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Astron. Astrophys. 357, 75-83 (2000)

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3. Results

Our sources were selected from the strong millimeter quasars that were seen in projection over one of the 17 HVC complexes identified, in the Wakker & van Woerden (1991) recapitulating map. Table 1 displays the characteristics of the sources observed, the name and expected velocity of the HVCs along each line of sight, following the notation of the HVC catalogue of Wakker & van Woerden (1991), as well as the measured continuum flux in Jy at 3 and 2 mm, for all 24 sources observed at SEST at the date of observation (6-10 November 1999). The 4 sources observed with the IRAM interferometer (also in Table 1), have been selected to have MgII detected in absorption by Savage et al. (1993), at very high velocities. The HI in emission at these high velocities was measured by the NRAO-43m HI survey towards 143 quasars of Lockman & Savage (1995). The detection of MgII insures the presence of a minimum metallicity towards these lines of sight (larger than 0.32 solar, in 3C 454.3 for instance), favorable for the detection of HCO+. The positions of the observed sources are plotted in Fig. 1 vis-a-vis an HI map of HVCs in Aitoff projection. Note that one source has been observed both with IRAM and SEST (3C 454.3).

[FIGURE] Fig. 1. a All-sky map of the high-velocity clouds, in HI 21cm line, in Aitoff projection, from Wakker & van Woerden (1997). b Location in the same projection of the quasars observed in this work.


Table 1. List of objects observed at SEST, and their detected 3 and 2 mm continuum. The 4 objects in the secondary list have been observed with IRAM interferometer.

The spectra were then normalized to the continuum flux, and the upper limit at 3[FORMULA] of the optical depth [FORMULA] in absorption was computed in each case assuming that the surface filling factor is f=1, i.e., that the absorbing molecular material covers completely the extent of the mm continuum source, in projection. If [FORMULA] is the observed continuum antenna temperature of the background source, and [FORMULA] the amplitude in temperature of the absorption signal, then the optical depth is:


The 3[FORMULA] upper limits of [FORMULA] as measured in 0.56 km s-1 channels is listed in Table 2.


Table 2. Derived column densities (upper limits assuming 1.1 km s-1 linewidth)

The total column density of HCO+, observed in absorption between the levels [FORMULA] with an optical depth [FORMULA] at the center of an observed line of width [FORMULA] at half-power, is:


where [FORMULA] is the frequency of the transition, [FORMULA] the statistical weight of the upper level (= 2 Ju+1), [FORMULA] the Einstein coefficient of the transition (here [FORMULA] = 3[FORMULA]10-5 s-1), [FORMULA] the excitation temperature, and


where [FORMULA] is the partition function. We assumed statistical equilibrium an excitation temperature, close to the cosmic background temperature, i.e. [FORMULA] = 3 K, because of the large critical density needed to excite HCO+. We also expect a very narrow linewidth, comparable to the detected lines, and adopted dv = 1.1 km s-1 to derive the upper limits in Table 2. The HCO+/H2 abundance was conservatively taken as 6[FORMULA]10-9, but it must be kept in mind that it could be more than an order of magnitude higher (e.g. Lucas & Liszt 1994; Wiklind & Combes 1997), and therefore the derived H2 column densities could be correspondingly lower. However, if the metallicity is lower than solar, the corrections should go in the reverse sense.

An indicative HI column density of the high-velocity gas was also estimated using the HI surveys of Stark et al. (1992) and Hartmann & Burton (1997) for the northern hemisphere, and Bajaja et al. (1985) for our 5 southernmost sources. This column density is rather uncertain since it has been smoothed over large areas (at least one half of a degree) while the material that could appear in absorption extends over milli-arcseconds, similar to the background millimeter sources. At small scales the HI column density could be much higher than the values presented.

Fig. 2 shows some of the low velocity detections. All of them have already been discovered by Lucas & Liszt (1996), except the one at 1923+210 which is new. In the low-resolution backends, though, they are barely resolved and have somewhat reduced peak intensity. To check this, we have re-tuned to their central velocity, and centered the high-resolution spectrograph at V[FORMULA] 0. In the HRS AOS, the absorptions had indeed stronger intensities and were in all cases compatible with previously reported values (the integration time though, for the retuned spectra, was not enough to obtain a high S/N).

[FIGURE] Fig. 2. Galactic clouds seen in absorption in the HCO+(1-0) line in front of the observed sources. One of the absorption lines is new (1923+210). The lines are very narrow, and are diluted in velocity, therefore appear of lower intensity than previously reported (LL96). The channel spacing is 2.3 km s-1 and the velocity resolution is 4.7 km s-1. The spectra are normalized to the total continuum detected.

Towards the source 1923+210, a tentative detection at the expected HVC velocity was obtained, as shown in Fig. 3. It is very narrow, and was detected clearly only in the HRS back-end.

[FIGURE] Fig. 3. Tentative detection of HCO+(1-0) absorption in the HVC in front of 1923+210 (at 5.5[FORMULA]). The channel spacing is 0.56 km s-1 and the velocity resolution is 1.1 km s-1. The spectrum is normalized to the total continuum detected (2.36 Jy).

Akeson & Blitz (1999) have recently reported upper limits in CO absorption with the BIMA and OVRO interferometers towards 7 continuum sources. We have no sources in common, and therefore we increase the statistical significance of the upper limits. They also searched for HI absorption in HVCs with the VLA, and have positive results only in the gas associated to the outer arm of our Galaxy. They conclude that true HVCs are very weak HI and molecular absorbers. HI absorption in HVCs has been searched with single dish or interferometer several times, without much success (Payne et al. 1980; Colgan et al. 1990; Wakker et al. 1991). The fact that only [FORMULA] 5% of the lines of sight towards HVCs show HI absorptions, while this frequency reaches 100% for normal galactic gas (Dickey et al. 1983), sheds some light on the physical structure of the clouds.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000