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

We present an analysis of near-infrared integral field unit spectroscopy for the gravitationally lensed Lyman break galaxy, the 8 o’clock arc, taken with SINFONI on the Very Large Telescope. We explore the shape of the spatially-resolved Hβ profile and demonstrate that we can decompose it into three main components that partially overlap (spatially) but are distinguishable when we include the dynamical information. To study the de-lensed morphology of the galaxy we make use of existing B & H imaging from the Hubble Space Telescope and construct a rigorous lens model using a Bayesian grid based lens modeling technique. We apply this lens model to the SINFONI data cube to construct the de-lensed Hβ line map and the velocity and velocity dispersion maps of the galaxy. We explore the dynamical state of the galaxy and find that the 8 o’clock arc has a complex velocity field that is not simply explained by a single rotating disk. The Hβ profile of the galaxy shows a blue-shifted wing suggesting gas outflows of ∼ 200 km s. We confirm previous findings 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 interstellar medium properties. We show that the gas surface density of the 8 o’clock arc is a factor of 2–4 higher than similar lowredshift galaxies selected from the Sloan Digital Sky Survey. We also find that the electron density in the ionized gas is approximately five times higher than in the comparison sample, which implies a higher H ii-region pressure as well as likely a higher density interstellar medium than in similar nearby galaxies.