1,068 research outputs found
A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction
The XA region of the Cygnus Loop is a complex zone of radiative and
nonradiative shocks interacting with interstellar clouds. We combine five far
ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT),
a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to
study the spatial emission line variations across the region. These spectral
data are placed in context using ground-based, optical emission line images of
the region and a far-UV image obtained by the Ultraviolet Imaging Telescope
(UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock
velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is
likely that a large range of shock velocities may exist at a spatial scale
smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII]
1909, we explore resonance scattering across the region. We find that a
significant column depth is present at all positions, including those not near
bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an
average optical depth of about unity in that ion while flux measurements of
[SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given
the brightness of the OVI emission and the age of the interaction, we rule out
the mixing layer interpretation of the UV emission. Furthermore, we formulate a
picture of the XA region as the encounter of the blast wave with a finger of
dense gas protruding inward from the pre-SN cavity.Comment: 21 pages, 9 figures, accepted by the Astronomical Journal, July 2001
Full resolution figures available at http://fuse.pha.jhu.edu/~danforth/xa
Experience with statically-generated proxies for facilitating Java runtime specialisation
Issues pertaining to mechanisms which can be used to change the behaviour of Java classes at runtime are discussed. The proxy mechanism is compared to, and contrasted with other standard approaches to this problem. Some of the problems the proxy mechanism is subject to are expanded upon. The question of whether statically-developed proxies are a viable alternative to bytecode rewriting was investigated by means of the JavaCloak system, which uses statically-generated proxies to alter the runtime behaviour of externally-developed code. The issues addressed include ensuring the type safety, dealing with the self problem, object encapsulation, and issues of object identity and equality. Some performance figures are provided which demonstrate the load the JavaCloak proxy mechanism places on the system
Studying the Warm-Hot Intergalactic Medium in Emission
We assess the possibility to detect the warm-hot intergalactic medium (WHIM)
in emission and to characterize its physical conditions and spatial
distribution through spatially resolved X-ray spectroscopy, in the framework of
the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are
equipped with microcalorimeter-based detectors. For this purpose we analyze a
large set of mock emission spectra, extracted from a cosmological
hydrodynamical simulation. These mock X-ray spectra are searched for emission
features showing both the OVII K alpha triplet and OVIII Ly alpha line, which
constitute a typical signature of the warm hot gas. Our analysis shows that 1
Ms long exposures and energy resolution of 2.5 eV will allow us to detect about
400 such features per deg^2 with a significance >5 sigma and reveals that these
emission systems are typically associated with density ~100 above the mean. The
temperature can be estimated from the line ratio with a precision of ~20%. The
combined effect of contamination from other lines, variation in the level of
the continuum, and degradation of the energy resolution reduces these
estimates. Yet, with an energy resolution of 7 eV and all these effects taken
into account, one still expects about 160 detections per deg^2. These line
systems are sufficient to trace the spatial distribution of the line-emitting
gas, which constitute an additional information, independent from line
statistics, to constrain the poorly known cosmic chemical enrichment history
and the stellar feedback processes.Comment: 19 pages, 10 figures, ApJ in press; revised version according to
revie
Probing the IGM/Galaxy Connection IV: The LCO/WFCCD Galaxy Survey of 20 Fields Surrounding UV Bright Quasars
We publish the survey for galaxies in 20 fields containing ultraviolet bright
quasars (with z_em 0.1 to 0.5) that can be used to study the association
between galaxies and absorption systems from the low-z intergalactic medium
(IGM). The survey is magnitude limited (R~19.5 mag) and highly complete out to
10' from the quasar in each field. It was designed to detect dwarf galaxies (L
~ 0.1 L*) at an impact parameter rho 1Mpc (z=0.1) from a quasar. The complete
sample (all 20 fields) includes R-band photometry for 84718 sources and
confirmed redshifts for 2800 sources. This includes 1198 galaxies with 0.005 <
z < (z_em - 0.01) at a median redshift of 0.18, which may associated with IGM
absorption lines. All of the imaging was acquired with cameras on the Swope 40"
telescope and the spectra were obtained via slitmask observations using the
WFCCD spectrograph on the Dupont 100" telescope at Las Campanas Observatory
(LCO). This paper describes the data reduction, imaging analysis, photometry,
and spectral analysis of the survey. We tabulate the principal measurements for
all sources in each field and provide the spectroscopic dataset online.Comment: Accepted to the Astrophysical Journal Supplements; 20 pages, only 6
figures shown in this version. See
http://www.ucolick.org/~xavier/WFCCDOVI/index.html for a full-length
manuscript and other supportive materia
Tracing baryons in the warm-hot intergalactic medium with broad Ly alpha absorption
We discuss physical properties and baryonic content of broad Ly alpha
absorbers (BLAs) at low redshift. These absorption systems, recently discovered
in high-resolution, high-signal to noise quasar absorption line spectra,
possibly trace the warm-hot intergalactic medium (WHIM) in the temperature
range between 10^5 and 10^6 K. To extend previous BLA measurements we have
analyzed STIS data of the two quasars H 1821+643 and PG 0953+415 and have
identified 13 BLA candidates along a total (unblocked) redshift path of
dz=0.440. Combining our measurements with previous results for the lines of
sight toward PG 1259+593 and PG 1116+215, the resulting new BLA sample consists
of 20 reliably detected systems as well as 29 additional tentative cases,
implying a BLA number density of dN/dz=22-53. We estimate that the contribution
of BLAs to the baryon density at z=0 is Omega_b(BLA)>0.0027 h_70^-1 for
absorbers with log (N/b)>11.3. This number indicates that WHIM broad Ly alpha
absorbers contain a substantial fraction of the baryons in the local Universe.
(Abridged abstract)Comment: 17 pages, 7 figures; Accepted for publication in A&
Probing the IGM/Galaxy Connection V: On the Origin of Lya and OVI Absorption at z<0.2
We analyze the association of galaxies to Lya and OVI absorption, the most
commonly detected transitions in the low-z intergalactic medium (IGM), in the
fields of 14 quasars with z_em = 0.06-0.57. Confirming previous studies, we
observe a high covering fraction for Lya absorption to impact parameter rho =
300kpc: 33/37 of our L>0.01L* galaxies show Lya equivalent width W_Lya>50mA.
Galaxies of all luminosity L>0.01L* and spectral type are surrounded by a
diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is
estimated at ~10^(10.5 +/- 0.3) Msun for a constant N_H. The virialized halos
and extended CGM of present-day galaxies are responsible for most strong Lya
absorbers (W_Lya > 300mA) but cannot reproduce the majority of observed lines
in the Lya forest. We conclude that the majority of Lya absorption with
W_Lya=30-300mA occurs in the cosmic web predicted by cosmological simulations
and estimate a characteristic width for these filaments of ~400kpc. Regarding
OVI, we observe a near unity covering fraction to rho=200kpc for L>0.1L*
galaxies and to rho = 300kpc for sub-L* (0.1 L*<L<L*) galaxies. Similar to our
Lya results, stronger OVI systems (W_OVI > 70mA) arise in the virialized halos
of L>0.1L* galaxies. Unlike Lya, the weaker OVI systems (W_OVI~30mA) arise in
the extended CGM of sub-L* galaxies. The majority of OVI gas observed in the
low-z IGM is associated with a diffuse medium surrounding individual galaxies
with L~0.3L*, and rarely originates in the so-called warm-hot IGM (WHIM)
predicted by cosmological simulations.Comment: Submitted to the Astrophysical Journal; 26 pages, 9 figures. See
http://www.ucolick.org/~xavier/WFCCDOVI/index.html for mor
Interstellar absorptions towards the LMC: Small-scale density variations in Milky Way disc gas
Observations show that the ISM contains sub-structure on scales less than 1
pc, detected in the form of spatial and temporal variations in column densities
or optical depth. Despite the number of detections, the nature and ubiquity of
the small-scale structure in the ISM is not yet fully understood. We use UV
absorption data mainly from FUSE and partly from STIS of six LMC stars
(Sk-67{\deg}111, LH54-425, Sk-67{\deg}107, Sk-67{\deg}106, Sk-67{\deg}104, and
Sk-67{\deg}101), all located within 5 arcmin of each other, and analyse the
physical properties of the Galactic disc gas in front of the LMC on sub-pc
scales. We analyse absorption lines of a number of ions within the UV spectral
range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and
fine-structure levels of neutral carbon have been used in order to study
changes in the density and the physical properties of the Galactic disc gas
over small angular scales. While most species do not show any significant
variation in their column densities, we find an enhancement of almost 2 dex for
H_2 from Sk-67{\deg}111 to Sk-67{\deg}101, accompanied by only a small
variation in the OI column density. Based on the formation-dissociation
equilibrium, we trace these variations to the actual density variations in the
molecular gas. On the smallest spatial scale of < 0.08 pc, between
Sk-67{\deg}107 and LH54-425, we find a gas density variation of a factor of
1.8. The line of sight towards LH54-425 does not follow the relatively smooth
change seen from Sk-67{\deg}101 to Sk-67{\deg}111, suggesting that
sub-structure might exist on a smaller spatial scale than the linear extent of
our sight-lines. Our observations suggest that the detected H_2 in these six
lines of sight is not necessarily physically connected, but that we are
sampling molecular cloudlets with pathlengths < 0.1-1.8 pc and possibly
different densities.Comment: 17 pages, 8 figures. Accepted for publication in A&
Multiphase Gas In Galaxy Halos: The OVI Lyman-limit System toward J1009+0713
We have serendipitously detected a strong O VI-bearing Lyman limit system at
z_abs = 0.3558 toward the QSO J1009+0713 (z_em = 0.456) in our survey of
low-redshift galaxy halos with the Hubble Space Telescope's Cosmic Origins
Spectrograph. Its rest-frame equivalent width of W_r = 835 +/- 49 mA is the
highest for an intervening absorber yet detected in any low-redshift QSO
sightline, with absorption spanning 400 km s^-1 in its rest frame. HST/WFC3
images of the galaxy field show that the absorber is associated with two
galaxies lying at 14 and 46 kpc from the QSO line of sight. The bulk of the
absorbing gas traced by H I resides in two strong, blended component groups
that possess a total logN(HI) = 18 - 18.8. The ion ratios and column densities
of C, N, O, Mg, Si, S, and Fe, except the O VI, can be accommodated into a
simple photoionization model in which diffuse, low-metallicity halo gas is
exposed to a photoionizing field from stars in the nearby galaxies that
propagates into the halo at 10% efficiency. We constrain the metallicity firmly
within the range 0.1 - 1 Zsun, and photoionization modeling indirectly
indicates a subsolar metallicity of 0.05 - 0.5 Zsun. The appearance of strong O
VI and nine Mg II components and our review of similar systems in the
literature support the "interface" picture of high-velocity O VI: the total
strength of the O VI shows a positive correlation with the number of detected
components in the low-ionization gas, however the total O VI column densities
still far exceed the values expected from interface models for the number of
detected clouds.Comment: 20 pages, 11 figures, accepted for publication in Ap
O VI Absorbers Tracing Hot Gas Associated with a Pair of Galaxies at z = 0.167
High signal-to-noise (S/N) observations of the QSO PKS 0405-123 (zem = 0.572)
with the Cosmic Origins Spectrograph from 1134 to 1796 A with a resolution of
17 km s-1 are used to study the multi-phase partial Lyman limit system (LLS) at
z = 0.16716 which has previously been studied using relatively low S/N spectra
from STIS and FUSE. The LLS and an associated H I-free broad O VI absorber
likely originate in the circumgalactic gas associated with a pair of galaxies
at z = 0.1688 and 0.1670 with impact parameters of 116 h70-1 and 99 h70-1. The
broad and symmetric O VI absorption is detected in the z = 0.16716 restframe
with v = -278 +/- 3 km s-1, log N(O VI) = 13.90 +/- 0.03 and b = 52 +/- 2 km
s-1. This absorber is not detected in H I or other species with the possible
exception of N V . The broad, symmetric O VI profile and absence of
corresponding H I absorption indicates that the circumgalactic gas in which the
collisionally ionized O VI arises is hot (log T ~ 5.8-6.2). The absorber may
represent a rare but important new class of low z IGM absorbers. The LLS has
strong asymmetrical O VI absorption with log N(O VI) = 14.72 +/- 0.02 spanning
a velocity range from -200 to +100 km s-1. The high and low ions in the LLS
have properties resembling those found for Galactic highly ionized HVCs where
the O VI is likely produced in the conductive and turbulent interfaces between
cool and hot gas.Comment: 68 pages including 13 figures. Accepted by ApJ (2010 Aug 20, vol 719
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