List of Past Astronomy Colloquia : 01-Jan-2010 to 01-Jun-2010

Date:   Wednesday 27-Jan-2010
Speaker:   Dr. David Latham (Harvard-Smithsonian)
Title:  Super-Earths and Life

Transiting planets are special. The amount of light blocked by the planet as it passes in front of its host star sets the size of the planet (relative to the star). If an orbit can be derived from Doppler spectroscopy of the host star, the light curve also provides the orientation of the orbit, leading to the mass of the planet (again relative to the star). The resulting density for the planet can be used to constrain models for its structure and bulk properties. We are on the verge of using these techniques to characterize super-Earths, planets in the range 1 to 10 Earth masses that may prove to be rocky or water worlds. An exciting example is the recent detection by the ground-based MEarth project of a probable water world orbiting Gliese 1214. Space missions such as Kepler and TESS promise to play key roles in the discovery and characterization of super-Earths.

Transiting planets also provide remarkable opportunities for spectroscopy of planetary atmospheres: transmission spectra during transit events and thermal emission throughout the orbit, calibrated during secondary eclipse. Spectroscopy of super-Earths will not be easy, but is not out of the question for the James Webb Space Telescope. Our long-range vision is to attack big questions, such as "Does the diversity of planetary environments map onto a diversity of biochemistries, or is there only one chemistry for life?" A giant first step would be to study the diversity of global geochemistries on super-Earths and Earth analogs.


Additional reading:

Charbonneau, D., et al. Nature 462, 891, 2009

Date:   Wednesday 03-Feb-2010
Speaker:   Dr. Eli Dwek (University of Maryland)

The detection of large amount of dust (M ~ 1e8 Msun) in SDSS J1148+5251, a galaxy at z=6.4, when the universe was less than ~ 1 Gyr old, provides astronomers with a unique opportunity to study the origin and evolution of dust in this object. In the local universe, supernovae and AGB stars are the main sources of newly formed dust. However, in the early universe, most AGB stars did not have time to evolve off the main sequence, limiting their role as important dust sources. Can supernovae alone account for the massive amount of dust observed in these galaxies, do AGB stars have to play a role as well, or do we need to resort to other more exotic sources to explain the presence of the observed dust in this galaxy? This talk will discuss these problems and examine possible solutions.

Date:   Wednesday 10-Feb-2010

Date:   Wednesday 17-Feb-2010
Speaker:   Dr. Fred Adams (University of Michigan)
Title:  Effects of Young Star Clusters on their Constituent Solar Systems

Most stars -- and hence most solar systems -- form within groups and clusters. This talk explores how these star forming environments affect the solar systems forming within them. The discussion starts with the dynamical evolution of young clusters with N = 100 - 1000 members. We use N-body simulations to study how evolution depends on system size and initial conditions. Multiple realizations of equivalent cases are used to build up a robust statistical description of these systems, e.g., distributions of closest approaches and radial locations. These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models then determine the efficacy of radiation in removing gas from the systems and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine the probability of solar system disruption. The main result of this work is thus a quantitative determination of the effects of clusters on forming solar systems. Along the way, we set constraints on the possible birth environments for our solar system. By studying orbits in these extended mass distributions, we have also discovered a robust orbit instability that operates when the potential is sufficiently triaxial. This instability not only affects young star clusters, but also arises in the dynamics dark matter halos.

Date:   Wednesday 24-Feb-2010
Speaker:   Dr. Britt Lundgren (Yale University)
Title:  " Revealing the Galaxies Traced by 77,000 SDSS Quasar Sightlines"

Owing to the advent of large spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS), tens of thousands of quasar absorption line systems have been identified to date. Absorber catalogs now extend to z ~ 4.6 and provide unique probes of the large scale structure of baryonic matter in the high-redshift universe; yet many critical questions regarding the origins and environments of these absorbers still remain. Metal absorption along the line of sight to quasars is believed to probe a wide variety of physical processes, including relativistic quasar outflows, star-forming regions in foreground galactic disks, and virialized gas in extended galactic haloes. Although progress has been made in observationally differentiating the physical phenomena producing absorption in individual systems, high-resolution spectroscopy is generally required for such work. Only recently has the vast dataset of the SDSS provided the means to disentangle some observational degeneracies in quasar absorption line studies statistically, with its large samples of medium resolution data.

I will present new results using a multi-ion absorber catalog extracted from the SDSS, which include: measurements of the large-scale clustering, and thereby the typical dark matter halo masses of intervening Mg II absorbers, and cross-correlations of SDSS galaxies with Ca II absorbers, the low-redshift analogues of strong Mg II absorption in optical spectra. Together, these analyses provide compelling evidence that the strongest Mg II absorbers trace actively star-forming galaxies to at least z~1.3.

Date:   Wednesday 03-March-2010
Speaker:   Dr. Jason Wright (Pennsylvania State University)
Title:  "Exoplanets Abound: "

I will review the latest statistics of the orbital elements of the nearly 350 giant planets detected by radial velocity surveys around normal stars. The distributions of orbital elements of these planets show several curious features including: the familiar "3-day pileup" among lower-mass singleton giant exoplanets; a "1 AU jump" among singleton super-Jupiters; indistinguishable eccentricity distributions between singleton planets and those in multiplanet systems; a flat log-orbital-distance distribution among planets in multiple systems; and a lack of close-in planets orbiting subgiant and giant stars. Since nearly all of the detected giant exoplanets have experienced significant migration, these features form important clues to the nature of migration,and ultimately provide tests for successful theories of planetary migration and dynamical evolution.

Date:   Wednesday 10-Mar-2010
Speaker:   Dr. Harald Krueger (Max Planck Institut)
Title:  Jupiter's Dust Disk: An Astrophysical Laboratory

Spacecraft investigations during the last 15 years have vastly improved our knowledge about dust around Jupiter. In-situ measurements with the dust detectors on board the Ulysses and Galileo spacecraft have detected, for the first time, i) the electromagnetic escape of tiny dust grains from Jupiter, ii) the production of impact ejecta from large moons, and iii) previously unseen structures in Jupiter's ring system.

i) The majority of the escaping 10 nanometer dust grains condense in the volcanic plumes of Jupiter's moon Io. They collect a positive electric charge in the Io plasma torus and are accelerated away from Jupiter by electromagnetic forces in bursts or streams that are then detected by the spacecraft.

ii) All of the Galilean moons are surrounded by tenuous clouds of mostly sub-micrometer ejecta grains generated by collisions of interplanetary micrometeoroids with the moons' surfaces. A tiny fraction of the ejecta gets sufficient energy to leave the moons' gravity and is dispersed in circum-jovian space. Very tenuous rings have been detected in-situ in the region between the Galilean moons and further beyond out to ~ 250 RJ from the planet (Jupiter radius RJ = 71492 km), showing that impact-ejecta derived from hypervelocity impacts onto moons are a major constituent of dusty planetary rings.

iii) Galileo dust measurements in Jupiter's gossamer ring have for the first time provided in-situ measurements in a dust ring also accessible with imaging techniques. They allow the first actual comparison of in-situ data with the ring structure and grain size distributions inferred from inverting optical images. Our results show that electromagnetic effects caused by variable dust charging on the day and night side of Jupiter are crucial in shaping the dusty jovian ring.

The Galileo dust investigations showed that the jovian system is a natural laboratory to study fundamental 'dusty' processes that are of primary importance in many astrophysical contexts far beyond our own solar system.

Date:   Wednesday 17-Mar-2010

Date:   Wednesday 24-Mar-2010
Speaker:   Dr. Elena Gallo (MIT Kavli Institute for Astrophysics & Space Research)
Title:  Black hole Accretion in the Nearby Universe: Evidence for Down-Sizing

An issue of fundamental importance in understanding the galaxy-black hole connection is the duty cycle of accretion. If black holes are indeed ubiquitous in galactic nuclei, little is known about the frequency and intensity of their activity, the more so at the low-mass/ low-luminosity end. I will present new results from AMUSE-Virgo, a Chandra survey of (formally) inactive early type galaxies in the Virgo cluster. Out of 100 objects, 32 show a nuclear X-ray source, including 6 hybrid nuclei which also host a massive nuclear cluster as visible from archival HST images. After carefully accounting for contamination from nuclear low mass X-ray binaries based on the shape and normalization of their X-ray luminosity function, we conclude that between 24-34% of the galaxies in our sample host a X-ray active super- massive black hole. This sets a firm lower limit to the black hole occupation fraction in nearby bulges within a cluster environment. At face value, the active fraction is found to increase with host stellar mass. However, taking into account selection effects, we find that the average Eddington-scaled X-ray luminosity scales with black hole mass to the power -0.62, with an intrinsic scatter of 0.46 dex. This finding can be interpreted as observational evidence for `down-sizing' of black hole accretion in local early types, that is, low mass black holes shine relatively closer to their Eddington limit than higher mass objects. As a consequence, the fraction of active galaxies, defined as those above a fixed X-ray Eddington ratio, decreases with increasing black hole mass.

Date:   Wednesday 31-Mar-2010
Speaker:   Dr. Jennifer Donley (Space Telescope Science Institute)
Title:  'Tracking down the Missing Population of Obscured AGN'.

While deep X-ray surveys provide the most efficient means of selecting AGN, they are substantially incomplete to the heavily obscured AGN predicted to dominate the active galaxy population both locally and in the distant universe. Fortunately, however, the high-energy emission absorbed by the dust and gas surrounding an AGN's central engine is re-emitted in the IR. Many methods have therefore been developed to use Spitzer to supplement the X-ray results. I will discuss my contributions to this field (e.g., the selection of radio-excess and power-law AGN), and will then critically review the full IR-selected AGN population. Finally, I will discuss new opportunities to track down the remainder of this missing population of AGN.

Date:   Wednesday 07-Apr-2010
Speaker:   Dr. Michael Shull (University of Colorado)
Title:  Missing Baryons: Searching between the Galaxies

I will review the current status of the "missing-baryons problem" in the low-redshift universe, and new results from Hubble/COS, the Cosmic Origins Spectrograph. 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 10^5 to 10^6 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.

Date:   Wednesday 14-Apr-2010
Speaker:   Dr. Jarita Holbrook (University of Arizona)
Title:  Hubble's Diverse Universe

A film focusing on the lives of nine African American and Hispanic American astronomers and astrophysicists. Questions and answers will follow with executive producer and interviewer Jarita C. Holbrook.

Date:   Wednesday 21-Apr-2010
Speaker:   Dr. John Hughes (Rutgers University)
Title:  "First Results from the Atacama Cosmology Telescope: SZ Detections of Galaxy Clusters"

The Atacama Cosmology Telescope (ACT) is a 6-m diameter telescope with a sensitive mm-wave band camera custom designed to survey the cosmic microwave background (CMB) on scales of 1.5 arcmin. The camera observes simultaneously in three bands at frequencies of 145 GHz, 220 GHz, and 280 GHz. One of the goals of the project is to find galaxy clusters through the Sunyaev-Zeldovich effect, which arises when hot electrons in the cluster inverse Compton scatter cold CMB photons. The SZ effect is manifest as a decrement in ACT's low frequency channel, an increment in its high frequency channel and a null in the middle channel. In this seminar I will present results on clusters detected by ACT and discuss our on-going multiwavelength follow-up studies of the ACT cluster sample.

Date:   Wednesday 28-Apr-2010
Speaker:   Dr. Randy Kimble (GSFC)
Title:  "Performance and Early Scientific Results of HST/Wide Field Camera 3"

Wide Field Camera 3 (WFC3) was installed into the Hubble Space Telescope during the highly successful Servicing Mission 4 in May, 2009. WFC3 offers sensitive, high resolution imaging over a broad wavelength range from the near UV through the visible to the near IR (200nm - 1700nm). Its capabilities in the near UV and near IR ends of that range represent particularly large advances vs. those of previous HST instruments. In this talk, I will review the purpose and design of the instrument, describe its performance in flight, and highlight some of the initial scientific results from WFC3, including its use in deep infrared surveys to detect galaxies at very high redshift, and in investigations of the global processes of star formation in nearby and medium-redshift galaxies.

Date:   Wednesday 05-May-2010
Speaker:   Dr. Tommaso Treu (University of California, Santa Barbara)
Title:  "New observational tests of the LCDM (with pesky baryons) paradigm"

I will presents new observational tests of the LCDM paradigm. In particular I will focus on the following questions: are there universal dark matter profiles? is there dark matter substructure? What is the equation of state of dark energy? I will discuss the results underlying the crucial need to uderstand the interplay between baryons and dark matter as part of the paradigm.

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