Infrared Galaxies in the Field of the Massive Cluster Abell S1063: Discovery of a Luminous Kiloparsec-Sized HII Region in a Gravitationally Lensed IR-Luminous Galaxy at $z=0.6$

Abstract

Using the Spitzer Space Telescope and Herschel Space Observatory, we have conducted a survey of infrared galaxies in the field of the galaxy cluster Abell S1063 (AS1063) at $z=0.347$, which is one of the most massive clusters known and a target of the HST CLASH and Frontier-Field surveys. The Spitzer/MIPS 24 $\hat{I}¼$m and Herschel/PACS & SPIRE images revealed that the core of AS1063 is surprisingly devoid of infrared sources, showing only a few detectable sources within the central r$\sim 1^{\prime }$. There is, however, one particularly bright source (2.3 mJy at 24 $\hat{I}¼$m; 106 mJy at 160 $\hat{I}¼$m), which corresponds to a background galaxy at $z=0.61$. The modest magnification factor (4.0$\times$) implies that this galaxy is intrinsically IR-luminous (L${}_{\mathrm{IR}}=3.1\times 10^{11}{\mathrm{L}}_{\odot }$). What is particularly interesting about this galaxy is that HST optical/near-infrared images show a remarkably bright and large (1 kpc) clump at one edge of the disk. Our follow-up optical/near-infrared spectroscopy shows Balmer (H$\hat{I}\pm$-H8) and forbidden emission from this clump ([OII] $\hat{I}»$3727, [OIII] $\hat{I}»\hat{I}»$4959,5007, [NII] $\hat{I}»\hat{I}»$6548,6583), indicating that it is a HII region. The HII region appears to have formed in-situ, as kinematically it is part of a rotating disk, and there is no evidence of nearby interacting galaxies. With an extinction correction of A${}_{\mathrm{V}}=1.5$ mag, the star formation rate of this giant HII region is $\sim$10 M${}_{\odot }$ yr${}^{-1}$, which is exceptionally large, even for high redshift HII regions. Such a large and luminous HII region is often seen at $z\sim 2$ but quite rare in the nearby Universe.

Accepted for publication in the Astrophysical Journal. 29 pages, 15 figures