1,111 research outputs found
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
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
Further Discoveries of 12CO in Low Surface Brightness Galaxies
Using the IRAM 30m telescope we have obtained seven new, deep CO J(1-0) and
J(2-1) observations of low surface brightness (LSB) galaxies. Five of the
galaxies have no CO detected to extremely low limits (0.1-0.4 K km/s at
J(1-0)), while two of the galaxies, UGC 01922 and UGC 12289, have clear
detections in both line transitions. When these observations are combined with
all previous CO observations taken of LSB systems, we compile a total of 34
observations, in which only 3 galaxies have had detections of their molecular
gas. Comparing the LSB galaxies with and without CO detections to a sample of
high surface brightness (HSB) galaxies with CO observations indicates that it
is primarily the low density of baryonic matter within LSB galaxies which is
causing their low CO fluxes. Finally, we note that one of the massive LSB
galaxies studied in this project, UGC 06968 (a Malin-1 `cousin'), has upper
limits placed on both M_H2 and M_H2/M_HI which are 10-20 times lower than the
lowest values found for any galaxy (LSB or HSB) with similar global properties.
This may be due to an extremely low temperature and metallicity within UGC
06968, or simply due to the CO distribution within the galaxy being too diffuse
to be detected by the IRAM beam.Comment: 18 pages, 8 figures, 4 tables. Accepted by Ap
First Detection of CO in a Low Surface Brightness Galaxy
We report on the first attempts at searching for CO in red low surface
brightness galaxies, and the first detection of molecular gas in a low surface
brightness (mu_B(0)_{obs} > 23 mag arcsec^{-2}) galaxy. Using the IRAM 30m
telescope, CO(1-0) and CO(2-1) lines were searched for in four galaxies --
P06-1, P05-5, C05-3, & C04-2. In three of the galaxies no CO was detected, to
T_{MB} ~ 1.8mK (at the 3 sigma level). In the fourth galaxy, P06-1, both lines
were detected. Comparing our findings with previous studies shows P06-1 to have
a molecular-to-atomic mass ratio considerably lower than is predicted using
theoretical models based on high surface brightness galaxy studies. This
indicates the N(H_2)/(int{T(CO)dv}) conversion factor for low surface
brightness galaxies may currently be consistently underestimated by a factor of
3 - 20.Comment: 8 pages, 2 figures, accepted by the ApJ
The bimodal spiral galaxy surface brightness distribution
We have assessed the significance of Tully and Verheijen's (1997) bimodal
Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution,
focussing on whether this bimodality is simply an artifact of small number
statistics. A Kolmogorov-Smirnov style of significance test shows that the
total distribution is fairly represented by a single-peaked distribution, but
that their isolated galaxy subsample (with no significant neighbours within a
projected distance of around 80 kpc) is bimodal at the 96 per cent level. We
have also investigated the assumptions underlying the isolated galaxy surface
brightness distribution, finding that the (often large) inclination corrections
used in the construction of this distribution reduce the significance of the
bimodality. We conclude that the Ursa Major Cluster dataset is insufficient to
establish the presence of a bimodal near-infrared surface brightness
distribution: an independent sample of around 100 isolated, low inclination
galaxies is required to establish bimodality at the 99 per cent level.Comment: 5 pages LaTeX; 2 embedded figures; re-submitted to MNRAS after
replying to referee's comment
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
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
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
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