1,683 research outputs found
Testing Modified Newtonian Dynamics with Rotation Curves of Dwarf and Low Surface Brightness Galaxies
Dwarf and low surface brightness galaxies are ideal objects to test modified
Newtonian dynamics (MOND), because in most of these galaxies the accelerations
fall below the threshold below where MOND supposedly applies. We have selected
from the literature a sample of 27 dwarf and low surface brightness galaxies.
MOND is successful in explaining the general shape of the observed rotation
curves for roughly three quarters of the galaxies in the sample presented here.
However, for the remaining quarter, MOND does not adequately explain the
observed rotation curves. Considering the uncertainties in distances and
inclinations for the galaxies in our sample, a small fraction of poor MOND
predictions is expected and is not necessarily a problem for MOND. We have also
made fits taking the MOND acceleration constant, a_0, as a free parameter in
order to identify any systematic trends. We find that there appears to be a
correlation between central surface brightness and the best-fit value of a_0,
in the sense that lower surface brightness galaxies tend to have lower a_0.
However, this correlation depends strongly on a small number of galaxies whose
rotation curves might be uncertain due to either bars or warps. Without these
galaxies, there is less evidence of a trend, but the average value we find for
a_0 ~ 0.7*10^-8 cm s^-2 is somewhat lower than derived from previous studies.
Such lower fitted values of a_0 could occur if external gravitational fields
are important.Comment: 12 pages, accepted for publication in Ap
Testing Modified Newtonian Dynamics with Low Surface Brightness Galaxies --Rotation curve fits-
We present MOND (Modified Newtonian Dynamics) fits to 15 rotation curves of
LSB galaxies. Good fits are readily found, although for a few galaxies minor
adjustments to the inclination are needed. Reasonable values for the stellar
mass-to-light ratios are found, as well as an approximately constant value for
the total (gas and stars) mass-to-light ratio. We show that the LSB galaxies
investigated here lie on the one, unique Tully-Fisher relation, as predicted by
MOND. The scatter on the Tully-Fisher relation can be completely explained by
the observed scatter in the total mass-to-light ratio. We address the question
of whether MOND can fit any arbitrary rotation curve by constructing a
plausible fake model galaxy. While MOND is unable to fit this hypothetical
galaxy, a normal dark halo fit is readily found, showing that dark matter fits
are much less selective in producing fits. The good fits to rotation curves of
LSB galaxies support MOND, especially as these are galaxies with large mass
discrepancies deep in the MOND regime.Comment: Accepted for publication in Astrophysical Journal 14 page
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
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
Star Formation and Tidal Encounters with the Low Surface Brightness Galaxy UGC 12695 and Companions
We present VLA H I observations of the low surface brightness galaxy UGC
12695 and its two companions, UGC 12687 and a newly discovered dwarf galaxy
2333+1234. UGC 12695 shows solid body rotation but has a very lopsided
morphology of the H I disk, with the majority of the H I lying in the southern
arm of the galaxy. The H I column density distribution of this very blue, LSB
galaxy coincides in detail with its light distribution. Comparing the H I
column density of UGC 12695 with the empirical (but not well understood) value
of Sigma_c = 10E21 atoms/cm^2 found in, i.e., Skillman's 1986 paper shows the
star formation to be a local affair, occurring only in those regions where the
column density is above this star formation threshold. The low surface
brightness nature of this galaxy could thus be attributed to an insufficient
gas surface density, inhibiting star formation on a more global scale.
Significantly, though, the Toomre criterion places a much lower critical
density on the galaxy (+/-10E20 atoms/cm^2), which is shown by the galaxy's low
SFR to not be applicable.
Within a projected distance of 300kpc/30kms of UGC 12695 lie two companion
galaxies - UGC 12687, a high surface brightness barred spiral galaxy, and
2333+1234, a dwarf galaxy discovered during this investigation. The close
proximity of the three galaxies, combined with UGC 12695's extremely blue color
and regions of localized starburst and UGC 12687's UV excess bring to mind
mutually induced star formation through tidal activity.Comment: 14 pages, 8 figures (2 color), To be published in A.J., May 2000
The Stellar Populations of Low Surface Brightness Galaxies
Near-infrared (NIR) K' images of a sample of five low surface brightness disc
galaxies (LSBGs) were combined with optical data, with the aim of constraining
their star formation histories. Both red and blue LSBGs were imaged to enable
comparison of their stellar populations. For both types of galaxy strong colour
gradients were found, consistent with mean stellar age gradients. Very low
stellar metallicities were ruled out on the basis of metallicity-sensitive
optical-NIR colours. These five galaxies suggest that red and blue LSBGs have
very different star formation histories and represent two independent routes to
low B band surface brightness. Blue LSBGs are well described by models with
low, roughly constant star formation rates, whereas red LSBGs are better
described by a `faded disc' scenario.Comment: 5 pages LaTeX; 2 embedded figures; MNRAS Letters, Accepte
A search for Low Surface Brightness galaxies in the near-infrared I. Selection of the sample
A sample of about 3,800 Low Surface Brightness (LSB) galaxies was selected
using the all-sky near-infrared (J, H and K_s-band) 2MASS survey. The selected
objects have a mean central surface brightness within a 5 arcsec radius around
their centre fainter than 18 mag/sq.arcsec in the K_s band, making them the
lowest surface brightness galaxies detected by 2MASS. A description is given of
the relevant properties of the 2MASS survey and the LSB galaxy selection
procedure, as well as of basic photometric properties of the selected objects.
The latter properties are compared to those of other samples of galaxies, of
both LSBs and `classical' high surface brightness (HSB) objects, which were
selected in the optical. The 2MASS LSBs have a (B_T_c)-(K_T) colour which is on
average 0.9 mag bluer than that of HSBs from the NGC. The 2MASS sample does not
appear to contain a significant population of red objects.Comment: accepted for publication in Astronomy and Astrophysics on 24/2/2003;
62 page
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
Local Group dSph radio survey with ATCA (III): Constraints on Particle Dark Matter
We performed a deep search for radio synchrotron emissions induced by weakly
interacting massive particles (WIMPs) annihilation or decay in six dwarf
spheroidal (dSph) galaxies of the Local Group. Observations were conducted with
the Australia Telescope Compact Array (ATCA) at 16 cm wavelength, with an rms
sensitivity better than 0.05 mJy/beam in each field. In this work, we first
discuss the uncertainties associated with the modeling of the expected signal,
such as the shape of the dark matter (DM) profile and the dSph magnetic
properties. We then investigate the possibility that point-sources detected in
the proximity of the dSph optical center might be due to the emission from a DM
cuspy profile. No evidence for an extended emission over a size of few arcmin
(which is the DM halo size) has been detected. We present the associated bounds
on the WIMP parameter space for different annihilation/decay final states and
for different astrophysical assumptions. If the confinement of electrons and
positrons in the dSph is such that the majority of their power is radiated
within the dSph region, we obtain constraints on the WIMP annihilation rate
which are well below the thermal value for masses up to few TeV. On the other
hand, for conservative assumptions on the dSph magnetic properties, the bounds
can be dramatically relaxed. We show however that, within the next 10 years and
regardless of the astrophysical assumptions, it will be possible to
progressively close in on the full parameter space of WIMPs by searching for
radio signals in dSphs with SKA and its precursors.Comment: 17 pages, 6 figure panels. Companion papers: arXiv:1407.5479 and
arXiv:1407.5482. v3: minor revision, matches published versio
Testing the Hypothesis of Modified Dynamics with Low Surface Brightness Galaxies and Other Evidence
The rotation curves of low surface brightness galaxies provide a unique data
set with which to test alternative theories of gravitation over a large dynamic
range in size, mass, surface density, and acceleration. Many clearly fail,
including any in which the mass discrepancy appears at a particular
length-scale. One hypothesis, MOND [Milgrom 1983, ApJ, 270, 371], is consistent
with the data. Indeed, it accurately predicts the observed behavior. We find no
evidence on any scale which clearly contradicts MOND, and a good deal which
supports it.Comment: Accepted for publication in the Astrophysical Journal. 35 pages
AAStex + 9 figures. This result surprised the bejeepers out of us, to
- …