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 $z=0.6$
  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 $\mu$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$\sim1^{\prime}$. There is, however, one
particularly bright source (2.3 mJy at 24 $\mu$m; 106 mJy at 160 $\mu$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$_{\rm
IR}=3.1\times10^{11}\ \rm L_{\odot}$). 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$\alpha$-H8) and forbidden
emission from this clump ([OII] $\lambda$3727, [OIII]
$\lambda\lambda$4959,5007, [NII] $\lambda\lambda$6548,6583), indicating that it
is a HII region. The HII region appears to have formed in-situ, as
kinematically it is part of a rotating disk, and there is no evidence of nearby
interacting galaxies. With an extinction correction of A$_{\rm 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 $z\sim2$ but quite rare in the nearby
Universe.
