Cosmology

Abstract:

The observed massive end of the galaxy stellar mass function is steeper than its predicted dark matter halo counterpart in the standard $\Lambda $CDM paradigm. In this paper, we investigate the impact of active galactic nuclei (AGN) feedback on star formation in massive galaxies. We isolate the impact of AGNs by comparing two simulations from the HORIZON suite, which are identical except that one also includes super massive black holes (SMBH), and related feedback models. This allows us to cross-identify individual galaxies between simulations and quantify the effect of AGN feedback on their properties, including stellar mass and gas outflows. We find that massive galaxies ($ \rm M_{*} \geq 10^{11} M_\odot $) are quenched by AGN feedback to the extent that their stellar masses decrease by up to 80% at $z=0$. SMBHs affect their host halo through a combination of outflows that reduce their baryonic mass, particularly for galaxies in the mass range $ \rm 10^9 M_\odot \leq M_{*} \leq 10^{11} M_\odot $, and a disruption of central gas inflows, which limits in-situ star formation. As a result, net gas inflows onto massive galaxies, $ \rm M_{*} \geq 10^{11} M_\odot $, drop by up to 70%. We measure a redshift evolution in the stellar mass ratio of twin galaxies with and without AGN feedback, with galaxies of a given stellar mass showing stronger signs of quenching earlier on. This evolution is driven by a progressive flattening of the $\rm M_{\rm SMBH}-M_* $ relation with redshift, particularly for galaxies with $\rm M_{*} \leq 10^{10} M_\odot $. $\rm M_{\rm SMBH}/M_*$ ratios decrease over time, as falling average gas densities in galaxies curb SMBH growth.

Abstract:

Sub-milllimetre galaxies (SMGs) are some of the most luminous star-forming galaxies in the Universe, however their properties remain hard to determine due to the difficulty of identifying their optical\slash near-infrared counterparts. One of the key steps to determining the nature of SMGs is measuring a redshift distribution representative of the whole population. We do this by applying statistical techniques to a sample of 761 850$\mu$m sources from the SCUBA-2 Cosmology Legacy Survey observations of the UKIDSS Ultra-Deep Survey (UDS) Field. We detect excess galaxies around $> 98.4$ per cent of the 850$\mu$m positions in the deep UDS catalogue, giving us the first 850$\mu$m selected sample to have virtually complete optical\slash near-infrared redshift information. Under the reasonable assumption that the redshifts of the excess galaxies are representative of the SMGs themselves, we derive a median SMG redshift of $z = 2.05 \pm 0.03$, with 68 per cent of SMGs residing between $1.07 < z < 3.06$. We find an average of $1.52\pm 0.09$ excess $K$-band galaxies within 12 arc sec of an 850$\mu$m position, with an average stellar mass of $2.2\pm 0.1 \times 10^{10}$ M$_\odot$. While the vast majority of excess galaxies are star-forming, $8.0 \pm 2.1$ per cent have passive rest-frame colours, and are therefore unlikely to be detected at sub-millimetre wavelengths even in deep interferometry. We show that brighter SMGs lie at higher redshifts, and use our SMG redshift distribution -- along with the assumption of a universal far-infrared SED -- to estimate that SMGs contribute around 30 per cent of the cosmic star formation rate density between $0.5 < z < 5.0$.