1.6
Outstanding questions
From the discussion above, it is clear that many questions regarding the nature of the red 2MASS AGN remain unanswered. This thesis focusses on the following issues. Is the red 2MASS AGN population homogeneous? It is important to estab- lish whether the red AGN population can be explained simply by one mechanism, or whether two or more are needed to explain the entire sample.
Why are their J-KS colours red?: It is possible that the 2MASS AGN have red
J-KS colours because dust extinction reduces the J-band flux relative to the K-band
flux such that the FK/FJ flux ratio is higher than that of a typical AGN; the obscuring
dust could be located in the outer layers of the torus, such that we are observing these objects at an angle where our line of sight grazes the edge of the torus, leading to the observed red colours. The large-scale dust structures in the host galaxy could also cause significant reddening.
Alternatively, rather than significant levels of dust extinction, a relatively large cov- ering factor of hot dust (T ∼ 1500K) could also result in red J-KS colours. Emission
from hot dust in the torus becomes significant at wavelengths which coincide with the KS-band. Therefore, a larger than average covering factor for this hot dust could in-
crease the flux measured in the KS-band relative to that of the J-band (the latter likely
dominated by accretion disk emission), thus producing the red J-KS colours measured
for the red AGN.
Are the 2MASS-selected objects young, dust obscured AGN? Further evidence is needed before classifying these red AGN as young, dust enshrouded AGN. Such ob- jects would be highly reddened when compared to typical UV/optically selected AGN. In addition, based on models for young AGN (e.g. Saunders et al. 1989; Hopkins et al. 2005; Narayanan et al. 2006; Hopkins et al. 2006), they would be expected to show greater evidence for powerful gas outflows than compared to ‘typical’ AGN. They would also have large quantities of gas available to the central supermassive black hole, lead- ing to large bolometric luminosities and therefore large Eddington ratios (Hopkins et al. 2006). Also, we might expect to detect a higher incidence of Narrow-Line Seyfert 1 objects (NLSy1), which are sometimes considered to represent young AGN (Mathur, 2000).
1.6. Outstanding questions 23
In order to address these questions I have used deep, wide spectral coverage William Herschel Telescope (WHT) Intermediate dispersion Spectrograph and Imaging System (ISIS) spectra of a representative sample of 29 red 2MASS quasars in the local Universe (z<0.29) with NIR colours of J-KS ∼> 2.0, and KS-band magnitudes: 11.0 < KS <
14.9, taken from Hutchings et al. (2003). In addition, for most of these objects, I have analysed Wide-field Infrared Survey Explorer (WISE) observations, which are useful for investigating the nature of the IR emission/colours in greater detail. In order to adequately test whether the 2MASS-selected quasars are different from UV/optically selected quasars, the properties of the 2MASS quasars have been compared with several comparison samples of ‘typical’ quasars throughout the thesis.
The thesis is structured as follows. Chapter 2 describes the sample, comparison samples, observations and data reduction. Chapter 3 discusses a unique object in the sample: Q1131+16. The spectrum of this object has the richest spectrum of FHILs (e.g. [FeVII], [FeX], [FeXI] and [NeV]) yet reported for an AGN, providing a rare opportunity to investigate the physical conditions and kinematics of the region(s) emitting the FHILs. Chapter 4 discusses what can be learnt from the optical spectra of the 2MASS sample as a whole using optical emission lines and continuum observations, comparing the results to those of the comparison samples. Chapter 5 investigates the near to mid-IR photometry of the 2MASS and comparison samples. In Chapter 6, a model is developed to reproduce the near- to mid-IR colours of the 2MASS AGN, and thereby test the feasibility of the hypotheses outlined in the previous chapters. Finally, Chapter 7 discusses the results found for the 2MASS AGN and puts the sample into context of the rest of the AGN population. In addition, any necessary future work is discussed in this chapter.
Chapter 2
Samples and Observations
2.1
Introduction
As described in Chapter 1, there have been several studies of red 2MASS AGN in the past. These studies have mainly focussed on samples of AGN which cover a wide range of redshift (0 < z < 3: Glikman et al. 2007; Georgakakis et al. 2009; Glikman at al. 2012). The problem with such samples is that it is difficult to investigate the intrinsic diversity of the red AGN population in the face of possible evolutionary and/or luminosity-dependent effects. Therefore, it is important to study a sample of 2MASS-selected AGN that covers a more limited range in redshift. This thesis reports observations of an RA-limited sample of 29 2MASS-selected quasars (J-KS ∼> 2.0) with a narrow range of redshifts: 0.09 < z <
0.28. The total sample studied in this thesis consists of 29 objects, however two of these objects have redshifts z>0.2. Although I present the data for the full 29 objects, I only consider the results for the 27 objects with redshifts z<0.2 in the analysis presented in Chapters 4, 5 and 6. This is because after redshift z = 0.2, the nearest 2MASS AGN in the sample has a redshift of z = 0.274. Therefore, including these objects when making comparisons to UV/optical selected AGN over the same wavelength range will have a significant effect on the conclusions, in the sense that the comparison samples (described below) may have a greater concentration of objects with redshifts z > 0.2 when compared to the 2MASS sample.
In order to answer the questions raised in Chapter 1, WHT/ISIS observations for 23 of the red 2MASS AGN were taken on the nights of the 8th and 9th of February 2007 under program ID P15 (P.I. Tadhunter), a further 2 objects were observed on the night starting on the 28th of July 2006 under program ID N15 (P.I. Rodr´ıguez Zaur´ın), and the final 4 objects were observed on the nights starting of the 26th and