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|Thu, Oct 02|
University of Arizona
"Evolution of quasars at high redshift from large optical surveys"
I will summarize the current state of optical quasar surveys, which are rapidly approaching a million quasars with spectroscopic redshifts, and now extend to z~7. The improved statistics provided by recent surveys show that quasars sharply decline in number density at z>3, as known for decades, but more surprisingly that the characteristic luminosity (the break in the luminosity function) evolves to continually higher luminosities out to z~6. This steady shift to more extreme systems suggests that the most massive black holes formed early and grew rapidly. I will discuss some limitations of current studies of quasar demographics and how these can be addressed with simulations of quasar properties and with observations that break degeneracies in theoretical models of quasar activity.
|Thu, Oct 09|
"Comparing apples with apples: forward-modeling observables from galaxy formation simulations"
Typically, physical properties of galaxies, such as their stellar masses and star formation rates, are inferred from observations and then compared with predictions of theoretical models. However, I will argue that it is often preferable to instead compare observable quantities predicted from simulations with observations via 'forward-modeling' techniques. Such techniques can sometimes yield more-accurate model comparisons and novel insights. I will discuss one such forward-modeling method, performing dust radiative transfer on hydrodynamical simulations of galaxies to generate spatially resolved spectra of the synthetic galaxies. I will then present some examples of the many applications of this method. In particular, I will demonstrate how this approach yielded novel predictions for the high-redshift submillimeter galaxy population that have since been confirmed observationally.
|Thu, Oct 16|
UC Santa Cruz
"The Large Reservoirs of Gas Around Galaxies"
The Circumgalactic Medium (CGM) is where infalling gas that feeds star formation meets outflowing, feedback-enriched materials. It is where satellites are stripped and disrupted, and where gas ejected from galaxies may eventually be recycled. This medium is seen primarily in absorption, taking the form of diffuse, ionized gas bound to the dark matter halo of its host galaxy and extending to at least 300 kpc. In this talk I will present two observationally-motivated puzzles requiring theoretical investigation. First, I will review the evidence that the CGM of quenched galaxies contains as much cold gas as their star-forming counterparts. This observation implies that galaxies are transformed from star-forming disks to quiescent spheroids while retaining a significant store of cold gas in their halos. Then, I will show that the cool and warm phases of the CGM (T < 10^6 K) account for most of the baryons purported to be missing from dark matter halos of both star-forming and passive galaxies with M_halo ~ 10^12. Yet, surprisingly, the cool (10^4 K) gas in the CGM is far from pressure equilibrium with a hot medium that could provide hydrostatic support.
|Thu, Oct 23|
University of Arizona
"The Surprisingly Complex Lives of Massive Galaxies and the Stability of the Mass Fundamental Plane"
Once thought to be relics of a much earlier epoch, the most massive local galaxies are red and dead ellipticals, with little ongoing star formation or organized rotation. In the last decade, observations of their assumed progenitors have demonstrated that the evolutionary histories of massive galaxies have been far from static. Instead, billions of years ago, massive galaxies were more compact and morphologically different, possibly with more disk-like structures and many were still forming stars. The details of this observed evolution can place constraints on the physical processes that have driven massive galaxy evolution through cosmic time. I will discuss recent observational studies of the structural and dynamical properties of massive high-redshift galaxies. Specifically, I will demonstrate that in spite of their dramatic structural evolution, the mass fundamental plane, or the empirical relation between dynamics, sizes, stellar mass surface density of massive galaxies, has been in place since z~2. This relation appears to hold for massive galaxies of all types, not just red, dead ellipticals. Therefore, this scaling relation is an ideal tool to follow the evolution of galaxy populations as it is minimally susceptible to progenitor biases due to the evolving stellar populations, structures, and dynamics of galaxies through cosmic time.
|Thu, Oct 30|
"Science with orbital phase curves in the space age"
Advancements in the field of observational astronomy are usually limited by technological capabilities. In the current era technology has allowed for space-based astronomical surveys delivering a growing sample of time series photometry of high-precision and long time span. This high quality photometry has finally enabled the detailed study of variability along the orbital motion, or orbital phase, of stellar binaries and star-planet systems. These orbital modulations are induced by a combination of gravitational and atmospheric processes. Gravitational processes include the beaming effect (also known as Doppler boosting) and tidal ellipsoidal distortion, so the photometric light curve shape is sensitive to the companion's mass and orbit's shape. Atmospheric processes include reflection of light and thermal emission by the companion, making phase curves a tool to study the companion's atmosphere. I will present the science done with phase curves, including the mass measurement of companion's to hot early-type stars (where the mass cannot be measured using radial velocities), the search for non-eclipsing systems, and the study of the companions' atmosphere. Finally, I will briefly discuss the study of phase curves with current and future space missions including K2, TESS, and PLATO.
|Thu, Nov 06|
University of Washington
"Mapping the Galaxy with Large Surveys: From SDSS to PanSTARRS to LSST"
The distributions of stars (and their properties) in our Galaxy contain a treasure trove of information about the formation, evolution, and present day structure of the Milky Way. Large surveys such as the SDSS or PanSTARRS, allow us to mine this rich resource to unprecedented scale and accuracy by directly mapping the distributions of stellar number density, metallicity, and kinematics, as well as ISM, in a representative volume of the Galaxy. These maps enable us to measure the shape of the Galaxy, observe the relationships between dynamical and physical properties of its stars, and identify and characterize streams and satellites in the Galactic halo. All of these provide new and valuable insights on the process of assembly, as well as the present day state of the Milky Way. In this talk I will discuss the photometric Milky Way studies with the SDSS and more recently PanSTARRS. I will present recent results, touching on inferences about cosmology and the theory of galaxy formation in general. I will also discuss and illustrate challenges for conducting such studies with multi-terabyte survey datasets, and ways to surmount them. Discussion of the latter is especially timely, given the accelerating march towards petascale surveys such GAIA, LSST, and others in between.
|Thu, Nov 13|
University of Washington
"The HeII Lyman-alpha forest"
Cosmological hydrodynamic simulations successfully reproduce most statistics in the z~3 hydrogen Lyman-alpha forest. However, studies have shown that the hydrogen Lyman-alpha forest is rather insensitive to fluctuations in the ionizing background, in the gas temperature, in the ionization state of helium, and to galactic feedback processes. There is a second diagnostic of the low density z~3 IGM, a forest of singly ionized helium that appears in the far ultraviolet. In fact, many of the seminal papers on this HeII Lyman-alpha forest have been written by past and current UW department members. The HeII Lyman-alpha forest is great for studying fluctuations in the ionizing background, the hardness of this background, and helium reionization. Previous attempts to measure fluctuations in the ionizing background using the HeII forest found 1-2 dex fluctuations on 1-10 Mpc scales, which some have speculated indicate non-standard radiation sources/unknown radiative transfer effects. I will show that recent HST/COS data are in fact consistent with the expectation of an almost uniform 1 and 4 Rydberg background at z~2.5. This measurement also allows us to constrain the unknown contribution of stars and of quasars to the hydrogen ionizing background at z=2.5, and to constrain quasar lifetimes on very interesting ~10 Myr timescales.
|Thu, Nov 20|
"The main sequence of quasars and the (non)evolution of the M-sigma relation"
Broad-line quasars display a remarkable regularity in their diverse multi-wavelength properties, which follow a well-defined physical sequence called "Eigenvector 1" discovered by Boroson and Green 20 years ago. I will present some latest observations that demonstrate that the diversity and regularity of quasar phenomena can be unified by Eddington ratio and orientation. In the second half of my talk, I will introduce the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project, a multi-object RM program with the SDSS-BOSS spectrograph. I will present some early science results from SDSS-RM, focusing on the measurements of stellar velocity dispersion in quasar hosts at z>0.3, and the (non?)evolution of the M-sigma relation to z~1.
|Thu, Dec 04|
University of Colorado
"Pushing forward the frontiers of exoplanet characterization"
In recent years, ground-based high resolution spectroscopy has become a valuable tool to study the atmospheres of transiting and non-transiting exoplanets. The key aspect of this technique is the ability of resolve molecular bands into individual lines. This in turns enables reliable molecular detections via line matching, and the measurement of the planet orbital motion for planets orbiting close to their parent star (hot Jupiters). This technique has been successful not only in detecting molecules, but also in measuring their relative abundances. Furthermore, it allowed to solve for the mass and orbital inclination of non-transiting planets. High-resolution spectroscopy also promises to unveil the atmospheric dynamics of giant exoplanets, and ultimately to detect biomarkers in terrestrial planets with the next generation of ground-based telescopes. Finally, by combining the complementary information of high- and low-resolution spectroscopy, it will be possible to constrain the properties of exoplanet atmospheres to an unprecedented detail. This will initiate a stage of comparative planetology in exoplanet research, which is required to fully understand planet formation and evolution.