58 research outputs found
High velocity structures in, and the X-ray emission from the LBV nebula around Eta Carinae
The Luminous Blue Variable star Eta Carinae is one of the most massive stars
known. It underwent a giant eruption in 1843 in which the Homunculus nebula was
created. ROSAT and ASCA data indicate the existence of a hard and a soft X-ray
component which appear to be spatially distinct: a softer diffuse shell of the
nebula around Eta Carinae and a harder point-like source centered on the star
Eta Car. Astonishingly the morphology of the X-ray emission is very different
from the optical appearance of the nebula. We present a comparative analysis of
optical morphology, the kinematics, and the diffuse soft X-ray structure of the
nebula around Eta Carinae. Our kinematic analysis of the nebula shows extremely
high expansion velocities. We find a strong correlation between the X-ray
emission and the knots in the nebula and the largest velocities, i.e. the X-ray
morphology of the nebula around Eta Carinae is determined by the interaction
between material streaming away from Eta Car and the ambient medium.Comment: 12 pages, 7 figures, A&A in press, same paper with images at full
resolution available from
http://www.ita.uni-heidelberg.de/~kweis/publications.htm
The kinematics of the diffuse ionized gas in NGC 4666
The global properties of the interstellar medium with processes such as
infall and outflow of gas and a large scale circulation of matter and its
consequences for star formation and chemical enrichment are important for the
understanding of galaxy evolution. In this paper we studied the kinematics and
morphology of the diffuse ionized gas (DIG) in the disk and in the halo of the
star forming spiral galaxy NGC 4666 to derive information about its kinematical
properties. Especially, we searched for infalling and outflowing ionized gas.
We determined surface brightness, radial velocity, and velocity dispersion of
the warm ionized gas via high spectral resolution (R ~ 9000) Fabry-P\'erot
interferometry. This allows the determination of the global velocity field and
the detection of local deviations from this verlocity field. We calculated
models of the DIG distribution and its kinematics for comparison with the
measured data. In this way we determined fundamental parameters such as the
inclination and the scale height of NGC 4666, and established the need for an
additional gas component to fit our observed data. We found individual areas,
especially along the minor axis, with gas components reaching into the halo
which we interpret as an outflowing component of the diffuse ionized gas. As
the main result of our study, we were able to determine that the vertical
structure of the DIG distribution in NGC 4666 is best modeled with two
components of ionized gas, a thick and a thin disk with 0.8 kpc and 0.2 kpc
scale height, respectively. Therefore, the enhanced star formation in NGC 4666
drives an outflow and also maintains a thick ionized gas layer reminiscent of
the Reynold's layer in the Milky Way.Comment: 12 pages, 10 figures, 3 table
Cosmic-ray driven dynamo in the medium of irregular galaxy
We investigate the cosmic ray driven dynamo in the interstellar medium of
irregular galaxy. The observations (Chyzy et al. 2000, 2003) show that the
magnetic field in irregular galaxies is present and its value reaches the same
level as in spiral galaxies. However the conditions in the medium of irregular
galaxy are very unfavorable for amplification the magnetic field due to slow
rotation and low shearing rate.
In this work we present numerical model of the interstellar medium in
irregular galaxies. The model includes magnetohydrodynamical dynamo driven by
cosmic rays in the interstellar medium provided by random supernova explosions.
We describe models characterized by different shear and rotation. We find that
even slow galactic rotation with low shearing rate gives amplification of the
magnetic field. Simulations have shown that high amount of the magnetic energy
flow out off the simulation region becoming an efficient source of
intergalactic magnetic fields.Comment: 2 pages, 2 figures, To be published in "Cosmic Magnetic Fields: From
Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E.
Beckman, eds., Proc. IAU Symp. 259, CU
Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from HeII
Contemporary research suggests that the reionisation of the intergalactic
medium (IGM) in the early Universe was predominantly realised by star-forming
(proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the
origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the
mechanisms facilitating their transport into the IGM remains sparse. Recent
efforts have thus focussed on the study of local analogues to these
high-redshift objects.
We used archival spectroscopic SDSS DR12 data to select a sample of low-z He
II 4686 emitters and restricted it to a set of SFGs with an emission line
diagnostic sensitive to the presence of an AGN, which serves as our only
selection criterion. Our final sample consists of eighteen low-mass,
low-metallicity dwarf galaxies which appear to be predominantly ionised by
stellar sources. We find large O32 ratios and [S II] deficiencies, which
provide strong indications for these galaxies to be LyC Emitters (LCEs). At
least 40% of these objects are candidates for featuring cosmologically
significant LyC escape fractions >10%. Their SFHs exhibit strong similarities
and almost all galaxies appear to contain an old (>1 Gyr) stellar component,
while also harbouring a young, two-stage (~10 Myr and <1 Myr) starburst, which
we speculate might be related to LyC escape.
The properties of the compact emission line galaxies presented here align
well with those observed in many local LCEs. In fact, our sample may prove as
an extension to the rather small catalogue of local LCEs, as the extreme
interstellar medium (ISM) conditions we find are assumed to facilitate LyC
leakage. Notably, all of our eighteen candidates are significantly closer
(z<0.1) than most established LCEs. If the inferred LyC photon loss is genuine,
this demonstrates that selecting SFGs from He II 4686 is a powerful selection
criterion in the search for LCEs.Comment: 28 pages, 22 figures. Accepted for publication in Astronomy &
Astrophysic
Sharpening up Galactic all-sky maps with complementary data - A machine learning approach
Galactic all-sky maps at very disparate frequencies, like in the radio and
-ray regime, show similar morphological structures. This mutual
information reflects the imprint of the various physical components of the
interstellar medium. We want to use multifrequency all-sky observations to test
resolution improvement and restoration of unobserved areas for maps in certain
frequency ranges. For this we aim to reconstruct or predict from sets of other
maps all-sky maps that, in their original form, lack a high resolution compared
to other available all-sky surveys or are incomplete in their spatial coverage.
Additionally, we want to investigate the commonalities and differences that the
ISM components exhibit over the electromagnetic spectrum. We build an
-dimensional representation of the joint pixel-brightness distribution of
maps using a Gaussian mixture model and see how predictive it is: How well
can one map be reproduced based on subsets of other maps? Tests with mock data
show that reconstructing the map of a certain frequency from other frequency
regimes works astonishingly well, predicting reliably small-scale details well
below the spatial resolution of the initially learned map. Applied to the
observed multifrequency data sets of the Milky Way this technique is able to
improve the resolution of, e.g., the low-resolution Fermi LAT maps as well as
to recover the sky from artifact-contaminated data like the ROSAT 0.855 keV
map. The predicted maps generally show less imaging artifacts compared to the
original ones. A comparison of predicted and original maps highlights
surprising structures, imaging artifacts (fortunately not reproduced in the
prediction), and features genuine to the respective frequency range that are
not present at other frequency bands. We discuss limitations of this machine
learning approach and ideas how to overcome them
Probing The Multiphase Interstellar Medium Of The Dwarf Starburst Galaxy NGC 625 With FUSE Spectroscopy
We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These
observations probe multiple phases of the interstellar medium, including the
coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc)
system shows a clear detection of outflowing coronal gas as traced by OVI 1032
Angstrom absorption. The centroid of the OVI profile is blueshifted with
respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a
low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20
km/sec, which is fully consistent with the detected HI outflow velocity found
in radio synthesis observations. We detect multiple lines of diffuse H2
absorption from the ISM of NGC 625; this is one of only a few extragalactic
systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find
a potential abundance offset between the neutral and nebular gas that exceeds
the errors on the derived column densities. Since such an offset has been found
in multiple dwarf galaxies, we discuss the implications of a lower-metallicity
halo surrounding the central star forming regions of dwarf galaxies. The
apparent offset may be due to saturation of the observed OI line, and higher
S/N observations are required to resolve this issue.Comment: ApJ, in press; full-resolution version may be obtained at
http://www.astro.umn.edu/~cannon/n625.fuse.p
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