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Kunyang (Lily) Li

Postdoctoral Research Fellow

Institut d'Astrophysique de Paris (IAP)

likun[@]iap.fr (remove “[]”)

Office: 307 Paris Observatory, 98bis Bd Arago, 75014 Paris

The merger of a pair of galaxies, along with the orbital evolution and coalescence of their MBHs, simulated using RAMSES code at a resolution of 100 pc. The red and blue lines represent the trajectories of the primary and secondary MBH, respectively. The black and magenta lines illustrate the sub-grid orbital evolution of two MBHs below the resolution limit (400 pc) simulated using the RAMCOAL sub-grid model (see: https://arxiv.org/abs/2410.07856).

Current Research

My current research with Marta Volonteri and Yohan Dubois, centers on modeling the evolution and coalescence of MBHs "on-the-fly" in cosmological simulations and on developing optimal strategies for detecting gravitational-wave counterparts through electromagnetic searches. With the goal to answer some of the key questions in the origin and growth of MBHs: How and when do MBHs forms? How can we learn from gravitational-wave signals about the formation and evolution of MBHs? How can we model the formation and evolution of MBHs in cosmological simulations for theoretical predictions that can be used to compare with observations of the multi-messenger universe?    

Exploring the universe through multi-messenger astronomy has the potential to solve the long-standing puzzle of the origin and growth of MBHs, a critical question in understanding galaxy evolution. The Laser Interferometer Space Antenna (LISA) and future ground-based gravitational wave detectors are expected to detect MBH mergers during the earliest stages of galaxy formation, occurring less than a billion years after the Big Bang. Pulsar timing arrays (PTAs) and LISA will also help constrain the MBH merger rate over cosmic time. By combining these with deep electromagnetic observations of MBH accretion, we will significantly enhance our understanding of MBH formation and growth. The foundation for successful multi-messenger observation strategies lies in a solid theoretical framework that should be developed before gravitational wave detection of MBHs. Cosmological simulations allow us peer deep into the universe, providing theoretical predictions that can be used to compare with observations of the multi-messenger universe. 

I developed RAMCOAL: a sub-grid model in RAMSES which simulates MBH evolution that incorporates orbital decay, preferential accretion, and active galactic nuclei (AGN) feedback. RAMCOAL also tracks EM luminosity, dual AGN variability, and GW emission from MBHBs below the resolution limit of cosmological simulations, assessing the potential for coalescence detection and localization through both GW and EM signals. This investigation bridges cosmological simulations and multi-messenger astronomy to better understand conditions for MBHB coalescences. These efforts provide a new theoretical framework for uncovering the hidden drivers of galaxy evolution, in preparation for the next generation of observational advances.

Timeline

Gruber Postdoctoral Fellow

Institut d'Astrophysique de Paris

With: Marta Volonteri, Yohan Dubois

2022-2025

Ph.D. in Physics (Amelio Fellow)

Georgia Institute of Technology

With: Tamara Bogdanović, David Ballantyne

2017-2022

Summer Undergraduate Research Fellowship

California Institute of Technology

LIGO collaboration

Bachelor of Science in Astrophysics and Mathematics (Distinguished Student Scholar)

Florida Institute of Technology

With: Eric Perlman

2013-2017

2016

CV/ Contact

My CV can be found at: CV_KunyangLi

My papers: paper_library

My email: likun[@]iap.fr (remove “[ ]”)

Skills

Cosmological simulation code: RAMSES

Programming: Python, C/C++, Fortran90, ChatGPT, MATLAB, Mathematica

Astro softwares: SAOImageDS9, IRAF, TOPCAT

WP: LATEX, OpenOffice, MS Word

Languages: English, French, Chinese

Previous Research

MBH coalescence probability as a function of the host galaxy and MBH pair properties with and without radiative feedback: binary mass (top left), mass ratio (top right), gas fraction (bottom left), and gas disk rotational speed (bottom right).

Detectability of dAGNs in the models without (top) or with (bottom) the radiation feedback effects.

MBHs are typically found at the centers of most massive galaxies. When galaxies merge, their respective MBHs begin orbiting within the gravitational potential of the resulting merger remnant. Through interactions with the stars and gas in the remnant galaxy, some MBH pairs gradually migrate toward the galactic core, eventually coalescing and becoming powerful sources of GWs. To predict these signals, I modeled the physical processes within the remnant galaxy that drive MBHs to coalesce. I constructed a parameter space of approximately 40,000 model galaxies, which spans a broad range of initial orbital eccentricities and includes both prograde and retrograde MBH orbits. These models allowed me to comprehensively assess how different orbital decay mechanisms affect MBHB evolution across diverse galaxy types and orbital configurations. 

I quantified the effect of radiation feedback from the innermost regions of accretion flows on both the decay time and the probability of MBHB coalescence. My findings indicate that radiation feedback can have a significant influence on the evolution of MBH pairs in post-merger galaxies.

Dual active galaxy nuclei (dAGNs) trace post-merger galaxies and serve as precursors to MBH mergers. Another application of the model I developed is quantifying the detectability of dAGNs. This study is crucial for both current and future EM and GW observatories, as it highlights where to search for possible MBH coalescences or the most detectable dAGNs. I incorporated in my semi-analytical model a prescription to model accretion onto both MBHs and examined the observational properties of these dAGNs. By analyzing how detectability varies with galaxy properties and orbital characteristics, I identified the types of post-merger galaxy remnants most likely to host EM-detectable dAGNs. This provides a practical approach for selecting dAGN candidates for follow-up observations.

I estimated LISA detection rates for various binary orbital configurations, with and without radiation feedback, and investigated the characteristics of MBHBs most likely to be detected by applying my model to MBHBs from the IllustrisTNG cosmological simulation. Figure on the left illustrates the expected cosmological merger rate during the first four years of the LISA mission, with hot pixels highlighting the parameter values where mergers are most likely to occur. The overlaid contours represent the LISA signal to noise ratio and indicate the portion of these cosmological mergers detectable by LISA. These contours largely overlap when no radiation feedback (RF) is included (top panels) because most of the mergers in this scenario involve lower-mass MBHBs, a population to which LISA is highly sensitive. However, when RF is included (bottom panels), the merger evolution for lower-mass MBHBs is significantly delayed, shifting the locus of high merger rates toward higher-mass MBHBs. These higher-mass MBHBs can still merge without being significantly impeded by RF effects. Unfortunately, LISA is unable to detect these mergers, resulting in a significantly lower detection rate when RF is considered. This figure thus serves as a predictive tool for identifying the galaxy properties most likely to host LISA-detectable coalescence.

Outreach

I’m actively involved in astronomy outreach. I’m an active member of the International Dark-Sky Association, and I have been promoting awareness of the value of dark, star-filled night skies and encourage their protection and restoration through organizing and participating public stargazing and dark sky events since high school. I aim to bring my knowledge and dedication to the community, especially to people from communities that are under-represented in science and technology.

Life

  • Hiking

  • Protect the nature

  • Grow it

  • Fight against the light pollution

  • Bring back the dark starry sky

  • Into the nature

  • More hiking

  • Greens

  • Gigantic mushroom!