Jan 8 |
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Jan 15 |
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Jan 22 |
Graca Rocha |
Constraining variations of fundamental constants with CMB data
The Λ CDM model assumes the validity of General Relativity on cosmological scales,
as well as the physics of the standard model of particle physics. One possible extension, which may
have motivations in fundamental physics, is to consider variations of dimensionless constants.
Such variations can be constrained through tests on astrophysical scales.
A number of physical systems have been used, spanning different
time scales, to set constraints on variations of the fundamental constants.
These range from atomic clocks in the laboratory at a redshift z = 0 to BBN at z∼108. However,
apart from the claims of varying based on high resolution quasar absorption-line spectra,
there is no other evidence for time-variable fundamental constants.
CMB temperature anisotropies have been used extensively to constrain the variation of fundamental constants over cosmic
timescales. In this talk I will give an overview of the constraints on the fine structure constant, α, using CMB data with special focus on Planck data.
In summary, Planck data improve the constraints on α with respect to those from WMAP-9 by a factor of about five. Our
analysis of Planck data limits any variation in the fine-structure constant from z ∼103 to the present day to be less than approximately
0.4%. Given the apparent tension between the base Λ-CDM parameters from Planck and direct measurements of H0, we include further information from the H0 prior and
BAO data. I will show that adding a varying α does not resolve the tension between direct measurements of H0 and the value determined from the CMB.
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Jan 29 |
Matthew Kunz |
Thanatology in Protoplanetary Disks
The magnetorotational instability (MRI) is the most promising mechanism
by which angular momentum is efficiently transported outwards in
astrophysical disks. However, its application to protoplanetary disks
remains problematic. These disks are so poorly ionized that they may not
support magnetorotational turbulence in regions referred to as "dead
zones". While purported dead zones have captured the attention of disk
theorists for nearly 20 years, there is generally no consensus regarding
whether they are, in fact, dead. The problem is complicated by two
non-ideal MHD processes -- ambipolar diffusion and the Hall effect -- the
latter having been largely ignored in numerical studies of the MRI until
recently. In this seminar, I will present several surprising results on
how these effects are likely to modify the magnetic and turbulent behavior
of protoplanetary disks. In disks dominated by a vertical magnetic flux,
the MRI can saturate by producing large-scale, long-lived, axisymmetric
structures in the magnetic field that cause a steep reduction in turbulent
transport and instigate dust-corralling zonal flows. In disks dominated
by an azimuthal flux, the dead zone can be eliminated by large-scale
magnetic stresses and a strong outflow produced. These results call into
question contemporary models of layered accretion.
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Feb 5 |
Joo Heon Yoon |
Circumcluster and circumgalactic medium in the Virgo Cluster
I present the first systematic QSO absorption line survey for a galaxy cluster.
Using COS onboard HST, we find 43 Lyα absorbers toward 23 background QSOs in and
around the Virgo Cluster. We find Lyα absorbers are predominantly in the cluster
outskirts where the infalling substructures of the Virgo Cluster are. The cluster
is surrounded by this diffuse, warm gas in contrast with field environment. Our
finding is consistent with cosmological simulations. We also examine the CGM by
connecting the Lyα absorbers to their closest galaxies. The CGM of galaxies in
the cluster is suppressed compared with circumcluster and field environments.
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Feb 12 |
Ruth Murray-Clay |
Origins of Gas Giant Planets
Several giant planets have now been directly imaged, offering the first view
of extrasolar planets at wide separations from their host stars. Formation of
these objects by either leading theory--core accretion or gravitational
instability--presents substantial difficulties. These challenges may be
reinterpreted as opportunities. In this talk, I will demonstrate how to use
upcoming constraints from direct imaging to distinguish between theories of
giant planet formation. Along the way, I will discuss whether gravitational
instability could have formed the iconic directly-imaged planetary system
HR 8799, present a new theory of planetary core growth in the presence of gas
that extends the reach of core accretion to large stellocentric distances, and
show how the atmospheric compositions of giant planets record signatures of
their formation locations.
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Feb 19 |
Jo Bovy |
MAPping the Milky Way: Precision
measurements of the chemo-dynamical structure of the Milky Way's disk
Observations of the structure and dynamics of different stellar
populations in the Milky Way's disk provide a unique perspective on disk
formation, evolution, and dynamics. I will review our current knowledge of
the chemo-orbital structure of the disk and its implications for our
understanding of how the Milky Way formed and evolved. In particular, I
will show recent results from a dissection into mono-abundance populations
(MAPs) of the Galactic disk based on SDSS/SEGUE data. These results show
that the individual components are simple, but exhibit very different
spatial and kinematic structure, with important implications for the
formation and evolution of the Milky Way's disk. I will further discuss
how these measurements are being extended over a large part of the disk by
the APOGEE survey and how Gaia will lead to a greatly improved mapping of
the age-abundance-orbital structure.
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Feb 26 |
Robert Feldmann |
Star formation and Molecules in Cosmological Simulations
Star formation rates and gas masses of nearby galaxies are strongly
correlated. The true functional form of this relation is a matter of
debate and widely different relationships are discussed in the observational
and theoretical literature. I will discuss the physical origin of the star
formation - gas correlation and show how it can be modeled in the context
of cosmological, galaxy evolution simulations. I will highlight various
lines of evidence that point to an approximately linear relationship
between star formation rates and molecular gas masses in galaxies and
discuss implications for the balance between gas accretion and star
formation in high redshift galaxies.
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Mar 5 |
Anna Pancoast |
XXI century reverberation mapping: inferring black hole mass, geometry and dynamics of the broad line region in active galaxies
Constraining the inner regions of active galactic nuclei (AGN) is
difficult due to the very small scales involved. By using reverberation
mapping, we can substitute time resolution for spatial resolution and
begin to probe AGN on the scales of the broad line region. However
traditional reverberation mapping analysis uses only a fraction of the
available information. We have been developing advanced analysis methods
that give us new insights into the properties of AGN, including the
absolute black hole mass and the geometry and dynamics of the broad line
region. Applying this technique to five AGNs from the Lick AGN Monitoring
Project 2008 sample, we find that the BLR geometry is consistent with a
thick disk and the dynamics are a combination of elliptical orbits and inflow.
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Mar 12 |
Iair Arcavi |
Two Possible Types of Tidal Disruption Events and Their Fancy for E+A Galaxies
A Tidal Disruption Event (TDE) is what happens to a star which gets
too close to a supermassive black-hole and is consequently torn apart
by gravitational tidal forces. Optical (and possibly also UV and X-ray)
flares are thought to accompany such events, but the expected observational
spectroscopic signature is still a topic of debate. In 2012, a TDE
candidate published in Nature (Gezari et al), surprisingly showed no
hydrogen, but only helium, in its spectrum, contrary to previous
expectations. I will present the results of an archival search for TDEs
in the Palomar Transient Factory database, where we find three such
events - one similar to the Gezari et al. (He-rich) TDE, and two which
do display hydrogen. Also, we find that these events seem to show a
surprising preference for rare post-starburst (E+A) galaxies.
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Mar 19 |
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