Browsing by Author "Yang, Qian"

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    A Transient "Changing-look" Active Galactic Nucleus Resolved on Month Timescales from First-year Sloan Digital Sky Survey V Data
    (2022) Zeltyn, Grisha; Trakhtenbrot, Benny; Eracleous, Michael; Runnoe, Jessie; Trump, Jonathan R.; Stern, Jonathan; Shen, Yue; Hernandez-Garcia, Lorena; Bauer, Franz E.; Yang, Qian; Dwelly, Tom; Ricci, Claudio; Green, Paul; Anderson, Scott F.; Assef, Roberto J.; Guolo, Muryel; MacLeod, Chelsea; Davis, Megan C.; Fries, Logan; Gezari, Suvi; Grogin, Norman A.; Homan, David; Koekemoer, Anton M.; Krumpe, Mirko; LaMassa, Stephanie; Liu, Xin; Merloni, Andrea; Martinez-Aldama, Mary Loli; Schneider, Donald P.; Temple, Matthew J.; Brownstein, Joel R.; Ibarra-Medel, Hector; Burke, Jamison; Pellegrino, Craig; Kollmeier, Juna A.
    We report the discovery of a new "changing-look" active galactic nucleus (CLAGN) event, in the quasar SDSS J162829.17+432948.5 at z = 0.2603, identified through repeat spectroscopy from the fifth Sloan Digital Sky Survey (SDSS-V). Optical photometry taken during 2020-2021 shows a dramatic dimming of Delta g approximate to 1 mag, followed by a rapid recovery on a timescale of several months, with the less than or similar to 2 month period of rebrightening captured in new SDSS-V and Las Cumbres Observatory spectroscopy. This is one of the fastest CLAGN transitions observed to date. Archival observations suggest that the object experienced a much more gradual dimming over the period of 2011-2013. Our spectroscopy shows that the photometric changes were accompanied by dramatic variations in the quasar-like continuum and broad-line emission. The excellent agreement between the pre- and postdip photometric and spectroscopic appearances of the source, as well as the fact that the dimmest spectra can be reproduced by applying a single extinction law to the brighter spectral states, favor a variable line-of-sight obscuration as the driver of the observed transitions. Such an interpretation faces several theoretical challenges, and thus an alternative accretion-driven scenario cannot be excluded. The recent events observed in this quasar highlight the importance of spectroscopic monitoring of large active galactic nucleus samples on weeks-to-months timescales, which the SDSS-V is designed to achieve.
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    Data-driven methods for building reduced kinetic Monte Carlo models of complex chemistry from molecular dynamics simulations
    (2019) Yang, Qian; Sing-Long C., Carlos A.; Chen, Enze; Reed, Evan J.; Goldman, Nir
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    Exploring Changing-look Active Galactic Nuclei with the Sloan Digital Sky Survey V: First Year Results
    (2024) Zeltyn, Grisha; Trakhtenbrot, Benny; Eracleous, Michael; Yang, Qian; Green, Paul; Anderson, Scott F.; LaMassa, Stephanie; Runnoe, Jessie; Assef, Roberto J.; Bauer, Franz E.; Brandt, W. N.; Davis, Megan C.; Frederick, Sara E.; Fries, Logan B.; Graham, Matthew J.; Grogin, Norman A.; Guolo, Muryel; Hernandez-Garcia, Lorena; Koekemoer, Anton M.; Krumpe, Mirko; Liu, Xin; Martinez-Aldama, Mary Loli; Ricci, Claudio; Schneider, Donald P.; Shen, Yue; Sniegowska, Marzena; Temple, Matthew J.; Trump, Jonathan R.; Xue, Yongquan; Brownstein, Joel R.; Dwelly, Tom; Morrison, Sean; Bizyaev, Dmitry; Pan, Kaike; Kollmeier, Juna A.
    "Changing-look" active galactic nuclei (CL-AGNs) challenge our basic ideas about the physics of accretion flows and circumnuclear gas around supermassive black holes. Using first-year Sloan Digital Sky Survey V (SDSS-V) repeated spectroscopy of nearly 29,000 previously known active galactic nuclei (AGNs), combined with dedicated follow-up spectroscopy, and publicly available optical light curves, we have identified 116 CL-AGNs where (at least) one broad emission line has essentially (dis-)appeared, as well as 88 other extremely variable systems. Our CL-AGN sample, with 107 newly identified cases, is the largest reported to date, and includes similar to 0.4% of the AGNs reobserved in first-year SDSS-V operations. Among our CL-AGNs, 67% exhibit dimming while 33% exhibit brightening. Our sample probes extreme AGN spectral variability on months to decades timescales, including some cases of recurring transitions on surprisingly short timescales (less than or similar to 2 months in the rest frame). We find that CL events are preferentially found in lower-Eddington-ratio (f Edd) systems: Our CL-AGNs have a f Edd distribution that significantly differs from that of a carefully constructed, redshift- and luminosity-matched control sample (Anderson-Darling test yielding p AD approximate to 6 x 10-5; median f Edd approximate to 0.025 versus 0.043). This preference for low f Edd strengthens previous findings of higher CL-AGN incidence at lower f Edd, found in smaller samples. Finally, we show that the broad Mg ii emission line in our CL-AGN sample tends to vary significantly less than the broad H beta emission line. Our large CL-AGN sample demonstrates the advantages and challenges in using multi-epoch spectroscopy from large surveys to study extreme AGN variability and physics.
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    High-quality Extragalactic Legacy-field Monitoring (HELM) with DECam: Project Overview and First Data Release
    (2024) Zhuang, Ming-Yang; Yang, Qian; Shen, Yue; Adamow, Monika; Friedel, Douglas N.; Gruendl, R. A.; Stone, Zachary; Li, Junyao; Liu, Xin; Martini, Paul; Abbott, Timothy M. C.; Anderson, Scott F.; Assef, Roberto J.; Bauer, Franz E.; Bielby, Richard; Brandt, W. N.; Burke, Colin J.; Casares, Jorge; Chen, Yu-Ching; De Rosa, Gisella; Drlica-Wagner, Alex; Dwelly, Tom; Eltvedt, Alice; Alvarez, Gloria Fonseca; Fu, Jianyang; Fuentes, Cesar; Graham, Melissa L.; Grier, Catherine J.; Golovich, Nathan; Hall, Patrick B.; Hartigan, Patrick; Horne, Keith; Koekemoer, Anton M.; Krumpe, Mirko; Li, Jennifer I.; Lidman, Chris; Malik, Umang; Mangian, Amelia; Merloni, Andrea; Ricci, Claudio; Salvato, Mara; Sharp, Rob; Trilling, David E.; Tucker, Brad E.; Wen, Di; Wideman, Zachary; Xue, Yongquan; Yu, Zhefu; Zucker, Catherine
    High-quality Extragalactic Legacy-field Monitoring (HELM) is a long-term observing program that photometrically monitors several well-studied extragalactic legacy fields with the Dark Energy Camera (DECam) imager on the CTIO 4 m Blanco telescope. Since 2019 February, HELM has been monitoring regions within COSMOS, XMM-LSS, CDF-S, S-CVZ, ELAIS-S1, and SDSS Stripe 82 with few-day cadences in the (u)gri(z) bands, over a collective sky area of similar to 38 deg2. The main science goal of HELM is to provide high-quality optical light curves for a large sample of active galactic nuclei (AGNs), and to build decades-long time baselines when combining past and future optical light curves in these legacy fields. These optical images and light curves will facilitate the measurements of AGN reverberation mapping lags, as well as studies of AGN variability and its dependencies on accretion properties. In addition, the time-resolved and coadded DECam photometry will enable a broad range of science applications from galaxy evolution to time-domain science. We describe the design and implementation of the program and present the first data release that includes source catalogs and the first similar to 3.5 yr of light curves during 2019A-2022A.
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    Tikhonov regularization as a nonparametric method for uncertainty quantification in aggregate data problems
    (2024) Villalon, Elena; Yang, Qian; Long, Carlos A. Sing
    Dynamical systems are widely used in mathematical models in engineering and in the applied sciences. However, the parameters of these systems are usually unknown and they must be estimated from measured data. Performing parameter estimation and quantifying the uncertainty in these estimates becomes critical to realize the predictive power of dynamical systems. In some applications, only measurements about the states of an ensemble of systems are available, where each one evolves according to the same model but for different parameter values, leading to aggregate data problems. In this case, an approach proposed in the literature is to estimate a density that is consistent with some estimates of the expected value of some quantities of interest. To solve the problem in practice, the density is discretized and, to solve the resulting ill-posed problem, Tikhonov regularization is used. In this work, we propose a Bayesian model that shows this approach can be interpreted as a maximum a posteriori estimate for the density. We show that the infinite-dimensional problem defining the MAP can be reformulated as a finite-dimensional problem that does not require discretizing the density. In several cases of interest, this problem is convex, unconstrained, and the objective function is smooth. Thus, it can be solved using algorithms with optimal convergence rates. The trade -off is that the objective is defined by an integral. However, our results characterize the regularity of the integrand, allowing the use of tailored numerical schemes to approximate it. Furthermore, our theoretical results characterize the form of the optimal density, whereas our numerical results illustrate the performance of our method and confirm our theoretical findings.