A survey of molecular gas in luminous sub-millimetre galaxies

Abstract

We present the results from a survey for 12 CO emission in 40 luminous sub-millimetre galaxies (SMGs), with 850-µm fluxes of S 850µm = 4 − 20 mJy, conducted with the Plateau de Bure Interferometer. We detect 12 CO emission in 32 SMGs at z ∼ 1.2 -4.1, including 16 SMGs not previously published. Using multiple 12 CO line (J up = 2-7) observations, we derive a median spectral line energy distribution for luminous SMGs and use this to estimate a mean gas mass of (5.3 ± 1.0) × 10 10 M . We report the discovery of a fundamental relationship between 12 CO FWHM and 12 CO line luminosity in high-redshift starbursts, which we interpret as a natural consequence of the baryon-dominated dynamics within the regions probed by our observations. We use far-infrared luminosities to assess the star-formation efficiency in our SMGs, finding a steepening of the L CO -L FIR relation as a function of increasing 12 CO J up transition. We derive dynamical masses and molecular gas masses, and use these to determine the redshift evolution of the gas content of SMGs, finding that they do not appear to be significantly more gas rich than less vigorously star-forming galaxies at high redshifts. Finally, we collate X-ray observations, and study the interdependence of gas and dynamical properties of SMGs with their AGN activity and supermassive black hole masses (M BH ), finding that SMGs lie significantly below the local M BH -σ relation. We conclude that SMGs represent a class of massive, gas-rich ultraluminous galaxies with somewhat heterogeneous properties, ranging from starbursting disc-like systems with L∼ 10 12 L , to the most highly star-forming mergers in the Universe.

Dusty star-forming galaxies at high redshift

“…Lotz et al, 2008;Kartaltepe et al, 2010Kartaltepe et al, , 2012. Similarly, much of the work on SMGs indicate that most have major merger histories with short duty cycles (Engel et al, 2010;Alaghband-Zadeh et al, 2012;Bothwell et al, 2013a). These results are closely tied with whether or not a main sequence galaxy can have a merger origin.…”

“…L CO and CO line widths are correlated for unlensed galaxies (Bothwell et al, 2013a), while lensed galaxies show a clear departure from this relation with a flat trend (Harris et al, 2012) (Fig. 37).…”

“…At higher redshifts, the best measurements of DSFG timescales come from 850µm-selected SMGs with CO measurements Greve et al, 2005;Tacconi et al, 2006Tacconi et al, , 2008Coppin et al, 2008b;Bothwell et al, 2010;Casey et al, 2011a;Bothwell et al, 2013a). Bothwell et al (2013a) provides a summary of all CO surveys directed at 850µm-selected sources, and more recently summarizes molecular gas surveys at high-z.…”

“…Bothwell et al (2013a) provides a summary of all CO surveys directed at 850µm-selected sources, and more recently summarizes molecular gas surveys at high-z. As described later in § 8, these surveys measure typical depletion timescales of ∼100-200 Myr for SMGs versus much longer ∼1 Gyr timescales for normal galaxies (e.g.…”

“…For example, CO dynamical mass measurements (e.g. Greve et al, 2005;Tacconi et al, 2008;Bothwell et al, 2013a) in combination with an assumed dark halo profile can provide a constraint on the remaining stellar mass. This neglects any contribution to the mass by HI, and comes with the uncertainty of an assumed dynamical state of SMGs.…”