Probing star formation across cosmic time with absorption-line systems

We present an empirical connection between cold (∼104 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 W0 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, dN/dW0, 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.