625 research outputs found
Direct Mapping of Massive Compact Objects in Extragalactic Dark Halos
A significant fraction of non-baryonic or baryonic dark matter in galactic
halos may consist of MASsive Compact Objects (MASCOs) with mass
M=10^{1-4}M_{sun}. Possible candidates for such compact objects include
primordial black holes or remnants of primordial (Population III) stars. We
propose a method for directly detecting MASCOs in extragalactic halos, using
the VLBI techniques with extremely high resolution. If a galactic halo
comprising a large number of MASCOs produces multiple images of a background
radio-loud QSO by gravitational lensing, then a high-resolution radio map of
each macro-lensed image should reveal microlensing effects by MASCOs. To assess
their observational feasibility, we simulate microlensing of the radio-loud,
four-image lensed QSO, B1422+231, assuming angular resolution of ~0.01 mas.
MASCOs are represented by point masses. For comparison, we also simulate
microlensing of B1422+231 by singular isothermal spheres. We find that the
surface brightness of the macro-lensed images shows distinct spatial patterns
on the scale of the Einstein radius of the perturbers. In the case of
point-mass perturbers, many tiny dark spots also appear in the macro-lensed
images associated with a decrease in the surface brightness toward the fringe
of the original QSO image, whereas no such spots are available in the SIS
models. Based on the size, position and magnified or demagnified patterns of
images, we shall be able to determine the mass and density profile of a MASCO
as well as its spatial distribution and abundance in a galactic halo.Comment: 4 pages, 3 figure
Constraints on the Inner Mass Profiles of Lensing Galaxies from Missing Odd Images
Most gravitational lens systems consist of two or four observable images. The
absence of detectable odd images allows us to place a lower limit on the
power-law slope of the inner mass profile of lensing galaxies. Using a sample
of six two-image radio-loud lens systems and assuming a singular power-law
surface density (Sigma proportional to r^{-beta}) for the inner several kpc of
the mass distribution, we find that there is less than a 10% probability that
the data are consistent with profile slopes beta < 0.80. Furthermore,
individual mass modeling yields beta > 0.85 for B0739+366 and beta > 0.91 for
B1030+074. Modeling central black holes as additional point masses changes the
constraints in these systems to beta > 0.84 and beta > 0.83, respectively. The
inner mass profiles of lensing galaxies are therefore not much shallower than
isothermal.Comment: Final published version, minor typos corrected, 13 page
Three-dimensional Mapping of CDM Substructure at Submillimeter Wavelengths
The cold dark matter (CDM) structure formation model predicts that about 5-10
percent of a typical galactic halo of mass \sim 10^{12} \ms is in
substructures with masses \lesssim 10^8 \ms. To directly detect such
substructures, we propose to observe dust continuum emission from a strongly
lensed QSO-host galaxy using a large submillimeter interferometer array with a
high angular resolution of arcsec such as the planned Atacama Large
Submillimeter Array (ALMA). To assess their observational feasibility, we
numerically simulate millilensing of an extended circular source by a CDM
substructure modeled as a tidally truncated singular isothermal sphere (SIS)
embedded in a typical QSO-galaxy lens system, B1422+231, modeled as a singular
isothermal ellipsoid (SIE) with an external constant shear and a constant
convergence. Assuming an angular resolution of 0.01arcsec, we find that the
angular positions of \sim 10^8 \ms substructures at several kpc from the
center of the macrolens halo can be directly measured if the size of the dust
continuum emission region and the gradient of the surface brightness at the
position of the perturber are sufficiently large. From the astrometric shift on
a scale of a few times mas of an image perturbed by a subhalo with respect
to an unperturbed macrolensed image, we can break the degeneracy between
subhalo mass and distance provided that macrolensing parameters are determined
from positions and fluxes of multiple images.Comment: 7 pages, 7 EPS files. An assessment of our assumption of constancy in
shear and convergence has been included. Version accepted for publication in
Ap
CFHT AO Imaging of the CLASS Gravitational Lens System B1359+154
We present adaptive optics imaging of the CLASS gravitational lens system
B1359+154 obtained with the Canada-France-Hawaii Telescope (CFHT) in the
infrared K band. The observations show at least three brightness peaks within
the ring of lensed images, which we identify as emission from multiple lensing
galaxies. The results confirm the suspected compound nature of the lens, as
deduced from preliminary mass modeling. The detection of several additional
nearby galaxies suggests that B1359+154 is lensed by the compact core of a
small galaxy group. We attempted to produce an updated lens model based on the
CFHT observations and new 5-GHz radio data obtained with the MERLIN array, but
there are too few constraints to construct a realistic model at this time. The
uncertainties inherent with modeling compound lenses make B1359+154 a
challenging target for Hubble constant determination through the measurement of
differential time delays. However, time delays will offer additional
constraints to help pin down the mass model. This lens system therefore
presents a unique opportunity to directly measure the mass distribution of a
galaxy group at intermediate redshift.Comment: 12 pages including 3 figures; ApJL accepte
Probing Dark Matter Substructure in Lens Galaxies
We investigate the effects of numerous dark matter subhalos in a galaxy-sized
halo on the events of strong lensing, to assess their presence as expected from
the cold dark matter scenario. Lens galaxies are represented by a smooth
ellipsoid in an external shear field and additional cold dark matter subhalos
taken from Monte Carlo realizations which accord with recent N-body results. We
also consider other possible perturbers, globular clusters and luminous dwarf
satellites, for comparison. We then apply the models to the particular lens
systems with four images, B1422+231 and PG1115+080, for which smooth lens
models are unable to reproduce both the positions of the images and their radio
flux ratios or dust-free optical flux ratios simultaneously. We show that the
perturbations by both globular clusters and dwarf satellites are too small to
change the flux ratios, whereas cold dark matter subhalos are most likely
perturbers to reproduce the observed flux ratios in a statistically significant
manner. This result suggests us the presence of numerous subhalos in lens
galaxies, which is consistent with the results of cosmological N-body
simulations.Comment: 19 pages, including 5 figures, ApJ in pres
Analytic Cross Sections for Substructure Lensing
The magnifications of the images in a strong gravitational lens system are
sensitive to small mass clumps in the lens potential; this effect has been used
to infer the amount of substructure in galaxy dark matter halos. I study the
theory of substructure lensing to identify important general features, and to
compute analytic cross sections that will facilitate further theoretical
studies. I show that the problem of a clump anywhere along the line of sight to
a lens can be mapped onto an equivalent problem of a clump in a simple
convergence and shear field; clumps at arbitrary redshifts are therefore not
hard to handle in calculations. For clumps modeled as singular isothermal
spheres (SIS), I derive simple analytic estimates of the cross section for
magnification perturbations of a given strength. The results yield two
interesting conceptual points. First, lensed images with positive parity are
always made brighter by SIS clumps; images with negative parity can be
brightened but are much more likely to be dimmed. Second, the clumps need not
lie within the lens galaxy; they can be moved in redshift by several tenths and
still have a significant lensing effect. Isolated small halos are expected to
be common in hierarchical structure formation models, but it is not yet known
whether they are abundant enough compared with clumps inside lens galaxies to
affect the interpretation of substructure lensing.Comment: accepted in ApJ; minor text changes from original versio
The Quintuple Quasar: Mass Models and Interpretation
The strange morphology of the six-component gravitational lens PMN J0134-0931
has resisted explanation. We present the first successful quantitative models
for the system, based on the idea that there are two lens galaxies and two
components of the background source. One source is quintuply imaged and
corresponds to the five brightest observed radio components. The other source
is triply imaged and corresponds to the sixth component, along with two others
too faint to have been detected. The models reproduce the observed image
positions and fluxes, and make falsifiable predictions about other properties
of the system. Some of these predictions have been confirmed by high-resolution
radio and optical observations, as described in the companion paper by Winn et
al. (2003). Although we cannot determine the lens model uniquely with current
data, we predict that the lens galaxies are spiral galaxies with roughly equal
velocity dispersions sigma~120km/s and a projected separation of only 0.4" (2/h
kpc at z_l=0.76). This system is the first known lens with five images of a
single quasar, and the second with more than four images.Comment: ApJ, in press. 22 pp, 9 fig
The Importance of Lens Galaxy Environments
While many strong gravitational lens galaxies are suspected to lie in groups
or clusters of galaxies, environmental effects in lens models are often
unconstrained and sometimes ignored. We show that this creates significant
biases in a variety of lensing applications, by creating mock lenses associated
with each of 13 galaxies in a realistic model group, and then analyzing them
with standard techniques. We find that standard models of double lenses, which
neglect environment, grossly overestimate the ellipticity of the lens galaxy
(de/e~0.5) and the Hubble constant (dh/h~0.22). Standard models of quad lenses,
which approximate the environment as a tidal shear, recover the ellipticity
reasonably well (|de/e|<~0.24) but overestimate the Hubble constant
(dh/h~0.15), and have significant (~30%) errors in the millilensing analyses
used to constrain the amount of substructure in dark matter halos. For both
doubles and quads, standard models slightly overestimate the velocity
dispersion of the lens galaxy (d(sigma)/sigma~0.06), and underestimate the
magnifications of the images (d(mu)/mu ~ -0.25). Standard analyses of lens
statistics overestimate Omega_Lambda (by 0.05-0.14), and underestimate the
ratio of quads to doubles (by a factor of 2). These biases help explain some
long-standing puzzles (such as the high observed quad/double ratio), but
aggravate others (such as the low value of H_0 inferred from lensing). Most of
the biases are caused by neglect of the convergence from the mass associated
with the environment, but additional uncertainty is introduced by neglect of
higher-order terms. Fortunately, we show that directly observing and modeling
lens environments should make it possible to remove the biases and reduce the
uncertainties associated with environments to the few percent level. (Abridged)Comment: 14 emulateapj pages; accepted in Ap
Analytic Time Delays and H_0 Estimates for Gravitational Lenses
We study gravitational lens time delays for a general family of lensing
potentials, which includes the popular singular isothermal elliptical potential
and singular isothermal elliptical density distribution but allows general
angular structure. Using a novel approach, we show that the time delay can be
cast in a very simple form, depending only on the observed image positions.
Including an external shear changes the time delay proportional to the shear
strength, and varying the radial profile of the potential changes the time
delay approximately linearly. These analytic results can be used to obtain
simple estimates of the time delay and the Hubble constant in observed
gravitational lenses. The naive estimates for four of five time delay lenses
show surprising agreement with each other and with local measurements of H_0;
the complicated Q 0957+561 system is the only outlier. The agreement suggests
that it is reasonable to use simple isothermal lens models to infer H_0,
although it is still important to check this conclusion by examining detailed
models and by measuring more lensing time delays.Comment: 16 pages with 2 embedded figures; submitted to Ap
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