1,909 research outputs found
The dark and baryonic matter content of low surface brightness disk galaxies
We present mass models of a sample of 19 low surface brightness (LSB)
galaxies and compare the properties of their constituent mass components with
those of a sample of high surface brightness (HSB) galaxies.We find that LSB
galaxies are dark matter dominated. Their halo parameters are only slightly
affected by assumptions on stellar mass-to-light ratios. Comparing LSB and HSB
galaxies we find that mass models derived using the maximum disk hypothesis
result in the disks of LSB galaxies having systematically higher stellar
mass-to-light ratios than HSB galaxies of similar rotation velocity. This is
inconsistent with all other available evidence on the evolution of LSB
galaxies. We argue therefore that the maximum disk hypothesis does not provide
a representative description of the LSB galaxies and their evolution. Mass
models with stellar mass-to-light ratios determined by the colors and stellar
velocity dispersions of galactic disks imply that LSB galaxies have dark matter
halos that are more extended and less dense than those of HSB galaxies. Surface
brightness is thus related to the halo properties. LSB galaxies are slowly
evolving, low density and dark matter dominated galaxies.Comment: 23 pages Latex, 12 postscript figures, uses mn.sty. Accepted for
publication in MNRA
Submillimeter observations of OH and CH in M42
The (sup 2) pi sub 1/2 (J = 3/2 to 1/2) transitions of OH at 163.12 and 163.40 micro m have been detected and upper limits have been obtained for the (sup 2) pi sub 3/2 (J = 3/2 to 1/2) transitions of CH at 149.09 and 149.39 micro m, in observations of the Kleinmann-Low Nebula of Orion. All four flux levels lie between 1 and 1.2 x 10 to the 17th power/sq.cm. The OH lines are bright when compared to the lower, (sup 2) pi sub 3/2 (J = 5/2 to 3/2) fluxes reported and imply that the 119 micro m emission observed is partially self-absorbed. The combined results provide strong constraints. Taken together with existing data on molecular hydrogen and CO and recent data on other OH transition, they suggest OH emission from post-shock regions at temperatures T approx 1000 k, densities approx. 7 x 10 to the 6th powr/cu cm N sub OH approx 80/cu cm optically thick for the (sup 2) pi sub 3/2 (J = 5/2 to 3/2), 119 micro m but only partially self-absorbing in the (J = 7/2 to 3/2), 84 micro m transitions over a Doppler velocity bandwidth of 30 km/sec. The OH column density is N sub OH approx 4 x 10 to the 16th powr/sq cm. in the emitting regions which occupy a fraction of approx 0.1 of a 1' x 1' field of view centered on the Becklin-Neugebauer source. The CO (J = 31 to 30), 84 micro m transition appears to lie sufficiently close to one of the 84 micro m OH line components to be partially absorbed as well, through a Bowen-type mechanism
Two Dimensional Velocity Fields of Low Surface Brightness Galaxies
We present high resolution two dimensional velocity fields from integral
field spectroscopy along with derived rotation curves for nine low surface
brightness galaxies. This is a positive step forward in terms of both data
quality and number of objects studied. We fit NFW and pseudo-isothermal halo
models to the observations. We find that the pseudo-isothermal halo better
represents the data in most cases than the NFW halo, as the resulting
concentrations are lower than would be expected for LCDM.Comment: 2 pages, 1 figure, to appear in the XXIst IAP Colloquium "Mass
Profiles and Shapes of Cosmological Structures", Paris 4-9 July 2005, (Eds.)
G. Mamon, F. Combes, C. Deffayet, B. Fort, (EDP Sciences
COMPTEL Observations of AGN at MeV-Energies
The COMPTEL experiment aboard CGRO, exploring the previously unknown sky at
MeV-energies, has so far detected 10 Active Galactic Nuclei (AGN): 9 blazars
and the radio galaxy Centaurus A. No Seyfert galaxy has been found yet. With
these results COMPTEL has opened the field of extragalactic Gamma-ray astronomy
in the MeV-band.Comment: 4 pages, 2 figures including 1 color plot, to appear in the
Proceedings of the 3rd INTEGRAL Workshop "The Extreme Universe", held in
Taormina, Italy, 14-18 September 199
Mass Density Profiles of LSB Galaxies
We derive the mass density profiles of dark matter halos that are implied by
high spatial resolution rotation curves of low surface brightness galaxies. We
find that at small radii, the mass density distribution is dominated by a
nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a,
the distribution of inner slopes a is strongly peaked around a = -0.2. This is
significantly shallower than the cuspy a < -1 halos found in CDM simulations.
While the observed distribution of alpha does have a tail towards such extreme
values, the derived value of alpha is found to depend on the spatial resolution
of the rotation curves: a ~ -1 is found only for the least well resolved
galaxies. Even for these galaxies, our data are also consistent with constant
density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the
illusion of steep cusps when insufficiently resolved. Consequently, there is no
clear evidence for a cuspy halo in any of the low surface brightness galaxies
observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and
emulateapj5.sty Typo in Eq 1 fixe
The Pricing of Housing and Mortgage Services for First-Time Versus Repeat Homebuyers
This study examines efficiency in the pricing of housing and mortgage services across first-time and repeat homebuyers. A logit model is used to test a number of variables for significant differences across first-time buyers and repeat buyers for a sample of brokered real estate sales. The results show that the housing market is somewhat less than completely efficient in providing its services. The logit results for the adjustable-rate mortgage segment show that first-time homebuyers are more likely to be associated with a higher sale price per square foot and higher discount points than repeat buyers. The results show that first-time homebuyers tend to be younger and have less household income than repeat homebuyers. For the full sample of data, the results show that higher sale price/square foot ratios and lower downpayment/sale price ratios are more likely to be associated with first-time homebuyers. For the fixed-rate mortgage segment of the data, the only distinguishing variables are downpayment/sale price and buyer age (both lower for first-time homebuyers). Type of buyer cannot be distinguished by sale price/square foot, contract interest rate, discount points, and other variables. For the adjustable-rate mortgage segment of the data, type of buyer can be distinguished by sale sprice/square foot (higher for first-time buyers), downpayment/sale price (lower for first-time buyers), and discount points (higher for first-time buyers).
COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission
The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy
range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have
been analyzed. During the first two years PKS 0528+134 was most significantly
detected at energies above 3 MeV. During the last year there is only evidence
for the quasar at energies below 3 MeV indicating a spectral change. The
time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well
represented by a power-law shape. Spectra collected from different
observational periods reveal different power-law shapes: a hard state during
flaring observations reported by EGRET, and a soft state otherwise. The
combined simultaneous EGRET and COMPTEL spectra indicate these two spectral
states as well. During low intensisty gamma-ray phases no spectral break is
obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray
flaring phases however, the combined COMPTEL and EGRET data require a spectral
bending at MeV-energies. By fitting broken power-law functions the best-fit
values for the break in photon index range between 0.6 and 1.7, and for the
break energy between ~5 MeV and ~20 MeV. Because the flux values measured by
COMPTEL below 3 MeV in both states are roughly equal, the observations would be
consistent with an additional spectral component showing up during gamma-ray
flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a
high-energy electron-positron population with a low-energy cutoff in their bulk
Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for
gamma-ray flaring phases shows that the major energy release across the entire
electro-magnetic spectrum is measured at MeV-energies.Comment: 10 pages, 8 postscript figures, latex, to appear in: A&A 328, 33
(1997
Gas Mass Fractions and the Evolution of Spiral Galaxies
We show that the gas mass fraction of spiral galaxies is strongly correlated
with luminosity and surface brightness. It is not correlated with linear size.
Gas fraction varies with luminosity and surface brightness at the same rate,
indicating evolution at fixed size.
Dim galaxies are clearly less evolved than bright ones, having consumed only
of their gas. This resolves the gas consumption paradox, since there
exist many galaxies with large gas reservoirs. These gas rich galaxies must
have formed the bulk of their stellar populations in the last half of a Hubble
time. The existence of such immature galaxies at indicates that either
galaxy formation is a lengthy or even ongoing process, or the onset of
significant star formation can be delayed for arbitrary periods in tenuous gas
disks.Comment: Accepted for publication in the Astrophysical Journal. 18 pages
AASLaTeX + 9 figure
A Multi-Criteria Model for Corporate Property Evaluation
A critical concern for corporations pursuing geographical expansion strategies involves property evaluation. In order for expansion to be successful, corporations must identify cities and properties that offer a good fit with the firm's corporate strategy. Unfortunately, little has been written to aid corporations in making complex property selection decisions. This paper presents a model that combines the concepts of strategic management, the management science technique of goal programming, and micro computer technology to provide managers with a more effective and efficient method for evaluation property and making selection decisions.
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