Very Large Array observations of the 8 o'clock arc lens system: Radio emission and a limit on the star-formation rate

TL;DR

This study uses high-resolution VLA observations to investigate the radio emission of the 8 o'clock arc lens system, finding that the radio emission is dominated by an AGN and setting an upper limit on the star-formation rate consistent with optical estimates.

Contribution

The paper provides the first high-resolution radio imaging of the 8 o'clock arc, distinguishing AGN emission from star formation and establishing a new SFR upper limit based on radio data.

Findings

Radio emission is dominated by a radio-loud AGN in the lensing galaxy.

No radio emission detected from the lensed star-forming galaxy at the given sensitivity.

The SFR upper limit from radio data (<750 solar-mass/year) agrees with optical/NIR estimates.

Abstract

The 8 o'clock arc is a gravitationally lensed Lyman Break Galaxy (LBG) at redshift z=2.73 that has a star-formation rate (SFR) of 270 solar-mass/year (derived from optical and near-infrared spectroscopy). Taking the magnification of the system ~12 and the SFR into account, the expected flux density of any associated radio emission at 1.4 GHz is predicted to be just 0.1 mJy. However, the lens system is found to be coincident with a radio source detected in the NRAO Very Large Array (VLA) Sky Survey with a flux density of ~5 mJy. If this flux density is attributed to the lensed LBG then it would imply a SFR ~11000 solar-mass/year, in contrast with the optical and near-infrared derived value. We want to investigate the radio properties of this system, and independently determine the SFR for the LBG from its lensed radio emission. We have carried out new high resolution imaging with the VLA ain A and B-configurations at 1.4 and 5 GHz. We find that the radio emission is dominated by a radio-loud AGN associated with the lensing galaxy. The radio-jet from the AGN partially covers the lensed arc of the LBG, and we do not detect any radio emission from the unobscured region of the arc down to a 3 sigma flux-density limit of 108 micro-Jy/beam. Using the radio data, we place a limit of <750 solar-mass/year for the SFR of the LBG, which is consistent with the results from the optical and near-infrared spectroscopy. We expect that the sensitivity of the Expanded VLA will be sufficient to detect many high redshift LBGs that are gravitationally lensed after only a few hours of observing time. The high angular resolution provided by the EVLA will also allow detailed studies of the lensed galaxies and determine if there is radio emission from the lens.