Alma submillimeter continuum imaging of the host galaxies of GRB 021004 and GRB 080607

Abstract

We report 345 GHz continuum observations of the host galaxies of gamma-ray bursts (GRBs) 021004 and 080607 at z > 2 using the Atacama Large Millimeter/Submillimeter Array (ALMA) in Cycle 0. Of the two bursts, GRB 021004 is one of the few GRBs that originate in a Lyman limit host, while GRB 080607 is classified as a "dark burst" and its host galaxy is a candidate of dusty star-forming galaxy at z ∼ 3. With an order of magnitude improvement in the sensitivities of the new imaging searches, we detect the host galaxy of GRB 080607 with a flux of S 345 = 0.31 ± 0.09 mJy and a corresponding infrared luminosity of L IR = (2.4-4.5) × 10 11 L⊙. However, the host galaxy of GRB 021004 remains undetected and the ALMA observations allow us to place a 3σ upper limit of L IR < 3.1 × 1011 L⊙ for the host galaxy. The continuum imaging observations show that the two galaxies are not ultraluminous infrared galaxies, but are at the faintest end of the dusty galaxy population that gives rise to the submillimeter extragalactic background light. The derived star formation rates of the two GRB host galaxies are less than 100 M⊙ yr-1, which are broadly consistent with optical measurements. The result suggests that the large extinction (AV ∼ 3) in the afterglow of GRB 080607 is confined along its particularly dusty sight line, and not representative of the global properties of the host galaxy. © 2012. The American Astronomical Society. All rights reserved.