We have observed the 12CO (Ishizuki et al 1990) and 13CO emission of the central 65″ region of the galaxy IC342 at the high resolutions of 2.4″ for 12CO and 5.1″×4.2″ for 13CO using the Nobeyama Millimeter Array. The integrated intensities of 12CO and 13CO emission are shown in Fig.1 and 2, respectively.

With the help of a series of N-body simulations made with J. Sellwood's code I have studied the properties of spirals and bars in interacting galaxies. I will give here only a brief summary of the main results. For more information see Athanassoula (1990 and in prep.)

We propose a physical mechanism to explain the origin of the intense burst of massive-star formation seen in physically colliding/ merging, field, spiral galaxies. We consider the detailed evolution of a realistic, two-component interstellar medium (consisting of H2 and Hi) within each galaxy. Also note that, in a typical spiral galaxy - like our Galaxy, the Giant Molecular Clouds (GMCs) are in a near-virial equilibrium and form the current sites of massive-star formation, but the star formation rate is low. We show that this star formation rate is greatly increased following a collision between galaxies. During a collision between two field spiral galaxies, the Hi clouds from the two galaxies undergo collisions at a relative velocity of ∼ 300 km · s−1. However, the GMCs, with their lower volume filling factor, do not collide. The collisions among the Hi clouds lead to the formation of hot, high-pressure remnant gas. The overpressure due to this hot gas causes a radiative shock compression of the outer layers of pre-existing GMCs in the overlapping wedge region. This triggers a burst of massive-star formation in the initially barely stable GMCs. For details, see Jog and Solomon (1990).

A uniform gaseous ellipsoid in external force field is applied as a dynamical model for massive gas cloud rotating at the galactic center of deep gravitational potential. An initially oblate ellipsoid elongates and rotates end-over-end when its contraction proceeds and the gas density becomes more than twenty times as large as the background matter density. A condition for third-order (pear-shaped) deformation of the gaseous cloud is easily satisfied, suggesting its split into two parts, separated on diametrically opposite sides of the center. Numerical simulations were made to confirm the fission: The two symmetric peaks observed in the 12CO (J=1−0) gas cloud on the galactic center (IC342, NGC6946, and Maffei 2) seem to be the result of the elongation and subsequent fission of a massive (107 to 108M⊙) high-density (102 to 103 H2cm−3) gas cloud accreted onto the galactic center.

A spectacular morphology, including two diffuse tidal tails, and high infrared luminosity suggest that the starburst NGC 3256 is a merger of two gas-rich disk galaxies. NGC 3256 exhibits strong and extended 12CO emission. A conventional interpretation of the observed 12CO and 13CO line ratios yields contradictory results.

The characteristic features of star-forming systems with prevalence of effects of induced star-formation (SF) are considered. Several mechanisms of induced SF are known: stimulation by stellar activity, by spiral density waves, by galactic mergers and tidal interactions. Properties of such systems can be described by four basic models: (i) self-regulated wave of SF; (ii) self-regulated starburst; (iii) SF-wave and (iv) starburst stimulated by external influence. In the first two cases, excitation of SF is caused by activity of stars of previous generation.

The active galaxy NGC 1808 has been studied in 12CO(1-0) and 12CO(2-1) transitions with the SEST telescope and in HI and radio continuum with the VLA.

We present images of M82 in six passbands spanning the 1-30 ym spectral region. JHK images obtained with the KPNO InSb array show that the center of M82 contains a bar about 1 kpc long. Analysis of the near-IR colors indicates that this bar morphology does not result from interstellar extinction (Fig. 1), but is intrinsic to the red stellar population. The bar probably resulted from the interaction of M82 with M81, and provides a hitherto unrecognized link between the interaction and the starburst. The 10-30 μm images, obtained at the IRTF with the MSFC 20-pixel bolometer array, show color variations with position that imply that the emission originates in a population of heated dust grains extending to very small sizes. The striking spatial variations of these colors also provide evidence for spatially dependent depletion of the small grains which can result in significant spatial variations in the elemental abundances and hence the molecular content. A paper describing these results has been accepted for publication in the Astrophysical Journal.

We have observed the nuclear regions of the galaxies NGC 2146 and IC 342 in 12CO and 13CO J=1−0 and 2−1, as well as CS J=2−1 lines using the FCRAO 14m telescope. The multi-line data are used, in conjunction with radiative transfer model calculations, to estimate the physical conditions of the gas with the purpose of determining the difference between the two galaxies in terms of their gas component. We hope to examine if the gas temperature and column density are dependent on the infrared properties of a galaxy, and whether a warm and optically thin gas component exists in these galaxies, as has been suggested in the case of M82 (Knapp et al.1980), especially for NGC 2146 for which the starburst has been proposed to be a scaled-up version of M82 (Jackson and Ho 1988).

High-spatial-resolution CO observations of galaxies have shown detailed distribution and dynamics of the molecular gas in spiral galaxies. We review observations, and show that the molecular gas distributions in the nuclear regions can be classified into three types: a core type with high concentration toward the nucleus; compact-ring type with ring radius of ∼200 pc; and a broad-ring type with ring radius of several hundred pc to ∼1 kpc. We also discuss their implications.

Studies of the central parts of nearby galaxies as well as of distant luminous galaxies, made with the IRAM 30 m telescope, show that the CO 2−1/1−0 ratio is 0.6 ± 0.2. Many galaxies in the distant luminous sample are new detections. In contrast to the distant luminous sample, the galaxy NGC 3147, at cz = 2800 km/s, has a more modest infrared luminosity, but a large mass of molecular gas, which is not concentrated in the center of the galaxy.

We report the preliminary results of a search for HCN and HCO+ in the nucleus of 14 IR bright galaxies. The line intensity ratio HCN/HCO+ is of the order of or larger than unity, except for M82 which has a line ratio of 0.5. The line ratios are discussed in terms of a model of chemistry of dense interstellar clouds. There is a correlation between the HCN and HCO+ line intensities and that of CO.

1) C18O(1−0) OBSERVATIONS. We observed 13 points of C18O(1−0) (resolution 22″) around the nucleus of M82 and obtained typical main beam brightness temperatures of 20-50 mK. The intensity distribution reveals a clear double–peaked structure with the maxima seperated by 25″. The relative strength of the peaks w.r.t. the emission from the center is consistent with the presence of two point–like sources located at the peak positions. We compared our data with the 17″ CO(1−0) observations of Nakai et al. (1987) and found a strong contrast for the CO(1−0)/C18O(1−0) ratio with high values (≃30) in the center and low values (≃15) at the peak positions. This result is consistent with that of Loiseau et al. (1990) who also found with 12″ resolution for the ratios of CO(2−1)/13CO(2−1) minima at the ring locations, although not as strong as we found for the CO(1−0)/C18O(1−0) ratio. This high contrast indicates the presence of opically thick gas within the molecular ring and supports the view that the main part of the star forming process occurs in the ring.

Using the IRAM interferometer we have mapped HCN(1 → 0), HCO+(1 → 0), and 3.5 mm continuum emission from Arp 220, the prototype infrared ultraluminous galaxy. The extended molecular emission and velocity gradient are aligned with the optical dust lane. The HCN and HCO+ lines are exceptionally strong, suggesting a connection between the dense molecular gas they trace and Arp 220's enhanced infrared emission. The inferred gas density and filling factor imply the nucleus of Arp 220 may be a giant molecular cloud.

A search for CS J=7−6 emission from the core of the galaxy IC342 was undertaken with the Caltech Submillimeter Observatory. At the line frequency of 342.883 GHz, the beamsize is 20″. An upper limit to TMBΔv of 0.4 K km s−1 was obtained. This upper limit can be compared directly to the CS J=3−2 integrated line flux (3.1 K km s−1; Mauersberger and Henkel 1989), because the beam used for that observation was similar (17″). The J=7−6 to J=3−2 line ratio limit is 0.13 with no correction for the slight difference in beamsize, and ≈ 0.15 with this correction included. Assuming Tkin = 70 K (Martin and Ho 1986), this leads to a density upper limit of about 5 × 105 cm−3 for this gas.

In 1986 three papers claimed that for bright IRAS galaxies their luminosity L is proportional to the amount of interstellar matter Mgas they contain. Mgas was derived from the CO luminosity and from the submm/mm dust emission. Both methods roughly agree and it is found that L/Mgas ≈ 5…10 in solar units. However, detailed comparison is still far from satisfactory. For example, for the five objects detected in both samples the dust emission gives (for comparable beams and using the same distances) on the average 2.5 larger gas masses than CO. This may be partially explained by the fact that the dust also emits from regions of neutral hydrogen, whereas CO emission is restricted to H2 clouds. Nevertheless, gas mass are still controversial.

Images with subarcsec spatial resolution in the light of near infrared atomic (Brγ) and molecular (1-0 and 2-1 S(1)) hydrogen emission lines have been obtained for some southern starburst and Seyfert galaxies for the first time. The data were obtained with the Max-Planck-Institut für extraterrestrische Physik (MPE) near-infrared array spectrometer (image scale 0.8″ /px, resolving power 950) during an observation run in January 1990 at the ESO/MPI 2.2m telescope. We investigated NGC253, NGC5253, Cen A, M83, LMC, He2-10, NGC1068 and NGC1808. Some results on the last two galaxies are presented.

We have observed 2.6 mm CO J=1−0 emission in a complete sample of galaxies with starburst nuclei using the 14 m millimeter telescope of the Five College Radio Astronomy Observatory (HPBW=45″). The galaxies in this sample have distances between 15 and 40 Mpc, and central 10 μm luminosities (R < 500 pc) comparable to the starburst galaxy M82 (Devereux 1989). Molecular gas was detected in 16 of the 18 galaxies observed, with global H2 masses in the range 109 to 2.5×1010M⊙.

Four systems selected from a sample of southern interacting IRAS galaxies are discussed. They have been studied with near-infrared photometry, optical imaging and spectroscopy, and coadded IRAS data.

Summary The Berkeley-Illinois-Maryland millimeter array has been used to image the CO (J=1−0) emission of three Seyfert galaxies, NGC 3227, NGC 7469 and NGC 5033, selected for their strong single-dish CO detections. Extensive u-v coverage was obtained for all three galaxies resulting in 2″-3″resolution. The CO emission in NGC 3227 and NGC 7469 appears as compact structures centered on the active nuclei, containing substantial fractions (∼ 80% and ∼ 50%) of the single-dish flux. In NGC 3227, 10% of the CO flux detected by the interferometer is contained within the ionized narrow line region. The unresolved molecular gas concentrations in the nucleus of NGC 3227 imply a CO mass of 6.5 × 107 M⊙ concentrated within a diameter ≍ 50 pc. The CO emission in NGC 5033 is not detected at this resolution, implying a CO structure size of 20″-60″(1.2-3.6 kpc). Continuum emission at 2.7mm is not detected in any of the three galaxies. In the center of NGC 7469, the H2 mass is comparable to the dynamical mass. Kinematic studies of the detected gas reveal a rotational motion of the gas in NGC 3227 and NGC 7469, allowing identification of the gas in NGC 7469 with a nuclear starburst. These data are consistent with the idea that interactions between galaxies cause gas to concentrate in their nuclei thereby feeding starburst and Seyfert activity.