AzTEC half square degree survey of the SHADES fields - II. Identifications, redshifts and evidence for large-scale structure

Abstract

The Astronomical Thermal Emission Camera (AzTEC) 1.1 mm survey of the two SCUBA HAlf Degree Extragalactic Survey (SHADES) fields is the largest (0.7 deg2) blank‐field millimetre‐wavelength (mm‐wavelength) survey undertaken to date at a resolution of ≃18 arcsec and a depth of ≃1 mJy. We have used the deep optical to radio multiwavelength data in the SHADES Lockman Hole East and SXDF/UDS fields to obtain galaxy identifications for ≃64 per cent (≃80 per cent including tentative identifications) of the 148 AzTEC‐SHADES 1.1 mm sources reported by Austermann et al., exploiting deep radio and 24 μm data complemented by methods based on 8 μm flux density and red optical–infrared (i − K) colour. This unusually high identification rate can be attributed to the relatively bright mm‐wavelength flux density threshold, combined with the relatively deep supporting multifrequency data now available in these two well‐studied fields. We have further exploited the optical–mid‐infrared–radio data to derive an ≃60 per cent (≃75 per cent including tentative identifications) complete redshift distribution for the AzTEC‐SHADES sources, yielding a median redshift of z ≃ 2.2, with a high‐redshift tail extending to at least z ≃ 4. Despite the larger area probed by the AzTEC survey relative to the original SCUBA‐SHADES imaging, the redshift distribution of the AzTEC sources is consistent with that displayed by the SCUBA sources, and reinforces tentative evidence that the redshift distribution of mm/submm sources in the Lockman Hole field is significantly different from that found in the SXDF/UDS field. Comparison with simulated surveys of similar scale extracted from semi‐analytic models based on the Millennium simulation indicates that this is as expected if the mm/submm sources are massive (M > 1011 M⊙) star‐forming galaxies tracing large‐scale structures over scales of 10–20 Mpc. This confirms the importance of surveys covering several deg2 (as now underway with SCUBA2) to obtain representative samples of bright (sub)mm‐selected galaxies. This work provides a foundation for the further exploitation of the Spitzer and Herschel data in the SHADES fields in the study of the stellar masses and specific star formation rates of the most active star‐forming galaxies in cosmic history.

Simulated observations of sub-millimetre galaxies: the impact of single-dish resolution and field variance

“…We find that for the bright population the distribution of p-values is similar to that obtained if we perform the same operation with 50 random samplings of the parent field, though with a slightly more significant low p-value tail. Approximately 10 per cent of the field pairs exhibit p < 0.05, suggesting that it is not necessarily as uncommon as one would expect by chance to find that redshift distributions derived from non-contiguous pencil beams of sky fail the KS test, as in Michałowski et al (2012). For the faint population, 92 per cent of the field pairs have p < 0.05.…”

“…In particular, Michałowski et al (2012) found evidence that photometric redshift distributions of radio-identified counterparts of 1100 and 850 µm selected SMGs in the two non-contiguous SCUBA Half-Degree Extragalactic Survey (SHADES) fields are inconsistent with being drawn from the same parent distribution. This suggests that the SMGs are tracing different large-scale structures in the two fields.…”

A Machine-learning Method for Identifying Multiwavelength Counterparts of Submillimeter Galaxies: Training and Testing Using AS2UDS and ALESS

“…For the machine-learning analysis, we note that previous work has shown that SMGs are in general at high redshift, are relatively bright in the rest-frame near-infrared, and have red colors in the optical and near-infrared wavebands (e.g., Smail et al 2002;Chapman et al 2005;Hainline et al 2009;Michałowski et al 2012;Wang et al 2012;Alberts et al 2013;Simpson et al 2014;. To compare the properties of the SMGs to the field, we use as our ("non-SMG") control sample those K-band-detected sources that are located within the primary beams of our ALMA maps but are >1 6 away from an ALMA-identified SMG.…”

“…We will focus on the radio and redder optical and near-infrared bands, as the dusty, starforming SMGs are expected to be typically brighter in these wavebands than the bulk of the field population (e.g., Wardlow et al 2011;Michałowski et al 2012;Hodge et al 2013;Simpson et al 2014). …”

“…Traditionally, the likely counterparts for single-dish submillimeter sources were identified by using indirect tracers of the far-infrared/submillimeter emission, such as the radio, 24 μm, or mid-infrared properties (e.g., Ivison et al 1998;Smail et al 2002;Pope et al 2006;Ivison et al 2007;Barger et al 2012;Michałowski et al 2012;Cowie et al 2017). These properties roughly track the far-infrared luminosity of galaxies, and they have two additional advantages: that observations in these bands are typically at significantly higher angular resolution than the submillimeter, and that the surface densities of sources in these wavebands are relatively low, so that the rate of chance associations is also low.…”