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Astron. Astrophys. 332, L61-L64 (1998)

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2. Observations and data analysis

The LDS (Hartmann 1994, Hartmann & Burton 1997) is the first large-scale 21-cm line survey which has been corrected for stray radiation from the side- and back-lobes of the antenna pattern (Hartmann et al. 1996). We have further improved the quality of the LDS data by correcting the observations for spurious line emission caused by radiation reflected from the ground into the receiver (details are given in Sect. 3).

Fig. 1 shows profiles averaged over all longitudes and over [FORMULA] in latitude for [FORMULA] (bottom) to [FORMULA] (top). To avoid any systematical biases due to southern sky data missing in the LDS, Fig. 1 was restricted to positive galactic latitudes. Weak, extended profile wings are visible which cannot be seen in individual profiles due to the noise (typically 50 mK after Hanning smoothing). Differential galactic rotation causes the wings to get broader at lower latitudes.

[FIGURE] Fig. 1. Profiles from the Leiden/Dwingeloo Survey (LDS) covering all positive galactic latitudes, corrected for ground reflections and averaged over all galactic longitudes and over [FORMULA] in latitude. The bottom profile is centered at [FORMULA], the top one at [FORMULA]. The zero levels of subsequent profiles are spaced by 50 mK. The solid lines follow from the model discussed in Sect. 4, corresponding to the emission of a co-rotating H halo with a vertical scale height of [FORMULA] kpc and a radial scale length of [FORMULA] kpc.

[FIGURE] Fig. 2. Estimated baseline uncertainties of the profiles plotted in Fig. 1, derived after a re-analysis of the LDS database. Observations affected by interference have been excluded from the inter-comparison. Only the range [FORMULA] 25 [FORMULA] and [FORMULA] is given. The emission corresponding to the halo model (see Sect. 4) is plotted for comparison. Unlike Fig. 1, the full velocity ranges included in the analysis are plotted here.

The properties of the profile wings have been studied by Westphalen (1997) who averaged the LDS in boxes of [FORMULA] and calculated the variance of the line emission for each velocity channel. The variance emphasizes small-scale spatial structure, instrumental problems, and interference. Thus the variance spectra can be used to test whether the wings are due to smooth emission. In Fig. 1 emission from HVCs and IVCs is clearly visible at negative velocities, superposed on the extended profile wings. At positive velocities the wings are only marginally affected by HVCs. These wings predominantly originate from gas which is smoothly distributed over large angular scales.

Averaged spectra for [FORMULA] 250 different boxes have been decomposed into Gaussian components by fitting only those channels of the averaged spectra which were found to be uncontaminated by fluctuations. Thus the analysis was biased to be most sensitive to components of large angular extent. HVD HI lines were found in all of the averaged spectra. The mean velocity dispersion of these lines is [FORMULA] [FORMULA] at the north galactic pole and increases at lower latitudes (Fig. 3). Such an increase can be explained by Kolmogoroff turbulence. For a plane-parallel HI distribution the length of the line of sight increases as (sin b)-1. The velocity dispersion is then expected to vary as [FORMULA]. Thus [FORMULA] = 60 [FORMULA] at the pole is consistent with the value [FORMULA] [FORMULA] which we find at [FORMULA]. The average line broadening due to differential galactic rotation in Fig. 3 is negligible ([FORMULA] %). At the north galactic pole the column density of this component is [FORMULA] cm-2.

[FIGURE] Fig. 3. The mean velocity dispersion of the diffuse high velocity dispersion (HVD) component ([FORMULA]) and of the stray radiation ([FORMULA]) as a function of galactic latitude.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998