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We present an empirical connection between cold ( ∼10 4  K ) gas in galactic haloes and star formation. Using a sample of more than 8500 Mg  ii  absorbers from Sloan Digital Sky Survey (SDSS) quasar spectra, we report the detection of a  15σ  correlation between the rest equivalent width   W  0   of Mg  ii  absorbers and the associated [O  ii ] luminosity, an estimator of star formation rate.  This correlation has interesting implications: using only observable quantities we show that Mg  ii  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 ∼ 2 . We then show that the distribution function of Mg  ii  rest equivalent widths,  d N /d W  0  , inherits both its shape and amplitude from the [O  ii ] luminosity function  Φ( 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 , favouring outflows as the mechanism responsible for Mg  ii  absorption. We then argue that intervening Mg  ii  absorbers and blueshifted Mg  ii  absorption seen in the spectra of star‐forming galaxies are essentially the same systems, implying that the observed outflowing gas can reach radii of  ∼ 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.

Probing star formation across cosmic time with absorption‐line systems