We report far-infrared observations of [0 I], [C II] and [O III] fine structure emission lines toward the nuclei of M82 and 7 other galaxies with a high rate of star formation. The far-infrared line emission is bright, contains about 0.5% of the bolometric luminosity in the central 60″, and is spatially concentrated toward the nuclei. In these galaxies between 10 and 30% of the interstellar gas near the nuclei is contained in a warm, atomic component. This atomic gas is probably located at the UV photodissociated surfaces of molecular clouds. The neutral gas in M82 has a temperature of ∼ 200 K, hydrogen density of ∼ 3 × 104 cm−3 and is very clumpy, indicating that the interstellar medium in this star burst galaxy is very different from that in the disk of our own galaxy. We discuss the implications of the infrared observations for the interpretation of mm molecular lines and for star formation at the nuclei of star burst galaxies.

The CO (J=1-0) emission in M82 has been mapped with the Nobeyama 45-m telescope. The CO intensity distribution in the central region is resolved into two peaks. An axisymmetric model reveals a ring structure of molecular gas at a distance of 80-400 pc (centered near 200 pc) from the nucleus. This “200-pc ring” corresponds to just the region of a star formation burst. The molecular gas in M82 is also expanding out of the galactic plane with a velocity of 100-500 km s−1. The expansion energy of (0.1-1.4) x 1056 erg can be explained by the energy supply of supernovae in the central region.

7″-resolution CO (1-0) observations of M82 with the Owens Valley millimeter-wave interferometer have resolved 2 components of molecular gas in the central 1.5 arcmin of the galaxy: (1) a high plane of M82, and (2) shell-like or filamentary structures of molecular gas, with size-scale as large as 400 pc, extending most likely out of the plane of the galaxy.

In the relatively gas-rich, pure exponential disk galaxies, available data is accumulating which suggests that the distribution of starformation in the disk either shows no radial variation, or else has a scale-length which is much longer than that of the stars.

Conspicuous dust lanes define the spiral arm in the south of M31. The integrated HI line emission map shows several large cloudlike structures with sizes ranging from hundreds to thousands of parsecs, forming a spiral arm along the dust lanes (Figure 1). To investigate how such super clouds correlate with star formation phenomena, we present in Figure 2 a compilation of published data on: CO emission, dark clouds, HII regions, OB associations, and thermal infrared radiation from IRAS. The CO emission distribution is similar to that of HI and also forms large cloudlike structures. The large CO clouds P, Q, and R, which are located in the high density HI areas, have bright HII region complexes of several hundred parsecs at their outer edges. Further out large OB associations are found. In contrast, cloud B, which shows weaker CO emission, has a large and diffuse HII region which may be relatively old. This giant HII region is located in a large hole of HI and dark clouds (Brinks 1981). The IRAS maps show strong correlated thermal emission from the dust lanes. The luminosity from the star formation activity heats the dust in the molecular clouds. The local peaks at A, P, and Q in the 25 μm band, extending 200-300 pc, have revealed the existence of massive young stars embedded in the clouds.

The spiral galaxy M33 has been observed at several frequencies from the ultraviolet to the radio regions. The observations at 2030 A have been presented by Milliard (1984); the radio data obtained with the WSRT and the VLA are presented and analyzed in a series of papers (Viallefond and Goss 1986, and references therein).

VLA observations of the spiral galaxy M81 in the radio continuum at wavelengths of 6 and 20-cm have been used to check the predictions of the density wave theory. The non-thermal radiation from the arms has been detected and the arms are found to be broader than the predictions of the classical density wave theory. Their width does seem to agree with that predicted by models which take the clumpy nature of the interstellar medium into account. These data are also able to separate giant HII regions from the non-thermal arms. Collaborators have furnished optical Hα data on the HII regions and HI 21-cm data, from the VLA, which will be used to find and measure the location of the HII regions with respect to the spiral shock wave and to measure the visual extinction in the disk of M81.

VLA observations have been made of the continuum emission at 20-cm from the barred spiral galaxy NGC 4314 with an angular resolution of 3.5 arcseconds that corresponds to a linear scale of approximately 156 pc at a distance to the galaxy. This resolution was sufficient to resolve the central region into several compact sources. The radiation is linearly polarized which may indicate a non-thermal origin. No emission was detected from the extended bar to a level of 130 Jy.

Observations of the distribution of the CO-molecule in several prominent late-type galaxies indicate that the central HI depressions may very well be filled in with molecular gas. One such galaxy is M83 (NGC 5236) and, although the angular resolution of the CO-observations is insufficient to discern details on the scale of a spiral arm, it is known that CO is concentrated in the central regions within a radius of 1′. Furthermore, at a resolution of 50″, the CO profile at the position of the nucleus is as bright in M83 as it is for example in NGC 6946, IC 346 and M51.

CO observations of the nuclear region of the SABc galaxy M83 have been made with the 45-m telescope at NRO. A bar-like elongation of the CO emission along the optical bar and a velocity field which suggests noncircular motions are found. These results are consistent with predictions based on the theoretical model of barred spiral galaxies. The inflow and concentration of molecular gas in the nucleus of M83 may supply raw material which maintains a burst of star formation there.

We have mapped the nuclear region of an Scd galaxy IC 342 in the CO (J = 1-0) emission with an angular resolution of 15″. This galaxy has a bright optical nucleus with a dark lane elongated in the north-south direction. A prominent molecular bar which has been discovered by Lo et al. (1984) using the Owens Valley Interferometer lies just on the dark lane. Their CO map was spatially limited by the primary beam pattern and the velocity coverage of their spectrometer was not sufficient. It is not known, therefore, whether the molecular bar is connected to the outer spiral arms or not. We made the highest resolution CO map accessible by a single-dish telescope with a sufficient velocity coverage.

A stellar bar is an essential dynamical component for the behaviour of gas and molecular clouds in a spiral galaxy. It can trigger the formation of the arms and provokes radial transfer of gas. N-body simulations have shown that the bar forms easily and is very stable; according to classification, the majority of spiral galaxies are barred.

From a sample of Virgo Cluster and “field” disk galaxies, it is shown that a critical value of the HI surface density discriminates between RDDO anemic and “healthy” spirals. Below this threshold, at least massive stars do not form any more and the galaxy gets the anemic appearance. The influence of the HI content on the global star formation rate is discussed in the context of present models (Elmegreen 1979, Seiden and Gerola 1979, Dopita 1985), as well as the fate of disks in cluster and “field” environments. The existence of this threshold is an issue for the problem caused by the short gas consumption time scales derived from the observations of spiral galaxies.

NGC 4318, in the Virgo Cluster, has the following optical properties:

1. — morphological type: E (Nilson, 1973)

2. — BoT magnitude : 13.8 (Nilson, 1973)

3. — photometric axis : 0.8′ × 0.6′ (Nilson, 1973)

4. — heliocentric radial velocity:

   • −300 km s−1 (Arakelian et al., 1975)

   • 1247± 15 km s (Tonry, 1981)

   • 1215 km s−1 (Huchra et al., 1983)

NGC 2363 is one of the extragalactic HII regions with the highest surface brightness in Hα and in λ5007. We discuss the physical conditions within this star-forming region. In particular, from spectrophometric studies in the 1200-7400 A range we have determined its chemical abundance. We find that the O/H is an order of magnitude smaller than in the solar vicinity, and that the C/O ratio is a factor of two smaller.

Since the first detection of OH molecules in external galaxies, in 1971, hydroxyl observations have led to several detections of absorption lines in extragalactic systems. Though more seldom, OH maser emission has been observed in the 1667 and 1665 MHz main lines in NGC 253 and in the 1667 MHz line in M82. More recently, very powerful masers have been observed in IC 4553, NGC 3690 and Mrk 231.

The powerful OH megamasers detected in IC4553 (Baan et al. 1982), NGC3690, Mrk231 (Baan, 1985; Kazès and Dickey, 1985) and Mrk273 (Bottinelli et al. 1985a) show several common properties; in particular, they are strong IR emitters. We have thus undertaken a study of the strongest far IR (100 μm) uncatalogued IRAS sources and we report here on the discovery of a new strong extragalactic maser in the IR source IRAS 17208 −0014 (Bottinelli et al., 1985b)

Optical observations (Balzano 1983) have found many galaxies with unusually blue colors, stellar or semi-stellar nuclei, and emission line spectra with narrow line widths and line ratios implying stellar ionization. Balzano concluded that these properties could be explained by a recent burst of star formation; since almost all of the galaxies were selected from the Markarian lists, they are referred to as “Markarian starburst galaxies.” We have compared the far infrared luminosities of these galaxies with a sample of normal spiral galaxies (Scoville et al. 1983). We have also compared the far infrared luminosities of the Markarian starburst galaxies to their blue light and Hα luminosities.

A far-UV population synthesis is performed with the IUE Spectral Atlas on absolute spectrophotometry for nuclei of early-type galaxies. Two types of stellar populations may be distinguished in gas-poor galaxies: a) the bulk of evolved stars mostly contributing in the 2000 A – 3000 A wavelength range as well as in the visible. The Long Wavelength Range IUE spectra appear to be excellent indicators of the turn off age in galaxy nuclei; b) a possible young massive star population which could be the origin of the far UV excess in most cases and which is essentially contributing in the 1200 A – 2000 A wavelength range. Spectral features of massive stars may be identified.

U-G and G-R colors of clumps in Mrk 297 and Mrk 325 are presented. In addition, colors of other interesting objects and radiation sources were calculated for the Kiso UGR system. These objects are used to interpret the intrinsic characteristics of our sample. The observed colors of the clumps in both Mrk 297 and Mrk 325 are bluer than Im galaxies as well as main sequence stars and show scatter over the extent of the U-G vs. G-R diagram.