Much of what is known about molecular clouds and their relation to star formation and galactic structure has been obtained from the large scale CO surveys. For several reasons, however, these have been done less systematically and less completely than the early 21 cm surveys; a great deal remains to be done. After a very brief general summary of what has been learned, I will emphasize the work now underway on the outer Galaxy and at high and intermediate latitudes. I will also briefly describe work in progress on M31. It will be argued in conclusion that a complete Galactic CO survey at an angular resolution of 1′ is quite feasible with a dedicated multiple-feed telescope.

If the CO distribution of the Milky Way is described as a truncated exponential rather than as a molecular ring with some gas at large radii, it becomes easier to understand the evolution of the disk of stars. The star formation rate per unit molecular gas mass is constant as a function of radius, and the H2 depletion time turns out to be only a few percent of the Hubble time. This very short timescale requires that the atomic gas act as a reservoir for the active star forming gas. Because the HI has such a different radial distribution, there must either be infall from outside the Galaxy, an efficient way for the atomic gas in the disk to lose angular momentum, or both, leading to measurable infall or inflow velocities. The truncation radius of CO is probably due to the recently identified stellar bar.

Over the past four years, Pat Thaddeus and I have been using the CfA 1.2 m telescope to carry out a new CO survey of the first Galactic quadrant with a sensitivity per solid angle 3–10 times higher than previous CO surveys of the plane, and 16 times higher than the only previous extensive wide-latitude survey (Dame et al. 1987). Since a preliminary purpose of this survey is the study of molecular gas out of the plane in the inner Galaxy, the observations have so far been confined mainly to several vertical strips across the plane covering a few degrees in Galactic longitude and 6°–8° in Galactic latitude; details of this on-going survey are given in Dame & Thaddeus (1995).

EGRET, the high-energy γ-ray telescope on the Compton Gamma-Ray Observatory, has the sensitivity, angular resolution, and background rejection necessary to study diffuse γ-ray emission from the interstellar medium (ISM). High-energy γ rays produced in cosmic-ray (CR) interactions in the ISM can be used to determine the CR density and calibrate the CO line as a tracer of molecular mass. Dominant production mechanisms for γ rays of energies ∼30 MeV–30 GeV are the decay of pions produced in collisions of CR protons with ambient matter and Bremsstrahlung scattering of CR electrons.

CO observations indicate that molecular clouds have a complex multiphase structure, and this is compared with the multiphase structure of the diffuse interstellar medium. The trace ionization within the molecular gas is governed primarily by UV photoionization. Magnetic fields contribute a significantly larger fraction of the pressure in molecular clouds than in the diffuse interstellar medium. Observations suggest that the total Alfvén Mach number, mA tot, of the turbulence in the diffuse ISM exceeds unity; Zeeman observations are consistent with mA tot ≲ 1 in molecular clouds, but more data are needed to verify this. Most molecular clouds are self-gravitating, and they can be modeled as multi-pressure polytropes with thermal, magnetic, and wave pressure. The pressure and density within self-gravitating clouds is regulated by the pressure in the surrounding diffuse ISM.

The Ursa Major molecular cloud complex lies in the direction of an expanding HI shell known as the North Celestial Pole loop. The NCP loop, which is centered at (l, b) ∼ (138°,30°) and easily seen in IRAS 100 μm emission, is some 60 pc across and 150 pc distant (Meyerdierks et. al 1991). At 100 μm, the Ursa Major clouds appear in projection as “finger” (l ∼ 140, b ∼ 38) which “hangs down” towards the center of the loop and the plane of the Galaxy. Distance estimates to the molecular clouds (Penprase 1993) are consistent with that of the NCP loop, indicating that the clouds are physically associated with the loop.

FCRAO has initiated an imaging survey of CO J=1–0 emission from the outer Galaxy to better understand the physical properties of the molecular gas component of the interstellar medium and to determine the relationship of this component to spiral arm features. When completed in Spring 1996, the CO Survey will cover 103° < l II < 145°, −3° < b II < 5°, and −150 < VLSR < 50 km s–1 with angular sampling of 50″ and velocity resolution of 0.8 km s–1. During the first year of observations, we have focused upon two fields with longitude ranges of 103° < l II < 112° and 131° < l II < 139°. The first field corresponds to the Cepheus region while the second field contains the W3, W4, and W5 complex of HII regions. To date, 700,000 CO spectra have been collected. In this contribution, we present preliminary results from the first year of observations.