Dr. Cheng Li

Look at this page if you're interested about myself, and refer to the Research Page for a brief introduction to my research and lists of my students/postdocs. My publications are listed on this page. My courses are listed on the Teaching Page. You're welcome to Contact me, if you're interested to visit and talk with me on my research or courses, but before that you may want to consult my Weekly Schedule and Travel plan.

• On the Origin of Quenched but Gas-rich Regions at Kiloparsec Scales in Nearby Galaxies
  • We use resolved spectroscopy from MaNGA to investigate the significance of both local and global properties of galaxies to the cessation of star formation at kpc scales. Quenched regions are identified from a sample of isolated disk galaxies by a single-parameter criterion D n (4000) - EW(Hα) , and are divided into gas-rich quenched regions (GRQRs) and gas-poor quenched regions according to the surface density of cold gas (Σgas). Both types of quenched regions tend to be hosted by non-AGN galaxies with relatively high mass (M * ≳ 1010 M ⊙) and red colors (NUV ‑ r ≳ 3), as well as low star formation rate and high central density at fixed mass. They span wide ranges in other properties including structural parameters that are similar to the parent sample, indicating that the conditions responsible for quenching in gas-rich regions are largely independent of the global properties of galaxies. We train random forest classifiers and regressors for predicting quenching in our sample with 15 local/global properties. Σ* is the most important property for quenching, especially for GRQRs. These results strongly indicate the important roles of low-mass hot evolved stars, which are numerous and long-lived in quenched regions and can provide substantial radiation pressure to support the surrounding gas against gravitational collapse. The different feature importance for quenching, as found previously by A. F. L. Bluck et al., is partly due to the different definitions of quenched regions, particularly the different requirements on EW(Hα).

• On the Existence, Rareness, and Uniqueness of Quenched H I-rich Galaxies in the Local Universe
  • Using data from ALFALFA, xGASS, H I-MaNGA, and the Sloan Digital Sky Survey (SDSS), we identify a sample of 47 “red but H I-rich” (RR) galaxies with near-UV (NUV) ‑ r > 5 and unusually high H I-to-stellar mass ratios. We compare the optical properties and local environments between the RR galaxies and a control sample of “red and H I-normal” (RN) galaxies that are matched in stellar mass and color. The two samples are similar in the optical properties typical of massive red (quenched) galaxies in the local Universe. The RR sample tends to be associated with slightly lower-density environments and has lower clustering amplitudes and smaller neighbor counts at scales from several hundred kiloparsecs to a few megaparsecs. The results are consistent with the RR galaxies being preferentially located at the center of low-mass halos, with a median halo mass ∼1012 h ‑1 M ⊙ compared to ∼1012.5 h ‑1 M ⊙ for the RN sample. This result is confirmed by the SDSS group catalog, which reveals a central fraction of 89% for the RR sample, compared to ∼60% for the RN sample. If assumed to follow the H I size–mass relation of normal galaxies, the RR galaxies have an average H I-to-optical radius ratio of R HI/R 90 ∼ 4, four times the average ratio for the RN sample. We compare our RR sample with similar samples in previous studies, and quantify the population of RR galaxies using the SDSS complete sample. We conclude that the RR galaxies form a unique but rare population, accounting for only a small fraction of the massive quiescent galaxy population. We discuss the formation scenarios of the RR galaxies.

• Criss-cross Nebula: A Case Study of Shock Regions with Resolved Microstructures at Scales of ∼1000 au
  • Using integral field spectroscopy (IFS) from MaNGA, we study the resolved microstructures in a shocked region in the Criss-cross Nebula (CCN), with an unprecedentedly high resolution of ≲1000 au. We measure surface brightness maps for 34 emission lines, which can be broadly divided into three categories: (1) the [O III] λ5007-like group including seven high-ionization lines and two [O II] auroral lines that uniformly present a remarkable lane structure, (2) the Hα λ6563-like group, including 23 low-ionization or recombination lines that present a clump-like structure, and (3) [O II] λ3726 and [O II] λ3729 showing high densities at both the [O III] λ5007 lane and the Hα clump. We use these measurements to constrain resolved shock models implemented in MAPPINGS V. We find our data can be reasonably well fitted by a model that includes a plane-parallel shock with a velocity of 133 ± 5 km s ‑1, plus an isotropic two-dimensional Gaussian component, which is likely another clump of gas ionized by photons from the shocked region, and a constant background. We compare the electron density and temperature profiles as predicted by our model with those calculated using observed emission-line ratios. We find different line ratios to provide inconsistent temperature maps, and the discrepancies can be attributed to observational effects caused by limited spatial resolution and projection of the shock geometry, as well as contamination of the additional Gaussian component. Implications on shock properties and perspectives on future IFS-based studies of the CCN are discussed.

• Post-starburst Galaxies in SDSS-IV MaNGA: Two Broad Categories of Evolutionary Pathways
  • We study the size–mass relation (SMR) and recent star formation history (SFH) of post-starburst (PSB) galaxies in the local Universe using spatially resolved spectroscopy from the final data release of MaNGA. Our sample includes 489 PSB galaxies: 94 cPSB galaxies with central PSB regions, 85 rPSB galaxies with ringlike PSB regions, and 310 iPSB galaxies with irregular PSB regions. When compared to control galaxies of similar star formation rate, redshift, and mass, a similar SMR is found for all types of PSB samples except the cPSB galaxies, which have smaller sizes at intermediate masses ( $9.5\lesssim {\mathrm{log}}_{10}({M}_{* }/{M}_{\odot })\lesssim 10.5$ ). The iPSB galaxies in the star-forming sequence (iPSB-SF) show no/weak gradients in D n (4000), EW(Hδ A ), and EW(Hα), consistent with the global star-forming status of this type of galaxy, while the quiescent iPSB (iPSB-Q) sample shows negative gradients in D n (4000) and positive gradients in EW(Hδ A ), indicating older stellar populations in the inner regions. Both the cPSB and rPSB samples show positive gradients in D n (4000) and negative gradients in EW(Hδ A ), indicating younger stellar populations in the inner regions. These results imply that the four types of PSB galaxies can be broadly divided into two distinct categories in terms of evolutionary pathway: (1) iPSB-SF and iPSB-Q, which have SMRs and SFHs similar to control galaxies, preferring an inside-out quenching process, and (2) rPSB and cPSB, which appear to be different stages of the same event and likely to follow the outside-in quenching process driven by disruption events such as mergers that result in a more compact structure as quenching proceeds.

• Mapping Dust Attenuation and the 2175 Å Bump at Kiloparsec Scales in Nearby Galaxies
  • We develop a novel approach to measure the dust attenuation properties of galaxies, including the dust opacity, the shape of the attenuation curve, and the strength of the 2175 Å absorption feature. From an observed spectrum, the method uses a model-independent approach to derive a relative attenuation curve with an absolute amplitude calibrated using NIR photometry. The dust-corrected spectrum is fitted with stellar population models to derive the dust-free model spectrum, which is compared with the observed SED/spectrum from NUV to NIR to determine the dust attenuation properties. We apply this method to investigate dust attenuation on kiloparsec scales using a sample of 134 galaxies with integral field spectroscopy from MaNGA, NIR imaging from 2MASS, and NUV imaging from Swift/UVOT. We find that the attenuation curve slope and the 2175 Å bump in both the optical and NUV span a wide range at kiloparsec scales. The slope is shallower at higher optical opacity, regardless of the specific star formation rate (sSFR), the minor-to-major axis ratio (b/a) of the galaxies, and the location of spaxels within individual galaxies. The 2175 Å bump presents a strong negative correlation with the sSFR, while the correlations with the optical opacity, b/a, and the locations within individual galaxies are all weak. All of these trends appear to be independent of the stellar mass of the galaxies. Our results support the scenario that the variation of the 2175 Å bump is driven predominantly by processes related to star formation, such as the destruction of small dust grains by UV radiation in star-forming regions.