Host galaxy colour gradients and accretion disc obscuration in AEGIS z ∼ 1 X-ray-selected active galactic nuclei

Abstract

We describe the effect of active galactic nucleus (AGN) light on host galaxy optical and UV‐optical colours, as determined from X‐ray‐selected AGN host galaxies at z∼ 1, and compare the AGN host galaxy colours to those of a control sample matched to the AGN sample in both redshift and stellar mass. We identify as X‐ray‐selected AGNs 8.7+4−3 per cent of the red‐sequence control galaxies, 9.8 ± 3 per cent of the blue‐cloud control galaxies and 14.7+4−3 per cent of the green‐valley control galaxies. The nuclear colours of AGN hosts are generally bluer than their outer colours, while the control galaxies exhibit redder nuclei. AGNs in blue‐cloud host galaxies experience less X‐ray obscuration, while AGNs in red‐sequence hosts have more, which is the reverse of what is expected from general considerations of the interstellar medium. Outer and integrated colours of AGN hosts generally agree with the control galaxies, regardless of X‐ray obscuration, but the nuclear colours of unobscured AGNs are typically much bluer, especially for X‐ray luminous objects. Visible point sources are seen in many of these, indicating that the nuclear colours have been contaminated by AGN light and that obscuration of the X‐ray radiation and visible light are therefore highly correlated. Red AGN hosts are typically slightly bluer than red‐sequence control galaxies, which suggests that their stellar populations are slightly younger. We compare these colour data to current models of AGN formation. The unexpected trend of less X‐ray obscuration in blue‐cloud galaxies and more in red‐sequence galaxies is problematic for all AGN feedback models, in which gas and dust is thought to be removed as star formation shuts down. A second class of models involving radiative instabilities in hot gas is more promising for red‐sequence AGNs but predicts a larger number of point sources in red‐sequence AGNs than is observed. Regardless, it appears that multiple AGN models are necessary to explain the varied AGN host properties discussed in the current work. Finally, we find that integrated optical and UV − optical colours are not strongly affected by X‐ray‐selected AGNs except in rare cases (<10 per cent), where the AGN is very luminous, unobscured and/or visible as a point source.

Cosmic X-ray surveys of distant active galaxies

“…The distinct difference between the locations of the AGNs and the optically selected galaxy populations in the CMD implies that distant AGNs are found in a subset of the galaxy population and, tentatively, suggests that they are the catalysts for the transition of galaxies from the blue cloud to the red sequence (e.g., through the suppression of star formation via energetic winds, outflows, and jets; see Veilleux et al 2005;Fabian 2012 for reviews). However, later studies showed that clear distinctions between the host-galaxy colors of the AGN and the galaxy populations mostly disappear when the galaxy sample is matched in mass to the AGN sample (e.g., Silverman et al 2009b;Xue et al 2010;Pierce et al 2010b;Rosario et al 2013b), with broadly similar fractions of coeval galaxies and AGNs found in the red sequence, green valley, and blue cloud out to at least z ≈ 3.…”

“…A number of different approaches for measuring host-galaxy morphologies have been developed (e.g., automated classifications based on the distribution of light in the galaxy, two-dimensional fits to the surface brightness profiles to provide disk/bulge decompositions, and human "eyeball" classification), which appear to give broadly similar results (e.g., Huertas-Company et al 2014). The contribution of the AGN to the optical emission can significantly bias morphological measurements (e.g., Simmons and Urry 2008;Pierce et al 2010b) and, therefore, the majority of the studies focus on optically obscured or intrinsically weak AGNs. Fig.…”

Decreased specific star formation rates in AGN host galaxies

“…Green colors can also just be an indication of a mixture of old and new stellar populations and not necessarily a "transition" between the two population. Further, recent studies that mass-match non-AGN galaxy samples to AGN host galaxies reveal that the difference in optical colors virtually disappear (Silverman et al 2009;Pierce et al 2010;Rosario et al 2013a,b) Far-infrared (FIR) emission (λ > 40µm) is essentially immune to reddening effects while also being a direct tracer of recent star formation. Dust in the galaxy is heated by UV photons from recently formed OB stars, that then reemit in the mid-far infrared regime (Draine 2003) creating a strong correlation between the FIR luminosity and SFR of a galaxy (Kennicutt 1998;Kennicutt & Evans 2012).…”

The Young, the Old, and the Dusty: Stellar Populations of Active Galactic Nucleus Hosts

“…A Gaussian stellar mass (M * ) distribution was used with M min * = 10 9.5 M ⊙ and M max * = 10 12 M ⊙ . The average M * was set at 10 10.9 M ⊙ with a standard deviation of 0.4 dex, in agreement with the sample of X-ray selected, z ∼ 1 AGN hosts presented by Pierce et al (2010). It is assumed that the mass-to-light ratio of the host galaxy is constant within each waveband and thus the host galaxy flux scales linearly with the host M * .…”

“…According to Figure 6 of Pierce et al (2010), type 1 and type 2 AGN tend to be found in the 0.55 < U-B < 1.4 and -23.5 < M B < -19 region of the CMD. This region of the CMD is well populated by the model adopted here as many combinations of AGN L X and host galaxy M * fit this criteria.…”