Abstract
ABSTRACT We investigate the balance of power between stars and AGN across cosmic history, based on the comparison between the infrared (IR) galaxy luminosity function (LF) and the IR AGN LF. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. We find that at all redshifts (at least up to z ∼ 2.5), the high-luminosity tails of the two LFs converge, indicating that the most IR-luminous galaxies are AGN-powered. Our results shed light to the decades-old conundrum regarding the flatter high-luminosity slope seen in the IR galaxy LF compared to that in the UV and optical. We attribute this difference to the increasing fraction of AGN-dominated galaxies with increasing total IR luminosity (LIR). We partition the LIR−z parameter space into a star formation-dominated and an AGN-dominated region, finding that the most luminous galaxies at all epochs lie in the AGN-dominated region. This sets a potential ‘limit’ to attainable star formation rates, casting doubt on the abundance of ‘extreme starbursts’: if AGN did not exist, LIR > 1013 L⊙ galaxies would be significantly rarer than they currently are in our observable Universe. We also find that AGN affect the average dust temperatures (Tdust) of galaxies and hence the shape of the well-known LIR−Tdust relation. We propose that the reason why local ULIRGs are hotter than their high-redshift counterparts is because of a higher fraction of AGN-dominated galaxies amongst the former group.
Highlights
In star-forming galaxies, a significant fraction of the stellar UV and optical radiation is absorbed by dust and re-emitted in the infrared (IR)
Note that we examine the behaviour of the luminosity function (LF) up to z ∼ 2.5, because as discussed in G13, there is a severe lack of spectroscopic redshifts amongst the population that makes up the IR LF at z > 2.5
In the bins where the G13 and A15 results do not cover exactly the same redshift range, we evaluated the parametric model of the A15 LF at the the centre of the G13 bins, finding the mean shift to be negligible at the bright end, so we use the original redshift bins for φIR, AGN in Fig. 1, as the AGN luminosity densities were calculated in those bins in A15
Summary
In star-forming galaxies, a significant fraction of the stellar UV and optical radiation is absorbed by dust and re-emitted in the infrared (IR). Symeonidis et al (2016; hereafter S16) and Symeonidis (2017; hereafter S17) challenged the idea that far-IR emission is in all cases primarily powered by star formation by showing that powerful AGN can dominate the entire IR spectral energy distribution (SED). The implications of this are that the correlation between IR luminosity and SFR must break down at high luminosities, at which point SFRs derived from IR broad-band photometry would be significantly overestimated.
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