742 research outputs found
Cluster Cores, Gravitational Lensing, and Cosmology
Many multiply--imaged quasars have been found over the years, but none so far
with image separation in excess of 8\arcsec. The absence of such large
splittings has been used as a test of cosmological models: the standard Cold
Dark Matter model has been excluded on the basis that it predicts far too many
large--separation double images. These studies assume that the lensing
structure has the mass profile of a singular isothermal sphere. However, such
large splittings would be produced by very massive systems such as clusters of
galaxies, for which other gravitational lensing data suggest less singular mass
profiles. Here we analyze two cases of mass profiles for lenses: an isothermal
sphere with a finite core radius (density , and a Hernquist profile (). We find that small core radii
kpc, as suggested by the cluster data, or large a \gsim 300 h^{-1} kpc, as
needed for compatibility with gravitational distortion data, would reduce the
number of large--angle splittings by an order of magnitude or more. Thus, it
appears that these tests are sensitive both to the cosmological model (number
density of lenses) and to the inner lens structure, which is unlikely to depend
sensitively on the cosmology, making it difficult to test the cosmological
models by large--separation quasar lensing until we reliably know the structure
of the lenses themselves.Comment: 17 pages, uuencoded compressed tarred postscript file including text
and 1 figure. To appear in January 20, 1996 issue of ApJ Letter
A Collision of Subclusters in Abell 754
We present direct evidence of a collision of subclusters in the galaxy
cluster Abell 754. Our comparison of new optical data and archival ROSAT PSPC
X-ray data reveal three collision signatures predicted by n-body/hydrodynamical
simulations of hierarchical cluster evolution. First, there is strong evidence
of a non-hydrostatic process; neither of the two major clumps in the galaxy
distribution lies on the off-center peak of the X-ray emission from the
intracluster gas. Second, the peak of the X-ray emission is elongated
perpendicular to the collision axis defined by the centroids of the two galaxy
clumps. Third, there is evidence of compression-heated gas; one of A754's two
X-ray temperature components (Henry & Briel 1995) is among the hottest observed
in any cluster and hotter than that inferred from the velocity dispersion of
the associated galaxy clump. These signatures are consistent with the
qualitative features of simulations (Evrard 1990a,b) in which two subclusters
have collided in the plane of the sky during roughly the last Gyr. The
detection of such collisions is crucial for understanding both the dynamics of
individual clusters and the underlying cosmology. First, for systems like A754,
estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium
and isothermality is incorrect and may produce the discrepancies sometimes
found between X-ray masses and those derived from gravitational lens models
(Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in which
subclusters have collided in the last Gyr is especially sensitive to the mean
mass density parameter Omega_0 (cf. Richstone et al. 1992; Evrard et al. 1993;
Lacey & Cole 1993). With a large, well-defined cluster sample, it will be
possible to place a new and powerful constraint on cosmological models.Comment: 4 pages + 1 color figure (Postscript). Accepted for Publication in
ApJ Letter
Is there a relationship between the mass of a SMBH and the kinetic energy of its host elliptical galaxy?
We use a restricted sample of elliptical galaxies, whose kinematical
parameters inside the semimajor axis were calculated correcting the effect of
the integration of the light along the line of sight, in order to analyze a
possible relationship between the mass of a Supermassive Black Hole (SMBH) and
the kinetic energy of random motions in the host galaxy. We find with depending on
the different fitting methods and samples used. This result could be
interpreted as a new fundamental relationship or as a new way to explain the
old law. In fact, the relations of the velocity dispersion
both with the mass of the SMBH () and with the
mass of the host galaxy () induce us to infer an
almost direct proportionality: . A similar
relationship is found for the total kinetic energy involving the rotation
velocity too.Comment: 16 pages, 3 figures, 1 Table, accepted for publication in IJMP
Emotional expressiveness of 5â6 month-old infants born very premature versus full-term at initial exposure to weaning foods
Facial expressions of 5â6 month-old infants born preterm and at term were compared while tasting for the first time solid foods (two fruit and two vegetable purĂ©es) given by the mother. Videotapes of facial reactions to these foods were objectively coded during the first six successive spoons of each test food using Baby FACS and subjectively rated by naĂŻve judges. Infant temperament was also assessed by the parents using the Infant Behaviour Questionnaire. Contrary to our expectations, infants born preterm expressed fewer negative emotions than infants born full-term. NaĂŻve judges rated infants born preterm as displaying more liking than their full-term counterparts when tasting the novel foods. The analysis of facial expressions during the six spoonfuls of four successive meals (at 1-week intervals) suggested a familiarization effect with the frequency of negative expressions decreasing after tasting the second spoon, regardless of infant age, type of food and order of presentation. Finally, positive and negative dimensions of temperament reported by the parents were related with objective and subjective coding of affective reactions toward foods in infants born preterm or full-term. Our research indicates that premature infants are more accepting of novel foods than term infants and this could be used for supporting the development of healthy eating patterns in premature infants. Further research is needed to clarify whether reduced negativity by infants born prematurely to the exposure to novel solid foods reflects a reduction of an adaptive avoidant behaviour during the introduction of novel foods
Dark energy constraints and correlations with systematics from CFHTLS weak lensing, SNLS supernovae Ia and WMAP5
We combine measurements of weak gravitational lensing from the CFHTLS-Wide
survey, supernovae Ia from CFHT SNLS and CMB anisotropies from WMAP5 to obtain
joint constraints on cosmological parameters, in particular, the dark energy
equation of state parameter w. We assess the influence of systematics in the
data on the results and look for possible correlations with cosmological
parameters.
We implement an MCMC algorithm to sample the parameter space of a flat CDM
model with a dark-energy component of constant w. Systematics in the data are
parametrised and included in the analysis. We determine the influence of
photometric calibration of SNIa data on cosmological results by calculating the
response of the distance modulus to photometric zero-point variations. The weak
lensing data set is tested for anomalous field-to-field variations and a
systematic shape measurement bias for high-z galaxies.
Ignoring photometric uncertainties for SNLS biases cosmological parameters by
at most 20% of the statistical errors, using supernovae only; the parameter
uncertainties are underestimated by 10%. The weak lensing field-to-field
variance pointings is 5%-15% higher than that predicted from N-body
simulations. We find no bias of the lensing signal at high redshift, within the
framework of a simple model. Assuming a systematic underestimation of the
lensing signal at high redshift, the normalisation sigma_8 increases by up to
8%. Combining all three probes we obtain -0.10<1+w<0.06 at 68% confidence
(-0.18<1+w<0.12 at 95%), including systematic errors. Systematics in the data
increase the error bars by up to 35%; the best-fit values change by less than
0.15sigma. [Abridged]Comment: 14 pages, 10 figures. Revised version, matches the one to be
published in A&A. Modifications have been made corresponding to the referee's
suggestions, including reordering of some section
A Hot Spot in Coma
We study the temperature structure of the central part (r<18' ~0.7 h50**-1
Mpc) of the Coma cluster of galaxies using ASCA data. Two different analysis
methods produce results in good agreement with each other and reveal the
presence of interesting structures in the gas temperature distribution.
Globally, the average temperature in the center of the cluster is 9.0 +/- 0.6
keV in good agreement with previous results. Superimposed on this, we find a
cool area with temperatures of 4-6 keV associated with a filament of X-ray
emission extending southeast from the cluster center detected by Vikhlinin and
coworkers. We also find a hot spot with a temperature of around 13 keV
displaced north from the central peak of emission. The distribution of the gas
temperatures and relative specific entropies suggests that the cool features
are most likely gas stripped from a galaxy group centered on NGC 4874 falling
toward the core from outside, while the hot spot located ``ahead'' of this
in-falling gas is due to shock heating. Thus our results suggest that we are
observing Coma during a minor merger with a small group of galaxies associated
with NGC 4874 shortly before the initial core passage.Comment: 5 pages, 2 figures, accepted for publication in the Astrophysical
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