By Marco Chiaberge, ESA Astronomer at the Space Telescope Science Institute
One of the most important problems in modern astrophysics is to understand the co-evolution of galaxies and their central supermassive black holes (SMBH) (see e.g. Heckman & Best 2014 for a recent review). Since the matter that ultimately accretes onto the central black hole needs to lose almost all (~99.9%) of its angular momentum, studies of mergers, tidal interactions, stellar bars and disk instabilities are central for understanding the details of such a process. However, observational efforts to assess the importance of mergers in Active Galactic Nuclei (AGN) so far have led to conflicting results. A major issue is related to the so-called radio-loud/radio-quiet dichotomy of active nuclei. Radio-loud AGNs have powerful relativistic plasma jets that are launched from a region very close to the central SMBH. The most popular scenario among those proposed so far assumes that energy may be extracted from the black hole via the innermost region of a magnetized accretion disk around a rapidly spinning black hole Blandford & Znajek (1977). In such a framework, the radio-quiet/radio-loud dichotomy can be explained in terms of a corresponding low/high black hole spin separation (Blandford et al. 1990). It is also important to stress that radio-loud AGN are invariably associated with central black holes of masses larger than ~108 solar masses (e.g. Chiaberge & Marconi 2011). Therefore, the black hole mass must play a role.
With the aim of determining the importance of mergers in triggering different types of AGN activity, my collaborators and I selected 6 samples of both radio-loud (RLAGN) and radio-quiet (RQAGN) AGN, and of non-active galaxies matched to the AGN samples in magnitude (or stellar mass). We focused in particular on redshifts between z=1 and z=2.5. All objects were observed with HST/WFC3-IR at 1.4 or 1.6mm, in order to ensure appropriate sensitivity at rest-frame optical wavelengths, and to allow us to detect faint signatures of a merger event. Most of the objects were taken from large surveys performed with Hubble (CANDELS, 3D-HST). The images of the high-luminosity radio galaxies were taken from a “snapshot” program we performed as part of our 3CR-HST survey of radio-loud AGN.
Figure 1 HST/WFC3-IR images of 3 radio-loud AGNs (top, credits: NASA, ESA, M. Chiaberge) and 4 radio-quiet AGNs (from CANDELS, Koekemoer et al. 2011). The 3 radio-loud AGNs are all classified as mergers. Only the 2 radio-quiet AGN in the right-most panels are mergers.
The fun part of the work was to visually inspect the WFC3-IR image of each of the 168 objects (Fig. 1) and determine whether each object was or was not showing signatures of a merger, according to a pre-defined classification scheme. It was at that point that we found something really interesting. Without knowing what type of object we were looking at, we classified almost all (95%) of the RLAGNs as “mergers”. On the other hand, the RQAGN samples and the non-active galaxies had merger fractions between 20% and 37%. We performed a careful statistical analysis of the results, and we concluded that the merger fraction in RLAGN is significantly higher than that in RQAGNs and non-active galaxies (Fig. 2). It is possible that all RLAGNs are associated with mergers. This result was also confirmed for lower redshift samples of radio galaxies (one at z~0.5 and one at z<0.3), and for objects of both low and high power. On the other hand, the merger fraction of RQAGNs is statistically not different from that of non-active galaxies.
Figure 2 Merger fraction vs. average radio loudness parameter Rx (ratio of the radio to X-ray luminosity) for the different AGN samples. Radio-quiet AGNs are on the left of the dashed line, radio-loud AGN are on the right. The filled symbols are the radio-loud samples and the empty symbols are radio-quiet. The dashed line represents the radio loudness threshold for PG QSOs. The solid line marks the 60% merger fraction that appears to roughly separate radio-loud and radio-quiet samples.
This result has very important implications. Firstly, it shows a clear association between mergers and AGN with relativistic jets (the RLAGN subclass), with no dependence on either redshift or luminosity. Secondly, we firmly determined that not all AGNs are triggered by mergers. The question now is how do mergers trigger AGN with jets? A possible scenario we envisage is that when a galaxy merger happens, the central supermassive black holes merge as well. In general, the resulting spin of the BH after coalescence is lower than the original spin values. But for particular spin alignments and for BHs of similar masses, the spin can be significantly higher (see Schnittman 2013, for a recent review). In that case, if the mass of the BH is at least ~108 solar masses, the energy extracted through the Blandford-Znejek mechanism may be large enough to power the jet. This is not a completely new idea, since it was already proposed in a slightly different form by Wilson & Colbert (1995). In the near future, we will focus on confirming the strong connection between RLAGNs and mergers with a larger dataset of HST observations, ALMA observations, and integral-field spectroscopy.
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