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Register a new userPhoto-Chemical Evolution of Elliptical Galaxies I. The high-redshift formation scenario
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In this paper we compute new multi-zone photo-chemical evolution models for elliptical galaxies, taking into account detailed nucleosynthetic yields, feedback from supernovae and an initial infall episode. By comparing model predictions with observations, we derive a picture of galaxy formation in which the higher is the mass of the galaxy, the shorter are the infall and the star formation timescales. Therefore, in this scenario, the most massive objects are older than the less massive ones, in the sense that larger galaxies stop forming stars at earlier times. Each galaxy is created outside-in, i.e. the outermost regions accrete gas, form stars and develop a galactic wind very quickly, compared to the central core in which the star formation can last up to ~1.3 Gyr. In particular, we suggest that both the duration of the star formation and the infall timescale decrease with galactic radius. (abridged) By means of our model, we are able to match the observed mass-metallicity and color-magnitude relations for the center of the galaxies as well as to reproduce the overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass. Furthermore, we find that the observed Ca underabundance relative to Mg can be real, due to the non-neglibile contribution of type Ia SN to the production of this element. We predict metallicity and color gradients inside the galaxies which are in good agreement with the mean value of the observed ones. (abridged)
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We present deep near-IR images of high redshift radio galaxies obtained with NIRC on the Keck I telescope. In most cases, the near-IR data sample rest wavelengths at ~4000 Angstroms, free of strong emission lines. At z > 3, the rest frame optical morphologies generally have faint, large-scale emission surrounding multiple components of ~10 kpc size. The brightest of the small knots are often aligned with the radio structures. At z < 3, the morphologies change dramatically, showing single, compact structures without radio-aligned features. The sizes and luminosities of the individual components in the z > 3 radio galaxies are similar to those of the radio-quiet star-forming galaxies discovered at z ~ 3 by the Lyman dropout technique. The rest frame optical colors of the z > 3 radio galaxies are consistent with models in which recent star formation dominates the observed IR light, and in one case (4C 41.17) we have direct spectroscopic evidence for massive star formation (Dey et al. 1997a). Our results suggest that the z > 3 radio galaxies evolve into very massive elliptical galaxies at 2 < z < 3, in qualitative agreement with the hierarchical model of galaxy formation. We also discuss the Hubble diagram of radio galaxies, the possibility of a radio power dependence in the K-z relation, and the implications for radio galaxy formation.
We present new deep optical spectra of 9 high-z radio galaxies (HzRGs) at z > 2.7 obtained with FORS2 on VLT. These rest-frame ultraviolet spectra are used to infer the metallicity of the narrow-line regions (NLRs) in order to investigate the chemical evolution of galaxies in high-z universe. We focus mainly on the CIV/HeII and CIII]/CIV flux ratios that are sensitive to gas metallicity and ionization parameter. Although the NV emission has been widely used to infer the gas metallicity, it is often too weak to be measured accurately for NLRs. By combining our new spectra with data from the literature, we examine the possible redshift evolution of the NLR metallicity for 57 HzRGs at 1 < z < 4. Based on the comparison between the observed emission-line flux ratios and the results of our photoionization model calculations, we find no significant metallicity evolution in NLRs of HzRGs, up to z ~ 4. Our results imply that massive galaxies had almost completed their chemical evolution at much higher redshift (z > 5). Finally, although we detect strong NV emission lines in 5 HzRGs at z > 2.7, we point out that high NV/HeII ratios are not indicative of high metallicities but correspond to high ionization parameters of gas clouds in NLRs.
I review the observational characteristics of intermediate-to-high redshift star forming galaxies, including their star formation rates, dust extinctions, ISM kinematics, and chemical compositions. I present evidence that the mean rate of metal enrichment, Delta{Z}/Delta{z}, from z=0--3, as determined from nebular oxygen abundance measurements in star forming galaxies, is 0.15 dex per redshift unit for galaxies more luminous than M_B=-20.5. This rate of chemical enrichment is consistent with the chemical rise in Galactic disk stars. It is less dramatic than, but perhaps consistent with, the enrichment rate of 0.18--0.26+/-0.07 dex per redshift unit seen in Damped Ly alpha systems, and it is much less than predicted by many cosmological evolution models. The high-redshift galaxies observed to date are the most luminous examples from those epochs, and thus, trace only the greatest cosmological overdensities. Star formation in the first 1-2 Gyr appears sufficient to elevate ambient metallicities to near or above the solar value, implying efficient production and retention of metals in these densest environments.
Using a sample of 57 VLT FORS spectra in the redshift range 1.37<z<3.40 (selected mainly from the FORS Deep Field survey) and a comparison sample with 36 IUE spectra of local (z ~ 0) starburst galaxies we derive CIV and SiIV 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_sun at the cosmic epoch corresponding to z ~ 3.2 to about 0.42 Z_sun 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 fixed metallicity. Our observational results are in good agreement with published observational data by other authors and with theoretical predictions of the cosmic chemical evolution.
This paper is the first in a series in which we present the results of an ESO Large Program on the kinematics and internal dynamics of dwarf elliptical galaxies (dEs). We investigate the relations between the parameters that quantify the structure and internal dynamics of dEs such as the Faber_Jackson relation and the Fundamental Plane (FP). We show that the dE sequences in the various diagrams are disjunct from those traced by bright and intermediate-luminosity elliptical galaxies and bulges of spirals. It appears that semi-analytical models (SAMs) are able to reproduce the position of the dEs in those diagrams. While these findings are clearly a success for the hierarchical-merging picture of galaxy formation, they do not necessarily invalidate the alternative ``harassment scenario, which posits that dEs stem from perturbed and stripped late-type disk galaxies that entered clusters and groups of galaxies about 5 Gyr ago.
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