The Epochs of Early-Type Galaxy Formation

By means of population synthesis models with variable α/Fe ratios we derive average ages, metallicities, and [α/Fe] element enhancements for a sample of 126 field and cluster early-type galaxies. We find a clear positive relation between [α/Fe] and velocity dispersion. Zero-point, slope, and scatter of this correlation are the same for cluster and field galaxies. In particular, the [α/Fe] ratios and mean ages of cluster ellipticals are positively correlated. This strongly reinforces the view that the [α/Fe] element enhancement in ellipticals is produced by star formation timescales rather than by variations of the initial mass function. These results indicate that the more massive the galaxy, the shorter is its star formation timescale, and the higher is the redshift of the bulk of star formation. This finding is not compatible with the predictions from models of hierarchical galaxy formation. The lenticular and field galaxies of the investigated sample do not follow the correlation between age and [α/Fe], but contain a non-negligible fraction of galaxies with young average ages and high [α/Fe] ratios. This revised version was published online in September 2006 with corrections to the Cover Date.     

Spectroscopic surveys of cosmic evolution

ALMA will be the premier instrument for the study of galaxy evolution in the early universe—enabling studies of the gas content, dynamics and dynamical masses, and star formation with unparalleled resolution and sensitivity. Galaxy evolution and AGN growth in the early universe are believed to be strongly driven by merging and dynamical interactions. Thus, a full exploration of the environmental influence is absolutely essential. The Cosmic Evolution Survey (COSMOS) is specifically designed to probe the correlated coevolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) large-scale structure (LSS) over the redshift range z>0.5 to 3. In this contribution I review the characteristics of the COSMOS survey and very exciting initial results on mapping large scale structure in galaxies and dark matter. The survey includes multi-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 square degree equatorial field. Given the very high sensitivity and resolution of these datasets, COSMOS will provide unprecedented samples of objects at z>3 for followup studies wit ALMA.     

The evolution of [O ii] emission from cluster galaxies

We investigate the evolution of the star formation rate in cluster galaxies. We complement data from the Canadian Network for Observational Cosmology 1 (CNOC1) cluster survey  (0.15 < z < 0.6)  with measurements from galaxy clusters in the Two-degree Field (2dF) galaxy redshift survey  (0.05 < z < 0.1)  and measurements from recently published work on higher-redshift clusters, up to almost   z = 1  . We focus our attention on galaxies in the cluster core, i.e. galaxies with   r < 0.7  h ⁻¹₇₀ Mpc  . Averaging over clusters in redshift bins, we find that the fraction of galaxies with strong [O  ii ] emission is ≲20 per cent in cluster cores, and the fraction evolves little with redshift. In contrast, field galaxies from the survey show a very strong increase over the same redshift range. It thus appears that the environment in the cores of rich clusters is hostile to star formation at all the redshifts studied. We compare this result with the evolution of the colours of galaxies in cluster cores, first reported by Butcher and Oemler. Using the same galaxies for our analysis of the [O  ii ] emission, we confirm that the fraction of blue galaxies, which are defined as galaxies 0.2 mag bluer in the rest-frame B – V than the red sequence of each cluster, increases strongly with redshift. Because the colours of galaxies retain a memory of their recent star formation history, while emission from the [O  ii ] line does not, we suggest that these two results can best be reconciled if the rate at which the clusters are being assembled is higher in the past, and the galaxies from which it is being assembled are typically bluer.     

The nature of high-redshift galaxies

Using semi-analytic models of galaxy formation set within the cold dark matter (CDM) merging hierarchy, we investigate several scenarios for the nature of the high-redshift     ) Lyman-break galaxies (LBGs). We consider a 'collisional starburst' model in which bursts of star formation are triggered by galaxy–galaxy mergers, and find that a significant fraction of LBGs are predicted to be starbursts. This model reproduces the observed comoving number density of bright LBGs as a function of redshift and the observed luminosity function at     and     with a reasonable amount of dust extinction. Model galaxies at     have star formation rates, half-light radii,     colours and internal velocity dispersions that are in good agreement with the data. Global quantities such as the star formation rate density and cold gas and metal content of the Universe as a function of redshift also agree well. Two 'quiescent' models without starbursts are also investigated. In one, the star formation efficiency in galaxies remains constant with redshift, while in the other, it scales inversely with disc dynamical time, and thus increases rapidly with redshift. The first quiescent model is strongly ruled out, as it does not produce enough high-redshift galaxies once realistic dust extinction is accounted for. The second quiescent model fits marginally, but underproduces cold gas and very bright galaxies at high redshift. A general conclusion is that star formation at high redshift must be more efficient than locally. The collisional starburst model appears to accomplish this naturally without violating other observational constraints.     

266 E+A galaxies selected from the Sloan Digital Sky Survey Data Release 2: the origin of E+A galaxies

E+A galaxies are characterized as galaxies with strong Balmer absorption lines but without any [O  ii ] or Hα emission lines. The existence of strong Balmer absorption lines indicates that E+A galaxies have experienced starburst within the past one gigayear. However, the lack of [O  ii ] and Hα emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as post-starburst galaxies. For many years, however, it has been a mystery why E+A galaxies started starburst and why they quenched star formation abruptly. Using one of the largest samples of 266 E+A galaxies carefully selected from the Sloan Digital Sky Survey Data Release 2, we have investigated the environment of E+A galaxies from 50 kpc to 8 Mpc scale, i.e. from a typical distance to satellite galaxies to the scale of large-scale structures. We found that E+A galaxies have an excess of local galaxy density only at a scale of <100 kpc (with a 2σ significance), but not at the cluster scale (∼1.5 Mpc) nor at the scale of large-scale structure (∼8 Mpc). These results indicate that E+A galaxies are not created by the physical mechanisms associated with galaxy clusters or the large-scale structure, but are likely to be created by dynamical interaction with closely accompanying galaxies at a <100 kpc scale. The claim is also supported by the morphology of E+A galaxies. We have found that almost all E+A galaxies have a bright compact core, and that ∼30 per cent of E+A galaxies have dynamically disturbed signatures or tidal tails, which quite strongly suggest the morphological appearance of merger/interaction remnants.     

Star formation in galaxies falling into clusters along supercluster-scale filaments

With the help of a statistical parameter derived from optical spectra, we show that the current star formation rate of a galaxy, falling into a cluster along a supercluster filament, is likely to undergo a sudden enhancement before the galaxy reaches the virial radius of the cluster. From a sample of 52 supercluster-scale filaments of galaxies joining a pair of rich clusters of galaxies within the two-degree Field Redshift Survey region, we find a significant enhancement of star formation, within a narrow range between ∼2 and  3  h ⁻¹₇₀ Mpc  of the centre of the cluster into which the galaxy is falling. This burst of star formation is almost exclusively seen in the fainter dwarf galaxies  ( M _B ≥−20)  . The relative position of the peak does not depend on whether the galaxy is a member of a group or not, but non-group galaxies have on average a higher rate of star formation immediately before falling into a cluster. From the various trends, we conclude that the predominant process responsible for this rapid burst is the close interaction with other galaxies falling into the cluster along the same filament, if the interaction occurs before the gas reservoir of the galaxy gets stripped off due to the interaction with the intracluster medium.     

The growth and assembly of a massive galaxy at z∼ 2

We study the stellar mass assembly of the Spiderweb galaxy  (MRC 1138−262)  , a massive   z = 2.2  radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by   z = 0  , increasing its stellar mass by up to a factor of ≃2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.     

Dynamical stability and evolution of the discs of Sc galaxies

We examine the local stability of galactic discs against axisymmetric density perturbations with special attention to the different dynamics of the stellar and gaseous components. In particular, the discs of the Milky Way and of NGC 6946 are studied. The Milky Way is shown to be stable, whereas the inner parts of NGC 6946, a typical Sc galaxy from the Kennicutt sample, are dynamically unstable. The ensuing dynamical evolution of the composite disc is studied by numerical simulations. The evolution is so fierce that the stellar disc heats up dynamically on a short time-scale to such a high degree, which seems to contradict the morphological appearance of the galaxy. The star formation rate required to cool the disc dynamically is estimated. Even if the star formation rate in NGC 6946 is at present high enough to meet this requirement, it is argued that the discs of Sc galaxies cannot sustain such a high star formation rate for extended periods.     

Inhomogeneous chemical evolution of dwarf spheroidal galaxies

Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal (dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s^-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies. The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s^-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally ∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the remaining gas over the gravitational potential of the dSph galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stellar archaeology: Exploring the Universe with metal‐poor stars

The abundance patterns of the most metal‐poor stars in the Galactic halo and small dwarf galaxies provide us with a wealth of information about the early Universe. In particular, these old survivors allow us to study the nature of the first stars and supernovae, the relevant nucleosynthesis processes responsible for the formation and evolution of the elements, early star‐ and galaxy formation processes, as well as the assembly process of the stellar halo from dwarf galaxies a long time ago. This review presents the current state of the field of “stellar archaeology” – the diverse use of metal‐poor stars to explore the high‐redshift Universe and its constituents. In particular, the conditions for early star formation are discussed, how these ultimately led to a chemical evolution, and what the role of the most iron‐poor stars is for learning about Population III supernovae yields. Rapid neutron‐capture signatures found in metal‐poor stars can be used to obtain stellar ages, but also to constrain this complex nucleosynthesis process with observational measurements. Moreover, chemical abundances of extremely metal‐poor stars in different types of dwarf galaxies can be used to infer details on the formation scenario of the halo and the role of dwarf galaxies as Galactic building blocks. I conclude with an outlook as to where this field may be heading within the next decade. A table of ~ 1000 metal‐poor stars and their abundances as collected from the literature is provided in electronic format (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The evolution of the galaxy red sequence in simulated clusters and groups

N -body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a Λ cold dark matter (ΛCDM) cosmology are used in order to follow the building-up of the colour–magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a 'dead sequence' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z ∼ 1, after which major events only involve the brightest cluster galaxies.
The emerging scenario is that the evolution of the colour–magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp 'red and dead' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z ∼ 1.5) and environment.
Differently from the DS, the operatively defined RS evolves more evidently with z , the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star-forming one: this goes from z ≃ 1 in clusters down to 0.4 in normal groups.     

Radio continuum and far-infrared emission from the galaxies in the Eridanus group

The Eridanus galaxies follow the well-known radio—FIR correlation. The majority (70%) of these galaxies have their star formation rates below that of the Milky Way. The galaxies that have a significant excess of radio emission are identified as low luminosity AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (L _20cm > 10²³ W Hz⁻¹) in the group. The two most far-infrared and radio luminous galaxies in the group have optical and HI morphologies suggestive of recent tidal interactions. The Eridanus group also has two far-infrared luminous but radio-deficient galaxies. It is believed that these galaxies are observed within a few Myr of the onset of an intense star formation episode after being quiescent for at least a 100 Myr. The upper end of the radio luminosity distribution of the Eridanus galaxies (L _20cm ∼ 10²² W Hz⁻¹) is consistent with that of the field galaxies, other groups, and late-type galaxies in nearby clusters.     

History of Star Formation of the Galaxy Cluster Abell 85

Abell 85 is a cD galaxy cluster in the southern hemisphere and has a redshift of 0.055. Based on the spectra of 242 member galaxies provided by the Sloan spectral survey data, using the stellar population constituents and star formation history of these member galaxies obtained from the population synthesis software STARLIGHT, we study the regularities of the variations of star formation properties of galaxies (such as the ages, metal abundances and star formation rates of the characteristic stellar populations) with the local surface density of galaxies. As revealed by the results, the galaxies situated in the highdensity environments of the central region of the cluster possess higher population ages and metal abundances, and their rates of star formation are rather low, the recent activities of star formation are obviously suppressed. Besides, the correlations of the galaxy metal abundance and speci?c star formation rate with the stellar mass are asserted.     

Interaction-induced star formation in a complete sample of 105 nearby star-forming galaxies

We investigate the clustering properties of a complete sample of 10⁵ star-forming galaxies drawn from the data release 4 (DR4) of the Sloan Digital Sky Survey. On scales less than 100 kpc, the amplitude of the correlation function exhibits a strong dependence on the specific star formation rate (SSFR) of the galaxy. We interpret this as the signature of enhanced star formation induced by tidal interactions. We then explore how the average star formation rate (SFR) in a galaxy is enhanced as the projected separation r _p between the galaxy and its companions decreases. We find that the enhancement strongly depends on r _p, but very weakly on the relative luminosity of the companions. The enhancement is also stronger in low-mass galaxies than in high-mass galaxies. In order to explore whether a tidal interaction is not only sufficient, but also necessary to trigger enhanced star formation in a galaxy, we compute background subtracted neighbour counts for the galaxies in our sample. The average number of close neighbours around galaxies with low to average values of SFR/ M _* is close to zero. At the highest SSFRs, however, more than 40 per cent of the galaxies in our sample have a companion within a projected radius of 100 kpc. Visual inspection of the highest SFR/ M _* galaxies without companions reveals that more than 50 per cent of these are clear interacting or merging systems. We conclude that tidal interactions are the dominant trigger of enhanced star formation in the most strongly star-forming systems. Finally, we find clear evidence that tidal interactions not only lead to enhanced star formation in galaxies, but also cause structural changes such as an increase in concentration.     

The relation between stellar populations, structure and environment for dwarf elliptical galaxies from the MAGPOP-ITP

Dwarf galaxies, as the most numerous type of galaxy, offer the potential to study galaxy formation and evolution in detail in the nearby universe. Although they seem to be simple systems at first view, they remain poorly understood. In an attempt to alleviate this situation, the MAGPOP EU Research and Training Network embarked on a study of dwarf galaxies named MAGPOP-ITP. In this paper, we present the analysis of a sample of 24 dwarf elliptical galaxies (dEs) in the Virgo cluster and in the field, using optical long-slit spectroscopy. We examine their stellar populations in combination with their light distribution and environment. We confirm and strengthen previous results that dEs are, on average, younger and more metal-poor than normal elliptical galaxies, and that their [α/Fe] abundance ratios scatter around solar. This is in accordance with the downsizing picture of galaxy formation where mass is the main driver for the star formation history. We also find new correlations between the luminosity-weighted mean age, the large-scale asymmetry, and the projected Virgocentric distance. We find that environment plays an important role in the termination of the star formation activity by ram-pressure stripping of the gas in short time-scales, and in the transformation of discy dwarfs to more spheroidal objects by harassment over longer time-scales. This points towards a continuing infalling scenario for the evolution of dEs.     

Forming stars on a viscous time-scale: the key to exponential stellar profiles in disc galaxies?

We argue for implementing star formation on a viscous time-scale in hydrodynamical simulations of disc galaxy formation and evolution. Modelling two-dimensional isolated disc galaxies with the Bhatnagar–Gross–Krook (BGK) hydrocode, we verify the analytic claim of various authors that if the characteristic time-scale for star formation is equal to the viscous time-scale in discs, the resulting stellar profile is exponential on several scalelengths whatever the initial gas and dark matter profile. This casts new light on both numerical and semi-analytical disc formation simulations that either (a) commence star formation in an already exponential gaseous disc, (b) begin a disc simulation with conditions known to lead to an exponential, i.e. the collapse of a spherically symmetric nearly uniform sphere of gas in solid-body rotation under the assumption of specific angular momentum conservation, or (c) in simulations performed in a hierarchical context, tune their feedback processes to delay disc formation until the dark matter haloes are slowly evolving and without much substructure so that the gas has the chance to collapse under conditions known to give exponentials. In such models, star formation follows a Schmidt-like law, which for lack of a suitable time-scale, resorts to an efficiency parameter. With star formation prescribed on a viscous time-scale, however, we find gas and star fractions after ∼12 Gyr that are consistent with observations without having to invoke a 'fudge factor' for star formation. Our results strongly suggest that despite our gap in understanding the exact link between star formation and viscosity, the viscous time-scale is indeed the natural time-scale for star formation.     

The evolution of disk galaxies in clusters

We are carrying out a programme to measure the evolution of the stellar and dynamical masses and M/L ratios for a sizeable sample of morphologically-classified disk galaxies in rich galaxy clusters at 0.2 < z < 0.9. Using FORS2 at the VLT we are obtaining rotation curves for the cluster spirals so that their Tully-Fisher relation can be studied as a function of redshift and compared with that of field spirals. We already have rotation curves for ∼ 10 cluster spirals at z = 0.83, and 25 field spirals at lower redshifts and we plan to increase this sample by one order of magnitude. We present here the first results of our study, and discuss the implications of our data in the context of current ideas and models of galaxy formation and evolution. This revised version was published online in September 2006 with corrections to the Cover Date.     

Where do 'red and dead' early-type void galaxies come from?

Void regions of the Universe offer a special environment for studying cosmology and galaxy formation, which may expose weaknesses in our understanding of these phenomena. Although galaxies in voids are observed to be predominately gas rich, star forming and blue, a subpopulation of bright-red void galaxies can also be found, whose star formation was shutdown long ago. Are the same processes that quench star formation in denser regions of the Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy Redshift Survey, to those from a galaxy formation model built on the Millennium simulation. We show that a global star formation suppression mechanism in the form of low-luminosity 'radio-mode' active galactic nuclei (AGN) heating is sufficient to reproduce the observed population of void early types. Radio-mode heating is environment independent other than its dependence on dark matter halo mass, where, above a critical mass threshold of approximately   M _vir∼ 10^12.5 M_⊙  , gas cooling on to the galaxy is suppressed and star formation subsequently fades. In the Millennium simulation, the void halo mass function is shifted with respect to denser environments, but still maintains a high-mass tail above this critical threshold. In such void haloes, radio-mode heating remains efficient and red galaxies are found; collectively these galaxies match the observed space density without any modification to the model. Consequently, galaxies living in vastly different large-scale environments but hosted by haloes of similar mass are predicted to have similar properties, consistent with observations.     

The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum-likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and considering the importance of dust all compromise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy‐mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.     

星系NGC5018的星族合成研究

利用星团谱样本的星族合成方法,研究了邻近巨椭圆星系NGC5018中的星族成分和其内部的恒星形成历史,给出了星系中不同年龄和金属丰度星族的成分占有比.星族合成结果表明,NGC5018中不仅存在大量金属丰度低的年老恒星成分,而且较年轻的星族成分(T=5×108yr)对星系光度贡献也很重要.星系吞并和相互作用过程可能是触发这些较年轻星族形成的物理原因,椭圆星系内部的恒星形成历史可能是2次爆发或者多次爆发过程.这些结果可以很好地解释NGC5018颜色偏蓝、Mg2谱指数强度偏弱等观测特征.