The selection, analysis and implications of rare AGN in large surveys

Greenwell, Claire Louise (2023) The selection, analysis and implications of rare AGN in large surveys. University of Southampton, Doctoral Thesis, 317pp.

Record type: Thesis (Doctoral)

Abstract

There is a known connection between supermassive black hole growth and galaxy evolution (e.g., Kormendy and Ho, 2013). Grasping the nature of this connection requires understanding of both the growth of black holes via accretion, and how these processes can affect their host galaxies. In order to understand the feedback between them it is necessary to work towards a full census of the highly diverse population of AGN types. This thesis aims to address the work needed to complete this census by assessing the ‘missing’ AGN, applying different selection techniques to fill the gaps, and analysing the AGN found in this way.

Lack of a complete census of AGN means that knowledge of vital population statistics such as accretion rates or obscuration depths is incomplete. This thesis takes a first step to solving this problem through all-sky searches in the immediate, local universe via the Local AGN Survey (LASr; Asmus et al., 2020). In Chapter 2 we construct an unbiased sample of local AGN, including both known AGN and candidate AGN. We select these candidates using mid-infrared techniques, to identify AGN that have been missed in photometric surveys at other wavelengths. The aim of this is to work towards a complete sample of local AGN, and thus we must follow up the AGN candidates to confirm them as true AGN or discard them from the census. We construct the most complete all-sky galaxy sample within 100 Mpc (90% completeness for log(M_⋆/M_⊙)∼9.4), four times deeper than the current reference (the Two Micron All-Sky Survey Redshift Survey; 2MRS), which misses ∼20% of known luminous AGN. Using MIR selection with WISE, we find 221 galaxies with L_nuc (12 μm) >~10^42.3 erg s⁻¹. Among these are 61 new AGN candidates. We begin a follow-up campaign with these objects, aiming to use optical and near-infrared spectroscopy to perform AGN diagnostics and assess the results of our AGN candidate selection. Here we present results from the first three targets where we find two probable AGN, and analysis of an object believed to be a Compact Obscured Nucleus.

A major difficulty in selecting AGN - and hence an area where known AGN in LASr are scarce - is recognising heavily obscured AGN. Almost every selection technique is significantly biased against these objects and thus they are often missed from surveys, despite possibly representing periods of intense growth in AGN life cycles. The most promising method of selection is with hard X-rays ( > 10 keV). Selected from the 80-month NuSTAR Serendipitous survey (Klindt et al., 2022, submitted), and expanding on previous work on the 40-month catalogue (Lansbury et al., 2017a,b), in Chapter 3 we find sources with an excess of emission in the hard band (8-24 keV) compared to the soft band (3-8 keV), implying thick obscuration. We combine the NuSTAR data with available soft X-ray data (XMM-Newton, Chandra, or Swift-XRT depending on availability and exposure) to study the properties of these rare AGN. From 14 hard X-ray selected candidate Compton thick (CT) AGN we analyse the 9 with net sources > 100, finding 4 CT (log N_H > 24 cm⁻²) and the remainder heavily obscured (log N_H >∼23 cm^-2), implying a Compton thick fraction for NuSTAR selected AGN within z < 0.07 of 15⁺¹⁰₋₆ % (combined with previous results from Lansbury et al., 2017a). Of the CT AGN, 29⁺²¹₋₁₂ % appear to be in actively interacting systems.

In Chapter 4, at higher redshift we focus on one specific niche – Optically Quiescent Quasars (OQQs) – which are optically bright, IR-coloured AGN (WISE W1 − W2 > 0.8, and monochromatic luminosity above λL_λ(12 μm) ≈ 3×10⁴⁴ erg s⁻¹) that lack major optical signatures. Initially we examine SDSS J075139.06+402810.9, a prototypical candidate at z=0.587, with an [OIII] λ5007Å limiting flux about two dex below Type 2 quasars at similar IR power. The source is significantly detected over 0.5–16 keV with XMM-Newton and NuSTAR, unambiguously confirming the presence of current accretion activity. Spectral modelling yields an intrinsic luminosity L_{2−10 keV} ≈ 4.4×10⁴³ erg s⁻¹, well within the AGN regime, but underluminous relative to its infrared power. Expanding to the wider population in Chapter 5, we select 64 objects that show no significant [O III] λ5007 emission as our OQQ sample. This would typically be a strong optical emission line in AGN, and thus objects without it would normally be discarded. As a comparison sample, we examine SDSS-selected Type 2 quasars (QSO2s), which show a significant [O III] λ5007 line, but have otherwise similar properties to OQQs. We find a 1:16 ratio of OQQs compared to QSO2s, suggesting that the OQQ duty cycle is likely much shorter than the duty cycle of QSO2s (though selection biases are not fully quantified). These may represent the fully enshrouded or ‘cocooned’ phase of AGN growth, and comprehensive searches of such populations are thus vital to constrain AGN properties at important junctions in their evolution. Well-tested techniques based on searching for isotropically emitted AGN signatures would be ineffective to
find such ‘cocooned’ phases, so this research aims to combine multi-wavelength selection techniques to uncover these hidden black holes. Alternatively, the observed lack of optical signatures may represent a true intrinsic lack of emission lines; for example, from a ‘young’ AGN only recently ‘switched on’. Either of these possibilities make OQQs an interesting and under-studied avenue of AGN evolution.

With these three sections, I demonstrate the incomplete nature of our knowledge of the true AGN population. I show how different selection techniques can begin to fill in the gaps - firstly with ideal selection instruments operating over small areas to discover objects that are hard to find but not intrinsically rare, and secondly with more general vast surveys across large areas of the sky, finding rarer objects that may represent under-examined areas of AGN evolution. With NuSTAR I reveal three previously unknown Compton thick AGN and confirm another. Using a combination of MIR selection and optical non-AGN appearance I select a group of AGN and classify them as ‘Optically Quiescent Quasars’. I make a detailed study of their properties, including an in depth analysis of a prototype OQQ. Finally, in Chapter 6 I conclude by placing these chapters into context with each other and with the wider AGN zoo.