An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: high-resolution dust continuum morphologies and the link between sub-millimetre galaxies and spheroid formation

B Gullberg,Ian Smail,D Scott,A P Thomson,C Conselice,A Weiss,E A Cooke,U Dudzevičiūtė,O Almaini,J L Wardlow,S M Stach,C C Chen,R J Ivison,M J Michałowski,D Farrah,J M Simpson,D Maltby,A M Swinbank

doi:10.1093/mnras/stz2835

Abstract

ABSTRACT We present an analysis of the morphology and profiles of the dust continuum emission in 153 bright sub-millimetre galaxies (SMGs) detected with ALMA at signal-to-noise ratios of >8 in high-resolution 0.18 arcsec (∼1 kpc) 870 $\mu$m maps. We measure sizes, shapes, and light profiles for the rest-frame far-infrared emission from these luminous star-forming systems and derive a median effective radius (Re) of 0.10 ± 0.04 arcsec for our sample with a median flux of S870 = 5.6 ± 0.2 mJy. We find that the apparent axial ratio (b/a) distribution of the SMGs peaks at b/a ∼ 0.63 ± 0.02 and is best described by triaxial morphologies, while their emission profiles are best fitted by a Sérsic model with n ≃ 1.0 ± 0.1, similar to exponential discs. This combination of triaxiality and n ∼ 1 Sérsic index are characteristic of bars and we suggest that the bulk of the 870 $\mu$m dust continuum emission in the central ∼2 kpc of these galaxies arises from bar-like structures. As such we caution against using the orientation of shape of the bright dust continuum emission at $\eqsim$ resolution to assess either the orientation of any disc on the sky or tits inclination. By stacking our 870 $\mu$m maps we recover faint extended dust continuum emission on ∼4 kpc scales which contributes 13 ± 1 per cent of the total 870 $\mu$m emission. The scale of this extended emission is similar to that seen for the molecular gas and rest-frame optical light in these systems, suggesting that it represents an extended dust and gas disc at radii larger than the more active bar component. Including this component in our estimated size of the sources we derive a typical effective radius of ≃0.15 ± 0.05 arcsec or 1.2 ± 0.4 kpc. Our results suggest that kpc-scale bars are ubiquitous features of high star-formation rate systems at $z$ ≫ 1, while these systems also contain fainter and more extended gas and stellar envelopes. We suggest that these features, seen some 10–12 Gyr ago, represent the formation phase of the earliest galactic-scale components: stellar bulges.

Highlights

Sub-millimetre galaxies (SMGs) are a class of high-redshift dust obscured, but far-infrared luminous, galaxies with estimated starformation rates of ∼100–1000 M yr−1 (Smail, Ivison & Blain 1997; Barger et al 1998; Hughes et al 1998).The high star-formation rates are similar to those measured for local ultraluminous infrared galaxies (ULIRGs; e.g. Sanders & Mirabel 1996; Tacconi et al 2008; Engel et al 2010; Riechers et al 2011; Bothwell et al 2013)
We first assess the spatial extent of the dust continuum emission in our high-resolution observations of SMGs through measurement in both the uv-amplitude plane, and fitting models to the image plane maps
We analyse the dust continuum morphologies and light profiles of 153 well-detected (S/N > 8) SMGs observed with ALMA at 0.18 arcsec (∼1 kpc) resolution

Summary

Introduction

Sub-millimetre galaxies (SMGs) are a class of high-redshift dust obscured, but far-infrared luminous, galaxies with estimated starformation rates of ∼100–1000 M yr−1 (Smail, Ivison & Blain 1997; Barger et al 1998; Hughes et al 1998).The high star-formation rates are similar to those measured for local ultraluminous infrared galaxies (ULIRGs; e.g. Sanders & Mirabel 1996; Tacconi et al 2008; Engel et al 2010; Riechers et al 2011; Bothwell et al 2013). The high star-formation rates are similar to those measured for local ultraluminous infrared galaxies The spatial extents of local (U)LIRGs have been shown to vary strongly depending upon the observed wavelength: with the highest star-formation rate (U)LIRGs displaying the most extended emission in the optical, while at the same time showing the most compact emission in the mid-infrared, which is thought to trace the on-going star formation (Chen, Lowenthal & Yun 2010; Psychogyios et al 2016). Comparisons of the rest-frame optical and far-infrared sizes of high-redshift SMGs have suggested similar behaviour, with much more extended optical sizes, compared to those derived from interferometric observations in the submillimetre, which trace the bulk of the star-formation activity visible in the rest-frame far-infrared waveband (Ikarashi et al 2015; Simpson et al 2015b; Hodge et al 2016; Lutz et al 2016)

Results

Discussion

Conclusion