1,556 research outputs found
The Spectacular Ionized Interstellar Medium of NGC55
We present deep Halpha+[NII], [SII] (6716,6731A) and [OII] (3726,3729A)
images of the highly inclined, actively star--forming SBm galaxy NGC 55,
located in the nearby Sculptor Group. Due to its proximity, NGC 55 provides a
unique opportunity to study the disk--halo interface in a late--type galaxy
with unprecedented spatial resolution. Our images reveal a spectacular variety
of ionized gas features, ranging from giant HII region complexes, to supergiant
filamentary and shell features, to patches of very faint diffuse emission. Many
of these features protrude well above the plane of the galaxy, including a very
faint fragmented shell of emission which is visible at 2.6 kpc above the disk.
We identify candidate `chimneys' extending out of the disk, which could be the
conduits into the halo for hot gas around disk star-forming regions, and could
also provide low-density paths for the passage of UV photons from the disk to
the halo. Several of the identified chimneys are `capped' with clumps of
ionized gas, one of which, located at 1.5 kpc above the disk plane, appears to
be the site of recent star formation. Emission--line ratios
([OII]/Halpha+[NII], [SII]/Halpha+[NII]) constrain the ionization mechanism of
the gas, and our images allow the first measurement of [OII]/Halpha+[NII] in
extra-planar diffuse ionized gas. The diffuse gas is characterized by
emission--line ratios which are enhanced on average by a factor of two compared
to those of bright HII regions. Each line ratio increases in value smoothly
from the cores of HII regions, through the haloes of HII regions, into the
diffuse ionized gas. Such a continuous trend is predicted by models in which
the diffuse gas is ionized by photons produced by massive stars in HII regions.Comment: 19 pages, including 4 figures, plus 2 external tables. Accepted for
publication in the Astronomical Journal. Compressed, postscript versions of
the plates are available at ftp://skysrv.pha.jhu.edu/ferguson
Assessment and forecasting of lightning potential and its effect on launch operations at Cape Canaveral Air Force Station and John F. Kennedy Space Center
Lightning plays a pivotal role in the operation decision process for space and ballistic launches at Cape Canaveral Air Force Station and Kennedy Space Center. Lightning forecasts are the responsibility of Detachment 11, 4th Weather Wing's Cape Canaveral Forecast Facility. These forecasts are important to daily ground processing as well as launch countdown decisions. The methodology and equipment used to forecast lightning are discussed. Impact on a recent mission is summarized
Interactions of keV sterile neutrinos with matter
A sterile neutrino with mass of several keV is a well-motivated dark-matter
candidate, and it can also explain the observed velocities of pulsars via
anisotropic emission of sterile neutrinos from a cooling neutron star. We
discuss the interactions of such relic particles with matter and comment on the
prospects of future direct detection experiments. A relic sterile neutrino can
interact, via sterile-active mixing, with matter fermions by means of
electroweak currents, with the final state containing a relativistic active
neutrino. The recoil momentum impacted onto a matter fermion is determined by
the sterile neutrino mass and is enough to ionize atoms and flip the spins of
nuclei. While this suggests a possibility of direct experimental detection, we
calculate the rates and show that building a realistic detector of the required
size would be a daunting challenge.Comment: 5 pages, 1 figur
The Asymmetric Thick Disk: A Star Count and Kinematic Analysis. II The Kinematics
We report a kinematic signature associated with the observed asymmetry in the
distribution of thick disk/inner halo stars interior to the Solar circle
described in Paper I. In that paper we found a statistically significant excess
(20% to 25 %) of stars in quadrant I (l ~ 20 deg to 55 deg) both above and
below the plane (b ~ +/- 25 deg to +/- 45 deg) compared to the complementary
region in quadrant IV. We have measured Doppler velocities for 741 stars,
selected according to the same magnitude and color criteria, in the direction
of the asymmetry and in the corresponding fields in quadrant IV. We have also
determined spectral types and metallicities measured from the same spectra. We
not only find an asymmetric distribution in the V_LSR velocities for the stars
in the two regions, but the angular rate of rotation, w, for the stars in
quadrant I reveals a slower effective rotation rate compared to the
corresponding quadrant IV stars. We use our [Fe/H] measurements to separate the
stars into the three primary population groups, halo, thick disk, and disk, and
conclude that it is primarily the thick disk stars that show the slower
rotation in quadrant I. A solution for the radial, tangential and vertical
components of the V_LSR velocities, reveals a significant lag of ~ 80 to 90
km/s in the direction of Galactic rotation for the thick disk stars in quadrant
I, while in quadrant IV, the same population has only a ~ 20 km/s lag. The
results reported here support a rotational lag among the thick disk stars due
to a gravitational interaction with the bar as the most likely explanation for
the asymmetry in both the star counts and the kinematics. The affected thick
disk stars, however, may be associated with the recently discovered Canis Major
debris stream or a similar merger event (abridged).Comment: Accepted for publication in the Astronomical Journa
- …