863 research outputs found
Spherical Branes
We construct new solutions of ten-dimensional supergravity with sixteen
supercharges which describe the backreaction of D-branes with spherical
worldvolume. These solutions are holographically dual to the
-dimensional maximally supersymmetric Yang-Mills theory on .
The finite size of the sphere provides an IR cut-off for the gauge theory which
is manifested in the supergravity solution as a smooth cap-off of the geometry.
In the UV the size of the sphere plays no role and the backgrounds asymptote to
the well-known supergravity solutions that describe the near-horizon limit of
flat D-branes. We compute the on-shell action of our spherical brane
solutions and show that it is in agreement with recent supersymmetric
localization results for the free energy of maximal SYM theory on .Comment: 57 pages, 4 figures; v2: minor corrections, appendix adde
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
Discovery of a tidal dwarf galaxy in the Leo Triplet
We report discovery of a dwarf galaxy in the Leo Triplet. Analysis of the
neutral hydrogen distribution shows that it rotates independently of the tidal
tail of NGC 3628, with a radial velocity gradient of 35-40 km s over
approximately 13 kpc. The galaxy has a very high neutral gas content,
explaining large part of its total dynamic mass - suggesting small dark matter
content. As it is located at the tip of the gaseous tail, this strongly
suggests its tidal origin. Should it be the case, it would be one of the most
confident and closest (to the Milky Way) detections of a tidal dwarf galaxy
and, at the same time, a most detached from its parent galaxy (140
kpc) object of this type.Comment: 6 pages, 4 figures; The Astrophysical Journal, in pres
Outflow or galactic wind: The fate of ionized gas in the halos of dwarf galaxies
Context: H\alpha images of star bursting irregular galaxies reveal a large
amount of extended ionized gas structures, in some cases at kpc-distance away
from any place of current star forming activity. A kinematic analysis of
especially the faint structures in the halo of dwarf galaxies allows insights
into the properties and the origin of this gas component. This is important for
the chemical evolution of galaxies, the enrichment of the intergalactic medium,
and for the understanding of the formation of galaxies in the early universe.
Aims: We want to investigate whether the ionized gas detected in two
irregular dwarf galaxies (NGC 2366 and NGC 4861) stays gravitationally bound to
the host galaxy or can escape from it by becoming a freely flowing wind.
Methods: Very deep H\alpha images of NGC 2366 and NGC 4861 were obtained to
detect and catalog both small and large scale ionized gas structures down to
very low surface brightnesses. Subsequently, high-resolution long-slit echelle
spectroscopy of the H\alpha line was performed for a detailed kinematic
analysis of the most prominent filaments and shells. To calculate the escape
velocity of both galaxies and to compare it with the derived expansion
velocities of the detected filaments and shells, we used dark matter halo
models.
Results: We detected a huge amount of both small scale (up to a few hundred
pc) and large scale (about 1-2 kpc of diameter or length) ionized gas
structures on our H\alpha images. Many of the fainter ones are new detections.
The echelle spectra reveal outflows and expanding bubbles/shells with
velocities between 20 and 110 km/s. Several of these structures are in
accordance with filaments in the H\alpha images. A comparison with the escape
velocities of the galaxies derived from the NFW dark matter halo model shows
that all gas features stay gravitationally bound.Comment: 15 pages, 13 figures, accepted for publication in A&
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
A search for extended radio emission from selected compact galaxy groups
Context. Studies on compact galaxy groups have led to the conclusion that a
plenitude of phenomena take place in between galaxies that form them. However,
radio data on these objects are extremely scarce and not much is known
concerning the existence and role of the magnetic field in intergalactic space.
Aims. We aim to study a small sample of galaxy groups that look promising as
possible sources of intergalactic magnetic fields; for example data from radio
surveys suggest that most of the radio emission is due to extended, diffuse
structures in and out of the galaxies. Methods. We used the Effelsberg 100 m
radio telescope at 4.85 GHz and NRAO VLA Sky Survey (NVSS) data at 1.40 GHz.
After subtraction of compact sources we analysed the maps searching for
diffuse, intergalactic radio emission. Spectral index and magnetic field
properties were derived. Results. Intergalactic magnetic fields exist in groups
HCG 15 and HCG 60, whereas there are no signs of them in HCG 68. There are also
hints of an intergalactic bridge in HCG 44 at 4.85 GHz. Conclusions.
Intergalactic magnetic fields exist in galaxy groups and their energy density
may be comparable to the thermal (X-ray) density, suggesting an important role
of the magnetic field in the intra-group medium, wherever it is detected.Comment: 13 pages, 4 figures, 3 tables, accepted for publication in A&
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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