25,096 research outputs found
What do gas-rich galaxies actually tell us about modified Newtonian dynamics?
It has recently been claimed that measurements of the baryonic Tully-Fisher
relation (BTFR), a power-law relationship between the observed baryonic masses
and outer rotation velocities of galaxies, support the predictions of modified
Newtonian dynamics for the slope and scatter in the relation, while challenging
the cold dark matter (CDM) paradigm. We investigate these claims, and find
that: 1) the scatter in the data used to determine the BTFR is in conflict with
observational uncertainties on the data; 2) these data do not make strong
distinctions regarding the best-fit BTFR parameters; 3) the literature contains
a wide variety of measurements of the BTFR, many of which are discrepant with
the recent results; and 4) the claimed CDM "prediction" for the BTFR is a gross
oversimplification of the complex galaxy-scale physics involved. We conclude
that the BTFR is currently untrustworthy as a test of CDM.Comment: 5 pages, 2 figures; minor revisions to match published versio
The Shape of Dark Matter Haloes IV. The Structure of Stellar Discs in Edge-on Galaxies
We present optical and near-infrared archival observations of eight edge-on
galaxies. These observations are used to model the stellar content of each
galaxy using the FitSKIRT software package. Using FitSKIRT, we can
self-consistently model a galaxy in each band simultaneously while treating for
dust. This allows us to accurately measure both the scale length and scale
height of the stellar disc, plus the shape parameters of the bulge. By
combining this data with the previously reported integrated magnitudes of each
galaxy, we can infer their true luminosities. We have successfully modelled
seven out of the eight galaxies in our sample. We find that stellar discs can
be modelled correctly, but have not been able to model the stellar bulge
reliably. Our sample consists for the most part of slow rotating galaxies, and
we find that the average dust layer is much thicker than what is reported for
faster rotating galaxies.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-IV.pd
The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool
We present a new HI modelling tool called \textsc{Galactus}. The program has
been designed to perform automated fits of disc-galaxy models to observations.
It includes a treatment for the self-absorption of the gas. The software has
been released into the public domain. We describe the design philosophy and
inner workings of the program. After this, we model the face-on galaxy NGC2403,
using both self-absorption and optically thin models, showing that
self-absorption occurs even in face-on galaxies. It is shown that the maximum
surface brightness plateaus seen in Paper I of this series are indeed signs of
self-absorption. The apparent HI mass of an edge-on galaxy can be drastically
lower compared to that same galaxy seen face-on. The Tully-Fisher relation is
found to be relatively free from self-absorption issues.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-II.pd
The planetary nebula population in the halo of M87
We investigate the diffuse light in the outer regions of the nearby
elliptical galaxy M87 in the Virgo cluster, using planetary nebulas (PNs) as
tracers. The surveyed areas (0.43 squared degrees) cover M87 up to a radial
distance of 150 kpc, in the ransition region between galaxy halo and
intracluster light (ICL). All PNs are identified through the on-off band
technique using automatic selection criteria based on the distribution of the
detected sources in the colour-magnitude diagram and the properties of their
point-spread function. We extract a catalogue of 688 objects down to
m_5007=28.4, with an estimated residual contamination from foreground stars and
background Lyalpha galaxies, which amounts to ~35% of the sample. This is one
of the largest extragalactic PN samples in number of candidates, magnitude
depth, and radial extent, which allows us to carry out an unprecedented
photometric study of the PN population in the outer regions of M87. We find
that the logarithmic density profile of the PN distribution is shallower than
the surface brightness profile at large radii. This behaviour is consistent
with the superposition of two components associated with the halo of M87 and
with the ICL, which have different luminosity specific PN numbers, the ICL
contributing three times more PNs per unit light. Because of the depth of this
survey we are also able to study the shape of the PN luminosity function (PNLF)
in the outer regions of M87. We find a slope for the PNLF that is steeper at
fainter magnitudes than the standard analytical PNLF formula and adopt a
generalised model that treats the slope as a free parameter. Comparing the PNLF
of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87
PNLF contains fewer bright PNs and has a steeper slope towards fainter
magnitudes.Comment: 16 pages, 13 figures, 5 tables, accepted for publication in A&
Modern Approaches to Exact Diagonalization and Selected Configuration Interaction with the Adaptive Sampling CI Method.
Recent advances in selected configuration interaction methods have made them competitive with the most accurate techniques available and, hence, creating an increasingly powerful tool for solving quantum Hamiltonians. In this work, we build on recent advances from the adaptive sampling configuration interaction (ASCI) algorithm. We show that a useful paradigm for generating efficient selected CI/exact diagonalization algorithms is driven by fast sorting algorithms, much in the same way iterative diagonalization is based on the paradigm of matrix vector multiplication. We present several new algorithms for all parts of performing a selected CI, which includes new ASCI search, dynamic bit masking, fast orbital rotations, fast diagonal matrix elements, and residue arrays. The ASCI search algorithm can be used in several different modes, which includes an integral driven search and a coefficient driven search. The algorithms presented here are fast and scalable, and we find that because they are built on fast sorting algorithms they are more efficient than all other approaches we considered. After introducing these techniques, we present ASCI results applied to a large range of systems and basis sets to demonstrate the types of simulations that can be practically treated at the full-CI level with modern methods and hardware, presenting double- and triple-ζ benchmark data for the G1 data set. The largest of these calculations is Si2H6 which is a simulation of 34 electrons in 152 orbitals. We also present some preliminary results for fast deterministic perturbation theory simulations that use hash functions to maintain high efficiency for treating large basis sets
Probable Gravitational Microlensing towards the Galatic Bulge
The MACHO project carries out regular photometric monitoring of millions of
stars in the Magellanic Clouds and Galactic Bulge, to search for very rare
gravitational microlensing events due to compact objects in the galactic halo
and disk. A preliminary analysis of one field in the Galactic Bulge, containing
{} stars observed for 190 days, reveals four stars which show
clear evidence for brightenings which are time-symmetric, achromatic in our two
passbands, and have shapes consistent with gravitational microlensing. This is
significantly higher than the event expected from microlensing by
known stars in the disk. If all four events are due to microlensing, a 95\%
confidence lower limit on the optical depth towards our bulge field is , and a ``best fit" value is ,where is the detection efficiency of the
experiment, and . If the true optical depth is close to the
``best fit" value, possible explanations include a ``maximal" disk which
accounts for most of the galactic circular velocity at the solar radius, a halo
which is centrally concentrated, or bulge-bulge microlensing.Comment: submitted to Astrophysical Journal Letters, 10 pages text as
uuencoded compressed PostScript, 5 figures and paper also available via
anonymous ftp from merlin.anu.edu.au in /pub/kcf/mach
A Normal Stellar Disk in the Galaxy Malin 1
Since its discovery, Malin 1 has been considered the prototype and most
extreme example of the class of giant low surface brightness disk galaxies.
Examination of an archival Hubble Space Telescope I-band image reveals that
Malin 1 contains a normal stellar disk that was not previously recognized,
having a central I-band surface brightness of mu_0 = 20.1 mag arcsec^-2 and a
scale length of 4.8 kpc. Out to a radius of ~10 kpc, the structure of Malin 1
is that of a typical SB0/a galaxy. The remarkably extended, faint outer
structure detected out to r~100 kpc appears to be a photometrically distinct
component and not a simple extension of the inner disk. In terms of its disk
scale length and central surface brightness, Malin 1 was originally found to be
a very remote outlier relative to all other known disk galaxies. The presence
of a disk of normal size and surface brightness in Malin 1 suggests that such
extreme outliers in disk properties probably do not exist, but underscores the
importance of the extended outer disk regions for a full understanding of the
structure and formation of spiral galaxies.Comment: 13 pages, 7 figures. To appear in AJ. Typographical error correcte
XMM-Newton Archival Study of the ULX Population in Nearby Galaxies
We present the results of an archival XMM-Newton study of the bright X-ray
point sources (L_X > 10^38 erg/s) in 32 nearby galaxies. From our list of
approximately 100 point sources, we attempt to determine if there is a
low-state counterpart to the Ultraluminous X-ray (ULX) population, searching
for a soft-hard state dichotomy similar to that known for Galactic X-ray
binaries and testing the specific predictions of the IMBH hypothesis. To this
end, we searched for "low-state" objects, which we defined as objects within
our sample which had a spectrum well fit by a simple absorbed power law, and
"high-state" objects, which we defined as objects better fit by a combined
blackbody and a power law. Assuming that ``low-state'' objects accrete at
approximately 10% of the Eddington luminosity (Done & Gierlinski 2003) and that
"high-state" objects accrete near the Eddington luminosity we further divided
our sample of sources into low and high state ULX sources. We classify 16
sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic
black hole systems, the spectral indices, Gamma, of the low-state objects, as
well as the luminosities, tend to be lower than those of the high-state
objects. The observed range of blackbody temperatures for the high state is
0.1-1 keV, with the most luminous systems tending toward the lowest
temperatures. We therefore divide our high-state ULXs into candidate IMBHs
(with blackbody temperatures of approximately 0.1 keV) and candidate stellar
mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of
the candidate stellar mass BHs have spectra that are well-fit by a
Comptonization model, a property similar of Galactic BHs radiating in the
"very-high" state near the Eddington limit.Comment: 54 pages, submitted to ApJ (March 2005), accepted (May 2006); changes
to organization of pape
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