VIII/54 Atlas of Galactic Neutral Hydrogen (Hartmann+, 1997)
Atlas of Galactic Neutral Hydrogen
Hartmann D., Burton W.B.
<Cambridge University Press (1997)>
=1997agnh.book.....H 1997agnh.book.....H
=1999yCat.8054....0H 1999yCat.8054....0H
ADC_Keywords: H I data; Interstellar medium; Surveys; Radio sources; Atlases
Keywords: Galaxy: structure - ISM: clouds - ISM: general -
ISM: kinematics and dynamics - surveys
Description:
The Leiden/Dwingeloo HI survey mapped the 21-cm spectral line emission
over the entire sky above declinations of -30 degrees using a grid
spacing of ∼ 0.5 degree and a velocity sampling of ∼ 1.03 km/s. The
useful velocity (V_lsr) range is from -450 to +400 km/s. The Atlas
presents calibrated spectra in units of brightness temperature. Using
interpolation and averaging, the authors have placed their data on an
evenly-spaced grid in Galactic coordinates (l,b). A detailed
discussion of the instrument and calibration procedures is provided in
the published Atlas.
The average sensitivity level of the survey is 0.07 K (1-sigma, rms).
This sensitivity level depends critically on the success of the
stray-radiation correction as discussed in Hartmann et al.
(1996A&AS..119..115H 1996A&AS..119..115H). In that discussion, several caveats are offered
regarding the removal of stray radiation, in particular that component
which might be due to reflection from the ground. Some instances have
been found where there are residuals which are clearly larger than the
mean accuracy quoted as representative of the Leiden/Dwingeloo survey.
Users of the data are reminded that the stray-radiation correction was
applied conservatively, ensuring that no overestimate was calculated
and removed, thereby yielding spurious negative intensities. A
specific example of remaining spurious emission is evident towards the
North Galactic Pole, a direction notoriously difficult to observe. All
spectra taken towards b=+90 degrees should, of course, be identical,
no matter the longitude or the orientation of the telescope with
respect to the ground or to the meridian. Because the sky was sampled
in 5x5 degree boxes, a spectrum was recorded at b=+90 degrees for
every Nx5 degrees (N=0..72) in longitude. The spectra in the final
dataset were interpolated between these measured spectra to yield a
0.5x0.5 degree grid. So, only every 10th spectrum at this extreme
latitude corresponds to an observed spectrum. Comparing all spectra at
b=+90 reveals differences which are larger than expected. The origin
of this discrepancy is currently unknown.
There is also an instrumental effect which reveals itself as
correlated noise, showing a pattern which alternates sign at adjacent
channels when the very lowest levels of intensity are examined. This
effect is due to an offset in the DAS autocorrelator used as the
backend in the Leiden/Dwingeloo survey. The presence of this artifact
becomes noticeable only after averaging 50 or more spectra. Although a
Hanning convolution of the data would eliminate this effect, it would
also degrade the velocity resolution; as the correlated noise is
noticeable only at very low levels (about 15 mK), well below the mean
rms sensitivity of the survey itself, the original spectra have not
been Hanning smoothed. Excepted are those spectra which suffered from
sinc interference. These spectra were Hanning smoothed to enable the
elimination of the interference spike.
Dr. Lloyd Higgs has compared the HI spectra made with the DRAO 26-m
telescope in support of the Canadian Galactic Plane Survey with those
of the Leiden Dwingeloo Survey, and has pointed out what are evidently
calibration problems in a small number of isolated LDS spectra. Either
Hartmann, Burton, or Higgs could provide additional information.
The Leiden/Dwingeloo HI survey is intended primarily for studies of
the interstellar gas associated with our own Galaxy. There are,
however, a small number of spectra in which 'contaminating' signatures
from known external galaxies are present. Detections of roughly 50
such external galaxies were made; refer to table 4 of the Atlas for a
list.
The HI spectra from the Leiden/Dwingeloo survey are archived as 721
files. Each file is in FITS image format, and maps the 21-cm
brightness temperature at a fixed Galactic longitude for an
evenly-spaced rectangular grid of (Galactic latitude, velocity)
points. There is one FITS file for every 0.5 degree in Galactic
longitude in the "fits" subdirectory.
In addition to the 721 (b,v) FITS files, there is an (l,b) FITS image
named TOTAL_HI.FIT, which contains the integrated intensity map over
the velocity range -450 km/s ≤ V_lsr ≤ +400 km/s. The map units are
in [K.km/s] and the FITS header contains comments regarding the
conversion to column densities.
Included as a visual aid is the GIF image file total_hi.gif, which
depicts the velocity-integrated map.
The data were originally distributed on a CD-ROM enclosed with the
Atlas of Galactic Neutral Hydrogen (reference given above). The CD
also contains animations of velocity slices through the data cube.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
total_hi.gif 140194 1 761 x 361 GIF image of velocity integrated map
total_hi.fit 2880 363 Velocity integrated map for whole sky
maps.dat 23 721 Summary of latitude-velocity maps
fits/* . 721 Latitude-velocity maps for 0.5°-step longitude
--------------------------------------------------------------------------------
Byte-by-byte Description of file: maps.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 F5.1 deg GLON [0,360] Longitude of map
7- 13 F7.2 m/s DVel Velocity increment per pixel
15- 23 A9 --- FITSfile Name of FITS file in subdirectory "fits"
--------------------------------------------------------------------------------
Acknowledgements:
Thanks to the authors and Cambridge University Press for making the
data available for public distribution. The Leiden-Dwingeloo Galactic
H I survey project was supported by the Netherlands Foundation for
Research in Astronomy (NFRA) with financial backing from the
Netherlands Organization for Scientific Research (NWO).
References:
Dap Hartmann, Kalberla P.M.W, Burton W.B. & Mebold U.:
1996 A&AS 119, 115 =1996A&AS..119..115H 1996A&AS..119..115H
(End) J. Weiland [ADF/ADC] 19-Apr-1999