Research -- Intergalactic Medium (IGM)
Advisors: Prof. George Becker (UCR)
Tracing the filamentray IGM with QSO Pairs
For over fifty years, absorption-line studies of the intergalactic and circum-galactic medium (IGM and CGM) have provided a wealth of information about cosmic structures. The Ly$\alpha$ forest in the spectra of QSOs traces the filaments that contains most of the baryons in the Universe, however, absorption lines face a fundamental limitation: they are only one-dimensional. Thanks to the advent of large IFUs on 8–10m telescopes (KCWI and MUSE), it has finally become possible to study the IGM three dimensionally in emission. We are trying to study the IGM that combines the detailed line-of-sight information from the Ly$\alpha$ forest absorption from QSOs spectra with the three-dimensional information provided by Ly$\alpha$ emission at redshift z~2. We will determine whether the emission traces the same structures as the absorption, and measure the emission in a way that is conducive to modeling via hydrodynamical simulations. Directly comparing Ly$\alpha$ absorption and emission, for example, will give us greater leverage for evaluating the relative contributions of different emission mechanisms (eg. recombination, collisional excitation, UV background reprocess).
Research -- Galaxy formation & Evolution
Advisors: Prof. Huiyuan Wang (USTC), Prof. Houjun Mo (UMass)
One of the topic of my research is galaxy formation and evolution especially on galaxy quenching. I am trying to understand why a galaxy stops forming new stars.
Dynamical hotness, star formation quenching and growth of supermassive black holes
With Mappping Nearby Galaxies at APO (MaNGA) survey, we study galaxies in spatially resovled perspective. A stellar system is dynamically hot when its kinetic energy is dominated by random motion represented by the stellar mass-depedent velocity dispersion $\sigma_{\rm hot}(M_*)$. Both inner and outer velocity dispersion ($\sigma_{\rm in}$ and $\sigma_{\rm out}$) of a galaxy are taken to characterize its dynamical status and study its connection with star-formation quenching and the growth of supermassive black hole (SMBH). We classify galaxies into fully quenched galaxies (FQGs), partially quenched galaxies (PQGs), and fully star-forming galaxies (FSGs), and identify central quenched cores (QCCs), which reside in the center of PQGs and have the same quenching property as FQGs. We propose the two-$\sigma$ ($\sigma_{\rm in}/\sigma_{\rm hot}-\sigma_{\rm out}/\sigma_{\rm hot}$) diagram. The galaxy distribution on two-$\sigma$ diagram is L-shape, consisting of a horizontal sequence ($\sigma_{\rm out}/\sigma_{\rm hot}$ ∼ 0, with cold outskirt) and a vertical sequence ($\sigma_{\rm in}/\sigma_{\rm hot}$ ∼ 1, with hot center). FQGs and QCCs are located at the top of the vertical sequence, $\sigma_{\rm out}/\sigma_{\rm hot}$ ~1, and are thus dynamically hot over their entire bodies. PQGs reside along the vertical sequence, so they have hot center but cold outskirt. FSGs are diverse and can be found in both sequences. Galaxy structural properties, star formation and AGN activities make a transition along the horizontal sequence at $\sigma_{\rm in}/\sigma_{\rm hot}$ ∼ 0.5, and along the vertical sequence at $\sigma_{\rm out}/\sigma_{\rm hot}$ ∼ 0.5. The fractions of optical AGNs and barred galaxies start to increase rapidly in the first transition and decline rapidly in the second; radio galaxies are located at the top of the vertical sequence. Our results demonstrate that star formation quenching and the growth of supermassive black holes (SMBH) are effective only in dynamically hot systems . A simple model along this line can reproduce the observed scaling relations of the SMBH mass with galaxy mass and velocity dispersion. We discuss how secular processes and strong interactions can make a system dynamically hot, and lead to the growth of its SMBH and the quenching of its star formation. For more details, please see my paper (Hong et al. 2023) .
Differences in color profiles between satellite and central galaxies: evidence for outside-in quenching
We analyzed photometric images (g, r, and i bands) of galaxies from the Sloan Digital Sky Survey Data Release 7 (DR7). These galaxies were categorized into star-forming galaxies (SFGs), transition galaxies (TGs), and quiescent galaxies (QGs). Using a substantial dataset, we compared the differences in color profiles (specifically, g-r color) between satellite galaxies and their corresponding central galaxies with similar stellar mass and redshift. Our findings reveal that, on average, satellite galaxies are redder across all radii compared to central galaxies. The color difference increases with radius, suggesting that environmental effects are more pronounced in the outskirts of galaxies, where the gravitational potential is weaker. This observation supports an 'outside-in' quenching mode for environmental processes. Notably, we observed the most significant color differences in transition galaxies (TGs) in comparison to SFGs and QGs. This reaffirms that TGs represent a transitional phase toward quenching. Additionally, the color differences become more pronounced at smaller halo-centric distances and with larger halo mass, where environment is denser. Moreover, we found that the difference in mass surface density profiles between satellite and central galaxies is negligible. This implies that environmental processes predominantly affect gas, as opposed to stars, favoring the concept of ram pressure stripping over tidal stripping. This is the paper draft. Main figures are all included.
Dynamics and quenching of satellite galaxies
Based on Hong et al. 2023, we analyze the dynamics and quenching properites of satellite galaxies. The quenching mode in satellite galaxies is quite different from what we observed in central galaxies. There are many interseting results. I am writing this paper. Stay tuned!
Duty cycle of radio galaxies
How do massive quenched galaxies sustain quenching status? Radio galaxies are competitive candidates! The radio jets emitted by these galaxies can heat the gas in dark matter halos, preventing it from cooling and forming new stars. Hong et al. 2023 has found that radio galaxies are dynamically hot from inner to outer. Beyond their dynamical status, what other properties do radio galaxies share? To explore this question, we have employed a machine learning method to classify galaxies into potential radio galaxies and non-radio galaxies. I am the co-first author in collaboration with a computer science lab and an undergraduate student at USTC. Currently this paper is under submission. Stay tuned!