196 research outputs found
Current data on the globular cluster Palomar 14 are not inconsistent with MOND
Certain types of globular clusters have the very important property that the
predictions for their kinematics in the Newtonian and modified Newtonian
dynamics (MOND) contexts are divergent. Here, we caution the recent claim that
the stellar kinematics data (using 17 stars) of the globular cluster Palomar 14
are inconsistent with MOND. We compare the observations to the theoretical
predictions using a Kolmogorov-Smirnov test, which is appropriate for small
samples. We find that, with the currently available data, the MOND prediction
for the velocity distribution can only be excluded with a very low confidence
level, clearly insufficient to claim that MOND is falsified.Comment: Research note accepted for publication in A&
Observational evidence for a connection between SMBHs and dark matter haloes
We investigate the relation between circular velocity vc and bulge velocity
dispersion sigma in spiral galaxies, based on literature data and new
spectroscopic observations. We find a strong, nearly linear vc-sigma
correlation with a negligible intrinsic scatter, and a striking agreement with
the corresponding relation for elliptical galaxies. The least massive galaxies
(sigma < 80 km/s) significantly deviate from this relation. We combine this
vc-sigma correlation with the well-known MBH-sigma relation to obtain a tight
correlation between circular velocity and supermassive black hole mass, and
interpret this as observational evidence for a close link between supermassive
black holes and the dark matter haloes in which they presumably formed. Apart
from being an important ingredient for theoretical models of galaxy formation
and evolution, the relation between MBH and circular velocity has the potential
to become an important practical tool in estimating supermassive black hole
masses in spiral galaxies.Comment: 4 pages, 1 figure, to appear in "The Interplay among Black Holes,
Stars and ISM in Galactic Nuclei", IAU Symposium 222, eds. Th. Storchi
Bergmann, L.C. Ho & H.R. Schmit
Modified gravity models and the central cusp of dark matter haloes in galaxies
The N-body dark matter (DM) simulations point that DM density profiles, e.g. the Navarro Frenk White (NFW) halo, should be cuspy in its centre, but observations disfavour this kind of DM profile. Here we consider whether the observed rotation curves close to the galactic centre can favour modified gravity models in comparison to the NFW halo, and how to quantify such difference. Two explicit modified gravity models are considered, Modified Newtonian Dynamics (MOND) and a more recent approach renormalization group effects in general relativity (RGGR). It is also the purpose of this work to significantly extend the sample on which RGGR has been tested in comparison to other approaches. By analysing 62 galaxies from five samples, we find that (i) there is a radius, given by half the disc scale length, below which RGGR and MOND can match the data about as well or better than NFW, albeit the formers have fewer free parameters; (ii) considering the complete rotation curve data, RGGR could achieve fits with better agreement than MOND, and almost as good as a NFW halo with two free parameters (NFW and RGGR have, respectively, two and one more free parameters than MOND)
Modified Baryonic Dynamics: two-component cosmological simulations with light sterile neutrinos
In this article we continue to test cosmological models centred on Modified
Newtonian Dynamics (MOND) with light sterile neutrinos, which could in
principle be a way to solve the fine-tuning problems of the standard model on
galaxy scales while preserving successful predictions on larger scales. Due to
previous failures of the simple MOND cosmological model, here we test a
speculative model where the modified gravitational field is produced only by
the baryons and the sterile neutrinos produce a purely Newtonian field (hence
Modified Baryonic Dynamics). We use two component cosmological simulations to
separate the baryonic N-body particles from the sterile neutrino ones. The
premise is to attenuate the over-production of massive galaxy cluster halos
which were prevalent in the original MOND plus light sterile neutrinos
scenario. Theoretical issues with such a formulation notwithstanding, the
Modified Baryonic Dynamics model fails to produce the correct amplitude for the
galaxy cluster mass function for any reasonable value of the primordial power
spectrum normalisation.Comment: 11 pages, 2 figures. Submitted to JCA
Dynamical measurement of the stellar surface density of face-on galaxies
The DiskMass survey recently provided measurements of the vertical velocity dispersions of disk stars in a sample of nearly face-on galaxies. By setting the disk scale-heights to be equal to those of edge-on galaxies with similar scale-lengths, it was found that these disks must be sub-maximal, with surprisingly low K-band mass-to-light ratios of the order of M-star/L-K similar or equal to 0.3 M-star/L-circle dot. This study made use of a simple relation between the disk surface density and the measured velocity dispersion and scale height of the disk, neglecting the shape of the rotation curve and the dark matter contribution to the vertical force, which can be especially important in the case of sub-maximal disks. Here, we point out that these simplifying assumptions led to an overestimation of the stellar mass-to-light ratios. Relaxing these assumptions, we compute even lower values than previously reported for the mass-to-light ratios, with a median M-star/L-K similar or equal to 0.18 M-star/L-circle dot, where 14 galaxies have M-star/L-K 1.5 for the axis ratios of the potential) might help. The cross-terms in the Jeans equation are also generally negligible. These deduced K-band stellar mass-to-light ratios are even more difficult to reconcile with stellar population synthesis models than the previously reported ones
The Galactic potential and the asymmetric distribution of hypervelocity stars
In recent years several hypervelocity stars (HVSs) have been observed in the
halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic
center and then propagated in the Galactic potential up to their current
position. The recent survey for candidate HVSs show an asymmetry in the
kinematics of candidate HVSs (position and velocity vectors), where more
outgoing stars than ingoing stars (i.e. positive Galactocentric velocities vs.
negative ones) are observed. We show that such kinematic asymmetry, which is
likely due to the finite lifetime of the stars and Galactic potential
structure, could be used in a novel method to probe and constrain the Galactic
potential, identify the stellar type of the stars in the survey and estimate
the number of HVSs. Kinematics-independent identification of the stellar types
of the stars in such surveys (e.g. spectroscopic identification) could further
improve these results. We find that the observed asymmetry between ingoing and
outgoing stars favors specific Galactic potential models. It also implies a
lower limit of ~54+-8 main sequence HVSs in the survey sample (>=648+-96 in the
Galaxy), assuming that all of the main sequence stars in the survey originate
from the Galactic center. The other stars in the survey are likely to be hot
blue horizontal branch stars born in the halo rather than stars ejected from
the Galactic center.Comment: 7 pages, 3 figures. Added an appendix. Accepted to Ap
HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4565
We present 21-cm observations and models of the neutral hydrogen in NGC 4565,
a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion
in LOcal GAlaxieS (HALOGAS) survey. These models provide insight concerning
both the morphology and kinematics of HI above, as well as within, the disk.
NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer.
Our modeling provides no evidence for a massive, extended HI halo. We see
evidence for a bar and associated radial motions. Additionally, there are
indications of radial motions within the disk, possibly associated with a ring
of higher density. We see a substantial decrease in rotational velocity with
height above the plane of the disk (a lag) of -40 +5/-20 km/s/kpc and -30
+5/-30 km s/kpc in the approaching and receding halves, respectively. This lag
is only seen within the inner ~4.75' (14.9 kpc) on the approaching half and
~4.25' (13.4 kpc) on the receding, making this a radially shallowing lag, which
is now seen in the HI layers of several galaxies. When comparing results for
NGC 4565 and those for other galaxies, there are tentative indications of high
star formation rate per unit area being associated with the presence of a halo.
Finally, HI is found in two companion galaxies, one of which is clearly
interacting with NGC 4565.Comment: 17 pages, 16 figures, accepted for publication in the Astrophysical
Journal, modified affiliatio
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