Updated: 14 April 2011

MIT Kavli Institute for Astrophysics and Space Research 2011 Postdoc Symposium April 13−15, 2011       11:45 AM−1:20 PM       BLDG 37-252 (MARLAR ROOM)

General Information

This annual event features the MKI-affiliated postdocs presenting 15 minute talks on their past, present, and/or future research to the MKI community. Lunch will be provided, and the talks will promptly begin at noon. There will be four speakers each day; the short schedule with just speakers and titles is available here. The 2010 Postdoc Symposium schedule is available here (PDF).

Abstracts

April 13, 2011

The ground-based gravitational-wave telescopes LIGO and Virgo approach the era of first detections. In this talk, I will review the current knowledge of the coalescence rates and parameter distributions of merging neutron-star and black-hole binaries. I emphasize the bi-directional connection between gravitational-wave astronomy and conventional astrophysics. Astrophysical input will make possible informed decisions about optimal detector configurations and search techniques. Meanwhile, rate upper limits, detected merger rates, and the distribution of masses and spins measured by gravitational-wave searches will constrain astrophysical parameters through comparisons with astrophysical models. I report on ongoing efforts to develop a framework for converting gravitational-wave observations into improved constraints on astrophysical parameters and discuss future developments necessary to the success of gravitational-wave astronomy.

Gravitational waves... The final frontier...
These are the detection candidates for the Laser Interferometer Gravitational Wave Observatory (LIGO).
Its continuing mission:
To build ever-more sensitive interferometers...
To seek out colliding black holes; new physics...
To boldly observe what have never been directly observed before!

As gravitational wave detectors improve their sensitivity, quantum noise is an increasingly important limitation. I will give an overview of how shot noise and radiation pressure noise enter the measurement, and strategies for their mitigation, such as the application of squeezed light. I will also discuss the role of opto-mechanics in future detectors, and describe a series of experiments performed at MIT to demonstrate these effects.

I will give an overview of the data analysis methods used in the searches for gravitational waves from compact binary coalescence with the ground based interferometers. I will describe the challenges one is faced with in and strategies for overcoming them. I will also present the highlights of the most recent searches performed during the initial LIGO and Virgo operation and discuss prospects of the advanced detectors network.

April 14, 2011

Gravitational waves hold the promise to open a new window to the Universe. Detectors on Earth, such as LIGO and Virgo, and in space, such as LISA, hope to detect tens to hundreds of events from a variety of sources, prime among which are binary inspirals in their last parsec of evolution. Space detectors are sensitive in the milli-Hertz range, best suited to observe supermassive black hole coalescences and extreme-mass ratio inspirals of solar-mass black holes into supermassive ones. These waves carry a wealth of information about the sources that generate them, allowing us to extract invaluable astrophysical information, such as their mass and location. In this talk, I will discuss how the detection of extreme-mass ratio inspirals with LISA can allow us to localize other supermassive black holes that perturb the extreme-mass ratio binary, provided the former are within a sphere of radius one parsec.

The low-redshift (z < 1) IGM probes the last eight-billion years of metal enrichment from galactic feedback processes. Outflows and mergers return to the IGM enriched material for future generations of galaxies and stars. The signature of this process is etched in the recycled gas: metallicity, abundances, density, distribution, etc. Observations of intergalactic absorbers provide "end-product" constraints on the physics driving chemical evolution and, therefore, the physics adopted in cosmological hydrodynamic simulations. We conducted the largest survey for CIV and/or SiIV systems at z < 1, and we compare our results to those from high-redshift (1.5 < z < 6) studies. For example, we present the frequency distribution ƒ(N) and mass density of CIV and SiIV absorbers in the HST archival spectra of 49 quasars. The changes in the properties of the low-redshift systems, compared to the high-redshift ones, indicate that CIV and SiIV more commonly trace circum-galactic gas at z < 1. Indeed, this agrees with simulated observations from several OverWhelmingly Large Simulations.

A fundamental question in the field of galaxy evolution is the relationship between the growth of a galaxy's central black hole and that of the galaxy's stars. I describe new deep observations with Subaru in 18 medium-band optical filters and I leverage the wealth of existing data in the Extended Chandra Deep Field-South, in order to investigate AGN host galaxies at z ∼ 1. This study, part of the Multi-wavelength Survey by Yale-Chile (MUSYC), provides the community with highly accurate photometric redshifts and detailed spectral energy distributions for galaxies and AGN in this legacy field. AGN host galaxies are predominantly massive galaxies, most of which lie on the red sequence and in the green valley of the color-mass diagram. Because both passive and dusty galaxies can appear red in optical colors, I use rest-frame near-infrared colors to separate passively evolving stellar populations from those reddened by dust. As with the overall galaxy population, ∼25% of the 'red' AGN host galaxies and ∼75% of the 'green' AGN host galaxies have dust-corrected rest-frame optical colors of blue star-forming galaxies, which implies that they are not passively aging to the red sequence. After de-reddening, it becomes clear that at z ∼ 1, AGN activity is roughly evenly split between two modes of black hole growth: the first in passively evolving host galaxies, which may be heating up the galaxy's gas and preventing future episodes of star formation, and the second in dust-reddened young galaxies, which may be ionizing the galaxy's interstellar medium and shutting down star formation.

To date, thermonuclear bursts from neutron stars had a distinctive signature: cooling along the burst decay. We have found a set of bursts from a recently discovered source in Terzan 5 that do not show such cooling signature, yet we are able to identify them as thermonuclear bursts from an accreting neutron star. We conclude that the detection of cooling along the decay is a sufficient, but not a necessary condition to identify an X-ray burst as thermonuclear. We also compare these findings with X-ray bursts from other rapidly accreting neutron stars.

April 15, 2011

The cold dark matter (CDM) paradigm predicts that a significant number of substructures, with a steeply rising mass function towards lower masses, populates the dark halo of galaxies. In the Milky Way, however, of order 104 substructures are predicted inside the virial radius, whereas only few tens have been so far observed. This poses a major challenge to the CDM paradigm. New and independent methods are, therefore, required to assess the level of mass substructure in galaxies in the Local Universe and beyond.

One such method will be discussed in this talk, which consists of three parts. First, I will discuss a novel method, based on strong gravitational lensing, to uniquely address this problem, allowing us to probe substructure beyond the local Universe and in massive early-type galaxies. I will briefly present a fully Bayesian adaptive-grid method, that uses all the information contained in the surface brightness distribution of highly magnified Einstein rings and arcs, to detect and precisely quantify mass substructure in single lens galaxies, even in case of very high mass-to-light ratios. Second, I will discuss how to combine, in a statistical sense, the detections of mass substructures from multiple lens galaxies to constrain the substructure mass fraction ƒ_sub and the slope of the mass function α. Third, I will present new results on the level of mass substructure in the sample of SLACS lenses, as well a several other systems, and discuss possible implications for galaxy formation models.

If time is left, I will discuss possible future applications of the method.

Two different, previously released, Cryogenic Dark Matter Search (CDMS) data sets have been reanalyzed to improve sensitivity to low-mass Weakly Interacting Massive Particle (WIMP) signals. The first data set was obtained from 2001 to 2002 at the shallow-depth Stanford Underground Facility with four germanium and two silicon detectors. The second data set utilized eight germanium detectors at the deep Soudan Underground Laboratory from 2006 to 2008. The Stanford data excludes parameter space between 3 and 4 GeV / c² while the Soudan result excludes parameter space favored by the DAMA / LIBRA and CoGeNT data as light WIMP signals at the >90% confidence level. Expected performance of new detectors with an interleaved charge readout, also being commissioned at Soudan, will (time permitting) also be discussed.

If quintessence is rolling slowly now, we expect it to enter an oscillatory regime as it finds its way towards a local minimum in its potential. We investigate the observational consequences of quintessence rolling to and oscillating near a minimum in its potential, "if" it happens close to the present epoch (z < 0.3). We show that there is a resonant, rapid growth in the field fluctuations and in the gravitational potential at wave numbers k < m, where m is the mass of quintessence near its minimum. This effect is in addition to the deviations in the expansion history and corresponding changes in growth of structure. Furthermore, at linear order, no-anisotropic stress is generated and the shape of the WIMP and galaxy power spectrum is not affected significantly at these scales.