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 (DRL
4E19 Astronomy Seminar Room) Daniel Grin (Caltech) Cosmological
hydrogen recombination: the effect of high-n states and electric quadrupole transitions Thanks to the ongoing
Planck mission, a new window will be opened on the properties of the
primordial density field, the cosmological parameters, and the physics of
reionization. Much of Planck's new leverage on these quantities will come
from temperature measurements at small angular scales and from polarization
measurements. These both depend on the details of cosmological hydrogen
recombination; use of the CMB as a probe of energies greater than 1016
GeV compels us to get the ~eV
scale atomic physics right. One question that remains is how high in hydrogen
principal quantum number we have to go to make sufficiently accurate
predictions for Planck. Using sparse matrix methods to beat computational
difficulties, I have modeled the influence of very high (up to and including n=300)
excitation states of atomic hydrogen on the recombination history of the
primordial plasma, resolving all angular momentum sub-states separately and
including, for the first time, the effect of hydrogen quadrupole
transitions. I will review the basic physics, explain the resulting plasma
properties, discuss recombination histories, and close by discussing the
effects on CMB observables. |
|
Spring 2010
DRL A6, Wednesdays at 2 PM,
except for the dates of department colloquia
|
Date |
Speaker |
Host |
|
Jan 20 |
Kevork Abazajian ( Indirect Signals of
the Nature of Dark Matter Several observations have
drawn considerable interest as potential indications for indirect signatures
of the nature of dark matter. Radio synchrotron towards the galactic
center, the “WMAP haze”, high-energy cosmic ray electron/positron
observations, as well as the behavior of dark matter in small scale
structure, are potential signals for the fundamental properties of the dark
matter. I will discuss how current and future observations will
test the dark matter interpretation of these signals. |
Trodden |
|
Jan 27 |
Phil Korngut ( Understanding Galaxy
Cluster Dynamics through high resolution SZ measurements with MUSTANG and the
GBT Galaxy clusters are the
most massive virialized objects in the universe and, as such, are extensively
used as cosmological probes. The clusters are usually assumed to be relaxed,
spherical, and isothermal to simplify the analyses. I will present new
observations of the Sunyaev Zel’dovich
Effect (SZE) in RXJ1347-1145 made at 90GHz (3.3mm) with the MUSTANG receiver
on the 100m Green Bank Telescope (GBT). These data have an angular
resolution of 10 seconds of arc, making the resulting image the most resolved
map of the SZE made to date. This cutting edge new dataset has
confirmed a previously reported strong, localized enhancement of the SZE 20”
to the South-East of the center of X-ray emission. This enhancement of the
SZE has been interpreted as hot (> 20 keV) gas caused by a recent, violent
merger event and is one of only three confirmed shocks yet detected in the
intra-cluster medium of galaxy clusters. I will also show new and
developing work on the high redshift massive cluster CL1226+3332. |
|
|
Feb 3 |
Vincent Desjacques
(Institute for Theoretical Physics, University of Zürich) Halo clustering and primordial
non-Gaussianity In CDM cosmologies,
galaxies reside inside virialized halos of dark matter. Understanding halo
clustering is thus a crucial step towards an accurate description of the
spatial distribution of galaxies required to extract maximum information from
observational data. I will show that many insights can be gained from a study
of density peaks, particularly with regards to the scale dependence of the
bias in scenarios with Gaussian and non-Gaussian initial conditions. |
Sheth |
|
Feb 17 |
David Weinberg ( The Sloan Digital
Sky Survey: I, II, III I will describe some of
the scientific highlights from the Sloan Digital Sky Survey (SDSS),
concentrating on those connected to cosmology and galaxy formation. The
SDSS is the most ambitious survey of the universe ever undertaken. To
date, it has imaged 11,500 square degrees and detected more than 350 million
objects, measured the spectra of 930,000 galaxies, 120,000 quasars, and
460,000 stars to create the largest ever 3-dimensional maps of cosmic
structure, and discovered more than 500 |
Jain |
|
Feb 24 |
Jose Diego (Instituto de Fisica
de Cantabria) Sunyaev-Zel'dovich
Effect in WMAP data Using WMAP 5 yr data we stack the area around the position of 700
known X-ray clusters. The stacking procedure reduces drastically the CMB and instrumental
noise and allows to “see” the average SZ signal from
these clusters. Comparing the observed SZ profile with a variety of models,
both in the WMAP data and the ROSAT data (X-rays), we are able to constrain
the models that simultaneously fit the average SZ profile in WMAP and the
average X-ray ROSAT profile. Among our results we find that beta-models are
hard to reconcile with the data while more realistic (and steeper) profiles
agree much better with the SZ and X-ray data. We also study the possible
contamination from unresolved point sources in the cluster and conclude that
a significant contamination from radio and infrared sources exist (on
average) in the core of galaxy clusters. Our results are relevant for
missions such as Planck, ACT or SPT since they predict a smaller number of SZ
detections than previously thought. This is mostly due to the different
(steeper) gas profile that reduces the SZ signal by a factor about two when
compared with the beta-model profile.
|
Sheth |
|
Mar 3 |
Aubra Anthony ( Studying the Sun
with SNO: Searching for high-frequency variations in the solar neutrino flux Recent helioseismology results have pointed
to the possible detection of high-frequency (periods of minutes to days)
gravity-mode oscillation signals in the Sun.
Periodic fluctuations in density, pressure and temperature (as would
be caused by g-modes at the solar core) could potentially modulate the
outgoing flux of solar neutrinos, through the close relationship between
temperature and neutrino production.
Density fluctuations could also affect the propagation of neutrinos
through the sun, through the MSW effect, because periodically-shifting matter
densities could temporally vary the probability for neutrino oscillations to
occur. The Sudbury Neutrino
Observatory was an optimal laboratory for studying time dependence in the
solar neutrino flux, due to excellent background elimination and real-time
signal detection. I will discuss the
searches that we performed with SNO neutrino data to identify any
high-frequency periodic signal in the sun, both on broad time scales, as well
as those specifically relevant to recent g-mode detection claims. |
Klein |
|
Mar 24 |
Tamas Budavari ( Robust Statistical Techniques for Astronomical
Images and Spectra Dedicated surveys
provide unprecedented amounts of data. While these observations open up new
opportunities for scientific measurements and discoveries, they often also
render our favorite analysis tools useless. Principal Component Analysis is
frequently applied to astronomical spectra and images. A statistically robust
and incremental method is introduced to overcome the scalability problems.
Its performance is illustrated on the 4000 Ĺ region of VVDS
spectra. With the upcoming photometric telescopes, we face a serious
challenge with the co-addition of images. I will discuss an ongoing project
that addresses the issues using Richardson-Lucy-like deconvolution updates
enhanced by techniques of robust statistics. |
Sheth |
|
Mar 31 |
Christopher Reynolds ( The physics of black hole accretion For almost the past 20 years,
the paradigm for black hole accretion has highlighted the central role of
magnetohydrodynamic (MHD) turbulence.
However, only in recent years have high-resolution simulations started
to explore the subtle nature of MHD turbulent disks. I will discuss results from a series of
simulations focusing on geometrically-thin accretion disks. I shall focus on the dynamics of the disk
and the transition to the plunging flow close to the black hole, and explain
why the low density “corona” of the disk plays a previously unrecognized and
important role in shaping the dynamics of the disk. I shall also present a study of the
temporal properties of the accretion flow.
I shall end by connecting this theoretical work with a new XMM-Newton
observation of the Seyfert galaxy 1H0707-495. |
Jain |
|
Apr 7 |
Steven Furlanetto ( Cosmology at Low Radio Frequencies One of the last
frontiers of cosmology is the "dark age" during which the first
galaxies formed and lit up the Universe, roughly 200 million years after the
Big Bang. Existing telescopes provide
tantalizing clues about the complexities of this era, but a variety of
observational challenges stand in the way of a complete picture. I will summarize our current understanding
of this epoch as well as describe new techniques to probe the dark ages,
focusing on a suite of low-frequency radio telescopes that hope to unlock the
astrophysics of the first galaxies and black holes. |
Aguirre |
|
Apr 21 |
Laura Newburgh ( CMB
Polarization with the Q/U Imaging ExperimenT
(QUIET) Abstract: The Q/U Imaging Experiment (QUIET) is a ground-based CMB
polarization experiment observing angular scales where the polarization spectrum
from inflationary gravity waves is predicted to be maximal. QUIET operates at
two frequency bands centered at 40 GHz and 90 GHz. The 40 GHz receiver was
deployed in 2008 at 5100m altitude in the Atacama Desert, |
Aguirre |