A${}^3$COSMOS: The Dust Attenuation of Star-Forming Galaxies at $z=2.5-4.0$ from the COSMOS-ALMA Archive

Abstract

We present an analysis of the dust attenuation of star forming galaxies at $z=2.5-4.0$ through the relationship between the UV spectral slope (${\hat{I}}^2$), stellar mass ($M_{\ast }$) and the infrared excess (IRX$=L_{\mathrm{IR}}/L_{\mathrm{UV}}$) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A${}^3$COSMOS team, which includes an unprecedented sample of $\sim 1500$ galaxies at $z\sim 3$ as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 $\hat{\mathrm{I}}¼Jy/beam$ (1$\ddot{I}ƒ$). The detection rate is highly mass dependent, decreasing drastically below $\log (M_{\ast }/M_{\odot })=10.5$. The detected galaxies show that the IRX-${\hat{I}}^2$ relationship of massive ($\log M_{\ast }/M_{\odot }>10$) main sequence galaxies at $z=2.5-4.0$ is consistent with that of local galaxies, while starbursts are generally offset by $\sim 0.5\mathrm{dex}$ to larger IRX values. At the low mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX-$M_{\ast }$ relation at $\log ({\mathrm{M}}_{\ast }/{\mathrm{M}}_{\odot })>9$ exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at $\log M_{\ast }/M_{\odot }<10$. However, our results are consistent with early measurements at $z\sim 5.5$, indicating a potential redshift evolution between $z\sim 2$ and $z\sim 6$. Deeper observations targeting low mass galaxies will be required to confirm this finding.

12 pages, 5 figures, MNRAS submitted