Mapping the spatial extent of H I-rich absorbers using Mg II absorption along gravitational arcs

¹ NRC Herzberg Astronomy and Astrophysics Research Centre, 5071 West Saanich Road, Victoria, B.C. V9E 2E7, Canada
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ European Southern Observatory, Alonso de Cordova 3107, Casilla, 19001 Santiago, Chile
⁴ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
⁵ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁶ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS Santiago, Chile
⁷ Institut d’Astrophysique de Paris, CNRS-SU, UMR, 7095, 98bis bd Arago, 75014 Paris, France
⁸ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
⁹ Instituto de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁰ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile

^⋆ Corresponding authors, equal contribution to the paper; trystyn.berg@nrc-cnrc.gc.ca, andrea.afruni@unifi.it

Received: 10 September 2024
Accepted: 12 November 2024

Abstract

H I-rich absorbers seen within quasar spectra contain the bulk of neutral gas in the Universe. However, the spatial extent of these reservoirs are not extensively studied due to the pencil beam nature of quasar sightlines. Using two giant gravitational arc fields (at redshifts 1.17 and 2.06) as 2D background sources with known strong Mg II absorption observed with the Multi Unit Spectroscopic Explorer integral field spectrograph (IFS), we investigated whether spatially mapped Mg II absorption can predict the presence of strong H I systems, and determine both the physical extent and H I mass of the two absorbing systems. We created a simple model of an ensemble of gas clouds in order to simultaneously predict the H I column density and gas covering fraction of H I-rich absorbers based on observations of the Mg II rest-frame equivalent width in IFS spaxels. We first test the model on the lensing field with H I observations already available from the literature, finding that we can recover H I column densities consistent with the previous estimates (although with large uncertainties). We then use our framework to simultaneously predict the gas covering fraction, H I column density and total H I gas mass (M_HI) for both fields. We find that both of the observed strong systems have a covering fraction of ≈70% and are likely damped Lyman α systems (DLAs) with M_HI > 10⁹ M_⊙. Our model shows that the typical Mg II metrics used in the literature to identify the presence of DLAs are sensitive to the gas covering fraction. However, these Mg II metrics are still sensitive to strong H I, and can be still applied to absorbers towards gravitational arcs or other spatially extended background sources. Based on our results, we speculate that the two strong absorbers are likely representative of a neutral inner circumgalactic medium and are a significant reservoir of fuel for star formation within the host galaxies.