The physical nature of the 8 o'clock arc based on near-IR IFU spectroscopy with SINFONI

Abstract

We present an analysis of near-infrared integral field unit spectroscopy of the 8 o'clock arc, a gravitationally lensed Lyman break galaxy, taken with SINFONI. We explore the shape of the spatially resolved Hβ profile and demonstrate that we can decompose it into three components that partially overlap (spatially) but are distinguishable when we include dynamical information. We use existing B and H imaging from the Hubble Space Telescope to construct a rigorous lens model using a Bayesian grid-based lens modelling technique. We apply this lens model to the SINFONI data cube to construct the de-lensed Hβ line-flux velocity and velocity dispersion maps of the galaxy. We find that the 8 o'clock arc has a complex velocity field that is not simply explained by a single rotating disc. The Hβ profile of the galaxy shows a blueshifted wing suggesting gas outflows of ∼200 km s^-1. We confirm that the 8 o'clock arc lies on the stellar mass-oxygen abundance-star formation rate plane found locally, but it has nevertheless significantly different gas surface density (a factor of 2-4 higher) and electron density in the ionized gas (five times higher) from those in similar nearby galaxies, possibly indicating a higher density interstellar medium for this galaxy.