THE STAR FORMATION HISTORY AND CHEMICAL EVOLUTION OF STAR-FORMING GALAXIES IN THE NEARBY UNIVERSE

Abstract

We have determined the O/H and N/O of a sample of 122751 SFGs from the DR7 of\nthe SDSS. For all these galaxies we have also determined their morphology and\ntheir SFH using the code STARLIGHT. The comparison of the chemical abundance\nwith the SFH allows us to describe the chemical evolution in the nearby\nuniverse (z < 0.25) in a manner which is consistent with the formation of their\nstellar populations and morphologies. A 45% of the SFGs in our sample show an\nexcess of abundance in nitrogen relative to their metallicity. We also find\nthis excess to be accompanied by a deficiency of oxygen, which suggests that\nthis could be the result of effective starburst winds. However, we find no\ndifference in the mode of star formation of the nitrogen rich and nitrogen poor\nSFGs. Our analysis suggests they all form their stars through a succession of\nbursts of star formation extended over a few Gyr period. What produces the\nchemical differences between these galaxies seems therefore to be the intensity\nof the bursts: the galaxies with an excess of nitrogen are those that are\npresently experiencing more intense bursts, or have experienced more intense\nbursts in their past. We also find evidence relating the chemical evolution\nprocess to the formation of the galaxies: the galaxies with an excess of\nnitrogen are more massive, have more massive bulges and earlier morphologies\nthan those showing no excess. As a possible explanation we propose that the\nlost of metals consistent with starburst winds took place during the formation\nof the galaxies, when their potential wells were still building up, and\nconsequently were weaker than today, making starburst winds more efficient and\nindependent of the final mass of the galaxies. In good agreement with this\ninterpretation, we also find evidence consistent with downsizing, according to\nwhich the more massive SFGs formed before the less massive ones.\n