Sub-damped Lyman α systems in the XQ-100 survey – I. Identification and contribution to the cosmological H i budget

Abstract

Sub-damped Lyman α systems (subDLAs; Hi column densities of 19.0 logN(Hi) < 20.3) are rarely included in the cosmic Hi census performed at redshifts z abs 1.5, yet are expected to contribute significantly to the overall Hi mass budget of the Universe. In this paper, we present a blindly selected sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path length ∆X = 475) in the XQ-100 legacy survey to investigate the contribution of subDLAs to the Hi mass density of the Universe. The impact of X-Shooter's spectral resolution on Lyα absorber identification is evaluated, and found to be sufficient for reliably finding absorbers down to a column density of logN(Hi) 18.9. We compared the implications of searching for subDLAs solely using Hi absorption versus the use of metal lines to confirm the identification, and found that metal-selection techniques would have missed 75 subDLAs. Using a bootstrap-Monte Carlo simulation, we computed the column density distribution function (f (N, X)) and the cosmological Hi mass density (Ω Hi ) of subDLAs and compared with our previous work based on the XQ-100 damped Lyman α systems. We do not find any significant redshift evolution in f (N, X) or Ω Hi for subDLAs. However, subDLAs contribute 10-20 per cent of the total Ω Hi measured at redshifts 2 < z < 5, and thus have a small but significant contribution to the Hi budget of the Universe.

Sub-damped Lyman α systems in the XQ-100 survey – II. Chemical evolution at 2.4 ≤ z ≤ 4.3

“…The survey was designed for multiple science cases; from studying the associated absorbers near QSOs (Perrotta et al 2016(Perrotta et al , 2018 to characterizing the Ly α forest (Iršič et al 2017a, b). As the QSOs were purposefully chosen to be blind to any intervening absorption line systems, the XQ-100 data set provides a randomly selected sample of subDLAs and DLAs to assess their cosmological contribution to the evolution of neutral gas at redshifts modestly probed in past surveys (Sánchez-Ramírez et al 2016;Berg et al 2016aBerg et al , 2017Berg et al , 2019aChristensen et al 2017). Details about the XQ-100 survey design and data reduction are summarized in López et al (2016).…”

“…The XQ-100 absorber catalogue consists of 155 subDLAs and 43 DLAs (Sánchez-Ramírez et al 2016;Berg et al 2019a). In brief, the absorbers were visually identified in the XQ-100 spectra by searching for simultaneous Ly α, Ly β, and Ly γ absorption consistent with simulated Voigt profiles of column density of logN(H I) ≥ 19.0.…”

“…H I column densities and absorption redshifts (z abs ) for the identified systems were measured by simultaneously fitting the continuum and Voigt profiles to the corresponding absorption for several Lyman series lines (typically Ly α, Ly β, Ly γ , and Ly δ). For more details about the identification and H I properties of the XQ-100 subDLAs, see Berg et al (2019a).…”

“…Here, we present the chemical abundances of a blind sample of 155 H I-selected subDLAs across redshifts 2.4 < z abs < 4.3, as identified in the XQ-100 survey (López et al 2016). In a previous paper (Berg et al 2019a), we have assessed the contribution of subDLAs to the cosmic H I budget, finding that they contribute 10-20 per cent at 2.4 < z abs < 4.3. In this paper, we present metal column density measurements of these absorbers in order to quantify the contribution of subDLAs to the chemical enrichment of the Universe.…”

The Cosmic Baryon and Metal Cycles

“…We note that 21-cm emission surveys encompass the total H i-mass, while absorption techniques reported here are limited to systems with log N (H i)>20.3. Lower column density quasar absorbers contribute an additional 10-20% to the total H i content (Zafar et al 2013;Berg et al 2019). In the absence of a physical motivation which would favour a specific functional form, we fit the cosmological evolution of Ω neutral gas with a mathematically simple two-parameter power-law.…”