Johns Hopkins
Summary
My research at JHU was on a topic called AGN Feedback- the relationship between the massive, central black holes as they accrete matter, and the galaxies that host them. AGN stands for Active Galactic Nucleus, referring to a super massive black hole that resides at the center of a galaxy and is actively accreting matter and subsequently releasing huge amounts of energy and momentum back into the host galaxy. We perform super computer simulations of this process and analyze the impact on the host’s morphology and star formation.
My advisor was award winning astrophysicist Joe Silk. (Emeritus Fellow of New College, Oxford, Fellow of the Royal Society, 2011 Balzan Prize, Homewood Professor of Physics and Astronomy at Johns Hopkins University, Savilian Chair of Astronomy at the University of Oxford, Chair of Astronomy at Berkley)
I worked very closely with Volker Gaibler, who is an expert on relativistic AGN jets and astrophysical fluid super computer simulations. Winner of 2009 Golden Spike Award of the High Performance Computing Centre in Stuttgart (HLRS) for his contributions to High-Performance Computing.
Below I include descriptions and links to our three papers that I was first author on.
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Feedback by AGN Jets and Wide Angle Winds on a Galactic Scale
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To investigate the differences in mechanical feedback from radio-loud and radio-quiet Active Galactic Nuclei (AGN) on the host galaxy, we perform 3D AMR hydrodynamic simulations of wide angle, radio-quiet winds with different inclinations on a single, massive, gas-rich disk galaxy at a redshift of 2-3. We compare our results to hydrodynamic simulations of the same galaxy but with a jet. The jet has an inclination of 0 degrees (perpendicular to the galactic plane), and the winds have inclinations of 0, 45, and 90 degrees. We analyze the impact on the host's gas, star formation, and circum-galactic medium. We find that jet feedback is energy-driven and wind feedback is momentum-driven. In all the simulations, the jet or wind creates a cavity mostly devoid of dense gas in the nuclear region where star formation is then quenched, but we find strong positive feedback in all the simulations at radii greater than 3 kpc. All four simulations have similar SFRs and stellar velocities with large radial and vertical components. However, the wind at an inclination of 90 degrees creates the highest density regions through ram pressure and generates the highest rates of star formation due to its ongoing strong interaction with the dense gas of the galactic plane. With increased wind inclination, we find greater asymmetry in gas distribution and resulting star formation. Our model generates an expanding ring of triggered star formation with typical velocity of order 1/3 of the circular velocity, superimposed on the older stellar population. This should result in a potentially detectable blue asymmetry in stellar absorption features at kpc scales.
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AGN Outflow Shocks on Bonnor-Ebert Spheres
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Feedback from Active Galactic Nuclei (AGN) and subsequent jet cocoons and outflow bubbles can have a significant impact on star formation in the host galaxy. To investigate feedback physics on small scales, we perform hydrodynamic simulations of realistically fast AGN winds striking Bonnor-Ebert (BE) spheres and examine gravitational collapse and ablation. We test AGN wind velocities ranging from 300--3,000 km s^−1 and wind densities ranging from 0.5--10 mp cm^−3. We include heating and cooling of low- and high-temperature gas, self-gravity, and spatially correlated perturbations in the shock, with a maximum resolution of 0.01 pc. We find that the ram pressure is the most important factor that determines the fate of the cloud. High ram pressure winds increase fragmentation and decrease the star formation rate, but also cause star formation to occur on a much shorter time scale and with increased velocities of the newly formed stars. We find a threshold ram pressure of ∼2×10^−8 dyne cm^−2 above which stars are not formed because the resulting clumps have internal velocities large enough to prevent collapse. Our results indicate that simultaneous positive and negative feedback will be possible in a single galaxy as AGN wind parameters will vary with location within a galaxy.
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Stellar Signatures of AGN-jet-triggered Star Formation
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To investigate feedback between relativistic jets emanating from active galactic nuclei and the stellar population of the host galaxy, we analyze the long-term evolution of the orbits of the stars formed in the galaxy-scale simulations by Gaibler et al. of jets in massive, gas-rich galaxies at z ~ 2-3. We find strong, jet-induced differences in the resulting stellar populations of galaxies that host relativistic jets and galaxies that do not, including correlations in stellar locations, velocities, and ages. Jets are found to generate distributions of increased radial and vertical velocities that persist long enough to effectively augment the stellar structure of the host. The jets cause the formation of bow shocks that move out through the disk, generating rings of star formation within the disk. The bow shock often accelerates pockets of gas in which stars form, yielding populations of stars with significant radial and vertical velocities, some of which have large enough velocities to escape the galaxy. These stellar population signatures can serve to identify past jet activity as well as jet-induced star formation.
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Computer simulations performed on JHU High Powered Computer and analysis performed on JHU Datascope
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The Johns Hopkins Institute for Data Intensive Engineering and Science Datascope
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The Johns Hopkins High Powered Computer
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