323 research outputs found
High-resolution imaging of compact high-velocity clouds (II)
We have imaged five compact high-velocity clouds in HI with arcmin angular-
and km/s spectral-resolution using the WSRT. Supplementary total-power data,
which is fully sensitive to both the cool and warm components of HI, is
available for comparison for all the sources, albeit with angular resolutions
that vary from 3' to 36'. The fractional HI flux in compact CNM components
varies from 4% to 16% in our sample. All objects have at least one local peak
in the CNM column which exceeds about 10^19 cm^-2 when observed with arcmin
resolution. It is plausible that a peak column density of 1-2x10^19 cm^-2 is a
prerequisite for the long-term survival of these sources. One object in our
sample, CHVC120-20-443 (Davies' cloud), lies in close projected proximity to
the disk of M31. This object is characterized by exceptionally broad linewidths
in its CNM concentrations (more than 5 times greater than the median value).
These CNM concentrations lie in an arc on the edge of the source facing the M31
disk, while the diffuse HI component of this source has a position offset in
the direction of the disk. All of these attributes suggest that CHVC120-20-443
is in a different evolutionary state than most of the other CHVCs which have
been studied. Similarly broad CNM linewidths have only been detected in one
other object, CHVC111-07-466, which also lies in the Local Group barycenter
direction and has the most extreme radial velocity known. A distinct
possibility for Davies' cloud seems to be physical interaction of some type
with M31. The most likely form of this interaction might be the ram-pressure or
tidal- stripping by either one of M31's visible dwarf companions, M32 or
NGC205, or else by a dark companion with an associated HI condensation.Comment: 12 pages, 11 (low res.) png figs, accepted for pub. in A&
A FUSE survey of high-latitude Galactic molecular hydrogen
Measurements of molecular hydrogen (H_2) column densities are presented for
the first six rotational levels (J=0 to 5) for 73 extragalactic targets
observed with FUSE. All of these have a final signal-to-noise ratio larger than
\snlimit, and are located at galactic latitude |b|>20 deg. The individual
observations were calibrated with the FUSE calibration pipeline CalFUSE version
2.1 or higher, and then carefully aligned in velocity. The final velocity
shifts for all the FUSE segments are listed. H_2 column densities or limits are
determined for the 6 lowest rotational (J) levels for each HI component in the
line of sight, using a curve-of-growth approach at low column densities ~16.5),
and Voigt-profile fitting at higher column densities. Detections include 73
measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight
sightlines yield non-detections for Galactic H_2. The measured column densities
range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found
between log N(H_2) and T_01, the excitation temperature of the H_2, as well as
between log N(H_2) and the level population ratios (log (N(J')/N(J))). The
average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is
calculated; however, because there are many HI clouds in each sightline, the
physics of the transition from HI to H_2 can not be studied. Detections also
include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35
IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm
of the Magellanic Stream), although 19 high-velocity clouds are intersected;
this strongly suggests that dust is rare or absent in these objects. Finally,
there are five detections of H_2 in external galaxies.Comment: Accepted for ApJ Supplement. Note: figs 7 and 8 not included because
astro-ph rejects them as too bi
Hubble-COS Observations of Galactic High-Velocity Clouds: Four AGN Sight Lines through Complex C
We report ultraviolet spectra of Galactic high-velocity clouds (HVCs) in
Complex C, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST), together with new 21-cm spectra from the Green Bank Telescope.
The wide spectral coverage and higher S/N, compared to previous HST spectra,
provide better velocity definition of the HVC absorption, additional ionization
species, and improved abundances in this halo gas. Complex C has a metallicity
of 0.1-0.3 solar and a wide range of ions, suggesting dynamical and thermal
interactions with hot gas in the Galactic halo. Spectra in the COS
medium-resolution G130M (1133-1468 A) and G160M (1383-1796 A) gratings detect
ultraviolet absorption lines from 8 elements in low ionization stages (O I, N
I, C II, S II, Si II, Al II, Fe II, P II) and 3 elements in intermediate and
high-ionization states (Si III, Si IV, C IV, N V). Our four AGN sight lines
toward Mrk 817, Mrk 290, Mrk 876, and PG1259+593 have high-velocity H I and O
VI column densities, log N_HI = 19.39-20.05 and log N_OVI = 13.58-14.10, with
substantial amounts of kinematically associated photoionized gas. The high-ion
abundance ratios are consistent with cooling interfaces between photoionized
gas and collisionally ionized gas: N(C IV)/N(O VI) = 0.3-0.5, N(Si IV)/N(O VI)
= 0.05-0.11, N(N V)/N(O VI) = 0.07-0.13, and N(Si IV)/N(Si III) = 0.2.Comment: 43 pages, 11 figures (appearing in ApJ, Sept 1, 2011
Multiwavelength observations of the M15 intermediate velocity cloud
We present Westerbork Synthesis Radio Telescope HI images, Lovell Telescope
multibeam HI wide-field mapping, Wisconsin H-alpha Mapper facility images,
William Herschel Telescope longslit echelle CaII observations, and IRAS ISSA 60
and 100 micron coadded images towards the intermediate velocity cloud located
in the general direction of the M15 globular cluster. When combined with
previously-published Arecibo data, the HI gas in the IVC is found to be clumpy,
with peak HI column density of 1.5x10^(20) cm^(-2), inferred volume density
(assuming spherical symmetry) of 24 cm^(-3)/(D kpc), and maximum brightness
temperature at a resolution of 81x14 arcsec of 14 K. The HI gas in the cloud is
warm, with a minimum FWHM value of 5 km/s, corresponding to a kinetic
temperature, in the absence of turbulence, of 540 K. There are indications in
the HI data of 2-component velocity structure in the IVC, indicative of
cloudlets. This velocity structure is also tentatively seen in the CaK spectra,
although the SNR is low. The main IVC condensation is detected by WHAM in
H-alpha with intensities uncorrected for Galactic absorption of upto 1.3
Rayleigh, indicating that the cloud is partially ionised. The FWHM of the
ionised component, at a resolution of 1 degree, exceeds 30 km/s. The spatial
and velocity coincidence of the H-alpha and HI peaks in emission is
qualitatively good. Finally, the 100 and 60 micron IRAS images show spatial
coincidence over a 0.7 degree field, with low and intermediate-velocity gas,
respectively, indicating that the IVC may contain dust.Comment: MNRAS, in pres
Exploring the Origin and Fate of the Magellanic Stream with Ultraviolet and Optical Absorption
(Abridged) We present an analysis of ionization and metal enrichment in the
Magellanic Stream (MS), the nearest gaseous tidal stream, using HST/STIS and
FUSE ultraviolet spectroscopy of two background AGN, NGC 7469 and Mrk 335. For
NGC 7469, we include optical spectroscopy from VLT/UVES. In both sightlines the
MS is detected in low-ion and high-ion absorption. Toward NGC 7469, we measure
a MS oxygen abundance [O/H]_MS=[OI/HI]=-1.00+/-0.05(stat)+/-0.08(syst),
supporting the view that the Stream originates in the SMC rather than the LMC.
We use CLOUDY to model the low-ion phase of the Stream as a photoionized plasma
using the observed Si III/Si II and C III/C II ratios. Toward Mrk 335 this
yields an ionization parameter log U between -3.45 and -3.15 and a gas density
log (n_H/cm^-3) between -2.51 and -2.21. Toward NGC 7469 we derive sub-solar
abundance ratios for [Si/O], [Fe/O], and [Al/O], indicating the presence of
dust in the MS. The high-ion column densities are too large to be explained by
photoionization, but also cannot be explained by a single-temperature
collisional-ionization model (equilibrium or non-equilibrium). This suggests
the high-ion plasma is multi-phase. Summing over the low-ion and high-ion
phases, we derive conservative lower limits on the ratio N(total H II)/N(H I)
of >19 toward NGC 7469 and >330 toward Mrk 335, showing that along these two
directions the vast majority of the Stream has been ionized. The presence of
warm-hot plasma together with the small-scale structure observed at 21 cm
provides evidence for an evaporative interaction with the hot Galactic corona.
This scenario, predicted by hydrodynamical simulations, suggests that the fate
of the MS will be to replenish the Galactic corona with new plasma, rather than
to bring neutral fuel to the disk.Comment: Accepted for publication in ApJ. 18 pages, 7 figures, all in colo
Damped Lyman-alpha absorption from a nearby Low Surface Brightness galaxy
Ground-based & HST images of the nearby galaxy SBS 1543+593 (z=0.009) show it
to be a Low Surface Brightness (LSB) galaxy with a central surface brightness
of mu_B(0)=23.2 mag/arcsec-2 and scale length 0.9 h-1 kpc, values typical for
the local LSB galaxy population. The galaxy lies directly in front of the QSO
HS 1543+5921 (z=0.807); an HST STIS spectrum of the quasar reveals a damped
Lyman-alpha (DLA) line at the redshift of the interloper with an HI column
density of log N(HI) = 20.35, as well as several low-ionization metal lines
with strengths similar to those found in the Milky Way interstellar medium. Our
data show that LSB galaxies are certainly able to produce the DLA lines seen at
higher redshift, and fuels the speculation that LSB galaxies are a major
contributor to that population of absorbers.Comment: Submitted to A
Metal Abundances in the Magellanic Stream
We report on the first metallicity determination for gas in the Magellanic
Stream, using archival HST GHRS data for the background targets Fairall 9, III
Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new
Parkes Multibeam Narrowband observations, we have unequivocally detected the
MSI HI component of the Stream (near its head) in SII1250,1253 yielding a
metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC
or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also
detect the saturated SiII1260 line, but set only a lower limit of
[SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in
MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward
the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas
near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the
lack of an accurate HI column density and the uncertain MgIII ionization
correction limits the degree to which we can constrain [Mg/H]; a lower limit of
[MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density
determination exists, but the extant FOS spectrum limits the quality of the
MgII column density determination, and we conclude that [MgII/HI]>-1.5.
Ionization corrections associated with MgIII and HII suggest that the
corresponding [Mg/H] may range lower by 0.3-1.0 dex. However, an upward
revision of 0.5-1.0 dex would be expected under the assumption that the Stream
exhibits a dust depletion pattern similar to that seen in the Magellanic
Clouds. Remaining uncertainties do not allow us to differentiate between an LMC
versus SMC origin to the Stream gas.Comment: 30 pages, 8 figures, LaTeX (aaspp4), also available at
http://casa.colorado.edu/~bgibson/publications.html, accepted for publication
in The Astronomical Journa
The high-velocity cloud complex Galactic Center Negative as seen by EBHIS and GASS. I. Cloud catalog and global properties
Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the
Galactic All-Sky Survey (GASS), we present a revised picture of the
high-velocity cloud (HVC) complex Galactic Center Negative (GCN). Owing to the
higher angular resolution of these surveys compared to previous studies (e.g.,
the Leiden Dwingeloo Survey), we resolve Complex GCN into lots of individual
tiny clumps, that mostly have relatively broad line widths of more than 15
km/s. We do not detect a diffuse extended counterpart, which is unusual for an
HVC complex. In total 243 clumps were identified and parameterized which allows
us to statistically analyze the data. Cold-line components (i.e., w < 7.5 km/s)
are found in about 5% only of the identified cloudlets. Our analysis reveals
that Complex GCN is likely built up of several subpopulations that do not share
a common origin. Furthermore, Complex GCN might be a prime example for warm-gas
accretion onto the Milky Way, where neutral HI clouds are not stable against
interaction with the Milky Way gas halo and become ionized prior to accretion.Comment: 13 pages, 19 figures; accepted by A&
Analytical sun synchronous low-thrust manoeuvres
Article describes analytical sun synchronous low-thrust manoeuvres
CaII K interstellar observations towards early disc and halostars - Paper II; distances to IVCs and HVCs
We compare existing high spectral resolution (R=40,000) CaII K observations
towards 88 mainly B-type stars, and new observations at R=10,000 towards 3
stars, with 21-cm HI emission-line profiles, in order to search for optical
absorption towards known intermediate and high velocity cloud complexes. Given
certain assumptions, limits to the gas phase abundance of CaII are estimated
for the cloud components. We use the data to derive the following distances
from the Galactic plane (z); 1) Tentative lower z-height limits of 2800 pc and
4100 pc towards Complex C using lack of absorption in the spectra of HD 341617
and PG 0855+294. 2) A weak lower z-height of 1400 pc towards Complex WA-WB
using lack of absorption in EC 09470-1433 and weak lower limit of 2470 pc with
EC 09452-1403. 3) An upper z-height of 2470 pc towards a southern intermediate
velocity cloud (IVC) with v_LSR=-55 km/s using PG 2351+198. 4) Detection of a
possible IVC in CaK absorption at v_LSR=+52 km/s using EC 20104-2944. No
associated HI in emission is detected. At this position, normal Galactic
rotation predicts velocities of up to +25 km/s. The detection puts an upper
z-height of 1860 pc to the cloud. 5) Tentative HI and CaK detections towards an
IVC at +70 km/s in the direction of HVC Complex WE, sightline EC 06387-8045,
indicating that the IVC may be at a z-height lower than 1770 pc. 6) Detection
of CaK absorption in the spectrum of PG 0855+294 in the direction of IV20,
indicating that this IVC has a z-height smaller than 4100 pc. 7) A weak lower
z-height of 4300 pc towards a small HVC with v_LSR=+115 km/s at l,b=200,+52,
using lack of absorption in the CaK spectrum of PG 0955+291.Comment: 13 pages, 5 figures. Accepted for publication in MNRAS, May 13 200
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