46 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
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
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
On the nature of the peculiar superthin galaxy UGC 12281
UGC 12281 has been classified as having a pure disk and being a low surface
brightness galaxy (LSBG), thus being an obvious member of the so-called
superthin galaxies. At the same time it represents an extremely untypical type
of LSBG due to its remarkable amount of current star formation and evidence for
extraplanar ionized gas. This makes it become a perfect tool to investigate the
triggering of star formation in LSB galaxies, located in an alleged isolated
area. By means of deep photometry and long-slit spectroscopy we analyse the
H halo and verify the existence of a potential dwarf companion which we
found on processed SDSS images.Comment: 3 pages, 2 figures, to appear in: IAU Symp. 284, The Spectral Energy
Distribution of Galaxies, eds.: R.J. Tuffs & C.C. Popesc
3D model of magnetic fields evolution in dwarf irregular galaxies
Radio observations show that magnetic fields are present in dwarf irregular
galaxies (dIrr) and its strength is comparable to that found in spiral
galaxies. Slow rotation, weak shear and shallow gravitational potential are the
main features of a typical dIrr galaxy. These conditions of the interstellar
medium in a dIrr galaxy seem to unfavourable for amplification of the magnetic
field through the dynamo process. Cosmic-ray driven dynamo is one of the
galactic dynamo model, which has been successfully tested in case of the spiral
galaxies. We investigate this dynamo model in the ISM of a dIrr galaxy. We
study its efficiency under the influence of slow rotation, weak shear and
shallow gravitational potential. Additionally, the exploding supernovae are
parametrised by the frequency of star formation and its modulation, to
reproduce bursts and quiescent phases. We found that even slow galactic
rotation with a low shearing rate amplifies the magnetic field, and that rapid
rotation with a low value of the shear enhances the efficiency of the dynamo.
Our simulations have shown that a high amount of magnetic energy leaves the
simulation box becoming an efficient source of intergalactic magnetic fields.Comment: 4 pages, 4 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche