Do Unusually Cold Starburst Galaxies Exist? A Case Study

We report observations of CO($J$=9$\to$8) and OH$^+$($N$=1$\to$0) toward the four millimeter-selected lensed starburst galaxies SPT 2354-58, 0150-59, 0314-44, and 0452-50, using the Atacama Large Millimeter/submillimeter Array Compact Array (ALMA/ACA), as part of a larger study of OH$^+$ in the early universe. In this work, we use these observations for the main purpose of spectroscopic redshift measurements. For all sources except SPT 0452-50, we confirm the previously reported most likely redshifts, and we find typical CO and OH$^+$ properties for massive starbursts. For SPT 0452-50, we rule out the previously reported value of $z$=2.0105, measuring a firm redshift of $z$=5.0160 based on [OI], [CII], H$_2$O, and CO emission instead when adding in ancillary ALMA data. Previously, SPT 0452-50 was considered an outlier in relations between dust temperature, far-infrared luminosity and redshift, which may have hinted at an unusually cold starburst with a dust temperature of only $T_{\rm dust}$=(21$\pm$2) K. Instead, our new measurements suggest it to be among highly luminous massive dusty starbursts at $z$$>$5, with rather typical properties within that population. We find a revised dust temperature of $T_{\rm dust}$=(76.2$\pm$2.5) K, and an updated lensing-corrected far-infrared luminosity (42.5-122.5 $\mu$m) of (2.35$^{+0.09}_{-0.08}$)$\times$10$^{13}$ $L_\odot$ - i.e., about an order of magnitude higher than previously reported. We thus do not find evidence for the existence of unusually cold starburst galaxies in the early universe that were missed by previous selection techniques.