965 research outputs found
Extraction of main levels of a building from a large point cloud
Horizontal levels are references entities, the base of man-made environments. Their creation is the first step for various applications including the BIM (Building Information Modelling). BIM is an emerging methodology, widely used for new constructions, and increasingly applied to existing buildings (scan-to-BIM). The as-built BIM process is still mainly manual or semi-automatic and therefore is highly time-consuming. The automation of the as-built BIM is a challenging topic among the research community. This study is part of an ongoing research into the scan-to-BIM process regarding the extraction of the principal structure of a building. More specifically, here we present a strategy to automatically detect the building levels from a large point cloud obtained with a terrestrial laser scanner survey. The identification of the horizontal planes is the first indispensable step to produce an as-built BIM model. Our algorithm, developed in C++, is based on plane extraction by means of the RANSAC algorithm followed by the minimization of the quadrate sum of points-plane distance. Moreover, this paper will take an in-depth look at the influence of data resolution in the accuracy of plane extraction and at the necessary accuracy for the construction of a BIM model. A laser scanner survey of a three floors building composed by 36 scan stations has produced a point cloud of about 550 million points. The estimated plane parameters at different data resolution are analysed in terms of distance from the full points cloud resolution
Dynamical Cusp Regeneration
After being destroyed by a binary supermassive black hole, a stellar density
cusp can regrow at the center of a galaxy via energy exchange between stars
moving in the gravitational field of the single, coalesced hole. We illustrate
this process via high-accuracy N-body simulations. Regeneration requires
roughly one relaxation time and the new cusp extends to a distance of roughly
one-fifth the black hole's influence radius, with density rho ~ r^{-7/4}; the
mass in the cusp is of order 10% the mass of the black hole. Growth of the cusp
is preceded by a stage in which the stellar velocity dispersion evolves toward
isotropy and away from the tangentially-anisotropic state induced by the
binary. We show that density profiles similar to those observed at the center
of the Milky Way and M32 can regenerate themselves in several Gyr following
infall of a second black hole; the presence of density cusps at the centers of
these galaxies can therefore not be used to infer that no merger has occurred.
We argue that Bahcall-Wolf cusps are ubiquitous in stellar spheroids fainter
than M_V ~ -18.5 that contain supermassive black holes, but the cusps have not
been detected outside of the Local Group since their angular sizes are less
than 0.1". We show that the presence of a cusp implies a lower limit of
\~10^{-4} per year on the rate of stellar tidal disruptions, and discuss the
consequences of the cusps for gravitational lensing and the distribution of
dark matter on sub-parsec scales.Comment: Accepted for publication in The Astrophysical Journa
Updated Mass Scaling Relations for Nuclear Star Clusters and a Comparison to Supermassive Black Holes
We investigate whether nuclear star clusters and supermassive black holes
follow a common set of mass scaling relations with their host galaxy's
properties, and hence can be considered to form a single class of central
massive object. We have compiled a large sample of galaxies with measured
nuclear star cluster masses and host galaxy properties from the literature and
fit log-linear scaling relations. We find that nuclear star cluster mass,
M_{NC}, correlates most tightly with the host galaxy's velocity dispersion: log
M_{NC} = (2.11 \pm 0.31) log (\sigma/54) + (6.63 \pm 0.09), but has a slope
dramatically shallower than the relation defined by supermassive black holes.
We find that the nuclear star cluster mass relations involving host galaxy (and
spheroid) luminosity and stellar and dynamical mass, intercept with but are in
general shallower than the corresponding black hole scaling relations. In
particular M_{NC} \propto {M}_{Gal,dyn}^{0.55 \pm 0.15}; the nuclear cluster
mass is not a constant fraction of its host galaxy or spheroid mass. We
conclude that nuclear stellar clusters and supermassive black holes do not form
a single family of central massive objects.Comment: 8 pages, 3 figure
The ACS Fornax Cluster Survey. III. Globular Cluster Specific Frequencies of Early-Type Galaxies
The globular cluster (GC) specific frequency (), defined as the number
of GCs per unit galactic luminosity, represents the efficiency of GC formation
(and survival) compared to field stars. Despite the naive expectation that star
cluster formation should scale directly with star formation, this efficiency
varies widely across galaxies. To explore this variation we measure the z-band
GC specific frequency () for 43 early-type galaxies (ETGs) from the
Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) Fornax Cluster
Survey. Combined with the homogenous measurements of in 100 ETGs from
the HST/ACS Virgo Cluster Survey from Peng et al. (2008), we investigate the
dependence of on mass and environment over a range of galaxy
properties. We find that behaves similarly in the two galaxy
clusters, despite the clusters' order-of-magnitude difference in mass density.
The is low in intermediate-mass ETGs (), and increases
with galaxy luminosity. It is elevated at low masses, on average, but with a
large scatter driven by galaxies in dense environments. The densest
environments with the strongest tidal forces appear to strip the GC systems of
low-mass galaxies. However, in low-mass galaxies that are not in strong tidal
fields, denser environments correlate with enhanced GC formation efficiencies.
Normalizing by inferred halo masses, the GC mass fraction,
, is constant for ETGs with stellar masses
, in agreement with previous
studies. The lack of correlation between the fraction of GCs and the nuclear
light implies only a weak link between the infall of GCs and the formation of
nuclei.Comment: 16 pages, 7 figures, 6 tables; accepted by Ap
The ACS Virgo Cluster Survey. XVII. The Spatial Alignment of Globular Cluster Systems With Early-Type Host Galaxies
We study the azimuthal distribution of globular clusters (GCs) in early-type
galaxies and compare them to their host galaxies using data from the ACS Virgo
Cluster Survey. We find that in host galaxies with visible elongation (epsilon
> 0.2) and intermediate to high luminosities (M_z<-19), the GCs are
preferentially aligned along the major axis of the stellar light. The red
(metal-rich) GC subpopulations show strong alignment with the major axis of the
host galaxy, which supports the notion that these GCs are associated with
metal-rich field stars. The metal-rich GCs in lenticular galaxies show signs of
being more strongly associated with disks rather than bulges. Surprisingly, we
find that the blue (metal-poor) GCs can also show the same correlation. If the
metal-poor GCs are part of the early formation of the halo and built up through
mergers, then our results support a picture where halo formation and merging
occur anisotropically, and where the present day major axis is an indicator of
the preferred merging axis.Comment: 8 pages, 7 figures, accepted for publication in The Astrophysical
Journa
Galaxies at the extremes: Ultra-diffuse galaxies in the Virgo Cluster
We report the discovery of three large (R29 >~ 1 arcminute) extremely low
surface brightness (mu_(V,0) ~ 27.0) galaxies identified using our deep,
wide-field imaging of the Virgo Cluster from the Burrell Schmidt telescope.
Complementary data from the Next Generation Virgo Cluster Survey do not resolve
red giant branch stars in these objects down to i=24, yielding a lower distance
limit of 2.5 Mpc. At the Virgo distance, these objects have half-light radii
3-10 kpc and luminosities L_V=2-9x10^7 Lsun. These galaxies are comparable in
size but lower in surface brightness than the large ultradiffuse LSB galaxies
recently identified in the Coma cluster, and are located well within Virgo's
virial radius; two are projected directly on the cluster core. One object
appears to be a nucleated LSB in the process of being tidally stripped to form
a new Virgo ultracompact dwarf galaxy. The others show no sign of tidal
disruption, despite the fact that such objects should be most vulnerable to
tidal destruction in the cluster environment. The relative proximity of Virgo
makes these objects amenable to detailed studies of their structural properties
and stellar populations. They thus provide an important new window onto the
connection between cluster environment and galaxy evolution at the extremes.Comment: 7 pages, 4 figures, to appear in ApJ Letters. Updated with minor
revisions to match accepted versio
Galaxies at the extremes: Ultra-diffuse galaxies in the Virgo Cluster
We report the discovery of three large (R29 >~ 1 arcminute) extremely low
surface brightness (mu_(V,0) ~ 27.0) galaxies identified using our deep,
wide-field imaging of the Virgo Cluster from the Burrell Schmidt telescope.
Complementary data from the Next Generation Virgo Cluster Survey do not resolve
red giant branch stars in these objects down to i=24, yielding a lower distance
limit of 2.5 Mpc. At the Virgo distance, these objects have half-light radii
3-10 kpc and luminosities L_V=2-9x10^7 Lsun. These galaxies are comparable in
size but lower in surface brightness than the large ultradiffuse LSB galaxies
recently identified in the Coma cluster, and are located well within Virgo's
virial radius; two are projected directly on the cluster core. One object
appears to be a nucleated LSB in the process of being tidally stripped to form
a new Virgo ultracompact dwarf galaxy. The others show no sign of tidal
disruption, despite the fact that such objects should be most vulnerable to
tidal destruction in the cluster environment. The relative proximity of Virgo
makes these objects amenable to detailed studies of their structural properties
and stellar populations. They thus provide an important new window onto the
connection between cluster environment and galaxy evolution at the extremes.Comment: 7 pages, 4 figures, to appear in ApJ Letters. Updated with minor
revisions to match accepted versio
The Next Generation Virgo Cluster Survey (NGVS). XVIII. Measurement and Calibration of Surface Brightness Fluctuation Distances for Bright Galaxies in Virgo (and Beyond)
We describe a program to measure surface brightness fluctuation (SBF)
distances to galaxies observed in the Next Generation Virgo Cluster Survey
(NGVS), a photometric imaging survey covering of the Virgo cluster
in the bandpasses with the Canada-France Hawaii Telescope. We
describe the selection of the sample galaxies, the procedures for measuring the
apparent -band SBF magnitude , and the calibration of the absolute
as a function of observed stellar population properties. The
multi-band NGVS data set provides multiple options for calibrating the SBF
distances, and we explore various calibrations involving individual color
indices as well as combinations of two different colors. Within the color range
of the present sample, the two-color calibrations do not significantly improve
the scatter with respect to wide-baseline, single-color calibrations involving
. We adopt the calibration as reference for the present
galaxy sample, with an observed scatter of 0.11 mag. For a few cases that lack
good photometry, we use an alternative relation based on a combination
of and colors, with only a slightly larger observed scatter of
0.12 mag. The agreement of our measurements with the best existing distance
estimates provides confidence that our measurements are accurate. We present a
preliminary catalog of distances for 89 galaxies brighter than
mag within the survey footprint, including members of the background M and W
Clouds at roughly twice the distance of the main body of the Virgo cluster. The
extension of the present work to fainter and bluer galaxies is in progress.Comment: ApJ accepte
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