Overview

My current research is concentrated in the characterization of galaxies and their stellar population properties derived from spectroscopic data. I am also investigating the environmental dependence of galaxy properties and the role played by environment in driving the build up of the two distinct galaxy populations seen in the local universe. I expect to research further the question of star formation and chemical histories obtained from the "fossil record" enclosed in galaxy spectra and from data of high-redshift galaxies. The data mining based on existing galaxy surveys and a preparation for future observational projects is also in the scope of my research activities.

Stellar populations as tracers of galaxy formation and evolution

The observed spectrum of a galaxy encodes several informations about the properties of its stellar populations and the physical processes which have guided its formation and evolution. The age and metallicity distributions of the constituent stars, which in turn reflect their star-formation and chemical histories, besides the gaseous content required for star formation to occur and the metal enrichment of the gas, are typical examples of physical properties that affect the integrated light coming from a galaxy. Retrieving such informations from observational data is of paramount importance to constrain theories of galaxy formation and evolution.

Supported by the need to improve our understanding of the physical properties of galaxies and to determine a robust way to retrieve them from spectroscopic data available nowadays, Dr. Laerte Sodré Jr. and I, have initiated during my Ph.D. a collaboration with Dr. Roberto Cid Fernandes (UFSC, Brazil) and Dr. Grazyna Stasińska (Obs. Meudon, France). The main aim of this collaboration on Semi-Empirical Analysis of Galaxies (SEAGal) is the analysis of fundamental physical properties of galaxies obtained through the application of a spectral synthesis method to their data. Such approach is built on a solid computational foundation and is able to recover reliable stellar population properties out of galaxy spectra of realistic quality. We have applied this method to a large sample of galaxies drawn from the Sloan Digital Sky Survey (SDSS) and the success of this approach is discussed in Cid Fernandes et al. 2005, the first paper of a series. In particular, we have shown that spectral synthesis provides robust information on the distributions of stellar age (t_star) and stellar metallicity (Z_star), as well as on the extinction, velocity dispersion and stellar mass. The works carried out by this collaboration will complement the investigations made by other groups (e.g. Kauffmann et al. 2003a,b).

In the second paper of the SEAGal's series, Mateus et al. (2006), we have revisited the bimodality of the galaxy population found firstly by Strateva et al. (2001) and investigated further by Baldry et al. (2004), among others. In this work, we have shown that the main characteristic which distinguishes the two galaxy populations is the mean light-weighted age of their stellar populations, whereas the stellar mass has a coadjuvant role in the sense that only less massive galaxies are currently forming stars in the local universe. In addition, we confirm the existence of a downsizing in the star formation properties of galaxies, where massive galaxies seen nowadays have stopped to form stars at earlier times.

These observational findings leave us with the need to investigate the evolution of the stellar population properties along the galaxy life. In particular, as galaxy spectra can be considered as fossil records of the evolution of galaxy properties, they provide an useful source for the study of the global star formation history of the Universe (e.g. Heavens et al. 2004). Thus, the evolution of physical properties of galaxies can be investigated through a galaxy archaeology approach from the application of our spectral synthesis method to SDSS galaxies, as well as through a more traditional one, by studying the properties of galaxies in high redshifts and then tracing their evolution to z ~ 0. The observational results obtained by these two methods can put constraints into current semi-analytic models of galaxy formation. I am also interested in studying this methodology in detail, which will provide a theoretical counterpart for my general understanding of galaxy formation and evolution.

Effects of environment on galaxy evolution

A fundamental aspect of the evolution of galaxy populations in the Universe is the environmental dependence of their properties, mainly related to star formation and gaseous content. In Mateus & Sodré (2004), we have investigated the environmental dependence of the fraction of galaxies with star formation from a sample of field galaxies extracted from the 2dF Galaxy Redshift Survey (2dFGRS). Our main result was the discovery of the strong decline of the fraction of star-forming galaxies with increasing local galaxy density, even in rarefied regions. It has been suggested that tidal interactions should be an important mechanism acting on galaxies, which is able to reduce their star formation activity even in less dense regions. Moreover, the existence of galaxies showing bursts of star formation triggered in short time scales (the so-called short starburst galaxies) with a constant fraction in all environments also have supported our main finding.

We also explored the environmental dependence of the stellar population properties of galaxies from the SDSS. This dependence can be inspected with basis on two evolutionary paths by which galaxies can evolve. In the first one, galaxy properties (mainly related to star formation and gas properties) are affected by environment through well known physical mechanisms acting on galaxies. This path, linked directly to the environment, gives origin to a nurture perspective for galaxy evolution. The second path is related to the initial conditions established during galaxy formation, which could account for the relations between galaxy properties and environment. Thus, it is related to a nature perspective driving galaxy evolution. Our recent results suggest that a natural path for galaxy evolution proceeds via a nurture way mainly at high-redshifts: massive galaxies have been formed in denser regions and evolved in an accelerated way, contrasting with a more unsocial life of low-mass galaxies inhabiting low density regions of the universe. The complete work is discussed in Mateus et al. 2007.

I have a special interest on this issue since the environment of a galaxy has a fundamental role in defining its main properties. I expect to provide answers to questions related to this field by using high-redshift galaxy data. The study and application of semi-analytic models of galaxy formation will also provide a comprehensive view of the role of environment on defining galaxy properties.

References

• Baldry I.K., Glazebrook K., Brinkmann J., Ivezic Z., Lupton R.H., Nichol R.C., Szalay A.S., 2004, ApJ, 600, 681
• Cid Fernandes R., Mateus A., Sodré L., Stasinska G., Gomes J.M, 2005, MNRAS, 358, 363
• Cole S., Aragon-Salamanca A., Frenk C.S., Navarro J.F., Zepf S.E., 1994, MNRAS, 271, 781
• Heavens A., Panter B., Jimenez R., Dunlop J, 2004, Nature, 428, 625
• Kauffmann G., et al., 2003a, MNRAS, 341, 33
• Kauffmann G., et al., 2003b, MNRAS, 341, 54
• Mateus A., Sodré L., 2004, MNRAS, 349, 1251
• Mateus A., Sodré L., Cid Fernandes R., Stasinska G., Schoenell W., Gomes J.M, 2005, submitted to MNRAS, astro-ph/0511578
• Mateus A., Sodré L., Cid Fernandes R., Stasinska G., 2006, in preparation
• Stasinska G., Mateus A., Sodré L., Szczerba R., 2004, A&A, 420, 475
• Strateva I., et al., 2001, AJ, 122, 1861