Observing the effect of galaxy environment on the evolution of active galactic nuclei in the era of multiplexed wide-field fibre-optic spectroscopy

In this thesis, we use the environment of active galactic nuclei (AGN) to test AGN unification. Furthermore, we analyse how interaction between the environment and host galaxy can influence AGN evolution. This is done for a range of galaxy densities, from the field to the cores of galaxy clusters.We utilise the high spectroscopic completeness of the Galaxy And Mass Assembly survey (GAMA) to perform a comparative analysis of the pair-wise environments of broad- and narrow-line AGN. Our observations show no difference in the frequency of broad- and narrow line AGN in all but the tightest galaxy pairs. Furthermore, our observations show no difference in the colour or star-forming properties of the neighbouring galaxies of AGN, contrary to previous works. These observations are consistent with AGN unification and indicate that close galaxy interactions may increase nuclear obscuration.Optically selected AGN preferentially inhabit infall regions of massive clusters, and are rarely found in the cluster core. Using the depth and completeness of GAMA, we optically select 451 AGN from 695 groups in the halo mass range 11.5 . log10(M200/Mʘ) . 14.5 at z 13.5 our results are similar to previous works on clusters with a deficit of AGN observed in the group core at 3.6! confidence. At lower halo masses no preferred location for AGN is seen.Weakly accreting radio AGN are found in dense galactic environments, and minor-mergers have been invoked as a plausible mechanism to ‘drip-feed’ these low-excitation radio galaxies (LERGs). We use deep optical imaging from the Dark Energy Camera Legacy Survey (DECaLS) to search for low-surface brightness tidal remnants in a sample of 189 LERGs. We observe such tidal features in 27+3.5 −3.0 per cent of LERGs compared to 31.5+2.5 −2.3 per cent of a control sample. Our results are thus inconsistent with the hypothesis that minor-mergers play a significant role in LERG fuelling

The impact of disturbed galaxy clusters on the kinematics of active galactic nuclei

© 2020 The Author(s) We produce a kinematic analysis of AGN-hosting cluster galaxies from a sample of 33 galaxy clusters selected using the X-ray Clusters Database (BAX) and populated with galaxies from the Sloan Digital Sky Survey Data Release 8. The 33 galaxy clusters are delimited by their relative intensity of member galaxy substructuring as a proxy to core merging to derive two smaller sub-samples of 8 dynamically active (merging) and 25 dynamically relaxed (non-merging) states. The AGN were selected for each cluster sub-sample by employing the WHAN diagram to the strict criteria of log10([N II]/Hα) ≥ −0.32 and EWHα ≥ 6 Å, providing pools of 70 mergings and 225 non-merging AGN sub-populations. By co-adding the clusters to their respective dynamical states to improve the signal-to-noise ratio of our AGN sub-populations we find that merging galaxy clusters on average host kinematically active AGN between 0-1.5r200 as r200 → 0, where their velocity dispersion profile (VDP) presents a significant deviation from the non-AGN sub-population VDP by ≳3σ. This result is indicative that the AGN-hosting cluster galaxies have recently coalesced on to a common potential. Further analysis of the composite distributions illustrates non-merging AGN-hosting sub-populations have, on average, already been accreted and predominantly lie within backsplash regions of the projected phase-space. This suggests merging cluster dynamical states hold relatively younger AGN sub-populations kinematically compared with those found in non-merging cluster dynamical states

The Discovery of 63 Giant Radio Galaxies in the FIRST Survey

Giant Radio Galaxies (GRGs) are Active Galactic Nuclei (AGN) with radio emission that extends over projected sizes $>0.7\,$Mpc. The large angular sizes associated with GRGs complicate their identification in radio survey images using traditional source finders. In this Note, we use DRAGNhunter, an algorithm designed to find double-lobed radio galaxies, to search for GRGs in the Faint Images of the Radio Sky at Twenty cm survey (FIRST). Radio and optical images of identified candidates are visually inspected to confirm their authenticity, resulting in the discovery of $63$ previously unreported GRGs.Comment: 5 pages, 1 Table. Catalog data available from https://doi.org/10.5281/zenodo.1009417

The effect of minor and major mergers on the evolution of low excitation radio galaxies

We use deep, μ r lesssim 28 mag arcsec−2, r-band imaging from the Dark Energy Camera Legacy Survey to search for past, or ongoing, merger activity in a sample of 282 low-excitation radio galaxies (LERGs) at z 4σ excess of major mergers in the LERGs with M * lesssim 1011 M⊙, with 10 ± 1.5% of these active galactic nuclei involved in such large-scale interactions compared to 3.2 ± 0.4% of control galaxies. This excess of major mergers in LERGs decreases with increasing stellar mass, vanishing by M * > 1011.3 M⊙. These observations show that minor mergers do not fuel LERGs, and are consistent with typical LERGs being powered by accretion of matter from their halo. Where LERGs are associated with major mergers, these objects may evolve into more efficiently accreting active galactic nuclei as the merger progresses and more gas falls on to the central engine

Galaxy and Mass Assembly (GAMA): active galactic nuclei in pairs of galaxies

There exist conflicting observations on whether or not the environment of broad- and narrowline active galatic nuclei (AGN) differ and this consequently questions the validity of the AGN unification model. The high spectroscopic completeness of the Galaxy and Mass Assembly (GAMA) survey makes it ideal for a comprehensive analysis of the close environment of galaxies. To exploit this, and conduct a comparative analysis of the environment of broad- and narrow-line AGN within GAMA, we use a double-Gaussian emission line fitting method to model the more complex line profiles associated with broad-line AGN. We select 209 type 1 (i.e. unobscured), 464 type 1.5–1.9 (partially obscured), and 281 type 2 (obscured) AGN within the GAMA II data base. Comparing the fractions of these with neighbouring galaxies out to a pair separation of 350 kpc h−1 and Δz < 0.012 shows no difference between AGN of different type, except at separations less than 20 kpc h−1 where our observations suggest an excess of type 2 AGN in close pairs. We analyse the properties of the galaxies neighbouring our AGN and find no significant differences in colour or the star formation activity of these galaxies. Further to this, we find that Σ5 is also consistent between broad- and narrow-line AGN. We conclude that the observations presented here are consistent with AGN unification

Galaxy And Mass Assembly (GAMA): The effect of galaxy group environment on active galactic nuclei

In galaxy clusters, efficiently accreting active galactic nuclei (AGN) are preferentially located in the infall regions of the cluster projected phase-space, and are rarely found in the cluster core. This has been attributed to both an increase in triggering opportunities for infalling galaxies, and a reduction of those mechanisms in the hot, virialised, cluster core. Exploiting the depth and completeness (98 per cent at r9.9 in 695 groups with 11.53≤log 10 (M 200 /M ⊙ )≤14.56 at z13.5 , AGN are preferentially found in the infalling galaxy population with 3.6σ confidence. At lower halo masses we observe no difference in AGN fraction between core and infalling galaxies. These observations support a model where a reduced number of low-speed interactions, ram pressure stripping and intra-group/cluster medium temperature, the dominance of which increase with halo mass, work to inhibit AGN in the cores of groups and clusters with log 10 (M 200 /M ⊙ )>13.5 , but do not significantly affect nuclear activity in cores of less massive structures

The Evolutionary Map of the Universe Pilot Survey

We present the data and initial results from the first pilot survey of the Evolutionary Map of the Universe (EMU), observed at 944 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The survey covers 270 deg2 of an area covered by the Dark Energy Survey, reaching a depth of 25–30 μJy beam−1 rms at a spatial resolution of ∼11–18 arcsec, resulting in a catalogue of ∼220 000 sources, of which ∼180 000 are single-component sources. Here we present the catalogue of single-component sources, together with (where available) optical and infrared cross-identifications, classifications, and redshifts. This survey explores a new region of parameter space compared to previous surveys. Specifically, the EMU Pilot Survey has a high density of sources, and also a high sensitivity to low surface brightness emission. These properties result in the detection of types of sources that were rarely seen in or absent from previous surveys. We present some of these new results here