A bright z=5.2 lensed submillimeter galaxy in the field of Abell 773: HLSJ091828.6+514223

TL;DR

This paper reports the discovery and detailed analysis of a highly luminous, strongly lensed submillimeter galaxy at redshift 5.2 behind Abell 773, revealing its gas properties, excitation, and potential active nucleus through multi-line observations.

Contribution

It presents the first detailed gas excitation and ionized gas flow analysis of a bright high-redshift lensed SMG, including new detections of [NII]205μm at high-z.

Findings

High FIR luminosity (~10^{14} L_sun) with strong lensing magnification (~11)

Detection of multiple CO lines revealing gas dynamics and excitation

First high-z detection of [NII]205μm line indicating ionized gas flows

Abstract

During our Herschel Lensing Survey (HLS) of massive galaxy clusters, we have discovered an exceptionally bright source behind the z=0.22 cluster Abell 773, which appears to be a strongly lensed submillimeter galaxy (SMG) at z=5.2429. This source is unusual compared to most other lensed sources discovered by Herschel so far, because of its higher submm flux (\sim 200mJy at 500\micron) and its high redshift. The dominant lens is a foreground z=0.63 galaxy, not the cluster itself. The source has a far-infrared (FIR) luminosity of L_FIR= 1.1 10^{14}/\mu Lo, where \mu is the magnification factor, likely \sim 11. We report here the redshift identification through CO lines with the IRAM-30m, and the analysis of the gas excitation, based on CO(7-6), CO(6-5), CO(5-4) detected at IRAM and the CO(2-1) at the EVLA. All lines decompose into a wide and strong red component, and a narrower and weaker blue component, 540\kms apart. Assuming the ultraluminous galaxy (ULIRG) CO-to-H2 conversion ratio, the H2 mass is 5.8 10^{11}/\mu Mo, of which one third is in a cool component. From the CI line we derive a CI/H2 number abundance of 6 10^{-5} similar to that in other ULIRGs. The H2O line is strong only in the red velocity component, with an intensity ratio I(H_2O)/I(CO) \sim 0.5, suggesting a strong local FIR radiation field, possibly from an active nucleus (AGN) component. We detect the [NII]205\mics line for the first time at high-z. It shows comparable blue and red components, with a strikingly broad blue one, suggesting strong ionized gas flows.