1,404 research outputs found
Is Holmberg-II beyond MOND theory?
We compare the basic properties and kinematics of two gas-rich dwarf
galaxies: KK246 and Holmberg II (HoII). HoII is 20 times more luminous in the
blue-band than KK246 and its HI mass is a factor of 6 higher than in KK246.
However, the amplitudes of the rotation curves (at the last measured point) of
both galaxies are very similar, of about 40 km/s at a galactocentric radius of
7 kpc. This fact is challenging for modified theories of gravity that predict a
one-to-one relation between gravity at any radius and the enclosed baryonic
mass in galaxies. In particular, MOdified Newtonian Dynamics (MOND) predicts an
asymptotic flat velocity of 60 km/s in HoII. Since the baryonic mass of HoII is
dominated by the gas component, MOND overboosts its rotation speed even if the
mass of the stellar disk is taken negligibly small. We conclude the rotation
curve of HoII is probably inconsistent with MOND, unless the inclination and
distance are both fine-tuned to very unlikely values.Comment: 4 pages, 4 figure
Signatures of Neutrino Cooling in the SN1987A Scenario
The neutrino signal from SN1987A confirmed the core-collapse scenario and the
possible formation of a neutron star. Although this compact object has eluded
all observations, theoretical and numerical developments have allowed a glimpse
of the fate of it. In particular, a hypercritical accretion model has been
proposed to forecast the accretion of in two
hours and the subsequent submergence of the magnetic field in the newborn
neutron star. In this paper, we revisit the Chevalier's model in a numerical
framework, focusing on the neutrino cooling effect on the supernova fall-back
dynamics. For that, using a customized version of the FLASH code, we carry out
numerical simulations of the accretion of matter onto the newborn neutron star
in order to estimate the size of the neutrino-sphere, the emissivity and
luminosity of neutrinos. As a signature of this phase, we estimate the
neutrinos expected on SK neutrino experiment and their flavor ratios. This is
academically important because, although currently it was very difficult to
detect 1.46 thermal neutrinos and their oscillations, these fingerprints are
the only viable and reliable way to confirm the hypercritical phase. Perhaps
new techniques for detecting neutrino oscillations arise in the near future
allowing us to confirm our estimates.Comment: 15 pages, 10 figures. Accepted in MNRAS. Minor changes were
introduce
Incremental Compilation of Bayesian networks
Most methods of exact probability propagation in Bayesian networks do not
carry out the inference directly over the network, but over a secondary
structure known as a junction tree or a join tree (JT). The process of
obtaining a JT is usually termed {sl compilation}. As compilation is usually
viewed as a whole process; each time the network is modified, a new compilation
process has to be carried out. The possibility of reusing an already existing
JT, in order to obtain the new one regarding only the modifications in the
network has received only little attention in the literature. In this paper we
present a method for incremental compilation of a Bayesian network, following
the classical scheme in which triangulation plays the key role. In order to
perform incremental compilation we propose to recompile only those parts of the
JT which can have been affected by the networks modifications. To do so, we
exploit the technique OF maximal prime subgraph decomposition in determining
the minimal subgraph(s) that have to be recompiled, and thereby the minimal
subtree(s) of the JT that should be replaced by new subtree(s).We focus on
structural modifications : addition and deletion of links and variables.Comment: Appears in Proceedings of the Nineteenth Conference on Uncertainty in
Artificial Intelligence (UAI2003
Analysis of Fermi-LAT observations, UHECRs and neutrinos from the radio galaxy Centaurus B
Centaurus B (Cen B) is one of the closest and brightest radio-loud galaxy in
the southern sky. This radio galaxy, proposed as a plausible candidate for
accelerating ultra-high-energy cosmic rays (UHECRs), is near the highest-energy
neutrino event reported (IC35) in the High-Energy Starting Events catalog.
Pierre Auger observatory reported the highest energy comic rays during 10 years
of collecting data with some of them around this source. In this paper, the
analysis of the gamma-ray spectrum and the light curve above 200 MeV is
presented with nine years of cumulative Fermi-LAT data around Cen B. Taking
into consideration the multi-wavelength observations carried out about this
radio galaxy, leptonic and hadronic scenarios are introduced in order to fit
the spectral energy distribution, assuming that the gamma-ray flux is produced
in a region close to the core and in the extended lobes. Using the best-fit
values found, several physics properties of this radio galaxy are derived.
Furthermore, a statistical analysis of the cosmic ray distribution around Cen B
is performed, finding that this distribution is not different from the
background at a level of significance of 5%. Considering the UHECR event
associated to this source by Moskalenko et al. and extrapolating its luminosity
to low energies, we do not find enough evidence to associate the highest-energy
neutrino event (IC35) with this radio galaxy.Comment: 29 pages, 12 figures. Accepted for publication in JCA
Spectroscopic observations of a sample of dwarf spiral galaxies. II- Abundance gradients
The oxygen gradient of four dS galaxies has been determined using abundances
for several HII regions determined with four different methods. The gradient
slopes of the three non-barred galaxies in the sample are quite steep, larger
than -0.2 dex/kpc, while the gradient of the barred galaxy is shallower, only
-0.1 dex/kpc. Although these gradients are quite steep they are real, following
all the galaxies the same trend. Moreover, the results obtained here agree with
those marked by the late-type, non-dwarf spirals, particularly the relationship
between the gradient and the absolute magnitude and the optical size for
non-barred galaxies, and the surface density for barred ones.Comment: 26 pages, 10 figures, resubmitted to AJ on August 28th after minor
referee suggestion
Spectroscopic observations of a sample of dwarf spirals galaxies. I.Oxygen abundances
The oxygen abundances for four dwarf spiral galaxies have been determined
using long-slit spectroscopy. The abundances of these galaxies have not
previously reported in the literature. Several HII regions were detected in
each galaxy. The electronic temperature method could be used only in one region
because of the lack of the auroral lines of oxygen or sulfur. Therefore, four
different semi empirical methods were used in the abundance determinations and
a weighted-average abundance is obtained. The abundances of three of the
galaxies are sub solar for all the regions, one of them having a very low
metallicity. Only three HII regions in the fourth galaxy show slightly over
solar abundances. Three of the HII regions might have a PN embedded. A
comparison with the oxygen abundances among different types of late-type
galaxies (BCG, LSBG, dI, Sm) is made. The conclusion is that all of them show
the same range in metallicity. Also, the log(N/O) is similar, showing a
secondary behaviour for abundances larger than 8.2 dex in spite of the
morphological type of the mother galaxy.Comment: Resubmitted to AJ on August 28th after minor referee suggestion
Diffuse Ionized Gas inside the dwarf irregular galaxy NGC 6822
We have studied the differences between the diffuse ionized gas (DIG) and the HII regions along a slit position in the local dwarf irregular galaxy NGC 6822. The slit position passes through the two most prominent HII regions: Hubble V and Hubble X. Important differences have been found in the excitation, ionization, and [NII]6584/Ha and [SII]6717/Ha line ratios between the DIG and the HII locations. Moreover, the values of all the line ratios are not similar to those in the DIG locations of spiral galaxies but are very similar to the values in other irregular galaxies, such as IC 10. We also determined the rate of recombination using the HeI5875 line. Finally, we obtained a picture of the ionization sources of the DIG. We consider that the leakage of photons from the HII regions might explain most of the line ratios, except [NII]/Ha, which might be explained by turbulence
The inclination of the dwarf irregular galaxy Holmberg II
We provide constraints on the inclination angle of the H\,{\sc i} disk of the
dwarf irregular galaxy Holmberg II (Ho II) from stability analysis of the outer
gaseous disk. We point out that a mean inclination angle of 27 degrees and thus
a flat circular velocity of ~60 km/s, is required to have a level of
gravitational stability similar to that found in other galaxies. Adopting this
inclination angle, we find that Ho II lies on the right location in the
baryonic Tully-Fisher relation. Moreover, for this inclination, its rotation
curve is consistent with MOND. However, the corresponding analysis of the
stability under MOND indicates that this galaxy could be problematic for MOND
because its outer parts are marginally unstable in this gravity theory. We urge
MOND simulators to study numerically the non-linear stability of gas-rich dwarf
galaxies since it may provide a new key test for MOND.Comment: 11 pages, 12 figures, accepted for publication in RevMexA
Slowly rotating gas-rich galaxies in modified Newtonian dynamics (MOND)
We have carried out a search for gas-rich dwarf galaxies that have lower
rotation velocities in their outskirts than MOdified Newtonian Dynamics (MOND)
predicts, so that the amplitude of their rotation curves cannot be fitted by
arbitrarily increasing the mass-to-light ratio of the stellar component or by
assuming additional undetected matter. With presently available data, the
gas-rich galaxies UGC 4173, Holmberg II, ESO 245-G05, NGC 4861 and ESO 364-G029
deviate most from MOND predictions and, thereby, provide a sample of promising
targets in testing the MOND framework. In the case of Holmberg II and NGC 4861,
we find that their rotation curves are probably inconsistent with MOND, unless
their inclinations and distances differ significantly from the nominal ones.
The galaxy ESO 364-G029 is a promising target because its baryonic mass and
rotation curve are similar to Holmberg II but presents a higher inclination.
Deeper photometric and HI observations of ESO 364-G029, together with further
decreasing systematic uncertainties, may provide a strong test to MOND.Comment: 9 pages, 5 figures, accepted for publication in the Astronomical
Journa
Gap formation by inclined massive planets in locally isothermal three-dimensional discs
We study gap formation in gaseous protoplanetary discs by a Jupiter mass
planet. The planet's orbit is circular and inclined relative to the midplane of
the disc. We use the impulse approximation to estimate the gravitational tidal
torque between the planet and the disc, and infer the gap profile. For low-mass
discs, we provide a criterion for gap opening when the orbital inclination is
. Using the FARGO3D code, we simulate the disc response to an
inclined massive planet. The dependence of the depth and width of the gap
obtained in the simulations on the inclination of the planet is broadly
consistent with the scaling laws derived in the impulse approximation. Although
we mainly focus on planets kept on fixed orbits, the formalism permits to infer
the temporal evolution of the gap profile in cases where the inclination of the
planet changes with time. This study may be useful to understand the migration
of massive planets on inclined orbit, because the strength of the interaction
with the disc depends on whether a gap is opened or not.Comment: 16 pages, 13 figures, accepted for publication in MNRA
- …