My research focuses on spatially resolved studies of Polycyclic Aromatic Hydrocarbon (PAH) properties and their correlations with underlying physical conditions in different galaxies, as well as multi-wavelength diagnostics of star formation and AGN activities and their feedback effects including outflows. With PAH and other emission line diagnostics as tools, I would like to better understand the evolutionary processes (as illustrated) within galaxies.
Prior to the era of JWST, I focused on the study of spatially resolved PAH emission in nearby galaxies using IFU-like Spitzer/IRS mapping mode observations.
Specifically, my research includes the investigation of the effectiveness of PAH emission as a SFR indicator on subkiloparsec scales, as well as the contribution of evolved stars to PAH excitation, providing useful calibrations between PAH emission and SFR with the stellar mass can be a second parameter to account for the contribution of evolved stars (Zhang, L., et al., 2021, AJ, 161, 29; Zhang, L., & Ho, L. C., 2023, ApJ, 943, 60). Meanwhile, my research also includes the investigation of the feasibility of using PAH emission as an empirical proxy to estimate molecular gas mass on subkiloparsec scales, providing empirical calibrations with a total scatter of only ∼0.2–0.25 dex to estimate molecular gas mass from PAH luminosity (Zhang, L., & Ho, L. C., 2023, ApJ, 943, 1). All of these research topics, including the ones below, are becoming increasingly popular in the era of JWST.
Furthermore, by studying the physical properties of PAHs across a broad range of black hole accretion power, my research sheds more light on the underlying physics that are responsible for the observed PAH characteristics in different galaxies. Whereas the PAH emission correlates tightly with SFR in star-forming regions, the spatially resolved regions in AGN tend to be PAH deficient. Moreover, the observed PAH band ratios in low luminosity AGN cannot be explained by the effects of the AGN radiation field alone. Instead, small PAH grains therein may be destroyed by combined effects of radiative processes and collisional shocks. While quasars also present a PAH deficit and unusual PAH band ratios, their PAH characteristics differ in detail compared to those observed in more weakly accreting AGN, a possible indicator of fundamental differences in their modes of energy feedback (Zhang, L., et al., 2022, ApJ, 939, 22).
In the era of JWST, with the excellent capabilities of JWST IFU observations, I furthered the study of spatially resolved PAH emission in different galaxies and developed the study of multi-wavelength diagnostics of AGN activities and their feedback effects including outflows.
For example, by investigating the star formation rate and efficiency within the central ∼1.5 kpc × 1.3 kpc region of the Seyfert 1 galaxy NGC 7469 on ∼100 pc scales, I found that the active nucleus leaves a notable imprint on its immediate surroundings by elevating the temperature of the warm molecular gas, driving an ionized gas outflow on subkiloparsec scales, and selectively destroying small PAH grains. These effects, nevertheless, have relatively little impact on the cold circumnuclear medium or its ability to form stars. Most of the star formation in NGC 7469 is confined to a clumpy starburst ring, but the star formation efficiency remains quite elevated even for the nuclear region that is most affected by the active nucleus (Zhang, L., & Ho, L. C., 2023, ApJL, 953, L9).
Later on, within the GATOS collaboration, I analyzed the PAH and emission-line characteristics in regions of ∼500 pc scales over or around the AGN of three nearby Seyferts. Combing the measurements and model predictions, I found that the inhibited growth or preferential erosion of PAHs decreasing the average size and overall abundance of PAHs, as well as the severe photoionization because of the radiative effects from the radiative shock precursor besides the AGN, can all contribute to different PAH characteristics in observations. These results further deepen our understanding of the underlying physics that are responsible for the observed PAH characteristics in different galaxies and have promising application in the era of JWST, especially in diagnosing different (i.e., radiative and kinetic) AGN feedback modes (Zhang, L., et al., 2024, ApJL, 975, L2).
In addition, as an important manifestation of AGN feedback, I investigated the outflow properties of ionized gas in six nearby Seyfert galaxies included in the GATOS survey. Based on spatially resolved flux distributions and velocity fields of six ionized emission lines covering a large range of ionization potentials (15.8–97.1 eV), all of the six targets exhibit the evidence of ionized gas outflows, and NGC 5728, NGC 5506, and ESO137-G034 also exhibit some highly disturbed regions. AGN-driven radio jets plausibly play an important role in triggering these highly disturbed regions. According to the outflow rates estimated based on [Ne V] emission, the six targets tend to have ionized outflow rates converged to a narrower range than the previous finding, and the newly derived outflow rates are more consistent with their correlation with the AGN luminosity. These results have an important implication for the outflow properties in AGN of comparable luminosity (Zhang, L., et al. 2024, ApJ, 974, 195).