Abstracts
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"The Megamaser Cosmology Project: Angular-Diameter Distances to Megamaser
Galaxies and the Masses of Their Central BHs"
The Megamaser Cosmology Project aims to measure the Hubble constant (H0) to 3% accuracy
to constrain the Dark Energy equation of state parameter, w. The key to achieve this goal
is to measure distances accurately to galaxies well into the Hubble flow (50 - 200 Mpc).
To measure accurate distances, we adopt the proven H2O megamaser method pioneered by the
study of NGC 4258. An essential component of this technique involves sub-milliarcsecon
Very Long Baseline Interferometry (VLBI) imaging of the sub-parsec H2O megamaser disks
at the centers of active galaxies. We determine accurate angular-diameter distances to
these galaxies with precise astrometry and kinematics of the maser clumps in the disks.
For distance determination, we have concentrated mainly on six galaxies (UGC 3789,
Mrk 1419, NGC 6264, NGC 6323, NGC 2273, and NGC 1194) that show the necessary maser
spectral characteristics. In addition, we are pursuing VLBI imaging for several other
galaxies for which a distance measurement would be difficult, but we can still measure
masses of the black holes (BH) at their centers. In this talk, I will present our
latest work on distance determination for NGC 6323 and NGC 6264, and our results
on BH mass measurements for seven megamaser galaxies, along with the implications
for the universality of the M-sigma relation. |
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"Solving Active Galactic Nuclei Puzzles - a New Paradigm"
In order to study the evolution of supermassive black holes and galaxies it is necessary to understand how AGNs work. Although the general picture of how AGNs function has become clearer in recent years, major observational puzzles threaten to undermine our fundamental beliefs about the AGN phenomenon. These puzzles include AGNs with extremely asymmetric emission line profiles, inconsistent multi-wavelength variability, rapid apparent changes in the direction of gas flow and the sizes of emitting regions, a curious insensitivity of gas in some narrow velocity ranges to changes in the ionizing continuum, and differing dependences of polarization on gas velocity. I show that all these puzzles have a common unified explanation, but one that requires a major paradigm shift in our views of how AGNs work. I discuss the implications of this for the detection of close supermassive black hole binaries and gravitational recoil events. |
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"Direct Observations of Ionizing Radiation from High-z Galaxies"
I will discuss recent progress in direct searches for and constraints
on hydrogen-ionizing radiation escaping from star-forming galaxies
at z~3. The results will be discussed in the context of understanding
the nature of the sources that reionized the universe at z~10, and
which maintain the high ionization of the intergalactic medium through
through the first few Gyr post-reionization. |
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"Observationally Constraining the Nature of QSO Host Galaxies and Their Relationship to Merger-Driven Galaxy Evolution"
The last two decades have given rise to large advances in our understanding of galaxy evolution. Evidence now exists suggesting that gas-rich, disk galaxies; starbursting galaxies; active galactic nuclei (AGN); post-starburst galaxies; and gas-poor, dynamically hot, elliptical galaxies can be seen as different phases of major galaxy mergers. Many observations of small samples of these different types of galaxies hinted at such an evolution scenario, but not until large galaxy surveys (e.g. SDSS, COMBO-17, GOODS, DEEP/DEEP2) have discoveries been made to strongly support this idea. The hosts of bona fide QSOS (Mv >-23) are not generally detectable in these surveys, due to up to 3.5 magnitudes difference in brightness between the galaxy and the host. We present an ongoing study to examine these most observationally difficult pieces of the puzzle. We have developed an optical and NIR imaging and spectroscopy observational program that specifically addresses these observational difficulties in order to examine the following: 1) The stellar populations of the host galaxies in order to understand the star formation history and compare the age of the youngest population to the different merger scenarios; 2) The stellar velocity dispersion, surface brightness and effective radii of the hosts in order to place these objects on the fundamental plane and compare them quantitatively to quiescent galaxies and 3) The relationship between MBH and sigma* in order to compare this relation to that for quiescent and low luminosity active galaxies. |
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Physics Relativity Seminar (RLM 6.116): "The Search for Compact Binary Coalescence in Association with Short GRBs with the First-Generation LIGO and Virgo Detectors"
Gamma-ray bursts (GRBs) are fantastically powerful, faraway
explosions. Every day, we detect light from these explosions with the
several spacecraft that form the interplanetary network, but their
mysteries endure. For a subset of bursts, the short-duration GRBs, the
prevailing progenitor theory is that the explosions occur in the final
stages of the inspiral of a neutron star with either another neutron
star or a black hole. Such cosmic violence would make itself known
also through an outpouring of gravitational waves. Detection of these
gravitational waves would not only solve the astronomical puzzle of
the origin of short GRBs, but also provide measurements of the
equation of state for matter at super-nuclear densities, measure the
dark matter and dark energy content in the Universe, and allow tests
of Einstein's general relativity in the strong-field regime. I will
describe a search for these gravitational waves using detectors from
the Laser Interferometer Gravitational-wave Observatory (LIGO) and
Virgo in their first-generation configurations. |
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"The Diverse, yet Orderly Lives of Galaxies"
At first glance the galaxy population today and even more so at
earlier times exhibits a huge diversity. However, the well-known
correlations between different galaxy properties, such as spatial
structure, stellar population, stellar mass, stellar dynamics, and
environment suggest that galaxy formation is actually an orderly
process. With the recent large photometric and spectroscopic surveys
and new instrumentation on the Hubble Space Telescope, it is now
finally possible to study galaxies in a systematic way at earlier
times, so that we can see directly how these relations change over
cosmic time and what the physical processes are that drive them.
Until very recently, these studies were hampered by the small sizes of
spectroscopic galaxy samples, whereas much larger photometric samples
lack the required spectroscopic information. I will discuss a novel
approach, that makes use of medium-band photometry to perform detailed
spectroscopic studies of ~3500 galaxies at 0.5 |
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"How Much Mass is in Spiral Disks?"
We present science highlights of the DiskMass Survey, and an extension
of this survey to lower surface-brightness disks, enabled by two
high-etendue IFUs on 3.5m telescopes. Our results show that disks are
substantially sub-maximal and that the mass-to-light ratios (M/L) of
stellar populations are smaller than previously supposed. These
results rule out disk dark-matter, but imply that unless our theory of
gravity is wrong, dark halos are dynamically important even in the
inner regions of galaxies. Lower stellar M/L can be explained by
either a more truncated IMF or the addition of late (luminous) phases
of stellar evolution. In either event, the stellar component of the
baryon budget, and its evolution with redshift, must be revised. |
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"Surprise! Dwarf Starburst Galaxy Hosts Supermassive Black Hole"
Supermassive black holes are now thought to inhabit essentially every
modern galaxy with a bulge. However, the birth and growth of the first
"seed" black holes, back in the earlier universe, is poorly
constrained. Reines et al. (2011) have recently identified a
candidate million-solar mass black hole in a vigorously star-forming,
bulgeless dwarf galaxy called Henize 2-10. This serendipitous
discovery offers the first opportunity to study a growing black hole
in a nearby galaxy much like those in the earlier universe, and opens
up an entirely new class of host galaxies in which to search for local
analogues of primordial black hole growth. Moreover, this finding has
important implications for our understanding of the co-evolution of
galaxies and their central black holes. In particular, the lack of a
discernible bulge in Henize 2-10 indicates that black hole growth can
precede the build-up of galaxy spheroids, which has been a matter of
debate in the astronomical community. |
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"Understanding the Global Course of Galaxy Evolution at z < 1"
Evolution in the global galaxy population over the past 7 Gyr has been
dominated by two principal trends: a dramatic decline in the average
level of star-formation activity combined with a substantial growth in
the stellar mass density within the red galaxy population. While both
of these evolutionary trends are well measured at z < 1, the physical
mechanisms responsible remain somewhat poorly understood. Using data
from the DEEP2 Galaxy Redshift Survey in concert with complementary
observations spanning UV to radio wavelengths, I will present recent
results that directly constrain the physical processes driving the
global transformation in galaxy properties at z < 1. In particular, I
will discuss ongoing work to probe the cold gas component of
star-forming galaxies at high redshift, which is providing direct
constraints on the fuel supply for star formation when the Universe
was less than half its current age. Finally, I will conclude by
outlining the limitations of the current data sets and how they might
be overcome with future ground- and space-based facilities. |
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"The Dark Matter Halo - Galaxy Connection in the Local Universe and Local Group"
The canonical model of galaxy formation within the cold dark matter paradigm
places all galaxies in extended dark matter halos. While the relationship
between galaxy stellar mass and dark matter halo mass is potentially quite
complicated, recent work has shown that it may be remarkably simple in practice.
I will describe a basic theoretical model of how galaxies populate dark matter
halos statistically, explore its implications, and present results from tests of
this model in the low-redshift Universe. On smaller scales, the subhalo-galaxy
connection also has the potential to inform our understanding of dark matter
physics. I will discuss efforts to probe scales currently inaccessible at
cosmological distances by studying dwarf satellites of the Milky Way, which have
led to perplexing results. |
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