A New View of Cold HiClouds in the Milky Way

Abstract

We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21 cm synthesis maps, taken from the Canadian Galactic Plane Survey, show a numerous and diverse population of H I self-absorption (HISA) features in gas outside the solar circle. These objects vary in size, shape, and contrast against the background H I. All display a high level of angular and velocity structure, and most would appear signiÐcantly diluted, if not invisible, in lower resolution H I surveys. A number of Perseus arm features remain unresolved by the 1@ beam of our survey, with apparent diameters less than 0.6 pc at 2 kpc distance. The majority of HISA features we detect have no obvious 12CO emission counterparts. This suggests that either HISA is not found predominantly in molecular clouds, as has often been presumed in the past, or that CO is not a good tracer of Some HISA lacking CO shows far-infrared H 2 . dust emission, though whether this arises from shielded molecular gas or from di †use atomic clouds is not clear. Constraining the gas properties of HISA remains a difficult problem, but we introduce a new method that aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships and should prove powerful if the input variables are sufficiently well known. We explore the current allowed parameter ranges for three sample features of very di †erent appearance. We Ðnd spin temperatures

The Physical Properties of a Cold HiFeature Observed Transitioning from Absorption to Emission

“…2) in our Galaxy should have a layer of cool photodissociated H i that may be observable as HISA if a sufficient background of warm H i exists. In contrast, many of the HISA features shown in Gibson et al (2000) and Gibson (2002) have no associated CO emission down to the sensitivity limit (0.6 K rms) of the 12 CO Five College Radio Astronomy Observatory Outer Galaxy Survey. Although more sensitive observations may indeed detect CO emission, it is also possible that these clouds contain molecular hydrogen not traced by CO (Gibson et al 2000).…”

“…In contrast, many of the HISA features shown in Gibson et al (2000) and Gibson (2002) have no associated CO emission down to the sensitivity limit (0.6 K rms) of the 12 CO Five College Radio Astronomy Observatory Outer Galaxy Survey. Although more sensitive observations may indeed detect CO emission, it is also possible that these clouds contain molecular hydrogen not traced by CO (Gibson et al 2000). Such observations, showing a limited association between CO emission and HISA, combined with observations of the large-scale velocity (i.e., roughly spatial) distribution of HISA, have led to the interesting suggestion that in some cases the cold H i gas being traced by HISA may be the earliest stages of the formation of molecular clouds (Gibson 2002).…”

“…Although knowledge of this phenomenon is not new (e.g., see the summary in x 1 of Gibson et al [2000]), the high spatial dynamic range (ratio of area covered to resolution) of the new Galactic plane surveys has allowed the morphology and true extent of HISA features to be determined. Some HISA is undoubtedly associated with existing molecular clouds.…”

Dark Hydrogen in the Galactic Plane

“…21cm line data from the Canadian [12], Southern [8], and VLA [13] Galactic Plane Survey components of the IGPS reveal a rich and subtle population of HISA features, many of which are invisible at lower resolutions. Analyses of several CGPS and SGPS features [4,6,7] find temperatures of a few tens of Kelvins and densities of order ¡ cm5 7 6 . Some have obvious counterparts in CO emission, while others do not; this is also the case with the VGPS HISA in Figure 1.…”

“…Some have obvious counterparts in CO emission, while others do not; this is also the case with the VGPS HISA in Figure 1. While most inner-Galaxy HISA has associated CO, most outer Galaxy HISA does not [3,4,7]. Since inner Galaxy sight lines are more likely to have the bright backgrounds needed for HISA, more frequent association of HISA with CO is likely in the inner Galaxy.…”