502 research outputs found
Matched filter for multi-transducers resonant GW antennas
We analyze two kinds of matched filters for data output of a spherical
resonant GW detector. In order to filter the data of a real sphere, a strategy
is proposed, firstly using an omnidirectional in-line filter, which is supposed
to select periodograms with excitations, secondly by performing a directional
filter on such selected periodograms, finding the wave arrival time, direction
and polarization. We point out that, as the analytical simplifications
occurring in the ideal 6 transducers TIGA sphere do not hold for a real sphere,
using a 5 transducers configuration could be a more convenient choice.Comment: 15 pages and 4 figures, version accepted for publication in PR
A Population of Massive Globular Clusters in NGC 5128
We present velocity dispersion measurements of 14 globular clusters in NGC
5128 (Centarus A) obtained with the MIKE echelle spectrograph on the 6.5m
Magellan Clay telescope. These clusters are among the most luminous globular
clusters in NGC 5128 and have velocity dispersions comparable to the most
massive clusters known in the Local Group, ranging from 10 - 30 km/s. We
describe in detail our cross-correlation measurements, as well as simulations
to quantify the uncertainties. These 14 globular clusters are the brightest NGC
5128 globular clusters with surface photometry and structural parameters
measured from the Hubble Space Telescope. We have used these measurements to
derive masses and mass-to-light ratios for all of these clusters and establish
that the fundamental plane relations for globular clusters extend to an order
of magnitude higher mass than in the Local Group. The mean mass-to-light ratio
for the NGC 5128 clusters is ~3+/-1, higher than measurements for all but the
most massive Local Group clusters. These massive clusters begin to bridge the
mass gap between the most massive star clusters and the lowest-mass galaxies.
We find that the properties of NGC 5128 globular clusters overlap quite well
with the central properties of nucleated dwarf galaxies and ultracompact dwarf
galaxies. As six of these clusters also show evidence for extratidal light, we
hypothesize that at least some of these massive clusters are the nuclei of
tidally stripped dwarfs.Comment: ApJ Accepted, 15 pages, 9 figures, uses emulateapj.st
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
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
A 20 Thousand Solar Mass Black Hole in the Stellar Cluster G1
We present the detection of a 2.0(+1.4,-0.8)x10^4 solar mass black hole (BH)
in the stellar cluster G1 (Mayall II), based on data taken with the Space
Telescope Imaging Spectrograph onboard the Hubble Space Telescope. G1 is one of
the most massive stellar clusters in M31. The central velocity dispersion (25
kms) and the measured BH mass of G1 places it on a linear extrapolation of the
correlation between BH mass and bulge velocity dispersion established for
nearby galaxies. The detection of a BH in this low-mass stellar system suggests
that (1) the most likely candidates for seed massive BHs come from stellar
clusters, (2) there is a direct link between massive stellar clusters and
normal galaxies, and (3) the formation process of both bulges and massive
clusters is similar due to their concordance in the M_BH/sigma relation.
Globular clusters in our Galaxy should be searched for central BHs.Comment: 4 pages, accepted in The Astrophysical Journal Letters, October 200
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
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 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
- …