Research Interests
Host Galaxy & AGN Co-evolution
At the center of every massive galaxy lies a supermassive black hole (SMBH)—a quiet powerhouse whose growth is deeply intertwined with the evolution of its host. Studying how AGN and their galaxies co-evolve isn’t just about understanding black holes themselves; it’s about uncovering the processes that have shaped cosmic structures across billions of years.
AGN are anything but passive. Fueled by the accretion of gas and dust, they can drive energetic feedback that influences everything from star formation to the thermodynamic state of the circumgalactic medium. Through mechanisms like jets, winds, and radiation, AGN interact with their surroundings in complex and sometimes dramatic ways—sculpting the interstellar medium and regulating the galaxy’s ability to grow.
Credit: NASA, ESA, Leah Hustak (STScI).
We see evidence of this co-evolution in well-established scaling relations—such as the correlation between black hole mass and properties like bulge velocity dispersion or stellar mass. These relationships suggest a shared history, where black holes and galaxies grow in tandem, leaving observable imprints in both the local and distant Universe.
Understanding this relationship requires a multi-wavelength approach. Observatories across the electromagnetic spectrum reveal different layers of AGN-galaxy interaction. For example, in the case of Hercules A, X-rays trace hot gas, optical data reveals ionized structures, and radio observations show the extent of relativistic jets. Each wavelength adds a piece to the puzzle, helping us reconstruct the physical conditions and feedback processes at play.
This symbiosis between SMBHs and their hosts evolves over cosmic time—from the starbursting, merger-rich early Universe to the more quiescent galaxies we see today. From Cosmic Dawn, when the first black holes ignited, through Cosmic Noon and into the present era of galaxy quenching, AGN have played a central role in shaping how galaxies grow and change.
In some cases, the influence of AGN extends well beyond individual galaxies. X-ray cavities in galaxy clusters, for instance, offer striking evidence of feedback operating on vast scales, injecting energy into the surrounding environment and altering its evolution.
To study AGN-galaxy co-evolution is to explore a long-standing cosmic partnership—one that’s shaped the structure, dynamics, and star-forming potential of galaxies across time. My work focuses on tracing this relationship through observations and simulations, bridging the local Universe and the high-redshift cosmos to better understand the legacy of AGN feedback in galaxy evolution.
Credit: X-ray: NASA/CXC/SAO; optical: NASA/STScI; radio: NSF/NRAO/VLA.
AGN Feedback: Sculpting Galaxies and Their Environments
AGN aren’t just bystanders in the story of galaxy evolution—they’re powerful engines capable of reshaping their surroundings. As matter falls onto a supermassive black hole, it releases enormous amounts of energy, launching outflows, heating gas, and sometimes halting star formation altogether. This process, known as AGN feedback, plays a key role in regulating how galaxies grow and evolve over time.
Feedback can take many forms: relativistic jets, radiative winds, thermal energy injection, and more. These mechanisms can heat or expel gas from the galaxy, enrich the interstellar medium with metals, and carve out massive cavities in the circumgalactic medium. The figure below illustrates these different modes of feedback, capturing the physical processes that drive AGN-galaxy interactions on both small and large scales.
Schematic viewing of the different outflow scales, from accretion-disc (a), to galaxy- (b) and halo-scales (c). Panels d, e, f show the UFO of PDS 456 observed in the X-rays (Nardini et al. 2015), the molecular outflow of Mrk 231 observed in the radio and mm bands (Cicone et al. 2012; Morganti et al. 2016), and the kpc-scales outflow in NGC 1365 observed in the optical (Venturi et al. 2018). Credits: Cicone et al. 2018 Nat. As. 2, 176.
But feedback isn’t a one-size-fits-all phenomenon. Its impact depends on the host galaxy’s structure, gas content, and environment. In many massive galaxies, AGN-driven outflows are linked to suppressed star formation—supporting the idea that AGN activity helps regulate stellar growth and contributes to the quenching of galaxies. These connections are reflected in key scaling relations, where black hole mass and AGN luminosity correlate with galactic properties like stellar velocity dispersion and star formation rate.
Multi-wavelength observations are essential to understanding how AGN feedback operates. Optical data maps out the galaxy’s morphology and ionized structures; infrared observations reveal dust-obscured regions influenced by AGN radiation; and X-rays trace hot gas and energetic outflows. Each wavelength uncovers a different facet of feedback, allowing us to build a more complete picture of how AGN shape their environments.
Far from being a secondary process, AGN feedback is a fundamental mechanism in galaxy evolution. It can suppress, regulate, or even trigger star formation depending on the context—balancing the inflow of cold gas with the outflow of enriched material. As we continue to explore this phenomenon through simulations and observations across the electromagnetic spectrum, we’re uncovering how black holes influence not just their host galaxies, but the larger cosmic web around them.
Dual AGNs: When Galaxies Collide
Dual AGN offer a rare but powerful glimpse into the messy, dynamic process of galaxy mergers and black hole growth. These systems form when two galaxies—each hosting a supermassive black hole (SMBH) —collide, triggering accretion onto both SMBHs at the same time. While rare, dual AGN are key to understanding how galaxies and their central black holes evolve together, especially in the context of hierarchical structure formation.
In a merger, tidal forces disrupt the galaxies’ structure and funnel gas toward their centers. This inflow fuels both black holes, igniting AGN activity in each and setting the stage for complex feedback interactions. The image below highlights examples of dual AGN systems, capturing the energetic interplay between merging galaxies, infalling gas, and SMBH-driven outflows.
Identifying dual AGN isn’t easy. Their small physical separations and overlapping emission often mask their dual nature, especially at optical wavelengths. High-resolution imaging and spectroscopy—especially in X-rays and the infrared—are essential tools for confirming their presence. These systems frequently defy the expectations set by isolated AGN hosts, showing structural and spectral features shaped by the violent dynamics of a recent merger.
One of the most iconic examples is the merging galaxy pair NGC 6240. Multi-wavelength observations reveal two actively accreting SMBHs deeply embedded in a chaotic galactic core. Optical images show the highly disturbed morphology of the host galaxies, while Chandra’s X-ray vision captures the distinct AGN signatures from each black hole. Systems like this are natural laboratories for studying SMBH fueling, feedback, and the transformation of galaxies through interaction.
Credit: NASA, ESA, and M. Koss (Eureka Scientific, Inc.); Keck images: W. M. Keck Observatory and M. Koss (Eureka Scientific, Inc.); Pan-STARRS images: Panoramic Survey Telescope and Rapid Response System and M. Koss (Eureka Scientific, Inc.)
Dual AGN are more than snapshots of galactic collisions—they’re crucial waypoints on the path to black hole coalescence. As their host galaxies merge and the SMBHs spiral inward, these systems eventually become sources of low-frequency gravitational waves, detectable by future observatories like LISA. In the meantime, studying dual AGN gives us vital clues about how SMBHs grow, how feedback reshapes the surrounding galaxy, and how some of the most massive structures in the Universe come to be.
In my own research, I led the discovery of the closest-separation dual supermassive black hole system confirmed to date through multiwavelength observations. Located just 760 million light-years away, this rare pair was identified using data from both NASA’s Hubble Space Telescope and the Chandra X-ray Observatory. The system showcases two actively accreting black holes in a late-stage galaxy merger, offering an unprecedented glimpse into the final phases of SMBH pairing. It stands as a compelling local analog to the kinds of mergers that will eventually produce gravitational wave signals detectable by future observatories like LISA.
Credit: NASA, ESA, Anna Trindade Falcão (CfA); Image Processing Joseph DePasquale (STScI)