of early-types with the aim of directly observing the feedback from the BH as the transition mechanisms from star forming to passive systems. They have a limited sample, but argue for a possible quenching effect since the active galaxies in their sample are gas poorer than the star forming ones, and their depletion time too rapid to be due to pure star formation.
Aiming to establish a direct connection between AGN activity and gas content, in this chapter we measure the average Hicontent of a sample of∼2000 AGN host galaxies, using the stacking. We build control samples and we look for differences in Hi mass fraction (MHI/M?) between AGN and control galaxies. In addition, we use the data from the CO Legacy Database for GASS (COLD GASS, Saintonge et al. 2011a) survey of molecular gas, which has selection criteria analogous to ours, to repeat the analysis for the H2 content.
In Section 2 we describe the Hi sample selection and the data we use. In Section 3 we compare the gas content of AGN and inactive counterparts in bins of nuclear properties;
in Section 4 we study the molecular gas content. A discussion of the results is presented in Section 5.
5.2 AGN and control samples
We have shown in the previous chapter that the Hi content of massive galaxies is, to first order, most tightly correlated with optical/UV (NUV−r) colour and stellar mass surface density. For an unbiased comparison of the cold gas in AGN hosts and quiescent galaxies we must match the two classes in these two properties, to remove first order dependencies.
Starting from sample A (§2.1) we extract the AGN (§5.2.1) and a control sample of galaxies where there is no evidence for accretion onto a central black hole (§5.2.2). We present the selection criteria applied, and the additional parameters we use in the analysis but that have not yet been described in Sections 2.4 and 2.5.
5.2.1 The AGN sample
We identify the AGN in sample A using the diagnostic diagram first defined by Baldwin et al. (1981, hereafter BPT). They showed that it is possible to distinguish type 2 AGN (in which the emission is obscured by a dusty circumnuclear medium, see the unified model by Antonucci 1993) from normal star-forming galaxies using the intensity of the ratios of nebular emission lines. Since the different classes of objects are dominated by different excitation mechanisms, the relative line intensities are different. In particular, we consider the ratio [Oiii]/Hβ versus [Nii]/Hα to identify AGN, applying a cut in signal-to-noise ≥3 inall four line measurements in order to have reliable estimates. The fluxes
Figure 5.1: BPT diagram for galaxies in our sample with S/N≥3 for the four emission lines Hα,Hβ, [Oiii] and [Nii]. The solid curve we use to demarcate the boundary between AGN and “normal” star-forming (SF) galaxies is from Kauffmann et al. (2003a); the dashed line shows the demarcation boundary for “pure” AGN from Kewley et al. (2001). Objects are colour-coded according to their nuclear properties as labeled in the diagram.
are measured inside the SDSS 3-arcseconds fibre, which covers the inner regions of our galaxies, therefore we can safely state that the AGN properties derived from them are not significantly contaminated by outer star forming regions (see Kewley et al. 2006).
The locus of sample A galaxies that pass our signal-to-noise cut on the BPT diagram is shown in Figure 5.1. We adopt the Kauffmann et al. (2003a) prescription to separate AGN and star forming objects, shown as solid line in the Figure, so that active galaxies lie in the plane region identified by:
log ([OIII]/Hβ) ≥ 0.61/{log([NII]/Hα)−0.05}+ 1.3.
We note that Kewley et al. (2001) suggested a more stringent cut to separate galaxies where the nuclear signal is almost completely excited by emission from the gas accreting onto the black holes, rather than from star-forming regions. With this cut (shown as a dashed line in Figure 5.1), we obtain a subsample of 912 “pure” AGN, which is too small for our stacking analysis. We then consider the demarcation line suggested by Kauffmann et al. (2003a), which allows us to define a final sample of 1871 active galaxies (hereafter AGN sample).
We can further subdivide the AGN targets into Seyferts, LINERs (which make up the
5.2 AGN and control samples 73 bulk of our sample) or “transition”, as indicated by the different colours in Figure 5.1.
As a quick review (but see also Ho et al. 1997; Kauffmann et al. 2003a, and references therein), these are the classes in which type 2 AGN are commonly divided. LINERs (Heckman 1980b) display high values of the ratio of lower ionization lines [Nii]/Hαand are characterised by lower nuclear luminosities than Seyfert galaxies. The class of “transition”
objects has been introduced to separate galaxies with nuclear emission-line properties that are intermediate between normal star-forming galaxies and Seyferts and LINERs; they are actually likely to be normal AGN whose emission is contaminated by Hii regions close to the nucleus (Ho et al. 1993).
Finally, we plot the AGN objects in the NUV−r colour - stellar mass surface density plane, which are the two parameters we use to match AGN and quiescent targets. In the left panel of Figure 5.2, black dots represent the AGN sample, magenta dots the “pure”
AGN defined using the Kewley et al. (2001) cut (dashed line in Figure 5.1). We note that
“pure” AGN span the same range of parameter space as the larger AGN sample adopted in this work; the inclusion of transition objects does not bias our study toward bluer, star forming galaxies.
Additional parameters
Once we have identified the active galaxies in our sample, we need a tracer of their nuclear activity. As discussed in Kauffmann et al. (2003a), the luminosity of the [Oiii]λ5007 line should be a reasonably reliable tracer of black hole activity once corrected for dust extinction. Because it is a high excitation line, it is less contaminated by emission from Hii regions than other lines such as Hα. We correct the line fluxes for dust extinction using the Balmer decrement, as explained in §2.4, and then convert the [Oiii] fluxes into luminosities.
From the luminosity, we can estimate the Eddington ratio for the AGN, which is defined as the ratio between the bolometric luminosity and the Eddington luminosity:
LEDD = 1.26· 1038MBH/Merg s−1. The Eddington luminosity is the limit at which the gravitational attraction balances the radiation pressure of the infalling material. We actually estimate a proxy for the Eddington ratio, using: :
Lbol LEDD
∝ Lbol MBH
∝ L[OIII]
σ4
where Lbol is the bolometric luminosity, and MBH the central black hole mass. The Ed-dington luminosity LEDD scales with MBH, which we estimate using the stellar velocity dispersion σ measured from the fibre spectrum: MBH ∝ σ4 (Tremaine et al. 2002, and
Figure 5.2: Galaxies in our samples are plotted in the (NUV−r)-µ? plane. Left: AGN targets.
Black points show the whole AGN sample, but we overplot as magenta points galaxies that lie above the stronger cut defined by Kewley et al. (2001). Right: control galaxies, including
“inactive” galaxies with S/N([Oiii])<3 (red dots are galaxies for which the S/N in all the four lines is smaller than 3) plus star-forming objects (SFG; blue dots). Gray dots are the galaxies that are discarded when matching to the AGN sample (i.e. mainly very blue, low stellar surface density objects). Black and coloured dots show the final control sample extracted.
references therein). Lbol scales linearly with L[Oiii] (see Heckman et al. 2004), so to first order the ratio L[Oiii]/σ4 can be used as a proxy for the accretion rate. For a precise estimate of the Eddington parameter we refer the reader to Heckman et al. (2004) and Kauffmann & Heckman (2009).
5.2.2 The control sample
Our control sample is drawn from a combined sample of “inactive” galaxies and star-forming systems. Inactive galaxies are those whose spectrum exhibits a S/N of the [Oiii] line smaller than 3. We exclude 474 galaxies where the S/N of the [Oiii] line is greater than 3 but other lines lie below this S/N threshold, because these galaxies cannot be accurately classified.
By applying a cut in the [Oiii] line only, our control sample may be contaminated by weak LINERs. The contamination is anyway negligible: for our targets, less than 5% of the galaxies with S/N([Oiii])<3 have Log([NII]/Hα)>Log(0.6) (vertical dash dotted line in Figure 5.1).
Inactive galaxies, together with galaxies classified as star-forming on the BPT diagram,