Propriedades físicas de galáxias starburts em baixo redshift: “Lyman Break Analogs"

Abstract

Physical properties of starburts galaxies at low redshift: ``Lyman Break Analogs'' Lyman Break Analogs (LBAs) are starburst systems at low redshift undergoing intense and continuous processes of star formation, usually as a result of a merger. These galaxies are selected on the basis of high surface brightness and high luminosity in the far-UV. Previous studies have shown that these LBAs are relatively dust-free systems, with relatively low metallicity, low stellar mass and high sSFR. Through the study of their physical properties we can understand similar processes that happen in systems that dominate the cosmic star formation rate density at high redshift, such as the Lyman Break Galaxies (LBGs). These galaxies are the best tracers of galaxy evolution in the early universe, however their study is very complex because of their distances. One way to study these galaxies is through their analogues at low redshift, which offer us good local laboratories to discover the physical processes that are operating in these kind of starburst galaxies. We observed two samples of LBAs at z ~ 0.3 using the X-Shooter spectrograph at the VLT (Very Large Telescope) to obtain medium resolution spectra in the UV-blue (UVB), optical (VIS) and near-infrared (NIR). In this thesis we study the physical properties of LBAs from these spectra, including their star formation rates (SFR), oxygen and nitrogen abundances determined from the direct or temperature-sensitive method and the strong-line methods, dynamical mass, gas velocity dispersion and supernova rates. We also determined age, metallicity and stellar mass of these systems from Simple Stellar Population models fits. By means of the [MgII]2796,2803 absorption lines in the UV we analize the occurrence of outflows attributed to supernovae feedback processes, and the mass outflow rates. On average the direct method abundances agree with the strong line values, but there is a significant scatter. The LBAs furthermore have high star formation rates, high star formation densities and present very strong outflows. Finally, we find that the LBAs have offset line ratios in the BPT diagram and show that these offsets likely result from the combination of increased densities, ionization parameters and excess of N/O. We show that a small part of the excess N/O can be related to Wolf-Rayet activity, but that the majority is probably due to the accretion of metal poor gas. Our findings can explain similar characteristics observed in high redshift Lyman Break Galaxies.