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Register a new userChemical and mass evolution of galaxies
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Bodo L. Ziegler
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An introduction is given to projects investigating galaxy evolution quantitatively by spectroscopic observations of very distant galaxies that have weak apparent brightnesses and small sizes as it is feasible with 10m-class telescopes like SALT. Such methods encompass scaling relations like the Tully-Fisher and Fundamental Plane relations that can be utilized to determine the luminosity evolution and mass assembly of galaxies. The stellar populations can be analyzed with respect to age, metallicity, and chemical enrichment by measureing absorption line strengths. Possible effects on galaxy evolution of the environment in rich clusters of galaxies compared to the field are also addressed. For each method, recent applications are presented like the evolution of the TFR determined with 77 field spirals up to z=1, a study of the internal kinematics of distant cluster spirals and a comparison of the stellar populations of ellipticals in the field and in rich clusters at z=0.4.
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Understanding the different mechanisms of galaxy assembly at various cosmic epochs is a key issue for galaxy evolution and formation models. We present MASSIV (Mass Assembly Survey with SINFONI in VVDS) in this context, an on-going survey with VLT/SINFONI aiming to probe the kinematics and chemical abundances of a unique sample of 84 star-forming galaxies selected in the redshift range z ~ 1-2. This large sample, spanning a wide range of stellar masses, is unique at these high redshifts and statistically representative of the overall galaxy population. In this paper, we give an overview of the MASSIV survey and then focus on the spatially-resolved chemical properties of high-z galaxies and their implication on the process of galaxy assembly.
We use fossil record techniques on the CALIFA sample to study how galaxies in the local universe have evolved in terms of their chemical content. We show how the metallicity and the mass-metallicity relation (MZR) evolve through time for the galaxies in our sample and how this evolution varies when we divide them based on their mass, morphology and star-forming status. We also check the impact of measuring the metallicity at the centre or the outskirts. We find the expected results that the most massive galaxies got enriched faster, with the MZR getting steeper at higher redshifts. However, once we separate the galaxies into morphology bins this behaviour is not as clear, which suggests that morphology is a primary factor to determine how fast a galaxy gets enriched, with mass determining the amount of enrichment. We also find that star-forming galaxies appear to be converging in their chemical evolution, that is, the metallicity of star-forming galaxies of different mass is very similar at recent times compared to several Gyr ago.
The mean alpha-to-iron abundance ratio ([$alpha$/Fe]) of galaxies is sensitive to the chemical evolution processes at early time, and it is an indicator of star formation timescale ($tau_{{rm SF}}$). Although the physical reason remains ambiguous, there is a tight relation between [$alpha$/Fe] and stellar velocity dispersion ($sigma$) among massive early-type galaxies (ETGs). However, no work has shown convincing results as to how this relation behaves at low masses. We assemble 15 data sets from the literature and build a large sample that includes 192 nearby low-mass ($18<sigma<80$~kms) ETGs. We find that the [$alpha$/Fe]-$sigma$ relation generally holds for low-mass ETGs, except in extreme environments. Specifically, in normal galaxy cluster environments, the [$alpha$/Fe]-$sigma$ relation and its intrinsic scatter are, within uncertainties, similar for low-mass and high-mass ETGs. However, in the most massive relaxed galaxy cluster in our sample, the zero point of the relation is higher and the intrinsic scatter is significantly larger. By contrast, in galaxy groups the zero point of the relation offsets in the opposite direction, again with substantial intrinsic scatter. The elevated [$alpha$/Fe] of low-mass ETGs in the densest environments suggests that their star formation was quenched earlier than in high-mass ETGs. For the low-mass ETGs in the lowest density environments, we suggest that their more extended star formation histories suppressed their average [$alpha$/Fe]. The large scatter in [$alpha$/Fe] may reflect stochasticity in the chemical evolution of low-mass galaxies.
We have derived oxygen and nitrogen abundances of a sample of late-type, low surface brightness (LSB) galaxies found in the Sloan Digital Sky Survey (SDSS). Furthermore, we have computed a large grid (5000 models) of chemical evolution models (CEMs) testing various time-scales for infall, baryon densities and several power-law initial mass functions (IMFs) as well. Because of the rather stable N/O-trends found both in CEMs (for a given IMF) and in observations, we find that the hypotheses that LSB galaxies have stellar populations dominated by low-mass stars, i.e., very bottom-heavy IMFs (see Lee et al. 2004), can be ruled out. Such models predict much too high N/O-ratios and generally too low O/H-ratios. We also conclude that LSB galaxies probably have the same ages as their high surface brightness counterparts, although the global rate of star formation must be considerably lower in these galaxies.
We investigate in detail 13 early-type field galaxies with 0.2<z<0.7 drawn from the FORS Deep Field. Since the majority (9 galaxies) is at z~0.4, we compare the field galaxies to 22 members of three rich clusters with z=0.37 to explore possible variations caused by environmental effects. We exploit VLT/FORS spectra (R~1200) and HST/ACS imaging to determine internal kinematics, structures and stellar population parameters. From the Faber-Jackson and Fundamental Plane scaling relations we deduce a modest luminosity evolution in the B-band of 0.3-0.5mag for both samples. We compare measured Lick absorption line strengths (Hdelta, Hgamma, Hbeta, Mg_b, & Fe5335) with evolutionary stellar population models to derive light-averaged ages, metallicities and the element abundance ratios Mg/Fe. We find that all these three stellar parameters of the distant galaxies obey a scaling with velocity dispersion (mass) which is very well consistent with the one of local nearby galaxies. In particular, the distribution of Mg/Fe ratios of local galaxies is matched by the distant ones, and their derived mean offset in age corresponds to the average lookback time. This indicates that there was little chemical enrichment and no significant star formation within the last ~5Gyr. The calculated luminosity evolution of a simple stellar population model for the derived galaxy ages and lookback times is in most cases very consistent with the mild brightening measured by the scaling relations.
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