Gamma Ray Bursts detected by Swift (2004-2015)

Higher obscurations, e.g. Compton-thick columns, could have been detected but are not observed. The NH distribution is consistent with sources randomly populating a ellipsoidal gas cloud of major axis N_H_^major^=10^23^cm^-2^ with 0.22dex intrinsic scatter between objects. The unbiased SHOALS survey of afterglows and hosts allows us to constrain the relation between Spitzer-derived stellar masses and X-ray derived column densities NH.

We find a well-constrained power-law relation of NH=10^21.7^cm^-2^x(M*/10^9.5^M_{sun}_)^1/3^, with 0.5dex intrinsic scatter between objects. The Milky Way and the Magellanic clouds also follow this relation. From the geometry of the obscurer, its stellar mass dependence and comparison with local galaxies, we conclude that LGRBs are primarily obscured by galaxy-scale gas.

Ray tracing of simulated Illustris galaxies reveals a relation of the same normalization, but a steeper stellar-mass dependence and mild redshift evolution. Our new approach provides valuable insight into the gas residing in high-redshift galaxies.