supermassive black holes – Kayhan Gultekin

I am pleased to announce that Korbin Waters’s latest paper, A Supermassive Black Hole Mass Measurement in NGC 5102 with Schwarzschild Orbit-superposition Modeling, has been officially published in ApJ. The paper addresses one of the gaps in our knowledge: the low-mass end of the black hole mass function. There, dynamical mass measurements are still rare enough that we don’t know what’s going on with the scaling relations.

Korbin measured the mass in NGC 5102 using data from the VLT/MUSE IFU and HST/STIS observations of the Ca II triplet and HST/WFPC2 F547M imaging. The mass measured is $1.3^{+0.19}_{-0.18} \times 10^{6}\,M_{\scriptscriptstyle\odot}$ at an assumed distance of 3.66 Mpc.

What’s especially compelling about this result is the agreement between Korbin’s measurement and an earlier method using Jeans anisotropic modeling of the CO band heads.

Congratulations, Korbin!

This figure shows where the new measurement lands on the $M–\sigma_{e}$ relation. NGC 5102 (orange) sits in the sparsely populated low-mass end, just above the nominal 10⁶ M⊙ boundary between supermassive and intermediate-mass black holes, and in close agreement with the earlier Nguyen et al. (2019) measurement of the same galaxy (teal). Both are consistent with an extrapolation of the Kormendy & Ho (2013) relation, which is anchored almost entirely by the far more numerous measurements at $\sigma_e > 100\,\textrm{km}\,{\textrm{s}^{-1}}$ .

Two side-by-side blue-scale images of the lenticular galaxy NGC 5102, each overlaid with white contours of constant surface brightness and marked with north and east compass arrows and a 15-arcsecond scale bar. The left panel is a smaller, rotated high-resolution field showing a bright, mottled nucleus with visible dust structure. The right panel covers a wider field, showing the same galaxy as a smooth elliptical glow with concentric contours and a dark dust lane crossing just below the center. — NGC 5102 imaged with contours of constant surface brightness overlaid. The high-resolution HST view (left) resolves the dust and structure in the innermost region, while the wider MUSE field (right) traces the smooth, regular isophotes of the outer galaxy. Together they supply the light distribution that the dynamical models turn into a mass.

I am excited to share that my graduate student CJ Harris will be presenting their research on binary supermassive black holes (SMBHs) at the 16th International LISA Symposium this summer. The event will be held in College Park, Maryland, from July 6–10.

CJ’s work investigates the evolution and detection prospects of these massive systems, and their attendance is supported by travel funding awarded by the LISA Consortium. This is a great opportunity to engage with the community as the mission continues to take shape.

We look forward to sharing more specifics about the research results closer to the symposium dates!

We are pleased to share that group member Korbin Waters has received a Rackham Graduate Student Research Grant. These funds are designed to support Rackham graduate students in carrying out research that advances their progress toward their degree.

Smiling man with short hair — Korbin Waters

This grant will facilitate Korbin’s upcoming research visit to Chile to collaborate with Neil Nagar and his research group. The primary focus of the visit is to work on advanced data reduction techniques for the MUSE (Multi Unit Spectroscopic Explorer) instrument, located on the VLT (Very Large Telescope).

By using the high-spatial-resolution integral field spectroscopy provided by MUSE, Korbin will continue his efforts on SMBH mass measurements. This collaboration is a vital step in our group’s work exploring the scaling relations and evolution of black holes.

Congratulations to Korbin on securing this support for his dissertation research!

I am pleased to announce that Cayenne Matt’s latest paper—Inferring M_BH–M_bulge Evolution from the Gravitational-Wave Background—has been officially published in The Astrophysical Journal. This work addresses a major question in modern multi-messenger astronomy: why is the observed gravitational-wave background (GWB) amplitude significantly higher than standard astrophysical models predict?

By comparing semi-analytic models with the observed GWB spectrum, Cayenne’s results suggest that to reproduce the GWB amplitude seen by pulsar timing arrays, one must either assume a significantly higher number density of massive galaxies at high redshift than previously thought, or allow for a positively evolving M_BH–M_bulge relation. Specifically, the study finds evidence that the M_BH/M_bulge ratio may have been higher in the past, evolving as α(z) = α₀(1+z)^1.04±0.5.

A scientific line plot showing the posterior probability density for the parameter alpha-z, which represents the redshift evolution of the black hole-bulge mass relation. Four colored curves (red, blue, gold, and green) represent different model configurations. Most models show a peak in probability at positive values between 0.5 and 1.5, indicating that the ratio of black hole mass to bulge mass was likely higher in the early universe. Horizontal error bars indicate 68 percent confidence regions, and a gray dashed line represents the uniform prior. — This plot displays the posterior distributions for αz across the various models tested in the paper. The distributions fall into two categories based on how the Galaxy Stellar Mass Function (GSMF) parameters were handled. Models with fewer degrees of freedom converged to higher values of α*_z* with higher confidence, providing strong evidence for a positive evolution in the M_BH–M_bulge amplitude.

Full Citation: Matt, C. et al. 2026, ApJ, 997, 188.
DOI:10.3847/1538-4357/ae2480
NASA ADS:2026ApJ…997..188M

CJ Harris, astrophysicist

I am thrilled to highlight the exceptional work of CJ Harris, who has just submitted a significant new paper to AAS Journals. CJ is currently a PhD candidate at the University of Michigan and is officially on the postdoc market (see their CV). Their latest research provides a critical analysis of how stellar scattering is likely to be insufficient on its own to explain how supermassive black hole binaries harden.

Breaking the “Final Parsec” Bottleneck

Although we are now reasonably certain that supermassive black holes form binaries that harden sufficiently to coalesce through gravitational wave emission, the exact solution to the “Final Parsec Problem” is unknown to us, if not to nature.

In their new paper, “Core Scouring Dynamics and Gravitational Wave Consequences: Constraints on Supermassive Black Hole Binary Hardening,” CJ and collaborators (Laura Blecha and I) use a multi-messenger approach to tackle this. By making use of merger trees from the IllustrisTNG cosmological simulations with physically motivated models of core scouring, they were able to see how efficient stellar scattering is in hardening binaries. They were able to reproduce the relationship between mass deficit and galaxy total stellar mass, but only if scattering is more efficient than previously expected. But they were not able to reproduce the nanohertz gravitational wave background with the same efficiency. Meaning that there must be some other source of hardening, most likely gas hardening. If gas hardening plays a dominant role, then we can be far more optimistic about finding binary AGN.

Scientific scatter plot on logarithmic scales. The x-axis represents galaxy log stellar mass ranging from 10 to 13.5 in solar units. The y-axis represents log stellar mass deficit ranging from 6 to 11 in solar units. The plot features numerous colored data points with error bars: green diamonds (Brightest Cluster Galaxies), blue circles (Ellipticals), purple triangles (Lenticulars), a maroon square (Early Type Spirals), and a single yellow circle highlighting M87. These observational points sit atop a underlying density of small black dots and gray contour lines labeled 'TNG Cores,' showing that the simulated merger models closely track the observed trend of increasing mass deficit with increasing galaxy mass. — The figure shows stellar mass deficit as a function of stellar mass for both the observed set of core galaxies and a sample generated using our core scouring model with Ĥ = 1.64. Synthetic data-points are shown in black with the corresponding KDE and are in agreement with the observed data. We reproduce the slope and amplitude of the mass deficit–stellar mass relation, and recover some of the flattening seen in the BCG population at the high mass end. (Fig. 7 from Harris et al. 2026)

Key takeaways from the work:

Efficiency Gap: To match observations of real galaxies, the process of binary hardening via stellar scattering must be 1.6x faster than previous N-body experiments suggested.
The Role of Gas: Most importantly, the paper demonstrates that even the most efficient stellar scattering cannot fully explain the low-frequency turnover in the gravitational wave background.

A double-logarithmic line plot showing gravitational wave strain versus frequency. Three colored curves with shaded confidence intervals are compared against vertical gray violin plots of observational data. The curves show varying degrees of "turnover" at lower frequencies. — Shown on the y-axis is the characteristic strain amplitude of the GWB, with frequency in Hz on the x-axis. The gray violins show the GW spectrum derived from the NANOGrav HD-w/MP+DP+CURN models (G. Agazie et al. 2023a; W. G. Lamb et al. 2023). The colored curves are GW spectra resulting from SBHB populations generated by the holodeck semi-analytic modeling submodule (SAMs) over 1000 realizations. Shown in green is the spectrum expected if the hardening mechanism that drove all binaries to coalescence was GW emission alone. The orange spectrum is the G. Agazie et al. (2023b) fit to the GWB data. The pink hybrid model is comprised of the phenomenological model where the parameters in equation (24) are set by the results of our model. Shaded regions indicate the 68% confidence intervals. The phenomenological and hybrid models are in agreement in the high frequency regime, but diverge at low frequencies. The discrepancy implies that the attenuation of the GWB at low frequencies is not caused by energy being transferred into the environment through loss-cone scattering alone. Because the attenuation occurs at such low binary separations the effect is most likely caused by gas dynamics.

The “Bright” Side: This strongly suggests that gas dynamics must play a dominant role in bringing these giants together—providing a reason to be optimistic about finding electromagnetically visible binary AGN.

Community Leadership & Presence

Beyond the data, CJ continues to be a leader in the broader physics community. This past November, they presented these findings at the National Society of Black Physicists (NSBP) 2025 Annual Meeting. Their talk, “Multimessenger Constraints on the Supermassive Black Hole Binary Hardening Timescale,” was a deep dive into these results and showcased their ability to bridge complex theory with observational reality.

I’m thrilled to highlight the outstanding research contributions of four talented undergraduate students who have been working with me and CJ Harris this past academic year: Maria Herrmann (she/her), Eva Jaku (she/her), August Moser (he/him), and Olivia Korensky (he/him).

These students have been analyzing Hubble Space Telescope imaging data to search for indirect evidence of past binary supermassive black hole (SMBH) activity in large, nearby galaxies. As you may know, supermassive black holes, with masses millions to billions of times that of our Sun, reside at the centers of most galaxies. When galaxies merge, these black holes sink to the center of the resulting galaxy and eventually form a binary pair. These binaries are significant because they are expected to emit gravitational waves, and in the past year, experiments using pulsar timing arrays have announced tantalizing evidence for the detection of these waves.

Despite this monumental breakthrough, confirmed observations of binary supermassive black hole systems remain elusive. Our research aims to address this puzzle by looking for a telltale sign of these systems in action: “missing light” at the center of galaxies. Interactions between binary supermassive black holes and stars in the galactic center can eject stars outwards, creating this deficit that can be detected in Hubble images.

black and white image of elliptical galaxy with small black dots where stuff was masked out.

Maria, Eva, August, and Olivia have played a crucial role in analyzing Hubble images at infrared and optical wavelengths to identify these subtle features. Their diligence and hard work culminated in poster presentations at two recent events: the Astronomy Department Undergraduate Research Day (Eva, August, and Olivia) and the UROP symposium (Maria, Eva, and August).

I’m incredibly proud of their dedication and the valuable contributions they’ve made to our understanding of galaxy evolution and supermassive black hole binaries. I wish them all the best in their future endeavors, and I’m especially excited to share that Olivia will be continuing his research journey this summer in Krakow, Poland!

Picture of young man in front of a scientific poster. The man's nametag reads "August Moser" — August Moser

Picture of young woman in front of a scientific poster. The woman's nametag reads "Eva Jaku". — August Moser

It has been too long since I have put up research updates. I really need to get back to my promise of having a page for each major research product.

In the meantime, here is a mini update on two black hole mass measurements.

NGC 3258

Korbin Waters used MUSE A/O data to measure the black hole mass in NGC 3258 in our first paper together via stellar dynamical techniques. The great thing about this one, is that we got a mass of (2.2 ± 0.2) × 10⁹ M_⊙, which is in close agreement to the ALMA-based CO measurement found by Boizelle et al. (2019). The agreement between the two methods is a great verification of both methods of measuring black hole masses.

Two color-scale images with contours overplotted showing the surface brightness of the galaxy. The left image is labeled "MUSE NFM" with a 1 arcsec scalebar about one fifth of the width of the square image. The right image is labeled "MUSE WFM" with a 10 arcsec scalebar about one fifth of the width of the square image.

The Black Eye Galaxy

The second is a galaxy that I have been working on for years: NGC 4286, aka M64, aka the Black Eye Galaxy, aka the Evil Eye Galaxy, aka the Sleeping Beauty Galaxy. This one turned out to be so tricky because of a number of key technical issues. In the end, we were able to conclude that this galaxy has a complicated photometric structure that includes a composite classical bulge and pseudobulge. The black hole mass is M = 8.4_‑0.6^+1.7×10⁶ M_⊙, as we reported in our paper Gültekin et al. (2024). This may be the most beautiful galaxy I have ever worked on. It was highlighted by AAS Nova as a Featured Image.

Two-panel image showing a disk galaxy with a dusty inner obscuring area as well as ring-like structures. — Multicolor color composite images of NGC 4826. The left image is a *gri*-band composite from the Sloan Digital Sky Survey via NED. The right image is a color composite made using the sum of 2MASS J, H, and K images for the blue channel, the Spitzer Space Telescope Channel 1 image at 3.55 μm for red and the average of the two for green. Both panels show the same field of view, the width of which is 11.′9; north is up and east is left. The right panel shows the Figure HST Hubble Heritage field outlined in black. The dusty disk that dominates visible bandpasses is a slightly rim-brightened bright disk (a “lens”component) when seen in the infrared, where dust extinction is less important.

Iconic HST Heritage Image of M64, showing a spiral galaxy with a dark dust lane that some say resembles a black eye. — Multiband image of NGC 4826 from STScI’s Hubble Heritage project. HST data used to make the image come from two programs: GO-8591 and GO-9042. Colors are blue: F450W (B), cyan: F547M (Strömgren y), red: F656N (Hα), and pink: F814W (I). North is up; east is left, and the NE side of the galaxy is the nearside. The long axis of the rectangular bounding box is 137″. The prominent dust lanes, a clear view of the small bulge, and inclination angle of the disk are evident from the image. At small radii, there is a composite classical bulge plus pseudobulge. Decoupling these components is necessary to interpret correlations between black hole mass and bulge properties. Parameters of two overlapping components are strongly coupled even in the absence of dust, and deriving them here required special care. A key revelation from our analysis is that the internally absorbed and very dark parts of the inner galaxy (in this figure) are actually higher in stellar density than the outer disk.