arXiv subjects

Neil H. M. Crighton

Publications and source records attributed to Neil H. M. Crighton.

22 records · Page 2Linked to original sources

arXiv2011

Through Thick and Thin - HI Absorption in Cosmological Simulations

We investigate the column density distribution function of neutral hydrogen at redshift z = 3 using a cosmological simulation of galaxy formation from the OverWhelmingly Large Simulations (OWLS) project. The base simulation includes gravity, hydrodynamics, star formation, supernovae feedback, stellar winds, chemodynamics, and element-by-element cooling in the presence of a uniform UV background. Self-shielding and formation of molecular hydrogen are treated in post-processing, without introducing any free parameters, using an accurate reverse ray-tracing algorithm and an empirical relation between gas pressure and molecular mass fraction. The simulation reproduces the observed z = 3 abundance of Ly-A forest, Lyman Limit and Damped Ly-A HI absorption systems probed by quasar sight lines over ten orders of magnitude in column density. Self-shielding flattens the column density distribution for NHI > 10^18 cm-2, while the conversion to fully neutral gas and conversion of HI to H2 steepen it around column densities of NHI = 10^20.3 cm-2 and NHI = 10^21.5 cm-2, respectively.

astro-ph.CO↗

arXiv2010

The association between gas and galaxies III: The Cross-correlation of Galaxies and Ly-alpha Absorbers at z ~ 1

We have measured the 2D 2-point correlation function, Xi_AG, between low column density Ly-alpha absorbers and galaxies at a redshift z ~ 1. We measured Ly-alpha absorbers between redshifts z=0.68 to 1.51 over a total redshift path length of Delta z=1.08 from HST STIS E230M absorption spectra towards the quasars HE 1122-1648 (z=2.4) and PKS 1127-145 (z=1.187). The column density of the Ly-alpha absorbers ranged from 13.2< log(N_HI (cm^-2))<17.4, with a median column density of log(N_HI)=14.0 . A total of 193 galaxy redshifts within the surrounding 6.8' x 5.7' field of view of both quasars were identified in a R magnitude limited survey (21.5<R_Vega<24.5) using the FORS2 spectrograph at the VLT, of which 95 were higher than the minimum redshift z=0.68 to be used in the correlation function. A 3 sigma upper-limit of Xi_AG=2.8 was found when 145 Ly-alpha absorber-galaxy pairs were binned in redshift space, in a bin of size Delta sigma=1.0, Delta pi=2.0 h^-1 Mpc along the projected separation and line of sight distances respectively. The upper-limit in the cross-correlation was found to be 5.4 sigma lower than the central peak in the galaxy auto-correlation within the same redshift range, Xi_GG, which was in our data equal to 10.7+/-1.4. Thus we have shown for the first time that the clustering between low column density absorbers and galaxies at a redshift of 1 is weaker than that between galaxies at the same redshift.

astro-ph.CO↗

arXiv2009

Galaxies at Redshift ~0.5 Around Three Closely Spaced Quasar Sightlines

We examine the relationship between galaxies and the intergalactic medium at z < 1 using a group of three closely spaced background QSOs with z_em ~1 observed with the Hubble Space Telescope. Using a new grouping algorithm, we identify groups of galaxies and absorbers across the three QSO sightlines that may be physically linked. There is an excess number of such groups compared to the number we expect from a random distribution of absorbers at a confidence level of 99.9%. The same search is performed with mock spectra generated using a hydrodynamic simulation, and we find the vast majority of such groups arise in dense regions of the simulation. We find that at z<0.5, groups in the simulation generally trace the large-scale filamentary structure as seen in the projected 2-d distribution of the HI column density in a ~30 h^-1 Mpc region. We discover a probable sub-damped Lyman-alpha system at z=0.557 showing strong, low-ionisation metal absorption lines. Previous analyses of absorption across the three sightlines attributed these metal lines to HI. We show that even when the new line identifications are taken into account, evidence remains for planar structures with scales of ~1 Mpc absorbing across the three sightlines. We identify a galaxy at z=0.2272 with associated metal absorption in two sightlines, each 200 kpc away. By constraining the star formation history of the galaxy, we show the gas causing this metal absorption may have been enriched and ejected by the galaxy during a burst of star formation 2 Gyr ago.

astro-ph.CO↗

arXiv2003

The Deuterium Abundance in the z=0.7 absorber towards QSO PG1718+4807

We report a further analysis of the ratio of deuterium to hydrogen (D/H) using HST spectra of the z=0.701 Lyman limit system towards the QSO PG1718+481. Initial analyses of this absorber found it gave a high D/H value, 1.8 - 3.1 \times 10^{-4} (Webb et al. 1998), inconsistent with several higher redshift measurements. It is thus important to critically examine this measurement. By analysing the velocity widths of the DI, HI and metal lines present in this system, Kirkman et al. (2001) report that the additional absorption in the blue wing of the lya line can not be DI, with a confidence level of 98%. Here we present a more detailed analysis, taking into account possible wavelength shifts between the three sets of HST spectra used in the analysis. We find that the constraints on this system are not as strong as those claimed by Kirkman et al. The discrepancy between the parameters of the blue wing absorption and the parameters expected for DI is marginally worse than 1 sigma. Tytler et al.(1999) commented on the first analysis of Webb et al.(1997,1998), reporting the presence of a contaminating lower redshift Lyman limit system, with log[N(HI)] = 16.7 at z=0.602, which biases the N(HI) estimate for the main system. Here we show that this absorber actually has log[N(HI)] < 14.6 and does not impact on the estimate of N(HI) in the system of interest at z = 0.701. The purpose of the present paper is to highlight important aspects of the analysis which were not explored in previous studies, and hence help refine the methods used in future analyses of D/H in quasar spectra.

astro-ph↗