527 research outputs found
Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023
Extended Red Emission (ERE) was recently attributed to the photo-luminescence
of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH), or
charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust
emission in the North-West and South photo-dissociation regions of the
reflection nebula NGC 7023.Using a blind signal separation method, we extracted
the map of ERE from images obtained with the Hubble Space Telescope, and at the
Canada France Hawaii Telescope. We compared the extracted ERE image to the
distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral
and ionized PAHs (PAH and PAH) obtained with the Spitzer Space
Telescope and the Infrared Space Observatory. ERE is dominant in transition
regions where VSGs are being photo-evaporated to form free PAH molecules, and
is not observed in regions dominated by PAH. Its carrier makes a minor
contribution to the mid-IR emission spectrum. These results suggest that the
ERE carrier is a transition species formed during the destruction of VSGs.
Singly ionized PAH dimers appear as good candidates but PAH molecules
seem to be excluded.Comment: Accepted for publication in A&
The lower mass function of the young open cluster Blanco 1: from 30 M_(Jup) to 3 M_â
Aims. We performed a deep wide field optical survey of the young (~100â150 Myr) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.
Methods. The survey covers 2.3 square degrees in the I and z-bands down to I â z â 24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I, I â z) CMD
relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy.
Results. We find that our survey should contain about 57% of the cluster members in the 0.03â0.6 M_â mass range, including 30â40 brown dwarfs. The candidateâs radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03 M_â
and 0.6 M_â, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index α = 0.69 ± 0.15. Over the whole mass domain, from 0.03 M_â to 3 M_â, the mass function is better fitted by a log-normal function with m_0 = 0.36 ± 0.07 M_â and Ï = 0.58 ± 0.06.
Conclusions. Comparison between the Blanco 1 mass function, other young open clustersâ MF, and the galactic disc MF suggests that
the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications
of this result on theories developed to date to explain the origin of the mass distribution
The lower mass function of young open clusters
We report new estimates for the lower mass function of 5 young open clusters
spanning an age range from 80 to 150 Myr. In all studied clusters, the mass
function across the stellar/substellar boundary (~0.072 Mo) and up to 0.4 Mo is
consistent with a power-law with an exponent alpha of -0.5 +/- 0.1, i.e., dN/dM
~ M**(-0.5).Comment: 8 pages, 4 figure
Deep imaging of the shell elliptical galaxy NGC3923 with MegaCam
Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number
of stellar shells, probable remnants of an accreted galaxy. Despite its
uniqueness, the deepest images of its outskirts come from the 1980s. On the
basis of the modified Newtonian dynamics (MOND), it has recently been predicted
that a new shell lies in this region.
Aims. We obtain the deepest image ever of the galaxy, map the tidal features
in it, and search for the predicted shell.
Methods. The image of the galaxy was taken by the MegaCam camera at the
Canada-France-Hawaii Telescope in the g' band. It reached the
surface-brightness limit of 29 mag/arcsec2. In addition, we reanalyzed an
archival HST image of the galaxy.
Results. We detected up to 42 shells in NGC 3923. This is by far the highest
number among all shell galaxies. We present the description of the shells and
other tidal features in the galaxy. A probable progenitor of some of these
features was discovered. The shell system likely originates from two or more
progenitors. The predicted shell was not detected, but the new image revealed
that the prediction was based on incorrect assumptions and poor data.Comment: 14 pages, 2 tables, 19 figures, accepted for publication in A&
A weak lensing study of the Coma cluster
Due to observational constraints, dark matter determinations in nearby
clusters based on weak lensing are still extremely rare, in spite of their
importance for the determination of cluster properties independent of other
methods. We present a weak lensing study of the Coma cluster (redshift 0.024)
based on deep images obtained at the CFHT. After obtaining photometric
redshifts for the galaxies in our field based on deep images in the u (1x1
deg2), and in the B, V, R and I bands (42'x52'), allowing us to eliminate
foreground galaxies, we apply weak lensing calculations on shape measurements
performed in the u image. We derive a map of the mass distribution in Coma, as
well as the radial shear profile, and the mass and concentration parameter at
various radii. We obtain M_200c = 5.1+4.3-2.1 x10^14 Msun and
c_200c=5.0+3.2-2.5, in good agreement with previous measurements. With deep
wide field images it is now possible to analyze nearby clusters with weak
lensing techniques, thus opening a broad new field of investigation
Mass-detection of a matter concentration projected near the cluster Abell 1942: Dark clump or high-redshift cluster?
A weak-lensing analysis of wide-field - and -band images centered on
the cluster Abell 1942 has uncovered a mass concentration arcminutes
South of the cluster center. A statistical analysis shows that the detections
are highly significant. No strong concentration of bright galaxies is seen at
the position of the mass concentration, though a slight galaxy number
overdensity and a weak extended X-ray source are present about 1' away from its
center.
From the spatial dependence of the tangential alignment around the center of
the mass concentration, we inferred a lower bound on the mass inside a sphere
of radius \ts Mpc of , much higher
than crude mass estimates based on X-ray data. No firm conclusion can be
inferred about the nature of the clump. If it were a high-redshift cluster, the
weak X-ray flux would indicate that it had an untypically low X-ray luminosity
for its mass; if the X-ray emission were physically unrelated to the mass
concentration, this conclusion would be even stronger.
The search for massive halos by weak lensing enables us for the first time to
select halos based on their mass properties only and to detect new types of
objects, e.g., dark halos. The mass concentration in the field of A1942 may be
the first example of such a halo.Comment: Sumitted to A&A Main Journal. 15 pages, 11 figures. 75 Kb gzipped tar
file. Figures with images not included, but available on ftp.iap.fr
/pub/from_users/mellier/A1942: a1942darkclump.ps.gz (2.1 Mb
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