The first results based on SEST observations of CO and other molecules in the Magellanic Clouds are presented. The properties of the CO emission, CO isotopic ratios and the conversion factor between CO emission and H2 column density are discussed. Abundances of detected molecules are estimated and compared with those in Galactic molecular clouds.

Molecular clouds in the Small Magellanic Cloud (SMC) show characteristics different to those of Galactic molecular clouds. They have weak CO emission and low CO luminosities. Preliminary results of high resolution observations of SMC molecular clouds with the SEST telescope are presented.

In the SMC, the factor to convert the velocity integrated CO emission into molecular hydrogen column density appears to be different than the factor derived in our Galaxy, and in the Large Magellanic Cloud.

Since the detection of molecular line emission from Galactic sources in the seventies, the importance of molecular clouds as a major constituent of the interstellar medium in the Galaxy has become clear. A major problem in Galactic research has been the overall distribution of the molecular component and position-related variations in this distribution. Studies of this problem are greatly hampered by our eccentric location inside the Galaxy. The global view offered by external galaxies eliminates this problem, but now limitations on linear resolution take its place. Only the nearby Local Group galaxies (Magellanic Clouds: 54 and 63 kpc; M31, M33 and NGC 6822: 500 − 700 kpc) combine acceptable linear resolutions with a global view. Their angular size and the low brightness of their molecular component makes mapping of these galaxies slow process. Yet, this is a worthwile undertaking: not only the global properties of the molecular interstellar medium emerge, but also interesting physical differences due to variations in metallicity and ambient radiation field strength in the galaxies of the Local Group. This paper presents a brief review on molecules in the Local Group and preliminary results from ongoing projects.

A total of 38 molecular clouds have been mapped in the inner kiloparsec of the nearby galaxy M33. The properties of these clouds (diameters, velocity widths, brightness temperatures, and masses) are very similar to the properties of Galactic giant molecular clouds. The cloud mass spectrum is also similar in the two galaxies, except that the M33 mass spectrum is deficient in the highest mass clouds. A comparison of the interferometer and single dish fluxes shows that only 40% of the CO flux is contained in clouds more massive than 105 M⊙, while 60% of the flux is due to diffuse gas or a fairly smooth distribution of small clouds that cannot be detected with the interferometer. Comparing the CO maps with a high-resolution HI map and optical Hα photometry reveals that peaks in these three components are always found close together. The atomic gas peaks near the molecular clouds are probably produced via photo-dissociation of the molecular gas.

NGC 604 is by far the brightest HII nebula in the nearby spiral galaxy M33. The gas in this region has about the same metallicity as the gas in the Large Magellanic Cloud (Diaz et al. 1987). In this respect the physical conditions in this nebula may resemble those in 30 Doradus. A one arcmin field centred on the eastern part of the ionized nebula has been observed in CO(2-1) and CO(1-0) using the IRAM 30m antenna and the OVRO interferometer. In CO(1-0) the IRAM and the OVRO data have been combined to produce high resolution maps including the emission at low spatial frequencies. A CO(1-0) map at 7″ resolution (about 25 pc for the distance of M33) has been obtained; maps in both transitions at 12″ resolution have also been produced. The distribution of total line emission (see Figure 1) is characterized by two prominent clouds plus diffuse emission. These clouds are resolved with typical virial masses of a few 105 M⊙. The peak brightness is 8K and the velocity width ΔV1/2 8 km s-1. The CO(2-1)/CO(1-0) brightness temperature ratio is 0.7. Based on LVG models (e.g. Castet et al., 1990) we conclude that the density must be about or at least 1000 cm-3. The virial mass to CO luminosity ratio for these clouds is identical to the one for Galactic GMCs (Solomon et al., 1987). These two molecular clouds are located on the edge of an HI concentration along the HI ridge of a M33 spiral arm feature. From the comparison of sensitive radio continuum VLA maps and an Hα map (Viallefond et al., in preparation) we observe the counterpart of these gas concentrations in the distribution of the extinction; we conclude that one of the two molecular clouds, the brightest on the CO map in Figure 1, must be located inside the ionized halo of NGC 604; the position of this cloud also coincides with the peak of brightness on the near infrared map for the H2 lines obtained by Israel et al. (1990); part of the HI may be photodissociated molecular gas. We suggest that the intense radiation field in NGC 604 compensates the metallicity deficiency to give a NH2/WCO ratio similar to the Galactic one. The diffuse emission is detected over the entire field. The velocity fields as observed with the CO, the HI (data from Deul and van der Hulst, 1988), and the Hα (e.g. Hippelein and Fried, 1984) lines are very similar, the main characteristic is an East-West gradient unrelated with the general rotation of M33. High velocity molecular gas is also detected as a faint blue shifted wing of about 25 km s-1 on the global CO profile; its spatial distribution is diffuse. Finally double peaked profiles are observed in the vicinity of a large optical and radio continuum loop known to be associated with the presence of a Wolf-Rayet star. Obviously the kinematics is perturbed by the recent vigorous formation of massive stars in this unusually bright star forming region of M33.

The 1.2 m telescope at the Center for Astrophysics is being used to carry out the first complete CO survey of M31. To date, the entire galaxy out to a radius of at least 15 kpc —- well beyond the optical disk and H I ring — has been observed every half-beamwidth (4.5'), with ∼1 hr of integration per point yielding an rms of 20 mK in 1.3 km s—1 channels. Like H I and other Population I tracers, molecular clouds in M31 are mainly concentrated in a broad ring with a radius of ∼10 kpc, but beyond the peak of the ring molecular gas falls off more sharply than atomic gas. On the assumption of a standard Galactic value for the conversion of CO emission to H2 mass, the average molecular surface density at the peak of the ring is ∼1 M⊙ pc-2 and the total molecular mass of M31 is ∼3 × 108 M⊙. The survey will be essentially repeated during the coming year to improve the signal-to-noise.

We present preliminary results of a comparison of the spatial distributions of 100 μm, Hi and CO emission in M31. Using a scatter diagram of 100 μm intensity versus H i column density, we have derived an upper limit to the molecular gas column density. We have compared these values with the velocity-integrated CO intensity to find an upper limit to the CO-to-H2 conversion factor in the ring of M 31 of 5 1020 mol cm—2 [K km s—1]—1.

We present a map of a 3′ × 3′ section of the southwest arm of M31 in the CO(J=2 → 1) transition. We have detected emission in 25 positions out of 31 observed in a field centered at (—42.5′, +7.5) (major axis, minor axis). The map is sampled at intervals of one beamwidth, with an angular resolution of 30″ or 100 × 460 pc.

Observations of CO emission in far-IR bright early-type galaxies have shown that a substantial number of them contain 107—109 MO of molecular gas. The star formation efficiency in these galaxies is as high as in spiral galaxies. The molecular gas has properties similar to that in spirals and is usually distributed in a rotating disk. The origin of the molecular gas is discussed.

A powerful way to measure the mass distribution in elliptical galaxies is to study systems with a central disk of ionized gas. Such disks occur in a small fraction (perhaps 10–20%) of nearby ellipticals, and the gas is generally believed to have been accreted from outside the galaxy as the result of a merger or interaction. Once the gas disk has settled into a principal plane of the stellar figure, measurements of its orientation and kinematics set strong constraints on the shape and orientation of the galaxy and allow us to derive M/L as a function of radius. Indeed, the observations discussed here were made at the ESO/MPI 2.2-m telescope as part of our ESO KEY-PROGRAMME, in which we aim to determine the mass-to-light ratios of nearby elliptical galaxies and examine the evidence for dark matter.

We report on observations of Cen A using the SEST telescope (Swedish-ESO Submillimeter Telescope). In a previous paper (Eckart et al. 1990a, hereafter Paper I) we presented a fully sampled map of the 12CO(1-0) distribution in the disk which complements the 12CO(2-1) cuts obtained by Phillips et al. (1987, 1989). From these data we determined that the ISM in the disk of Centaurus A is cold (T<10 K), with densities of the order of the 12CO critical density (∼2 × 104 cm—3). Absorption against the nuclear continuum source has been reported for HI (van Gorkom 1987, van der Hulst et al. 1983, Whiteoak and Gardner 1971) and a number of molecular species, such as OH, H2CO, C3H2, 12CO, and 13CO (Whiteoak and Gardner 1971, Gardner and Whiteoak 1976, Bell and Seaquist 1988, Phillips et al. 1987, Israel et al. 1990). Here we summarize measurements of absorption in the lines of 12CO, 13CO, HCO+, HCN, CN and CS (see Eckart et al. 1990b; hereafter Paper II).

We have observed the early-type galaxy M104 (NGC 4594, Sombrero Galaxy) in the 12CO(1-0) transition with the 30-m IRAM telescope. We have detected CO in two positions which coincide with maxima of HI, dust and radio continuum. We failed to detect CO in the nuclear area of M104.

Bright Clumpy Irregular galaxies (CI) are characterized by intense star formation activity. At the present time, a few objects of this type is known; to search for new candidates, and to investigate the causes of this intense star formation, we observe a sample of galaxies selected mainly from Kiso Schmidt survey.

Self-Regulated star formation on large scale is an attractive notion in the study of galaxy evolution. Here we are interested in a self-regulating mechanism based on the sensitivity of the warm gas condensation on the UV interstellar radiation field (Parravano 1989). In this model the star formation rate (SFR) is assumed to be self-regulated in such a way that it maintains Pmax approximately equal to the interstellar warm gas pressure. Here, Pmax is the pressure in the marginal state of stability for the transition warm gas — > small clouds. Due to the dependence of Pmax on the gas density, gas composition, and dust content, the resulting self-regulated star formation rate also depends on this parameters. In term of global galactic parameters the self-regulated SFR (ψpp) can be expressed (Parravano 1989)

The recent single-dish and interferometric mapping of the disks of nearby spirals in the CO lines are reviewed. In all galaxies with a grand-design structure molecular spiral arms are prominent, with an arm/interarm contrast of 4 or more. CO-poor spirals seem to have more contrasted CO arms. In most galaxies (NGC6946, M31, M83, M81) the CO, HI and Hα ridges are closely associated while in some regions of M51, the CO is upstream and associated with the dust lane and spiral shock. This suggests that there is no unique sequence leading from the gas to the stars in the arms. The differences between these galaxies are probably linked to the strength of the density wave, the location of the resonances and the overall gas content of the galaxy.

M51, M101 and IC342 were all observed in the CO(1-0) line with the Onsala 20 m antenna. “The average pointing error for a 20 minute run is less than 5”, usually considerably less, and the receivers are stable. The error lobe is considerable with an efficiency of about 40%. With a width of about 20′ it is of little importance compared to an emission region of 2′ to 3′ however. Of more concern is an upper and a lower sidelobe with a combined efficiency of about 14%.

We have mapped the central ≈8'×5' of the spiral galaxy M101 with full beam width spacing (30″) in the CO(J=1−0) line using the Onsala 20m telescope. In total, observations were performed towards 153 positions, and 120 were definitely detected. We discuss briefly the distribution and kinematics of the CO gas.

The CO emission from the almost entire disk of M51 was mapped with the Nobeyama 45-m telescope. Using the high spatial resolution and high sensitivity data, we examine formation mechanisms of massive stars and molecular clouds both in arms and in interarms. The N(H2)/CO ratio is also evaluated in the galaxy.