Sep 10

"Tidal Imprints of Dark Sub-Halos on the Cold Gas in Galaxies"
Sukanya Chakrabarti, University of California, Berkeley

"I will discuss a new analysis of the observed perturbations in the HI disk of the Milky Way (Chakrabarti & Blitz 2009). Through analysis of a large set of high resolution hydrodynamical simulations, I find that the best-fit to these perturbations is produced by a perturber that tidally interacted with the Milky Way disk, having a mass one-hundredth of the Milky Way, and a pericentric approach distance of 5 kpc. I show that under certain conditions one can break the degeneracy between the mass and the pericentric approach in the evaluation of the tidal force to directly determine the masses of satellites. This is a new method of determining the masses of satellites and can provide an independent method of characterizing the dark sub-halo population. Secondly, I use my radiative transfer code RADISHE to observe simulations (both of quiescently evolving and tidally interacting galaxies). I calculate the SEDs and images of these simulations to compare to observed star formation profiles and the IR emission in external galaxies."

Sep 24

"Accurate AGN Black Hole Masses and the Dispersions in the Black Hole Mass - Bulge Luminosity and M-Sigma Relationships"
Martin Gaskell, University of Texas at Austin

"I present an improved simple method of estimating black hole masses in AGNs. This is substantially more accurate than the currently most widely used method. The improved black hole mass estimates show that the relationship between black hole mass and bulge luminosity is very tight at the high-mass end and that the intrinsic dispersion in both the M-L and M-sigma relationships increases towards lower masses. I argue that AGN black hole mass estimates are now as accurate as stellar dynamical mass estimates for inactive galaxies. This surprising accuracy of the AGN black hole mass estimates strongly supports the "bird's nest" model of the inner structure of all rapidly accreting AGNs. The increasing scatter in the M-L and M-sigma relationships in less massive galaxies gives important clues to the formation of these relationships."

Nov 6 Friday 3:30 pm

"Active galactic nuclei and the cosmic gamma-ray background radiation in the Fermi era"
Yoshiyuki Inoue, Kyoto University

"The origin of the extragalactic gamma-ray background (EGRB) has been a mystery for a long time. In this talk, I will discuss about the origin of the MeV and GeV EGRB. First, although the cosmic X-ray background can be accounted for by Active Galactic Nuclei (AGNs), AGNs are not able to explain the MeV background, because of the thermal exponential cutoff in the spectral models. We constructed a new spectral model by calculating the Comptonization process including nonthermal components. We show that the MeV background spectrum can be naturally explained by our model. Second, we present a new model of the blazar gamma-ray luminosity function and the EGRB spectrum. The unified sequence of blazar spectral energy distribution is taken into account to make a nontrivial prediction for the GeV EGRB spectrum. We then try to explain the EGRB data by the two AGN populations: one is blazars, and the other is non-blazar AGNs that are responsible for the MeV background. The predicted EGRB spectrum is in agreement with a wide range of the observed data from X-ray to GeV including the very recent EGRB data by the Fermi satellite. These results indicate that AGNs including blazars are the primary source of EGRB."

Nov 12

"Dual Supermassive Black Holes as Tracers of Galaxy Mergers"
Julie Comerford, University of California, Berkeley

"A wealth of observations have shown that galaxy mergers are common and that nearly all galaxies host a central supermassive black hole (SMBH); consequently, some galaxies host two SMBHs as the result of recent mergers. However, the full potential of these dual SMBHs for studies of galaxy evolution has not yet been realized, due the small number of known dual SMBHs. In this talk, I will describe a new technique I use to build a significantly larger sample of dual SMBHs, where I spectroscopically identify dual SMBHs that power active galactic nuclei (AGN). I search the DEEP2 Galaxy Redshift Survey for galaxy spectra that exhibit AGN emission lines that are offset in velocity relative to the mean velocity of the host galaxy's stars, suggesting bulk motion of the AGN within the host galaxies. Within the set of DEEP2 red galaxies at 0.3 < z < 0.8, I find 32 AGN with statistically significant (greater than 3 sigma) velocity offsets, ranging from ~50 km/s to ~300 km/s. After exploring physical effects such as AGN outflows that could cause such velocity offsets, I find that these offsets are most likely the result of dual SMBHs in merger-remnant galaxies. With this new technique of identifying galaxy mergers, I find that roughly half of red galaxies hosting AGN are merger-remnant galaxies. This result implies that galaxy mergers may trigger AGN activity in red galaxies and sets a merger rate of ~3 mergers/Gyr for red galaxies at 0.3 < z < 0.8. Finally, I will discuss the utility of HST imaging and optical slit spectroscopy in further increasing the number of known dual SMBHs."

Dec 3

"Distances to the 21 cm High Velocity Clouds"
John Barentine, University of Texas at Austin

"Clouds of neutral hydrogen of large angular extent on the sky were detected in the 21 cm transition in the early 1960's. Their observed velocities do not match those predicted by simple differential galactic rotation models, and their nature and origin remain enigmatic. The clouds are implicated in various astrophysical scenarios, including as components of galaxy assembly in Lambda-CDM cosmologies, a possible solution of the "G Dwarf Problem" in the Milky Way, and direct observation of the "Galactic Fountain" in action. They are also important in extragalactic studies as analogs of Galactic high velocity clouds have been observed in external galaxies. I will report initial results of multi-year effort to determine the distances to five cloud complexes or individual clouds, which in turn provide the first empirical constraints on properties such as their masses, radii, densities, and pressures. Our results support the interpretation of some clouds as pristine, low-metallicity gas falling into the Milky Way for the first time, and others as the outflow and return streams of the Galactic Fountain."