Evolution of Dust-obscured Star Formation and Gas to z = 2.2 from HiZELS

Abstract

We investigate the far-infrared (far-IR) properties of galaxies selected via deep, narrow-band imaging of the Hα emission line in four redshift slices from $z=0.40\mbox{--}2.23$ over ~1 deg2 as part of the High-redshift Emission Line Survey (HiZELS). We use a stacking approach in the Herschel PACS/SPIRE far-IR bands, along with $850\,\mu {\rm{m}}$ imaging from SCUBA-2 and Very Large Array 1.4 GHz imaging, to study the evolution of the dust properties of Hα-emitters selected above an evolving characteristic luminosity threshold, $0.2{L}_{{\rm{H}}\alpha }^{\star }(z)$. We investigate the relationship between the dust temperatures, T dust, and the far-infrared luminosities, L IR, of our stacked samples, finding that our Hα-selection identifies cold, low-L IR galaxies (${T}_{\mathrm{dust}}\sim 14$ K; $\mathrm{log}[{L}_{\mathrm{IR}}/{L}_{\odot }]\sim 9.9$) at z = 0.40, and more luminous, warmer systems (${T}_{\mathrm{dust}}\sim 34$ K; $\mathrm{log}[{L}_{\mathrm{IR}}/{L}_{\odot }]\sim 11.5$) at z = 2.23. Using a modified graybody model, we estimate "characteristic sizes" for the dust-emitting regions of Hα-selected galaxies of ~0.5 kpc, nearly an order of magnitude smaller than their stellar continuum sizes, which may provide indirect evidence of clumpy interstellar medium structure. Lastly, we use measurements of the dust masses from our far-IR stacking along with metallicity-dependent gas-to-dust ratios (${\delta }_{\mathrm{GDR}}$) to measure typical molecular gas masses of $\sim 1\times {10}^{10}\,{M}_{\odot }$ for these bright Hα-emitters. The gas depletion timescales are shorter than the Hubble time at each redshift, suggesting probable replenishment of their gas reservoirs from the intergalactic medium. Based on the number density of Hα-selected galaxies, we find that typical star-forming galaxies brighter than $0.2{L}_{{\rm{H}}\alpha }^{\star }(z)$ comprise a significant fraction (35 ± 10%) of the total gas content of the universe, consistent with the predictions of the latest state-of-the-art cosmological simulations.