The Hawaii SCUBA-2 Lensing Cluster Survey: Number Counts and Submillimeter Flux Ratios

TL;DR

This study measures deep submillimeter galaxy counts at 450 and 850 micrometers using SCUBA-2, revealing that faint sources significantly contribute to the extragalactic background light and exploring flux ratio evolution with redshift.

Contribution

First combined lensing and blank field data to extend submillimeter counts to fainter fluxes, constraining the faint end of galaxy populations and their contribution to the EBL.

Findings

Faint sources with L_IR < 10^12 L_sun dominate the EBL.

K-selected LIRGs and normal galaxies do not fully account for the EBL from faint sources.

Flux ratio at 850 μm correlates with flux and redshift, unlike at 450 μm.

Abstract

We present deep number counts at 450 and 850 $\mu$m using the SCUBA-2 camera on the James Clerk Maxwell Telescope. We combine data for six lensing cluster fields and three blank fields to measure the counts over a wide flux range at each wavelength. Thanks to the lensing magnification, our measurements extend to fluxes fainter than 1 mJy and 0.2 mJy at 450 $\mu$m and 850 $\mu$m, respectively. Our combined data highly constrain the faint end of the number counts. Integrating our counts shows that the majority of the extragalactic background light (EBL) at each wavelength is contributed by faint sources with $L_{\rm IR} < 10^{12} L_{\odot }$, corresponding to luminous infrared galaxies (LIRGs) or normal galaxies. By comparing our result with the 500 $\mu$m stacking of $K$-selected sources from the literature, we conclude that the $K$-selected LIRGs and normal galaxies still cannot fully account for the EBL that originates from sources with $L_{\rm IR} < 10^{12} L_{\odot }$. This suggests that many faint submillimeter galaxies may not be included in the UV star formation history. We also explore the submillimeter flux ratio between the two bands for our 450 $\mu$m and 850 $\mu$m selected sources. At 850 $\mu$m, we find a clear relation between the flux ratio and the observed flux. This relation can be explained by a redshift evolution, where galaxies at higher redshifts have higher luminosities and star formation rates. In contrast, at 450 $\mu$m, we do not see a clear relation between the flux ratio and the observed flux.