Baryonic Conversion Tree: The global assembly of stars and dark matter in galaxies from the SDSS

Abstract

Using the spectroscopic sample of the SDSS DR1 we measure how gas was transformed into stars as a function of time and stellar mass: the baryonic conversion tree (BCT). There is a clear correlation between early star formation activity and present-day stellar mass: the more massive galaxies have formed about 80% of their stars at $z>1$, while for the less massive ones the value is only about 20%. By comparing the BCT to the dark matter merger tree, we find indications that star formation efficiency at $z>1$ had to be about a factor of two higher than today ($\sim 10%$) in galaxies with present-day stellar mass larger than $2 \times 10^{11}M_\odot$, if this early star formation occurred in the main progenitor. Therefore, the LCDM paradigm can accommodate a large number of red objects. On the other hand, in galaxies with present-day stellar mass less than $10^{11}$ M$_{\odot}$, efficient star formation seems to have been triggered at $z \sim 0.2$. We show that there is a characteristic mass (M$_* \sim 10^{10}$ M$_{\odot}$) for feedback efficiency (or lack of star formation). For galaxies with masses lower than this, feedback (or star formation suppression) is very efficient while for higher masses it is not. The BCT, determined here for the first time, should be an important observable with which to confront theoretical