An ALMA survey of sub-millimetre Galaxies in the Extended Chandra Deep Field South: the far-infrared properties of SMGs

Abstract

We exploit ALMA 870-µm (345 GHz) observations of sub-millimetre sources in the Extended Chandra Deep Field South to investigate the far-infrared properties of highredshift sub-millimetre galaxies (SMGs). Using the precisely located 870 µm ALMA positions of 99 SMGs, together with 24µm and radio imaging of this field, we deblend the Herschel / SPIRE imaging of this region to extract their far-infrared fluxes and colours. The median redshifts for ALMA LESS (ALESS) SMGs which are detected in at least two SPIRE bands increases as expected with wavelength of the peak in their SEDs, with z = 2.3 ± 0.2, 2.5 ± 0.3 and 3.5 ± 0.5 for the 250, 350 and 500-µm peakers respectively. We find that 34 ALESS SMGs do not have a >3 σ counterpart at 250, 350 or 500µm. These galaxies have a median photometric redshift derived from the rest-frame UV-mid-infrared SEDs of z = 3.3 ± 0.5, which is higher than the full ALESS SMG sample; z = 2.5 ± 0.2. Using the photometric redshifts together with the 250-870µm photometry, we estimate the far-infrared luminosities and characteristic dust temperature of each SMG. The median infrared luminosity and characteristic dust temperature of the S 870µm > 2 mJy SMGs is L IR = (3.0 ± 0.3) × 10 12 L ⊙ (star formation rate of SFR = 300 ± 30 M ⊙ yr −1 ) and T d = 32 ± 1 K (λ peak = 93 ± 4 µm). At a fixed luminosity, the characteristic dust temperature of these high-redshift SMGs is ∆T d = 3-5 K lower than comparably luminous galaxies at z = 0, reflecting the more extended star formation occurring in these systems. By extrapolating the 870µm number counts to S 870 = 1 mJy, we show that the contribution of S 870µm ≥ 1 mJy SMGs to the cosmic star formation budget is 20% of the total over the redshift range z ∼ 1-4. We derive a median dust mass for these galaxies of M d = (3.6 ± 0.3) × 10 8 M ⊙ and by adopting an appropriate gas-to-dust ratio, we estimate that the typical molecular mass of the ALESS SMGs in our sample is M H2 = (4.2 ± 0.4) × 10 10 M ⊙ . Together with the average stellar masses of SMGs, M ⋆ = (8 ± 1) × 10 10 M ⊙ , this suggests an average molecular gas fraction of ∼ 40%. Finally, we use our estimates of the H 2 masses to show that SMGs with S 870µm > 1 mJy (L IR > ∼ 10 12 L ⊙ ) contain ∼ 10% of the z ∼ 2 volume-averaged H 2 mass density at this epoch.

Dusty star-forming galaxies at high redshift

“…A few high-z SMGs which have CO(1-0) observations have also been used to calibrate galaxies' gas-to-dust ratio ∼100, which has been subsequently used to estimate some high-z DSFGs' gas masses (e.g. Swinbank et al, 2013). It should be noted that there is a degeneracy between dust temperature and the emissivity spectral index β similar, yet not as pronounced as the degeneracy between dust temperature and assumed opacity model.…”

“…This is thought to be due to more extended dust distributions (e.g. Swinbank et al, 2013) in high-z DSFGs on scales > ∼ 2 kpc, versus the more compact ISM seen in local ULIRGs ≈1 kpc (see more on DSFGs' sizes in § 5.10).…”

Cold dust emission from X-ray AGN in the SCUBA-2 Cosmology Legacy Survey: dependence on luminosity, obscuration and AGN activity

“…The Herschel-SPIRE data in the COSMOS field is taken from the Herschel Multi-tiered Extragalactic Survey (Oliver et al 2012). Maps have been produced using the Level 2 data products from the ESA archive as detailed in Swinbank et al (2014). While the stacked 250-μm emission appears to be well centred at the radio position, the 500-μm emission shows an offset relative to the radio position.…”

“…Herschel fluxes for all 850-μm detected sources are extracted from the catalogues of Swinbank et al (2014) by matching the optical positions of the AGN in our catalogue to the 24-μm positions in the Herschel catalogues, using a matching radius of 3 arcsec. The Herschel catalogues have already been corrected for blending using the 24-μm positional priors (Swinbank et al 2014).…”

“…The Herschel catalogues have already been corrected for blending using the 24-μm positional priors (Swinbank et al 2014). These fluxes are presented in Appendix B together with other derived properties for these submillimeter bright X-ray AGN.…”