Dual Supermassive Black Holes

Since the vast majority of galaxies contain supermassive black holes (SMBHs) and galaxy interactions trigger nuclear gas accretion, a direct consequence of the hierarchical model of galaxy formation would be the existence of binary active galactic nuclei (AGNs). The existence, frequency, andcharacteristics of such binary AGNs have important astrophysical implications on the SMBH mass function and the interplay between SMBHs and its host galaxy. Despite decades of searching, and strong theoretical reasons that they should exist, observationally confirmed cases of binary AGNs are extremely rare, and most have been discovered serendipitously.

Our group has been carrying out an infrared and X-ray investigation of interacting galaxies to search for dual AGNs at the smallest pair separations currently known.  These studies employ observations from the Chandra X-ray Observatory and several infrared ground and space-based observations.

Illustration of supermassive black hole pair. Credits: NASA/CXC/A.Hobart

Supermassive Black Holes in Bulgeless and Low Mass Galaxies

We now know that supermassive black holes (SMBHs) one million to a few billion times the mass of the sun lurk in the centers of possibly all bulge-dominated galaxies in the local Universe and that their mass is strongly correlated with the galaxy’s bulge mass. These observations have formed the basis of the widely held view that galaxies and their central black holes are fundamentally linked: a link that can most simply be understood if galaxy mergers induce bulge growth and feed the central black hole in concert. While the SMBH and host galaxy properties in the high bulge mass regime have been studied extensively, very little is known about the existence and properties of SMBHs in galaxies with low masses and those with small bulges. This is a significant deficiency since the study of this population allows us to gain an understanding of merger-free pathways to black hole growth. Furthermore, the occupation fraction and properties of SMBHs in galaxies with low masses and in those with no evidence for a bulge provide one of the only observational constraints on the origin and growth efficiency of SMBH seeds, thought to have formed at high redshift. Since SMBHs in massive bulge-dominated galaxies have undergone significant accretion through multiple dynamical interactions over cosmic history, any information on the seed population will not be retained. In contrast, galaxies that lack significant bulges have undergone a more secular evolution and therefore the mass distribution and occupation fraction of SMBHs in these galaxies contains clues about the original seed population, allowing us to discriminate between lower mass seeds formed from stellar remnants or massive seeds formed directly out of dense gas . The study of black holes at the low bulge mass regime is therefore crucial to our understanding of both the origin of SMBHs and their growth and connection to galaxy evolution.

Intermediate Mass Black Holes

While we know that most, if not all, massive galaxies host a supermassive black hole, there is currently no direct evidence for black holes with masses between ~100 – 50,000 times the mass of our Sun. Black holes in this “mass desert” are crucial to our understanding of black hole seed formation, as their occupation fraction and mass function could hold vital clues that will allow us to differentiate between lower mass seeds formed from stellar remnants, or massive seeds formed directly out of the collapse of dense gas.  Mergers between black holes in this mass range are also one of the most promising sources of gravitational waves detectable with the Laser Interferometer Space Antenna (LISA), yet black hole pairs in this mass range have not been identified and their merger rate is unknown.

The search for intermediate mass black holes (IMBH) holds many challenges that are less present when observing larger black holes.  The sphere of influence of an IMBH is too small to be able to detect them kinematically outside our galaxy, so many can only be detected if they are accreting.  However, accreting IMBHs are likely to be found in the centers of low-mass galaxies, where star formation in the host galaxy can dominate the optical spectrum, and gas and dust can obscure the central engine at optical and X-ray wavelengths.  Even if an IMBH is highly accreting and unobscured, their low luminosity makes them indistinguishable from high mass X-ray binaries in the X-ray, or compact nuclear starbursts in the radio.  Observations of prominent high-excitation state infrared emission lines may offer us one of the only definitive tools to discover buried IMBHs.  Our group uses photoionization modeling and observations at Keck Observatory and the Large Binocular Telescope (LBT) to explore this possibility and determine the best diagnostics to identify these objects and measure their masses in preparation for the launch of the most sensitive infrared observatories to date, the James Webb Space Telescope (JWST).

Artist Rendition of JWST in Orbit. Credits: NASA/STSCI