Since the advent of large-scale ground-based spectroscopic campaigns, detailed studies of the brightest emission lines in the optical regime have been carried out on millions of galaxies. These studies have provided unequivocal evidence for the ubiquity of supermassive black holes (SMBHs) in massive galaxies and have revealed that when accreting, SMBHs can have a profound impact on the host galaxies in which they reside.

However, there have not yet been any systematic studies of the much fainter high ionization forbidden lines in a large sample of galaxies. This is a significant deficiency because these emission lines, often referred to as “coronal lines” due to their initial discovery in the sun’s corona, arise from collisionally excited forbidden fine-structure transitions from highly ionized species in dense gas, and are a powerful diagnostic in uncovering active galactic nuclei (AGNs) in galaxy populations in which all other available tools are often ineffective.

Coronal line emission can be used to constrain accretion disk models, study the properties of dense gas in close proximity to the SMBH, and trace outflows of ionized gas near their launch regions. Because the SED of an accretion disk hardens with decreasing black hole mass, coronal line ratios may even constrain black hole mass and reveal accreting intermediate-mass black holes (IMBHs).

Our group has been conducting an extensive exploration of coronal line emission in large galaxy samples and quasars, modeling their physical origin and diagnostic power.

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, the Gemini Observatory, and VLT/MUSE to explore this possibility and determine the best diagnostics to identify these objects and measure their masses.

Observing the earliest galaxies and their black holes during the epoch of reionization is one of the major goals of JWST. While JWST has delivered spectacular rest-frame optical and UV observations of galaxies at high redshift, these observations lack the spatial resolution, sensitivity, and access to longer wavelengths necessary to constrain the ionizing radiation field, and map the state and structure of the interstellar medium (ISM) of these galaxies. Thus, despite the tremendous progress in uncovering faint AGNs in the early universe, our understanding of the earliest black holes and their impact on their host galaxies remains incomplete.

While it is not possible to uncover faint AGNs in metal-poor dwarfs and study the detailed physics of their interaction with their host galaxies at high-z, objects like the faint AGNs discovered by JWST at high-z do exist in the local universe.

We are fortunate to be leading a pilot project on searching for accreting black holes in the least massive, most metal-poor galaxies in the local Universe, in which JWST can resolve in exquisite detail the state and structure of the ISM and determine if accreting IMBHs are lurking in their centers.

With JWST’s spectacular spatial resolution, we can see intricate details in the center of this compact galaxy. We can see young stars forming, and we can see a bright central source. There are well over 100 spectral lines we can see over only a narrow range in wavelength. On my days at Mauna Kea, we could only see maybe 3 or 4 lines in the same spectral region in galaxies a million times brighter. But in just a few minutes of staring at this smudge in the sky, JWST uncovered a treasure trove of spectral signatures.

You can read about our results in our publication.

According to the current cold dark matter cosmological paradigm, galaxy interactions are an integral part of the cosmic history of galaxies and play a critical role in their evolution. Theory predicts that these interactions funnel gas toward the central regions of galaxies, potentially fueling nuclear star formation and triggering gas accretion onto the central SMBH.

However, despite over three decades of extensive research, the observational merger–AGN connection is still a topic of vigorous debate. Many previous studies are limited by small sample sizes, lack of carefully selected controls, limited sampling across merger stages, and wavelength-dependent AGN identification. Since the inflowing material can obscure central regions of interacting galaxies, optical studies may miss obscured AGNs entirely.

We conducted the first large mid-infrared study of AGNs in mergers and galaxy pairs, finding that the fraction of AGNs increases with decreasing projected separation — peaking in post-mergers, where we observe a 10–20× enhancement compared to matched controls. These results support theoretical predictions that obscured, energetically dominant AGNs emerge in the late stages of mergers.

As most galaxies contain SMBHs, and interactions trigger gas inflows, the existence of dual AGNs — two accreting black holes in one system — is a natural consequence. Yet confirmed dual AGNs remain extremely rare.

Our group leads deep infrared and X-ray surveys targeting dual AGNs in tightly interacting pairs. Using Chandra, Spitzer, and ground-based telescopes, we probe the smallest separations currently observable