Dust formation in damped Ly-alpha systems

Damped Ly-α (DLA) systems are responsible to the strongest absorption in quasar spectra, and it is traditionally assumed that DLA systems are progenitors of present day galaxies. In this work we deal with spiral galaxies, considering the hypothesis that spirals from early to late types may represent part of observed DLA systems. We present a one-zone chemo-spectrophotometric evolution model of spiral galaxies with self-consistent inclusion of dust evolution, and compare chemical evolution of spiral galaxies corrected for the dust depletion with chemical abundances observed in DLA systems over available redshift range. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chemo-spectrophotometric evolution of spiral galaxies – V. Properties of galactic discs at high redshift

We explore the implications for the high-redshift universe of 'state-of-the-art' models for the chemical and spectrophotometric evolution of spiral galaxies. The models are based on simple 'scaling relations' for discs, obtained in the framework of cold dark matter models for galaxy formation, and were 'calibrated' so as to reproduce the properties of the Milky Way and of nearby discs (at redshift z ∼0) . In this paper, we compare the predictions of our 'hybrid' approach to galaxy evolution to observations at moderate and high redshift. We find that the models are in fairly good agreement with observations up to z ∼1 , while some problems appear at higher redshift (provided there is no selection bias in the data); these discrepancies may suggest that galaxy mergers (not considered in this work) played a non-negligible role at z >1 . We also predict the existence of a 'universal' correlation between abundance gradients and disc scalelengths, independent of redshift.     

Chemical Evolution in the Early Universe: Can Damp Lyα Systems Be Spiral Galaxies?

The distribution of element abundances with redshift in Damped Lyα (DLA) systems can be adequately reproduced by the same models reproducing the halo and disk components of the Milky Way Galaxy at different galactocentric distances. In this sense, DLA systems are well represented by normal spiral galaxies in their early evolutionary stages. This revised version was published online in July 2006 with corrections to the Cover Date.     

The first detection of cobalt in a damped Lyman alpha system

The study of elemental abundances in damped Lyman alpha systems (DLAs) at high redshift represents one of our best opportunities to probe galaxy formation and chemical evolution at early times. By coupling measurements made in high- z DLAs with our knowledge of abundances determined locally and with nucleosynthetic models, we can start to piece together the star formation histories of these galaxies. Here, we discuss the clues to galactic chemical evolution that may be gleaned from studying the abundance of Co in DLAs. We present high resolution echelle spectra of two quasi-stellar objects (QSOs), Q2206−199 and Q1223+17, both already known to exhibit intervening damped systems. These observations have resulted in the first ever detection of Co at high redshift, associated with the z _abs=1.92 DLA in the sightline towards Q2206−199. We find that the abundance of Co is approximately 1/4 solar and that there is a clear overabundance relative to iron, [Co/Fe]=+0.31±0.05 . From the abundance of Zn, we determine that this is a relatively metal-rich DLA, with a metallicity of approximately 1/3 Z_⊙ . Therefore, this first detection of Co is similar to the marked overabundance relative to Fe seen in Galactic bulge and thick-disc stars.     

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.     

The evolution of barred spiral galaxies in the Hubble Deep Fields North and South

The frequency of barred spiral galaxies as a function of redshift contains important information on the gravitational influence of stellar discs in their dark matter haloes and may also distinguish between contemporary theories for the origin of galactic bulges. In this paper we present a new quantitative method for determining the strength of barred spiral structure, and verify its robustness to redshift-dependent effects. By combining galaxy samples from the Hubble Deep Field North with newly available data from the Hubble Deep Field South, we are able to define a statistical sample of 46 low-inclination spiral systems with I _814 W<23.2 mag. Analysing the proportion of barred spiral galaxies seen as a function of redshift, we find a significant decline in the fraction of barred spirals with redshift. The redshift distribution of 22 barred and 24 non-barred spirals with suitable inclinations is inconsistent with their being drawn from the same distribution at the 99 per cent confidence level. The physical significance of this effect remains unclear, but several possibilities include dynamically hotter (or increasingly dark-matter-dominated) high-redshift discs, or an enhanced efficiency in bar destruction at high redshifts. By investigating the formation of the 'orthogonal' axis of Hubble's classification tuning fork, our result complements studies of evolution in the early–late sequence, and pushes to later epochs the redshift at which the Hubble classification sequence is observed to be in place.     

Structure through colour: a pixel approach towards understanding galaxies

We present a study of pixel colour–magnitude diagrams (pCMDs) for a sample of 69 nearby galaxies chosen to span a wide range of Hubble types. Our goal is to determine how useful a pixel approach is for studying galaxies according to their stellar light distributions and content. The galaxy images were analysed on a pixel-by-pixel basis to reveal the structure of the individual pCMDs. We find that the average surface brightness (or projected mass density) in each pixel varies according to galaxy type. Early-type galaxies exhibit a clear 'prime sequence' and some pCMDs of face-on spirals reveal 'inverse-L' structures. We find that the colour dispersion at a given magnitude is found to be approximately constant in early-type galaxies but this quantity varies in the mid and late types. We investigate individual galaxies and find that the pCMDs can be used to pick out morphological features. We discuss the discovery of 'Red Hooks' in the pCMDs of six early-type galaxies and two spirals and postulate their origins. We develop quantitative methods to characterize the pCMDs, including measures of the blue-to-red light ratio and colour distributions of each galaxy and we organize these by morphological type. We compare the colours of the pixels in each galaxy with the stellar population models of Bruzual & Charlot to calculate star formation histories for each galaxy type and compare these to the stellar mass within each pixel. Maps of pixel stellar mass and mass-to-light ratio are compared to galaxy images. We apply the pCMD technique to three galaxies in the Hubble Ultra Deep Field to test the usefulness of the analysis at high redshift. We propose that these results can be used as part of a new system of automated classification of galaxies that can be applied at high redshift.     

Photochemical evolution of elliptical galaxies – II. The impact of merging-induced starbursts

The effects of late gas accretion episodes and subsequent merger-induced starbursts on the photochemical evolution of elliptical galaxies are studied and compared to the picture of galaxy formation occurring at high redshift with a unique and intense starburst modulated by a very short infall, as suggested by Pipino and Matteucci in Paper I. By means of the comparison with the colour–magnitude relations (CMRs) and the  [〈Mg/Fe〉 _V ]–σ  relation observed in ellipticals, we conclude that either bursts involving a gas mass comparable to the mass already transformed into stars during the first episode of star formation (SF) and occurring at any redshift, or bursts occurring at low redshift (i.e. z ≤ 0.2) and with a large range of accreted mass, are ruled out. These models fail in matching the above relations even if the initial infalling hypothesis is relaxed, and the galaxies form either by means of more complicated SF histories or by means of the classical monolithic model. On the other hand, galaxies accreting a small amount of gas at high redshift (i.e. z ≥ 3) produce a spread in the model results, with respect to the best model of Paper I, which is consistent with the observational scatter of the CMRs, although there is only marginal agreement with the  [〈Mg/Fe〉 _V ]–σ  relation. Therefore, only small perturbations to the standard scenario seem to be allowed. We stress that the strongest constraints to galaxy-formation mechanisms are represented by the chemical abundances, whereas the colours can be reproduced under several different hypotheses.     

Evolutionary synthesis models for galaxy transformation in clusters

The galaxy population in rich local galaxy clusters shows a ratio of one quarter elliptical galaxies, two quarters S0 galaxies, and one quarter spiral galaxies. Observations of clusters at redshift 0.5 show a perspicuously different ratio, the dominant galaxy type are spiral galaxies with a fraction of two quarters while the number of S0 galaxies decreases to a fraction of one quarter (Dressler et al. 1997). This shows an evolution of the galaxy population in clusters since redshift 0.5 and it has been suspected that galaxy transformation processes during the infall into a cluster are responsible for this change. These could be merging, starburst or ram-pressure stripping. We use our evolutionary synthesis models to describe various possible effects of those interactions on the star formation of spiral galaxies infalling into clusters. We study the effects of starbursts of various strengths as well as of the truncation of star formation at various epochs on the color and luminosity evolution of model galaxies of various spectral types. As a first application we present the comparison of our models with observed properties of the local S0 galaxy population to constrain possible S0 formation mechanisms in clusters. Application to other types of galaxies is planned for the future. This revised version was published online in August 2006 with corrections to the Cover Date.     

The contribution of faint blue galaxies to the submillimetre counts and background

Observations in the submillimetre (submm) waveband have recently revealed a new population of luminous sources. These are proposed to lie at high redshift and to be optically faint because of their high intrinsic dust obscuration. The presence of dust has been previously invoked in optical galaxy count models which use the Bruzual & Charlot evolution models with an exponential τ =9 Gyr star formation rate (SFR) for spirals, and these fit the count data well from U to K . We now show that by using either a 1/ λ or Calzetti absorption law for the dust and re-distributing the evolved spiral galaxy ultraviolet (UV) radiation into the far-infrared (FIR), these models can account for all of the 'faint' ( 1 mJy) 850-μm galaxy counts, but fail to fit 'bright' ( 2 mJy) sources, indicating that another explanation for the submm counts may apply at brighter fluxes, e.g., quasi-stellar objects (QSOs) or ultraluminous infrared galaxies (ULIRGs). We find that the main contribution to the faint, submm number counts is in the redshift range 0.5< z <3, peaking at z ≈1.8. The above model, using either dust law, can also explain a significant proportion of the extragalactic background at 850 μm, as well as producing a reasonable fit to the bright 60-μm IRAS counts.     

Galaxy Evolution from Deep Multicolor Surveys

A composite sample of NIR-selected galaxies having extended multicolor coverage has been used to probe the cosmological evolution of the blue luminosity function and of the stellar mass function. The bright fraction of the sample has spectroscopic redshifts, and the remaining fraction well-calibrated photometric redshifts. The resulting blue luminosity function shows an increasing brightening with redshift respect to the local luminosity function. Hierarchical CDM models predictions are in agreement only at low and intermediate redshifts but fail to reproduce the observed brightening at high redshifts (z ∼ 2–3). This brightening marks the epoch where starburst activity triggered by galaxy interactions could be an important physical mechanism for the galaxy evolution. At the same time the NIR galaxy sample has been used to trace the evolution of the cosmological stellar mass density up to ∼3. A clear decrease of the average mass density is apparent with a fraction ∼15% of the local value at z ∼ 3. UV bright star-forming galaxies are substancial contributors to the evolution of the stellar mass density. Although these results are globally consistent with Λ–CDM scenarios, they tend to underestimate the mass density produced by more massive galaxies present at z > 2.     

星系形态分类的研究进展

传统的哈勃星系形态分类法可以很好地对近邻的亮星系进行分类,但对低面亮度星系、矮椭球星系、矮旋涡星系以及高红移星系等都已无能为力。德沃古勒分类系统、叶凯士分类系统和范登伯分类系统是在哈勃分类法的基础上进行了发展和细化,利用光的中心聚集度或光度级等作为星系形态分类的参数。模型化分类系统试图定量地测量星系形态参数,但需要假定星系面亮度分布满足一定的形式,如r~(1/4)律、指数律等。最近几年,又有一些学者提出了非模型化分类系统,给出了若干个可以直接测量星系形态的结构参数,如:聚集度指数C、非对称指数A、簇聚指数S、基尼系数G及矩指数M_(20)。这些参数可以反映星系的形成历史、恒星形成、与其他星系的相互作用、已经发生或正在进行的并合活动等。它们不仅可以有效地给出近邻星系的分类特性,还能用于测量高红移星系的形态。该文介绍了不同的星系形态分类方法,比较了各类方法的优点和不足。在此基础上,最后介绍了基于非模型化分类系统的星系形态分类的研究进展。     

Gas dynamics and structure of galaxies

Between gas dynamics and structure of galaxies is a two-way relation. On one hand, gas dynamics in a galaxy is largely determined by the structure of the galaxy, and on the other hand, gas dynamics can gradually alter the galaxy structure through redistribution of mass and angular momentum within the galaxy. The first half of this relation should mostly determine gas distribution and regulate star formation in undisturbed spirals, and the second half has been suggested to cause secular evolution of spiral galaxies—a slow mode of galaxy evolution in the absence of major mergers. Our knowledge on this relation is going to be greatly deepened by the ALMA. Focusing on the galaxy evolution through gas dynamics, I briefly review what we know about the subject. Then I try to look out what the ALMA can do to answer open questions in the field. It is pointed out that the ALMA will be able to fully map all the spiral galaxies between 1 and 25 Mpc at 1″ resolution in 1000 hours.     

An HI Survey of Clusters in the Local Universe:

We outline the project of multifrequency observation of five clusters of galaxies spanning a redshift between 0.05 and 0.2. The core of the project is an HI survey of clusters accomplished with the VLA in its C configuration, and complemented with GMRT data. The 21 cm imaging is being combined with optical spectroscopy and deep NIR imaging with the aim to obtain a database on galaxy evolution in the nearby universe. We choose a sample of clusters with different degrees of dynamical evolution, some containing an important population of starburst or/and post–starburst galaxies, and a hot intracluster medium with emission in X–ray. This revised version was published online in July 2006 with corrections to the Cover Date.     

Testing the nature of S0 galaxies using planetary nebula kinematics in NGC 1023

We investigate the manner in which lenticular galaxies are formed by studying their stellar kinematics: an S0 formed from a fading spiral galaxy should display similar cold outer disc kinematics to its progenitor, while an S0 formed in a minor merger should be more dominated by random motions. In a pilot study, an attempt to distinguish between these scenarios, we have measured the planetary nebula (PN) kinematics of the nearby S0 system NGC 1023. Using the Planetary Nebula Spectrograph, we have detected and measured the line-of-sight velocities of 204 candidate planetary nebulae (PNe) in the field of this galaxy. Out to intermediate radii, the system displays the kinematics of a normal rotationally supported disc system. After correction of its rotational velocities for asymmetric drift, the galaxy lies just below the spiral galaxy Tully–Fisher relation, as one would expect for a fading system. However, at larger radii the kinematics undergo a gradual but major transition to random motion with little rotation. This transition does not seem to reflect a change in the viewing geometry or the presence of a distinct halo component, since the number counts of PNe follow the same simple exponential decline as the stellar continuum with the same projected disc ellipticity out to large radii. The galaxy's small companion, NGC 1023A, does not seem to be large enough to have caused the observed modification either. This combination of properties would seem to indicate a complex evolutionary history in either the transition to form an S0 or in the past life of the spiral galaxy from which the S0 formed. More data sets of this type from both spirals and S0s are needed in order to definitively determine the relationship between these types of system.     

Metallicity in damped Lyman-α systems: evolution or bias?

Assuming that damped Lyman- α (DLA) systems are galactic discs, we calculate the corresponding evolution of metal abundances. We use detailed multizone models of galactic chemical evolution (reproducing successfully the observed properties of disc galaxies) and appropriate statistics (including geometrical probability factors) to calculate the average metallicity as a function of redshift. The results are compatible with available observations, provided that observational biases are taken into account , as suggested by Boissé et al. In particular, high column density and high-metallicity systems are not detected because the light of backround quasars is severely extinguished, while low column density and low-metallicity systems are not detectable through their absorption lines by current surveys. We show that these observational constraints lead to a 'no-evolution' picture for the DLA metallicity, which does not allow us to draw strong conclusions about the nature of those systems or about their role in 'cosmic chemical evolution'.     

The evolutionary history of early-type galaxies as derived from the fundamental plane

The fundamental plane (FP) scaling relations and their evolution are a powerful tool for studying the global properties of early-type galaxies and their evolutionary history. The form of the FP, as derived by surveys in the local Universe at wavelengths ranging from the U to the K band, cannot be explained by metallicity variations alone among early-type galaxies; systematic variations in age, dark matter content, or homology breaking are required. A large-scale study of early-type galaxies at 0.1 < z < 0.6demonstrates that the SB intercept of the FP, the rest frame (U-V) colour, and the absorption line strengths all evolve passively, thereby implying a high mean formation redshift for the stellar content. The slope of the FP evolves with redshift, which is broadly consistent with systematic age effects occurring along the early-type galaxy sequence. The implication that the least luminous early-type galaxies formed later than the luminous galaxies is discussed in the context of the evolution of thecolour–magnitude relation, the Butcher–Oemler effect and hierarchical galaxy formation models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectro-morphology of galaxies

We present a quantitative method of classification of the multi-wavelength morphology of galaxies, elaborated from the nearby galaxies and usable to high redshift objects. We use the morphological parameters of concentration of light at the galaxy center and 180^°-rotationnal asymmetry, computed in several wavelengths, from ultraviolet (UV) to R band. The variation of these indices with λ reflects the proportion of young and old stellar populations in galaxies, that is characteristic of every morphological type: elliptical/lenticular, spiral and irregular. We need to elaborate some templates of these variation spectra from the study of nearby objects, for an application to distant galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Galaxy Evolution from Deep Field Galaxy Counts

We present deep galaxy number counts and colours of K—band selected galaxy surveys. We argue that primeval galaxies are present within the survey data, but have remained unidentified. There are few objects with the colours of an L ^* elliptical galaxy at a redshift of z ≈ 1, in contradiction to standard luminosity evolution models. We present K—band photometry of the objects in a spectroscopic redshift survey selected at 21 < B < 22.5. The absolute K magnitudes of the galaxies are consistent with the no-evolution or pure luminosity evolution models. The excess faint blue galaxies seen in the B—band number counts at intermediate magnitudes are a result of a low normalization, and do not dominate the population until B ≈ 25. Extreme merging or excess dwarf models are not needed at z < 1. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of Bar on Star-forming Activities in Nuclear Regions of Nearby Spiral Galaxies

By using the SDSS spectra, we have studied the star formation properties of the nearby spiral galaxies selected from the Revised Bright Galaxy Sample, and tried to find the effect of bar structure on the star formation activity in the nuclear regions of nearby galaxies. The stellar population composition and the intensity of star formation activities of each sample galaxy are acquired by using the stellar population synthesis code—STARLIGHT, and the star formation properties of nuclear regions are compared with those of integral sample galaxies. We find that the star formation in barred spiral galaxies is more active than that of unbarred spirals, and that barred spirals have younger stellar populations.