Molecular Hydrogen in High-redshift Damped Lyman-α Absorbers

TL;DR

Damped Lyman-$\alpha$ absorbers at high redshift probe neutral gas in the CGM, and this work examines whether neutral CGM can host detectable molecular hydrogen. The authors develop a post-processing framework that combines a two-sided 1D H$_2$ formation–destruction balance with dust attenuation and SKIRT-based radiative transfer, applied to the FIRE-2 zoom-in simulations to predict H$_2$ columns in the CGM. They find that H$_2$-bearing sightlines are typically limited to $\sim0.1\,R_{\rm vir}$ and that molecular fractions in CGM gas with $N_{\text{HI}}\gtrsim2\times10^{20}\ \rm cm^{-2}$ are usually $f_{\rm H_2}\ll0.01$, consistent with many DLA non-detections; redshift trends in gas density, metallicity, and LW flux balance to yield little evolution in the detectable H$_2$ extent. While the model captures the general paucity of H$_2$ in DLAs, it overpredicts H$_2$ at intermediate columns and suggests that ISM sightlines contribute in some cases, pointing to the need for higher ISM resolution and larger-volume simulations. Overall, the work provides testable predictions for future H$_2$ searches in DLAs and clarifies the CGM’s role in shaping molecular content at high redshift.

Abstract

Simulations predict that circumgalactic hydrogen gas surrounding massive ($M_{\rm{halo}}^{z=1}=10^{12}-10^{13}\ M_{\odot}$) galaxies at $z\sim4$ may be predominantly neutral, and could produce damped Ly$α$ absorbers (DLAs) along sight-lines to background quasars \citep{Stern2021}. A circumgalactic medium (CGM) origin for DLAs naturally explains high redshift HI absorption-selected galaxy detections at physical separations much greater than the likely extents of the galaxy disks \citep{Neeleman2017, Neeleman2019}. The observed $z\sim 4$ DLA HI column densities are large and comparable to interstellar (ISM) gas columns at which substantial molecular hydrogen (H$_2$) abundances occur. We therefore investigate the possible molecular content of high-redshift CGM gas, and its potential detectability via (rest-frame) far-ultraviolet (UV) absorption line studies. For this purpose we develop an analytic sub-grid model for HI-to-H$_2$ transitions and incorporate the model with zoom-in FIRE-2 simulations of evolving high-$z$ galaxies. We include dust absorption and scattering computations for the transfer of photodissociating Lyman-Werner (LW) band radiation. We find that the typical extents of detectable H$_2$ sightlines are $\approx 0.1\, R_{\rm vir}$, independent of redshift from $z=2.5$ to 5. We argue that a CGM origin for DLAs naturally explains the low detection rates of H$_2$ in DLA observations, as the low CGM densities and relatively strong far-UV fields lead to molecular fractions much lower than observed in the ISM at comparable HI columns.

Figures (11)

• Figure 1: Total H$_2$ column density, $N_{\rm{H_2,tot}}$, as a function of total (atomic+molecular) hydrogen column density, $N_{\rm tot}$, for our two-sided 1D models, for various densities $n$ (top panel), LW band fluxes $I_{\rm LW}$ (middle panel), and dust abundances $Z^{\prime}_d$ (bottom panel). Colored curves show variations on our fiducial ISM model with $n=100\ \rm cm^{-3}$, $I_{\rm LW}=1$, and $Z^{\prime}_d=0.1$. Black dashed lines show our representative high-$z$ CGM model with $n=1\ \rm cm^{-3}$, $I_{\rm LW}=10$, and $Z^{\prime}_d=0.1$. Grey dotted lines indicate constant $f_{\rm H_2}\equiv 2N_{\rm H _{2,tot}}/N_{\rm tot}$.
• Figure 2: Top panel: H$_2$ photodissociation rate for a Haardt2012 UV background spectrum as a function of redshift. The right $y$-axis shows the value normalized to the solar neighborhood Draine1978 field. The shaded regions show the redshift range we study and the corresponding range in $I_{\rm LW}$ of 0.13-0.16 associated with the metagalactic field. Bottom panel: radial median profiles of $I_{\rm LW}$ for each redshift. Shaded regions show the 25-75% range and vertical dashed lines show the radial distance at which the median contribution from the galaxy and the metagalactic radiation field are equal.
• Figure 3: 2D maps of H$_2$ column density (top left), H $I$1.2ex column density (top left), mass-weighted average $I_{\rm LW}$, and $f_{\rm H _2}=N_{\rm H _2}/N_{\text{H\scaleto{$I$}{1.2ex}}}$, for halo A4 at $z=4$ (halo mass $5\times10^{11}\ M_{\odot}$, stellar mass $0.8\times10^{10} \ M_{\odot}$, virial radius $50\ \rm kpc$, and stellar half mass radius $2.3\ \rm kpc$). White contours show $N_{\rm H_2,detec}=2\times 10^{14}\;\rm cm ^{-2}$, black contours show $N_{\text{H\scaleto{$I$}{1.2ex}},\rm DLA}=2\times10^{21}\;\rm cm ^{-2}$, the inner circle is the stellar half-mass radius, and the outer circle is the halo virial radius.
• Figure 4: Radial profiles of different quantities for halo A4 at $z=4$. Top panel: projected H $I$1.2ex and H$_2$ column density profiles. Red solid and dashed lines show $N_{\rm \text{H\scaleto{$I$}{1.2ex}}{},DLA}=2\times10^{21}\rm cm^{-2}$ and $N_{\rm H_2,detec}=10^{14} \ \rm cm^{-2}$, respectively. Middle panel: radial profiles of normalized LW band flux $I_{\rm{LW}}$, dust metallicity $Z_d^{\prime}$, and atomic+molecular hydrogen density $n$. Bottom panel: $R_{\rm mol}$ as is computed by our model (solid black) and using the optically thin approximation described in Section \ref{['sec: maps']} (dashed black). The vertical dotted line marks radial distance at which the dust optical depth $\tau_d=\left(N_{\text{H\scaleto{$I$}{1.2ex}}{},\rm p}+2N_{\rm H_2 ,p}\right)\,\sigma_{\rm g}$ drops below 1, beyond which $R_{\rm mol}$ computed from our model agrees with the optically thin approximation. Shaded regions show the interquartile range of the corresponding curves.
• Figure 5: Top: radial median H $I$1.2ex column density profiles using all halos at a given redshift. The shaded region shows that interquartile range for the $z=4$ curve. Middle: as top, but for H$_2$ column density, dashed curves showing the 90th percentile. The red dashed lines correspond to the $2\times10^{21}$ (top) and $2\times10^{14}\;\rm cm^{-2}$ (middle). Bottom: the normalized radius at which the radial H$_2$ (H $I$1.2ex) profiles drop below their respective thresholds. The extent of H $I$1.2ex, as represented by $r_{\rm DLA}$ clearly increases with redshift, while the extent of detectable H$_2$ does not show a clear redshift dependence.