Clustering of galaxies at high redshifts

Recent observations show a large concentration of galaxies at high redshift. At first sight, strong clustering of galaxies at high redshifts seems to be in contradiction with the models of structure formation. In this paper we show that such structures are a manifestation of the strong clustering of rare peaks in the density field. We compute the frequency of occurrence of such large concentrations of galaxies in some models of structure formation.     

Evidence for chemical evolution in spectra of high redshift galaxies

Using a sample of 57 VLT FORS spectra in the redshift range 1.37< z < 3.40 and a comparison sample with 36 IUE spectra of local ( ) starburst galaxies we derive CIV equivalent width values and estimate metallicities of starburst galaxies as a function of redshift. Assuming that a calibration of the CIV equivalent widths in terms of the metallicity based on the local sample of starburst galaxies is applicable to high-z objects, we find a significant increase of the average metallicities from about 0.16 Z _⊙ at the cosmic epoch corresponding to z ≈ 3.2 to about 0.42 Z _⊙ at z ≈ 2.3. A significant further increase in metallicity during later epochs cannot be detected in our data. Compared to the local starburst galaxies our high-redshift objects tend to be overluminous for a given metallicity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Search for and study of extremely metal-deficient galaxies

We summarize the progress in identifying and observational study of extremely metal-deficient (XMD) gas-rich galaxies (BCGs, dIr and LSBDs). Due to volume limitation only the following issues are addressed: sample creation, some statistical data, Colour-Magnitude Diagrams (CMD) and ages, the case of SBS 0335–052 system, and probable evolution paths of various XMD objects. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Understanding dwarf galaxies as galactic building blocks

This is a summary of a general discussion held during the third EuroConference on galaxy evolution. Various observational features of the stellar populations in present-day dwarf galaxies were presented to introduce the discussion on the possibility that these systems be the main building blocks of spiral and elliptical galaxies. Many people in the audience turned out to think that the inconsistencies among the observed properties of large and dwarf galaxies are too many to believe that the former are built up only by means of successive accretions of the latter. However, theorists of hierarchical galaxy formation suggested that present-day dwarfs are not representative of the galactic building blocks, which may be completely invisible nowadays. Some of them suggested that, contrary to what is usually assumed in hierarchical modelling, the actual building blocks were still fully gaseous systems when their major mergers occurred. If this is the case, then most of the inconsistencies can be overcome, and the scenario of hierarchical galaxy formation becomes not too different from that of a slow gas accretion. This revised version was published online in August 2006 with corrections to the Cover Date.     

Recombination emission from protoglobular clouds on high redshifts

Fluxes in hydrogen and helium lines from Proto-Globular Clusters (PGCs) at redshifts z > 1 are estimated. A supernovae remnant in PGCs is considered as a possible source. Intensities in Lyα,Hα and 1640 Å HeI are calculated. It is shown that such PGCs can be detectable for the ongoing and future projects.     

Tidal dwarf galaxies at intermediate redshifts

We present the first attempt at measuring the production rate of tidal dwarf galaxies (TDGs) and estimating their contribution to the overall dwarf population. Using HST/ACS deep imaging data from GOODS and GEMS surveys in conjunction with photometric redshifts from COMBO-17 survey, we performed a morphological analysis for a sample of merging/interacting galaxies in the Extended Chandra Deep Field South and identified tidal dwarf candidates in the rest-frame optical bands. We estimated a production rate about 1.4×10⁻⁵ per Gyr per comoving volume for long-lived TDGs with stellar mass 3×10⁸⁻⁹ M_⊙ at 0.5<z<1.1. Together with galaxy merger rates and TDG survival rate from the literature, our results suggest that only a marginal fraction (less than 10%) of dwarf galaxies in the local universe could be tidally-originated. TDGs in our sample are on average bluer than their host galaxies in the optical. Stellar population modelling of optical to near-infrared spectral energy distributions (SEDs) for two TDGs favors a burst component with age 400/200 Myr and stellar mass 40%/26% of the total, indicating that a young stellar population newly formed in TDGs. This is consistent with the episodic star formation histories found for nearby TDGs.     

Chemical and dynamical evolution of spiral galaxies

We investigate the consequences of the hypothesis of the secular evolution (growth of the bulge from disc material via a bar and temporal evolution of the Hubble sequence) on the chemical evolution of a galaxy. We present the first dynamical and chemical results of our 3D tree-SPH simulations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light-cone distortion of the clustering and abundance of massive galaxies at high redshifts

Observational surveys of galaxies are not trivially related to single-epoch snapshots from computer simulations. Observationally, an increase in the distance along the line of sight corresponds to an earlier cosmic time at which the properties of the surveyed galaxy population may change. The effect of observing a survey volume along the light cone must be considered in the regime where the mass function of galaxies varies exponentially with redshift. This occurs when the haloes under consideration are rare, that is either when they are very massive or observed at high redshift. While the effect of the light cone is negligible for narrow-band surveys of Lyα emitters, it can be significant for dropout surveys of Lyman-break galaxies (LBGs) where the selection functions of the photometric bands are broad. Since there are exponentially more haloes at the low-redshift end of the survey, the low-redshift tail of the selection function contains a disproportionate fraction of the galaxies observed in the survey. This leads to a redshift probability distribution for the dropout LBGs with a mean less than that of the photometric selection function (PHSF) by an amount of order the standard deviation of the PHSF. The inferred mass function of galaxies is then shallower than the true mass function at a single redshift with the abundance at the high-mass end being twice or more as large as expected. Moreover, the statistical moments of the count of galaxies calculated ignoring the light-cone effect deviate from the actual values.     

Young nuclei in dwarf elliptical galaxies

We report the discovery of young embedded structures in three diffuse elliptical galaxies (dE) in the Virgo cluster: IC 783, IC 3468, and IC 3509. We performed 3D spectroscopic observations of these galaxies with the MPFS spectrograph at the 6-m Special Astrophysical Observatory telescope and obtained spatially resolved distributions of kinematic and stellar population parameters by fitting high-resolution PEGASE. HR synthetic single stellar populations (SSP) in pixel space. In all three galaxies, the luminosity-weighted age of the nuclei (∼4 Gyr) is considerably younger than that of the population in the outer regions of the galaxies. We discuss two possibilities for the formation of such structures—a dissipative merger event and a different ram pressure stripping efficiency during two consecutive crossings of the Virgo cluster centre. The article was translated by the authors.     

Global properties of nearby galaxies in various environments

We analyze a sample of the Local Volume that contains 451 galaxies within 10 Mpc. We compare the various global parameters of these galaxies with their tidal index that characterizes the local density of the environment. The closest correlation is observed between the density of the galaxy’s environment and its morphological type. The abundance of neutral hydrogen in the members of close groups was found to be, on average, a factor of 3 lower than that in isolated galaxies. However, much of this difference is attributable to different morphological composition for the group members and field galaxies. The total mass-to-luminosity ratio is virtually independent of the tidal index of the galaxy, which indirectly indicates a low percentage of tidal systems among dwarf galaxies. All of the galaxies with three or more companions in the Local Volume are shown to have masses above the threshold value of 10¹⁰M_⊙.     

The role of AGN in the colour transformation of galaxies at redshifts z≈ 1

We explore the role of active galactic nuclei (AGN) in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1 Gyr) the termination of their star formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the All-wavelength Extended Groth strip International Survey (AEGIS). Stacking the X-ray photons at the positions of galaxies  (0.4 < z < 0.9)  not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-infrared (IR) properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that black hole (BH) accretion outlives the termination of the star formation. This is also supported by our finding that post-starburst galaxies at z ≈ 0.8 and AGN are associated, in agreement with recent results at low z . A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after quenching their star formation.     

Chemically decoupled nuclei in five lenticular galaxies from SAURON data

We analyze data from the SAURON integral-field spectrograph of the William Herschel 4-m telescope for five lenticular galaxies in which we previously found chemically decoupled nuclei from observations with the Multipupil Fiber Spectrograph of the 6-m Special Astrophysical Observatory telescope. In a larger field of view, we confirmed the presence of peaks of the equivalent width of the Mg Ib λ5175 absorption line in the nuclei of all five galaxies. However, the structure of the chemically decoupled regions turned out to be highly varied even in such a small sample: from compact unresolved knots to disks with an extent of several hundred parsecs and, in one case, a triaxial compact minibar-type structure. We confirmed the presence of an inner gaseous polar ring in NGC 7280 and found it in NGC 7332. In their outer parts, the planes of these polar rings are warped toward the plane of stellar rotation in such a way that the gas counterrotates with respect to the stars. This behavior of the gas in a triaxial potential was predicted by several theoretical models.     

High resolution spectra of galaxies

We present inversion techniques which aim at recovering the composite nature and the kinematics of a stellar population from its high resolution absorption line spectrum. The originality of the combined inversion is its potential to recover both the stellar content and the kinematics simultaneously. These techniques use new synthetic high resolution spectra produced by PéGASE and minimization algorithms. We apply them to mock data representing the bulge and disk population of the inner region of spiral galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Scaling relations of field spirals at intermediate redshift

In the last few years, galaxies at redshifts up to z ∼ 1 have become accessible for medium-resolved spectroscopy thanks to the new generation of 10 m-class telescopes. With kinematic and photometric information on spiral galaxies in this regime, well-known scaling relations like the Tully-Fisher relation (TFR) can be studied over half a Hubble time. By comparison to local samples, these studies facilitate simultaneous tests of the hierarchical merging scenario and stellar population models. Using the Very Large Telescope, we obtained spatially resolved rotation curves of 78 spiral galaxies in the FORS Deep Field (FDF), covering all Hubble types from Sa to Sm/Irr at redshifts 0.1 < z < 1.0. We find evidence for a B-band luminosity increase of up to 2 mag for low-mass spirals, whereas the most massive galaxies are of the same luminosity as their local counterparts. In effect, the TFR slope decreases significantly. This would explain the discrepant results of previous observational studies. We also present the velocity-size relation and compare it to the predictions of numerical simulations based on the hierarchical merging scenario. This revised version was published online in August 2006 with corrections to the Cover Date.     

A survey searching for the epoch of assembling of Hubble types

We have started a survey of galaxies at intermediate redshifts using the HST-STIS parallel fields. Our main goal is to analyse the morphology of faint galaxies in order to estimate the epoch of formation of the Hubble classification sequence. The high resolution of STIS images (0.05″) is ideal for this work and enables us to perform a morphological classification and to analyse the internal structures of galaxies. We find that 40% of the 290 galaxies are early types and that there are more irregulars and ellipticals at the fainter magnitudes. This revised version was published online in August 2006 with corrections to the Cover Date.