A. Mateus

Semi-empirical analysis of Sloan Digital Sky Survey galaxies – II. The bimodality of the galaxy population revisited

We revisit the bimodal distribution of the galaxy population commonly seen in the local universe. Here, we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample, with magnitude limit cut-off M(r ) =− 20.5, containing about 50 000 luminous galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2). In addition, galaxies are classified according to their emission-line properties in three distinct spectral classes: star-forming galaxies, with young stellar populations; passive galaxies, dominated by old stellar populations; and hosts of active nuclei, which comprise a mix of young and old stellar populations. We show that the extremes of the distribution of some galaxy properties, essentially galaxy colours, 4000 A break index and mean stellar ages, are associated to star-forming galaxies at one side, and passive galaxies at another. We find that the mean light-weighted stellar age of galaxies is directly responsible for the bimodality seen in the galaxy population. The stellar mass, in this view, has an additional role since most of the star-forming galaxies present in the local universe are low-mass galaxies. Our results also give support to the existence of a ‘downsizing’ in galaxy formation, where massive galaxies seen nowadays have stellar populations formed at early times.

Uncovering the chemical enrichment and mass-assembly histories of star-forming galaxies

We explore the mass-assembly and chemical enrichment histories of star-forming galaxies by applying a population synthesis method to a sample of 84 828 galaxies from the Sloan Digital Sky Survey Data Release 5. Our method decomposes the entire observed spectrum in terms of a sum of simple stellar populations spanning a wide range of ages and metallicities, thus allowing the reconstruction of galaxy histories. A comparative study of galaxy evolution is presented, where galaxies are grouped on to bins of nebular abundances or mass. We find that galaxies whose warm interstellar medium is poor in heavy elements are slow in forming stars. Their stellar metallicities also rise slowly with time, reaching their current values (Z � ∼ 1/ 3Z � ) in the last ∼100 Myr of evolution. Systems with metal-rich nebulae, on the other hand, assembled most of their mass and completed their chemical evolution long ago, reaching Z � ∼

Semi-empirical analysis of Sloan Digital Sky Survey galaxies -IV. A nature via nurture scenario for galaxy evolution

We investigate the environmental dependence of stellar population properties of galaxies in the local Universe. Physical quantities related to the stellar content of galaxies are derived from a spectral synthesis method applied to a volume-limited sample containing more than 60000 galaxies (0.04 < z < 0.075; M r ≤-19.9), extracted from the Data Release 4 of the Sloan Digital Sky Survey. Mean stellar ages, mean stellar metallicities and stellar masses are obtained from this method and used to characterize the stellar populations of galaxies. The environment is defined by the projected local galaxy density estimated from a nearest neighbour approach. We recover the star formation-density relation in terms of the mean light-weighted stellar age, which is strongly correlated with star formation parameters derived from Hα. We find that the age-density relation is distinct when we divide galaxies according to luminosity or stellar mass. The relation is remarkable for galaxies in all bins of luminosity. On the other hand, only for an intermediate stellar mass interval (associated to a transition in galaxy properties) the relation shows a change in galaxy properties with environment. Such distinct behaviours are associated to the large stellar masses of galaxies with the same luminosity in high-density environments. In addition, the well-known star formation-density relation results from the prevalence of massive systems in high-density environments, independently of galaxy luminosity, with the additional observed downsizing in galaxy formation, in which the star formation is shifted from massive galaxies at early times to low-mass galaxies as the Universe evolves. Finally, our results support that a natural path for galaxy evolution proceeds via a nurture way, in the sense that galaxy evolution is accelerated in denser environments, that took place mainly at high redshifts.

The evolution of the mass–metallicity relation in SDSS galaxies uncovered by astropaleontology

We have obtained the mass–metallicity (M–Z) relation at different lookback times for the same set of galaxies from the Sloan Digital Sky Survey, using the stellar metallicities estimated with our spectral synthesis code STARLIGHT. We have found that this relation steepens and spans a wider range in both mass and metallicity at higher redshifts. We have modelled the time evolution of stellar metallicity with a closed-box chemical evolution model, for galaxies of different types and masses. Our results suggest that the M–Z relation for galaxies with present

Star formation and the environment of nearby field galaxies

We investigate the environmental dependence of galaxies with star formation from a volume-limited sample of 4782 nearby field galaxy spectra extracted from the 2dF Galaxy Redshift Survey final data release. The environment is characterized by the local spatial density of galaxies, estimated from the distance to the fifth nearest neighbour. Extensive simulations have been made to estimate local density correction factors resulting from sample incompleteness. We discriminate the galaxies in distinct spectral classes – passive, star-forming and short starburst galaxies – by the use of the equivalent widths (EWs) of [O ii]λ3727 and Hδ. The frequency of galaxies of different classes is then evaluated as a function of the environment. We show that the fraction of star-forming galaxies decreases with increasing density, whereas passive galaxies present the opposite behaviour. The fraction of short starburst galaxies, which suffered a starburst at ∼200 Myr ago, do not present strong environmental dependence. The fraction of this class of galaxies is also approximately constant with galaxy luminosity, except for the faintest bins in the sample, where their fraction seems to increase. We find that the star formation properties are affected in all ranges of densities present in our sample (which excludes clusters), which supports the idea that star formation in galaxies is affected by the environment everywhere. We suggest that mechanisms like tidal interactions, which act in all environments, do play a relevant role in star formation in galaxies.

Emission-Line Taxonomy and the Nature of AGN-Looking Galaxies in the SDSS

Massive spectroscopic surveys like the SDSS have revolutionized the way we study AGN and their relations to the galaxies they live in. A first step in any such study is to define samples of different types of AGN on the basis of emission line ratios. This deceivingly simple step involves decisions on which classification scheme to use and data quality censorship. Galaxies with weak emission lines are often left aside or dealt with separately because one cannot fully classify them onto the standard Star-Forming, Seyfert of LINER categories. This contribution summarizes alternative classification schemes which include this very numerous population. We then study how star-formation histories and physical properties of the hosts vary from class to class, and present compelling evidence that the emission lines in the majority of LINER-like systems in the SDSS are not powered by black-hole accretion. The data are fully consistent with them being galaxies whose old stars provide all the ionizing power needed to explain their line ratios and luminosities. Such retired galaxies deserve a place in the emission line taxonomy.

What stellar populations can tell us about the evolution of the mass–metallicity relation in SDSS galaxies

During the last three decades, many papers have reported the existence of a luminosity metallicity or mass metallicity ( M – Z ) relation for all kinds of galaxies: The more massive galaxies are also the ones with more metal-rich interstellar medium. We have obtained the mass-metallicity relation at different lookback times for the same set of galaxies from the Sloan Digital Sky Survey (SDSS), using the stellar metallicities estimated with our spectral synthesis code starlight . Using stellar metallicities has several advantages: We are free of the biases that affect the calibration of nebular metallicities; we can include in our study objects for which the nebular metallicity cannot be measured, such as AGN hosts and passive galaxies; we can probe metallicities at different epochs of a galaxy evolution. We have found that the M – Z relation steepens and spans a wider range in both mass and metallicity at higher redshifts for SDSS galaxies. We also have modeled the time evolution of stellar metallicity with a closed-box chemical evolution model, for galaxies of different types and masses. Our results suggest that the M – Z relation for galaxies with present-day stellar masses down to 10 10 M ⊙ is mainly driven by the star formation history and not by inflows or outflows.

The Evolution of the Mass–Metallicity Relation in Seyferts

N. Vale Asari1,2, G. Stasińska2, R. Cid Fernandes1, J. M. Gomes1,3, M. Schlickmann1, A. Mateus4, and W. Schoenell1 Dpto. de Fı́sica CFM Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil LUTH, Observatoire de Paris, CNRS, Université Paris Diderot; Place Jules Janssen 92190 Meudon, France GEPI, Observatoire de Paris, CNRS, Université Paris Diderot; Place Jules Janssen 92190 Meudon, France 4 IAG, Universidade de São Paulo, São Paulo, SP, Brazil

The Chemical Enrichment and Mass Assembly Histories of SDSS Galaxies

We explore the mass-assembly and chemical enrichment histories of star forming galaxies by applying a population synthesis method to a sample of nearly 70k galaxies culled from over 500k galaxies from the Sloan Digital Sky Survey Data Release 5. Our method decomposes the entire observed spectrum in terms of a sum of simple stellar populations spanning a wide range of ages and metallicities, thus allowing the reconstruction of galaxy histories. A comparative study of galaxy evolution is presented, where galaxies are grouped onto bins of nebular abundances or mass. We find that galaxies whose warm interstellar medium is poor in heavy elements are slow in forming stars. Their stellar metallicities also rise slowly with time, reaching their current values ( Z ⋆ ~ 1/4 Z ⊙ ) in the last ~100 Myr of evolution. Systems with metal rich nebulae, on the other hand, assembled most of their mass and completed their chemical evolution long ago, reaching Z ⋆ ~ Z ⊙ already at lookback times of a few Gyr. These same trends, which are ultimately a consequence of galaxy downsizing, appear when galaxies are grouped according to their stellar mass. The reconstruction of galaxy histories to this level of detail out of integrated spectra offers promising prospects in the field of galaxy evolution theories.

Star Forming Galaxies and AGN Hosts: The Seagull Wings

Using photoionization models applied to the data from the Sloan Digital Sky Survey (SDSS) we propose a physically motivated dividing line in the [OIII]/Hβ vs [NII]/Hα (BPT) diagram between normal star forming (NSF) galaxies and AGN hosts. We also propose a new diagnostic diagram which can be used for optical spectra of galaxies with redshifts up to z= 1.3.