The nature of submillimetre and highly star-forming galaxies in the EAGLE simulation

Abstract

We exploit eagle, a cosmological hydrodynamical simulation, to reproduce the selection of the observed sub-millimeter (submm) galaxy population by selecting the model galaxies at z ≥ 1 with mock submm fluxes S 850µm ≥ 1 mJy (computed in post processing using radiative transfer techniques). There is a reasonable agreement between the galaxies within this sample and the properties of the observed submm population, such as their star formation rates (SFRs) at z < 3, redshift distribution and many integrated galaxy properties. We find that the bulk of the S 850µm ≥ 1 mJy model population is at z ≈ 2.5, and that they are massive galaxies (M * ∼ 10 11 M ) with high dust masses (M dust ∼ 10 8 M ), gas fractions ( f gas ≈ 50%) and SFRs ( M * ≈ 100 M yr −1 ). They have major and minor merger fractions similar to the general population, suggesting that mergers are not the primary driver of the model submm galaxies. Instead, the S 850µm ≥ 1 mJy model galaxies yield high SFRs primarily because they maintain a significant gas reservoir as a result of hosting an undermassive black hole. In addition, we find that not all 'highly star-forming' ( M * ≥ 80 M yr −1 ) eagle galaxies have submm fluxes S 850µm ≥ 1 mJy. Thus, we investigate the nature of z ≥ 1 highly star-forming 'Submm-Faint' galaxies (i.e., M * ≥ 80 M yr −1 but S 850µm < 1 mJy). We find they are similar to the model submm galaxies; being gas rich and hosting undermassive black holes, however they are typically lower mass (M * ∼ 10 10 M ) and are at higher redshifts (z > 4). These typically higher-redshift galaxies show stronger evidence for having been triggered by major mergers, and critically, they are likely missed by current submm surveys due to their higher dust temperatures. This suggests a potentially even larger contribution to the SFR density at z 3 from dust-obscured systems than implied by current observations.

An ALMA survey of the SCUBA-2 CLS UDS field: physical properties of 707 sub-millimetre galaxies

“…To select a sample of galaxies from the EAGLE model we use the largest volume in the simulation set -Ref-L0100N1504, which is a 100 cMpc on-a-side periodic box (total volume 10 6 cMpc 3 ). However, we note that the volume of even the largest published EAGLE simulation is too small to include significant numbers of high-redshift galaxies with star-formation rates (or predicted 870-µm flux densities) comparable to those seen in AS2UDS (McAlpine et al 2019). As a result, to match the observations as closely as possible, but also provide a statistical sample for our comparison, we select all 9,431 galaxies from EAGLE with SFR > 10 M yr −1 and z > 0.25, but also isolate the 100 most strongly star-forming galaxies in the redshift range z = 1.8-3.4 (the 16-84th percentile redshift range of our survey).…”

“…Finally, in Fig. 7, we overlay the predicted redshift distribution for SMGs with S 850 ≥ 1 mJy from the EAGLE simulation (McAlpine et al 2019). The median redshift for the EAGLE SMGs is z = 2.4 ± 0.1, with a sharp decrease above z ∼ 2.5, driven in part by an increasing dust temperature in sources at higher redshifts.…”

“…This yields predicted SEDs of model galaxies covering the rest-frame UV-to-radio wavelengths (e.g. Camps et al 2018;McAlpine et al 2019), and is calibrated against far-infrared observations from the Herschel Reference Survey (Boselli et al 2010). Our primary goal here is to run magphys on the model photometry of EAGLE galaxies and so test whether the uncertainties on the derived quantities from magphys encompass the known physical properties of the model galaxies.…”

“…The dashed line indicates the prior distribution. For comparison, we also overlay theoretical predictions for SMG type galaxies from McAlpine et al (2019) Middle: The stacked likelihood redshift probability distribution of a complete sample of 364 AS2UDS SMGs that have S 870 ≥ 3.6 mJy. We overlay a simple analytic model assuming that SMGs reside in haloes whose mass crosses a characteristic threshold of ∼4 × 10 12 M and accounting for the evolution of molecular gas fraction with redshift ( f mol , scaled by a factor of 2×, is shown as dashed line) in the halos.…”

“…Alternatively, samples of highly star-forming galaxies could represent a heterogeneous mix of these two classes of systems, encompassing both physical processes (e.g. Hayward et al 2011;Narayanan et al 2015;McAlpine et al 2019).…”