J. M. Gomes

The history of star-forming galaxies in the Sloan Digital Sky Survey

This paper, the sixth in the Semi-Empirical Analysis of Galaxies series, studies the evolution of 82 302 star-forming (SF) galaxies from the Sloan Digital Sky Survey. Star formation histories (SFHs) are derived from detailed spectral fits obtained with our publicly available spectral synthesis code STARLIGHT. Our main goals are to explore new ways to derive SFHs from the synthesis results and apply them to investigate how SFHs vary as a function of nebular metallicity (Zneb). A number of refinements over our previous work are introduced, including (1) an improved selection criterion; (2) a careful examination of systematic residuals around Hβ; (3) self-consistent determination of nebular extinctions and metallicities; (4) tests with several Zneb estimators; (5) a study of the effects of the reddening law adopted and of the relation between nebular and stellar extinctions and the interstellar component of the Na I D doublet.

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.

Can retired galaxies mimic active galaxies? Clues from the Sloan Digital Sky Survey

The classification of galaxies as star forming or active is generally done in the ([O III]/Hβ, [N II]/Hα) plane. The Sloan Digital Sky Survey (SDSS) has revealed that, in this plane, the distribution of galaxies looks like the two wings of a seagull. Galaxies in the right wing are referred to as Seyfert/LINERs, leading to the idea that non-stellar activity in galaxies is a very common phenomenon. Here, we argue that a large fraction of the systems in the right wing could actually be galaxies which stopped forming stars. The ionization in these ‘retired’ galaxies would be produced by hot post-asymptotic giant branch stars and white dwarfs. Our argumentation is based on a stellar population analysis of the galaxies via our STARLIGHT code and on photoionization models using the Lyman continuum radiation predicted for this population. The proportion of LINER galaxies that can be explained in such a way is, however, uncertain. We further show how observational selection effects account for the shape of the right wing. Our study suggests that nuclear activity may not be as common as thought. If retired galaxies do explain a large part of the seagull’s right wing, some of the work concerning nuclear activity in galaxies, as inferred from SDSS data, will have to be revised.

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

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.

Synthesis of composite stellar populations models

Spectral synthesis is largely used in the literature to decompose stellar populations with integrated light of galaxies as if the star formation histories (SFH) could be approximated by single bursts. In the case of our method (see http://www.starlight.ufsc.br/ for the SEAGal Semi Empirical Analysis of Galaxies collaboration), the starlight code combines the spectra of simple stellar populations (SSP) of different ages and metallicities, computed with high spectral resolution evolutionary synthesis models of Bruzual & Charlot (2003), to reproduce the observed spectrum of a given galaxy from which we can derived a huge amount of galaxy properties such as: the population vector, stellar mass, extinction and others. We have done that for all galaxies of the SDSS database. Despite all the results of astrophysical interest, we have decided to use continuous composite stellar models (CSP) with a single metallicity and a star formation rate ∝ τ−1e−t/ τ , where t stands for the time that the star formation started (1, 5 and 13 Gyr ago) and τ is the attenuation factor chosen to be 1, 5, 10 and 99 Gyr. When the attenuation with respect to the time t is very low, this mimics a single burst, and when we choose it to be very large (99 Gyr), this is almost a constant star formation rate. We have perturbed each composite model spectrum 10 times with three distinct signal/noise ratios equal to 10, 15 and 30 in λ0 = 4020 A. These models were inserted into our code to verify how a picture of single bursts deal with continuous composite models of galaxies. Our CSP models can be easily integrated in an analytical form. Therefore, we have derived theoretically the mean ages and metallicities and compared them to the output derived by the synthesis. We can see that the synthesized mean ages weighted by light tend to be lower than the models, due to the degeneracies involved in the problem. The same thing can be found for the mean metallicities weighted by light, which tend to be higher for the output values.

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