A high signal-to-noise HST spectrum towards J1009+0713: precise absorption measurements in the CGM of two galaxies

Abstract

High signal-to-noise spectra towards background quasars are crucial for uncovering weak absorption in the circumgalactic medium (CGM) of intervening galaxies, such as the diagnostic lines of N V that provide insight to the ionization process of warm gas but typically have low equivalent widths. We present a new spectrum from the Hubble Space Telescope with a signal-to-noise ratio of ∼20-35 towards the quasar SDSS J1009+0713 and analyse absorption systems in the CGM of two L^⋆ galaxies close to the line of sight. We identify additional absorption in the CGM of these galaxies that was not reported by the previous lower signal-to-noise spectrum, as well as Milky Way absorbers and quasar outflows from J1009+0713. We measure log (N_{N V}/N_{O vi}) ∼ -1.1 for two CGM absorbers, inconsistent with gas in collisional ionization equilibrium and consistent with a radiatively cooling bulk flow of ∼50-150 km s^-1, which could be produced by galactic winds. These column density ratios are also consistent with those found for other L^⋆ galaxies and for some gas in the Milky Way's halo. We place upper limits of log (N_{N V}/N_{O VI}) < -1.8 to -1.2 for other O VI absorbers in the same haloes, which suggests that O VI is produced by different processes in different parts of the CGM, even within the same galactic halo. Together with the kinematically different structure of high- and low-ionization lines, these results indicate there are many components to a single galaxy's gaseous halo. We find the redshift number density of Ly α forest absorbers and broad Ly α absorbers are consistent with expectations at this redshift.