179 research outputs found
MOND and the Galaxies
We review galaxy formation and dynamics under the MOND hypothesis of modified
gravity, and compare to similar galaxies in Newtonian dynamics with dark
matter. The aim is to find peculiar predictions both to discriminate between
various hypotheses, and to make the theory progress through different
constraints, touching the interpolation function, or the fundamental
acceleration scale. Galaxy instabilities, forming bars and bulges at longer
term, evolve differently in the various theories, and help to bring
constraints, together with the observations of bar frequency. Dynamical
friction and the predicted merger rate could be a sensitive test of theories.
The different scenarios of galaxy formation are compared within the various
theories and observations.Comment: 8 pages, 7 figures, Invited paper to "The Invisible Universe
International Conference", ed. J-M. Alimi, A. Fuzfa, P-S. Corasaniti, AIP pu
Polar ring galaxies as tests of gravity
Polar ring galaxies are ideal objects with which to study the
three-dimensional shapes of galactic gravitational potentials since two
rotation curves can be measured in two perpendicular planes. Observational
studies have uncovered systematically larger rotation velocities in the
extended polar rings than in the associated host galaxies. In the dark matter
context, this can only be explained through dark halos that are systematically
flattened along the polar rings. Here, we point out that these objects can also
be used as very effective tests of gravity theories, such as those based on
Milgromian dynamics (MOND). We run a set of polar ring models using both
Milgromian and Newtonian dynamics to predict the expected shapes of the
rotation curves in both planes, varying the total mass of the system, the mass
of the ring with respect to the host, as well as the size of the hole at the
center of the ring. We find that Milgromian dynamics not only naturally leads
to rotation velocities being typically higher in the extended polar rings than
in the hosts, as would be the case in Newtonian dynamics without dark matter,
but that it also gets the shape and amplitude of velocities correct. Milgromian
dynamics thus adequately explains this particular property of polar ring
galaxies.Comment: 9 pages, 8 Figures, 1 Table, Accepted for publication by MNRA
Evolution of spiral galaxies in modified gravity: II- Gas dynamics
The stability of spiral galaxies is compared in modified Newtonian Dynamics
(MOND) and Newtonian dynamics with dark matter (DM). We extend our previous
simulations that involved pure stellar discs without gas, to deal with the
effects of gas dissipation and star formation. We also vary the interpolating
function between the MOND and Newtonian regime. Bar formation is compared in
both dynamics, from initial conditions identical in visible component. One
first result is that the MOND galaxy evolution is not affected by the choice of
the mu-function, it develops bars with the same frequency and strength. The
choice of the mu-function significantly changes the equivalent DM models, in
changing the dark matter to visible mass ratio and, therefore, changing the
stability. The introduction of gas shortens the timescale for bar formation in
the DM model, but is not significantly shortened in the MOND model. As a
consequence, with gas, the MOND and DM bar frequency histograms are now more
similar than without gas. The thickening of the plane occurs through vertical
resonance with the bar and peanut formation, and even more quickly with gas.
Since the mass gets more concentrated with gas, the radius of the peanut is
smaller, and the appearance of the pseudo-bulge is more boxy. The bar strength
difference is moderated by saturation, and feedback effects, like the bar
weakening or destruction by gas inflow due to gravity torques. Averaged over a
series of models representing the Hubble sequence, the MOND models have still
more bars, and stronger bars, than the equivalent DM models, better fitting the
observations. Gas inflows driven by bars produce accumulations at Lindblad
resonances, and MOND models can reproduce observed morphologies quite well, as
was found before in the Newtonian dynamics.Comment: 9 pages, 11 figures, accepted in A&
Loss of mass and stability of galaxies in MOND
The self-binding energy and stability of a galaxy in MOND-based gravity are
curiously decreasing functions of its center of mass acceleration towards
neighbouring mass concentrations. A tentative indication of this breaking of
the Strong Equivalence Principle in field galaxies is the RAVE-observed escape
speed in the Milky Way. Another consequence is that satellites of field
galaxies will move on nearly Keplerian orbits at large radii (100 - 500 kpc),
with a declining speed below the asymptotically constant naive MOND prediction.
But consequences of an environment-sensitive gravity are even more severe in
clusters, where member galaxies accelerate fast: no more Dark-Halo-like
potential is present to support galaxies, meaning that extended axisymmetric
disks of gas and stars are likely unstable. These predicted reappearance of
asymptotic Keplerian velocity curves and disappearance of "stereotypic
galaxies" in clusters are falsifiable with targeted surveys.Comment: 4 pages, 2 figures, ApJ Letter
Tidal dwarf galaxies as a test of fundamental physics
Within the cold dark matter (CDM) framework tidal dwarf galaxies (TDGs)
cannot contain dark matter, so the recent results by Bournaud et al. (2007)
that 3 rotating TDGs do show significant evidence for being dark matter
dominated is inconsistent with the current concordance cosmological theory
unless yet another dark matter component is postulated. We confirm that the TDG
rotation curves are consistent with Newtonian dynamics only if either an
additional dark matter component is postulated, or if all 3 TDGs happen to be
viewed nearly edge-on, which is unlikely given the geometry of the tidal
debris. We also find that the observed rotation curves are very naturally
explained without any free parameters within the modified Newtonian dynamics
(MOND) framework if inclinations are adopted as derived by Bournaud et al. We
explore different inclination angles and two different assumptions about the
external field effect. The results do not change significantly, and we conclude
therefore that Newtonian dynamics has severe problems while MOND does
exceedingly well in explaining the observed rotation curves of the 3 TDGs
studied by Bournaud et al.Comment: Accepted for publication in A&A Letters, 5 pages, 3 figure
Galactic kinematics with modified Newtonian dynamics
We look for observational signatures that could discriminate between
Newtonian and modified Newtonian (MOND) dynamics in the Milky Way, in view of
the advent of large astrometric and spectroscopic surveys. Indeed, a typical
signature of MOND is an apparent disk of "phantom" dark matter, which is
uniquely correlated with the visible disk-density distribution. Due to this
phantom dark disk, Newtonian models with a spherical halo have different
signatures from MOND models close to the Galactic plane. The models can thus be
differentiated by measuring dynamically (within Newtonian dynamics) the disk
surface density at the solar radius, the radial mass gradient within the disk,
or the velocity ellipsoid tilt angle above the Galactic plane. Using the most
realistic possible baryonic mass model for the Milky Way, we predict that, if
MOND applies, the local surface density measured by a Newtonist will be
approximately 78 Msun/pc2 within 1.1 kpc of the Galactic plane, the dynamically
measured disk scale-length will be enhanced by a factor of 1.25 with respect to
the visible disk scale-length, and the local vertical tilt of the velocity
ellipsoid at 1 kpc above the plane will be approximately 6 degrees. None of
these tests can be conclusive for the present-day accuracy of Milky Way data,
but they will be of prime interest with the advent of large surveys such as
GAIA.Comment: 5 page
The inner structure of very massive elliptical galaxies: implications for the inside-out formation mechanism of z~2 galaxies
We analyze a sample of 23 supermassive elliptical galaxies (central velocity
dispersion larger than 330 km s-1), drawn from the SDSS. For each object, we
estimate the dynamical mass from the light profile and central velocity
dispersion, and compare it with the stellar mass derived from stellar
population models. We show that these galaxies are dominated by luminous matter
within the radius for which the velocity dispersion is measured. We find that
the sizes and stellar masses are tightly correlated, with Re ~ M*^{1.1}$,
making the mean density within the de Vaucouleurs radius a steeply declining
function of M*: rho_e ~ M*^{-2.2}. These scalings are easily derived from the
virial theorem if one recalls that this sample has essentially fixed (but
large) sigma_0. In contrast, the mean density within 1 kpc is almost
independent of M*, at a value that is in good agreement with recent studies of
z ~ 2 galaxies. The fact that the mass within 1 kpc has remained approximately
unchanged suggests assembly histories that were dominated by minor mergers --
but we discuss why this is not the unique way to achieve this. Moreover, the
total stellar mass of the objects in our sample is typically a factor of ~ 5
larger than that in the high redshift (z ~ 2) sample, an amount which seems
difficult to achieve. If our galaxies are the evolved objects of the recent
high redshift studies, then we suggest that major mergers were required at z >
1.5, and that minor mergers become the dominant growth mechanism for massive
galaxies at z < 1.5.Comment: 11 pages, 8 figures, accepted in MNRA
The PN.S Elliptical Galaxy Survey: a standard LCDM halo around NGC 4374?
As part of our current programme to test LCDM predictions for dark matter
(DM) haloes using extended kinematical observations of early-type galaxies, we
present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84)
based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph,
along with extended long-slit stellar kinematics. This is the first such
analysis of a galaxy from our survey with a radially constant velocity
dispersion profile. We find that the spatial and kinematical distributions of
the PNe agree with the field stars in the region of overlap. The velocity
kurtosis is consistent with zero at almost all radii. We construct a series of
Jeans models, fitting both velocity dispersion and kurtosis to help break the
mass-anisotropy degeneracy. Our mass models include DM halos either with
shallow cores or with central cusps as predicted by cosmological simulations -
along with the novel introduction in this context of adiabatic halo contraction
from baryon infall. Both classes of models confirm a very massive dark halo
around NGC 4374, demonstrating that PN kinematics data are well able to detect
such haloes when present. Considering the default cosmological mass model, we
confirm earlier suggestions that bright galaxies tend to have halo
concentrations higher than LCDM predictions, but this is found to be solved if
either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed.
Thus for the first time a case is found where the PN dynamics may well be
consistent with a standard dark matter halo. A cored halo can also fit the
data, and prefers a stellar mass consistent with a Salpeter IMF. The less
dramatic dark matter content found in lower-luminosity "ordinary" ellipticals
suggests a bimodality in the halo properties which may be produced by divergent
baryonic effects during their assembly histories.Comment: 22 pages, 14 figures. MNRAS, accepte
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