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05/2007: pages 424-431;
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ABSTRACT: We analyzed the soft X-ray background (SXRB) source intensity distribution of the 3/4 keV and 1/4 keV photons by correlating
the public ROSAT PSPC All-Sky Survey data with the new Leiden/Dwingeloo HI Survey. We found that the 3/4 keV and 1/4 keV distant background source intensity distribution can be modelled by an isothermal
(T=1.5 · 106 K) disk-like (scale length A
1=15 kpc) X-ray halo in addition to the radiation of the extragalactic X-ray background.
05/2007: pages 471-474;
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ABSTRACT: Diffuse excess 1/4 keV soft X-ray emission was found to be positionally correlated with the column density distribution of
the high velocity cloud (HVC) complex C (Kerp et al. 1996). Here we point out that the detected diffuse X-ray emission is
indeed associated with the HVC phenomenon. For this purpose we study the 1/4 keV radiation transfer as well as the HI column density distribution of HVCs and intermediate velocity clouds (IVCs) towards HVC complex C in detail. We present evidence
that on arcmin scales the 3/4 keV soft X-ray emission is positionally anticorrelated with the HVC column density distribution
of an individual HVC filament of complex C.
05/2007: pages 455-466;
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ABSTRACT: We find indications for diffuse Hi gas at substantial z heights in our Galaxy, with a velocity dispersion of 60 km s−1 and a vertical projected column density of 1.4·1019 cm−2. This pervasive component of the emission spectrum could be identified in the Leiden/Dwingeloo 21 cm Survey (LDS) after increasing
the accuracy further by correcting the observations for reflections from ground. Assuming hydrostatic equilibrium an exponential
scale height of 4.4 kpc for the observed diffuse high-dispersion H1 component is deduced. This differs from the scale height
of 1 kpc derived by Lockman & Gehman (1991), which corresponds to a velocity dispersion of 34 km s−, based on an analysis of the the Bell Laboratories H1 Survey (BLS). A comparison of BLS and LDS data explains the differences
in the derived model parameters in terms of baseline uncertainties at a level of ≈30 mK. We find additional indications for
baseline uncertainties in the BLS data. Concerning the LDS we cannot, however, exclude that this survey may also be affected
by baseline uncertainties. Receiver bandpass and stray-radiation effects need a more thorough analysis before drawing from
conclusions.
05/2007: pages 479-482;
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ABSTRACT: We present the first dynamical study of the globular cluster system of NGC 4636. This giant elliptical galaxy is claimed to be extremely dark matter dominated, according to X-ray observations. Using the VLT with FORS2/MXU, we obtained velocities for 174 globular clusters. The clusters have projected galactocentric distances in the range 4 to 70 kpc, the overwhelming majority lie within 30 kpc. We find some indication for a rotation of the red (metal-rich) clusters about the minor axis. Out to a radius of 30 kpc, we find a roughly constant projected velocity dispersion for the blue clusters of ~200 km/s. The red clusters exhibit a distinctly different behavior: at a radius of about 13 kpc, the velocity dispersion drops by ~50 km/s to about 170 km/s which then remains constant out to a radius of 30 kpc. Using only the blue clusters as dynamical tracers, we perform Jeans-analyses for different assumptions of the orbital anisotropy. Depending on the anisotropy and the adopted M/L-values, we find that the dark matter fraction within one effective radius can vary between 20% and 50% with most a probable range between 20% and 30%. A main source of uncertainty is the ambiguity of the velocity dispersion in the outermost bin. Although the dark halo mass still cannot be strongly constrained, NGC 4636 does not seem to be extremely dark matter dominated. The derived circular velocities are also consistent with Modified Newtonian Dynamics. Comment: 19 pages, 18 figures. Accepted for publication in A&A. Appendix A (velocity tables) will be published in the online version of the journal
Astronomy and Astrophysics 11/2006; 459(2):391. · 4.59 Impact Factor
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ABSTRACT: We present the first fully and uniformly sampled, spatially complete HI survey of the entire Magellanic System with high velocity resolution, performed with the Parkes Telescope. The final data-cubes have an rms noise of sigma ~ 0.05 K and an effective angular resolution of 16 arcmin. The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are associated with huge gaseous features with a total HI mass of M(HI) = 4.87 10^8 M_sun [d/55 kpc]^2, if all HI gas is at the same distance of 55 kpc. Approximately two thirds of this HI gas is located close to the Magellanic Clouds (Magellanic Bridge and Interface Region), and 25% of the HI gas is associated with the Magellanic Stream. The Leading Arm has a four times lower HI mass than the Magellanic Stream, corresponding to 6% of the total HI mass of the gaseous features. We have analyzed the velocity field of the Magellanic Clouds and their neighborhood introducing a LMC-standard-of-rest frame. The HI in the Magellanic Bridge shows low velocities relative to the Magellanic Clouds suggesting an almost parallel motion, while the gas in the Interface Region has significantly higher relative velocities indicating that this gas is leaving the Magellanic Bridge building up a new section of the Magellanic Stream. The clouds in the Magellanic Stream and the Leading Arm show significant differences, both in the column density distribution and in the shapes of the line profiles. The HI gas in the Magellanic Stream is more smoothly distributed than the gas in the Leading Arm. These morphological differences can be explained if the Leading Arm is at considerably lower z-heights and embedded in a higher pressure ambient medium. Comment: 23 pages, 18 figures, accepted for publication in A&A
11/2004;
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ABSTRACT: We present new observational results on high-velocity clouds (HVCs) based on an analysis of the Leiden/Dwingeloo HI survey. We cataloged all HVCs with NHI > 1 10^19 cm^-2 and found 252 clouds that form a representative flux limited sample. The detailed analysis of each individual HVC in this sample revealed a significant number of HVCs (nearly 20%) having simultaneously a velocity and a column density gradient. These HVCs have a cometary appearance in the position-velocity representation and are called henceforward head-tail HVCs (HT HVCs). The head is the region with the highest column density of the HVC, while the column density of the tail is in general much lower (by a factor of 2 - 4). The absolute majority of the cataloged HVCs belongs to the well known HVC complexes. With exception of the very faint HVC complex L, all HVC complexes contain HT HVCs. The HT HVCs were analyzed statistically with respect to their physical parameters like position, velocity (v_LSR,v_GSR), and column density. We found a linear correlation between the fraction of HVCs having a head-tail structure and the peak column density of the HVCs. While there is no correlation between the fraction of HT HVCs and v_LSR, we found a dependence of the fraction of HT HVCs and v_GSR. There is no significant correlation between the fraction of HT HVCs and the parameters galactic longitude and latitude. The HT HVCs may be interpreted as HVCs that are currently interacting with their ambient medium. In the context of this model the tails represent material that is stripped off from the HVC core. We discuss the implications of this model for galactic and extragalactic HVCs.
04/2000;
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ABSTRACT: The cool atomic interstellar medium of the Large Magellanic Cloud (LMC)
seems to be quite different from that in the Milky Way. In a series of
three papers we study the properties of the cool atomic hydrogen in the
LMC (Paper I), its relation to molecular clouds using
SEST-CO-observations (Paper II) and the cooling mechanism of the atomic
gas based on ISO-[\CII]-investigations (Paper III). In this paper we
present the results of a third 21 cm absorption line survey toward the
LMC carried out with the Australia Telescope Compact Array (ATCA). 20
compact continuum sources, which are mainly in the direction of the
supergiant shell LMC 4, toward the surroundings of 30 Doradus and toward
the eastern steep \HI\ boundary, have been chosen from the 1.4 GHz
snapshot continuum survey of Marx et al. We have identified 20
absorption features toward nine of the 20 sources. The properties of the
cool \HI\ clouds are investigated and are compared for the different
regions of the LMC taking the results of Dickey et al. (survey 2) into
account. We find that the cool \HI\ gas in the LMC is either unusually
abundant compared to the cool atomic phase of the Milky Way or the gas
is clearly colder (\Tc\ ~ 30 K) than that in our Galaxy (\Tc\ ~ 60 K).
The properties of atomic clouds toward 30 Doradus and LMC 4 suggest a
higher cooling rate in these regions compared to other parts of the LMC,
probably due to an enhanced pressure near the shock fronts of LMC 4 and
30 Doradus. The detected cool atomic gas toward the eastern steep \HI\
boundary might be the result of a high compression of gas at the leading
edge.
The Australia Telescope is funded by the Commonwealth of Australia for
operation as a National Facility managed by CSIRO.
Astronomy and Astrophysics 01/2000; 354:787-801. · 4.59 Impact Factor
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ABSTRACT: We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI survey, looking for soft X-ray signatures of prominent high-velocity-cloud (HVC) complexes. We study the transfer of 1/4 keV photons through the interstellar medium in order to distinguish variations in the soft X-ray background (SXRB) intensity caused by photoelectric absorption effects from those due to excess X-ray emission. The X-ray data are modelled as a combination of emission from the Local Hot Bubble (LHB) and emission from a distant plasma in the galactic halo and extragalactic sources. The X-ray radiation intensity of the galactic halo and extragalactic X-ray background is modulated by the photoelectric absorption of the intervening galactic interstellar matter. We show that large- and small-scale intensity variations of the 1/4 keV SXRB are caused by photoelectric absorption which is predominantly traced by the total N(HI) distribution. The extensive coverage of the two surveys supports evidence for a hot, X-ray emitting corona. We show that this leads to a good representation of the SXRB observations. For four large areas on the sky, we search for regions where the modelled and observed X-ray emission differ. We find that there is excess X-ray emission towards regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray emission is positionally correlated with the high-velocity clouds. Some lines of sight towards HVCs also pass through significant amounts of intermediate-velocity gas, so we cannot constrain the possible role played by IVC gas in these directions of HVC and IVC overlap, in determining the X-ray excesses.
11/1998;
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ABSTRACT: We have used the ROSAT (1)/(4) keV all-sky survey together with H I
observations to derive the total column density of hydrogen nuclei,
N(H), of the Draco nebula [= G91+38 (v_LSR = -21 km s(-1) )], which
casts a deep shadow in the soft X-ray background. Adopting a
two-component model for the X-ray plasma in which one component is
located behind the Draco nebula, the other in front of all the absorbing
material (the so-called Local Hot Bubble, LHB), we fit the parameters of
the radiation transport equation to the observed X-ray count rates. The
optical depth in this equation is derived from H I column densities
obtained with the 100-m telescope and the appropriate X-ray absorption
cross sections. The solutions obtained by this approach are biased since
H I column densities underestimate the absorption in regions where
molecular hydrogen is abundant. The bias is avoided by excluding regions
with strong X-ray shadowing from the fit and by comparing fits which are
obtained on the basis of hydrogen column densities derived from IRAS 100
mu m data. We find that the absorbing column densities at the deepest
X-ray shadows are up to about 3 10(20) cm(-2) larger than the observed H
I column densities. At the edge towards low galactic latitudes and
longitudes, up to 70% of the hydrogen is in molecular form. In other
parts of the nebula the molecular abundance is la25 %. We also find an
approximately constant FIR-emissivity per hydrogen nucleon (H I + 2H_2)
of about 1.0 10(-20) MJy sr(-1) cm(2) . This is close to the mean value
for the galactic cirrus (0.86 10(-20) MJy sr(-1) cm(2) ). In contrast,
the FIR-emissivity per H I atom is varying strongly across the nebula.
The xWCO values (equiv N(H_2)/W((12) CO)) found in the Draco
nebula are typically in the range 0.34 < xWCO < 0.52
10(20) cm(-2) (K km s(-1) )(-1) , similar to other cirrus clouds. We
find a very low xWCO ratio of 0.17 cm(-2) (K km s(-1) )(-1)
at the edge of the Draco nebula towards low galactic coordinates where
the CO abundance could be altered in a low-velocity shock. Finally, the
X-ray emission measure for the distant component of the X-ray emitting
plasma is found to be about 5 times larger than that for the LHB,
assuming constant plasma temperatures of 10(6.3) K and 10(5.85) K
respectively. Since the Draco nebula (distance > 300 pc) is located
outside the galactic gas layer, this is evidence of a bright Galactic
X-ray corona or an extended coronal hot spot. The intensity of this
coronal emission is constant over the observed 7degr -field within the
uncertainties of our analysis (< 15%).
Astronomy and Astrophysics 07/1998; 336:682-696. · 4.59 Impact Factor
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ABSTRACT: We analyze mosaiked ROSAT PSPC pointings towards the Large Magellanic
Cloud (LMC). Focussing on the area south-east of 30Doradus roughly at
the position of RA(2000)=5h40m and DEC(2000)=-70°, we verify the
existence of an X-ray absorption feature. The ROSAT data imply that
X-ray attenuating material is located in front of a diffuse X-ray source
that extends from the eastern boundary of the LMC to west of the giant
molecular cloud south of 30 Dor. The difference of the absorbing
hydrogen column densities, derived from the ROSAT data, between the
X-ray bright ``wedge'' region and dark ``shadow'' region is about
N_H_=(7.0+/-1.8).10^21^cm^-2^. This column density difference is
consistent with the value derived from IRAS 100μm emission map of
this region which leads to total hydrogen column densities of
N_Wedge_=(5.2+/-0.9)x10^21^cm^-2^ towards the X-ray bright and
N_Shadow_=(11.1+/-2.2)x10^21^cm^-2^ towards the X-ray dark regions. The
agreement between these values derived from the ROSAT and IRAS data
suggest that a part of the extended diffuse X-ray emission is located on
the far side of the giant molecular cloud. This kind of 3-D structure is
independently verified by radio continuum observations. The X-ray
attenuating gas is positionally associated with the polarized radio
continuum emission of a giant magnetic loop emerging out of the plane of
the LMC. Faraday depolarization implies that this magnetized loop
penetrates deep into - or beyond - the LMC, possibly commencing in the
X-ray emitting plasma of the LMC.
Astronomy and Astrophysics 06/1997; 323:585-592. · 4.59 Impact Factor
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ABSTRACT: This article describes the stray–radiation correction that was applied to the Hi observations of the Leiden/Dwingeloo survey of Hartmann & Burton. This correction involved convolving the empirically–determined antenna pattern with the measured all–sky Hi distribution. The importance of the correction is demonstrated and practice regarding its application described. The general algorithm used here is presented. The results obtained with this algorithm are compared to those following from other methods. The 0.07 K sensitivity level of the survey depends critically on the success of the stray–radiation correction.
Astronomy and Astrophysics Supplement Series 09/1996;
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07/1996; 12:212.
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ABSTRACT: We present observational evidence in ROSAT all-sky survey data that
large parts of the high-velocity cloud (HVC) complex C are associated
with soft X-ray radiation. Moreover the new Leiden/Dwingeloo
northern-sky survey shows very faint HI 21-cm line emission which
connects the HVC velocity regime with the low-velocity atomic hydrogen
of the galactic disk by velocity bridges. Finally we find a close
positional correlation of soft X-ray enhancements and the atomic
hydrogen velocity bridges. Thus we obtain for the first time direct
evidence that large parts of the HVC complex C are impacting onto the
galactic disk. As a consequence, the HVCs of complex C are closer to the
galactic disk than previously assumed. Most probably the gas of the
Reynolds and/or Lockman layers are the collision partners of the HVCs.
Astronomy and Astrophysics 07/1996; 312:67-73. · 4.59 Impact Factor
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04/1996; 28:900-901.
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ABSTRACT: We present observational evidence for the existence of "velocity
bridges" between high-velocity clouds (HVCs) and neutral gas at lower
velocities towards HVC complex C. The velocity bridges (VBs
henceforward) show up as faint 21-cm line emission filaments (T_B_~0.2K)
connecting the two velocity regimes over velocity gaps of 20 to 100km/s.
We interpret VBs as indicators of energetic interactions of HVCs with
gas in the galactic halo or the disk-halo interface.
Astronomy and Astrophysics 03/1996; 308:L37-L40. · 4.59 Impact Factor
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ABSTRACT: By analyzing ROSAT PSPC pointings and H I 21-cm line observations the
authors investigate the soft X-ray background (SXRB) distribution
towards (l,b) = (90°,60°). Comparing the SXRB intensity with the
column density distribution of neutral absorbing matter they found
evidence for a soft X-ray shadow. The soft X-ray radiation transport
equation can be accurately solved by a two X-ray plasma component model
which consists of a cooler plasma (T ≤ 106K) located
between us and the bulk of the absorbing matter and a hotter plasma (T
≥ 106.2K) located beyond the neutral clouds casting the
shadow. The authors use ROSAT spectra to check for absorbing column
densities against observed H I column densities and find consistent
numbers in the range NH I =
(0.7-5.6)×1020cm-2. Fits of their
two-component plasma model to the scatter diagrams of X-ray intensity
versus NH I yield a foreground count rate of
5×10-4counts s-1arcmin-2 and an
unabsorbed count rate of the distant plasma of about
9×10-4counts s-1arcmin-2. While
the foreground count rate is typically for the Local Hot Bubble, the
distant plasma is roughly 2.5 times fainter than detected at galactic
latitudes of about 40°. This result indicates that the distant X-ray
emission is not entirely produced by the superposition of extragalactic
source because an intensity variation with galactic latitude is not
expected. Thus, a significant fraction of the SXRB is caused by a hot
plasma in the galactic halo.
01/1996; -1:335-336.
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ABSTRACT: Observations with ROSAT have established that the emission of a diffuse
hot plasma in the galactic halo contributes to the X-ray background at
1/4keV. This paper addresses the question of the extent of this hot
galactic corona and discusses the origin of the soft X-rays emitted
there. Based on the Wisconsin soft X-ray surveys, Hirth et al. suggested
that the interaction of Hi high-velocity clouds with the galactic disk
gas is at least partly responsible for the soft X-ray background. Here
we compare the ROSAT 1/4keV survey data towards the high-velocity-cloud
Complex M with the new Leiden/Dwingeloo survey of galactic Hi. The
distance of this cloud complex with respect to the galactic plane has
recently been constrained to lie between 1.5 and 4.4kpc. To account for
the observed distribution of soft X-ray emission, we consider the
absorption of the X-ray photons by the neutral foreground gas including
intermediate- and high-velocity neutral clouds. We find that the
high-velocity clouds attenuate the soft X-ray background. This shows
that the galactic corona extends at least up to several kiloparsecs from
the galactic plane. Furthermore, we find evidence for a soft X-ray
edge-brightening of parts of the high-velocity clouds, suggesting that
these clouds are contributing to the heating of the galactic corona. The
heating mechanism itself remains unclear.
Astronomy and Astrophysics 05/1995; 298:606. · 4.59 Impact Factor
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ABSTRACT: We have used te Australia Telescope Compact Array to survey 21-cm
absorption toward compact continuum sources in and behind the Large
Magellanic Cloud. We find many absorption lines: out of a sample of 30
lines of sight observed we detect absorption in 19. This shows that the
cool atomic phase of the interstellar medium is abundant in the LMC,
more so relative to the warm neutral medium than in the Milky Way. This
abundance of cool atomic clouds contrasts with the relative scarcity of
molecular clouds in the LMC, suggesting a difference in the cloud
population compared with what is found in the Milky Way. An alternative
interpretation which is consistent with our results is that the
temperature of the cool atomic clouds is much lower in the LMC than in
the Milky Way, perhaps 40 K compared to 60 K in the solar neighborhood,
but with a similar abundance. Further observations will resolve this
ambiguity. The spatial distribution of the absorption suggests that the
abundance of cool-phase gas in the LMC decreased with distance away from
the region of intense star formation in the vicinity of 30 Dor at the
north end of the giant molecular cloud. This region of intense star
formation and abundant molecular gas around and south of 30 Dor weighs
so heavily in the cool gas statistics that it accounts for all the
difference between the LMC and the Milky Way. Considering only our lines
of sight that do not pass within one kpc of 30 Dor, we find exactly the
same distribution of warm and cool H I phases as in the solar
neighborhood. Most of the absorption lines detected are quite narrow,
with half power widths of only 2 to 43 km/s. They thus allow us to probe
the kinematics of the cloud populationin detail.
Astronomy and Astrophysics 08/1994; 289:357-380. · 4.59 Impact Factor
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ABSTRACT: We have analysed three complete data sets, the IRAS 100 μm intensity,
I100, the H I column density, N(H I), and the integral of the
J = 1 → 0 line of the 12C0 molecule, W(12CO),
for the Draco nebula [=IVC G091.0+38.0 (VLSR = -21)] to trace the total
column density of hydrogen nuclei, N(H), and to derive the extinction,
AV, and the total gas mass of this nebula. Adopting the
relations between these quantities which were set up by Heithausen &
Mebold we have analysed the [I100, N(H I),
W(12CO)] data cube by the method of the principal component
transformation. The results are tested by the statistical F- and
t-tests. We derive the FIR-emissivity per H I nucleon at 100 μm,
α100, and the ratio
χWCO≡N(H2)/W(12CO) for
different regions of the Draco nebula. A difference of the
FIR-emissivities for the predominantly molecular cloud cores and their
predominantly atomic halos is established and taken into account for the
determination of χWCO.
The FIR-emissivities found in the Draco nebula are larger than the mean
value for the galactic cirrus of 110-20 MJy ster-1
em-2. They depend on the molecular content as well as on the
60 μm/l00 μm colour temperature. This dependence is probably due
to screening of the interstellar radiation field, variations of the dust
grain size, and/or a non-constant gas-to-dust ratio.
The χWCO values in the Draco nebula are typically in the
range 0.02 1020 cm-2 < χWCO <
0.26 l02O (K km s-1)-1, i.e. more than
a factor of 10 smaller than in the galactic plane and smaller than in
most other cirrus clouds. The lowest values are found in regions where
we suspect interaction with high velocity clouds. The low
χWCO values are only partly due to optical depth effects
of the 12CO J = 1 → 0 lines or excitation conditions of
the CO molecules by hydrogen atoms. They result probably from the
combined effects of high temperatures and an increase of the CO
abundance in the shocks of the interaction zone.
Our results arc used to derive the atomic and the molecular content
[M(H2)=45MsunD2 (500 pc)-2]
and the total mass of the Draco nebula [M =
630MsunD2 (500 pc)-2] and its different
molecular clumps. The consequences for the visual extinction and the
distance, D, are discussed.
Astronomy and Astrophysics 04/1993; 272:514. · 4.59 Impact Factor
