Harvard University

Astronomy 200 - Spring'2004: Gravitational Lensing

Catalog Number: 8574
Krzysztof Z. Stanek, Scott Gaudi, Joshua N. Winn and members of the CfA

Wednesdays, 3:30-4:30 p.m. in Pratt, unless otherwise noted.

Graduate seminar on topical areas in modern astrophysics and cosmology. Each semester a different topic of current special interest is selected. Participants in this seminar discuss papers given by seminar members (in rotation). Several faculty members also participate.

Note: Participation for three semesters is required to obtain credit for this course.

Schedule for Spring'2004:

*0. February 4th, 2004

``Organizational meeting'' - Kris Stanek, Scott Gaudi and Josh Winn

Abstract:

How the course is organized. Motivation for the topic. Scheduling the talks.

General reading material and useful links for the Semester:

Always useful are the NASA Astrophysics Data System and the astro-ph preprint archive

Gravitational Lensing Demonstration

CASTLe - CfA-Arizona Space Telescope Lens Survey

- Ongoing microlensing project (see also OGLE III Early Warning System)

*1. February 4th, 2004

``History, Overview and Basic Theory of Gravitational Lensing'' - Randall Cooper

Abstract:

Though the basic idea has existed since the eighteenth century, gravitational lensing has been used regularly as an astronomical tool for only the past 25 years. Gravitational lensing, the attraction of light by matter, has recently become a hot topic of research. I will begin with an historical introduction to the field. Next, I will explain some of the basic theory of gravitational lensing. Finally, I will conclude with an overview of this topic's many subfields.

Reading material:

*2. February 11th, 2004

``General Theory of Gravitational Lensing'' - Xavier Koenig

Abstract:

Following on from the first talk, which briefly introduced the theory of Gravitational Lensing, I will recap and build upon what was shown then, in order to develop and demonstrate the tools and concepts that will be needed as the semester of talks progresses. Particularly, I will explain what is meant by caustics and critical lines, and how the lens equation naturally leads us to apply Fermat's Principle to the problem of Gravitational Lensing as an extension of geometrical optics. I will sum up by considering the specific applications of these tools to the various sub-fields - microlensing, strong lensing and others.

Reading material:

*3. February 18th, 2004

``Strong Lensing'' - Kaloyan Penev

Abstract:

I will start with a historical introduction, briefly covering the most important steps in strong lensing. Afterwards, following up on the general gravitational lensing expressions I will introduce the conditions for strong lensing and how one identifies them. Then I will switch over to modelling and reconstructing the lens mass. I will introduce both types of models used today - parametric and free form. I will describe the effects of nearby clusters and briefly introduce microlensing and the effects it has on the images observed.

Reading material:

*4. February 25th, 2004

``Cosmology and Strongly-Lensed QSOs" - Andrew Friedman

Abstract:

In this talk I will build upon our previous discussions of strong gravitational lensing and discuss the major applications of strong lensing of quasars for cosmology, namely determining Ho from time delays and constraining Omega_Lambda from quasar lensing statistics. I will outline the history and theoretical background for both approaches, discuss the most current cosmological lensing constraints on Ho and Omega_Lambda, and place them in the context of other cosmographic constrains from Cepheids, CMB, SNe Ia, and LSS measurements.

Reading material:

General
* Blandford & Narayan, 1992 (Cosmological applications of gravitational lensing)...excel lent review.
Blandford & Narayan, 1986, (Fermat's principle, caustics, and the classification of gravitational lens images)
Courbin, Saha, & Schechter, 2002 (Another good review on Quasar Lensing)
Hogg, 2000, astro-ph/9905116, (Distance Measures in Cosmology...very useful cheat sheet for cosmological distance formulae)

The Hubble Constant – Ho
Kochanek & Schechter, 2003, eprint arXiv:astro-ph/0306040, (The Hubble Constant from Gravitational Lens Time Delays)...The Ho part of my talk primarily followed this review.
Courbin, F. eprint arXiv:astro-ph/0304497 (Quasar Lensing: the Observer's Point of View)
Refsdal, S. 1964, MNRAS, Vol. 128, p.307. (On the possibility of determining Hubble's parameter and the masses of galaxies from the gravitational lens effect)...seminal paper

The Cosmological Constant - Omega_Lambda
Mitchell, Keeton, Frieman, & Sheth, 2004, eprint arXiv:astro-ph/0401138, (Robust Cosmological Constraints from Gravitational Lens Statistics)...The Lambda part of my talk primarily followed this review.
Kochanek, C. 1996, (Is There a Cosmological Constant?)...provocative early paper - pre SNe Ia or WMAP
Kochanek, C. 1993, (Analytic results for the gravitational lens statistics of singular isothermal spheres in general cosmologies)
Carroll, Press, & Turner, 1992, ARA&A. (The cosmological constant)...massive annual review article
Turner, E. 1990, (Gravitational lensing limits on the cosmological constant in a flat universe)
Gott, Park, & Lee, 1989. (Setting limits on q0 from gravitational lensing)
Turner, Ostriker, & Gott, (The statistics of gravitational lenses - The distributions of image angular separations and lens redshifts)...original formalism

Web Links :
CASTLe - CfA-Arizona Space Telescope Lens Survey
CLASS – Cosmic Lens All-Sky Survey

*5. March 3rd, 2004

``Other Applications of QSO Lensing'' - Eliza Miller-Ricci

Abstract:

This talk focuses on the applications from observed systems of lensed QSOs. I will mainly be highlighting three applications of QSO lensing. First I will discuss how one can use gravitational lensing to constrain the distribution of cold dark matter (CDM) in the lensing galaxy. The second application is the use of lensing systems to measure masses of lens galaxies and to study galaxy evolution with redshift. I will finish by discussing the use of galaxy lenses as "natural telescopes" with which to observe QSOs. I will also briefly mention some other applications of QSO lensing.

Reading material:

*6. March 10th, 2004

``Microlensing in the Local Group'' - Joel Hartman

Abstract:

Over the last decade gravitational microlensing has proved to be a powerful tool in understanding the structure of the local group of galaxies. In this talk I will overview the subject emphasizing the applications of microlensing to the study of dark matter. I will briefly describe the history of microlensing, beginning with Paczynski's seminal work in 1986, and then outline the basic theory, focusing on how one can probe the distribution of matter with massive photometric searches for microlensing candidates. I will describe some of the complications involved in this technique and finish by discussing results from several of the major searches (MACHO, EROS, and OGLE), showing how combinations of these have provided strong constraints on the contribution of MACHOs to the dark matter halo of the Milky Way, and led to a surprisingly high value for the optical depth to lensing toward the Galactic Bulge.

Reading material:

*7. March 17th, 2004

``Other Science from Microlensing Surveys I'' - Jonathan Devor

Abstract:

Microlenses provide observers nothing short of a treasure trove of effects from which one can deduce information, which is extremely difficult to get using direct methods. This is especially true for the study of stellar astrophysics. In my talk I will explain and demonstrate four such microlensing applications, for probing stellar atmospheres and finding extrasolar planets. I will outline a few such observations that have already been made, their successes and pitfalls, and how future missions will be able to do even more.

Reading material:

Microlensing movies by Scott Gaudi.

Powerpoint version of the talk.

*8. March 24th, 2004 (in Phillips)

``Other Science from Microlensing Surveys II'' - Christopher Pilman

Abstract:

Ambitious surveys (such as OGLE, MACHO and EROS) conducted in the last 12 years, searching for microlensing events, have contributed to observational astronomy beyond the field of microlensing itself. I shall discuss the impact which these surveys have had on several other areas of astronomy, including variable stars and distance indicators. I shall also discuss the new techniques which must be used to analyze and distribute the vast amount of data obtained by these surveys.

Reading material:

Spring Break

*9. April 7th, 2004

``Theory of weak lensing'' - Daniel Babich

Abstract:

This lecture will begin our series of talks on weak gravitational lensing. After discussing the statistical nature of the effect, I will review the propagation of light through an inhomogeneous universe. Then I will apply these concepts to the coherent distortion of the shapes of galaxies. I will conclude by discussing gravitational lensing of CMB anisotropies.

Reading material:

*10. April 14th, 2004

``Weak lensing - application to galaxy clusters'' - Jonathan Foster

Abstract:

In part two of the weak-lensing trilogy I will discuss using weak gravitational-lensing to determine the mass distribution and total mass of galaxy clusters. This method relies on fewer assumptions than other methods of cluster mass reconstruction, but is observationally challenging. The small signal makes it important to use all available information in the inversion, and there are several different methods which attempt to do this. The results, while roughly similar to other methods, show small but significant differences which may tell us something important about galaxy clusters.

Reading material:

*11. April 21st, 2004 (in Phillips)

``Cosmological applications of weak lensing'' - Kamson Lai

Abstract:

Weak lensing by large scale structure is a promising new tool for precision cosmology. Since the first detections of cosmic shear signals in 2000, much progress has been made and it is now possible to derive strong cosmological constraints from weak lensing surveys. I will review the basic principles of cosmological weak lensing, and present results from recent surveys. I will also discuss the exciting possibilities, such as probing dark energy, as well as the many challenges that lie in the future of weak lensing.

Reading material:

*12. April 28th, 2004

``Gravitational Lensing in the Strong Field Limit'' - Deborah Freedman

Abstract:

Gravitational lensing describes the apparent bending of light rays by massive objects. To determine photon trajectories near compact objects, a full general relativistic treatment is required because the usual approximation of small deflection angle breaks down. In this talk, I will discuss applications of general relativity for the interpretation and prediction of observations involving compact objects. The applications that I will include are measurements of self-lensing in neutron stars, proposed observations of Sag A*, and proposed studies of iron line reverberations in accretion disks.

Reading material:

*13. May 5th, 2004

``Macro-, milli-, micro- and nano-lensing of quasars'' - Prof. Paul L* Schechter

Abstract:

The 0th, 1st and 2nd derivatives of a "Fermat potential" give the three D's of gravitational lensing: delay, deflection and distortion. Observations of these delays, deflections and distortions make it possible to model the gravitational potentials of the intervening galaxies which produce multiple images of distant quasars. Simple models for lensing potentials that successfully reproduce image positions to high accuracy fail ABYSMALLY in reproducing the flux ratios of the multiple images, suggesting the presence of small scale structure within the lensing galaxies. It has been argued that the flux ratio anomalies observed at radio wavelengths signal the presence of CDM mini-halos. We argue that the yet larger anomalies observed at optical wavelengths can result from microlensing by stars, but ONLY if a substantial fraction of the projected mass is in a smooth, dark component.

Reading material:

Quasar Microlensing at High Magnification and the Role of Dark Matter: Enhanced Fluctuations and Suppressed Saddlepoints, Paul L. Schechter & Joachim Wambsganss (2002, ApJ, 580, 685-695)

Questions or Comments? Send an e-mail to kstanek@cfa.harvard.edu.

This page was last updated on Mon Nov 22 18:14:23 EST 2004