A survey of the cold molecular gas in gravitationally lensed star-forming galaxies atz> 2

Abstract

Using the Australia Telescope Compact Array (ATCA), we conducted a survey of CO J = 1 − 0 and J = 2 − 1 line emission towards strongly lensed high-redshift dusty star forming galaxies (DSFGs) previously discovered with the South Pole Telescope (SPT). Our sample comprises 17 sources that had CO-based spectroscopic redshifts obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX). We detect all sources with known redshifts in either CO J = 1 − 0 or J = 2 − 1. Twelve sources are detected in the 7-mm continuum. The derived CO luminosities imply gas masses in the range (0.5 − 11) × 10 10 M ⊙ and gas depletion timescales t dep < 200 Myr, using a CO to gas mass conversion factor α CO = 0.8 M ⊙ (K km s −1 pc 2 ) −1 . Combining the CO luminosities and dust masses, along with a fixed gas-to-dust ratio, we derive α CO factors in the range 0.4 − 1.8 M ⊙ (K km s −1 pc 2 ) −1 , similar to what is found in other starbursting systems. We find small scatter in α CO values within the sample, even though inherent variations in the spatial distribution of dust and gas in individual cases could bias the dust-based α CO estimates. We find that lensing magnification factors based on the CO linewidth to luminosity relation (µ CO ) are highly unreliable, but particularly when µ < 5. Finally, comparison of the gas and dynamical masses suggest that the average molecular gas fraction stays relatively constant at z = 2 − 5 in the SPT DSFG sample.

A Total Molecular Gas Mass Census in Z ∼ 2–3 Star-Forming Galaxies: Low-J Co Excitation Probes of Galaxies’ Evolutionary States

“…We do not find a statistically significant correlation and find that our sample spans a larger range in line luminosities after correcting by the known lensing magnification. This result is likely due to the inhomogeneous way in which our sample is selected when compared to Harris et al (2012), and is in line with the results of Carilli & Walter (2013) and Aravena et al (2016).…”

“…Harris et al (2012), Bothwell et al (2013), and Goto & Toft (2015) find a correlation between the CO line luminosity and the CO line FWHM for SMGs. However, Aravena et al (2016) donot find a significant correlation for their sample of dusty star-forming galaxies, nor doCarilli & Walter (2013) when evaluating a variety of different star-forming galaxy populations. A correlation between the line brightness and FWHM could occur if the CO line is tracing the gravitational potential occupied by the molecular gas,especially if the molecular gas is dominating the dynamical mass.…”

“…The relatively recent availability of Ka-band receivers on the Karl G. Jansky Very Large Array (VLA) and the Robert C. Byrd Green Bank Telescope (GBT) has led to a small but growing number of CO detections inz 2-3 galaxies that are complete down to the lowest rotational line, CO(1-0), which is crucial for tracing the coldest gas components (e.g., Harris et al 2010Harris et al , 2012Swinbank et al 2010;Danielson et al 2011;Ivison et al 2011Ivison et al , 2012Ivison et al , 2013Riechers et al 2011bRiechers et al , 2011cRiechers et al , 2011dThomson et al 2012Thomson et al , 2015Fu et al 2013;Sharon et al 2013Sharon et al , 2015; for CO(1-0) detections at other redshifts using other bands, see Carilli et al 2002;Greve et al 2003;Hainline et al 2006;Riechers et al 2006Riechers et al , 2009Riechers et al , 2011dRiechers et al , 2013Dannerbauer et al 2009;Aravena et al 2010Aravena et al , 2014Aravena et al , 2016Carilli et al 2010;Emonts et al 2011). , indicative of a multiphase molecular ISM including a previously unaccounted for cold gas reservoir (e.g., Harris et al 2010;Swinbank et al 2010;Danielson et al 2011;Ivison et al 2011;Thomson et al 2012;Bothwell et al 2013; see Riechers et al 2011c;Sharon et al 2013Sharon et al , 2015.…”

The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-Redshift Galaxies

“…with redshift (see also Riechers et al 2010;Casey et al 2011;Geach et al 2011;Aravena et al 2012Aravena et al , 2016Magnelli et al 2012;Bothwell et al 2013;Saintonge et al 2013;Tacconi et al 2013;Chapman et al 2015a;Genzel et al 2015). …”

“…To date, only some several hundred sources at > z 1 have been detected in a molecular gas tracer (typically the rotational transitions of the carbon monoxide 12 CO molecule; e.g., Carilli & Walter 2013). This sample is dominated by "extreme" sources, such as QSO host galaxies (e.g., Bertoldi et al 2003;Walter et al 2003;Weiß et al 2007;Wang et al 2013) or submillimeter galaxies (e.g., Frayer et al 1998;Neri et al 2003;Greve et al 2005;Bothwell et al 2013;Riechers et al 2013;Aravena et al 2016), with IR luminosities  L 10 IR 12  L and star formation rates (SFRs) 100  M yr −1 . These extreme sources might contribute significantly to the star formation budget in the universe at > z 4, but their role declines with cosmic time (Casey et al 2014).…”