My results, especial in Chapter 4, suggest that QSOs are associated with powerful star- forming events. Simulations which include AGN feedback and postulate that AGN form during galaxy mergers (e.g.Hopkins et al., 2007) have received much attention because of their success in reproducing a range of observed properties of both AGN and galaxies. In this scenario, galaxy interactions trigger the AGN and also produce nuclear starbursts that both feed and obscure the central engine for much of its active lifetime. AGN-driven outflows eventually develop, which at later stages become strong enough to rapidly quench the star- formation, allowing the AGN to shine unobscured for a short period (∼ 108 yr), leaving
FIGURE 6.3: Optical images and spectra and X-ray images of the two components of SDSSJ1254+0846. Redshift, continuum and broad-line shapes are remarkably similar. X-rays re- veal the two bright QSO nuclei while the optical images clearly shows the tidal arms of a host galaxy merger. Figures fromGreen et al.(2010).
behind a red, passive remnant.
A clear prediction of these facts suggests not only that binary SMBHs should be com- monplace, but also that merging galaxies with binary quasars should be common. However, examples of binary quasars are rather isolated in the literature. Most major mergers may have occurred at high-z (e.g. Silverman et al., 2005). Active mergers may be heavily ob- scured and as a result detectable only as ULIRGs Cui et al. (2001) or, the lifetime of the interacting phase may be extremely short Mortlock et al. (1999). A handful of new close (sub-kpc) binary AGN have been found recently (e.g.Liu & Shapiro, 2010), however these intriguing lower-luminosity objects are not homogeneously selected. It takes sensitive, wide- area searches to find binary quasars. In the wake of wide, deep SDSS photometry, the number of known binary quasars has mushroomed, thanks to the statistical surveys ofHennawi et al. (2006) andMyers et al. (2008). In particular, the latter survey targets quasars separated by <40kpc over 0.4 < z < 2.5, from which the first, luminous, spatially resolved binary QSO in a disturbed host galaxy merger showing obvious tidal tails was observed (Green et al., 2010; Fig.6.3).
A major problem is explaining the observed excess of binary quasars with separations <200 kpc. It has been proposed that this excess is evidence for nuclear triggering in galaxies during dissipative mergers (e.g.Myers et al.,2007). According toHopkins et al.(2007), the excess measured clustering (Myers et al., 2007, 2008), represents compelling evidence for the merger-driven origin of QSOs. However, they also suggested that attaching all QSOs to moderately rich dark matter environments in which mergers are likely to occur is sufficient to explain the observed excess of quasars with small (<200 kpc) separations. That is, they just happen to be neighbors where the observed∆z could represent ∼Mpc separations rather than dynamical velocities and their properties should be statistically indistinguishable from those of single QSOs. As a result, the discovery of binary QSOs whose hosts are clearly
interacting with evidence of enhanced star-formation, presents rare opportunities to study the merging/triggering mechanisms. Submillimetre photometry can help us test these ideas by revealing the presence of triggered ongoing star-formation. The aim is to obtain sub- millimetre observations of a complete sample of spectroscopically confirmed binary QSOs from SDSS and compare its submillimetre properties to the nearly ideal comparison sample of single QSOs observed by HerMES (Hatziminaoglou et al.,2010).
The suggested sample is drawn from the complete sample of binary quasars discussed in Myers et al.(2008) andGreen et al.(2011), restricted to velocity differences−500 < ∆V < 500 km s−1 and proper separations R
P < 200 kpc, resulting in a total of 28 pairs. The
separation criterion selects hosts likely to be interacting on their first or second pass while the velocity criterion removes most chance projections but still allows for hosts in a vari- ety of environments from isolated pairs to massive clusters. Binary quasars were selected as having g < 20.85 and either the UVX and/or low-redshift quasar Bayesian classifica- tion of Richards et al. (2009). These cuts ensure a high efficiency of quasar pairs in the targets and a reasonably homogeneous sample from 0.4 < z < 2.4. Pairs of quasar can- didates were then followed up spectroscopically and with X-rays confirming their binary QSO nature. The parent sample thus reflects typical SDSS optical QSO selection and as a result is therefore directly comparable to the sample of 156 SDSS spectroscopically identi- fied Type-1 AGNHatziminaoglou et al.(2010), observed by Herschel as part of the shallow HerMES-Lockman Science Demonstration Phase field. For all 28 pairs in our sample we have available deep u, g, r, i, z photometry and spectroscopy, archival SDDS and 2MASS photometry plus X-rays either from ChaMP or Chandra archival. With this project I will try to address the following issues:
Test the merger hypothesis: Submillimetre detections will provide the first direct evidence
for the presence of cool dust in these systems, a tracer of ongoing star-formation. Star- formation rates can be estimated by combining broad-band SEDs and available spectroscopy using the methods described inBuat et al.(2010) andRowan-Robinson et al.(2010). These rates will then be compared with the rates estimated in the Hatziminaoglou et al. (2010) sample of single QSOs. A substantial difference in the SFR among our targets and sin- gle quasars would be direct evidence of triggered star-formation. The absence of enhanced star-formation events will suggest that the link between AGN and galaxy formation is more complex than current merger models assume and might be evidence for a time-lag between the peak of the star-formation and the black hole accretion. In addition, the SFR and mass of the cool dust present in these systems will provide a better constraint on the properties of the host galaxies used for the merger simulations we present in Fig.6.4, enabling us to check whether these systems are consistent with a merger-triggered scenario for QSO formation.
The starburst-AGN connection: It has been suggested that obscuration in AGN may often
not be from star forming regions in the host galaxy (e.g.Fabian et al., 1998). Indeed, many observations point to a starburst-AGN connection, which can be most easily understood if
FIGURE6.4: Numerical simulation of a merger like SDSSJ1254+0846, a pair typical of the sources in our sample (Green et al.,2010). Left, middle and right panels show the gas distribution, the stars in the xy and xz planes and the model star-formation rates, nuclear black hole masses and accre- tion rates for the two components respectively. Red diamonds mark the nuclear separation, position and extent of tidal features, black hole masses and accretion rates at 2.3 Gyr, similar to those of SDSSJ1254+0846.
both forms of activity have a common origin, such as in a merger. A major problem in assess- ing the level of star-formation in AGN is the difficulty in decomposing the star-forming from the AGN emission, especially in the case of dust enshrouded systems. Combining X-ray data with longer submillimetre wavelengths can provide a handle on this issue. The X-rays provide the most efficient way of identifying active BHs, least biased by obscuration, while the submillimetre band is considered a clean star-formation diagnostic. Galaxies undergoing a burst of star formation should show an excess in the long wavelength data, from which the star formation rate can be estimated (e.g.Rowan-Robinson et al., 2010). I can examine the starburst-AGN connection by seeing if the SFRs in the binary sources are typically enhanced with respect to single QSOs. I can also perform a qualitative test of the starburst obscuration hypothesis comparing the SFR with obscuration measures from the X-ray, or from optical SED/NIR SED fitting.
Spectral Energy Distributions: From the X-ray to submillimetre broad-band SEDs and
LBol, I will be able to calculate the probability that the merger scenario is viable, i.e., that
these quasars show evidence for triggered, highL/LEdd accretion. I will also correlate the
SEDs andLBolwith binary characteristics likeRP and∆V and test whether smaller separa-
tions and/or lower velocities result in luminous, high column systems. The broad emission lines from the spectra, can help us estimateMBH(e.g.Vestergaard & Peterson,2006). From
the SEDs and MBH, I can measure the Eddington ratio (Warner et al., 2004), to constrain
whether these quasars are in especially high mass accretion phases (analog to spectroscopic binary stars;White et al.,1983).