The main purpose of this paper is to give an overview of the complex structure
of HI absorption towards Cas A. The data cube is displayed in various cuts.
The "Perseus Arm'' HI absorption was discussed in
Bieging et al. (1991) based
on their higher resolution VLA data.
Our short discussion is therefore focussed on the local gas,
previously not given in such detail. We found a 2 arcmin feature at a velocity
of -1.5 km s
of very small velocity width. This width corresponds
to kinetic temperatures well below 10 K. The mean profile of this feature
corresponds with lines of molecules (as CO) with the same velocity and
velocity width. A new value of the ratio 
of 0.05 is derived.
Acknowledgements
We thank the staff of the WSRT for making the observations and the calibration of the data.The Westerbork Radio Synthesis Telescope is operated by the Netherlands Foundation for Research in Astronomy which is financially supported by the Netherlands Foundation for Scientific Research (NWO). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
We thank E.M. Arnal for help and the "GIPSY-team'' in Groningen (K.G. Begeman, J.P. Terlouw and Martin Vogelaar) for their support and for providing many options in the plotting routines and J. Keohane (Minnesota), T. Troland (Kentucky), L. Rudnick (Minnesota) for the critical reading of the paper.
Figure 2: HI Optical depth images for the velocities indicated. In the first frame
the continuum image is shown (units Jy/beam), in the second frame the errors in 
.
The third panel is the map of the "equivalent width'' in HI, 
; the units
are km s
. The contours and grayscale are between the given minimum and maximum value
as indicated in the individual plots
Figure 2: a) b) c) d)
Figure 3: Velocity-right ascension diagram of the Perseus arm HI feature
optical depth in the interval -56 to -30 km s
, at various
declinations given in offsets from phase and pointing centre (cf. Table 1 (click here))
Figure 4: Velocity-right ascension diagram of optical depth of the local gas, with
velocities in the interval -16 to 10 km s
, at various
declinations given in offsets from phase and pointing centre (cf. Table 1 (click here))
Figure 5: HI spectra of the Perseus arm feature
optical depth, in the interval -56 to -30 km s
.
Only every second pixel of the grid
is displayed in steps of -19.7'' (about 2/3 beamsize). Offsets in right
ascension and declination are with respect to the phase and pointing centre
(cf. Table 1 (click here)). The intensity scale
is given in the top right panel
Figure 6: As Fig. 5 (click here) but of the local gas ( -10 < v < 10 km s
)
Figure 7: HI spectra derived from 
-images from which the mean optical
depth has been subtracted. Because the low velocity images are quite uniform
across the source, small scale features are enhanced by this presentation.
Most notably is the
feature at 
(-14 grids),
.
This HI feature has an extremely small observed velocity width of
0.8 km s
with an instrumental resolution of 0.62 km s
.The
coordinates are the same as in Fig. 5 (click here)
Figure 8: Average HI spectrum in the low velocity range (-10, 
),
symbols "+'', plotted together with
three fitted Gaussian (crosses) and the sum of the Gaussians, all in dotted
lines. The full line spectrum is HI emission of the surrounding
of Cas A, as measured by Mebold et al. (1974) and
corrected for stray-radiation,
Kalberla et al. (1980). The thick dashed line is the average
spectrum across Cas A obtained by
Wilson et al. (1993); their velocity resolution was 
Figure 9: Images of the parameters of the Gaussians fitted to the
-1.5 km s
feature. The upper two panels
have units km s
. 
(units K) is
derived from the velocity width, after correction
for instrumental broadening. 
is in units
of 
at cm
Figure 10: Optical depth images with velocity v
below those of the Perseus
Arm. The clumps found by Reynoso et al. (1997),
are plotted as circles