Star formation rate and extinction in faint z 4 Lyman break galaxies

Abstract

We present a statistical detection of 1.5 GHz radio continuum emission from a sample of faint z 4 Lyman break galaxies (LBGs). To constrain their extinction and intrinsic star formation rate (SFR), we combine the latest ultradeep Very Large Array 1.5 GHz radio image and the Hubble Space Telescope Advanced Camera for Surveys (ACS) optical images in the GOODS-N. We select a large sample of 1771 z 4 LBGs from the ACS catalog using BF435W- dropout color criteria. Our LBG samples have I F775W 25-28 (AB), 0-3 mag fainter than at z 4. In our stacked radio images, we find the LBGs to be point-like under our 2″ angular resolution. We measure their mean 1.5 GHz flux by stacking the measurements on the individual objects. We achieve a statistical detection of S 1.5 GHz = 0.210 ± 0.075 μJy at 3σ for the first time on such a faint LBG population at z 4. The measurement takes into account the effects of source size and blending of multiple objects. The detection is visually confirmed by stacking the radio images of the LBGs, and the uncertainty is quantified with Monte Carlo simulations on the radio image. The stacked radio flux corresponds to an obscured SFR of 16.0 ± 5.7 M yr-1, and implies a rest-frame UV extinction correction factor of 3.8. This extinction correction is in excellent agreement with that derived from the observed UV continuum spectral slope, using the local calibration of Meurer et al. This result supports the use of the local calibration on high-redshift LBGs to derive the extinction correction and SFR, and also disfavors a steep reddening curve such as that of the Small Magellanic Cloud. © 2014. The American Astronomical Society. All rights reserved.