An interferometric CO survey of luminous submillimetre galaxies

Abstract

In this paper, we present results from an Institut de Radio Astronomie Millimétrique (IRAM) Plateau de Bure millimetre‐wave Interferometer (PdBI) survey for carbon monoxide (CO) emission towards radio‐detected submillimetre galaxies (SMGs) with known optical and near‐infrared spectroscopic redshifts. Five sources in the redshift range z∼ 1–3.5 were detected, nearly doubling the number of SMGs detected in CO. We summarize the properties of all 12 CO‐detected SMGs, as well as six sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the submillimetre galaxy (SMG) population as a whole. The median CO line luminosity of the SMGs is 〈L′CO〉= (3.8 ± 2.0) × 1010 K km s‐1 pc2. Using a CO‐to‐H2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass 〈M(H2)〉= (3.0 ± 1.6) × 1010 M⊙ within an ∼ 2 kpc radius, approximately 4 times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high‐redshift radio galaxies (HzRGs) and quasi‐sellar objects (QSOs). The median CO FWHM linewidth is broad, 〈FWHM〉= 780 ± 320 km s−1, and the SMGs often have double‐peaked line profiles, indicative of either a merger or a disc. From their median gas reservoirs (∼ 3 × 1010 M⊙) and star formation rates (≳ 700 M⊙ yr−1), we estimate a lower limit on the typical gas‐depletion time‐scale of ≳ 40 Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs and suggests that negative feedback processes may play an important role in prolonging the gas consumption time‐scale. We find a statistically significant correlation between the far‐infrared and CO luminosities of the SMGs, which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts. The non‐linear nature of the correlation implies that SMGs have higher far‐infrared to CO luminosity ratios and possibly higher star formation efficiencies (SFEs), than local ULIRGs. Assuming a typical CO source diameter of θ∼ 0.5 arcsec (D∼ 4 kpc), we estimate a median dynamical mass of 〈Mdyn〉≃ (1.2 ± 1.5) × 1011 M⊙ for the SMG sample. Both the total gas and stellar masses imply that SMGs are very massive systems, dominated by baryons in their central regions. The baryonic and dynamical properties of these systems mirror those of local giant ellipticals and are consistent with numerical simulations of the formation of the most massive galaxies. We have been able to impose a lower limit of ≳ 5 × 10−6 Mpc−3 to the comoving number density of massive galaxies in the redshift range z∼ 2–3.5, which is in agreement with results from recent spectroscopic surveys and the most recent model predictions.

Dusty star-forming galaxies at high redshift

“…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.…”

“…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.…”

“…have allowed for a significant expansion in the number of high Σ SFR galaxies, and thus a window into star formation in more extreme environments accordingly, though the interpretation of the results is steeply dependent on the form of the CO-H 2 conversion factor assumed. The first major CO surveys that examined the KS relation at high-z were presented by Greve et al (2005) and Bouché et al (2007) which were limited to only the brightest sources (i.e. SMGs).…”

The nature of the [C ii] emission in dusty star-forming galaxies from the SPT survey

“…Hainline et al 2011;Micha lowski et al 2012) and gas-rich (Mgas ∼ 3 − 5 × 10 10 M ; e.g. Greve et al 2005;Tacconi et al 2010;Bothwell et al 2013b), and have star formation rates 1000 M yr −1 (e.g. Chapman et al 2005).…”

Molecular Gas in Galaxies at all Redshifts

“…The molecular content is derived from the CO luminosity, after applying a conversion factor adapted for ultraluminous galaxies (ULIRG). Their star formation efficiency, as traced by the ratio between the FIR and CO luminosities is strongly increasing with redshift (Greve et al 2005). In Figure 2.…”