WHAT

Dr. Zhen-Yi CAI （蔡振翼，Zin-Iak CUA）

Credit. NAOJ

Research field

Major interests and Representative works

◦ Cai & Wang (2023, Nature Astronomy, 7, 1506): In the extreme ultraviolet, average quasar SEDs are independent of luminosity and redder than all previous mean composite quasar spectra.
◦ Cai (2024, Universe, 10, 431; here are supplementary materials): Being independent of many physical properties, such as black hole mass and Eddington ratio, the universality of the average quasar SED exists since cosmic noon.

◦ EUCLIA: a new thermal fluctuation model indicating a novel mechanism for the inter-band continuum lags for AGN, rather than the light echoing (Cai et al. 2018, ApJ, 855, 117), successfully explains a broad phenomena of AGN variability, including the multi-wavelength correlation and the puzzling large UV to X-ray lags (Cai et al. 2020, ApJ, 892, 63).
◦ New clues between jet launching and disk fluctuation: the inner disks in radio loud quasars may be more stable (Cai et al. 2019, SCPMA, 62, 69511), supported by following observations (2024, ApJ, 971, 60).

◦ Progenitors of early-type elliptical galaxies and massive bulges of Sa galaxies, i.e., proto-spheroids at z > 1.5, are forming stars in a self-regulated way, rather than galaxy merging (Cai et al. 2013, ApJ, 768, 21).
◦ Dust extinction is a key ingredient of understanding the cosmic evolution of Lyman break galaxies and Lyα emitters, and galaxies are the dominant ionizing source for the cosmic hydrogen reionization (Cai et al. 2014, ApJ, 785, 65).
◦ Part of the excess of z > 4 sub-millimeter galaxies can be understood in terms of an initial mass function (IMF) somewhat top-heavier than Chabrier, but not the extreme top-heavy IMFs commonly assumed in the galaxy merging model (Cai et al. 2020, ApJ, 891, 74).
◦ Our model prediction without any adjustment of the parameters was verified by dusty star-forming galaxies detected by the South Pole Telescope, and a similar IMF is suggested for star formations across cosmic time (Cai et al. 2022, ApJ, 932, 13).

Students (current)

✓ Su et al. (2025, ApJ, 990, 10): Analyzing the interband lags of 94 bright and massive (~10^8 M_sun) AGNs at z < 0.8 using gri-band light curves from Zwicky Transient Facility Data Release 22, he found that more than half of these AGNs exhibit significant seasonal (one-year) variations in their interband lags. Furthermore, he revealed that the short-term lags (measured over one year) are consistently shorter than the long-term lags (measured over six years)—a result that supports his earlier theoretical simulations.
✓ Su et al. (2024, ApJ, 976, 155): He demonstrated that, due to the random nature of AGN variability, the measured interband lags for an individual AGN can differ between observational campaigns with a finite baseline. This conclusion was drawn using two distinct scenarios for AGN variability, i.e., the thermal fluctuation of accretion disk and the reprocessing of both the accretion disk and clouds in the broad line region.
✓ Su et al. (2024, ApJ, 969, 78): For the first time at intermediate-mass scales, he reported prominent short-timescale (≲ hours) departures from the damped random walk (DRW) process in AGN optical variation. He further proposed a new timescale–mass scaling that bridges the intermediate-mass and supermassive regimes.

✓ Cai et al. (2024, Universe, 10, 282): His work showed that applying the flux variation gradient (FVG) method tends to overestimate the host galaxy fluxes of AGNs, with the effect being more pronounced in luminous AGNs and quasars.

◦ Dr. Mr. Wen-Yong Kang (2019-2022; co-supervised with Prof. Jun-Xian Wang; USTC博后)

✓ Kang et al. (2024, ApJ, 971, 60): Unveiling a prominent excess of radio emission in extremely stable quasars, he suggested that a strong magnetic field may not only have stabilized the inner disks of quasars but also promoted jet launching.
✓ Kang et al. (2018, ApJ, 868, 58): He found that quasars exhibiting more intense X-ray radiation also display greater variability in the UV/optical, and that this intrinsic connection between X-ray loudness and UV/optical variation becomes progressively more pronounced over longer timescales (>100 days). This trend challenges the X-ray reprocessing paradigm and instead suggests an interesting scenario in which both the heating of the X-ray corona (innermost disk) and UV/optical variability (outer disk) in quasars are closely linked to magnetic disc turbulence.

◦ Dr. Mr. Fei-Fan Zhu (2015-2019; co-supervised with Prof. Jun-Xian Wang; 中电32所)

✓ Zhu et al. (2018, ApJ, 860, 29): For the first time in an individual AGN, his findings revealed that the color variations in NGC 5548 are clearly dependent on timescale, challenging the widely accepted reprocessing paradigm.

✓ Hu et al. (2024, ApJ, 961, 5): His new optimized statistical method enables unbiased measurement of AGN variability properties (like the characteristic timescale), achieving 2% accuracy with a minimal baseline of just ~8 times the intrinsic timescale.

◦ Mr. Zhen-Bo Su (2020-2021; 毕设; co-supervised with Prof. Yang Huang from YNU; USTC读研)