Searching Far and Long I: Pilot ALMA 2mm Follow-up of Bright Dusty Galaxies as a Redshift Filter

Abstract

A complete census of dusty star-forming galaxies (DSFGs) at early epochs is necessary to constrain the obscured contribution to the cosmic star formation rate density (CSFRD), however DSFGs beyond $z\sim 4$ are both rare and hard to identify from photometric data alone due to degeneracies in submillimeter photometry with redshift. Here, we present a pilot study obtaining follow-up Atacama Large Millimeter Array (ALMA) $2$mm observations of a complete sample of 39 $850\mu \mathrm{m}$-bright dusty galaxies in the SSA22 field. Empirical modeling suggests $2$mm imaging of existing samples of DSFGs selected at $850\mu \mathrm{m}-1$mm can quickly and easily isolate the "needle in a haystack" DSFGs that sit at $z>4$ or beyond. Combining archival submillimeter imaging with our measured ALMA $2$mm photometry ($1\sigma \sim 0.08$mJybeam${}^{-1}$ rms), we characterize the galaxies' IR SEDs and use them to constrain redshifts. With available redshift constraints fit via the combination of six submillimeter bands, we identify 6/39 high-$z$ candidates each with $>50\%$ likelihood to sit at $z>4$, and find a positive correlation between redshift and $2$mm flux density. Specifically, our models suggest the addition of $2$mm to a moderately constrained IR SED will improve the accuracy of a millimeter-derived redshift from $\Delta z/(1+z)=0.3$ to $\Delta z/(1+z)=0.2$. Our IR SED characterizations provide evidence for relatively high emissivity spectral indices ($⟨\beta ⟩=2.4\pm 0.3$) in the sample. We measure that especially bright ($S_{850\mu \mathrm{m}}>5.55$mJy) DSFGs contribute $\sim 10$% to the cosmic-averaged CSFRD from $2<z<5$, confirming findings from previous work with similar samples.

22 pages, 7 figures, accepted for publication in ApJ

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