Research Interests
Host Galaxy & AGN Co-evolution
At the heart of every massive galaxy lies a supermassive black hole (SMBH), a silent architect whose growth and energy output are intricately entwined with the life story of its host galaxy. Understanding the co-evolution of active galactic nuclei (AGN) and their hosts is not just about unraveling the mysteries of black holes themselves—it is about unlocking the processes that have shaped the cosmic structures we observe today.
AGN are far from passive passengers in their galactic homes. Their growth, fueled by the accretion of gas and dust, can influence everything from the formation of stars to the distribution of dark matter. This complex interplay is illustrated conceptually in the figure to the right, which highlights the dynamic relationship between AGN feedback mechanisms and galactic environments, showing jets, winds, radiation, and the mutual impact on interstellar matter.
Credit: NASA, ESA, Leah Hustak (STScI).
The co-evolution of AGN and host galaxies is supported by scaling relations observed in the local and distant Universe. These include the tight correlation between black hole mass and host galaxy properties, such as bulge velocity dispersion ( ) and stellar mass ( ), as depicted in the figure below. These relationships reveal how AGN influence the structural evolution of galaxies over billions of years, leaving a legacy imprinted in observable data.
Ting Xiao et al 2011 ApJ 739 28
The symbiosis between AGN and host galaxies is best understood through multi-wavelength observations. For example, in the figure to the right, the galaxy Hercules A showcases AGN emission observed in optical, X-rays, and radio wavelengths. Each wavelength unveils unique facets of this interaction—from the heating of gas to the obscuration of central activity by dust. Together, these observations paint a more complete picture of AGN feedback’s impact on its host.
This intricate relationship unfolds across cosmic time, from the chaotic starbursts of the early Universe to the quiescent galaxies of today. This evolutionary journey can be traced throughout key epochs, from Cosmic Dawn, when the first SMBHs ignited, to Cosmic Noon, the peak of star formation and AGN activity, and into the present era of galaxy quenching.
Nicholas J. McConnell and Chung-Pei Ma 2013 ApJ 764 184
Credit: X-ray: NASA/CXC/SAO; visual: NASA/STScI; radio: NSF/NRAO/VLA.
Finally, the large-scale impact of AGN is vividly captured in the figure below, where X-ray cavities in a galaxy cluster showcase how AGN influence not just their hosts but their larger environments.
To study host galaxy and AGN co-evolution is to delve into a cosmic symbiosis that has persisted for over 13 billion years. It is an investigation that spans the full electromagnetic spectrum, piecing together clues from distant quasars to the quiet SMBHs lurking in nearby galaxies. Each observation and simulation brings us closer to understanding not only the origins of this complex dance but its enduring legacy in the Universe we inhabit today.
Credit: NASA/CXC/SAO
AGN Feedback: Sculpting Galaxies and Their Environments
AGN are not merely observers in the story of galaxy evolution; they are dynamic agents of change. Fueled by the accretion of matter onto a supermassive black hole, AGN can release vast amounts of energy into their surroundings. This process, known as AGN feedback, has emerged as a cornerstone of modern astrophysics, shaping galaxies and their environments across cosmic time.
AGN feedback manifests in diverse ways, from powerful relativistic jets to radiative winds and intense thermal energy deposition. The figure below provides a conceptual illustration of these mechanisms, highlighting how AGN-driven outflows can heat, expel, or redistribute gas in a galaxy. These processes regulate star formation, enrich the interstellar medium (ISM) with metals, and create large-scale cavities in the circumgalactic medium (CGM).
The impact of AGN feedback is not uniform; its effectiveness depends on the host galaxy’s properties and environment. Observational studies reveal strong correlations between AGN activity and galaxy quenching, with massive galaxies often showing signs of reduced star formation in the presence of AGN-driven outflows. Such correlations also underscore the importance of AGN feedback in explaining observed scaling relations, where AGN activity regulates black hole growth and the galaxy’s stellar velocity dispersion.
The evidence for AGN feedback is clearest when viewed through the lens of multi-wavelength observations. Optical imaging reveals the galactic structure, infrared highlights dust-enshrouded regions affected by AGN radiation, and X-rays trace the high-energy outflows that impact the surrounding gas. Together, these observations offer a comprehensive view of how AGN feedback sculpts its environment.
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.
AGN feedback is more than a cosmic curiosity; it is a critical regulator of galaxy evolution. By balancing the inflow of cold gas and the outflow of enriched material, AGN ensure that galaxies grow in a controlled manner, preventing runaway star formation in some cases and triggering bursts of activity in others. As we continue to study AGN feedback through simulations, scaling relations, and multi-wavelength observations, we gain a deeper understanding of the interplay between black holes and the galaxies they inhabit—a dynamic partnership that has shaped the Universe we see today.
Dual AGNs: When Galaxies Collide
Dual active galactic nuclei (AGN) represent a rare but powerful window into the dynamics of galaxy mergers and the growth of supermassive black holes (SMBHs). Formed when two galaxies, each hosting an SMBH, collide and trigger simultaneous accretion onto both black holes, dual AGNs offer a unique opportunity to study the interplay of black holes, their host galaxies, and their shared environment. These systems are not merely a curiosity of astrophysics; they are pivotal in understanding the hierarchical nature of galaxy and SMBH evolution.
The existence of dual AGNs arises from the chaotic environment of a galaxy merger. As the two galaxies interact, tidal forces disrupt their structure, funneling large quantities of gas toward their central SMBHs. This gas inflow ignites intense accretion and triggers the simultaneous activity of both black holes. The image below illustrates a few examples of dual AGN, showcasing how the merging galaxies drive the inflow of material while their SMBHs carve out dynamic and energetic regions through feedback processes.
Detecting dual AGNs is a challenge due to their typically small separations and overlapping emission. Observational campaigns using high-resolution imaging and spectroscopy have been instrumental in identifying these systems. The host galaxies of dual AGNs often deviate from traditional single-AGN trends, reflecting the influence of their merger-driven environment. Such deviations highlight the unique conditions that drive dual activity, including dense gas reservoirs and strong tidal interactions.
Pfeifle et al., ApJ, Volume 875, Issue 2, i.d. 117, 2019
One of the most compelling examples of dual AGNs is shown in the figure below, featuring a multi-wavelength observation of the merging galaxy pair NGC 6240. Optical imaging reveals the disrupted structure of the host galaxies, while X-ray observations from Chandra detect the simultaneous emission from both SMBHs. This system demonstrates how dual AGNs can act as laboratories for studying not only SMBH growth but also the broader impact of mergers on star formation and galactic structure.
X-ray (NASA/CXC/MIT/C.Canizares, M.Nowak); Optical (NASA/STScI)
Dual AGNs are not just snapshots of cosmic collisions; they are key players in the evolutionary path of SMBHs. As the two black holes continue to spiral toward coalescence, they pave the way for gravitational wave events detectable with next-generation observatories. In the meantime, the study of dual AGNs offers invaluable insights into how galaxies and SMBHs grow together, how feedback shapes their environments, and how the Universe’s most massive objects evolve through cosmic time.