This preliminary HISA census was carried out with Higgs' two new HISA ID algorithms, HI_ABS and HI_DIPS, which find small-scale dark structures in channel maps and spectra, respectively. The intersection of these two sets are shown here. The flagged HISA voxels (3-D pixels) have been analyzed with code written by Gibson, using the 4-component radiative transfer model (see AAS poster), and assuming p = 1, T_c = 5 K, and T_s = 60 K to obtain maps of HISA optical depth. The amount of material detected sums to roughly 10 million solar masses over the area considered. However this preliminary census has only identified the darkest HISA. Search parameters will require adjustment to locate the weaker features. What is shown here represents at most 50% of the total HISA population.
The fields surveyed were MV1, MV2, MW1, MW2, MX1, and the preliminary AB mosaic. The Pilot HI cube was examined, but proved incompatible with the rest of the sample due to differing sensitivity. Thus the area covered by the awaited MX2, MY1, and MY2 fields is uncharted at present. This is shown by a red boundary line. Other field edges and names are given in green. HISA features are dark blue, while 12CO data from the FCRAO survey is outlined with magenta contours. The general HI emission is indicated with a light blue background, and was taken from the MVWXY HI cube assembled by Chris Brunt. FCRAO coverage ends in the MW region, and is indicated with a magenta line.
The four images below show the HISA distribution on the sky, first by itself, and then in comparison to CO and HI emission at various scales. Both CO and HI data shown here are restricted to Perseus Arm velocities. The HISA is not restricted, but is nearly all at Perseus velocities anyway. Note the HISA exhibits a degree of clumpiness similar to the CO, but appears independent of it much of the time.
The remaining four images show the same data collapsed along Galactic latitude rather than velocity. Recall the survey is incomplete in the XY region. The red line shows the predicted velocity behavior of the spiral shock ridge in Roberts' (1972) model of the Perseus Arm (see his Figure 6). The densest HISA seems to track this line rather well in the MV-MX range, falling at velocities 5-10 km/s more positive. Much of the CO occupies the same velocities, though again, the two are only partly juxtaposed.
Though there is no systematic velocity difference between HISA and CO, this is still consistent with the hypothesis that the HISA clouds are a stage in molecular condensation (see 1999 March newsletter and AAS poster). Gas reaccelerates as it leaves the shock, and then appears to slow down again at larger distances due to differential rotation and perspective. A consistent radiative transfer geometry puts the HISA shortly after the shock, with the CO further downstream. Both could have similar velocities due to the multi-valued nature of the Roberts model. The ``hill'' where velocities reach a maximum value downstream of the shock before diminishing again is large enough in space for gas to take ~10 Myr to pass through, which may be sufficient time for molecular formation.
One discrepancy with the Roberts model is found in the AB region, where HISA appears scattered about the predicted shock ridge line rather than nearly all at velocities more positive than the line. However the HI and CO also exhibit this behavior, starting in the MX & MY areas. Perhaps the spiral density wave is perturbed here by large-scale star formation energetics, namely the W3/4/5/HB3 complex and the adjacent superbubble recently announced by Wallace et al. Or we may be seeing an inadequacy of the Roberts model. Additional longitude coverage will be required to address this question.
