Molecular Hydrogen in High-redshift Damped Ly <i>α</i> Absorbers

Abstract Simulations predict that circumgalactic hydrogen gas surrounding massive ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>M</mml:mi> <mml:mi>halo</mml:mi> <mml:mrow> <mml:mi>z</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>12</mml:mn> </mml:msup> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mn>13</mml:mn> </mml:msup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:math> ) galaxies at z ∼ 4 may be predominantly neutral and could produce damped Ly α absorbers (DLAs) along sight lines to background quasars. A circumgalactic medium (CGM) origin for DLAs naturally explains H i absorption-selected galaxy detections at z ∼ 4 with physical separations much greater than the likely extents of the galaxy disks. These H i column densities observed in z ∼ 4 DLA 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 subgrid model for H i -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-band radiation. We find that the typical extents of detectable H 2 sightlines are ≈0.1 R 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 H i columns.