Probing star formation across cosmic time with absorption line systems
  We present an empirical connection between cold gas in galactic halos and
star formation. Using a sample of more than 8,500 MgII absorbers from SDSS
quasar spectra, we report the detection of a 15 sigma correlation between the
rest equivalent width W0 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 OII luminosity density and recover the overall star formation history of
the Universe derived from classical emission estimators up to z~2. We then show
that the distribution function of MgII rest equivalent widths, dN/dW0 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>0.5, favoring outflows as the mechanism responsible for MgII
absorption. We then argue that intervening MgII absorbers and blue-shifted MgII
absorption seen in the spectra of star forming galaxies are essentially the
same systems. These results not only shed light on the nature of MgII 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 dataset for which emission
lines are not detected in individual objects. This is of particular interest
for high redshift studies.
