physics & astronomy
The gas-phase metallicity and ionization parameter are two primary parameters that drive the variations of emission line spectra in HII regions. There is an increasing amount of evidence that these two parameters are correlated. Theoretical works on the dynamical evolution of HII regions predict an anti-correlation between metallicity and ionization parameter. However, observations of HII region spectra have yielded a variety of different correlations, including anti-correlations, positive correlations, and even no correlation. All these measurements of the correlation rely on photoionization modeling. Also, different combinations of emission lines are used for calibrating metallicity and ionization parameter in different works. To solve the above problem, we examine how the derived correlation between metallicity and ionization parameter depends on the choice of models and emission lines. In this talk, I will show that it is important to constrain the model parameters using observational data before making any measurement. With the MaNGA IFU data, we compute a best-fit photoionization model for general HII regions and use a Bayesian method to fit metallicities and ionization parameters simultaneously. Our result shows that a positive correlation exists between the gas-phase metallicity and ionization parameter, regardless of which combination of emission lines is used for the fitting. This result clearly contradicts the prediction by the dynamical model. I will discuss potential mechanisms that lead to this discrepancy, and how it might be solved with future IFU surveys on HII regions.
Elements condensing into dust grains is an important physical process that occurs in the ISM. Through studies on extinction, scattering, heating and depletion of elements out of gas-phase, it has long been known that grains have a major effect on observed spectra of galaxies. Despite the large amount of observations available on extinction, scatter and heating, details of depletions onto grains and their effect on observed emission-line spectra are rarely studied. We explore the effects of grain depletions on strong spectral lines, and find non-trivial results. Our results suggest that the level of depletion, not only affects the emission line strength of corresponding ions, but also changes the abundance of coolants in the ISM gas, hence affecting its temperature. In addition, we include new code into Cloudy, allowing a user to alter the degree of grain depletions in a given model.
The environment in which a galaxy lives plays a key role in driving its evolution. As the most tenuously bound component of galaxies, neutral atomic hydrogen (HI) is a valuable tracer of both the interaction history of a galaxy with its environment and a measure of its future star formation potential. As galaxies move from the low-density field to high-density clusters, they lose their gas and star formation is quenched, but how exactly this happens is still poorly understood. In fact, perhaps most galaxies spend a large fraction of their life in the intermediate-density group environment where the signatures of galaxy evolution are more subtle and widely varied. In this talk I will present what my work on both wide-area HI surveys and individual observations have revealed about galaxy evolution, from the low mass group environment to the outskirts of massive galaxy clusters. Statistical studies of the HI provide insight on not only the gas processing and ongoing evolution within galaxy groups, but also the growth of large-scale structure. In addition, I'll present the first of the next generation of HI surveys, Apertif, which is observing 3500 square degrees at 14 times the spatial resolution of previous HI surveys and better HI mass sensitivity. Apertif will allows us to resolve and take the inventory, for the first time, of the physical mechanisms that remove gas from galaxies, across the full range of galaxy environments from poor groups to galaxy clusters.
The cosmic downsizing of quasars is still a big puzzle in astronomy and it is commonly believed that the central active galactic nucleus (AGN) must have played a significant role in quenching itself, in a self-regulatory mechanism popularly termed “AGN feedback” . The AGN feedback also plays a crucial role in black hole and host galaxy co-evolution across cosmic time (the M-sigma relation). Here I will discuss the nature and impact of pc scale outflows from AGN, detected in X-rays. On the other hand, the feeding of the supermassive black hole (SMBH) at the center of AGNs is an equally interesting puzzle. We still do not know how matter from the host galaxy loses their angular momentum and falls into the accretion disk, finally feeding the SMBH. In an extensive X-ray spectral variability study of Compton-thin Type-II AGN, we found the presence of clumpy gas in the near vicinity (<pc) of the SMBH which are likely candidates of matter which fall into the black hole and feed them, creating the luminous AGN.
The recently demonstrated skipper-CCDs have the ability to count single electronic with minimal noise. I will discuss the current status of the electron counting skipper-CCD technology, and its applications. Including low threshold experiments for dark matter and neutrinos, photon starved imaging in QIS and astronomy and more. I will also discuss the challenges and ongoing R&D effort for developments of the next generation experiments using this technology.
As the search for dark matter continues, a growing number of theories predict that dark matter is a supermassive particle or composite state. Discovering dark matter at this high mass extreme requires new approaches. I will survey some recent developments, including dark matter that forms black holes in the sun and earth, dark matter that would make old white dwarfs explode, and supermassive dark matter detected through its fusion of nuclei in Antarctic ice.
The Southern Stellar Stream Spectroscopic Survey (S5) is an ongoing spectroscopic program that maps the newly discovered stellar streams with the fiber-fed AAOmega spectrograph on the Anglo-Australian Telescope (AAT). S5 is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere, providing a uniquely powerful sample for understanding the building blocks of the Milky Way's stellar halo, the progenitors and formation of stellar streams, the mass and shape of the Milky Way's halo, and ultimately the nature of dark matter. The survey started in Summer 2018 and has mapped ~20 streams with over 50 nights on AAT. In this talk, I will give a brief overview of the current status of the program, highlighting the latest science results from the survey, and end the talk with the public data release plan.