Constraining the properties of gas around galaxies through absorption along multiple lines-of-sight

Abstract

In this thesis we study the relationship between galaxies and the surrounding gas at redshifts < 1 using absorption spectroscopy along multiple, closely-spaced lines-of-sight. We use high-resolution UV spectroscopy of a projected quasar triplet at z ∼ 1 to detect absorption due to the Lyman-α transition of neutral hydrogen across the redshift range 0 < z < 1, as well as absorption lines attributed to numerous metal species. Alongside a variety of imaging and spectroscopic data covering foreground galaxies in this field in optical and near-infrared wavelengths, we attempt to constrain the likely origins of this absorbing gas in a range of galaxy environments.

Using our catalogue of ≈ 270 identified H i absorption systems, ≈ 150 metal line absorption systems, and ≈ 1000 galaxies with confident redshift measurements within a few Mpc of the sightlines, we apply statistical tests utilising this large sample size, and also attempt to model in detail the absorption around a smaller number of galaxies and galaxy groups.

We find that absorption is detected at a similar redshift in multiple lines-of-sight (separated by ∼ 400 kpc on average) significantly more frequently than in a sample
of mock absorbers with randomised redshifts. This is driven primarily by absorbers with column densities greater than 10 ^14 cm −2 , whilst weaker absorbers do not appear in multiple sightlines more often than the random sample, suggesting that they arise in the intergalactic medium.

Using H i, a bimodality is found in the azimuthal angle distribution of galaxy-absorber pairs with impact parameters < 300 kpc. We attribute this primarily to minor-axis outflows and major-axis accretion. This picture is supported by the observations that absorbers near the major axis show a significant preference towards line-of-sight velocities aligned with the kinematics of the galaxy (where available), and that those near the minor axis more frequently exhibit metal absorption (most notably O vi) at the same redshift.

By probing the gas around galaxies at multiple locations, we rule out simple disk, halo and outflow models around several isolated galaxies, and provide constraints on the velocities, opening angles or scale heights for any such structures around several additional galaxies. This can provide an additional diagnostic for helping to distinguish outflowing and infalling material in the circumgalactic medium. These simple models can plausibly fit ≈ 60 % of observed absorption within 300 km s −1 of our isolated galaxies.

We can also reproduce a large fraction (≈ 75 %) of absorption around pairs and groups of galaxies in this field using our ‘toy’ disk and outflow models, with
parameters generally similar to those around isolated galaxies. However, given the large number of model parameters available in groups of galaxies, it is difficult to rule out tidally-stripped or intra-group material resulting from the interactions between group galaxies.

Combined, these results indicate that outflowing and disk-like structures containing detectable levels of neutral gas are commonplace, although not ubiquitous, around galaxies with a wide variety of properties in a wide range of environments.