640 research outputs found
A Framework for the Flexible Integration of a Class of Decision Procedures into Theorem Provers
The role of decision procedures is often essential in theorem proving. Decision procedures can reduce the search space of heuristic components of a prover and increase its abilities. However, in some applications only a small number of conjectures fall within the scope of the available decision procedures. Some of these conjectures could in an informal sense fall âjust outsideâ that scope. In these situations a problem arises because lemmas have to be invoked or the decision procedure has to communicate with the heuristic component of a theorem prover. This problem is also related to the general problem of how to exibly integrate decision procedures into heuristic theorem provers. In this paper we address such problems and describe a framework for the exible integration of decision procedures into other proof methods. The proposed framework can be used in different theorem provers, for different theories and for different decision procedures. New decision procedures can be simply âplugged-inâ to the system. As an illustration, we describe an instantiation of this framework within the Clam proof-planning system, to which it is well suited. We report on some results using this implementation
Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks
High--resolution 2D--MHD numerical simulations have been carried out to
investigate the effects of continuing infall of clumpy gas in extended HI
galactic disks. Given a certain accretion rate, the response of the disk
depends on its surface gas density and temperature. For Galactic conditions at
a galactocentric distance of ~20 kpc, and for mass accretion rates consistent
with current empirical and theoretical determinations in the Milky Way, the
rain of compact high velocity clouds onto the disk can maintain transonic
turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width
is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only
mechanism of driving turbulence. Some statistical properties of the resulting
forcing flow are shown in this Letter. The radial dependence of the gas
velocity dispersion is also discussed.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter
XMM-Newton observations of the spiral galaxy M74 (NGC 628)
The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February
2. In total, 21 sources are found in the inner 5' from the nucleus (after
rejection of a few sources associated to foreground stars). Hardness ratios
suggest that about half of them belong to the galaxy. The higher-luminosity end
of the luminosity function is fitted by a power-law of slope -0.8. This can be
interpreted as evidence of ongoing star formation, in analogy with the
distributions found in disks of other late-type galaxies. A comparison with
previous Chandra observations reveals a new ultraluminous X-ray transient (L_x
\~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We
find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5'
North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also
found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see
http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm
Diffuse HI Disks in Isolated Galaxies
In order to investigate the contribution of diffuse components to their total
HI emission, we have obtained high precision HI line flux densities with the
100m Green Bank Telescope for a sample of 100 isolated spiral and irregular
galaxies which we have previously observed with the 43m telescope. A comparison
of the observed HI line fluxes obtained with the two different telescopes,
characterized by half-power beam widths of 9 arcmin and 21 arcmin respectively,
exploits a ``beam matching'' technique to yield a statistical determination of
the occurrence of diffuse HI components in their disks. A simple model of the
HI distribution within a galaxy well describes ~75 % of the sample and accounts
for all of the HI line flux density. The remaining galaxies are approximately
evenly divided into two categories: ones which appear to possess a
significantly more extensive HI distribution than the model predicts, and ones
for which the HI distribution is more centrally concentrated than predicted.
Examples of both extremes can be found in the literature but little attention
has been paid to the centrally concentrated HI systems. Our sample has
demonstrated that galaxies do not commonly possess extended regions of low
surface brightness HI gas which is not accounted for by our current
understanding of the structure of HI disks. Eight HI-rich companions to the
target objects are identified, and a set of extragalactic HI line flux density
calibrators is presented.Comment: 26 page
A Digital Archive of HI 21 cm Line Spectra of Optically-targeted Galaxies
We present a homogeneous compilation of HI spectral parameters extracted from
global 21 cm line spectra for some 9000 galaxies in the local universe
(heliocentric velocity -200 < V_Sun < 28,000 km/s) obtained with a variety of
large single dish radio telescopes but reanalyzed using a single set of
parameter extraction algorithms. Corrections to the observed HI line flux for
source extent and pointing offsets and to the HI line widths for instrumental
broadening and smoothing are applied according to model estimates to produce a
homogenous catalog of derived properties with quantitative error estimates.
Where the redshift is available from optical studies, we also provide flux
measurements for an additional 156 galaxies classified as marginal HI
detections and rms noise limits for 494 galaxies classified as nondetections.
Given the diverse nature of the observing programs contributing to it, the
characteristics of the combined dataset are heterogeneous, and as such, the
compilation is neither integrated HI line flux nor peak flux limited. However,
because of the large statistical base and homogenous reprocessing, the spectra
and spectral parameters of galaxies in this optically targeted sample can be
used to complement data obtained at other wavelengths to characterize the
properties of galaxies in the local universe and to explore the large scale
structures in which they reside.Comment: 13 pages, 9 figures, 3 external online tables, accepted for
publication in ApJ
Ongoing Gas Stripping in the Virgo Cluster Spiral NGC 4522
The Virgo cluster galaxy NGC 4522 is one of the best spiral candidates for
ICM-ISM stripping in action. Optical broadband and H-alpha images from the WIYN
telescope of the highly inclined galaxy reveal a relatively undisturbed stellar
disk and a peculiar distribution of H-alpha emission. Ten percent of the
H-alpha emission arises from extraplanar HII regions which appear to lie within
filamentary structures >3 kpc long above one side of the disk. The filaments
emerge from the outer edge of a disk of bright H-alpha emission which is
abruptly truncated beyond 0.35R(25). Together the truncated H-alpha disk and
extraplanar H-alpha filaments are reminiscent of a bow shock morphology, which
strongly suggests that the interstellar medium (ISM) of NGC 4522 is being
stripped by the gas pressure of the intracluster medium (ICM). The galaxy has a
line-of-sight velocity of 1300 km/sec with respect to the mean Virgo cluster
velocity, and thus is expected to experience a strong interaction with the
intracluster gas. The existence of HII regions apparently located above the
disk plane suggests that star formation is occuring in the stripped gas, and
that newly formed stars will enter the galaxy halo and/or intracluster space.
The absence of HII regions in the disk beyond 0.35R(25), and the existence of
HII regions in the stripped gas suggest that even molecular gas has been
effectively removed from the disk of the galaxy.Comment: to appear in The Astronomical Journal, 16 pages, 5 figures, 1 tabl
Gas Rich Dwarfs from the PSS-II III. HI Profiles and Dynamical Masses
We present Arecibo neutral hydrogen data on a sample of optically selected
dwarf galaxies. The sample ranges in HI mass from 10^6 M_sun to 5x10^9 M_sun,
with a mean of 7.9x10^8 M_sun. Using estimated HI radii, the HI surface
densities range from 0.6 to 20 M_sun pc^-2, all well below the critical
threshold for star formation (Kennicutt 1998). M_HI/L values of the LSB dwarfs
range from 0.3 to 12 with a mean value of 2.0. Dynamical masses, calculated
from the HI profile widths, range from 10^8 M_sun to 10^11 M_sun. There is a
strong correlation between optical luminosity and dynamical mass for LSB dwarfs
implying that the dark matter (whether baryonic or non-baryonic) follows the
detectable baryonic matter.Comment: 53 pages, AASTeX v4.0, 8 figures, to be published in ApJ Suppl,
images, tables and referee report can be found at http://zebu.uoregon.edu/~j
Kinematics and morphology of ionized gas in Hickson Compact Group 18
We present new observations of emission in the Hickson Compact
Group 18 (HCG 18) obtained with a scanning Fabry-Perot interferometer. The
velocity field does not show motions of individual group members but, instead,
a complex common velocity field for the whole group. The gas distribution is
very asymmetric with clumps of maximum intensity coinciding with the optically
brightest knots. Comparing and HI data we conclude that HCG 18 is
not a compact group but instead a large irregular galaxy with several clumps of
star formation.Comment: Accepted for publication in Astronomical Journal (13p 6 figures
Starburst Galaxies: Why the Calzetti Dust Extinction Law?
The empirical reddening function for starburst galaxies generated by Calzetti
and her co-workers has proven very successful, and is now used widely in the
observational literature. Despite its success, however, the physical basis for
this extinction law, or more correctly, attenuation law remains weak. Here we
provide a physical explanation for the Calzetti Law based on a turbulent
interstellar medium. In essence, this provides a log-normal distribution of
column densities, giving a wide range of column densities in the dusty
foreground screen. Therefore, extended sources such as starburst regions or HII
regions seen through it suffer a point-to-point stochastic extinction and
reddening. Regions of high column densities are "black" in the UV, but
translucent in the IR, which leads to a flatter extinction law, and a larger
value of the total to selective extinction, R_V. We fit the Calzetti Law, and
infer that the variance sigma of the log-normal distribution lies in the range
0.6<sigma<2.2. The absolute to selective extinction R_V is found to be in the
range 4.3 to 5.2 consistent with R_V=4.05+/-0.80 of the Calzetti Law.Comment: accepted for publication in ApJ
HI Observations of the Stephan's Quintet
Using the VLA, we have made spectral-line and continuum observations of the
neutral hydrogen in the direction of the compact group of galaxies Stephan's
Quintet. The high-velocity clouds between 5600 and 6600 km/s, the disk of the
foreground galaxy, NGC 7320, at 800 km/s, the extended continuum ridge near the
center of the group, and 3 faint dwarf-like galaxies in the surrounding field
were imaged with C, CS, and D arrays. Four of the HI clouds previously detected
are confirmed. The two largest HI features are coincident with and concentrated
mainly along separate large tidal tails that extend eastward. The most diffuse
of the four clouds is resolved into two clumps, one coincide with tidal
features south of NGC 7318a and the other devoid of any detectable stellar or
Halfa sources. The two compact clouds, along the same line of sight, have peak
emission at luminous infrared and bright Halfa sources probably indicative of
star-forming activity. The total amount of HI detected at high redshifts is ~
10**10Msol. As in previous HI studies of the group, no detectable emission was
measured at the positions of any high-redshift galaxies so that any HI still
bound to their disks must be less than 2.4 x 10**7Msol.Comment: Accepted for publication in AJ and tentatively scheduled for the May
2002 issue. High-resolution ps figures are available at:
http://www.iaa.es/~lourdes/bw/bw-paper.tar.g
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