On the Dusty Proximate Damped Lyman-α System toward Q2310-3358 at z = 2.40

TL;DR

This study investigates a dust-reddened quasar with a proximate damped Lyman-alpha system by leveraging Gaia-based selection and VLT/X-Shooter spectroscopy. The proximate DLA at z ≈ 2.4007 shows near-solar metallicity after dust depletion corrections and hosts CI and H2, indicating a cold, dense, self-shielded ISM under a strong UV field. The absorber sits at a large velocity offset (~864 km s^-1) from the quasar, suggesting physical connection within the same galaxy, likely in a merger-driven environment that fuels star formation and AGN activity. The work highlights the importance of accounting for dust in quasar surveys, extends existing DLAs-metallicity relations to proximate systems, and provides detailed constraints on dust, metals, and molecular gas in the ISM of early galaxies.

Abstract

Quasar absorption systems not only affect the way quasars are selected, but also serve as key probes of galaxies, providing insight into their chemical evolution and interstellar medium (ISM). Recently, a method based on Gaia astrometric measurements has aided the selection of quasars reddened by dust hitherto overlooked. We conducted a spectroscopic study using VLT/X-Shooter on one such dust-reddened quasar, Q2310-3358. This quasar, at $z = 2.3909\pm0.0022$, is associated with a damped Lyman-$α$ absorber (DLA) at nearly the same redshift $2.4007\pm0.0003$, with a neutral hydrogen column density of $\log N(HI)=21.214\pm0.003$. The DLA is very metal-rich (close to solar metallicity after correction for depletion on dust grains). Its properties align with the metal-to-dust ratio and the mass-metallicity relation established in previous large samples of DLAs. Surprisingly, given its proximity to the quasar in redshift, the absorber has strong cold gas characteristics, including CI and H$_2$. Based on the derived kinetic temperature of $71^{+28}_{-15}$~K, we infer the presence of a strong UV radiation field, which in turn suggests that the quasar and the DLA are in close proximity, i.e., part of the same galaxy and not just different objects in the same overdensity of galaxies. We used the line ratios of the fine-structure lines to constrain the density of the cold gas, yielding $n_{\rm H} \sim 10^{3}~\mathrm{cm}^{-3}$. Our analysis extends the understanding of $z_{abs} \approx z_{em}$ absorption line systems and provides valuable constraints on the interplay between dust, metals, and neutral gas in the ISM of early galaxies.

Figures (8)

• Figure 1: Spectrum of Q 2310-3358 obtained with VLT/X-Shooter (black curve), with the positions of the prominent quasar emission lines indicated by dashed purple lines. The blue curve represents the composite quasar spectrum from 2016AA...585A..87S. A reddened template is plotted in red using the best-fit extinction parameters, along with the addition of a Ly$\alpha$ absorption line corresponding to $\log N(\ion{H}{I}) = 21.21\pm0.003$. The inset provides a zoomed-in view of the Ly$\alpha$ line from the absorber. Gray bands mark the regions where the spectrum is unreliable due to X-Shooter arm overlaps and telluric absorption.
• Figure 2: Upper panel: Two single-Gaussian profiles fit to the H$\alpha$ emission line. The red curve shows the fit after masking the artifacts (marked by the red bands), while the blue curve displays the result when the artifacts are not masked. Bottom: Fitting residuals, where the gray shading denotes the $1\sigma$ uncertainty of the spectrum. In both cases, the sky-line regions are masked out, as indicated by the blue bands. The mean ($\mu_G$), standard deviation ($\sigma_G$), and $\mathrm{FWHM}_G$ (corrected for the instrument resolution of 39.377) of the fit are listed in the plot.
• Figure 3: Metallicities of the system corrected for dust using the refractory index $B_X$, with $B_X$ values for all elements taken from Table 3 of 2016AA...596A..97D and Table 1 of 2024AA...691A.129K. The blue curve represents a linear fit to Fe and Zn, with the slope $\delta_Z$ indicating dust depletion, and the intercept $M_{\rm tot}$ representing the corrected total metallicity of the system. Notably, Al and S lie above the curve, which can be attributed to $\alpha$-enrichment.
• Figure 4: Mass–metallicity relation of Q 2310$-$3358 derived through the correlation between logarithmic velocity width and metallicity, measured by two unsaturated metal lines, FeII $\lambda2374$ and SiII $\lambda1808$. The two dashed blue lines show the linear relation fit by 2006AA...457...71L based on 70 DLAs in the redshift range $1.7 < z < 4.3$, while the solid red line illustrates the linear relation from 2013MNRAS.430.2680M, based on 110 DLAs spanning $0.11 < z < 5.06$.
• Figure 5: Voigt profile fitting of four multiplets of CI. The spectrum is shown as the black line with the best-fitting model overlaid as the solid red curve. Each of the blue tick marks indicates the position of a velocity component of either the ground level ($J=0$) or the two excited fine-structure levels ($J=1$ and $J=2$). The small panel on top of each sub-figure shows the residuals of the fit in the given region.