arXiv subjects

Shao-Hua Zhang

Publications and source records attributed to Shao-Hua Zhang.

3 recordsLinked to original sources

arXiv2026

Revealing the Physical Driver of the Baldwin Effect: Gas Density in the Broad-Line Region

The Baldwin effect --- the inverse correlation between the equivalent width of emission lines and the continuum luminosity in active galactic nuclei (AGNs) --- has been known for nearly five decades, yet its physical origin remains poorly understood. Using a sample of 41,159 radio quasars constructed from the Sloan Digital Sky Survey and the Low-Frequency Array Two-metre Sky Survey, we investigate the origin and underlying physics of the Baldwin effect of MgII broad emission lines in both radio-quiet (RQ) and radio-loud (RL) quasars. We find that the slope $β$ of the Baldwin effect is positively correlated with the Eddington ratio $λ_{\rm Edd}$ in both populations, and RL quasars exhibit steeper $β$ than their RQ quasars at fixed $λ_{\rm Edd}$. Photoionization simulations reveal that the $β$ is primarily governed by the gas density in the broad-line region (BLR): lower gas densities yield steeper slopes. This density-driven mechanism naturally connects the Baldwin effect to the broader AGN evolutionary context. Specifically, higher $λ_{\rm Edd}$ drive stronger accretion disk winds, leading to denser BLRs and shallower $β$. Our findings indicate that BLR gas density serves as the primary physical driver underlying the "global" Baldwin effect, offering a physically grounded framework for interpreting AGN accretion states and their coupled evolution with host galaxies.

astro-ph.GA↗

arXiv2016

The Redshifted Hydrogen Balmer and Metastable He I Absorption Line System in Mini-FeLoBAL Quasar SDSS J112526.12+002901.3: A Parsec Scale Accretion Inflow?

The accretion of interstellar medium onto the central super massive black holes is widely accepted as the source of the gigantic energy released by the active galactic nuclei. But few pieces of observational evidence have been confirmed directly demonstrating the existence of the inflows. The absorption line system in the spectra of quasar SDSS J112526.12+002901.3 presents an interesting example, in which the rarely detected hydrogen Balmer and metastable He I absorption lines are found redshifted to the quasar's rest frame along with the low-ionization metal absorption lines Mg II, Fe II, etc. The repeated SDSS spectroscopic observations suggest a transverse velocity smaller than the radial velocity. The motion of the absorbing medium is thus dominated by infall. The He I* lines present a powerful probe to the strength of ionizing flux, while the Balmer lines imply a dense environment. With the help of photoionization simulations, we find the absorbing medium is exposed to the radiation with ionization parameter $U\approx 10^{-1.8}$, and the density is $n(\mathrm{H})\approx 10^9\ \mathrm{cm}^{-3}$. Thus the absorbing medium is located $\sim 4\ \mathrm{pc}$ away from the central engine. According to the similarity in the distance and physical conditions between the absorbing medium and the torus, we strongly propose the absorption line system as a candidate for the accretion inflow which originates from the inner surface of the torus.

astro-ph.GA↗

arXiv2013

Quasar-Galaxy Clustering through Projected Galaxy Counts at z=0.6-1.2

We investigate the spatial clustering of galaxies around quasars at redshifts from 0.6 to 1.2 using the photometric data from Sloan Digital Sky Survey (SDSS) Stripe 82. The quasar and galaxy cross-correlation functions are measured through the projected galaxy number density n(r_p) on scales $0.05<r_p<20 h^-1 Mpc around quasars for a sample of 2300 quasars from Schneider et al. (2007). We detect strong clustering signals at all redshifts, and find that the clustering amplitude increases significantly with redshift. We examine the dependence of the quasar-galaxy clustering on quasar and galaxy properties and find that the clustering amplitude is significantly larger for quasars with more massive black holes, or with bluer colors, while the dependence on quasar luminosity is absent. We also show that quasars have a stronger correlation amplitude with blue galaxies than with red galaxies. We finally discuss the implication of our finding.

astro-ph.CO↗