Metal absorption lines in the CGM

Advisor: Dr. Thorsten Naab

The circumgalactic medium (CGM) is the gas that surrounds galaxies, at distances between the galaxies' disk or interstellar medium and the virial radii. It fuels a galaxy's star formation and regulates the gas supply of a galaxy. The CGM is known to be a multiphase medium, and it can be probed using absorption lines technique. Nevertheless, complex processes are difficult to disentangle observationally, given the limited number of sightlines.

To counter this limitation, astronomers turn to employ simulations to bridge the observed absorption line properties with the physical and dynamical state of the CGM gas. One such method is examining how simulated LyA absorption traces the conditions of the CGM gas in simulations. Unfortunately, LyA absorption lines get saturated fairly easily and thus, do not show clear internal structure. Saturated lines make it impossible to disentangle the main contributors to the broadening of LyA lines -- we need more information beyond the LyA line and its shape to distinguish the contributions of various factors to the broadened LyA line.

Therefore, we turn to probe into metal absorption lines -- which are typically less likely to be saturated -- as a means to disentangle the contributors to line broadening. The combination of information from multiple metal ion lines should allow us to distinguish the various CGM phases and trace the baryon cycle better via absorption lines. We aim to trace the recycled gas and the newly accreted gas into the galaxy, building a clearer picture of the baryon cycle of the galaxy. This clearer tracing could help us quantify the amount of "missing halo baryons" hiding in the CGM gas.

TDE-powered AGNs in low-mass galaxies

Advisors: Dr. Guang Yang, Prof. Jonelle Walsh

Tidal-disruption events (TDEs) can contribute to the growth of black holes (BH; at galactic centers) across cosmic time, independent of galaxy luminosity. The amount of BH growth is negligible for massive BHs, whose dominant growth channel should be cold-gas accretion. However, for low-mass BHs, the TDE-related BH growth could account for a significant fraction of total BH mass, making TDEs to possibly be a crucial ingredient for the growth of low-mass AGNs hosted by dwarf galaxies. We investigate the temporal behavior of AGNs detected in dwarf galaxies to determine if they are powered by TDEs.

Using X-ray data from the CDF-S Survey, we fit different models to the selected sources' light curve and apply statistical methods to determine the model that has the better quality of fit. A source is considered a potential TDE if it shows strong preference for the canonical TDE model; using this, we can estimate the fraction of TDE-powered AGNs in our sample. I also simulate TDE and AGN light curves, and use the same fitting method to compute the TDE fractions from simulations, which are then used to compute the true and false positive rates for each source. Using these rates, I perform Bayesian analysis to obtain the posterior for the TDE fraction, which helps constrain the fraction of AGNs in our sample that are powered by TDEs.

Stellar kinematics measurements from Gemini NIFS spectral data

Advisor: Prof. Jonelle Walsh

under construction...

Estimating rotation periods of Kepler/Gaia stars

Advisor: Prof. Gibor Basri

under construction...

Extracting astronomical data to date Neo-Assyrian letters

Advisors: Dr. Gil Breger, Dr. Laurie Pearce

In ancient Mesopotamia, scholars would observe the skies, day and night, and report their findings to the ruler. The reports would also include their predictions and words of caution to the ruler. Typically, astronomical observations/events such as lunar phases, eclipses (both lunar and solar), rising and setting of planets, etc, are reported by scholars. Tapping into their astrological knowledge, they would then make predictions of what might happen, and provide suggestions on how the ruler can circumvent bad omens. These letters were written on clay tablets and were typically undated and/or unsigned. Some of them are also chipped due to the passage of time.

Assyriologist Simo Parpola has conducted a series of studies on these letters from the Neo-Assyrian court. He assigned authorship and dates to unsigned letters based on paleographic features, prosopographic evidence, and the computed dates of astronomical phenomena (in particular solar and lunar eclipses). Our project focuses on exploring the heuristics used to identify the astronomical phenomena that form the backbone for dating several of these letters. While Parpola relied on the then-standard handbooks of astronomical phenomena (e.g., ephemerides), the computational tools at his disposal then were hardly as refined as those available to us today. Thus, our aim is to assess the reliability and reproducibility of Parpola's results, in an attempt to demonstrate the value of modern computational tools in developing reproducible humanities research. We also utilize widely-accessible, open-source digital tools for our purpose.

Our project combines astronomical and prosopographic information from these tablets to provide potential dates for the clay tablets. We selected tablets that included astronomical observations as listed above, and extracted the relevant information that is useful in dating. Using personnel information in the letters, such as the name of sender/writer and names of citizens (merchants, scholars, etc.), is where prosopographic knowledge comes into play -- it helps narrow down the possible timeframe, from the entire length of the Neo-Assyrian period to mere decade or two. We then used computer programs to compute ephemeris of astronomical objects during the selected timeframe, and investigate if there are potential dates in which the astronomical observations match the calculations. Typically, we would have a few potential dates for each tablet. Further information will be required to narrow down the dates to just one. We find that our selected dates are usually a match with the dates that Parpola has assigned. We find that, in general, we are able to reproduce Parpola's results using modern digital tools.