About me

Born and raised in Taipei. I began my first Astronomy project as a summer intern at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), where I used molecular tracers to probe the physical conditions in IR-selected candidate massive young stellar objects. Immediately after receiving my bachelor's degree, I moved to metropolitan Washington DC to pursue graduate studies in astronomy at the University of Maryland. I then shifted my research toward time domain astronomy, whereby I began my journey with transient phenomena around supermassive black holes.
For my research, I use both ground- and space-based instruments to study these rare tidal disruption events – the result of a star being torn apart and consumed by a supermassive black hole – discovered by the intermediate Palomar Transient Factory (iPTF). I take an active role in iPTF and its successor, the Zwicky Transient Facility (ZTF), by developing pipelines to enable timely discoveries of unusual nuclear transients.

I enjoy reading, baking, and travelling.

Research

Tidal Disruption Events

When a star passes close enough to a supermassive black hole, the tidal force of the black hole overpowers the self-gravity of the star and shreds the star into streams of gas. Tidal disruption events (TDEs) allow us to study supermassive black holes residing in distant and quiescent galaxies that cannot be probed by other means.
However, TDEs are hard to find – the observational rate of TDEs is only ~2 per year with the capability of the current surveys. With only photometric data, we are always swamped by other interloping nuclear transient events such as SN Ia and AGN variability that are 100x more common.
Therefore, a huge part of my thesis is to find ways to effectively curate TDEs from the nightly influx of photometric data from the intermediate Palomar Transient Factory (iPTF), which utilizes the 48-inch telescope at Palomar observatory to observe the sky down to 20.5 mag.
In 2016, we successfully discovered 2 TDEs, iPTF16axa [1] and iPTF16fnl [2], from a 4-month long experiment. We reviewed the optically detected TDEs discovered so far and concluded that they are confined to a very narrow range of luminosity and share a similar decline rate that loosely follows t-5/3 (see figure below).

However, iPTF16fnl does not share these similarities with the other TDEs. It is the nearest, faintest, and fastest TDE that has been found. Combining these two studies, we conclude that there is at least a different population of TDEs. The design of current optical surveys may be biased against finding them because they evolve more rapidly than previously discovered TDEs.

Changing-look AGN

A byproduct of our search of TDEs is another rare class of objects called changing-look AGNs. iPTF16bco [3] was observed to undergo a transition in spectral shape from a quiescent galaxy to a booming Type 1 (broad line) quasar. The continuum in this object has increased by 10 times over a year, making this the tightest time constraint ever placed on this phenomenon.

AGN variability in UV and optical

We investigated UV and optical variability in AGNs on the timescale of ~1 year with a sample of variability-selected sources from Galaxy Evolution Explorer (GALEX) and Pan-STARRS 1 Medium Deep Survey [4]. Our sample includes both AGNs in morphologically galaxies and point-like quasars. Using image subtraction, we can separate the variable component that originate from the AGN from the star light. Our main finding is that UV and optical photometric variability can be well-described by a simple Shakura-Sunyaev thin disk with changing accretion rate. The goodness-of-fit of this thin disk model even outperforms a simple power-law for the majority of our sample. Below is a figure showing the best-fit average characteristic temperatures of the disk between two selected epochs (separated by ~1 yr). The bottom panel shows the best-fit power-law indices with the red dashed line marking the power-law index from a classic thin disk model.


Please see the papers here:
  • [1] Revisiting Optical Tidal Disruption Events with iPTF16axa.
  • [2] iPTF16fnl: A Faint and Fast Tidal Disruption Event in an E+A Galaxy.
  • [3] iPTF Discovery of the Rapid "Turn-on" of a Luminous Quasar.
  • [4] The GALEX Time Domain Survey. II. Wavelength-Dependent Variability of Active Galactic Nuclei in the Pan-STARRS1 Medium Deep Survey.
  • Curriculum Vitae

  • See here for my CV.
  • Contact

    Tiara (Tzu-Yu) Hung
    Office: ATL 1229
    Department of Astronomy, University of Maryland College Park, MD 20742-2421
    Email: tiarahung[at]astro.umd.edu

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