833 research outputs found
2-10 keV luminosity of high-mass binaries as a gauge of ongoing star-formation rate
Based on recent work on spectral decomposition of the emission of
star-forming galaxies, we assess whether the integrated 2-10 keV emission from
high-mass X-ray binaries (HMXBs), L_{2-10}^{HMXB}, can be used as a reliable
estimator of ongoing star formation rate (SFR). Using a sample of 46 local (z <
0.1) star forming galaxies, and spectral modeling of ASCA, BeppoSAX, and
XMM-Newton data, we demonstrate the existence of a linear SFR-L_{2-10}^{HMXB}
relation which holds over ~5 decades in X-ray luminosity and SFR. The total
2-10 keV luminosity is not a precise SFR indicator because at low SFR (i.e., in
normal and moderately-starbursting galaxies) it is substantially affected by
the emission of low-mass X-ray binaries, which do not trace the current SFR due
to their long evolution lifetimes, while at very high SFR (i.e., for very
luminous FIR-selected galaxies) it is frequently affected by the presence of
strongly obscured AGNs. The availability of purely SB-powered galaxies - whose
2-10 keV emission is mainly due to HMXBs - allows us to properly calibrate the
SFR-L_{2-10}^{HMXB} relation. The SFR-L_{2-10}^{HMXB} relation holds also for
distant (z ~ 1) galaxies in the Hubble Deep Field North sample, for which we
lack spectral information, but whose SFR can be estimated from deep radio data.
If confirmed by more detailed observations, it may be possible to use the
deduced relation to identify distant galaxies that are X-ray overluminous for
their (independently estimated) SFR, and are therefore likely to hide strongly
absorbed AGNs.Comment: Astronomy & Astrophysics, in press (15 pages, 7 figures, 4 tables
Gravitomagnetic corrections to the lensing deflection angle for spiral galaxy models
We investigate the effects of the gravitomagnetic corrections to the usual
gravitational lens quantities for a specific lensing mass distribution modelled
after spiral galaxies. An exponential disk is embedded into two different
spherical halo models where disk and haloes parameters are fixed according to
the observed mass to light ratios, galaxy magnitudes and rotation curves. The
general expressions for the lensing deflection angle are given also taking into
account the orientation of the galaxy disk plane with respect to the lens
plane. It is found that the gravitomagnetic term changes the deflection angle
by a typical amount of the order of ten microarcseconds.Comment: 7 pages, 2 figures, accepted for publication on MNRA
Probing neutralino dark matter in the MSSM & the NMSSM with directional detection
We investigate the capability of directional detectors to probe neutralino
dark matter in the Minimal Supersymmetric Standard Model and the
Next-to-Minimal Supersymmetric Standard Model with parameters defined at the
weak scale. We show that directional detectors such as the future MIMAC
detector will probe spin dependent dark matter scattering on nucleons that are
beyond the reach of current spin independent detectors. The complementarity
between indirect searches, in particular using gamma rays from dwarf spheroidal
galaxies, spin dependent and spin independent direct search techniques is
emphasized. We comment on the impact of the negative results on squark searches
at the LHC. Finally, we investigate how the fundamental parameters of the
models can be constrained in the event of a dark matter signal.Comment: 21 pages, 16 figure
XMM-Newton observations of ULIRGs I: A Compton-thick AGN in IRAS19254-7245
We present the XMM-Newton observation of the merging system IRAS 19254-7245,
also known as The Superantennae, whose southern nucleus is classified as a
Seyfert 2 galaxy. The XMM-Newton data have allowed us to perform a detailed
X-ray imaging and spectral analysis of this system. We clearly detect, for the
first time in this system, a strong EW ~ 1.4 keV Fe emission line at 6.49+/-0.1
keV (rest-frame). The X-ray spectrum requires a soft thermal component (kT~0.9
keV; L(0.5-2) ~ 4E41 cgs), likely associated with the starburst, and a hard
power-law continuum above 2 keV (observed L(2-10) ~ 4E42 cgs). We confirm the
flatness of this latter component, already noted in previous ASCA data. This
flatness, together with the detection of the strong Fe-Kalpha line and other
broad band indicators, suggest the presence of a Compton-thick AGN with
intrinsic luminosity > 1E44 cgs. We show that a Compton-thick model can
perfectly reproduce the X-ray spectral properties of this object.Comment: 6 pages, 4 figures, Latex manuscript, Accepted for publication in
Astronomy and Astrophysic
Quantum Phase Shift in Chern-Simons Modified Gravity
Using a unified approach of optical-mechanical analogy in a semiclassical
formula, we evaluate the effect of Chern-Simons modified gravity on the quantum
phase shift of de Broglie waves in neutron interferometry. The phase shift
calculated here reveals, in a single equation, a combination of effects coming
from Newtonian gravity, inertial forces, Schwarzschild and Chern-Simons
modified gravity. However the last two effects, though new, turn out to be too
tiny to be observed, and hence only of academic interest at present. The
approximations, wherever used, as well as the drawbacks of the non-dynamical
approach are clearly indicated.Comment: 16 pages, minor errors corrected. Accepted for publication in Phys.
Rev.
Comparing Galaxies and Lyman Alpha Absorbers at Low Redshift
A scenario is explored in which Lyman alpha absorbers at low redshift arise
from lines of sight through extended galaxy disks, including those of dwarf and
low surface brightness galaxies. A population of galaxies is simulated based
upon observed distributions of galaxy properties, and the gas disks are modeled
using pressure and gravity confinement. Some parameter values are ruled out by
comparing simulation results with the observed galaxy luminosity function, and
constraints may be made on the absorbing cross sections of galaxies. Simulation
results indicate that it is difficult to match absorbers with particular
galaxies observationally since absorption typically occurs at high impact
parameters (>200 kpc) from luminous galaxies. Low impact parameter absorption
is dominated by low luminosity dwarfs. A large fraction of absorption lines is
found to originate from low surface brightness galaxies, so that the absorbing
galaxy is likely to be misidentified. Low redshift Lyman alpha absorber counts
can easily be explained by moderately extended galaxy disks when low surface
brightness galaxies are included, and it is easily possible to find a scenario
which is consistent with observed the galaxy luminosity function, with low
redshift Lyman limit absorber counts, and with standard nucleosynthesis
predictions of the baryon density, Omega_Baryon.Comment: 17 pages, 8 figures, accepted to the Astrophysical Journa
The rotation curve and mass-distribution in highly flattened galaxies
A new method is developed which permits the reconstruction of the
surface-density distribution in the galactic disk of finite radius from an
arbitrary smooth distribution of the angular velocity via two simple
quadratures. The existence of upper limits for disk's mass and radius during
the analytic continuation of rotation curves into the hidden (non-radiating)
part of the disk is demonstrated.Comment: 13 pages, 2 figure
Formation of Disk Galaxies: Warm Dark Matter and the Angular Momentum problem
We have performed TreeSPH simulations of disk galaxy formation in various
warm dark matter (WDM) cosmologies. Our results indicate that for a range of
WDM free-streaming masses, the disk galaxy formation angular momentum problem
can be completely resolved by going to the WDM structure formation scenario,
without having to invoke stellar feedback processes at all. We also confirm our
previous suspicion, that part of the angular momentum problem is due to
numerical effects, most likely related to the shock capturing, artificial
viscosity used in SPH. Furthermore we find that we can match the observed
I-band Tully-Fisher (TF) relation, provided that the I-band mass-to-light ratio
of disk galaxies is about 0.8. We argue that this is quite a reasonable value
in comparison with various dynamical and spectrophotometric estimates,
including one given in this paper. We speculate that our success in matching
the TF relation may be due to WDM halos being less centrally concentrated than
CDM halos and suggest to check this exciting possibility with high resolution
simulations, in particular in low Omega_M, WDM cosmologies. Finally, we discuss
possible physical candidates for WDM particles extensively. We find that the
most promising are neutrinos with weaker or stronger interactions than normal,
majorons (light pseudogoldstone bosons) or mirror or shadow world neutrinos.Comment: 50 pages incl. 17 figures. Accepted for publication in Ap
Relation Between the Thickness of Stellar Disks and the Relative Mass of Dark Halo in Galaxies
We consider a thickness of stellar disks of late-type galaxies by analyzing
the R and K_s band photometric profiles for two independent samples of edge-on
galaxies. The main goal is to verify a hypotesis that a thickness of old
stellar disks is related to the relative masses of the spherical and disk
components of galaxies. We confirm that the radial-to-vertical scale length
ratio for galactic disks increases (the disks become thinner) with the
increasing of total mass-to-light ratio of the galaxies, which characterize the
contribution of dark halo to the total mass, and with the decreasing of central
deprojected disk brightness (surface density). Our results are in good
agreement with numerical models of collisionless disks evolved from subcritical
velocity dispersion state to a marginally stable equilibrium state. This
suggests that in most galaxies the vertical stellar velocity dispersion, which
determine the equilibrium disk thickness, is close to the minimum value, that
ensures disk stability. The thinnest edge-on disks appear to be low brightness
galaxies (after deprojection) in which a dark halo mass far exceeds a mass of
the stellar disk.Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002
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