Tereasa G. Brainerd
Recent and Ongoing Research Projects
Weak Lensing
General Relativity tells us that any mass
will cause a curvature
of spacetime in its vicinity. Therefore, any mass located
along the line of sight to a distant luminous object will act as
a gravitational lens by deflecting light rays emanating from the object
as they propagate through the universe. The
most striking instances of gravitational lensing (e.g.,
multiple images, rings, arcs) are
examples of phenomena caused by
strong
gravitational lenses, which greatly distort the images of distant
galaxies. By contrast,
weak gravitational lenses distort the images of distant galaxies
very mildly but produce a net coherent pattern of image distortions in
which there is a slight preference for the lensed galaxies to be
oriented tangentially with respect to the direction vector that
connects their centroids with the center of the gravitational
potential of the lens.
While weak lenses do not give rise to stunning individual
images, they are detectable in a statistical sense via ensemble
averages over many mildly-distorted images.
Also, unlike strong
lensing which occurs very rarely,
weak lensing is commonplace and it is
routinely detectable in high quality data.
Weak Lensing by Flattened Halos
Galaxy-galaxy lensing, in which the images of distant galaxies are
marginally distorted by foreground galaxies, has recently developed
into a useful tool by which the characteristic physical parameters
of the halos of galaxies may be constrained directly. In collaboration
with my former PhD student, Candace Oaxaca Wright, I have worked on
galaxy-galaxy lensing by flattened dark matter halos in order to determine
whether or not the aniostropic lensing signal which would be induced
by such objects would be detectable in a realistic ground-based data
set. Candace's thesis work showed that in
the case of deep data sets (limiting magnitudes of less than about
I=23), multiple weak deflections
induce correlations between the observed images of foreground-background
pairs of galaxies and this, in turn, leads to systematic effects in
the galaxy-galaxy lensing signal. These effects can be removed by
imposing a simple redshift cut on the data such that only lenses with
z < 0.5 and sources with z > 0.5 are considered in the analysis.
If dark matter halos are flattened to a "reasonable"' extent (a median
ellipticity of order 0.3) and if lens-source separation is done via
photometric redshifts with a typical accuracy of less than about 0.1,
a 4-sigma detection of the effects
of halo flattening on the galaxy-galaxy lensing signal in
a deep survey
which covers 22 to 32 square degrees should be possible.
Multiple Deflections in the HDF-North
I have recently investigated the theoretical weak shear field
in the Hubble Deep Field North (HDF-N) which would be yielded
solely by the galaxies, and have found
a tremendous sensitivity to
the predictions for the shear when the characteristic velocity dispersion
of the halos of L* galaxies is varied only modestly.
Source galaxies located behind the HDF have a high
probability of being lensed at a significant
level by more than one foreground galaxy. That is, multiple weak
deflections are important in such deep data sets.
The effects of multiple deflections
serve to increase the mean tangential shear about the lens galaxies
in comparison to a simple calculation where only single deflections
are considered (i.e., calculations in which each source is assumed to be
lensed solely by the foreground galaxy
which is nearest to it in projection on the sky). In addition,
multiple deflections in galaxy--galaxy lensing
induce correlated ellipticities in
the images of the source galaxies.
Dynamical Mass Estimators for Isolated Galaxies
In collaboration with a BU undergraduate student, Mike Specian, I
have been investigating the validity of a number of common dynamical
mass estimators for isolated galaxies. Mike has been using a numerical
simulation that incorporates semi-analytic galaxy formation to select
"satellite" and "host" galaxies in a manner consistent with a number
of observational selection criteria. Using the galaxy pairs extracted
from the simulation, then, Mike is assessing the typical level of error
in the masses which are derived from the dynamical mass estimators.
He is in the process of quantifying the effects of "interloper" galaxies
(i.e., galaxies which are selected as satellites but which are not
physically associated with the host galaxy), and is applying his
best mass estimators to a sample of isolated spiral galaxies which were
drawn from the June 2001 2dF data release.
Graduate students who are interesting in carrying out research in
theoretical cosmology might be interested in the following projects:
Weak Lensing
by Numerical Clusters
Weak Lensing in
the HDF North