HI-IRAS comparisons at 12 arcmin scales

The 60 and 100 brightnesses meaured by IRAS are known to correlate well with the Galactic HI surface density. However this correlation is not perfect and reflects a range of physical conditions in the ISM which we have investigated by studying well-defined 100 clouds at high Galactic latitudes with 100 surface brightness in the range 0.5 to 5 MJy . By using adjacent off-cloud references it is possible to establish accurately the ratio of far-infared to neutral hydrogen surface density /N(HI) without dependance on the unknown FIR zero level. The individual FIR clouds were directly indentifiable in HI and each has its particular velocity (Malawi & Davies 1996). When the data from the independent observed points in all 12 observed clouds were plotted in the vs /N(HI) plane, a significant scatter was found. This scatter suggests that the clouds are subject to different interstellar radiation fields and that they consist of different mixes of dust grain sizes. The influence of the radiation field can be seen in the factor 2 difference in /N(HI) between the Northern and Southern Galactic pole regions in the sense expected from the Sun's 20 pc offset from the mid-layer of the Galactic disk. Further, no difference was found between the FIR-HI properties of low velocity Galactic disk gas and intermediate velocity clouds (IVCs) at v(lsr) km . It should also be emphasized that stray-radiation corrections are essential in any such intermediate and high latitude studies.

A survey by Willacy et al. (1993) of a 6 2 field in the low HI surface brightness region centred on RA = , Dec = has been made with the 12' beam of the Lovell Telescope. This map of HI surface brightness was used in a study of the soft X-ray distribution from ROSAT. Again HI features were found on all scales fown to the resolution limit of 12'

© Copyright Astronomical Society of Australia 1997