24 research outputs found
On the origin of anomalous velocity clouds in the Milky Way
We report that neutral hydrogen (HI) gas clouds, resembling High Velocity
Clouds (HVCs) observed in the Milky Way (MW), appear in MW-sized disk galaxies
formed in high-resolution Lambda Cold Dark Matter (LCDM) cosmological
simulations which include gas-dynamics, radiative cooling, star formation,
supernova feedback, and metal enrichment. Two such disk galaxies are analyzed,
and HI column density and velocity distributions in all-sky Aitoff projections
are constructed. The simulations demonstrate that LCDM is able to create
galaxies with sufficient numbers of anomalous velocity gas clouds consistent
with the HVCs observed within the MW, and that they are found within a
galactocentric radius of 150 kpc. We also find that one of the galaxies has a
polar gas ring, with radius 30 kpc, which appears as a large structure of HVCs
in the Aitoff projection. Such large structures may share an origin similar to
extended HVCs observed in the MW, such as Complex C.Comment: Accepted by ApJL, 08 Jun 2006. 5 pages, 5 figures, 1 table. LaTeX
(emulateapj.cls). File with high resolution images available at
http://astronomy.swin.edu.au/~tconnors/publications/ . References added;
discussion added to, but conclusions unchange
A Very Sensitive 21cm Survey for Galactic High-Velocity HI
Very sensitive HI 21cm observations have been made in 860 directions at dec
>= -43deg in search of weak, Galactic, high-velocity HI emission lines at
moderate and high Galactic latitudes. One-third of the observations were made
toward extragalactic objects. The median 4-sigma detection level is NHI =
8x10^{17} cm^-2 over the 21' telescope beam. High-velocity HI emission is
detected in 37% of the directions; about half of the lines could not have been
seen in previous surveys. The median FWHM of detected lines is 30.3 km/s. High-
velocity HI lines are seen down to the sensitivity limit of the survey implying
that there are likely lines at still lower values of NHI. The weakest lines
have a kinematics and distribution on the sky similar to that of the strong
lines, and thus do not appear to be a new population. Most of the emission
originates from objects which are extended over several degrees; few appear to
be compact sources. At least 75%, and possibly as many as 90%, of the lines are
associated with one of the major high-velocity complexes. The Magellanic Stream
extends at least 10 deg to higher Galactic latitude than previously thought and
is more extended in longitude as well. Although there are many lines with low
column density, their numbers do not increase as rapidly as NHI^-1, so most of
the HI mass in the high-velocity cloud phenomenon likely resides in the more
prominent clouds. The bright HI features may be mere clumps within larger
structures, and not independent objects.Comment: 88 pages includes 22 figures Accepted for Publication in ApJ Suppl.
June 200
Highly-Ionized High-Velocity Gas in the Vicinity of the Galaxy
We report the results of an extensive FUSE study of high velocity OVI
absorption along 102 complete sight lines through the Galactic halo. The high
velocity OVI traces a variety of phenomena, including tidal interactions with
the Magellanic Clouds, accretion of gas, outflow from the Galactic disk,
warm/hot gas interactions in a highly extended Galactic corona, and
intergalactic gas in the Local Group. We identify 85 high velocity OVI features
at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines.
Approximately 60% of the sky (and perhaps as much as 85%) is covered by high
velocity H+ associated with the high velocity OVI. Some of the OVI is
associated with known high velocity HI structures (e.g., the Magellanic Stream,
Complexes A and C), while some OVI features have no counterpart in HI 21cm
emission. The smaller dispersion in the OVI velocities in the GSR and LGSR
reference frames compared to the LSR is necessary (but not conclusive) evidence
that some of the clouds are extragalactic. Most of the OVI cannot be produced
by photoionization, even if the gas is irradiated by extragalactic background
radiation. Collisions in hot gas are the primary OVI ionization mechanism. We
favor production of some of the OVI at the boundaries between warm clouds and a
highly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3]
Galactic corona or Local Group medium. A hot Galactic corona or Local Group
medium and the prevalence of high velocity OVI are consistent with predictions
of galaxy formation scenarios. Distinguishing between the various phenomena
producing high velocity OVI will require continuing studies of the distances,
kinematics, elemental abundances, and physical states of the different types of
high velocity OVI features found in this study. (abbreviated)Comment: 78 pages of text/tables + 31 figures, AASTeX preprint format. All
figures are in PNG format due to astro-ph space restrictions. Bound copies of
manuscript and two accompanying articles are available upon request.
Submitted to ApJ
Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption line measurements
for the high- and intermediate velocity clouds (HVCs and IVCs). New values are
derived for N(HI) in the direction of observed probes, in order to arrive at
reliable abundances and abundance limits (the HI data are described in Paper
II). Distances to stellar probes are revisited and calculated consistently, in
order to derive distance brackets or limits for many of the clouds, taking care
to properly interpret non-detections. The main conclusions are the following.
1) Absolute abundances have been measured using lines of SII, NI and OI, with
the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar
for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~ solar for two
northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the
range 0.5-2 solar are found for three more IVCs. 2) Depletion patterns in IVCs
are like those in warm disk or halo gas. 3) Most distance limits are based on
strong UV lines of CII, SiII and MgII, a few on CaII. Distance limits for major
HVCs are >5 kpc, while distance brackets for several IVCs are in the range
0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major
HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5
within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies
by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be
useful for determining both lower and upper distance limits, but NaI only
yields upper limits.Comment: To appear in the "Astrophysical Journal Supplement"; 82 pages;
figures 6, 9 and 10 are in color; degraded figures (astro-ph restriction) -
ask for good version
Analytical tools for environmental design and management in a systems perspective
The aim of this book is to link demand and supply of environmental information in the field of Life Cycle Management
Mise au point d'un appareillage d'analyse des surfaces : application a l'etude de la corrosion des verres et a l'etude des films minces deposes par le procede sol-gel
SIGLECNRS T 59591 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc