Quasar Sightline and Galaxy Evolution (QSAGE) Survey - I. The Galaxy Environment of OVI Absorbers up to z=1.4 around PKS 0232-04

We present the first results from a study of OVI absorption around galaxies at $z<1.44$ using data from a near-infrared grism spectroscopic Hubble Space Telescope Large Program, the Quasar Sightline and Galaxy Evolution (QSAGE) survey. QSAGE is the first grism galaxy survey to focus on the circumgalactic medium at $z\sim1$, providing a blind survey of the galaxy population. Using the first of 12 fields, we provide details of the reduction methods, in particular the handling of the deep grism data which uses multiple position angles to minimise the effects of contamination from overlapping traces. The resulting galaxy sample is H$\alpha$ flux limited ($f({\rm H\alpha}) > 2\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$) at 0.68<z<1.44, corresponding to $\gtrsim0.2-0.8$ M$_\odot$ yr$^{-1}$. We combine the galaxy data with high-resolution STIS and COS spectroscopy of the background quasar to study OVI in the circumgalactic medium. At z>0.68, we find 5 OVI absorption systems along the line of sight with identified galaxies lying at impact parameters of $b\approx100-350$ kpc (proper), whilst we find a further 13 galaxies with no significant associated OVI absorption (i.e. $N({\rm OVI})<10^{13.5-14}$ cm$^{-2}$) in the same impact parameter and redshift range. We find a large scatter in the stellar mass and star-formation rates of the closest galaxies with associated OVI. Whilst one of the OVI absorber systems is found to be associated with a low mass galaxy group at $z\approx1.08$, we infer that the detected OVI absorbers typically lie in the proximity of dark matter halos of masses $10^{11.5} {\rm M_\odot}\lesssim M_{\rm halo}\lesssim10^{12} {\rm M_\odot}$.
Comment: 22 pages, 18 figures, 4 tables, submitted to MNRAS 14th Sept 2018