456 research outputs found
ASTErIsM - Application of topometric clustering algorithms in automatic galaxy detection and classification
We present a study on galaxy detection and shape classification using
topometric clustering algorithms. We first use the DBSCAN algorithm to extract,
from CCD frames, groups of adjacent pixels with significant fluxes and we then
apply the DENCLUE algorithm to separate the contributions of overlapping
sources. The DENCLUE separation is based on the localization of pattern of
local maxima, through an iterative algorithm which associates each pixel to the
closest local maximum. Our main classification goal is to take apart elliptical
from spiral galaxies. We introduce new sets of features derived from the
computation of geometrical invariant moments of the pixel group shape and from
the statistics of the spatial distribution of the DENCLUE local maxima
patterns. Ellipticals are characterized by a single group of local maxima,
related to the galaxy core, while spiral galaxies have additional ones related
to segments of spiral arms. We use two different supervised ensemble
classification algorithms, Random Forest, and Gradient Boosting. Using a sample
of ~ 24000 galaxies taken from the Galaxy Zoo 2 main sample with spectroscopic
redshifts, and we test our classification against the Galaxy Zoo 2 catalog. We
find that features extracted from our pipeline give on average an accuracy of ~
93%, when testing on a test set with a size of 20% of our full data set, with
features deriving from the angular distribution of density attractor ranking at
the top of the discrimination power.Comment: 20 pages, 13 Figures, 8 Tables, Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
Do Globular Clusters Harbor Black Holes?
It has been firmly established that there exists a tight correlation between
the central black hole mass and velocity dispersion (or luminosity) of
elliptical galaxies, ``pseudobulges'' and bulges of galaxies, although the
nature of this correlation still remains unclear. In this letter, we explore
the possibility of extrapolating such a correlation to less massive, spherical
systems like globular clusters. In particular, motivated by the apparent
success in globular cluster M15, we present an estimate of the central black
hole mass for a number of globular clusters with available velocity dispersion
in the literature.Comment: 6 pages, 2 figures, 1 table; accepted for publication in CJA
Masses and M/L Ratios of Bright Globular Clusters in NGC 5128
We present an analysis of the radial velocities and velocity dispersions for 27 bright globular clusters in the nearby elliptical galaxy NGC 5128 (Centaurus A). For 22 clusters we combine our new velocity dispersion measurements with the information on the structural parameters, either from the literature when available or from our own data, in order to derive the cluster masses and mass-to-light (M/L) ratios. The masses range from 1.2 × 105M, typical of Galactic globular clusters, to 1.4 × 107M, similar to more massive dwarf globular transition objects (DGTOs) or ultra compact dwarfs (UCDs) and to nuclei of nucleated dE galaxies. The average M/LV is 3±1, larger than the average M/LV of globular clusters in the Local Group galaxies. The correlations of structural parameters, velocity dispersion, masses and M/LV for the bright globular clusters extend the properties established for the most massive Local Group clusters towards those characteristic of dwarf elliptical galaxy nuclei and DGTOs/UCDs. The detection of the mass-radius and the mass-M/LV relations for the globular clusters with masses greater than ~ 2 × 106M provides the link between "normal” old globular clusters, young massive clusters, and evolved massive object
Chemical evolution of star clusters
I discuss the chemical evolution of star clusters, with emphasis on old
globular clusters, in relation to their formation histories. Globular clusters
clearly formed in a complex fashion, under markedly different conditions from
any younger clusters presently known. Those special conditions must be linked
to the early formation epoch of the Galaxy and must not have occurred since.
While a link to the formation of globular clusters in dwarf galaxies has been
suggested, present-day dwarf galaxies are not representative of the
gravitational potential wells within which the globular clusters formed.
Instead, a formation deep within the proto-Galaxy or within dark-matter
minihaloes might be favoured. Not all globular clusters may have formed and
evolved similarly. In particular, we may need to distinguish Galactic halo from
Galactic bulge clusters.Comment: 27 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 6 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
Central Proper-Motion Kinematics of NGC 6752
We present proper motions derived from WFPC2 imaging for stars in the core of
the peculiar globular cluster NGC 6752. The central velocity dispersion in both
components of the proper motion is 12 km/s. We discuss the implications of this
result as well as the intrinsic difficulties in making such measurements. We
also give an alternative correction for the 34-row problem in the WFPC2 CCDs.Comment: 25 pages, 7 figures, 1 table included. Accepted for publication in A
Probing the presence of a single or binary black hole in the globular cluster NGC 6752 with pulsar dynamics
The five millisecond pulsars that inhabit NGC 6752 display locations or
accelerations that are quite unusual compared to all other pulsars known in
globular clusters. In particular PSR-A, a binary pulsar, lives in the cluster
halo, while PSR-B and PSR-E, located in the core, show remarkably high negative
spin derivatives. This is suggestive that some uncommon dynamical process is at
play in the cluster core that we attribute to the presence of a massive
perturber. We here investigate whether a single intermediate-mass black hole,
lying on the extrapolation of the Mass versus Sigma relation observed in galaxy
spheroids, or a less massive binary consisting of two black holes could play
the requested role. To this purpose we simulated binary-binary encounters
involving PSR-A, its companion star, and the black hole(s). Various scenarios
are discussed in detail. In our close 4-body encounters, a black hole-black
hole binary may attract on a long-term stable orbit a millisecond pulsar.
Timing measurements on the captured satellite-pulsar, member of a hierarchical
triplet, could unambiguously unveil the presence of a black hole(s) in the core
of a globular cluster.Comment: 13 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
The PCA Lens-Finder: application to CFHTLS
We present the results of a new search for galaxy-scale strong lensing
systems in CFHTLS Wide. Our lens-finding technique involves a preselection of
potential lens galaxies, applying simple cuts in size and magnitude. We then
perform a Principal Component Analysis of the galaxy images, ensuring a clean
removal of the light profile. Lensed features are searched for in the residual
images using the clustering topometric algorithm DBSCAN. We find 1098 lens
candidates that we inspect visually, leading to a cleaned sample of 109 new
lens candidates. Using realistic image simulations we estimate the completeness
of our sample and show that it is independent of source surface brightness,
Einstein ring size (image separation) or lens redshift. We compare the
properties of our sample to previous lens searches in CFHTLS. Including the
present search, the total number of lenses found in CFHTLS amounts to 678,
which corresponds to ~4 lenses per square degree down to i=24.8. This is
equivalent to ~ 60.000 lenses in total in a survey as wide as Euclid, but at
the CFHTLS resolution and depth.Comment: 21 pages, 12 figures, accepted for publication on A&
XTE J1550-564: INTEGRAL Observations of a Failed Outburst
The well known black-hole X-ray binary transient XTE J1550-564 underwent an
outburst during the spring of 2003 which was substantially underluminous in
comparison to previous periods of peak activity in that source. In addition,
our analysis shows that it apparently remained in the hard spectral state over
the duration of that outburst. This is again in sharp contrast to major
outbursts of that source in 1998/1999 during which it exhibited an irregular
light curve, multiple state changes and collimated outflows. This leads us to
classify it as a "failed outburst." We present the results of our study of the
spring 2003 event including light curves based on observations from both
INTEGRAL and RXTE. In addition, we studied the evolution of the high-energy
3-300 keV continuum spectrum using data obtained with three main instruments on
INTEGRAL. These spectra are consistent with typical low-hard-state thermal
Comptonization emission. We also consider the 2003 event in the context of a
multi-source, multi-event period-peak luminosity diagram in which it is a clear
outlyer. We then consider the possibility that the 2003 event was due to a
discrete accretion event rather than a limit-cycle instablility. In that
context, we apply model fitting to derive the timescale for viscous propagation
in the disk, and infer some physical characteristics.Comment: 22 pages, 8 figures, to be published in The Astrophysical Journa
Structural Parameters and Dynamical Masses for Globular Clusters in M33
Using high-dispersion spectra from the HIRES echelle spectrograph on the Keck
I telescope, we measure velocity dispersions for 4 globular clusters in M33.
Combining the velocity dispersions with integrated photometry and structural
parameters derived from King-Michie model fits to WFPC2 images, we obtain
mass-to-light ratios for the clusters. The mean value is M/LV = 1.53 +/- 0.18,
very similar to the M/LV of Milky Way and M31 globular clusters. The M33
clusters also fit very well onto the fundamental plane and binding energy -
luminosity relations derived for Milky Way GCs. Dynamically and structurally,
the four M33 clusters studied here appear virtually identical to Milky Way and
M31 GCs.Comment: 25 pages, including 7 figures and 4 tables. Accepted for AJ, Nov 200
The INTEGRAL/SPI response and the Crab observations
The Crab region was observed several times by INTEGRAL for calibration
purposes. This paper aims at underlining the systematic interactions between
(i) observations of this reference source, (ii) in-flight calibration of the
instrumental response and (iii) the development and validation of the analysis
tools of the SPI spectrometer. It first describes the way the response is
produced and how studies of the Crab spectrum lead to improvements and
corrections in the initial response. Then, we present the tools which were
developed to extract spectra from the SPI observation data and finally a Crab
spectrum obtained with one of these methods, to show the agreement with
previous experiments. We conclude with the work still ahead to understand
residual uncertainties in the response.Comment: 4 pages, 4 figures, Proc. of the 5th INTEGRAL Workshop (Feb. 16-20
2004), to be published by ES
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