Center for Relativistic Astrophysics

CRA Seminar: James Casey & Life In Antarctica
2018-05-10 15:00 - 16:00 (America/New_York)

End of Term - Spring 2018
2018-05-05

Faculty in the hotseat with Gongjie Li
2018-05-03 15:00 - 16:00

CRA Seminar: Neil Cornish (Montana State)
2018-05-01 15:00 - 16:00 | Boggs 1-90 VizLab
Measuring the polarization state of gravitational waves
Alternative theories of gravity predict additional polarization states to the two tensor modes of general relativity. LIGO/Virgo have provided the first observation constraints on polarization. Pulsar Timing and Astrometry can be used to set even stronger constraints. I will give an update of the current status of polarization studies and discuss the near future prospects.
Note special day: Tuesday

Dr. Sam Hadden (Harvard-Smithsonian Center for Astrophysics)
2018-04-27 15:00 - 16:00 | CULC 102

Cosmic Coffee - TBD
2018-04-25 15:00 - 16:00

Cosmic Coffee - Yuya Sakurai
2018-04-18 15:00 - 16:00

Dr. Mojegan Azadi (Harvard-Smithsonian Center for Astrophysics, Cambridge)
2018-04-12 15:00
Hunting for Monsters! Active supermassive black holes identification and their host galaxies’ physical properties

Active galactic nuclei (AGN) are powered by the accretion activity of supermassive black holes residing at the centers of galaxies. While observations propose that growth of AGN and galaxies are globally tied, I investigate whether this connection exists in individual galaxies. I also investigate various AGN selection techniques and star formation rate (SFR) estimates using multi-wavelength data from Chandra, Spitzer and rest-frame optical spectra from the Keck telescope.

I find that combining multi-wavelength identification techniques provides a complete AGN sample, as each selection method suffers from selection biases. In particular, all selection techniques are biased against identifying AGN in low mass galaxies. Once stellar mass selection biases are taken into account, I find that AGN reside in galaxies with similar physical properties (i.e., SFR) as inactive galaxies. I find that while AGN are prevalent in both star-forming and quiescent galaxies, they are more likely to reside in star-forming galaxies. The probability of fueling an AGN does not strongly depend on SFR for a star-forming galaxy, though it decreases when star formation is shut down in quiescent galaxies. I find no evidence for a strong correlation between SFR of the host galaxy and AGN luminosity. These results indicate that while both AGN and galaxy growth are reliant on the same fuel, enhanced star formation activity does not necessarily go hand-in-hand with increased AGN activity.

While the star formation activity of galaxies can be traced with various indicators, my investigations indicate that at z~2 a combination of mid-infrared and far-infrared data provide a more reliable SFR estimation than the mid-infrared data alone. I also find that the robustness of UV-based SFRs depends on the extinction correction method used. I find that about 30% of z ∼ 2 galaxies have SFRs from infrared observations that are elevated relative to the dust corrected UV-based SFR. My investigations show that this infrared excess is not due to any contribution from AGN, and is primarily due to polycyclic aromatic hydrocarbon (PAH) emission.

Cosmic Coffee - Aaron Smith
2018-04-11 15:30 - 16:00

Aaron Smith (UT-Austin): Simulating Lyman-Alpha Sources in the Epoch of Reionization
2018-04-11 15:00 - 16:00 | Boggs 1-90 VizLab
Title: Simulating Lyman-Alpha Sources in the Epoch of Reionization
Abstract: Radiation from the first stars and galaxies initiated the dramatic phase transition marking an end to the cosmic dark ages. The emission and absorption signatures from the Lyman-alpha (Lyα) transition of neutral hydrogen have been indispensable in extending the observational frontier for high-redshift galaxies into the epoch of reionization. Lyα radiative transfer provides clues about the processes leading to Lyα escape from individual galaxies and the subsequent transmission through the intergalactic medium. Cosmological simulations incorporating Lyα radiative transfer enhance our understanding of fundamental physics by supplying the inferred spectra and feedback on the gas. In this talk, I will discuss recent advances in Lyα modeling based on current state-of-the-art simulations and observational insights. As a case study I will present post-processing results of cosmological “zoom-in” simulations of z > 5 galaxies created with the hydrodynamics code GIZMO under the framework of Feedback In Realistic Environments (FIRE). I will also discuss my new resonant discrete diffusion Monte Carlo (rDDMC) method designed to break the efficiency barrier of Monte Carlo Lyα radiative transfer in the high optical depth regime. Such efficient and robust algorithms will enable fully coupled 3D Lyα radiation hydrodynamics in the near future.

CRA Seminar: Sam Finn (Penn State)
2018-04-05 15:00 - 16:00 | Boggs 1-90 VizLab

Prof. Konstantin Batygin (Caltech)
2018-04-04 15:00 - 16:00 | Boggs 1-90 VizLab

CRA Seminar ( Prof. Ann-Marie Madigan (University of Colorado, Boulder)
2018-03-29 15:00

Cosmic Coffee - Bhavesh Khamesra
2018-03-28 15:00 - 16:00

Spring Break
2018-03-19 - 2018-03-23

Professor Avi Loeb (Harvard-Smithsonian Center for Astrophysics)
2018-03-12 18:00 - 19:00
Avi’s most recent initiative is the Starshot project, which explores possibility of interstellar travel (https://breakthroughinitiatives.org/Initiative/3).

Dr. Billy Quarles (University of Oklahoma)
2018-03-08 15:00 - 16:00 | Boggs 1-90 VizLab

Cosmic Coffee - Chris Evans
2018-03-07 15:00 - 16:00

Cosmic Coffee - Karelle Siellez
2018-02-28 15:00 - 16:00

Matt Holman (Harvard CfA, director of Minor Planet Center)
2018-02-22 15:00 - 16:00 | Boggs 1-90 VizLab
100,000 Asteroids Down the Back of the Couch:
A Novel Approach to the Minor Planet Linking Problem
A number of ongoing surveys, such as Pan-STARRS, the Catalina Sky
Survey, OSSOS, and NEOWISE, as well as planned surveys such as ZTF,
LSST, and NEOCam, are designed to pursue goals ranging from
constraining models of planet formation, through finding evidence of
additional planets in our solar system, to fulfilling the US
Congressional mandate to discover 90% of the potential hazardous
asteroids with diameters exceeding 140m.
The typical asteroid search strategy is based on identifying
`tracklets’, a sequence of two or more observations that are taken
over a time span that is short enough that it is likely that the
detections correspond to the same moving object, and a long enough to
distinguish solar system objects from stationary background sources.
A primary goal is to obtain enough tracklets for each object that the
corresponding orbit is accurately determined. By design, most objects
are naturally re-observed in the course of these surveys. However,
which observations correspond to which object must still be identified
before the orbits of those objects can be determined. This is known
as the `linking problem.’
The best current solution to the linking problem, the Pan-STARRS Moving
Object Processing System (MOPS), employs a sophisticated variation of
the brute force approach, bringing groups of three tracklets together
to be tested with orbit fitting. The computational load of MOPS
scales as $mathcal{O}(N_t^3)$, where $N_t$ is the number of tracklets.
We present a novel approach, heliocentric linking and clustering, that
scales as $mathcal{O}(N_t log N_t)$. We use this approach to
identify thousands of new objects within the Minor Planet Center’s
“Isolated Tracklet File”. Finally, we discuss the implications of
our results for ongoing and future surveys.

Cosmic Coffee - Chris Tung
2018-02-21 15:00 - 16:00

CRA Special Seminar: Next Generation of Science Instruments for SOFIA (Alan Rhodes)
2018-02-15 15:00 - 16:00

Cosmic Coffee - KwangHo Park
2018-02-14 15:00 - 16:00

Cosmic Coffee - Cancelled
2018-02-07 15:00 - 16:00

CRA Seminar: Dr. Chanda Prescod-Weinstein (University of Washington, Seattle) **Special Time**
2018-02-07 13:00 - 14:00 | Boggs 1-90 VizLab
Making a Universe with (Rel)axions
The discovery of the Higgs boson reinforces the possibility that other similar, scalar particles may exist in nature. In this talk, I will begin by discussing one such dark matter and sometime inflaton candidate, the axion. I will describe the claim that dark matter axions form an exotic state of matter called a Bose-Einstein condensate and my work on this exciting prospect. Along the way I will draw connections with the interesting relaxion idea and possible phenomenologies for the proposed NASA STROBE-X experiment.

Please note special day (Wednesday) AND time (1pm). Coffee and cookies will be served in the CRA kitchen after the seminar to allow participants to attend the Black History Month lecture at 3pm: http://www.diversity.gatech.edu/2018-black-history-month-lecture

CRA Seminar: Prof. Justin Vandenbroucke (UW Madison)
2018-02-01 15:00 - 16:00 | Boggs 1-90 VizLab
Cosmic-ray anisotropy searches with the Fermi Large Area Telescope
Cosmic rays arrive at Earth nearly isotropically due to magnetic deflection during propagation. However, small anisotropy has now been measured in cosmic-ray nuclei over eight orders of magnitude in energy, from 100 GeV to 10^10 GeV. This anisotropy is an essential observable in the study of cosmic ray origins and propagation. The Fermi Large Area Telescope (LAT), although primarily a gamma-ray instrument, is making unique measurements at the low end of this range. With a large acceptance and a decade of flight, it has recorded the largest set of cosmic rays ever collected at the 100 GeV energy scale. The LAT has set stringent limits on cosmic-ray electron and positron anisotropy, with implications for dark matter and the mysterious positron excess. Moreover for cosmic-ray nuclei, it has excellent ability to identify composition and to constrain the full-sky anisotropy, neither of which has been possible by the ground-based instruments that have performed all anisotropy measurements to date.

Cosmic Coffee - Raj Gohil
2018-01-31 15:00 - 16:00

CRA Seminar **Special Time** ( Myoungwon Jeon, U. Arizona )
2018-01-30 15:00 - 16:00 | Boggs 1-42 (LIGO HIVE)
Title: Connecting the first galaxies with ultra-faint dwarfs in the local Universe.

Abstract :
Ultra-Faint Dwarf Galaxies (UFDs) are the least luminous systems and also the least metal- enriched systems.
Their simple, ancient nature, gives us an opportunity to look back at the star formation activities in early galaxies.
In this talk, I will present the results of our highly-resolved cosmological ab-initio simulations to show how first galaxies

can be connected with their local descendants. Our work has, for the first time, traced the detailed enrichment history of
stellar populations in local dwarf galaxies, reaching back to the first generation of stars. In this talk, I will discuss such
questions: what kind of star formation history did UFDs experience? Which physical mechanism regulates star formation
in UFDs? Are they true fossils of first galaxies?

Cosmic Coffee - Qi Ge
2018-01-24 15:00 - 16:00

Prof. Stephan Rosswog (Stockholm University)
2018-01-23 15:00 - 16:00
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Multi-messenger signals from gravitational wave sources

Neutron star mergers had long been suspected to produce gravitational
wave “chirps”, gamma ray bursts and produce r-process elements.
While overall convincing, all these conjectures were based on indirect
arguments and none was proven directly.
This changed on August 17, 2017: a gravitational wave signal from a
merging neutron star binary was detected, closely followed by a
short gamma-ray burst and transients accross the electromagnetic
spectrum coming from the radioactive decay of freshly synthesised r-process
elements. In this talk I will give an overview over these recent, very
exciting developments.

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