Research

 

My work focuses primarily on the evolution of supermassive black holes and their host galaxies. I'm especially interested in the role of galaxy mergers, the signatures of black hole inspiral, merger, and recoil, and the population of black holes in dwarf galaxies. Much of my work involves numerical modeling that allows direct comparisons with observations, and I collaborate on a number of observational projects as well. Here are a few highlights:


  1.   Signatures of gravitational-wave recoil

  2.   Candidate recoiling black holes

  3.   Signatures of active black holes in merging galaxies

  4.   Active black holes in low-mass galaxies


Signatures of gravitational-wave recoil


Supermassive black hole binaries inevitably form during galaxy mergers. If the black holes themselves merge, copious amounts of energy is released in "gravitational waves", and a "recoil kick" is imparted to the merged remnant. This kick can even eject the BH from the galaxy entirely in some cases. If actively accreting, recoiling BHs could be detected as offset from their host galaxy nuclei.


I have done extensive modeling of recoiling BHs, their observable signatures, and the effects of recoil on host galaxies. Most recently, as part of the Illustris Project, I used cosmological hydrodynamic simulation data to predict that a population of recoiling AGN could be detectable even with ground-based all-sky surveys.


Left: slice of the Illustris simulation volume at z=0, showing dark matter density transitioning to gas density (left to right) (Credit: the Illustris collaboration). Left inset: artist's impression of gravitational waves from inspiraling BHs (Credit: K. Thorne & T. Carnahan, NASA GSFC). Right: Distribution of spatial and velocity offsets for recoiling active BHs, from merger events across cosmic time in the Illustris simulation. Recoil trajectories and AGN luminosities are calculated in post-processing. The top panel corresponds to an "optimistic" kick distribution, and the bottom panel shows a more realistic model where large kicks are suppressed in gas-rich mergers. Both models predict that a population of recoiling BHs could be detected in the near future. (From Blecha et al. 2015.)





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Candidate recoiling black holes


I'm actively involved in searches for and follow-up of candidate recoiling black holes. Thus far, two recoiling black hole candidates appear particularly promising.


SDSS 1133 is a candidate recoiling black hole offset from the dwarf galaxy Mrk 177 (Koss, Blecha et al. 2014). This galaxy merger simulation illustrates how a merger and recoil event could produce the observed offset (credit: NASA GSFC/L. Blecha):




Another strong candidate recoiling black hole, CID-42, is clearly hosted in a recent galaxy merger:

 

Left: HST F814W image of CID-42. (Credit: Civano et al. 2010.) The simulated models (lower panels) show two possible scenarios: a recoiling black hole (left) or an inspiraling black hole pair (right). Only the candidate recoiling black hole is actively accreting; if a black hole is present in the other nucleus, it must be quiescent. (From Blecha et al. 2013a).





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Signatures of active black holes in merging galaxies


Mergers between galaxies are important drivers of galaxy evolution, and they can simultaneously trigger rapid fueling of their central black holes. Inevitably, these black holes will also form a binary pair. Hundreds of candidate pairs have been identified, but the confirmed sample is still small and strongly influenced by selection effects. Theoretical studies are therefore crucial for overcoming these biases and understanding the link between mergers and black hole fueling. To this end, I work on simulating various multi-wavelength signatures of active black holes during galaxy mergers.


    

Top left: projected stellar density, showing a double-core structure (5.5 kpc scale). Bottom left: corresponding velocity distribution of NL gas (1 kpc scale). Arrows show projected BH velocity and are color-coded according to line-of-sight velocity. Right: resulting velocity profile for each NL region separately, and the combined double-peaked profile. (From Blecha et al. 2013b.)




    

Example of a double-peaked NL AGN produced by dual SMBH motion on kpc scales. The top panels show the Hbeta line profile with and without attenuation/scattering, respectively; the middle panels show mock slit spectra for the same, and the bottom panels show the Hbeta images for the same (white lines denote the “slit” position). (From Blecha et al., in prepration.)



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Active black holes in low-mass galaxies


While supermassive black holes are virtually ubiquitous in massive galaxies, we know little about their existence in low-mass, or “dwarf” galaxies. In collaboration with Richard Mushotzky and Michael Koss, I’m studying ultra-hard X-ray selected AGN in low-mass hosts, which constitute a uniquely unbiased sample of AGN in the local universe. I’m also working on understanding the effects of AGN feedback in the shallow potential wells of dwarf galaxies.


NGC 4395, a nearby dwarf galaxy with an active central black hole. Credit: David W. Hogg, Michael R. Blanton, and the SDSS Collaboration. (Image source)


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CV