Jan 7 |
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Jan 14 |
Matt McQuinn |
Understanding Cosmological Perturbation Theory
This is not a talk about precision cosmology, but rather about different
attempts to understand how nonlinear structure developments using perturbation
theory. Linear order perturbation theory has been fantastically successful at
describing the CMB and large-scale structure smoothed on >> 10Mpc scales, but
the standard methods to go beyond linear order have not met much success.
I will explain why this is the case, and how to more rigorously formulate
perturbation theory, specializing mostly to the simplified case of 1D dynamics. |
Jan 21 |
Xuening Bai |
Protoplanetary Disks and Planet Formation: A Microphysical Perspective
Planet formation is intimately connected to the gas dynamics in protoplanetary disks (PPDs), where a central role is played by magnetic fields. Due to extremely weak level of ionization, PPDs suffer from strong non-ideal magnetohydrodynamic (MHD) effects including Ohmic resistivity, the Hall effect and ambipolar diffusion, and they are the key to understanding the level of disk turbulence, angular momentum transport, and the overall disk structure and evolution. Via local shearing-box simulations that self-consistently include all non-ideal MHD effects, we show that in the inner region of PPDs (<10 AU), the magneto-rotational instability (MRI) is suppressed, and disk accretion is mainly driven by magnetocentrifugal wind. The gas dynamics also strongly depend on the polarity of the external magnetic field threading the disk as a result of the Hall effect. In addition, we predict that PPD wind is heavily loaded, with wind mass loss rate a substantial fraction of the disk accretion rate. In the outer region of PPDs (>15 AU), the MRI operates in the surface layer due to far-UV ionization, and is damped near the midplane due to ambipolar diffusion. In addition, external magnetic flux strongly concentrates into thin, axisymmetric shells, leading to enhanced radial pressure variations known as zonal flows. Implications on core accretion and planet-disk interaction will be discussed. Our simulation results provide key ingredients for a new paradigm on PPD gas dynamics, and shed new lights on the theory of planet formation. |
Jan 23 |
Tucker Jones - Special Friday Astro Lunch Seminar |
The heavy element abundance ("metallicity") of a galaxy is regulated by star formation, gas accretion, and galactic-scale outflows. Metallicity is therefore a powerful diagnostic of galaxy formation history. Significant observational efforts have been invested in measuring metallicity evolution out to redshifts z=3 with intriguing results. I will highlight discrepancies in current large data sets, and argue that we desperately need a better understanding of the physical conditions in high redshift galaxies. I will describe my work toward this goal using accurate measurements of physical properties at z=1, and discuss implications for metallicity evolution studies. In the later part of this talk I will discuss measurements and broader implications of the gaseous outflows which regulate metallicity.
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Jan 28 |
Gwen Rudie |
Gas and Galaxies at High Redshift: Intergalactic, Circumgalactic, and Interstellar Matter
In this seminar, I will discuss three projects that have focused on
early galaxy formation and its effect on the surrounding gaseous environment.
First, I will discuss how detailed observations of the 2 < z < 3
intergalactic medium can provide important clues about how the earliest
galaxies may have reionized the Universe at z > 6. Next, I will discuss
a first look at observations of ionized metal species in the circumgalactic
medium surrounding z~2.3 star-forming galaxies and explain how we can measure
metallicities, enrichment patterns, and gas temperatures as a function of
distance and velocity from galaxies using the Keck Baryonic Structure Survey
(KBSS). This study will provide a high-fidelity probe of the nature and
sphere of influence of galaxy-scale outflows at high redshift and will
constrain the properties of gas inflows. Finally, I will show new
observations of the rest-frame optical spectrum of star-forming galaxies
during the peak of cosmic star formation. I will discuss first results from
KBSS-MOSFIRE, a rest-frame optical spectroscopic survey of more than 800
galaxies in the same QSO fields. These data provide new insight into the
physical properties of star-forming regions at high redshift, which show
remarkable differences in their ionization and excitation conditions
compared to low-redshift star-forming regions. These results have
significant implications for both diagnostics of the chemical abundances
of high-z galaxies as well as our understanding of massive stars during
the peak of cosmic star formation.
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Feb 4 |
Björn Benneke |
Probing the Formation of Giant Planets Through
Measurements of Carbon-to-Oxygen Ratios in Hot Jupiters
Spectroscopic observations of hot Jupiters - gas giant exoplanets orbiting
within 0.1 AU from their host stars - offer an invaluable window into planet
formation and migration. Thanks to their high atmospheric temperatures, virtually
all carbon- and oxygen-bearing molecules are in gaseous form and accessible to
infrared remote sensing. This makes hot Jupiters excellent targets to probe the
carbon-to-oxygen ratios (C/O) in giant planets - measurements that are currently
not even available for the Solar System giant planets, but which are critical to
trace their origins and to distinguish between competing planet formation theories.
In this talk, I will present the main conclusions from a comprehensive study
of hot Jupiter transmission spectra using the novel self-consistent atmospheric
retrieval framework, SCARLET. The main finding is that the C/O ratios of at
least four hot Jupiters are robustly below 0.9, favoring a formation within the
CO2 ice line, i.e. within ~10 AU from sun-like stars. Finally, I will present a
short overview of my ongoing multi-semester Keck/NIRSPEC program (6 nights
allocated to date) to acquire the first well-constrained measurements of C/O
ratios in hot Jupiters. This program will bring the field of giant exoplanet
atmospheres from mostly confirming the presence of "to-be-expected" molecular
species to quantitatively inferring the elemental abundances in giant planets.
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Feb 11 |
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Feb 18 |
George Becker |
A Consensus Picture of Reionization?
When and how the intergalactic medium (IGM) became reionized carries
fundamental implications for the formation of the first stars and galaxies.
New results from the Planck satellite now suggest that the bulk of
reionization occurred somewhat later than previously thought, potentially
easing tensions with observed galaxy populations at high redshifts. A wide
range of reionization histories are still allowed, however, and it is
unclear whether the simplest models truly match observations. I will
present new constraints on the ionizing output from galaxies and the
timing of reionization based on quasar absorption line studies of the
IGM over 2 < z < 7. The results help to clarify how and when
reionization ended, but also pose significant challenges to current models.
I will describe the next steps forward, and the opportunities for IGM
science with upcoming facilities.
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Feb 25 |
Drew Newman |
Tracing the Growth of Dead Galaxies
Observations show that star formation has already begun to shut down
in many massive galaxies at z > 2. After these galaxies are "dead" in
terms of new star formation, they nonetheless continue to grow in mass
and especially size: their average size, at a given stellar mass, has
increased by a remarkable factor of 5 over the last 11 Gyr. I will present
results from a program aimed at tracking the growth of dead galaxies over
z=0.4-2.5 and testing its physical drivers. This work is based on
observations from both HST/WFC3 and a Keck spectroscopic survey designed
to measure the stellar populations and internal kinematics of quiescent
galaxies at z > 1. With these data we have addressed the controversial
question of disentangling genuine galaxy growth from other concurrent
changes in the galaxy population. The main cause of this growth is widely
suspected to be the accretion of satellite galaxies, particularly lower
mass systems, but it is not certain whether such "minor" mergers are
actually frequent enough to account for the relatively rapid growth that
is observed. We have conducted a statistical study of the satellite systems
surrounding massive galaxies to z=2 designed to test the viability of this
mechanism. Finally, I will conclude with several ongoing and future
observations aimed at revealing the internal structure of compact galaxies
at z=2, their star-forming progenitors, and their evolution into today's
early-type galaxies.
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Mar 4 |
Sirio Belli |
The most effective probe of the
physical nature of quiescent galaxies is absorption line spectroscopy,
which is particularly challenging at high redshift. Using the improved
sensitivity of optical and infrared detectors at the Keck observatory,
and the multiplex advantage of its new MOSFIRE spectrograph, we have
undertaken a new spectroscopic survey in the redshift range 1 < z < 2.5.
Velocity dispersions and stellar ages derived from our spectra, together
with HST-based sizes, provide valuable insight into the mass assembly of
quiescent galaxies. We find that the stellar to dynamical mass ratio
evolves with redshift, which might imply a change in the dark matter
fraction or in the stellar initial mass function. Our main conclusion
is that the population of compact quiescent galaxies at high redshift
grows in size partly via minor mergers (physical growth) and partly because
of the increasing contribution of recently quenched, larger galaxies
(progenitor bias). Finally, by fitting stellar population models to the
spectroscopic and photometric data, we are able to robustly constrain,
for the first time, the mass assembly and star formation histories of z > 2
quiescent systems.
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Mar 11 |
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Mar 18 |
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