SDSS J0952+2552: A Spectroscopically Confirmed Dual AGN
In the current Λ-CDM cosmological model (Rees et al. 1984; Davis et al. 1985; Jenkins et al. 1998), the growth of galaxies at low and intermediate redshift must occur primarily viacollisions between galaxies. This fact has been used to explain a number of observations of distant galaxies, in particular the existence of active galactic nuclei (AGN). The argument goes that collisions between galaxies torques the atomic gas in the outskirts of the galaxies, causing the gas to flow in toward the center, providing fuel for central starbursts and AGN outbursts (García-Burillo et al. 2006). However, if we take the current model as it stands, we are left with a slight problem: in the form of “dual-Quasars“. Simulations predict that in the stage between the initial collision between equal-mass galaxies and their final coalescence, there should be a high probability of seeing both galaxies as quasars (Rosalie et al. 2011).
While there are many examples of AGN pairs with relatively large separations, there are currently only six known closely separated pairs (Junkkarinen et al. 2001; Komossa et al. 2003; Comerford et al. 2009b; Fu et al. 2011; Rosario et al. 2011; Liu et al. 2011). Unless there are many more undiscovered closely separated AGN pairs, something must be seriously wrong with the current theoretical understanding of galaxy mergers events or the growth of black holes.
This brings up an interesting question. Observationally, how exactly can one tell that a pair of galaxies is indeed a dual AGN? For example, one could look in large catalogues of galaxy spectra (e.g. the SDSS) for objects with dual sets of high excitation emission lines with modest velocity offsets (e.g. Smith et al. 2010; Zhou et al. 2004; Wang et al. 2009; Xu & Komossa 2009; Gerke et al. 2007). These objects may be dual AGN, but may also be a chance superposition, a signature of an outflow emanating from a single AGN, or be due to an AGN surrounded by an ongoing central starburst. One could also perform imaging of objects known to host double-peaked emission lines and look for objects with distinct but closely separated cores of emission. Unfortunately, this doesn’t confirm an object as a dual AGN, and can only be used to rule objects out.
What is really needed is spatially resolved spectroscopy, e.g. with an Integral Field Unit (IFU). An IFU observation yields a position-position-velocity “data cube”, that is, an image of the sky with a corresponding spectrum at each pixel. One can then combine both of the methods discussed above: an AGN with double, offset emission lines can be spatially resolved into two clumps of emission and that can then be individually inspected to pick out the AGN lines emitted by each source.
Recently McGurk et al. (2011) have obtained observations of the SDSS quasar J0952+2552 with OSIRIS, an IFU on the Keck 2 telescope. Taking advantage of laser guide star adaptive optics, they are able to obtain extremely high spatial resolution imaging and spectroscopy to uniquely and beautifully confirm this object as a dual AGN. In the figure to the right, the authors compare both the broadband near IR adaptive optics imaging with the OSIRIS IFU observations. Clearly, the IFU observes at lower spatial resolution, however, it is also clearly able to separate two blobs of emission, corresponding to two distinct galaxies at redshifts of z = 0.34.
McGurk et al. (2011) conclude that both galaxies host distinct AGN by comparing the spectra of the central regions. The primary galaxy is a Type 1 Seyfert galaxy, showing both broad and narrow emission lines, while the companion is a Type 2 Seyfert, showing only weaker narrow emission lines. Since the companion only shows narrow lines, it is possible that the emission emanates from star forming regions in a nuclear starburst rather than in the region near an AGN.
However, AGN and young stars have very different ionizing spectra (AGN emit more of their ionizing radiation as X-rays and in the extreme ultraviolet than young stars) therefore, one can use the ratio of the flux in certain emission lines of nitrogen, hydrogen, and oxygen to determine whether the ionization is due primarily to a starburst or due to an AGN, a so-called BPT analysis. While the authors cannot perform a full BPT analysis, they can measure one of the line ratios that goes into the analysis; luckily, they can conclusively rule out star formation as the cause of the ionization.
These observations required some of the most cutting edge techniques ever used in observational astronomy. By leveraging new observing technologies, they have expanded the dual AGN sample by a staggering 20 percent! (In one paper!!) More observations like those described here will further expand the sample and allow theorists to pin down the physical mechanisms at work when two galaxies collide. However, there are still questions that still remain to be answered regarding the nature and low observing rates of dual-AGN. For example, how does the AGN duty cycle affect the detectability of dual AGN and at what separations are both of the AGN bright, and for how long? What fracton of AGN are actually currently in mergers? One thing is for sure, the questions that do remain will require a lot more observations if anyone intends on categorically answering them to a statistically significant level.
- Blumenthal, G. R.; Faber, S. M.; Primack, J. R.; Rees, M. J. (1984) Formation Of Galaxies & Large-Scale Structure With Cold Dark Matter. Nature, 311 pp. 517-525.
- Kauffmann, G. et al. (2003) The Host Galaxies Of Active Galactic Nuclei. Monthly Notices Royal Astronomical Society, 346 (4) pp. 1055-1077.
- Rodriguez, C. et al. (2006) A Compact Supermassive Binary Black Hole System. The Astrophysical Journal, 646 (1) pp. 49-60.
- Green, P.J. et al. (2010) SDSS J1254+0846: A Binary Quasar Caught In The Act of Merging. The Astrophysical Journal, 710 (2) pp. 1578-1588.
- McGurk, R.C. et al. (2011) Spatially-Resolved Spectroscopy Of SDSS J0952+2552: A Confirmed Dual AGN. The Astrophysical Journal: Letters, 738, (1): Article I.D. L2.