arXiv subjects

Zhe-Geng Chen

Publications and source records attributed to Zhe-Geng Chen.

4 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↗

arXiv2025

The infrared traced star formation rates of associated absorption lines quasars

Some optically selected quasars exhibit Mg II assoicated absorption lines (AALs), and its origin remains unclear. In this paper, we compile a sample of 1769 quasars, with or without Mg II AALs. Of which 1689 are Far-Infrared (FIR) detected quasars and the rest are not detected in FIR. For the FIR undetected quasars, we obtain stacks for both with and without Mg II AAL quasars. Then we estimate the star formation rates (SFRs) within quasar host galaxies based on their FIR luminosities derived from their FIR greybody components, and find that, although quasars with Mg II AALs have significantly redder median composite spectra than those without Mg II AALs, the SFR distributions of the two types of quasars are statistically indistinguishable. These results do not require an evolutionary link between the quasars with and without Mg II AALs, and would be reconciled if an orientation effect cannot be ignored among the quasars hosting different types of absorption lines.

astro-ph.GA↗

arXiv2024

Narrow [O III] emission lines as a potential proxy for the evolutionary stage of quasars

Radio spectral shape of quasars can provide insight into the ages of quasars. We have compiled data for 1804 quasars with $z\lesssim1$ from the Sloan Digital Sky Survey (SDSS). Additionally, these quasars were also mapped by the Low-Frequency Array at 144 MHz and the Very Large Array Sky Survey at 3000 MHz. The radio spectral index, designated as $α^{\rm 144}_{\rm 3000}$ (with $S(ν)\proptoν^α$), is analyzed between 144 MHz and 3000 MHz as a proxy for the ages of quasars. We measure the [O III] $λ$5007 emission line in the SDSS spectra. A strong correlation was found between the equivalent width of the core component of the [O III] $λ$5007 emission line and $α^{\rm 144}_{\rm 3000}$. This relationship suggests that the core component of the [O III] $λ$5007 emission line could potentially serve as a surrogate for the evolutionary stage of a quasar. The quasars at an early stage of evolutions tend to show weaker [O III] $λ$5007 emission, while older quasars exhibit stronger [O III] $λ$5007 emission.

astro-ph.GA↗

arXiv2023

The Study of Circumgalactic Medium with Quasar Pairs

We have collected 10025 foreground-background quasar pairs with projected distances $d_p<500$ kpc from the large quasar catalog of the SDSS DR16Q. We investigate the properties of the Mg II absorption lines with $W_r>0.15$ Å around foreground quasars, including both the LOS (line-of-sights of foreground quasars) and transverse (TRA, perpendicular to the LOS) absorptions. Both the equivalent width (the correlation coefficient $ρ=-0.915$ and the probability $P < 10^{-4}$ of no correlation) and incident rate ($ρ=-0.964$ and $P < 10^{-6}$) of TRA \Mgii\ absorption lines are obviously anti-correlated with projected distance. The incident rate of TRA \Mgii\ absorption lines is obviously ($>4σ$) greater than that of LOS \Mgii\ absorption lines at projected distances $d_p<200$ kpc, while the TRA and LOS \Mgii\ both have similar ($<3σ$) incident rates at scales $d_p>200$ kpc. The anisotropic radiation from quasars would be the most possible interpretation for the anisotropic absorption around quasars. This could also indicate that the quasar radiation is not obviously impacting the gas halos of quasars at scales $d_p>200$ kpc.

astro-ph.GA↗