203 research outputs found
A study of the compact group of galaxies Shahbazian 4
The radial velocities of members of Shakhbazian 4 are determined. It is found that the dispersion of the radial velocities is 440 km/s. The apparent and absolute magnitudes of galaxies in V color are obtained. It is found that the M/L ratio of the group is about 220 solar mass/solar luminosity. The crossing time for the Shakhbazian 4 group is equal to 47 Myr
High-Resolution Mid-Infrared Morphology of Cygnus A
We present subarcsecond resolution mid-infrared images at 10.8 and 18.2
microns of Cygnus A. These images were obtained with the University of Florida
mid-IR camera/spectrometer OSCIR at the Keck II 10-m telescope. Our data show
extended mid-IR emission primarily to the east of the nucleus with a possible
western extension detected after image deconvolution. This extended emission is
closely aligned with the bi-conical structure observed at optical and near-IR
wavelengths by the HST. This emission is consistent with dust heated from the
central engine of Cygnus A. We also marginally detect large-scale low level
emission extending > 1.5 kpc from the nucleus which may be caused by in-situ
star formation, line emission, and/or PAH contamination within the bandpass of
our wide N-band filter.Comment: 20 pages, 8 figures, accepted for publication in the Astrophysical
Journa
MS 2053.7-0449: Confirmation of a bimodal mass distribution from strong gravitational lensing
We present the first strong lensing study of the mass distribution in the
cluster MS 2053-04 based on HST archive data. This massive, X-ray luminous
cluster has a redshift z=0.583, and it is composed of two structures that are
gravitationally bound to each other. The cluster has one multiply imaged system
constituted by a double gravitational arc.
We have performed a parametric strong lensing mass reconstruction using NFW
density profiles to model the cluster potential. We also included perturbations
from 23 galaxies, modeled like elliptical singular isothermal sphere, that are
approximately within 1'x1' around the cluster center. These galaxies were
constrained in both the geometric and dynamical parameters with observational
data. Our analysis predicts a third image which is slightly demagnified. We
found a candidate for this counter-image near the expected position and with
the same F702W-F814W colors as the gravitational arcs in the cluster. The
results from the strong lensing model shows the complex structure in this
cluster, the asymmetry and the elongation in the mass distribution, and are
consistent with previous spectrophotometric results that indicate that the
cluster has a bimodal mass distribution. Finally, the derived mass profile was
used to estimate the mass within the arcs and for comparison with X-ray
estimates.Comment: To be published in ApJ (accepted
Perturbative reconstruction of a gravitational lens: when mass does not follow light
The structure and potential of a complex gravitational lens is reconstructed
using the perturbative method presented in Alard 2007, MNRAS, 382L, 58; Alard
2008, MNRAS, 388, 375. This lens is composed of 6 galaxies belonging to a small
group. The lens inversion is reduced to the problem of reconstructing
non-degenerate quantities: the 2 fields of the perturbative theory of strong
gravitational lenses. Since in the perturbative theory the circular source
solution is analytical, the general properties of the perturbative solution can
be inferred directly from the data. As a consequence, the reconstruction of the
perturbative fields is not affected by degeneracy, and finding the best
solution is only a matter of numerical refinement. The local shape of the
potential and density of the lens are inferred from the perturbative solution,
revealing the existence of an independent dark component that does not follow
light. The most likely explanation is that the particular shape of the dark
halo is due to the merging of cold dark matter halos. This is a new result
illustrating the structure of dark halos at the scale of galaxies.Comment: Final version (Astronomy and Astrophysics in press
Ringing effects reduction by improved deconvolution algorithm Application to A370 CFHT image of gravitational arcs
We develop a self-consistent automatic procedure to restore informations from
astronomical observations. It relies on both a new deconvolution algorithm
called LBCA (Lower Bound Constraint Algorithm) and the use of the Wiener
filter. In order to explore its scientific potential for strong and weak
gravitational lensing, we process a CFHT image of the galaxies cluster Abell
370 which exhibits spectacular strong gravitational lensing effects. A high
quality restoration is here of particular interest to map the dark matter
within the cluster. We show that the LBCA turns out specially efficient to
reduce ringing effects introduced by classical deconvolution algorithms in
images with a high background. The method allows us to make a blind detection
of the radial arc and to recover morphological properties similar to
thoseobserved from HST data. We also show that the Wiener filter is suitable to
stop the iterative process before noise amplification, using only the
unrestored data.Comment: A&A in press 9 pages 9 figure
Hubble Space Telescope Planetary Camera Images of NGC 1316
We present HST Planetary Camera V and I~band images of the central region of
the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by
a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an
assumed distance of 16.9 Mpc, the deprojected luminosity density reaches \sim
2.0 \times 10^3 L_{\sun} pc.
Outside the inner two or three arcseconds, a constant mass-to-light ratio of
is found to fit the observed line width measurements. The
line width measurements of the center indicate the existence of either a
central dark object of mass 2 \times 10^9 M_{\sun}, an increase in the
stellar mass-to-light ratio by at least a factor of two for the inner few
arcseconds, or perhaps increasing radial orbit anisotropy towards the center.
The mass-to-light ratio run in the center of NGC 1316 resembles that of many
other giant ellipticals, some of which are known from other evidence to harbor
central massive dark objects (MDO's).
We also examine twenty globular clusters associated with NGC 1316 and report
their brightnesses, colors, and limits on tidal radii. The brightest cluster
has a luminosity of 9.9 \times 10^6 L_{\sun} (), and the
faintest detectable cluster has a luminosity of 2.4 \times 10^5 L_{\sun}
(). The globular clusters are just barely resolved, but their core
radii are too small to be measured. The tidal radii in this region appear to be
35 pc. Although this galaxy seems to have undergone a substantial merger
in the recent past, young globular clusters are not detected.Comment: 21 pages, latex, postscript figures available at
ftp://delphi.umd.edu/pub/outgoing/eshaya/fornax
The Ray Bundle method for calculating weak magnification by gravitational lenses
We present here an alternative method for calculating magnifications in
gravitational lensing calculations -- the Ray Bundle method. We provide a
detailed comparison between the distribution of magnifications obtained
compared with analytic results and conventional ray-shooting methods. The Ray
Bundle method provides high accuracy in the weak lensing limit, and is
computationally much faster than (non-hierarchical) ray shooting methods to a
comparable accuracy.
The Ray Bundle method is a powerful and efficient technique with which to
study gravitational lensing within realistic cosmological models, particularly
in the weak lensing limit.Comment: 9 pages Latex, 8 figures, submitted to MNRA
Gravitational Lensing by Dark Matter Caustics
Dark matter caustics have specific density profiles and, therefore, precisely
calculable gravitational lensing properties. We present a formalism which
simplifies the relevant calculations, and apply it to four specific cases. In
the first three, the line of sight is tangent to a smooth caustic surface. The
curvature of the surface at the tangent point is positive, negative or zero. In
the fourth case the line of sight passes near a cusp. For each we derive the
map between the image and source planes. In some cases, a point source has
multiple images and experiences infinite magnification when the images merge.
Unfortunately, for the dark matter caustics expected in realistic galactic halo
models, the angular resolution required to resolve the multiple images is not
presently achievable. A more promising approach aims to observe the distortions
caused by dark matter caustics in the images of extended sources such as radio
jets.Comment: 36 pages, 11 figure
An S-shaped arc in the galaxy cluster RXJ0054.0-2823
The center of the galaxy cluster RX J0054.0-2823 at z = 0.292 is a
dynamically active region which includes an interacting system of three
galaxies surrounded by a large halo of intra-cluster light. We report here the
discovery of an S-shaped feature of total length 11 arcsec in the central
region of this cluster and discuss its physical nature. We test the
gravitational lensing assumption by doing a mass modelling of the central part
of the galaxy cluster. We very naturally reproduce position and form of this
S-shape feature as a gravitationally lensed background object at redshift
between 0.5 and 1.0. We conclude that the lensing nature is the very probable
explanation for this S-shaped arc; the ultimate proof will be the spectroscopic
confirmation by measuring the high redshift of this elongated feature with
surface brightness V~24mag/arcsec2.Comment: 6 pages, accepted for publication in A&
Low-extinction windows in the inner Galactic Bulge
We built K band extinction maps in the area of two candidate low-extinction
windows in the inner Bulge: W0.2-2.1 at (l,b) = (0.25o,-2.15o), and W359.4-3.1
at (l,b) = (359.40o,-3.10o). We employed JHKs photometry from the 2MASS Point
Source Catalog. Extinction values were determined by fitting the upper giant
branch found in the present 2MASS Ks x J-Ks diagrams to a de-reddened bulge
stellar population reference giant branch. We tested the method on the well
known Baade's and Sgr I windows: the 2MASS mean extinction values in these
fields agreed well with those of previous studies. We confirm the existence of
low-extinction windows in the regions studied, as local minima in the A_K maps
reaching A_K values about 2 standard deviations below the mean values found in
the neighbouring areas. Schlegel et al.'s (1998) FIR extinction maps, which
integrate dust contributions throughout the Galaxy, are structurally similar to
those derived with 2MASS photometry in the two studied windows. We thus
conclude that the dust clouds affecting the 2MASS and FIR maps in these
directions are basically the same and are located on foreground of the bulk of
bulge stars. However, the A_K absolute values differ significantly. In
particular, the FIR extinction values for W359.4-3.1 are a factor ~1.45 larger
than those derived from the 2MASS photometry. Possible explanations of this
effect are discussed. The lower Galactic latitudes of the low-extinction
windows W359.4-3.1 and W0.2-2.1, as compared to Baade's Window, make them
promising targets for detailed studies of more central bulge regions.Comment: 8 pages, 10 figures, LaTeX, aa.cls. To appear in Astron. & Astroph.,
in pres
- …