We have analyzed the first ten square degrees of the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (GRS), a new, fully-sampled survey of ¹³CO emission over a large portion of the inner Galaxy. We compare the GRS ¹³CO maps to 21 cm H I maps from the Boston University-Arecibo Observatory Galactic HI Survey. Toward many molecular clouds, we find a remarkable association between ¹³CO emission and HI self-absorption, both in velocity and morphology. In particular, G45.6+0.3, a large filamentary cloud, shows deep HI self-absorption throughout the ¹³CO emitting region. This cloud is also associated with pronounced optical extinction and thus lies at the near kinematic distance. Another cloud, G43.3-0.3, also shows a pronounced correlation between ¹³CO emission and 21 cm H I self-absorption.

Position-velocity diagrams also show a number of additional molecular clouds that correspond with HI self-absorption.

Photodissociation theory suggests that the skins of typical molecular clouds exposed to normal Galactic UV radiation fields will exhibit H I self-absorption against strong H I backgrounds. Because H I emission is ubiquitous throughout the Galaxy, with sufficient angular resolution, the cold H I can be used to determine whether the molecular cloud lies at the near or far kinematic distance. Clouds at the near distance will show H I self-absorption, whereas clouds at the far distance will show H I in emission. This technique can resolve the kinematic distance ambiguity by cleanly separating near clouds from far clouds.

We compare the GRS ¹³CO molecular data and 21 cm H I data taken from the Boston University-Arecibo Observatory (BUAO) Galactic H I
Survey. The BUAO Survey parameters are:

Table. 1: BUAO Galactic HI Survey

Galactic Longitude	30.5 to 60.5 degrees
Galactic Latitude	-0.5 to 0.5 degrees
Angular resolution	4' HPBW
Angular sampling	2' or 4' intervals
LSR velocity range	-264.0 to +264.0 km/s
Velocity resolution	0.52km/s
Sensitivity	0.5 K

In the course of this comparison, we found that a number of clouds show a clear correspondence between ¹³CO emission and H I self-absorption, both in morphology and in velocity. Although H I self-absorption toward molecular clouds has been known for some time, this is the first clear correspondence between H I self-absorption and CO emission in both 2-dimensional morphology and in velocity. This correspondence suggests that these molecular clouds contain significant amounts of cold atomic hydrogen.

As a special consequence of the geometry of the inner Galaxy, any particular value of radial velocity corresponds to two distances along the line of sight, called the ``near'' and ``far'' distances. Thus, kinematic information alone cannot establish a reliable distance estimate, due to this so-called ``distance ambiguity.''

The likelihood of detecting H I self-absorption, however, favors the geometry in which a molecular cloud lies at the near kinematic distance. Since H I is ubiquitous throughout the Galaxy, warm H I emits at essentially every velocity. For clouds at the near kinematic distance, there is an ample amount of H I at the far distance against which a cold molecular foreground cloud can be seen in silhouette. If the same cloud were placed at the far distance, there would be very little background H I, and the self-absorption feature would be absent. Even if a ``far'' molecular cloud were to have HI self-absorption, it would not be observed since its characteristic spectral signature would be filled in by emission from H I at the foreground near position. The geometry is illustrated in a face-on sketch of the Inner Galaxy in Fig. 1.

Fig. 1: Face-on sketch of the geometry of H I self-absorption in the First Quadrant of the Milky Way. Statistically, only H I associated with molecular clouds located at the near kinematic distance will show the characteristic spectral signature of H I self-absorption. This is seen schematically in the two plots which show gray scale H I spectra with ¹³CO spectra superimposed as either red or blue line profiles.

The cloud with the deepest self-absorption line, G45.6+0.3, shows pronounced visual extinction, and is therefore unequivocally located at the near kinematic distance. A second cloud with deep H I self absorption, G43.3-0.3, probably also lies at the near kinematic distance. We suggest that, since the
geometry of self-absorption favors clouds located at the near kinematic distance, then the presence or absence of self-absorption may prove to be a powerful technique to resolve the near-far distance ambiguity.

Fig. 2: Overlay of GRS ¹³CO J=1-0 contours towards G45.6+0.3 integrated over the velocity range 25-30 km/s on a false color image of the appropriate region from the BUAO H I Survey. Black indicates H I self-absorption. The ¹³CO contours start at the 3sigma level of the map. Higher intensity contours are drawn in yellow to emphasize the locations of peak ¹³CO emission.

Fig. 3: Overlay of the GRS ¹³CO J=1-0 contours from Fig. 2 towards G45.6+0.3 on a false color optical image of the appropriate region from the POSS plate (Digital Sky Survey, DSS).

Fig. 4: A comparison of GRS ¹³CO J=1-0 (black line) and H I (solid gray) emission towards a selected position in G45.6+0.3. To enhance the signal to noise and properly compare the data sets, the 13CO data have been resampled to the H I resolution and grid 4'. The presence (or absence) of H I self-absorption at the 13CO velocity places the cloud at the near (or far) kinematic distance.

Fig. 5: Shown is a comparison between GRS ¹³CO and BUAO H I emission in longitude-LSR velocity space. Color indicates the H I brightness temperature distribution at three galactic latitudes for the longitude range 40 < l < 46.3. Red corresponds to the strongest H I emission, blue to the weakest. The GRS ¹³CO emission distribution is drawn as black to white contours as intensities increase. It is clear that ¹³CO features correlate both with H I peaks and H I valleys. Analysis of the H I valley spectra shows that many have the characteristic signature of H I self-absorption, indicating that the CO cloud is located at the near kinematic distance. CO clouds associated with H I peaks are located at the far kinematic distance (see Figs. 1 through 4). Based on H I self-absorption, we indicate here with the symbols ``N'' and ``F'' whether a given ¹³CO emission clump is located at the Near or Far kinematic distance.

In comparing the ¹³CO Galactic Ring Survey with the BUAO H I survey, we have found that many molecular clouds exhibit both ¹³CO emission and H I self-absorption. We conclude the following:

H I self-absorption matches the spatial extent, velocity, and line width of ¹³CO emission for the molecular cloud G45.6+0.3 Visual extinction confirms that this cloud lies at the near kinematic distance.
The theory of Photodissociation regions suggests that cold H I is associated with typical molecular clouds (n ~ 103 cm-2, X ~ 1 Habing fields). This cold H I will have sufficient opacity to exhibit self-absorption if the cloud lies in front of typical H I backgrounds.
The geometry of the Inner Galaxy suggests that the presence or absence of H I self-absorption toward a molecular cloud depends on whether it lies at the near or far kinematic distance. Thus, the distances to molecular clouds in the Inner Galaxy can be well determined.

Position-velocity diagrams show a large number of molecular clouds with H I self-absorption. We conclude that these clouds lie at the near distance as well. The clouds that do not show H I self-absorption lie at the far kinematic distance. We can thus use this technique to produce an accurate map of the molecular gas distribution in the Milky Way.