2,847 research outputs found
Gaussian decomposition of HI surveys. IV. Galactic intermediate- and high-velocity clouds
Traditionally IVC and HVC were defined to be concentrations of HI gas, with
line-of-sight velocities that are inconsistent with data on the differential
rotation of the Galaxy. We demonstrate that IVCs and HVCs can be identified
from certain density enhancements in (V_C, FWHM) distribution of Gaussians,
representing the Galactic HI 21 cm radio lines. We study the Gaussians, which
parameters fall into the regions of the phase-space density enhancements about
$(V_C, FWHM) = (-131, 27), (164, 26) and (-49 km/s, 23 km/s). The sky
distribution of the Gaussians, corresponding to the first two concentrations,
very well represents the sky distribution of HVCs, as obtained on the basis of
the traditional definition of these objects. The Gaussians of the last
concentration correspond to IVCs. Based on this identification, the division
line between IVCs and HVCs can be drawn at about |V_C| = 74 km/s, and IVCs can
be identified down to velocities of about |V_C| = 24 km/s. Traces of both IVCs
and HVCs can also be seen in the sky distribution of Gaussians with FWHM = 7.3
km/s. In HVCs, these cold cores have small angular dimensions and low observed
brightness temperatures T_b. In IVCs, the cores are both larger and brighter.
This definition of IVCs and HVCs is less dependant than the traditional one, on
the differential rotation model of the Galaxy. The consideration of line-width
information may enable IVCs and HVCs to be better distinguished from each
other, and from the ordinary Galactic HIComment: 9 pages, 7 figures. Accepted for publication in A&A. High-resolution
version available at http://www.aai.ee/~urmas/ast/HVCc.pdf (12.4 MBaits
Molecular gas in high-velocity clouds: revisited scenario
We report a new search for 12CO(1-0) emission in high-velocity clouds (HVCs)
performed with the IRAM 30 m telescope. This search was motivated by the recent
detection of cold dust emission in the HVCs of Complex C. Despite a spatial
resolution which is three times better and sensitivity twice as good compared
to previous studies, no CO emission is detected in the HVCs of Complex C down
to a best 5 sigma limit of 0.16 K km/s at a 22'' resolution. The CO emission
non-detection does not provide any evidence in favor of large amounts of
molecular gas in these HVCs and hence in favor of the infrared findings. We
discuss different configurations which, however, allow us to reconcile the
negative CO result with the presence of molecular gas and cold dust emission.
H2 column densities higher than our detection limit, N(H2) = 3x10^{19} cm^{-2},
are expected to be confined in very small and dense clumps with 20 times
smaller sizes than the 0.5 pc clumps resolved in our observations according to
the results obtained in cirrus clouds, and might thus still be highly diluted.
As a consequence, the inter-clump gas at the 1 pc scale has a volume density
lower than 20 cm^{-3} and already appears as too diffuse to excite the CO
molecules. The observed physical conditions in the HVCs of Complex C also play
an important role against CO emission detection. It has been shown that the
CO-to-H2 conversion factor in low metallicity media is 60 times higher than at
the solar metallicity, leading for a given H2 column density to a 60 times
weaker integrated CO intensity. And the very low dust temperature estimated in
these HVCs implies the possible presence of gas cold enough (< 20 K) to cause
CO condensation onto dust grains under interstellar medium pressure conditions
and thus CO depletion in gas-phase observations.Comment: 9 pages, 4 figures, Accepted for publication in A&
Quantum pumping in a ballistic graphene bilayer
We investigate quantum pumping of massless Dirac fermions in an ideal
(impurity free) double layer of graphene. The pumped current is generated by
adiabatic variation of two gate voltages in the contact regions to a weakly
doped double graphene sheet. At the Dirac point and for a wide bilayer with
width W much larger then length L, we find that the pumped current scales
linearly with the interlayer coupling length l_{perp} for L/l_{perp} << 1, is
maximal for L/l_{perp} ~ 1, and crosses over to a
log(L/l_{perp})/(L/l_{perp})-dependence for L/l_{perp} >> 1. We compare our
results with the behavior of the conductance in the same system and discuss
their experimental feasibility.Comment: 6 pages, 3 figure
High-resolution 21-cm observations of low-column density gas clumps in the Milky Way halo
We study the properties of low-column density gas clumps in the halo of the
Milky Way based on high-resolution 21-cm observations.
Using interferometric data from the WSRT and the VLA we study HI emission at
low-, intermediate- and high radial velocities along four lines of sight
towards quasars. Along these sightlines we previously detected weak CaII and
NaI absorbers in their optical spectra.
The analysis of the high-resolution HI data reveals the presence of several
compact and cold clumps of neutral gas at velocities similar to the optical
absorption. The clumps have narrow HI line widths in the range of 1.8 to 13
km/s, yielding upper limits for the kinetic temperature of the gas of 70 to
3700 K. The neutral gas has low HI column densities in the range of 5E18 to
3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes.
Our high-resolution 21-cm observations indicate that many of the CaII and NaI
absorbers seen in our optical quasar spectra are associated with low-column
density HI clumps at small angular scales. This suggests that next to the
massive, high-column density neutral gas clouds in the halo (the common 21-cm
LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas
structures in the halo that remain mostly unseen in the existing 21-cm all-sky
surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected
HI clumps are consistent with what is expected from theoretical models of gas
in the inner and outer Milky Way halo.Comment: 12 pages, 7 figure
High-Velocity Clouds in the Nearby Spiral Galaxy M 83
We present deep HI 21-cm and optical observations of the face-on spiral
galaxy M 83 obtained as part of a project to search for high-velocity clouds
(HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M
83, with 5.6x10^7 Msolar of HI contained in a disk rotating 40-50 km/s more
slowly in projection than the bulk of the gas. We interpret this as a
vertically extended thick disk of neutral material, containing 5.5% of the
total HI within the central 8 kpc. Using an automated source detection
algorithm to search for small-scale HI emission features, we find eight
distinct, anomalous-velocity HI clouds with masses ranging from 7x10^5 to
1.5x10^7 Msolar and velocities differing by up to 200 km/s compared to the HI
disk. Large on-disk structures are coincident with the optical spiral arms,
while unresolved off-disk clouds contain no diffuse optical emission down to a
limit of 27 r' mag per square arcsec. The diversity of the thick HI disk and
larger clouds suggests the influence of multiple formation mechanisms, with a
galactic fountain responsible for the slowly-rotating disk and on-disk discrete
clouds, and tidal effects responsible for off-disk cloud production. The mass
and kinetic energy of the HI clouds are consistent with the mass exchange rate
predicted by the galactic fountain model. If the HVC population in M 83 is
similar to that in our own Galaxy, then the Galactic HVCs must be distributed
within a radius of less than 25 kpc.Comment: 30 pages, 23 figures; accepted for publication in ApJ. Some figures
have been altered to reduce their siz
Infrared Search for Young Stars in HI High-velocity Clouds
We have searched the IRAS Point Source Catalog and HIRES maps for young
stellar objects (YSOs) in the direction of five \HI high-velocity clouds
(HVCs). In agreement with optical searches in the halo, no evidence was found
for extensive star-forming activity inside the high-latitude HVCs.
Specifically, we have found no signs of star formation or YSOs in the direction
of the A IV cloud or in the very-high-velocity clouds HVC~110-7-465 and
HVC~114-10-440. We have identified only one young star in the direction of the
M~I.1 cloud, which shows almost perfect alignment with a knot of \HI emission.
Because of the small number of early-type stars observed in the halo, the
probability for such a positional coincidence is low; thus, this young star
appears to be physically associated with the M~I.1 cloud. We have also
identified a good YSO candidate in the \HI shell-like structure observed in the
core region of the low-latitude cloud complex H (HVC~131+1-200). This region
could be a supernova remnant with several other YSO candidates formed along the
shock front produced by the explosion. In agreement with recent theoretical
estimates, these results point to a low but significant star-formation rate in
intermediate and high Galactic latitude HVCs. For M~I.1 in particular, we
estimate that the efficiency of the star-formation process is M(YSO)/M(\HI)\ga
10^{-4}-10^{-3} by mass. Such efficiency is sufficient to account for (a) the
existence of the few young blue stars whose ages imply that they were born in
the Galactic halo, and (b) the nonprimordial metallicities inferred for some
HVCs if their metal content proves to be low.Comment: 9 pages, 4 JPEG figures. PostScript figures available from author
A Single-Stage Approach to Anscombe and Aumann's Expected Utility
expected utility theory;decision analysis;revealed preference
Comonotonic Book-Making with Nonadditive Probabilities
This paper shows how de Finetti's book-making principle, commonly used to justify additive subjective probabilities, can be modi-ed to agree with some nonexpected utility models.More precisely, a new foundation of the rank-dependent models is presented that is based on a comonotonic extension of the book-making principle.The extension excludes book-making only if all gambles considered induce a same rank-ordering of the states of nature through favorableness of their associated outcomes, and allows for nonadditive probabilities.Typical features of rank-dependence, hedging, ambiguity aversion, and pessimism and optimism, can be accommodated.Book-making;comonotonic;Choquet expected utility;ambiguity aversion;ordered vector space
Origin(s) of the Highly Ionized High-Velocity Clouds Based on Their Distances
Previous HST and FUSE observations have revealed highly ionized high-velocity
clouds (HVCs) or more generally low HI column HVCs along extragalactic
sightlines over 70-90% of the sky. The distances of these HVCs have remained
largely unknown hampering to distinguish a "Galactic" origin (e.g., outflow,
inflow) from a "Local Group" origin (e.g., warm-hot intergalactic medium). We
present the first detection of highly ionized HVCs in the Cosmic Origins
Spectrograph (COS) spectrum of the early-type star HS1914+7134 (l = 103, b=+24)
located in the outer region of the Galaxy at 14.9 kpc. Two HVCs are detected in
absorption at v_LSR = -118 and -180 km/s in several species, including CIV,
SiIV, SiIII, CII, SiII, OI, but HI 21-cm emission is only seen at -118 \km.
Within 17 degrees of HS1914+7134, we found HVC absorption of low and high ions
at similar velocities toward 5 extragalactic sight lines, suggesting that these
HVCs are related. The component at -118 km/s is likely associated with the
Outer Arm of the Milky Way. The highly ionized HVC at -180 km/s is an HVC
plunging at high speed onto the thick disk of the Milky Way. This is the second
detection of highly ionized HVCs toward Galactic stars, supporting a "Galactic"
origin for at least some of these clouds.Comment: Submitted to the ApJ Letter
- …