14 research outputs found
The shape of "dark matter" halos in disc galaxies according to the modified dynamics
Analyses of halo shapes for disc galaxies are said to give incongruous
results. I point out that the modified dynamics (MOND) predicts for disc
galaxies a distribution of fictitious dark matter that comprises two
components: a pure disc and a rounder halo. The former dominates the true disc
in regions of small accelerations, where it controls the z-dynamics in the disc
(disc flaring etc.); it has a finite total mass. It also dominates the round
component near the centre where the geometry is nearly planar. The second
component controls motions far from the plane, has a total enclosed mass that
diverges linearly with radius, and determines the rotation curve at large
radii. Its ellipticity may be appreciable at small radii but vanishes
asymptotically. This prediction of MOND differs from what one expects from
galaxy-formation scenarios with dark matter.
Analyses to date, which, as they do, assume one component--usually with a
constant ellipticity, perforce give conflicting results for the best-value
ellipticity, depending on whether they probe the disc or the sphere, small
radii or large ones.Comment: 8 page
The Flattened Dark Matter Halo of M31 as Deduced from the Observed HI Scale Heights
In this paper, we use the outer-galactic HI scale height data as well as the
observed rotation curve as constraints to determine the halo density
distribution of the Andromeda galaxy (M31). We model the galaxy as a
gravitationally-coupled system of stars and gas, responding to the external
force-field of a known Hernquist bulge and the dark matter halo, the density
profile of the latter being characterized by four free parameters. The
parameter space of the halo is optimized so as to match the observed HI
thickness distribution as well as the rotation curve on an equal footing,
unlike the previous studies of M31 which were based on rotation curves alone.
We show that an oblate halo, with an isothermal density profile, provides the
best fit to the observed data. This gives a central density of 0.011 M_sun
/pc^3, a core radius of 21 kpc, and an axis ratio of 0.4. The main result from
this work is the flattened dark matter halo for M31, which is required to match
the outer galactic HI scale height data. Interestingly, such flattened halos
lie at the most oblate end of the distribution of halo shapes found in recent
cosmological simulations.Comment: 21 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way
An all high-latitude sky survey for cool carbon giant stars in the Galactic
halo has revealed 75 such stars, of which the majority are new detections. Of
these, more than half are clustered on a Great Circle on the sky which
intersects the center of Sagittarius dwarf galaxy (Sgr) and is parallel to its
proper motion vector, while many of the remainder are outlying Magellanic Cloud
C-stars. A pole-count analysis of the carbon star distribution clearly
indicates that the Great Circle stream we have isolated is statistically
significant, being a 5-6 sigma over-density. These two arguments strongly
support our conclusion that a large fraction of the Halo carbon stars
originated in Sgr. The stream orbits the Galaxy between the present location of
Sgr, 16 kpc from the Galactic center, and the most distant stream carbon star,
at ~60 kpc. It follows neither a polar nor a Galactic plane orbit, so that a
large range in both Galactic R and z distances are probed. That the stream is
observed as a Great Circle indicates that the Galaxy does not exert a
significant torque upon the stream, so the Galactic potential must be nearly
spherical in the regions probed by the stream. We present N-body experiments
simulating this disruption process as a function of the distribution of mass in
the Galactic halo. A likelihood analysis shows that, in the Galactocentric
distance range 16 kpc < R < 60 kpc, the dark halo is most likely almost
spherical. We rule out, at high confidence levels, the possibility that the
Halo is significantly oblate, with isodensity contours of aspect q_m < 0.7.
This result is quite unexpected and contests currently popular galaxy formation
models. (Abridged)Comment: 26 pages, 13 figures (6 in color, 8 chunky due to PS compression),
minor revisions, accepted by Ap
The cold gaseous halo of NGC 891
We present HI observations of the edge-on galaxy NGC 891. These are among the
deepest ever performed on an external galaxy. They reveal a huge gaseous halo,
much more extended than seen previously and containing almost 30 % of the HI.
This HI halo shows structures on various scales. On one side, there is a
filament extending (in projection) up to 22 kpc vertically from the disk. Small
(M_HI ~ 10^6 Msol) halo clouds, some with forbidden (apparently
counter-rotating) velocities, are also detected. The overall kinematics of the
halo gas is characterized by differential rotation lagging with respect to that
of the disk. The lag, more pronounced at small radii, increases with height
from the plane. There is evidence that a significant fraction of the halo is
due to a galactic fountain. Accretion from intergalactic space may also play a
role in building up the halo and providing low angular momentum material needed
to account for the observed rotation lag. The long HI filament and the
counter-rotating clouds may be direct evidence of such accretion.Comment: Accepted for publication in The Astronomical Journal. High-resolution
version available at http://www.astron.nl/~oosterlo/n891Pape
A Pressure Anomaly for HII Regions in Irregular Galaxies
The pressures of giant HII regions in 6 dwarf Irregular galaxies are a factor
of ~10 larger than the average pressures of the corresponding galaxy disks,
obtained from the stellar and gaseous column densities. Either the visible HII
regions in these dwarfs are all so young that they are still expanding, or
there is an unexpected source of disk self-gravity that increases the
background pressure. We consider the possibility that the additional
self-gravity comes from disk dark matter, but suggest this is unlikely because
the vertical scale heights inferred for Irregular galaxies are consistent with
the luminous matter alone. Some of the HII region overpressure is probably the
result of local peaks in the gravitational field that come from large gas
concentrations, many of which are observed directly. These peaks also explain
the anomalously low average column density thresholds for star formation that
were found earlier for Irregular galaxies, and they permit the existence of a
cool HI phase as the first step toward dense molecular cores. Many of the HII
regions could also be so strongly over-pressured that they will expand for a
long time. In this case, the observed population would be only 7% of the total,
and the aging HII regions, now too faint to see, should occupy nearly the
entire dwarf galaxy volume. Such prolonged HII region expansion would explain
the origin of the giant HI shells that are seen in these galaxies, and account
for the lack of bright central clusters inside these shells.Comment: 27 pages, 4 figures, Astrophysical Journal, 540, Sep 10, 2000, in
pres
The Shape and Figure Rotation of NGC 2915's Dark Halo
NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showing
a short central bar and extended spiral arms, both reaching far beyond the
optical component. We use Tremaine & Weinberg (1984) method to measure the
pattern speed of the bar from HI radio synthesis data. Our measurements yield a
pattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), in
disagreement with the general view that corotation in barred disks lies just
outside the end of the bar, but consistent with recent models of barred
galaxies with dense dark matter halos. Our adopted bar semi-length puts
corotation at more than 1.7 bar radii. The existence of the pattern is also
problematic. Because NGC 2915 is isolated, interactions cannot account for the
structure observed in the HI disk. We also demonstrate that the low observed
disk surface density and the location of the pseudo-rings make it unlikely that
swing amplification or bar-driven spiral arms could explain the bar and spiral
pattern.
Based on the similarity of the dark matter and HI surface density profiles,
we discuss the possibility of dark matter distributed in a disk and following
closely the HI distribution. The disk then becomes unstable and can naturally
form a bar and spiral pattern. However, this explanation is hard to reconcile
with some properties of NGC 2915. We also consider the effect of a massive and
extended triaxial dark matter halo with a rotating figure. The existence of
such halos is supported by CDM simulations showing strongly triaxial dark halos
with slow figure rotation. The observed structure of the HI disk can then arise
through forcing by the rotating triaxial figure. We associate the measured
pattern speed in NGC 2915 with the figure rotation of its dark halo.Comment: 37 pages, including 8 figures and 2 tables (AASTeX, aaspp4.sty).
Fig.1 and 2 available as jpg. Accepted for publication in The Astronomical
Journal. Online manuscript with PostScript figures available at:
http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
The thickness of HI in galactic discs under MOND: theory and application to the Galaxy
The outskirts of galaxies are a very good laboratory for testing the nature
of the gravitational field at low accelerations. By assuming that the neutral
hydrogen gas is in hydrostatic equilibrium in the gravitational potential of
the host galaxy, the observed flaring of the gas layer can be used to test
modified gravities. For the first time we construct a simple framework to
derive the scaleheight of the neutral hydrogen gas disc in the MOND scenario
and apply this to the Milky Way. It is shown that using a constant gas velocity
dispersion of ~9 km/s, MOND is able to give a very good fit to the observed HI
flaring beyond a galactocentric distance of 17 kpc up to the last measured
point (~40 kpc). Between 10 and 16 kpc, however, the observed scaleheight is
about 40% more than what MOND predicts for the standard interpolating function
and 70% for the form suggested by Famaey & Binney. Given the uncertainties in
the non-thermal pressure support by cosmic rays and magnetic fields, MOND seems
to be a plausible alternative to dark matter in explaining the Milky Way
flaring. Studying the flaring of extended HI discs in external edge-on galaxies
may be a promising approach to assess the viability of MOND.Comment: 13 pages, 4 figure
On the Spiral Structure of NGC 2915 and Dark Matter
NGC 2915 is a blue compact dwarf galaxy embedded in an extended, low surface
brightness HI disk exhibiting a two-armed spiral structure and a central
bar-like component. Commonly accepted mechanisms are unable to explain the
existence of these patterns and Bureau et al. proposed disk dark matter
(scaling with the HI distribution) or a rotating triaxial dark halo as
alternative solutions. In an attempt to explore these mechanisms,
hydrodynamical simulations were run for each case and compared to observations
using customized column density and kinematic constraints. The spiral structure
can be accounted for both by an unseen bar or triaxial halo, the former fitting
the observations slightly better. However, the large bar mass or halo pattern
frequency required make it unlikely that the spiral wave is driven by an
external perturber. In particular, the spin parameter is much higher than
predicted by current cold dark matter (CDM) structure formation scenarios. The
massive disk models show that when the observed gas surface density is scaled
up by a factor about 10, the disk develops a spiral structure resembling
closely the observed one, in perturbed density as well as perturbed velocity.
This is consistent with more limited studies in other galaxies and suggests
that the disk of NGC 2915 contains much more mass than is visible, tightly
linked to the neutral hydrogen. A classic (quasi-)spherical halo is
nevertheless still required, as increasing the disk mass further to fit the
circular velocity curve would make the disk violently unstable. Scaling the
observed surface density profile by an order of magnitude brings the disk and
halo masses to comparable values within the disk radius.Comment: Accepted for publication in ApJ. Full resolution figures available at
http://www.star.qmul.ac.uk/~masset/publications.htm