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We estimate the galaxy density along lines of sight to hard extragalacticgamma-ray sources by correlating source positions on the sky with a voidcatalog based on the Sloan Digital Sky Survey (SDSS). Extragalactic gamma-raysources that are detected at very high energy (VHE; E>100 GeV) or have beenhighlighted as VHE-emitting candidates in the Fermi Large Area Telescope hardsource catalog (together referred to as "VHE-like" sources) are distributedalong underdense lines of sight at the 2.4 sigma level. There is also a lesssuggestive correlation for the Fermi hard source population (1.7 sigma). Acorrelation between 10-500 GeV flux and underdense fraction along the line ofsight for VHE-like and Fermi hard sources is found at 2.4 sigma and 2.6 sigma,respectively. The preference for underdense sight lines is not displayed bygamma-ray emitting galaxies within the second Fermi catalog, containing sourcesdetected above 100 MeV, or the SDSS DR7 quasar catalog. We investigate whetherthis marginal correlation might be a result of lower extragalactic backgroundlight (EBL) photon density within the underdense regions and find that, even inthe most extreme case of a entirely underdense sight line, the EBL photondensity is only 2% less than the nominal EBL density. Translating this intogamma-ray attenuation along the line of sight for a highly attenuated sourcewith opacity tau(E,z) ~5, we estimate that the attentuation of gamma-raysdecreases no more than 10%. This decrease, although non-neglible, is unable toaccount for the apparent hard source correlation with underdense lines ofsight.

A Correlation Between Hard Gamma-ray Sources and Cosmic Voids Along the Line of Sight