Krishnan, Charutha
(2020)
Tracking the activity of supermassive black holes and the habitats of their hosts.
PhD thesis, University of Nottingham.
Abstract
In this thesis, we explore the connections between active galactic nuclei (AGN) and the environments of their host galaxies across cosmic time, as viewed using three complementary angles. Much of this thesis is based on recently available, state-of- the-art data. Our galaxy catalogues comprise deep, multi-wavelength data provided by the UKIDSS Ultra Deep Survey (UDS) and Cosmic Evolution Survey (COSMOS). The UDS field contains a dense structure at z ∼ 1.6 with additional narrow-band imaging tailored to its redshift, allowing for the selection of protocluster members using precise photometric redshifts (Hatch et al. 2016). Both the UDS and COSMOS have coverage from X-ray telescopes such as Chandra and XMM-Newton, enabling the clean selection of AGN. This thesis also makes use of galaxy classifications (such as star-forming, passive, and post-starburst) based on principal component analysis (PCA) of broad-band photometry.
In our first study, we investigate the prevalence of AGN in the Cl 0218.3–0510 protocluster at z = 1.62. Analyses using imaging from the Chandra X-ray Telescope reveal a large overdensity of AGN in the protocluster by a factor of 23 ± 9 times the field density of AGN. The overdensity of massive galaxies in the protocluster is a factor of 11 ± 2, accounting for roughly half of the measured AGN overdensity. Likewise, we find that 17+6% of massive galaxies (M∗ > 1010 M⊙) in the protocluster host an X-ray luminous AGN, compared to 8 ± 1% in the field, corresponding to an enhancement of AGN activity in massive protocluster galaxies by a factor of 2.1 ± 0.7. This AGN overdensity is centrally concentrated, located within 3 arcmin and most pronounced within 1 arcmin of the centre of the protocluster. We find no significant differences in the distributions of AGN properties such as X-ray luminosity and hardness ratio, between AGN in the protocluster and the field. Using visually classified morphologies, we find tentative evidence that the fraction of “irregular” galaxies is also enhanced in the protocluster with respect to the field. From these results, we conclude that there is a reversal in the local anti-correlation between galaxy density and AGN activity, and that there is tentative evidence for a correlation between galaxy interactions and AGN activity.
Adopting a statistical approach, in our second study we investigate the relationship between AGN and dark matter halo mass using clustering techniques. We present evidence that X-ray selected AGN in the UDS and COSMOS fields show a clustering signal likely determined by the properties of their host galaxies, at all epochs from z ∼ 4.5 to z ∼ 0.5. Consistent with previous studies, we find that AGN are on average hosted by galaxies in dark matter halos of 10^12 − 10^13 M⊙, corresponding to group-like environments. However, we show that the same clustering signal can be produced by inactive (i.e. non-AGN) galaxies closely matched to the AGN in spectral class, stellar mass and redshift. We find that AGN in higher mass galaxies have a higher clustering signal, but that this stellar mass dependence disappears when passive host galaxies are removed. The strength of clustering is also largely independent of AGN X-ray luminosity. Therefore, the most important property that determines the clustering in a given AGN population appears to be the fraction of passive host galaxies. From these results we infer that AGN luminosity is likely not driven by environmental triggering, and conclude that AGN may be a stochastic phenomenon without a strong dependence on large-scale environment.
Finally, our third study presents a preliminary analysis of the properties of galaxy neighbours within 500 kpc of X-ray selected AGN between z ∼ 2.5 and z ∼ 0.5 in the UDS and COSMOS fields. At all epochs, we find consistent number densities of neighbours around AGN and control galaxies, suggesting that AGN do not live in special environments. At the highest redshifts (1.5 < z < 2.5), the neighbours of AGN are indistinguishable from those of control galaxies, since the star-forming properties of neighbours around AGN in passive/star-forming hosts are consistent with control passive/star-forming galaxies. We find that the star-formation activity of neighbours of AGN is more complex at lower redshifts, since we see opposite trends below and above z = 1. At 1.0 < z < 1.5 we find that the star-formation activity in the neighbours of AGN is enhanced (at 2.3σ) with respect to neighbours of control galaxies, while this is reversed at 0.5 < z < 1.0 (at 4.3σ). This study must be repeated in smaller redshift intervals in order to develop a deeper understanding of these trends and to draw robust conclusions.
In conclusion of the work presented in this thesis, the majority of AGN (hosted by normal, star-forming galaxies) are likely triggered by the stochastic accretion of cold gas, without a dominant influence from the environment. It appears that galaxy mergers and interactions can play a role in triggering AGN, especially in passive host galaxies, but that they are not a crucial parameter for the vast majority of the AGN population.
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