Probing star formation across cosmic time with absorption-line systems

We present an empirical connection between cold (similar to 10(4) K) gas in galactic haloes and star formation. Using a sample of more than 8500 MgII absorbers from Sloan Digital Sky Survey (SDSS) quasar spectra, we report the detection of a 15 sigma correlation between the rest equivalent width W-0 of MgII absorbers and the associated [OII] luminosity, an estimator of star formation rate. This correlation has interesting implications: using only observable quantities we show that MgII absorbers trace a substantial fraction of the global [O II] luminosity density and recover the overall star formation history of the Universe derived from classical emission estimators up to z similar to 2. We then show that the distribution function of MgII rest equivalent widths, dN/dW(0), inherits both its shape and amplitude from the [OII] luminosity function Phi (L). These distributions can be naturally connected, without any free parameter. Our results imply a high covering factor of cold gas around star-forming galaxies: C greater than or similar to 0.5, favouring outflows as the mechanism responsible for MgII absorption. We then argue that intervening MgII absorbers and blueshifted MgII absorption seen in the spectra of star-forming galaxies are essentially the same systems, implying that the observed outflowing gas can reach radii of similar to 50 kpc. These results not only shed light on the nature of Mg II absorbers but also provide us with a new probe of star formation, in absorption, i.e. in a way which does not suffer from dust extinction and with a redshift-independent sensitivity. As shown in this analysis, such a tool can be applied in a noise-dominated regime, i.e. using a data set for which emission lines are not detected in individual objects. This is of particular interest for high-redshift studies.