Ciclo Otto

(2002) selected objects detected in all 3 2MASS bands with J-KS > 2.0, with Galactic

latitudes |b| > 30◦ _{to avoid Galactic AGB stars (Cutri et al., 2002). In addition, K}

S-band

magnitudes in the range 11.0 < KS < 14.9 were used. The latter criterion was chosen

so that bright (KS < 11.0) Galactic AGB and carbon stars were excluded, along with

‘normal’ galaxies in the redshift range 0.4 < z < 0.5 (KS > 14.9; Cutri et al. 2002).

These selection criteria produced over 17000 red AGN candidates, most of which were previously undiscovered.

However, there are a few caveats with this selection. One such caveat is that, because of host galaxy contamination, low luminosity and redshift AGN are missed. This is because the host galaxy stars contribute more flux at shorter wavelengths than at the longer wavelengths. The other caveat is that the SEDs of quiescent galaxies at redshifts between z=0.4–0.5 may also produce the red NIR colours (Cutri et al., 2002).

A spectroscopic follow up of 704 of the red AGN candidates was carried out. It was found that 55% of the objects are Type 1/Quasar objects, 14% are Type 2/LINER objects, 15% are quiescent galaxies, 10% are unusual stars such as carbon stars, and the remaining objects could not be identified due to insufficient signal-to-noise in their optical spectra (Cutri et al., 2002). The red AGN candidates selected from 2MASS represent a population of objects that have remained hidden from surveys at other wavelengths, and understanding this population is critical to our overall understanding of the entire AGN population.

1.5

Recent studies

Because the red 2MASS AGN potentially make up a large fraction of the AGN popula- tion, it is important that we understand their relationship to the unobscured population: are they young, dust obscured versions of ‘typical quasars’ (Hutchings et al. 2003; Geor- gakakis et al. 2009; Glikman et al. 2007; Glikman at al. 2012)? Or can their properties be explained by a specific viewing angle in the orientation-based unified schemes (Wilkes et al., 2002)? Are they high-redshift galaxies whose dominant emission has been shifted into the near-IR? Or do they perhaps represent a new population altogether? Are these objects all luminous quasars, or are some less luminous AGN?

There have been several studies of red 2MASS AGN in the past that have focussed on different wavelength regions (e.g. UV, optical or IR). The basic properties of the samples used for the previous studies are summarised in Table 1.1. Many of these past studies concluded that the red 2MASS quasars are young, dust-enshrouded, transitional

1.5. Recent studies 19

objects, based on the following five strands of evidence.

- Morphological disturbance of the host galaxies. Urrutia, Lacy & Becker (2008) followed up a representative sub-sample of 13 of the Glikman et al. (2007) sample (see Table 1.1), with 0.4 < z < 1, using imaging observations with the ACS Wide Field Camera on the Hubble Space Telescope (HST) with the F475W and F814W filters. Their sub-sample is relatively luminous, with intrinsic AGN absolute magnitudes in the range -26.2 ≤ MB ≤ -23.5. They found that 85%

of the host galaxies in their sample show evidence for morphological disturbance, with the level of disturbance appearing to increase with the measured reddening of the AGN. Similarly, using CFHT snapshot R-band data, Hutchings et al. (2003) report a high incidence of host galaxy interaction (>70%) in their sample, based on a qualitative analysis of their images. These findings are consistent with the idea that the AGN are triggered by mergers, as gas and dust is driven into the near-nuclear regions of the merger remnant. In this scenario AGN start their lives obscured by a natal cocoon of dust, and appear in UV/optical surveys once the obscuring dust has been removed by AGN winds (e.g. Saunders et al. 1989; Hopkins et al. 2005; Narayanan et al. 2006; Hopkins et al. 2006).

- High rate of occurrence of outflows based on absorption line stud- ies. Broad absorption line (BAL) quasars are defined as objects with blueshifted (∼3000–25000 km s−1_{), broad absorption features (FWHM > 2000 km s}1_{) asso-}

ciated with UV resonance lines (Weymann et al., 1991). These can be divided into objects that lack MgII absorption — high-ionization BALs (HiBALs) — and objects with MgII absorption — low-ionization BALs (LoBALs). While the overall incidence of BALs in the population of red 2MASS quasars is similar to the general population of UV/optical selected quasars (63% and 61% for Urrutia et al. (2009) and Glikman at al. (2012) respectively), for objects with redshifts that allow the detection of BALs, both Urrutia et al. (2009) and Glikman at al. (2012) find that 95% of their BAL objects are LoBALs. This is in contrast to rate of occurrence in UV/optical selected quasars where LoBALs make only 10-20% of the BAL popula- tion (Trump, Hall & Reichard, 2006). A high fraction of LoBALs is thought to be consistent with a population of objects representing a transitional stage in quasar evolution. This is because the presence of LoBALs is believed to be evidence that AGN are ‘casting off’ their cocoons of dust and gas (e.g. see Voit, Weymann & Korista 1993).

1.5. Recent studies 20

- High star formation rates. Mid- to far-IR observations of the red quasars show evidence for higher star-formation rates than measured in samples of UV/optical selected quasars. Using Spitzer photometric data, Georgakakis et al. (2009) de- termined that their sample of red quasars shows evidence for relatively high star formation rates, based on 60/12 µm flux ratios which are high compared to the PG quasars. Similarly, using IRS spectra, Shi et al. (2007) detect strong Polycyclic Aromatic Hydrocarbon (PAH) features in 40% of their 2MASS sample at 7.7 µm and 11.3 µm — signatures of star formation in galaxies. By comparing their results to the PG quasars and 3CR objects, Shi et al. (2007) find that the star formation rates in the red 2MASS quasars are higher than those of the UV/optical selected quasar samples.

- High accretion rates. Based on both broad hydrogen emission line luminosities (Balmer, Paschen etc.) and SED fitting, Urrutia et al. (2012) find that a large fraction of their sample (∼54%) have relatively high Eddington ratios (LBOL/LEDD

> 0.3). Higher than in UV/optical selected samples.

- Rapid growth of Black Holes. Canalizo et al. (2012) measured black hole masses and host galaxy properties for 29 red 2MASS AGN with 0.17 < z < 0.37, finding that the majority of red 2MASS AGN have significantly more massive black holes than those in local AGN. Canalizo et al. (2012) believe that this indicates that the SMBH had undergone a period of rapid growth.

The majority of these previous studies of red AGN have focussed on samples covering a wide range of redshift (0 < z < 3). The problem with such samples is it is difficult to: (a) investigate the intrinsic diversity of the red quasar population in the face of possible evolutionary and/or luminosity-dependent effects, (b) obtain accurate estimates of the levels of dust extinction in the high redshift AGN because of the challenges faced with measuring the Balmer decrements in high redshift objects, and (c) to define appropriate comparison samples of ‘typical’ AGN, given such a large range of redshifts. Therefore, to limit these effects, it is important to study a sample of 2MASS-selected AGN that covers a more limited range in redshift. In this thesis, I discuss the results of observations of an RA-limited sample of 29 2MASS-selected quasars (J-KS _∼> 2.0) with a narrow range

of redshifts (0.07 < z < 0.28), and attempt to address the issues outlined in the next section.

1 .5 . R e c e n t st u d ie s 2 1

Table 1.1: Basic properties of red AGN samples.

Sample Size Z Data used in Study Key Selection Criteria

J-KS _∼> 2.0,

This thesis 29 0.06 < z < 0.28 Optical Spectra 11.0 < KS < 14.9,

selected from Hutchings et al. (2003) J-KS _∼> 2.0,

Hutchings et al. (2003) 76 z < 0.3 R-band Images 11.0 < KS < 14.9,

previously unidentified

Glikman et al. 2004, 2007, 120 0.1 < z < 2.5 Optical & IR Spectra J-KS ≥ 1.7 & R-K ≥ 4.0,

2011 & 2012 selected by cross-correlating the 2MASS

PSC with the FIRST radio catalogue

Georgakakis et al. 2009 10 0.29 < z < 3.1 UV-FIR SED fitting J-KS ≥ 1.5 & R-K ≥ 5.0,

selected by cross-correlating the 2MASS PSC with SDSS DR3 spectra

Urrutia et al. 2008, 2009 13 0.1 < z < 1.0 Optical Spectra Selected from the

& 2012 Glikman et al. (2007) sample

Shi et al. 2007 25 0.08 < z < 0.37 IR Spectra J-KS _∼> 2.0,

selected from Cutri et al. (2001)

Kuraszkiewicz 44 0.07 < z < 0.37 X-ray-FIR SED fitting, J-KS > 2.0 and B-KS > 4.3

et al. 2009a,b PCA analysis

Canalizo et al. 2012 9 0.13 < z < 0.37 Optical/NIR spectroscopy, J-KS > 2.0 and MK .-25

HST images

Marble et al. 2003 29 0.13 < z < 0.6 HST images J-KS > 2.0 and MK .-23

Smith et al. 2002 20 0.06 < z < 0.6 Optical broadband J-KS > 2.0 and MK .-23