arXiv subjects

Zhenyi Cai

Publications and source records attributed to Zhenyi Cai.

12 recordsLinked to original sources

arXiv2024

The Host Galaxy Fluxes of Active Galaxy Nuclei Are Generally Overestimated by the Flux Variation Gradient Method

In terms of the variable nature of normal active galaxy nuclei (AGN) and luminous quasars, a so-called flux variation gradient (FVG) method has been widely utilized to estimate the underlying non-variable host galaxy fluxes. The FVG method assumes an invariable AGN color, but this assumption has been questioned by the intrinsic color variation of quasars and local Seyfert galaxies. Here, using an up-to-date thermal fluctuation model to simulate multi-wavelength AGN variability, we theoretically demonstrate that the FVG method generally overestimates the host galaxy flux; that is, it is more significant for brighter AGN/quasars. Furthermore, we observationally confirm that the FVG method indeed overestimates the host galaxy flux by comparing it to that estimated through other independent methods. We thus caution that applying the FVG method should be performed carefully in the era of time-domain astronomy.

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arXiv2023

Extreme Variability Quasars in Their Various States. II: Spectral Variation Revealed with Multi-epoch Spectra

We previously built a sample of 14,012 extremely variable quasars (EVQs) based on SDSS and Pan-STARRS1 photometric observations. In this work we present the spectral fitting to their SDSS spectra, and study the spectral variation in 1,259 EVQs with multi-epoch SDSS spectra (after prudently excluding spectra with potentially unreliable spectroscopic photometry). We find clear "bluer-when-brighter" trend in EVQs, consistent with previous findings of normal quasars and AGNs. We detect significant intrinsic Baldwin effect (iBeff, i.e., smaller line EW at higher continuum flux in individual AGNs) in the broad MgII and CIV lines of EVQs. Meanwhile, no systematical iBeff is found for the broad Hb line, which could be attributed to strong host contamination at longer wavelengths. Remarkably, by comparing the iBeff slope of EVQs with archived changing-look quasars (CLQs), we show that the CLQs identified in literature are mostly likely a biased (due to its definition) sub-population of EVQs, rather than a distinct population of quasars. We also found no significant broad line breathing of either Hb, MgII or CIV, suggesting the broad line breathing in quasars may disappear at longer timescales ($\sim$ 3000 days).

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arXiv2023

Sciences with the 2.5-meter Wide Field Survey Telescope (WFST)

The Wide Field Survey Telescope (WFST) is a dedicated photometric surveying facility being built jointly by the University of Science and Technology of China and the Purple Mountain Observatory. It is equipped with a 2.5-meter diameter primary mirror, an active optics system, and a mosaic CCD camera with 0.73 gigapixels on the primary focal plane for high-quality image capture over an FOV of 6.5-square-degree. It is anticipated that WFST will be set up at the Lenghu site in the summer of 2023 and begin to observe the northern sky in four optical bands (u, g, r, and i) with a range of cadences, from hourly/daily in the Deep High-Cadence Survey (DHS) program to semiweekly in the Wide-Field Survey (WFS) program, three months later. During a photometric night, a nominal 30 s exposure in the WFS program will reach a depth of 22.27, 23.32, 22.84, and 22.31 (AB magnitudes) in these four bands, respectively, allowing for the detection of a tremendous amount of transients in the low-z universe and a systematic investigation of the variability of Galactic and extragalactic objects. In the DHS program, intranight 90 s exposures as deep as 23 (u) and 24 mag (g), in combination with target of opportunity follow-ups, will provide a unique opportunity to explore energetic transients in demand for high sensitivities, including the electromagnetic counterparts of gravitational wave events, supernovae within a few hours of their explosions, tidal disruption events and fast, luminous optical transients even beyond a redshift of unity. In addition, the final 6-year co-added images, anticipated to reach g=25.8 mag in WFS or 1.5 mags deeper in DHS, will be of fundamental importance to general Galactic and extragalactic science. The highly uniform legacy surveys of WFST will serve as an indispensable complement to those of LSST that monitor the southern sky.

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arXiv2021

Extreme Variability Quasars in Their Various States. I: The sample selection and composite SDSS spectra

Extremely variable quasars (EVQs) are a population of sources showing large optical photometric variability revealed by time-domain surveys. The physical origin of such extreme variability is yet unclear. In this first paper of a series, we construct the largest-ever sample of 14,012 EVQs using photometric data spanning over $>$ 15 years from SDSS and Pan-STARRS1. We divide them into five sub-samples according to the relative brightness of an EVQ during SDSS spectroscopic observation compared to the mean brightness from photometric observations. Corresponding control samples of normal quasars are built with matched redshift, bolometric luminosity and supermassive black hole mass. We obtain the composite SDSS spectra of EVQs in various states and their corresponding control samples. We find EVQs exhibit clearly bluer (redder) SDSS spectra during bright (dim) states, consistent with the "bluer-when-brighter" trend widely seen in normal quasars. We further find the line EWs of broad MgII, CIV and OIII (but not broad Hb which is yet puzzling) gradually decrease from dim state to bright state, similar to the so-called intrinsic Baldwin effect commonly seen in normal AGNs. Meanwhile, EVQs have systematically larger line EWs compared with the control samples. We also see that EVQs exhibit subtle excess in the very broad line component compared with control samples. Possible explanations for the discoveries are discussed. Our findings support the hypothesis that EVQs are in the tail of a broad distribution of quasar properties, but not a distinct population.

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arXiv2021

The effect of the morphological quenching mechanism on star formation activity at 0.5 < z < 1.5 in 3D-HST/CANDELS

Several mechanisms for the transformation of blue star-forming to red quiescent galaxies have been proposed, and the green valley (GV) galaxies amid them are widely accepted in a transitional phase. Thus, comparing the morphological and environmental differences of the GV galaxies with early-type disks (ETDs; bulge dominated and having a disk) and late-type disks (LTDs; disk dominated) is suitable for distinguishing the corresponding quenching mechanisms. A large population of massive ($M_* \geqslant 10^{10}M_\odot$) GV galaxies at $0.5 \leqslant z \leqslant 1.5$ in 3D-HST/CANDELS is selected using extinction-corrected $(U-V)_{\rm rest}$ color. After eliminating any possible active galactic nucleus candidates and considering the "mass-matching", we finally construct two comparable samples of GV galaxies with either 319 ETD or 319 LTD galaxies. Compared to the LTD galaxies, it is found that the ETD galaxies possess higher concentration index and lower specific star formation rate, whereas the environments surrounding them are not different. This may suggest that the morphological quenching may dominate the star formation activity of massive GV galaxies rather than the environmental quenching. To quantify the correlation between the galaxy morphology and the star formation activity, we define a dimensionless morphology quenching efficiency $Q_{\rm mor}$ and find that $Q_{\rm mor}$ is not sensitive to the stellar mass and redshift. When the difference between the average star formation rate of ETD and LTD galaxies is about 0.7 $M_\odot \rm \;yr^{-1}$, the probability of $Q_{\rm mor}\gtrsim 0.2$ is higher than 90\%, which implies that the degree of morphological quenching in GV galaxies might be described by $Q_{\rm mor}\gtrsim 0.2$.

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arXiv2021

Faint AGNs Favor Unexpectedly Long Inter-band Time Lags

Inconsistent conclusions are obtained from recent active galactic nuclei (AGNs) accretion disk inter-band time-lag measurements. While some works show that the measured time lags are significantly larger (by a factor of $\sim 3$) than the theoretical predictions of the Shakura \& Sunyaev disk (SSD) model, others find that the time-lag measurements are consistent with (or only slightly larger than) that of the SSD model. These conflicting observational results might be symptoms of our poor understanding of AGN accretion physics. Here we show that sources with larger-than-expected time lags tend to be less-luminous AGNs. Such a dependence is unexpected if the inter-band time lags are attributed to the light-travel-time delay of the illuminating variable X-ray photons to the static SSD. If, instead, the measured inter-band lags are related not only to the static SSD but also to the outer broad emission-line regions (BLRs; e.g., the blended broad emission lines and/or diffuse continua), our result indicates that the contribution of the non-disk BLR to the observed UV/optical continuum decreases with increasing luminosity ($L$), i.e., an anti-correlation resembling the well-known Baldwin effect. Alternatively, we argue that the observed dependence might be a result of coherent disk thermal fluctuations as the relevant thermal timescale, $τ_{\mathrm{TH}}\propto L^{0.5}$. With future accurate measurements of inter-band time lags, the above two scenarios can be distinguished by inspecting the dependence of inter-band time lags upon either the BLR components in the variable spectra or the timescales.

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arXiv2021

An Extraordinary Response of Iron Emission to the Central Outburst in a Tidal Disruption Event Candidate

Understanding the origin of \feii\ emission is important because it is crucial to construct the main sequence of Active Galactic Nuclei (AGNs). Despite several decades of observational and theoretical effort, the location of the optical iron emitting region and the mechanism responsible for the positive correlation between the \feii\ strength and the black hole accretion rate remain open questions as yet. In this letter, we report the optical \feii\ response to the central outburst in PS1-10adi, a candidate tidal disruption event (TDE) taking place in an AGN at $z = 0.203$ that has aroused extensive attention. For the first time, we observe that the \feii\ response in the rising phase of its central luminosity is significantly more prominent than that in the decline phase, showing a hysteresis effect. We interpret this hysteresis effect as a consequence of the gradual sublimation of the dust grains situating at the inner surface of the torus into gas when the luminosity of the central engine increases. It is the iron element released from the sublimated dust that contributes evidently to the observed \feii\ emission. This interpretation, together with the weak response of the \hb\ emission as we observe, naturally explains the applicability of relative \feii\ strength as a tracer of the Eddington ratio. In addition, optical iron emission of this origin renders the \feii\ time lag a potential "standard candle" with cosmological implications.

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arXiv2020

Corona-Heated Accretion-disk Reprocessing (CHAR): A Physical Model to Decipher the Melody of AGN UV/optical Twinkling

Active galactic nuclei (AGNs) have long been observed to "twinkle" (i.e., their brightness varies with time) on timescales from days to years in the UV/optical bands. Such AGN UV/optical variability is essential for probing the physics of supermassive black holes (SMBHs), the accretion disk, and the broad-line region. Here we show that the temperature fluctuations of an AGN accretion disk, which is magnetically coupled with the corona, can account for observed high-quality AGN optical light curves. We calculate the temperature fluctuations by considering the gas physics of the accreted matter near the SMBH. We find that the resulting simulated AGN UV/optical light curves share the same statistical properties as the observed ones as long as the dimensionless viscosity parameter $α$, which is widely believed to be controlled by magnetohydrodynamic (MHD) turbulence in the accretion disk, is about $0.01$---$0.2$. Moreover, our model can simultaneously explain the larger-than-expected accretion disk sizes and the dependence of UV/optical variability upon wavelength for NGC 5548. Our model also has the potential to explain some other observational facts of AGN UV/optical variability, including the timescale-dependent bluer-when-brighter color variability and the dependence of UV/optical variability on AGN luminosity and black hole mass. Our results also demonstrate a promising way to infer the black-hole mass, the accretion rate, and the radiative efficiency, thereby facilitating understanding of the gas physics and MHD turbulence near the SMBH and its cosmic mass growth history by fitting the AGN UV/optical light curves in the era of time-domain astronomy.

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arXiv2019

The mass dependence of structure, star formation rate, and mass assembly mode at 0.5<z< 2.5

To investigate the mass dependence of structural transformation and star formation quenching, we construct three galaxy samples using massive ($M_* > 10^{10} M_{\odot}$) red, green, and blue galaxy populations at $0.5<z<2.5$ in five 3D--{\it HST}/CANDELS fields. The structural parameters, including effective radius ($r_{\rm e}$), galaxy compactness ($Σ_{1.5}$), and second order moment of 20\% brightest pixels ($M_{20}$) are found to be correlated with stellar mass. Sérsic index ($n$), concentration ($C$), and Gini coefficient ($G$) seem to be insensitive to stellar mass. The morphological distinction between blue and red galaxies is found at a fixed mass bin, suggesting that quenching processes should be accompanied with transformations of galaxy structure and morphology. Except for $r_e$ and $Σ_{1.5}$ at high mass end, structural parameters of green galaxies are intermediate between red and blue galaxies in each stellar mass bin at $z < 2$, indicating green galaxies are at a transitional phase when blue galaxies are being quenched into quiescent statuses. The similar sizes and compactness for the blue and green galaxies at high-mass end implies that these galaxies will not appear to be significantly shrunk until they are completely quenched into red QGs. For the green galaxies at $0.5<z<1.5$, a morphological transformation sequence of bulge buildup can be seen as they are gradually shut down their star formation activities, while a faster morphological transformation is verified for the green galaxies at $1.5<z<2.5$.

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arXiv2018

Evolution of quasar stochastic variability along its main sequence

We explore the evolution of the time variability (in the optical $g$-band and on timescales of weeks to years) of SDSS Stripe 82 quasars along the quasar main sequence. A parent sample of $1004$ quasars within $0.5\leq z \leq 0.89$ are used for our statistical studies, we then make subsamples from our parent sample: a subsample of $246$ quasars with similar luminosities, and a subsample of $399$ quasars with similar Rfe (i.e., the ratio of the equivalent width of FeII within $4435$--$4685\ \mathrmÅ$ to that of \Hbeta). We find the variability amplitude decreases with luminosity ($L_{\mathrm{bol}}$). The anti-correlation between the variability amplitude and Rfe is weak but statistically significant. The characteristic timescale, $τ$, correlates mostly with quasar luminosity, its dependence on Rfe is statistically insignificant. After controlling luminosity and Rfe, the high- and low-FWHM samples have similar structure functions. These results support the framework that Rfe is governed by Eddington ratio and FWHM of Hbeta is mostly determined by orientation. We then provide new empirical relations between variability parameters and quasar properties (i.e., luminosity and Rfe). Our new relations are consistent with the scenario that quasar variability is driven by the thermal fluctuations in the accretion disk, $τ$ seems to correspond to the thermal timescale. From our new relations, we find the short-term variability is mostly sensitive to $L_{\mathrm{bol}}$. Basing on this, we propose that quasar short-term (a few months) variability might be a new type of "Standard Candle" and can be adopted to probe cosmology.

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arXiv2018

The morphological evolution, AGN fractions, dust content, environments, and downsizing of massive green valley galaxies at 0.5<z<2.5 in 3D-HST/CANDELS

To explore the evolutionary connection among red, green, and blue galaxy populations, based on a sample of massive ($M_* > 10^{10} M_{\odot} $) galaxies at 0.5 2. A significant environmental difference is found between green and red galaxies at z<1.5. Green and blue galaxies at z>0.5 seem to have similar local density distributions, suggesting that environment quenching is not the major mechanism to cease star formation at z>0.5. The fractions of three populations as functions of mass support a "downsizing" quenching picture that the bulk of star formation in more massive galaxies is completed earlier than that of lower mass galaxies.

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arXiv2018

A falling Corona model for the anomalous behavior of the broad emission lines in NGC 5548

NGC 5548 has been intensively monitored by the AGN Space Telescope and Optical Reverberation Mapping collaboration. Approximately after half of the light curves, the correlation between the broad emission lines and the lag-corrected ultraviolet continua becomes weak. This anomalous behavior is accompanied by an increase of soft X-ray emission. We propose a simple model to understand this anomalous behavior, i.e., the corona might fall down, thereby increasing the covering fraction of the inner disk. Therefore, X-ray and extreme ultraviolet emission suffer from spectral variations. The ultraviolet continua variations are driven by both X-ray and extreme ultraviolet variations. Consequently, the spectral variability induced by the falling corona would dilute the correlation between the broad emission lines and the ultraviolet continua. Our model can explain many additional observational facts, including the dependence of the anomalous behavior on velocity and ionization energy. We also show that the time lag and correlation between the X-ray and the ultraviolet variations change as NGC 5548 displays the anomalous behavior. The time lag is dramatically longer than the expectation from disk reprocessing if the anomalous behavior is properly excluded. During the anomalous state, the time lag approaches the light-travel timescale of disk reprocessing albeit with a much weaker correlation. We speculate that the time lag in the normal state is caused by reprocessing of the broad line region gas. As NGC 5548 enters the abnormal state, the contribution of the broad line region gas is smaller; the time lag reflects disk reprocessing. We also discuss alternative scenarios.

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