Sub-Kiloparsec Imaging of Cool Molecular Gas in Two Strongly Lensed Dusty, Star-Forming Galaxies

Abstract

We present spatially-resolved imaging obtained with the Australia Telescope Compact Array (ATCA) of three CO lines in two high-redshift gravitationally lensed dusty star-forming galaxies, discovered by the South Pole Telescope. Strong lensing allows us to probe the structure and dynamics of the molecular gas in these two objects, at z = 2.78 and z = 5.66, with effective source-plane resolution of less than 1 kpc. We model the lensed emission from multiple CO transitions and the dust continuum in a consistent manner, finding that the cold molecular gas as traced by low-J CO always has a larger half-light radius than the 870 µm dust continuum emission. This size difference leads to up to 50% differences in the magnification factor for the cold gas compared to dust. In the z=2.78 galaxy, these CO observations confirm that the background source is undergoing a major merger, while the velocity field of the other source is more complex. We use the ATCA CO observations and comparable resolution Atacama Large Millimeter/submillimeter Array dust continuum imaging of the same objects to constrain the CO-H 2 conversion factor with three different procedures, finding good agreement between the methods and values consistent with those found for rapidly star-forming systems. We discuss these galaxies in the context of the star formation -gas mass surface density relation, noting that the change in emitting area with observed CO transition must be accounted for when comparing high-redshift galaxies to their lower redshift counterparts.

NEW CONSTRAINTS ON THE MOLECULAR GAS IN THE PROTOTYPICAL HyLIRGs BRI 1202–0725 AND BRI 1335–0417

“…Since both are seen in CO emission, this suggests that the northern area is not starbursting. This size discrepancy between low order CO and star formation has been noted for other high redshift SFGs (star forming galaxies; e.g., as compiled by Spilker et al 2015). Alternatively, the different sizes could simply be a result of low signal-to-noise continuum observations not detecting the lower optical depth outer sections of the star forming regions.…”

ISM Properties of a Massive Dusty Star-forming Galaxy Discovered at z ∼ 7

“…We use a fixed CO abundance of 8 10 5 -relative to H 2 (Frerking et al 1982), but keep the C I [ ] abundance and the GDMR as free parameters. For the frequency dependence of the dust absorption coefficient we adopt 0.04 250 GHz & Siebenmorgen 1994), which is in good Spilker et al 2015 and references therein), for our bestfitting β.…”

The Strong Gravitationally Lensed Herschel Galaxy HLock01: Optical Spectroscopy Reveals a Close Galaxy Merger with Evidence of Inflowing Gas

“…It is thus no surprise that most high-z close mergers (10 kpc projected separation) have been discovered through the gravitational lensing effect (e.g., Ivison et al 2010b;MacKenzie et al 2014;Messias et al 2014;Rawle et al 2014;Wuyts et al 2014;Spilker et al 2015;Marrone et al 2018), with a few exceptions of unlensed, well separated, SMG-SMG, SMG-QSO or SMG-LBG bright interacting pairs resolved with interferometric observations (e.g., Ivison et al 2002Ivison et al , 2008Smail et al 2003;Salomé et al 2012;Oteo et al 2016;Lu et al 2017;Riechers et al 2017). …”