Evidence for Extended, Obscured Starbursts in Submillimeter Galaxies

Abstract

We compare high-resolution optical and radio imaging of 12 luminous submillimeter (submm) galaxies at a median z = 2.2 ± 0.2 observed with Hubble Space Telescope (HST) and the MERLIN and VLA radio interferometers at comparable spatial resolution, ∼ 0.3 ′′ (∼ 2 kpc). The radio emission is used as a tracer of the likely far-infrared morphology of these dusty, luminous galaxies. In ∼30% of the sample the radio emission appears unresolved at this spatial scale, suggesting that the power source is compact and may either be an obscured AGN or a compact nuclear starburst. However, in the majority of the galaxies, ∼70% (8/12), we find that the radio emission is resolved by MERLIN/VLA on scales of ∼ 1 ′′ (∼10 kpc). For these galaxies we also find that the radio morphologies are often broadly similar to their restframe UV emission traced by our HST imaging. To assess whether the radio emission may be extended on even larger scales, ≫ 1 ′′ , resolved out by the MERLIN+VLA synthesized images, we compare VLA B-array (5 ′′ beam) to VLA A-array (1.5 ′′ beam) fluxes for a sample of 50 µJy radio sources, including 5 submm galaxies. The submm galaxies have comparable fluxes at these resolutions and we conclude that the typical radio emitting region in these galaxies are unlikely to be much larger than ∼ 1 ′′ (∼10 kpc). We discuss the probable mechanisms for the extended emission in these galaxies and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median star formation rates for these galaxies are ∼ 1700 M ⊙ yr −1 (M> 0.1M ⊙ ) occuring within regions with typical sizes of ∼40 kpc 2 , giving a star formation density of 45 M ⊙ yr −1 kpc −2 . Such vigorous and extended starburst appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission in these systems shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance between the structures seen in the two wavebands. We interpret these differences as resulting from highly structured internal obscuration within the submm galaxies, suggesting that their vigorous activity is producing wind-blown channels through their obscuring dust clouds. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Lyα emission in the spectra of these very dusty galaxies.

On the abundance of gravitational arcs produced by submillimeter galaxies at radio and submm wavelengths

“…In particular, Biggs & Ivison (BI08 hereafter) combined 1.4 GHz data from the VLA and MERLIN to produce high resolution radio maps of 12 SMGs detected in the Lockman Hole. The deconvolved sizes derived from these radio maps (obtained by fitting each map with an elliptical Gaussian) are consistent with Chapman et al (2004) (see also Muxlow et al 2005) and Tacconi et al (2006). Figure 1 shows the effective radius and axis ratio for each of the 12 radio sources reported in BI08 (black circles).…”

Dusty star-forming galaxies at high redshift

“…The first comprehensive rest-frame optical study of SMGs was done by Swinbank et al (2004) who spectroscopically confirm 30 galaxies in Hα emission whose redshifts were originally reported via detection of Lyα in Chapman et al (2003a) and Chapman et al (2004b). Swinbank et al measure [Nii]/Hα ratios and Hα line widths to deduce presence of AGN in at least 40% of the sample.…”

Cosmic evolution of submillimeter galaxies and their contribution to stellar mass assembly

“…Such extreme SFRs likely result from major mergers (e.g. Chapman et al 2004;Swinbank et al 2004;Greve et al 2005;Tacconi et al 2006Tacconi et al , 2008Younger et al 2007Younger et al , 2008Berciano Alba et al 2010;Narayanan et al 2009Narayanan et al , 2010 and cannot be sustained for a long period (after a few hundred Myr at most the gas reservoir should be depleted; see Greve et al 2005;Hainline et al 2006).…”