Jul 012015
 

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.

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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.

blog_fig2Figure 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.

These results have been published in an ApJ paper, in an ESA press release, and in a Nature “News and Comments” article.

 

References

Blandford R. D., Netzer H., Woltjer L., Courvoisier T. J.-L. and Mayor M. 1990 Active Galactic Nuclei, Vol. 280 (Berlin, Heidelberg, New York: Springer)

Blandford R. D. and Znajek R. L. 1977 MNRAS 179 433

Chiaberge M. and Marconi A. 2011 MNRAS 416 917

Heckman T. M. and Best P. N. 2014 ARA&A 52 589

Koekemoer A. M., Faber S. M., Ferguson H. C. et al 2011 ApJS 197 36

Schnittman J. D. 2013 CQGra 30 244007

Wilson A. S. and Colbert E. J. M. 1995 ApJ 438 62

Feb 112014
 

Shooting movies of nature’s great particle accelerators with the Hubble Space Telescope

By Eileen Meyer, Postdoctoral Fellow at STScI

We know that the relativistic jets spewing out of the centers of disks around supermassive black holes start out very fast. VLBI studies in the radio of hundreds of jets1 in active galaxies have shown super-luminal apparent velocities of up to 50c, implying Lorentz factors of at least 50 (real speeds over 99.9% c) in the fastest jets. However, these studies are limited to measuring jet speeds on scales close to the black hole (typically < 1 pc), while the extent of the jet can range from a few kpc up to a Mpc, in some cases greatly beyond the scale of the host galaxy.

The long lifetime of Hubble has given us an opportunity to use over 13 years of archival images of one of nature’s most photogenic jets, M87, to map the complete velocity structure of a relativistic jet on kpc scales2.  Using state-of-the art astrometry (thanks to Jay Anderson and others of the HST Proper Motions3 Team), I was able to register over 400 images of M87 (d: 16.4 Mpc, or 78pc/”) taken by 3 cameras on Hubble: WFPC2, ACS/HRC, and ACS/WFC (all F814W filter) using the positions of over 1000 globular clusters spread throughout the host galaxy.  The resulting systematic astrometric error was only 0.17 mas, corresponding to an unprecendented 0.003c over the 13 year span of the study.  Our goal was to get high-precision speeds of individual knots in the jet in order to measure not only the speeds along the jet as it extends out into the host galaxy, but  also look for subtle accelerations and transverse motions which might give us insights into the jet structure and how it evolves on human timescales.

The most striking presentation of the data is in the form of movies (available at my personal webpage) , in which component speeds of up to 4.5c yield motions easily distinguished by eye. We found an impressive variety of behaviors in the jet, with some knots apparently stationary, others rapidly decelerating, and some with superluminal transverse speeds, challenging the previous picture of a jet that “smoothly decelerates” (see Figure 1 where our measurements are compared to previous studies in the radio and optical).   We also found evidence for the first time of helical motions in the outermost part of the jet, where speeds were still superluminal nearly 2 kpc (projected) from the core.  By overlaying the velocity vectors onto an image of the M87 jet, the apparent alignment of the vectors gives the impression of side-to-side motion (i.e., a flattened helix) as shown in Figure 2.  Helical, ordered  magnetic fields have long been suggested as a possible feature of relativistic jets in AGN4.

Fig4_vel_envelope.eps

 

Figure 1: Velocities along the jet (upper panel) and transverse to the jet (lower panel) as a function of distance from the core. Previous measurements shown for comparison taken from Biretta et al. (1995), Biretta et al. (1999), Cheung et al. (2007), and Kovalev et al. (2007). Figure from Meyer et al., 2013

 

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Figure 2: Various knots in the outer part of the M87 jet are shown, in 4 different epochs (1995, 1998, 2001, 2008), with vertical lines to guide the eye. Bottom panel: a depiction of velocities as vectors from their positions along the jet.

 

This work is ongoing, and the next publication will discuss the theoretical implications of our study and release the full dataset of multiwavelength spectra and positions as a function of time for almost 2 dozen individual components in the jet. In addition, we were recently awarded time in Cycle 21 for deep imaging of 3 more nearby jets in order to measure their kpc-scale proper motions. The most distant, 3C 273, is over 500 Mpc from Earth, making it the most distant optical proper motions target ever studied.

Further information on this study is available in Meyer et al. (2013), as well as the NASA/ESA press release.

References:

  1. Lister et al 2009 (AJ, 138, 1874)
  2. Meyer et al 2013 (ApJ, 774, 21)
  3. HSTPROMO
  4. Blandford & Znajek 1977 (MNRAS, 179, 433)