Radio Constraints on Heavily-Obscured Star-Formation within Dark Gamma-Ray Burst Host Galaxies

TL;DR

This study uses VLA radio observations to investigate star-formation rates in heavily dust-obscured GRB host galaxies, finding most are not forming stars at extreme rates and that optical extinction does not predict radio detectability.

Contribution

First radio survey of heavily obscured GRB hosts, revealing that most do not exhibit extreme star-formation rates and challenging assumptions about dust extinction and star-formation correlation.

Findings

Most obscured GRB hosts are not forming stars at extreme rates.

Optical extinction along GRB sightlines does not predict radio detectability.

Some submillimeter galaxies produce GRBs, but often not heavily obscured ones.

Abstract

Highly dust-obscured starbursting galaxies (submillimeter galaxies and their ilk) represent the most extreme sites of star-formation in the distant universe and contribute significantly to overall cosmic star-formation beyond z>1.5. Some stars formed in these environments may also explode as GRBs and contribute to the population of "dark" bursts. Here we present VLA wideband radio-continuum observations of 15 heavily dust-obscured Swift GRBs to search for radio synchrotron emission associated with intense star-formation in their host galaxies. Most of these targets (11) are not detected. Of the remaining four objects, one detection is marginal and for two others we cannot yet rule out the contribution of a long-lived radio afterglow. The final detection is secure, but indicates a star-formation rate roughly consistent with the UV-inferred value. Most galaxies hosting obscured GRBs are therefore not forming stars at extreme rates, and the amount of optical extinction seen along a GRB afterglow sightline does not clearly correlate with the likelihood that the host has a sufficiently high star-formation rate to be radio-detectable. While some submillimeter galaxies do readily produce GRBs, these GRBs are often not heavily obscured - suggesting that the outer (modestly obscured) parts of these galaxies overproduce GRBs and the inner (heavily obscured) parts underproduce GRBs relative to their respective contributions to star-formation, hinting at strong chemical or IMF gradients within these systems.