Talks & Events
Astronomy Colloquia: 2009
Braneworld Black Holes
Braneworlds are a fascinating way of hiding extra dimensions by confining ourselves to live on a brane. One particular model (Randall-Sundrum) has a link to string theory via living in anti de Sitter space. I'll describe how the ads/cft correspondence has been used to claim that a braneworld black hole would tell us how Hawking radiation back reacts on spacetime, thus solving one of the outstanding problems of quantum gravity - the ultimate fate of an evaporating black hole. I'll review evidence for this
conjecture, ending with some recent work that shows it may be problematic.
Expectations, and Probes of Intergalactic Magnetic Fields
It is likely that intergalactic magnetic fields are naturally seeded by (1) “normal” stellar processes in galaxies, and (2) by central galactic black holes. There may also be (3) a primordial component from the pre-galactic Universe, but this is probably overwhelmed by the first two processes except possibly in cosmic voids. I discuss some methods for probing intergalactic fields, and some recent results. Some are new and tentative, and serve to focus on what better data of the same kinds could be obtained with present instruments. I present a brief overview of what we know about galactic and extragalactic magnetic fields from the local universe from “here” up to z ~ 3. This includes some global a priori calculations of IGM field strengths based on known facts of galactic magnetic energy outflows. I briefly include ideas on of where some distributed UHECR acceleration sites may be found.
Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs
Suggestive evidence has accumulated that intermediate mass black holes (IMBH) exist in dwarf galactic nuclei and some globular clusters. As stars diffuse in the cluster, some will inevitable wander sufficiently close to the hole that they suffer tidal disruption. An attractive feature of the IMBH hypothesis is its potential to disrupt not only solar-type stars but also compact white dwarf stars. Attention is given to the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such extent that explosive nuclear burning may be triggered. Consistent modeling of the gas dynamics together with the nuclear reactions allows for a realistic determination of the explosive energy release. Although the explosion will increase the mass fraction escaping on hyperbolic orbits, a good fraction of the debris remains to be swallowed by the hole, causing a bright soft X-ray flare lasting for about a year. Such transient signatures, if detected, would be a compelling testimony for the presence of a moderately mass black hole.
Ice Fishing for Cosmic Neutrinos
Cosmic rays at GZK energies produce neutrinos as a consequence of their interactions with the cosmic microwave background radiation. When these neutrinos interact in the Antarctic ice sheet, the resulting showers produce coherent Cherenkov radiation at radio frequencies via the Askaryan mechanism. A balloon instrument, the Antarctic Impulsive transient Antenna (ANITA), has flown over Antarctica twice, most recently during the past austral summer. Results from the first flight and information about the second flight will be presented.
A Dustier Universe Than You'd Like
Using galaxies and quasars from the Sloan Digital Sky Survey, we have extended previous work measuring the weak lensing magnification. We can now compare our measurements directly to those made using galaxy-galaxy shear lensing and find that the results agree remarkably well. Our new technique also allows us to make the first measurements of the extended dust halos associated with the galaxies in our sample, detecting a level of intergalactic dust roughly twice that expected from theoretical estimates. At a mean redshift of z ~ 0.35, we find that the dust halos of galaxies extend to Mpc scales, following a power-law density distribution and exhibiting a reddening slope equivalent to that seen in the LMC. From this we infer a smooth halo component to the dust that cannot be accounted for by radially averaging satellite galaxies and find a dust mass density for the universe roughly twice that from previous estimates. Finally, we present some preliminary estimates for the impact of this dustier universe on current and future supernova cosmology measurements.
Constraining Dark Energy: First Results from the SDSS-II Supernova Survey
A decade ago, two teams studying distant type Ia supernovae discovered that the expansion of the Universe is speeding up. Since then, supernova surveys from the ground and from space have brought major improvements in the quality and quantity of SN Ia data, confirming the discovery of cosmic acceleration. This talk will focus on early cosmological results from the Sloan Digital Sky Survey-II Supernova Survey, which operated for 9 months in 2005-7, discovering and measuring light curves and spectra for over 500 SNe Ia. I will discuss the observational challenges to determining supernova distances for cosmology and how those challenges are being addressed. These results inform the prospects for using supernovae to obtain improved cosmology measurements from planned and proposed surveys in the future.
Empirical Constraints on the Formation and Evolution of Low-Mass Stars and Brown Dwarfs: A Data-Intensive Approach
Recent and ongoing large surveys, both from the ground and from space, are enabling new data-intensive approaches to a variety of problems in stellar astrophysics. This talk describes three such projects, each serving as a vignette of a different but complementary mode of data-intensive research into low-mass star formation and evolution. The X10000 Project takes a panchromatic, time-domain approach to study the structures of young stellar coronae in order to understand the role of extreme coronal mass ejections in the angular momentum evolution of young stars. SLoWPoKES takes an ensemble, data-mining approach to extract from the Sloan Digital Sky Survey the largest sample of wide low-mass binaries ever assembled, which can be used to constrain binary formation theory and for refining the fundamental mass-age-activity-rotation-metallicity relations for low-mass stars. The EB Factory project takes a time-domain, data-mining approach to identify rare, but astrophysically very interesting, case studies from among the large numbers of eclipsing binaries being harvested by surveys for transiting exoplanets. We will highlight recent discoveries from this work, and will draw these results together to elucidate the physical interrelationships between stellar rotation, magnetic field generation, and stellar structure during the star-formation process.
Astronomy from Dome A, Antarctica
The Antarctic plateau provides many exciting possibilities for astronomical observations. The next decade may see a significant astronomical buildup on the Antarctic Plateau. Dome A and Dome C are currently the two most promising sites. These high points on the plateau have unique properties for astronomical observations. Two of these arise from the extreme cold: the column density of water vapor is lower than at any other site, thus opening unique windows at infrared and submillimeter wavelengths; and the ambient temperature, and thus the thermal background emission of telescope mirrors, is lower than at any other site. Two more advantages arise from the unique character of the atmospheric turbulence: the atmospheric boundary layer is extremely thin, only tens of meters, which opens the possibility of wide field, high resolution imaging by either adaptive correction of the thin ground layer or by raising the telescope above the boundary layer; the wind speeds at all levels of the atmosphere are low, which is highly favorable for adaptive correction. It will likely be possible to form diFFraction limited images over a good fraction of the sky down to visible light wavelengths three times HST resolution for an 8 m telescope. Dome A, being the highest and coldest point in Antarctica, is especially promising based on the results of recent theoretical models and site surveys. More comprehensive site monitoring should be planned for the next decade. Based on existing data, the site has certain areas of astronomical observations can already be planned with little risks. Wide field near-IR imaging, for example, relies critically on the thermal background and the low temperature at Dome A makes it an ideal site. Another key area is likely to be exoplanet imaging and spectroscopy in the L-band, where the combination of super-diFFraction limited AO correction and the very low thermal background will enable very high contrast imaging at very close inner working angle, for example 0.15 arcsec for an 8 m telescope.
Lessons from the Nearby Universe about Star Formation in Galaxies
The study of the relation between gas and star formation in galaxies is a matter of great current interest, and a crucial piece of information in our understanding of galaxy evolution. In order to determine how primordial density fluctuations become observable structures in the present day universe, it is crucial to characterize the processes that drive star formation on galactic scales. In turn, I will argue, this requires considering the creation of the self-gravitating cold molecular phase of the interstellar medium: Giant Molecular Clouds (GMCs).
In this talk I will present multi-scale measurements of the relation between gas and star formation in galaxies (the "star formation law"), and discuss our current knowledge of this relation. I will also discuss our new results on dust formation by supernovae, frequently assumed to be the major dust creation pathway in the early universe. The properties of GMCs, the major reservoirs of star-forming gas, play a key role in setting the initial conditions for the formation of stars. I will discuss the results from a comprehensive study of the resolved GMC properties in a number of extragalactic systems, including both normal and dwarf galaxies, and I will contrast the results of the virial and high-resolution far-infrared studies and what they tell use about molecular clouds in primitive galaxies and the relation between gas and the formation of stars.
Strong Lensing by Optically-Selected Galaxy Clusters
Gravitational lensing by galaxy clusters was predicted in the 1930s, and finally discovered in 1980s. In the two decades following the initial discovery, several dozen significant cluster lenses were found, though only a handful of these have been studied extensively. Lensing clusters probe the distribution of massive halos in the universe; the expected arc production frequency can be predicted from simulations and compared to existing data. Massive lensing clusters act as 'natural telescopes', providing highly magnified images of background sources which cannot otherwise be studied using the current generation of telescopes. The details of the observed lensing in clusters also probes the internal properties of these massive halos. Most cluster strong lens studies to date have been limited by the small number and heterogeneous nature of the sample of known lenses (most of which are one-off discoveries). I will report on efforts to take the study of strong lensing clusters to a new statistical regime, by identifying and studying two new samples of strong lenses within large catalogs of optically selected galaxy clusters from the RCS-2 and SDSS surveys; in total we have found hundreds of new giant arcs. These efforts are now approximately three-quarters-complete; in this progress report I will describe some of the spectacular successes of these studies, and the remaining challenges.
Gravitational Waves: A New Window onto the Universe
Over the next decade or so, the gravitational-wave window onto the Universe will be opened in four frequency bands that span 22 orders of magnitude: The high-frequency band, 10 to 10,000 Hz (ground-based interferometers such as LIGO), the low-frequency band, 10^-5 to 0.1 Hz (the space-based interferometer LISA), the very-low-frequency band, 10^-9 to 10^-7 Hz (pulsar timing arrays), and the extremely-low-frequency band, 10^-18 to 10^-16 Hz (polarization of the cosmic microwave background). This lecture will describe these four bands, the detectors that are being developed to explore them, and what we are likely to learn about black holes, neutron stars, white dwarfs and early-universe exotica from these detectors' observations.
[I will focus largely on LIGO and LISA but, unless you advise otherwise, I think it useful to include PTAs and CMB polarization as well.]
Exoplanets and their Odd Orbital Orientations
In the Solar system, the planets follow orbits that are aligned with the Sun's equatorial plane to within about 7 degrees. What about planets around other stars? Recently we have measured the orbital orientations (relative to their parent stars' equators) of more than a dozen different exoplanets, using a technique first theorized in the 19th century. Many systems have good alignment, as in the Solar system -- but there are a few surprises. I will discuss these results and their implications for theories of planet formation and migration.
Before the Big Bang
This talk will explore the idea that our universe existed before the big bang, as an alternative to the standard big bang/inflationary model. I will show how a phase of slow contraction before the big bang can explain the observed degree of flatness and homogeneity of our universe, as well as generate a nearly scale-invariant spectrum of primordial density perturbations. I will contrast the predictions of this model with those of inflationary cosmology, and discuss observational prospects for distinguishing between the two scenarios.
Reionization and Cosmology with 21 cm Fluctuations