University of Calgary
1998, Bull. A.A.S., 30, 1341, #65.12
A press release was made for this poster.
We have begun a systematic study of HISA within a 73 x 9 degree region (Fig. 2) mapped by the Dominion Radio Astrophysical Observatory's Synthesis Telescope (DRAO-ST) for the ongoing Canadian Galactic Plane Survey (CGPS; Taylor et al. 1999; English et al. 1998). The ~1 arcminute DRAO-ST beam reveals considerable substructure within detected clouds (see Figs. 1, 3 & 5). Our investigation has uncovered a wealth of remarkable features in both the Local and Perseus Arms. Some have clear 12CO and dust counterparts, while many others do not.
Below, we investigate properties of sample CGPS HISA features and also consider aspects of the population as a whole. Their partial correlation with CO and dust, narrow linewidths, and intermediate appearance between diffuse HI and compact CO morphologies suggest the HISA features may represent HI in the act of molecular condensation. This possibility is augmented by a substantial abundance of HISA at the same velocities where molecular clouds are likely to form after encountering the Perseus Arm density wave shock.
Here T_on and T_off are continuum-subtracted ON and OFF brightnesses, and p is the fraction of T_off emission lying behind the HISA feature. Constraining p leaves two unknowns, tau_hisa and T_spin. Inclusion of other relations, such as that for Gaussian line column density, uniform gas density, and an ideal gas law then produce the transcendental equation
where L_|| is the HISA line-of-sight pathlength, and a canonical pressure such as P_therm / k = 4000 K cm^-3 is used (only thermal pressure is relevant).
In Table 1, we estimate properties for the objects in Figures 3 & 5. Both have T_c = 5 K from the general Galactic synchrotron background. Representative T_on & T_off values were taken from Figures 4 & 6. In more sophisticated future analyses, these will be measured separately for each HISA voxel, with T_off taken from a volume interpolation of non-HISA voxels surrounding the feature in space and velocity.
We assume the smallest high-contrast filaments in each object are roughly cylindrical (within a factor of 2) to obtain L_||; the pathlength will be that of a single filament if their filling factor inside the object is low, which typically appears to be the case. The ~2 kpc distance to the Perseus feature is based on the sightline velocity model of Roberts (1972), while the 200-500 pc distance for the Local feature is constrained merely to lie within the Local Arm, but outside the Local Bubble. p = 1 was used for both objects; if p < 1, T_spin, tau_hisa, and related values become lower limits. For the Perseus feature, M_hisa refers only to the central 10 x 45 arcminute component; the entire complex may be 10-100 times more massive.
Though our methods of estimating distances and background spectra are still crude at this stage, the values obtained serve as useful rough measures of the class of cold atomic structures revealed by our ongoing investigation.
The darker HISA features are quite easy to identify by eye but less so by computer, due to confusion imposed by noise and complex background HI emission structure. An initial method which traces at least the darkest HISA is to smooth each velocity channel in the cube spatially to improve S / N, and then convolve each sightline spectrum with a narrow (FWHM = 2 km/s) 1-D ``Mexican Hat'' wavelet function to pick out sharp local spectral minima. We have used this technique to flag the darkest HISA voxels in the velocity cube, interpolating non-HISA neighbors in the surrounding volume to estimate T_off. Resulting aggregate properties for each channel are shown in Figure 7.
The top panel shows the average T_off - T_on contrast. For some HISA voxels this can be 40 K or more, but for most it is only a few kelvins. Since the limit of detectability is 1-2 K, this suggests many more HISA features may exist which are too optically thin to see. The middle panel shows the average of T_off itself, with <T_B(HI)> overplot for comparison. Though <T_off> is a measure of average HISA neighbor brightness rather than average total HI brightness, a correlation between the two is apparent. Comparison of <T_B(HI)> with <T_off - T_on> above shows most detected features have a contrast of a few percent of background. The minimum <T_off> value is ~40 K (values of zero occur in channels where no HISA was found). Consideration of the radiative transfer equation suggests this may represent a lower limit to T_spin, as features become invisible when T_spin = p * T_off + T_c. 40 K is warmer than most HISA detected in previous searches of molecular clouds, but cooler than canonical temperatures of 80 K for ``cold'' neutral hydrogen. We use T_spin = 60 K to compute optical depths integrated over each channel in the bottom panel. This serves as an estimate of HISA mass vs. velocity. Since our present algorithm flags only the darkest HISA voxels, it uses many of their less-dark HISA neighbors to find T_off. This underestimation of background brightness makes all current contrast, optical depth, and mass measurements lower limits.
Figure 7 shows a significant peak of integrated HISA opacity at a radial velocity of -41 km/s LSR. This peak coincides with others in mean CO and HI emission brightness, both of which in fact correspond to gas constituents of the Perseus Spiral Arm. A plum pudding distribution would also show HISA peaks coincident with background HI peaks, since the fraction of HISA to HI is constant. However the HISA peak at -41 km/s is much greater relative to the general HISA level than the HI emission peak is to other HI, arguing against a homogeneous distribution.
Instead, the peak in integrated opacity suggests a substantial abundance in HISA in the Perseus Arm, in full agreement with visual impressions of the amount of HISA in different channels of the velocity cube. While most velocities appear to have a certain low level of HISA, perhaps indicating a general faint homogeneous population, Perseus velocities show a major enhancement.
These velocities are the same predicted by Roberts (1972) to contain gas encountering the density wave shock of the Perseus Arm. Gas falling into the arm's gravitational potential slows abruptly upon colliding with gas already there, and picks up speed again on its way out, appearing to slow once more at larger distances, due to the perspective effects of a given sightline (Figures 8 & 9). The shock itself induces cloud compression, leading to the condensation of HI into H_2, and the eventual formation of new stars. We believe we may be seeing evidence of this initial phase transformation. The radiative transfer requirement of additional HI emission beyond the HISA is satisfied by the Roberts spiral shock geometry, which places the HI leaving the arm further along the sightline at the same velocity as gas in the shock. Though our measurements are currently only lower limits tracing the darkest of the HISA, we feel it is likely that a more thorough census will produce the same result.
Figure 4 (Upper Right): ON (solid) and OFF (dashed) HI and CO representative spectra for the Perseus HISA feature, extracted from boxes marked in Fig. 5.
Figure 5 (Lower Right): A much fainter feature in the Local Arm at $+1\kms$, with brightness from 55-80 K. Yellow CO contours (Dame et al. 1987) are at 0.33, 0.67, 1.0, and 1.33 K. No correspondence in dust emission was found. ON (white) and OFF (black) spectrum boxes used in Fig. 6 are shown.
Figure 6 (Lower Left): ON (solid) and OFF (dashed) HI and CO representative spectra for the Local HISA feature, extracted from boxes marked in Fig. 5. The CO detection is marginal but suggestive, being limited in S/N by low spatial resolution.
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