Probing ISM Models with H observations

Does a Galactic Fountain exist ?

One of the major predictions and elements in favor of the Galactic Fountain (GF) model (Shapiro & Field 1976, subsequently detailed by Bregman 1980) is the existence of the High Velocity Clouds (HVCs, recently reviewed by Wolfire etal. 1995) in the halo, as a by-product of the cooling of the hot fountain gas. Any constraint on the origin of HVCs would be highly valuable in terms of understanding the global disk/halo circulation. Ferrara & Field (1994) have investigated the ionization and thermal structure of HVCs due to the extragalactic background radiation field and calculated the H emission from the partially ionized edge of a given cloud. Comparing the model results to the available H observations, the authors found that the observed H intensity is larger than the predicted one for all the different cases considered. One way to reconcile this dicotomy is that some of the observed emission is H light coming from the disk of the Galaxy and subsequently back-scattered by dust in the clouds. Assessing the presence of dust in HVCs is a young research field and very little is known about this issue. However, there are theoretical bases to expect dust to be present. The efficiency of dust destruction in a shock (presumably the heating source for the fountain gas) is likely to be less than 10% (McKee 1989); the subsequent thermal sputtering in the hot gas might destroy some of the smallest grains: in a  K gas, the sputtering time is shorter than the gas cooling time only for grains sizes smaller than m. However, previous low sensitivity searches using IRAS data have produced only upper limits (Wakker & Boulanger 1986) on HVCs dust content. In addition to the unlikely complete dust depletion, there are at least two other possibilities which might explain the non-detection: (i) HVCs are far above the galactic plane ( kpc); (ii) the dust is too cold to emit substantially in the IRAS bands. Since, at least for some of the clouds, the distance is bracketed around a much lower value for the distance, (i) is an unlikely explanation. It is quite possible, instead, that the dust is cold, due to the very diluted halo radiation field. The expected IR emission from a  cm HVC, assuming either a given temperature of the grains or a galactic ISRF diluted by a given factor (1-100 times) is shown in Fig. 4 together with ISOPHOT surface brightness limits (128 s of integration time, S/N=10) in the various bands. Clearly, dust hotter than  K and/or heated by a ISRF diluted by at most 10 times, could be detectable. Such a detection would bring strong support to the idea that HVCs are not of intergalactic origin and they are very likely part of a global disk/halo circulation.

  
Figure 4: IR surface brightness from dust in HVCs with HI column density  cm. Numbers refer to fixed grain temperatures (solid)) or radiation flux intensity (dotted), where is the Galactic ISRF. Also shown are the Wakker & Boulanger limits and ISOPHOT sensitivity (see text)

© Copyright Astronomical Society of Australia 1997