977 research outputs found
Estimating extragalactic Faraday rotation
(abridged) Observations of Faraday rotation for extragalactic sources probe
magnetic fields both inside and outside the Milky Way. Building on our earlier
estimate of the Galactic contribution, we set out to estimate the extragalactic
contributions. We discuss the problems involved; in particular, we point out
that taking the difference between the observed values and the Galactic
foreground reconstruction is not a good estimate for the extragalactic
contributions. We point out a degeneracy between the contributions to the
observed values due to extragalactic magnetic fields and observational noise
and comment on the dangers of over-interpreting an estimate without taking into
account its uncertainty information. To overcome these difficulties, we develop
an extended reconstruction algorithm based on the assumption that the
observational uncertainties are accurately described for a subset of the data,
which can overcome the degeneracy with the extragalactic contributions. We
present a probabilistic derivation of the algorithm and demonstrate its
performance using a simulation, yielding a high quality reconstruction of the
Galactic Faraday rotation foreground, a precise estimate of the typical
extragalactic contribution, and a well-defined probabilistic description of the
extragalactic contribution for each data point. We then apply this
reconstruction technique to a catalog of Faraday rotation observations. We vary
our assumptions about the data, showing that the dispersion of extragalactic
contributions to observed Faraday depths is most likely lower than 7 rad/m^2,
in agreement with earlier results, and that the extragalactic contribution to
an individual data point is poorly constrained by the data in most cases.Comment: 20 + 6 pages, 19 figures; minor changes after bug-fix; version
accepted for publication by A&A; results are available at
http://www.mpa-garching.mpg.de/ift/faraday
Turbulence and Radio Mini-halos in the Sloshing Cores of Galaxy Clusters
A number of relaxed, cool-core galaxy clusters exhibit diffuse,
steep-spectrum radio sources in their central regions, known as radio
mini-halos. It has been proposed that the relativistic electrons responsible
for the emission have been reaccelerated by turbulence generated by the
sloshing of the cool core gas. We present a high-resolution MHD simulation of
gas sloshing in a galaxy cluster coupled with subgrid simulations of
relativistic electron acceleration to test this hypothesis. Our simulation
shows that the sloshing motions generate turbulence on the order of 50-200 km s on spatial scales of 50-100 kpc and below in the
cool core region within the envelope of the sloshing cold fronts, whereas
outside the cold fronts, there is negligible turbulence. This turbulence is
potentially strong enough to reaccelerate relativistic electron seeds (with
initial ) to via damping of
magnetosonic waves and non-resonant compression. The seed electrons could
remain in the cluster from, e.g., past AGN activity. In combination with the
magnetic field amplification in the core, these electrons then produce diffuse
radio synchrotron emission that is coincident with the region bounded by the
sloshing cold fronts, as indeed observed in X-rays and the radio. The result
holds for different initial spatial distributions of preexisting relativistic
electrons. The power and the steep spectral index () of the
resulting radio emission are consistent with observations of minihalos, though
the theoretical uncertainties of the acceleration mechanisms are high. We also
produce simulated maps of inverse-Compton hard X-ray emission from the same
population of relativistic electrons.Comment: 28 pages, 29 figures, in emulateapj format. Revised version accepted
by the referee, conclusions unchange
Polycyclic aromatic hydrocarbons in the dwarf galaxy IC 10
Infrared observations from the Spitzer Space Telescope archive are used to
study the dust component of the interstellar medium in the IC~10 irregular
galaxy. Dust distribution in the galaxy is compared to the distributions of
H and [SII] emission, neutral hydrogen and CO clouds, and ionizing
radiation sources. The distribution of polycyclic aromatic hydrocarbons (PAH)
in the galaxy is shown to be highly non-uniform with the mass fraction of these
particles in the total dust mass reaching 4%. PAHs tend to avoid bright HII
regions and correlate well with atomic and molecular gas. This pattern suggests
that PAHs form in the dense interstellar gas. We propose that the significant
decrease of the PAH abundance at low metallicity is observed not only globally
(at the level of entire galaxies), but also locally (at least, at the level of
individual HII regions). We compare the distribution of the PAH mass fraction
to the distribution of high-velocity features, that we have detected earlier in
wings of H and SII lines, over the entire available galaxy area. No
conclusive evidence for shock destruction of PAHs in the IC~10 galaxy could be
found.Comment: Accepted for publication in Astronomy Report
The internal dynamical equilibrium of HII regions: a statistical study
We present an analysis of the integrated Halpha emission line profiles for
the HII region population of the spiral galaxies NGC 1530, NGC 6951 and NGC
3359. We show that 70% of the line profiles show two or three Gaussian
components. The relations between the Halpha luminosity and non-thermal line
width for the HII regions of the three galaxies are studied and compared with
the relation found taken all the HII regions of the three galaxies as a single
distribution. A clearer envelope in non-thermal line width is found when only
those HII regions with non-thermal line width bigger than 13kms are considered.
The linear fit for the envelope is logL=36.8+2.0*log(sigma). The masses of the
HII regions on the envelope using the virial theorem and the mass estimates
from the Halpha luminosity are comparable, which offers evidence that the HII
regions on the envelope are virialized systems, while the remaining regions,
the majority, are not in virial equilibrium.Comment: 19 pages, 10 figures,accepted for publication in A&
New Models for Wolf-Rayet and O Star Populations in Young Starbursts
Using the latest stellar evolution models, theoretical stellar spectra, and a
compilation of observed emission line strengths from Wolf-Rayet (WR) stars, we
construct evolutionary synthesis models for young starbursts. We explicitly
distinguish between the various WR subtypes (WN, WC, WO), and we treat O and Of
stars separately. We provide detailed predictions of UV and optical emission
line strengths for both the WR stellar lines and the major nebular hydrogen and
helium emission lines, as a function of several input parameters related to the
starburst episode. We also derive the theoretical frequency of WR-rich
starbursts. We then discuss: nebular HeII 4686 emission, the contribution of WR
stars to broad Balmer line emission, techniques used to derive the WR and O
star content from integrated spectra, and explore the implications of the
formation of WR stars through mass transfer in close binary systems in
instantaneous bursts. The observational features predicted by our models allow
a detailed quantitative determination of the massive star population in a
starburst region (particularly in so-called "WR galaxies") from its integrated
spectrum and provide a means of deriving the burst properties (e.g., duration,
age) and the parameters of the initial mass function of young starbursts.
(Abridged abstract)Comment: Accepted by ApJ Supplements. LaTeX using aasmp4, psfigs macros. 49
pages including 23 figures. Paper (full, or text/figures separated) and
detailed model results available at
http://www.stsci.edu/ftp/science/starburst/sv97.htm
Mid-infrared spectral evidence for a luminous dust enshrouded source in Arp220
We have re-analyzed the 6-12 micron ISO spectrum of the ultra-luminous
infrared galaxy Arp220 with the conclusion that it is not consistent with that
of a scaled up version of a typical starburst. Instead, both template fitting
with spectra of the galaxies NGC4418 and M83 and with dust models suggest that
it is best represented by combinations of a typical starburst component,
exhibiting PAH emission features, and a heavily absorbed dust continuum which
contributes ~40% of the 6-12 micron flux and likely dominates the luminosity.
Of particular significance relative to previous studies of Arp220 is the fact
that the emission feature at 7.7 micron comprises both PAH emission and a
broader component resulting from ice and silicate absorption against a heavily
absorbed continuum. Extinction to the PAH emitting source, however, appears to
be relatively low. We tentatively associate the PAH emitting and heavily
dust/ice absorbed components with the diffuse emission region and the two
compact nuclei respectively identified by Soifer et al. (2002) in their higher
spatial resolution 10 micron study. Both the similarity of the absorbed
continuum with that of the embedded Galactic protostars and results of the dust
models imply that the embedded source(s) in Arp220 could be powered by, albeit
extremely dense, starburst activity. Due to the high extinction, it is not
possible with the available data to exclude that AGN(s) also contribute some or
all of the observed luminosity. In this case, however, the upper limit measured
for its hard X-ray emission would require Arp220 to be the most highly obscured
AGN known.Comment: 11 pages, 9 figures. Accepted for publication in A&A. Also available
at http://www.astro.rug.nl/~spoon/publications.htm
Near-IR spectroscopic ages of massive star clusters in M82
Like other starburst galaxies, M82 hosts compact, massive young star clusters
that are interesting both in their own right and as benchmarks for population
synthesis models. Can spectral synthesis models at resolutions around 1000
adequately reproduce the near-IR spectral features and the energy distribution
of these clusters between 0.8 and 2.4 microns? How do the derived cluster
properties compare with previous results from optical studies?
We analyse the spectra of 5 massive clusters in M82, using data acquired with
the spectrograph SpeX on the InfraRed Telescope Facility (NASA/IRTF) and a new
population synthesis tool with a highly improved near-IR extension, based on a
recent collection of empirical and theoretical spectra of red supergiant stars.
We obtain excellent fits across the near-IR with models at quasi-solar
metallicity and a solar neighbourhood extinction law. Spectroscopy breaks a
strong degeneracy between age and extinction in the near-IR colours in the red
supergiant-dominated phase of evolution. The estimated near-IR ages cluster
between 9 and 30 Myr, i.e. the ages at which the molecular bands due to
luminous red supergiants are strongest in the current models. They do not
always agree with optical spectroscopic ages. Adding optical data sometimes
leads to the rejection of the solar neighbourhood extinction law. This is not
surprising considering small-scale structure around the clusters, but it has no
significant effect on the near-IR based spectroscopic ages. [abridged]Comment: 14 pages, 20 figures, uses aa.cl
Tomographic weak lensing shear spectra from large N-body and hydrodynamical simulations
Forthcoming experiments will enable us to determine tomographic shear spectra
at a high precision level. Most predictions about them have until now been
biased on algorithms yielding the expected linear and non-linear spectrum of
density fluctuations. Even when simulations have been used, so-called Halofit
(Smith et al 2003) predictions on fairly large scales have been needed. We wish
to go beyond this limitation. We perform N-body and hydrodynamical simulations
within a sufficiently large cosmological volume to allow a direct connection
between simulations and linear spectra. While covering large length-scales, the
simulation resolution is good enough to allow us to explore the high-l
harmonics of the cosmic shear (up to l ~ 50000), well into the domain where
baryon physics becomes important. We then compare shear spectra in the absence
and in presence of various kinds of baryon physics, such as radiative cooling,
star formation, and supernova feedback in the form of galactic winds. We
distinguish several typical properties of matter fluctuation spectra in the
different simulations and test their impact on shear spectra. We compare our
outputs with those obtainable using approximate expressions for non--linear
spectra, and identify substantial discrepancies even between our results and
those of purely N-body results. Our simulations and the treatment of their
outputs however enable us, for the first time, to obtain shear results taht are
fully independent of any approximate expression, also in the high-l range,
where we need to incorporate a non-linear power spectrum of density
perturbations, and the effects of baryon physics. This will allow us to fully
exploit the cosmological information contained in future high--sensitivity
cosmic shear surveys, exploring the physics of cosmic shears via weak lensing
measurements.Comment: 13 pages, 19 figures, A&A in pres
Identification of the ionizing source of NGC 2024
We propose the late-O, early-B star IRS2b as the ionizing source of the Flame
Nebula (NGC 2024). It has been clear that such a hot, massive star must be
present in this heavily obscured region, and now it has been identified. New
near-infrared photometry shows that IRS2b is the most luminous and hottest star
in the young star cluster embedded in the center of NGC 2024. The near-infrared
observations (5' x 5') cover ~90 % of the HII region detected in radio
continuum radiation, making the probability very low that the ionizing star is
not present in the field. A K-band spectrum of IRS2b obtained with ISAAC on the
Very Large Telescope indicates that the spectral type of IRS2b is in the range
O8V - B2V. Additional arguments indicate that its spectral type is likely
closer to O8 than to B2. The corresponding amount of ionizing radiation is
consistent with published radio continuum and recombination line observations.Comment: 7 pages, 4 figures. Figure 1 included as jpeg. Accepted for
publication in Astronomy & Astrophysic
The intracluster magnetic field power spectrum in A2199
We investigate the magnetic field power spectrum in the cool core galaxy
cluster A2199 by analyzing the polarized emission of the central radio source
3C338. The polarized radiation from the radio emitting plasma is modified by
the Faraday rotation as it passes through the magneto-ionic intracluster
medium. We use Very Large Array observations between 1665 and 8415 MHz to
produce detailed Faraday rotation measure and fractional polarization images of
the radio galaxy. We simulate Gaussian random three-dimensional magnetic field
models with different power-law power spectra and we assume that the field
strength decreases radially with the thermal gas density as n_e^{\eta}. By
comparing the synthetic and the observed images with a Bayesian approach, we
constrain the strength and structure of the magnetic field associated with the
intracluster medium. We find that the Faraday rotation toward 3C338 in A2199 is
consistent with a magnetic field power law power spectrum characterized by an
index n=(2.8 \pm 1.3) between a maximum and a minimum scale of fluctuation of
\Lambda_{max}=(35 \pm 28) kpc and \Lambda_{min}=(0.7 \pm 0.1) kpc,
respectively. By including in the modeling X-ray cavities coincident with the
radio galaxy lobes, we find a magnetic field strength of =(11.7 \pm 9.0)
\mu G at the cluster center. Further out, the field decreases with the radius
following the gas density to the power of \eta=(0.9 \pm 0.5).Comment: 17 pages, 12 figures, A&A accepte
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