Penn Astrophysics and Cosmology
Seminars
Academic Year 2009 – 2010
Fall 2009
DRL A6, Wednesdays at 2 PM
|
Date |
Speaker |
Host |
|
Sep 16 |
Peng Oh
( AGN Feedback Heating
in Galaxy Clusters Despite short central
cooling times, the gas in massive galaxy clusters shows no sign of developing
massive cooling flows. At the same time, Chandra observations have shown the
presence of large X-ray cavities, presumably injected by a central AGN or
radio galaxy. I discuss models of AGN feedback in galaxy clusters and how
they can alleviate the cooling flow problem. In particular, I discuss models
of cosmic-ray heating. I also discuss a model of the bimodal galaxy cluster
population, motivated by linear global stability calculations: cool core
clusters are stabilized primarily by AGN heating, while normal clusters are
stabilized primarily by conduction or mergers. AGN outbursts could
potentially regulate the transition between the two populations. |
Lidz |
|
Sep 23 |
J. Michael Shull ( Baryons and Metals
in the Low-Redshift Intergalactic Medium and Galactic Halo I will review the
current status of the “missing-baryons problem” in the low-redshift
universe. Models of Big Bang nucleosynthesis and acoustic peaks in
Cosmic Microwave Background each find that baryons make up 4.6 ±0.2% of the
critical (closure) density of the universe. However, fewer than 10% of
these baryons are found in galaxies. We find that most baryons reside
between the galaxies, in a highly structured, multi-phase intergalactic
medium (IGM). Ultraviolet spectrographs aboard the Hubble and FUSE satellites
detect half of the baryons in the “Cosmic Web”, a filamentary structure seen
as quasar absorption lines of diffuse neutral hydrogen (Lyman-alpha) and hot
ionized gas at 105 to 106 K, produced by large-scale
structure shocks and galactic winds. The Cosmic Origins Spectrograph
(COS) recently installed on the Hubble Space Telescope will further probe the
IGM content and evolution. COS key science projects include studies of
missing baryons, IGM heavy-element evolution and transport, galaxy halos, and
cosmology. We hope to study more than 10,000 filaments of the Cosmic
Web in Lyman-alpha and corresponding lines of elements such as C, N, O, Si,
and Fe. |
Aguirre |
|
Sep 30 |
Alessandra Silvestri
(MIT) Cosmological Tests
of General Relativity with Tomographic Surveys Future cosmological
surveys, combining galaxy counts and weak lensing measurements, will map the
evolution of matter perturbations and gravitational potentials from the
matter dominated epoch until today. In addition to tightening the constraints
on allowed expansion histories, the combination of these measurements will
test the relationships between matter overdensities, local curvature, and the
Newtonian potential. These relationships can be modified in alternative
theories of gravity and by exotic forms of Dark Energy. I will present a
study of the potential of upcoming and future tomographic surveys, such as
DES and LSST, with the aid of CMB and supernovae data, to detect departures
from the growth of perturbations expected within General Relativity with a
cosmological constant. |
Trodden |
|
Oct 7 |
Tyce DeYoung ( A Multimessenger View of the TeV Sky
The origins of the
cosmic rays remain uncertain nearly a century after their discovery. At
the TeV scale, alternative messenger particles such
as neutrinos and gamma rays must be used to study the sources of cosmic
rays. The IceCube observatory searches for
neutrinos from suspected cosmic ray accelerators such as supernova remnants,
active galactic nuclei, and gamma ray bursts, as well as from exotic sources
such as dark matter annihilations. We will discuss recent results from IceCube and describe the new Deep Core low energy
extension of IceCube. We will also discuss
efforts to correlate IceCube data with gamma ray,
X-ray, and other electromagnetic observations to produce a more complete
picture of the high energy universe. |
Devlin |
|
Oct 21 |
Fiona Hoyle ( Much Voids are among the
largest features in the Universe, with typical size 15 h-1 Mpc in radius, and they have been known to exist for over
twenty five years. However, they are very empty; typical densities are less
than 10% of average. These two facts make them very difficult to observe and
thus it is only in recent years that their properties have been examined in
detail. I will give an overview of how we observe nothing, describe how voids
can be found and present the latest results on voids detected in the Sloan
Digital Sky Survey. |
Aguirre |
|
Oct 28 |
Adam Lidz ( Hydrogen and Helium Reionization A key period in our
story of structure formation is the Epoch of Reionization
(EoR), when early populations of galaxies and/or
quasars formed, emitted ultraviolet light and ionized 'bubbles' of gas around
them, eventually filling the entire volume of the intergalactic medium (IGM)
with ionized gas. Reionization studies aim to
determine the filling factor and size distribution of ionized bubbles during
the EoR, which in turn constrain the properties of
the first luminous sources. Current observations suggest that hydrogen
is reionized sometime before z>~6 by
star-forming galaxies. These sources should simultaneously singly ionize
helium, but are unlikely to also doubly ionize it. Helium may be
doubly-ionized only later on, perhaps near z~3, by bright quasars. I
will describe efforts to theoretically model the Epochs of Hydrogen and
Helium Reionization, and focus on some of their
observational implications. First, I will forecast the prospects for learning
about hydrogen reionization from upcoming 21 cm
observations. I will then discuss an analysis of existing HI Ly-α
forest data aimed at identifying signatures of helium reionization
near z~3. |
|
|
Nov 4 |
Jaiyul Yoo ( A New Perspective on
Galaxy Clustering as a Cosmological Probe: General Relativistic Effects We present a general
relativistic description of galaxy clustering in a FLRW universe. The
observed redshift and position of galaxies are affected by the matter
fluctuations and the gravity waves between the source galaxies and the
observer, and the volume element constructed by using the observables differs
from the physical volume occupied by the observed galaxies. Therefore, the
observed galaxy fluctuation field contains additional contributions arising
from the distortion in observable quantities and these include tensor
contributions as well as numerous scalar contributions. We generalize the
linear bias approximation to relate the observed galaxy fluctuation field to
the underlying matter distribution in a gauge-invariant way. Our full
formalism is essential for the consistency of theoretical predictions.
As our first application, we compute the angular auto correlation of
large-scale structure and its cross correlation with CMB temperature
anisotropies. We comment on the possibility of detecting primordial gravity
waves using galaxy clustering and discuss further applications of our
formalism. |
Lidz |
|
Nov 11 |
Britt Reichborn-Kjennerud ( The E and B
Experiment (EBEX): Probing the History of the Universe by Measuring CMB
Polarization Anisotropies The E and B Experiment (EBEX)
is a NASA-funded balloon-borne microwave telescope designed to measure the
polarization of the cosmic microwave background. EBEX will probe the
first fraction of a second of the universe by either detecting or placing a
tighter constraint on the primordial B-mode signal resulting from
gravitational waves predicted by inflation. EBEX is also sensitive to
the lensing B-mode signal that is expected to be generated at late times due
to the shearing of primordial E-mode polarization by foreground matter.
EBEX's sensitivity to a wide range of scales and
frequencies will also allow it to make unprecedented measurements of galactic
polarized dust. I will provide an overview of the EBEX science and
instrument and a preliminary report from the EBEX North American flight in
June 2009. |
Sako |
|
Nov 18 |
Brice Ménard
(CITA) Cosmic Dust
After summarizing the
existing constraints on the opacity of the Universe, I will present the
detection of intergalactic dust obtained with the SDSS and discuss its
potential impact on the study of type Ia
Supernovae. The detection of dust is based on correlating the colors of
distant quasars with the density of foreground matter. It allows us to trace
the spatial distribution of the dust from 20 kpc to
several Mpc around galaxies. Its projected density
appears to follow that of the mass (obtained from magnification measurements)
but lower by 5 orders of magnitude. We quantify the amount of dust in
galactic halos, the wavelength dependence of its extinction and its
contribution to the overall opacity of the Universe. I will finally show how
this cosmic dust component can affect the estimation of cosmological
parameters from type Ia supernovae. |
Sheth |
|
Dec 1 |
SPECIAL SEMINAR Daniel Grin (Caltech) Cosmological hydrogen
recombination: the effect of high-n states and electric quadrupole transitions |
|
Spring 2010
DRL A6, Wednesdays at 2 PM,
except
for the dates of department colloquia
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Date |
Speaker |
Host |
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Jan 20 |
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Jan 27 |
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Feb 3 |
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Feb 17 |
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Feb 24 |
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Mar 3 |
Aubra Anthony ( SNO |
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Mar 24 |
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Mar 31 |
Christopher Reynolds ( |
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Apr 7 |
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Apr 21 |
Jason Glenn ( Herschel First
Science |
Bernstein |