4,676 research outputs found
The Case Against Warm or Self-Interacting Dark Matter as Explanations for Cores in Low Surface Brightness Galaxies
Warm dark matter (WDM) and self-interacting dark matter (SIDM) are often
motivated by the inferred cores in the dark matter halos of low surface
brightness (LSB) galaxies. We test thermal WDM, non-thermal WDM, and SIDM using
high-resolution rotation curves of nine LSB galaxies. We fit these dark matter
models to the data and determine the halo core radii and central densities.
While the minimum core size in WDM models is predicted to decrease with halo
mass, we find that the inferred core radii increase with halo mass and also
cannot be explained with a single value of the primordial phase space density.
Moreover, if the core size is set by WDM particle properties, then even the
smallest cores we infer would require primordial phase space density values
that are orders of magnitude smaller than lower limits obtained from the Lyman
alpha forest power spectra. We also find that the dark matter halo core
densities vary by a factor of about 30 from system to system while showing no
systematic trend with the maximum rotation velocity of the galaxy. This
strongly argues against the core size being directly set by large
self-interactions (scattering or annihilation) of dark matter. We therefore
conclude that the inferred cores do not provide motivation to prefer WDM or
SIDM over other dark matter models.Comment: Accepted to ApJL; additions to Figs 3 and 4; minor changes to tex
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
Halo Mass Profiles and Low Surface Brightness Galaxies Rotation Curves
A recent study has claimed that the rotation curve shapes and mass densities
of Low Surface Brightness (LSB) galaxies are largely consistent with
CDM predictions, in contrast to a large body of observational work. I
demonstrate that the method used to derive this conclusion is incapable of
distinguishing the characteristic steep CDM mass-density distribution from the
core-dominated mass-density distributions found observationally: even
core-dominated pseudo-isothermal haloes would be inferred to be consistent with
CDM. This method can therefore make no definitive statements on the
(dis)agreement between the data and CDM simulations. After introducing an
additional criterion that does take the slope of the mass-distribution into
account I find that only about a quarter of the LSB galaxies investigated are
possibly consistent with CDM. However, for most of these the fit parameters are
so weakly constrained that this is not a strong conclusion. Only 3 out of 52
galaxies have tightly constrained solutions consistent with CDM. Two
of these galaxies are likely dominated by stars, leaving only one possible dark
matter dominated, CDM-consistent candidate, forming a mere 2 per cent of the
total sample. These conclusions are based on comparison of data and simulations
at identical radii and fits to the entire rotation curves. LSB galaxies that
are consistent with CDM simulations, if they exist, seem to be rare indeed.Comment: Accepted for publication in Astrophysical Journa
The First CO Map of a Low Surface Brightness Galaxy
Using the Owens Valley Radio Observatory Millimeter-Wavelength Array (OVRO)
we have obtained the first CO map of a low surface brightness (LSB) galaxy. The
studied galaxy, UGC 01922, was chosen for these observations because both of
its previous CO detection with the IRAM 30m telescope and its classification as
a Malin 1 `cousin' - an LSB galaxy with M_HI > 10^10 Msol. The OVRO map
detected approximately 65% of the CO(1-0) flux found earlier with the single
dish measurements, giving a detected gas mass equivalent to M_H2 = 1.1X10^9
Msol. The integrated gas peak lies at the center of the galaxy and coincides
with both the optical and 1.4 GHz continuum emission peaks. The molecular gas
extends well beyond the OVRO beam size (~4'' or 3 kpc), covering ~25% of the
optical bulge. In all, perhaps the most remarkable aspect of this map is its
unexceptional appearance. Given that it took over ten years to successfully
detect molecular gas in any low surface brightness system, it is surprising
that the appearance and distribution of UGC 01922's CO is similar to what would
be expected for a high surface brightness galaxy in the same morphological
class.Comment: 5 pages, including 3 figures and 3 tables. also available online at
http://www.gb.nrao.edu/~koneil. Accepted by ApJ
Gas Content and Star Formation Thresholds in the Evolution of Spiral Galaxies
The gas mass fraction (fg) of spiral galaxies is strongly correlated with the
central surface brightness of their disks. There exist many dim galaxies with
long gas consumption time scales and fg > 0.5. This resolves the gas
consumption paradox.
The surface density of gas follows the optical surface brightness, but does
not vary by as large a factor. This is the signature of a critical density
threshold for star formation. Such a mechanism seems to be responsible for the
slow evolution of dim galaxies.Comment: 4 pages postscript including imbedded figures. Contribution to 1996
October conference "Star Formation, Near and Far.
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
Dynamical Stability and Environmental Influences in Low Surface Brightness Disk Galaxies
Using analytic stability criteria, we demonstrate that, due to their low
surface mass density and large dark matter content, LSB disks are quite stable
against the growth of global nonaxisymmetric modes such as bars. However,
depending on their (poorly constrained) stellar velocity dispersions, they may
be only marginally stable against local instabilities. We simulate a collision
between an LSB and HSB galaxy and find that, while the HSB galaxy forms a
strong bar, the response of the LSB disk is milder, manifesting weaker rings
and spiral features. The lack of sufficient disk self-gravity to amplify
dynamical instabilities naturally explains the rarity of bars in LSB disks. The
stability of LSB disks may also inhibit interaction-driven gas inflow and
starburst activity in these galaxies.Comment: 13 pages, 3 figures, LaTeX using AASTeX macros 4.0, accepted for
publication in the Astrophysical Journal Letter
- …