Nicolis, and Stubbs (2009) and Jain (2011) can be carried out with low-redshift galaxies, though care must be exercised in understandingpossible complications from astrophysical effects.

Tracing the evolution of nearby early-type galaxies in low density environments. The ultraviolet view from GALEX

We detected recent star formation in nearby early-type galaxies located in low density environments, with GALEX Ultraviolet (UV) imaging. Signatures of star formation may be present in the nucleus and in outer rings/arm like structures. Our study suggests that such star formation may be induced by different triggering mechanisms, such as the inner secular evolution driven by bars, and minor accretion phenomena. We investigate the nature of the (FUV-NUV) color vs. Mg2 correlation, and suggest that it relates to “downsizing” in galaxy formation.

Galaxy pairs in the Sloan Digital Sky Survey - II. The effect of environment on interactions

We use a sample of close galaxy pairs selected from the Sloan Digital Sky Survey Data Release 4 (SDSS DR4) to investigate in what environments galaxy mergers occur and how the results of these mergers depend on differences in local galaxy density. The galaxies are quantified morphologically using two-dimensional bulge-plus-disk decompositions and compared to a control sample matched in stellar mass, redshift and local projected density. Lower density environments have fractionally more galaxy pairs with small projected separations (r_p) and relative velocities (Delta v), but even high density environments contain significant populations of pairs with parameters that should be conducive to interactions. Metrics of asymmetry and colour are used to identify merger activity and triggered star formation. The location of star formation is inferred by distinguishing bulge and disk colours and calculating bulge fractions from the SDSS images. Galaxies in the lowest density environments show the largest changes in star formation rate, asymmetry and bulge-total fractions at small separations, accompanied by bluer bulge colours. At the highest local densities, the only galaxy property to show an enhancement in the closest pairs is asymmetry. We interpret these results as evidence that whilst interactions (leading to tidal distortions) occur at all densities, triggered star formation is seen only in low-to-intermediate density environments. We suggest that this is likely due to the typically higher gas fractions of galaxies in low density environments. Finally, by cross-correlating our sample of galaxy pairs with a cluster catalogue, we investigate the dependence of interactions on clustercentric distance. It is found that for close pairs the fraction of asymmetric galaxies is highest in the cluster centres.

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.

Stellar ages and metallicities of nearby elliptical galaxies

Stellar ages and metallicities are crucial for understanding the formation and evolution of elliptical galaxies. However, due to the age--metallicity degeneracy, it is hard to measure these two parameters accurately with broad-band photometry. In this paper, we observed high-resolution spectra for a sample of 20 nearby elliptical galaxies (EGs) with the NAOC 2.16 m telescope, and determined stellar ages and metallicities by using the empirical population synthesis and Lick/IDS index system methods. We found that stellar ages from these two methods are consistent with each other for purely old EGs; however, stellar metallicities show a zeropoint offset of 0.5 Z⊙. Our results confirm that stellar populations in low-density environment galaxies are more diverse compared to their high-density counterparts. We also investigated the element abundance--galaxy mass relation for nearby elliptical galaxies.

The role of E+A and post-starburst galaxies - I. Models and model results

Different compositions of galaxy types in the field in comparison to galaxy clusters as described by the morphology-density relation in the local universe is interpreted as a result of transformation processes from late- to early-type galaxies. This interpretation is supported by the Butcher-Oemler effect. We investigate E+A galaxies as an intermediate state between late-type galaxies in low density environments and early-type galaxies in high density environment to constrain the possible transformation processes. For this purpose we model a grid of post-starburst galaxies by inducing a burst and/or a halting of star formation on the normal evolution of spiral galaxies with our galaxy evolution code GALEV. From our models we find that the common E+A criteria exclude a significant number of post-starburst galaxies and propose that comparing their spectral energy distributions leads to a more sufficient method to investigate post-starbust galaxies. We predict that a higher number of E+A galaxies in the early universe can not be ascribed solely to a higher number of starburst, but is a result of a lower metallicity and a higher burst strength due to more gas content of the galaxies in the early universe. We find that even galaxies with a normal evolution without a starburst have a Hdelta-strong phase at early galaxy ages.

Current star formation in early-type galaxies and the KA phenomenon

We present the results of an effort to identify and study a sample of the likely progenitors of elliptical (E) and lenticular (S0) K+A galaxies. To achieve this, we have searched a sample ∼11 000 nearby (m r < 16) early-type galaxies selected by morphology from the Sloan Digital Sky Survey (SDSS) main spectroscopic sample for actively star-forming E and S0 galaxies. Using emission-line ratios and visual inspection of SDSS g-band images, we have identified 335 galaxies from the SDSS Fourth Data Release (DR4) as actively star-forming E and S0 galaxies. These galaxies make up about 3 per cent of the total early-type sample and less than 1 per cent of all main galaxies with m r < 16. We also identified a sample of ∼400 K+A galaxies from DR4 with m r < 16; more than half of these are E and S0 galaxies. We find that star-forming early-type galaxies and K+A galaxies have similar mass distributions; they are on average less massive than typical early-type galaxies but more massive than the average starforming galaxy. Both of these types of galaxies are found in higher fractions among all galaxies in lower density environments. The fractions of star-forming E and S0 galaxies and E and S0 K+A galaxies depend on environment in nearly the same way. Model spectra fit to the stellar continua of the star-forming E and S0 galaxies showed that their properties are consistent with star formation episodes of � 1 Gyr in duration. The modelling results imply that on average, the star formation episodes will increase the stellar masses by about 4 per cent. The results also imply that only episodes that increase the stellar mass by more than 2‐5 per cent will lead to K+A galaxies as we have defined them and that this is true for roughly 80 per cent of the star-forming E and S0 galaxies in our sample. The estimated typical increase in stellar mass

The Hot Interstellar Medium in Normal Elliptical Galaxies. III. The Thermal Structure of the Gas

This is the third paper in a series analyzing X-ray emission from the hot interstellar medium in a sample of 54 normal elliptical galaxies observed by Chandra. We focus on a subset of 36 galaxies with sufficient signal to compute radial temperature profiles. We distinguish four qualitatively different types of profile: positive gradient (outwardly rising), negative gradients (falling), quasi-isothermal (flat), and hybrid (falling at small radii and rising at larger radii). We measure the mean logarithmic temperature gradients in two radial regions: from 0 to 2 J-band effective radii RJ (excluding the central point source), and from 2 to 4 RJ. We find the outer gradient to be uncorrelated with intrinsic host galaxy properties, but strongly influenced by the environment: galaxies in low-density environments tend to show negative outer gradients, while those in high-density environments show positive outer gradients, suggesting the influence of circumgalactic hot gas. The inner temperature gradient, however, is largely unaffected by the environment, but strongly correlated with intrinsic host galaxy characteristics: negative inner gradients are more common for smaller, optically faint, low radio luminosity galaxies, whereas positive gradients are found in bright galaxies with stronger radio sources. There is no evidence for bimodality in the distribution of inner or outer gradients. We propose three scenarios to explain the inner temperature gradients: (1) weak AGNs heat the ISM locally, while higher luminosity AGNs heat the system globally through jets inflating cavities at larger radii; (2) the onset of negative inner gradients indicates a declining importance of AGN heating relative to other sources, such as compressional heating or supernovae; or (3) the variety of temperature profiles are snapshots of different stages of a time-dependent flow, cyclically reversing the temperature gradient over time.

The Galaxy Evolution Explorer UV emission in shell galaxies: tracing galaxy ‘rejuvenation’ episodes

We present the Galaxy Evolution Explorer (GALEX) far-ultraviolet (FUV) and near-ultraviolet (NUV) imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135 located in low-density environments. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV−NUV) ≈ 2 throughout the whole galaxy. NGC 7135 is blue in the centre (FUV−NUV) ≈ 0 and as red as (FUV−NUV) ≈ 1.5 in the outskirts, including the faint shell-like feature. The three shell galaxies are members of poor groups of galaxies. We compare GALEX NUV observations with available H i large-scale measurements, and determine the UV magnitudes of likely companions. Most of the known (and possible) companions are gas-rich late-type galaxies, suggesting that our shell galaxies inhabit the ideal environment for hosting rejuvenating episodes driven by accretion events. We investigate the ability of the nuclearGALEX (FUV−NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hβ, HγA, HδA Lick line-strength indices and the (FUV−NUV) colour. We extend the study to a sample of early-type galaxies in low-density environments which includes shell galaxies and/or galaxies with emission lines in their optical spectra. In the index versus (FUV−NUV) colour diagrams, most of the galaxies are well explained by passively evolving single stellar populations. On the average, ages and metallicities of the galaxies in our sample estimated from optical line-strength indices are consistent with those inferred from the (FUV−NUV) colour. We note that the GALEX (FUV−NUV) and (UV−V) colours have different response to age and metallicity. In general, all the colours but for (FUV−NUV) and (FUV−V), become nearly age-insensitive when 1–2 Gyr have elapsed from the last star-forming event. Finally, considering composite stellar population models with a recent burst of star formation, we suggest that the positions of the NGC 7135 and NGC 2865 nuclei in the (FUV−NUV)–Hβ plane could be explained in terms of a recent rejuvenation episode.

The dark haloes of early-type galaxies in low-density environments: XMM–Newton and Chandra observations of NGC 57, 7796 and IC 1531*

We present analysis of Chandra and XMM-Newton observations of three early-type galaxies, NGC 57, 7796 and IC 1531. All three are found in very low-density environments, and appear to have no neighbours of comparable size. NGC 57 has a halo of kT ∼ 0.9 keV, solar metallicity gas, while NGC 7796 and IC 1531 both have ∼0.55 keV, 0.5-0.6 Z ⊙ haloes. IC 1531 has a relatively compact halo, and we consider it likely that gas has been removed from the system by the effects of active galactic nucleus heating. For NGC 57 and 7796 we estimate mass, entropy and cooling time profiles and find that NGC 57 has a fairly massive dark halo with a mass-to-light ratio (M/L) of 44.7 +4.0 8.5 M ⊙ /L B⊙ (1σ uncertainties) at 4.75r e . This is very similar to the M/L found for NGC 4555 and confirms that isolated ellipticals can possess sizable dark matter haloes. We find a significantly lower M/L for NGC 7796, 10.6 +2.5 -2.3 M ⊙ /L B⊙ at 5r e , and discuss the possibility that NGC 7796 hosts a galactic wind, causing us to underestimate its mass.

The spatially resolved stellar populations of isolated early-type galaxies

We present radial stellar population parameters for a subsample of 12 galaxies from the 36 isolated early-type galaxies of Reda et al. Using new long-slit spectra, central values and radial gradients for the stellar age, metallicity [Z/H] and alpha-element abundance [E/Fe] are measured. Similarly, the central stellar population parameters are derived for a further 5 isolated early-type galaxies using their Lick indices from the literature. On average, the seventeen isolated galaxies have mean central [Z/H]o and [E/Fe]o of 0.29+/-0.03 and 0.17+/-0.03 respectively and span a wide range of ages from 1.7 to 15 Gyrs. We find that isolated galaxies follow similar scaling relations between central stellar population parameters and galaxy velocity dispersion to their counterparts in high density environments. However, we note a tendency for isolated galaxies to have slightly younger ages, higher [Z/H] and lower [E/Fe]. Such properties are qualitatively consistent with the expectation of an extended star formation history for galaxies in lower density environments. Generally we measure constant age and [E/Fe] radial gradients. We find that the age gradients anti-correlate with the central galaxy age. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies metallicity gradients are shallower with increasing mass. Such behaviour is not predicted in dissipational collapse models but might be expected in multiple mergers. The metallicity gradients correlate with the central age and metallicity, as well as to the age gradients. In conclusion, our stellar population data for isolated galaxies are more compatible with an extended merger/accretion history than early dissipative collapse.

Nearby early-type galaxies with ionized gas

In this paper we study the underlying stellar population of a sample of 65 nearby early-type galaxies predominantly located in low density environments. Ages, metallicities and [alpha/Fe] ratios have been derived through the comparison of Lick indices measured at different galacto-centric distances with new SSP models which account for the presence of alpha/Fe enhancement. The SSPs cover a wide range of ages, metallicities and [alpha/Fe] ratios. To derive the stellar population parameters we have devised an algorithm based on the probability density function. We derive a large spread in age ((1-15) Gyrs). Age does not show any significant trend with central velocity dispersion sigma_c but E galaxies appear on average older than S0. On the contrary, an increasing trend of metallicity and [alpha/Fe] with sigma_c is observed, testifying that the chemical enrichment was more efficient and the duration of the star formation shorter in more massive galaxies. We have also sought for possible correlations with the local galaxy density but neither metallicity nor alpha-enhancement show clear trends. However we find that while low density environments (LDE) contain very young objects (from 1 to 4 Gyr), none of the galaxies in the higher density environments (HDE) is younger than 5 Gyrs. Considering the lack of environmental effect on the [alpha/Fe] ratio and the high value of [alpha/Fe] in some young massive objects, we argue that young galaxies in LDE are more likely due to recent rejuvenation episodes. By comparing the number of rejuvenated objects with the total number of galaxies in our sample, and by means of simple two-SSP component models, we estimate that, on average, the rejuvenation episodes do not involve more than 25 % of the total galaxy mass.

Stellar populations of early-type galaxies in different environments

This is the second paper of a series devoted to the study of the stellar content of early-type galaxies. The goal of the series is to set constraints on the evolutionary status of these objects Methods. We use a new set of models that include an improved stellar library (MILES) to derive simple stellar population (SSP)-equivalent parameters in a sample of 98 early-type galaxies. The sample contains galaxies in the field, poor groups, and galaxies in the Virgo and Coma clusters. Results. We find that low-density environment galaxies span a larger range in SSP age and metallicity than their counterparts in high density environments, with a tendency for lower sigma galaxies to be younger. Early-type galaxies in low-density environments appear on average similar to 1.5 Gyr younger and more metal rich than their counterparts in high density environments. The sample of low-density environment galaxies shows an age-metallicity relation in which younger galaxies are found to be more metal rich, but only when metallicity is measured with a Fe-sensitive index. Conversely, there is no age-metallicity relation when the metallicity is measured with a Mg sensitive index. The mass-metallicity relation is only appreciable for the low-density environment galaxies when the metallicity is measured with a Mg-sensitive index, and not when the metallicity is measured with other indicators. On the contrary, this relation exists for the high-density environment galaxies independent of the indicator used to measure the metallicity. Conclusions. This suggests a dependence of the mass-metallicity relation on the environment of the galaxies. Our data favour a scenario in which galaxies in low density environments have suffered a more extended star formation history than the galaxies in the Coma cluster, which appear to host more homogenous stellar populations.

Central stellar populations of early-type galaxies in low-density environments

We have investigated the properties of a volume and magnitude limited sample of nearby early type galaxies that were carefully selected from the AAO two degree field galaxy redshift survey. We used images from the DSS to confirm the E/S0 morphologies, and augmented this sample with field galaxies from Colbert et al. 2001. We present spectroscopic observations of 22 galaxies from the combined sample, from which central velocity dispersions and the Lick stellar population indices were measured. After carefully correcting the spectra for nebular emission we derived luminosity-weighted ages, metallicities, and alpha-element abundance ratios. We compare these isolated galaxies with samples of early-type galaxies in the Virgo and Coma clusters, and also with the sample of galaxies in low-density regions of Kuntschner et al. (2002). We find that galaxies in low-density environments are younger and have a greater spread of ages compared to cluster galaxies. They also show a wider range of metallicities at a given velocity dispersion than cluster galaxies, which display only super-solar metallicities. On average cluster, as well as, isolated galaxies show non-solar abundance ratios in alpha-elements, suggesting that, independent of galactic environment, star formation occurred on short time-scales. We reason that early-type galaxies in low-density environments experienced merging-induced star-formation episodes over a longer and more recent period of time compared to a cluster environment, and speculate that a considerable fraction of their stars formed out of low-metallicity halo gaseous material during the slow growth of a stellar disk between merging events.

Evolution and Environment of Early-Type Galaxies

Chemical abundance indicators are studied using composite spectra, which we provide in tabular form. Tables of line strengths measured from these spectra and parameters derived from these line strengths are also provided. From these we find that at fixed luminosity, early-type galaxies in low-density environments are slightly bluer, with stronger O II emission and stronger Hδ and Hγ Balmer absorption lines, indicative of star formation in the not very distant past. These galaxies also tend to have systematically weaker D4000 indices. The Lick indices and α-element abundance indicators correlate weakly but significantly with environment. For example, at fixed velocity dispersion, Mg is weaker in early-type galaxies in low-density environments by 30% of the rms scatter across the full sample, whereas most Fe indicators show no significant environmental dependence. The galaxies in our sample span a redshift range that corresponds to look-back times of ∼1 Gyr. We see clear evidence for evolution of line-index strengths over this time. Since the low-redshift population is almost certainly a passively aged version of the more distant population, age is likely the main driver for any observed evolution. We use the observed redshift evolution as a model-independent clock to identify indicators that are more sensitive to age than to other effects such as metallicity. In principle, for a passively evolving population, comparison of the trends with redshift and environment constrain how strongly the luminosity-weighted ages and metallicities depend on environment. We develop a method for doing this that does not depend on the details of stellar population synthesis models. Our analysis suggests that the galaxies that populate the densest regions in our sample are older by ∼1 Gyr than objects of the same luminosity in the least dense regions, and that metallicity differences are negligible. We also use single-burst stellar population synthesis models, which allow for nonsolar α-element abundance ratios, to interpret our data. The combination of Hβ, Mg b, and ⟨Fe⟩ lines suggests that age, metallicity, and α-enhancement all increase with velocity dispersion. The objects at lower redshifts are older but have the same metallicities and α-enhancements as their counterparts of the same σ at higher redshifts, as expected if the low-redshift sample is a passively aged version of the sample at higher redshifts. In addition, objects in dense environments are less than 1 Gyr older and α-enhanced by ∼0.02 relative to their counterparts of the same velocity dispersion in less dense regions, but the metallicities show no dependence on environment. This suggests that in dense regions, the stars in early-type galaxies formed at slightly earlier times and on a slightly shorter timescale than in less dense regions. Using HγF instead of Hβ leads to slightly younger ages but the same qualitative differences between environments. In particular, we find no evidence that objects in low-density regions are more metal-rich.

Ages, Metallicities, and α-Element Enhancement for Galaxies in Hickson Compact Groups

Central velocity dispersions and eight line-strength Lick indices have been determined from 1.3 Å resolution long-slit spectra of 16 elliptical galaxies in Hickson compact groups. These data were used to determine galaxy properties (ages, metallicities, and α-element enhancements) and allowed a comparison with the parameters determined for a sample of galaxies in lower density environments studied by González. The stellar population parameters were derived by comparison to single stellar population models of Thomas et al. and to a new set of simple stellar population models for the indices Mg2, Fe5270, and Fe5335 based on synthetic spectra. These models, based on an updated version of the fitting functions presented in Barbuy et al., are fully described here. Our main results are (1) the two samples have similar mean values for the metallicities and [α/Fe] ratios and (2) the majority of the galaxies in compact groups seem to be old (median age of 14 Gyr for eight galaxies for which ages could be derived), in agreement with recent work by Proctor et al. These findings support two possible scenarios: compact groups are either young systems whose members have recently assembled and had not enough time to experience any merging yet, or they are old systems that have avoided merging since their time of formation.

The Evolution and Structure of Early‐Type Field Galaxies: A Combined Statistical Analysis of Gravitational Lenses

We introduce a framework for simultaneously investigating the structure and luminosity evolution of early-type gravitational lens galaxies. The method is based on the fundamental plane, which we interpret using the aperture mass-radius relations derived from lensed image geometries. We apply this method to our previous sample of 22 lens galaxies with measured redshifts and excellent photometry. Modeling the population with a single mass profile and evolutionary history, we find that early-type galaxies are nearly isothermal (logarithmic density slope n = 2.06 +/- 0.17, 68% C.L.), and that their stars evolve at a rate of dlog(M/L)_B/dz = -0.50 +/- 0.19 (68% C.L.) in the rest frame B band. For a Salpeter IMF and a concordance cosmology, this implies a mean star formation redshift of > 1.5 at 95% confidence. While this model can neatly describe the mean properties of early-type galaxies, it is clear that the scatter of the lens sample is too large to be explained by observational uncertainties alone. We therefore consider statistical models in which the galaxy population is described by a distribution of star formation redshifts. We find that stars must form over a significant range of redshifts (Delta z_f > 1.7, 68% C.L.), which can extend as low as z_f = 1 for some acceptable models. However, the typical galaxy will still have an old stellar population ( > 1.5). The lens sample therefore favors early star formation in field ellipticals -- even if we make no a priori assumption regarding the shape of the mass distribution in lenses, and include the range of possible deviations from homology in the uncertainties. Our evolution results call into question several recent claims that early-type galaxies in low-density environments have much younger stars than those in rich clusters.

Ram pressure stripping of disk galaxies

Galaxies in clusters and groups moving through the intracluster or intragroup medium (abbreviated ICM for both) are expected to lose at least a part of their interstellar medium (ISM) by the ram pressure they experience. We perform high resolution 2D hydrodynamical simulations of face-on ram pressure stripping (RPS) of disk galaxies to compile a comprehensive parameter study varying galaxy properties (mass, vertical structure of the gas disk) and covering a large range of ICM condi- tions, reaching from high density environments like in cluster centres to low density environments typical for cluster outskirts or groups. We find that the ICM-ISM interaction proceeds in three phases: firstly the instantaneous stripping phase, secondly the dynamic intermediate phase, thirdly the quasi-stable continuous viscous stripping phase. In the first phase (time scale 20 to 200 Myr) the outer part of the gas disk is displaced but only partially unbound. In the second phase (10 times as long as the first phase) a part of the displaced gas falls back (about 10% of the initial gas mass) despite the constant ICM wind, but most displaced gas is now unbound. In the third phase the galaxy continues to lose gas at a rate of about 1 Myr −1 by turbulent viscous stripping. We find that the stripping efficiency depends slightly on the Mach number of the flow, however, the main parameter is the ram pressure. The stripping efficiency does not depend on the vertical structure and thickness of the gas disk. We discuss uncertainties in the classic estimate of the stripping radius of Gunn & Gott (1972, ApJ, 176, 1), which compares the ram pressure to the gravitational restoring force. In addition, we adapt the estimate used by Mori & Burkert (2000, ApJ, 538, 559) for spherical galaxies, namely the comparison of the central pressure with ram pressure. We find that the latter estimate predicts the radius and mass of the gas disk remaining at the end of the second phase very well, and better than the Gunn & Gott (1972, ApJ, 176, 1) criterion. From our simulations we conclude that gas disks of galaxies in high density environments are heavily truncated or even com- pletely stripped, but also the gas disks of galaxies in low density environments are disturbed by the flow and back-falling material, so that they should also be pre-processed.

The Epochs of Early‐Type Galaxy Formation as a Function of Environment

The aim of this paper is to set constraints on the epochs of early-type galaxy formation through the archaeology of the stellar populations in local galaxies. Using our models of absorption-line indices that account for variable abundance ratios, we derive ages, total metallicities, and element ratios of 124 early-type galaxies in high- and low-density environments. The data are analyzed by comparison with mock galaxy samples created through Monte Carlo simulations taking the typical average observational errors into account, in order to eliminate artifacts caused by correlated errors. We find that all three parameters, age, metallicity, and ?/Fe ratio, are correlated with velocity dispersion. We show that these results are robust against recent revisions of the local abundance pattern at high metallicities. To recover the observed scatter we need to assume an intrinsic scatter of about 20% in age, 0.08?dex in [Z/H], and 0.05?dex in [?/Fe]. All low-mass objects with M* 1010 M? (? 130 km s-1) show evidence for the presence of intermediate-age stellar populations with low ?/Fe ratios. About 20% of the intermediate-mass objects with 1010 M*/M? 1011 [110 ?/(km s-1) 230; both elliptical and lenticular galaxies] must have either a young subpopulation or a blue horizontal branch. On the basis of the above relationships, valid for the bulk of the sample, we show that the Mg-? relation is mainly driven by metallicity, with similar contributions from the ?/Fe ratio (23%) and age (17%). We further find evidence for an influence of the environment on the stellar population properties. Massive early-type galaxies in low-density environments seem on average ~2?Gyr younger and slightly (~0.05-0.1?dex) more metal-rich than their counterparts in high-density environments. No offsets in the ?/Fe ratios are instead detected. With the aid of a simple chemical evolution model, we translate the derived ages and ?/Fe ratios into star formation histories. We show that most star formation activity in early-type galaxies is expected to have happened between redshifts ~3 and 5 in high-density environments and between redshifts 1 and 2 in low-density environments. We conclude that at least 50% of the total stellar mass density must have already formed at z ~ 1, in good agreement with observational estimates of the total stellar mass density as a function of redshift. Our results suggest that significant mass growth in the early-type galaxy population below z ~ 1 must be restricted to less massive objects, and a significant increase of the stellar mass density between redshifts 1 and 2 should be present, caused mainly by the field galaxy population. The results of this paper further imply the presence of vigorous star formation episodes in massive objects at z ~ 2-5 and evolved elliptical galaxies around z ~ 1, both observationally identified as SCUBA galaxies and extremely red objects, respectively.