Limits on dust emission from z~5 LBGs and their local environments

TL;DR

This study uses 1.2mm observations to set upper limits on dust emission in z~5 Lyman Break Galaxies, finding they are less dusty than lower-redshift submillimeter sources and suggesting different galaxy populations trace large-scale structures differently.

Contribution

First direct millimeter flux limits for z~5 LBGs, constraining their dust content and star formation activity, and comparing their properties to lower-redshift dusty galaxies.

Findings

No individual LBG detected at 1.2mm down to 1.6 mJy/beam.

Stacked flux limit implies LFIR<~3x10^11 Lsun for typical z~5 LBGs.

Dust mass constrained to Mdust<~10^8 Msun, an order of magnitude lower than z~1-3 sub-mm sources.

Abstract

We present 1.2mm MAMBO-2 observations of a field which is over-dense in Lyman Break Galaxies (LBGs) at z~5. The field includes seven spectroscopically-confirmed LBGs contained within a narrow (z=4.95+/-0.08) redshift range and an eighth at z=5.2. We do not detect any individual source to a limit of 1.6 mJy/beam (2*rms). When stacking the flux from the positions of all eight galaxies, we obtain a limit to the average 1.2 mm flux of these sources of 0.6mJy/beam. This limit is consistent with FIR imaging in other fields which are over-dense in UV-bright galaxies at z~5. Independently and combined, these limits constrain the FIR luminosity (8-1000 micron) to a typical z~5 LBG of LFIR<~3x10^11 Lsun, implying a dust mass of Mdust<~10^8 Msun (both assuming a grey body at 30K). This LFIR limit is an order of magnitude fainter than the LFIR of lower redshift sub-mm sources (z~1-3). We see no emission from any other sources within the field at the above level. While this is not unexpected given millimetre source counts, the clustered LBGs trace significantly over-dense large scale structure in the field at z = 4.95. The lack of any such detection in either this or the previous work, implies that massive, obscured star-forming galaxies may not always trace the same structures as over-densities of LBGs, at least on the length scale probed here. We briefly discuss the implications of these results for future observations with ALMA.