obtained with the SEST, H2 emiSSion . . from an unresolved nuclear source mea­ , .., • • • sured with the 3.6-m telescope at La Silla (Israel et al., 1989), and literature data suggest that the nucleus of Cen A is surrounded by a disk of 2 107 MG' Such a disk, with an outer edge radius of 160 pc and with a density distribution of n U 1'2, is consistent with all existing observations of the nuclear region of Centaurus A. ... Future observing programmes that are currently in progress 01' have already been scheduled will investigate the dis­ tribution of the moieculaI' material with the highest possible spatial resolution, the moieculaI' line emission in high den­ sity tracers - such as HCN, CS, and .. HCO+, as weil as the absorption features in those species. A combina­ I' tion and detailed analysis of these data will cast more light on the nature of the '. interstellar medium and star formation in • ' . . -.. Centaurus A and elliptical radio galaxies , . in general. Figure 1: Gontour map of the integrated 12GO J = 1-0 emission of Gentaurus A superimposed On an optical image. The emission is weil concentrated along the dust lane. Gontour intervals References are 17.5.22.5,27.5, ... K km s-'. The peak intensity is 54 K km S-I. von 8allmoos, P., Diehl, R. and Schoenfelder, V., 1987, Ap. J 312,134. Bland, J., 1985, Ph. D. Thesis, Universily of features in the CO lines against the nu­ the CPC instrument on board IRAS and Sussex. clear continuum source. show that the dust temperature in the Burns, J.O., Feigelson, E. D. and Schreier, The main results of these observa­ dust lane is about 42 K. The ratio be­ E. J., 1983, Ap. J 273, 128. Crawford, M. K., Genzel, R., Townes, C. H., tions are that the bulk moieculaI' mate­ tween the far-infrared luminosity and the and Watson, D. M., 1985, Ap. J, 291, 755. rial is closely associated with the dust total moieculaI' mass is 18 . IMG which Ebneter, K. and 8alick, 8., 1983, P.A.S.P. lane and contained in a disk of about is close to the mean value obtained for 95,675. 180" diameter with a total moieculaI' isolated galaxies. For giant moieculaI' Eckart, A., Cameron, M., Rothermel, H., Wild, mass of about 2 10B MG' The total cloud complexes in our Galaxy, this W., Zinnecker, H., Olberg, M., Rydbeck, moieculaI' mass of the disk and bulge is ratio is of the order of 1 to 10 . IMG . A G., Wiklind, T., 1989, in preparation. of the order of 3 10B MG' The moieculaI' comparison of the 12CO J = 1-0 and the Feigelson, A. E. D., Schreier, E. J., Devaille, gas In the nucleus is warm with a kinetic far-infrared data indicates that a consid­ J.P., Giaconni, R.E., Grindlay, J.E., and temperature of the order of 15 K and a erable amount (about 50 %) of the far­ Lightman, A.P., 1981, Ap.J., 251, 31. van Gorkom, J. H., 1987, in: Structure and nUmber density of 103 to 3 104 cm-3 . infrared emission at 100 ~m is not inti­ Dynamics of Elliptical Galaxies, Ed. T. de Absorption features in the 12CO and mately associated with massive star for­ 13C . Zeeuw, lAU Symp. 127, D. Reidel Publ. . 0 Iines against the nuclear con- mation. TI,is emission is larger in extent Co., Dortrecht, p. 421. t1~uum indicate that the properties of than the moieculaI' disk and is probably Israel, F. P., van Dishoeck, E. F., Baas, F., glant moieculaI' clouds are comparable due to diffuse gas clouds in Centau­ Koornneef, J., 8lack, J., de Graauw, Th., to those in our Galaxy. rus A, similar in nature to the "cirrus" 1989, in preparation. emission in our Galaxy. Kellermann, K.I., 1974, Ap.J Letters 194, The absorption features detected in L135. COmparison with Other Data the CO emission lines are coincident Malin, D. F., Quinn, P.J. and Graham, J.A., 1983, Ap. J Letters 272, L5. The moieculaI' data have been com­ with known H I, C H , and H CO absorp­ 3 2 2 Pllillips, T.G., Ellison, B.N., Keene, J.8., bined with 100 ~lm and 50 ~m far-in­ tion lines, although the moieculaI' con­ Leighton, R. 8., Howard, R. J., Masson, frared emission of Centaurus A in order tent of gas in red and blue shifted clouds C. R., Sanders, D. B., Veidl, B. and Young, to study the variations in the gas and (with respect to the centre velocity of K., 1987, Ap.J Letter 322, L73. dust distributions (Eckart et al., 1989). VLSR = 550 km S-1) seem to be different. Shaffer, D.8. and Schilizzi, R. 1., 1975, A. J. These far-infrared data were taken with A combination of new moieculaI' data 80,753.

Moleeules in External Galaxies

F. COMBES, Laboratoire de Radioastronomie Millimetrique, Meudon, France

The 15-m SEST telescope is the ject that galaxies in the northern sky ~nlque facility to study with high resolu­ al ready give a large enough sampie to ~Ion the moieculaI' component of galax­ investigate, but there are outstanding the southern hemisphere; apart from the les in the southern sky. One could ob- objects that can only be studied from obvious Magellanic Clouds, it is weil

21 known that most beautiful barred galax­ Way's; this is useful in particular to con­ molecules is still a complete mystery. ies, for example, are at low declinations firm the universality of the N(H 2)/I(CO) In the nearby and almost edge-on (NGC 1300, NGC 1365, NGC 1097 ...). conversion ratio, widely used by barred galaxy NGC 4945, J. B. Whiteoak For this first year of operation, ex­ millimetric radioastronomers (I (CO) is (CSIRO, Australia), M. Dahlem (MPlfR, tragalactic astronomers have explored the integrated CO emission). Bonn, FRG), J. Harnett (CSIRO) and R. all types of galaxies, at all distances, The NGC 300 galaxy, an Sc spiral in Wielebinski (MPlfR) have obtained 270 from the nearest spirals (the equivalent the Sculptor group, is a good object for CO (1-0) spectra. They have identified of Andromeda in the North), to the out­ such a study (45" = 360 pe). Albert Bos­ five velocity components in the central skirts of the detectable molecular sky, at ma, from Marseille Observatory region. Only three of them are in the redshifts of z = 0.1. (France), has observed 19 positions in nucleus, as shown by OH absorption this galaxy, corresponding to H 11 re­ spectra taken at 6 GHz with the Austra­ lian radio telescope. The two outside 1. Nearby Spirals gions. Signals are very weak, lower than 1 Kkm/s in integrated CO emission. The components seem to come from a rotat­ Nearby galaxies, like the Magellanic derived H2 masses in each complex are ing ring-like structure: they correspond Clouds (see the article by F. Israel in this between 1 and 5 . 105 MG' i. e. even less to two peaks at 460 and 640 km/s (on issue of the Messengef!, give the oppor­ massive than the GMCs in our own each side of the systemic velocity), tunity to study Giant Molecular Clouds Galaxy. Yet the surface explored in each separating the rigid rotation curve inside (GMC) one by one, almost un-diluted in beam is larger than a GMC's surface. 100" of radius, from the differential rota­ the 45" beam of the SEST at CO (1-0) This kind of weak and very narrow pro­ tion beyond. Such a ring at the radius of frequency, and even less with the 23" files has already been seen in northern maximum rotation velocity is expected CO (2-1) beam. One can then learn nearby galaxies, like in M 33 and M 81. It in barred galaxies, as discussed now. whether the physical nature of their is important to better study these ob­ clouds is similar or not to the Milky jects, since their weak abundance of 2. Barred Galaxies Barred galaxies possess most of the time strong spiral density waves, and the gas component is essential for the persistence of these waves. The gas behaviour in a barred potential has been the subject of many theoretical studies and hydrodynamic simulations. It is ex­ pected that c10ud collisions will deflect the cloud orbits with respect to those of stars, that are ellipses aligned with the bar. These gradual deflections produce the spiral wave. The gravitational torque exerted by the bar on the spiral will drive gaseous radial transport from the end of the bar towards the centre. When there exists an inner Lindblad resonance, the gas will accumulate in rings. As the inner resonance always occurs near the max­ imum of the rotation curve, the ring is predicted to be located there, in good agreement with the nuclear rings that are often observed in hot spot galaxies. The rings contain conspicuous H 11 re­ gions, tracers of intense star formation. The molecular clouds are therefore expected by theory to be highly centrally concentrated in barred galaxies, and to follow the spiral structure in the disko N. Loiseau (INPE, Brazil). J. Harnett (CSIRO, Australia), E. Bajaja (Argentina) and H.-P. Reuter (MPlfR, Bonn, FRG) have started observational projects on barred galaxies, to test these models, and in particular whether the gas reveals peculiar velocities that could be inter­ preted as infloW towards the centre. It is expected that starburst or even nuclear activity could be triggered by the effect of the bar. Results obtained towards the barred spiral NGC 613 have al ready been reported by Bajaja and Hummel in Figure 1: Gunn z c%ur image of NGC 1365 showing the dust (light areas) as weil as regions the Messenger No. 55 (March 1989). with hot stars and H 11 regions (dark areas). The over/ay shows the J = 1-0 CO profi/es observed A beautiful southern barred spiral is with SEST on a 20" grid. The ve/ocity scale goes from 950 to 2250 km/s, the antenna the Seyfert galaxy NGC 1365. Sandqvist temperature sca/e from -0.12 to 0.36 K. et al. (1989) are in the process of map- 22 • • • • • •

•

"

• f . .~'. . Figure 2: Oplical pl1olograph of NGC 7252, reproduced from 111 a-F + RG 630 plate, oblained for lhe ESO red halfof lhe ESO/SERC Survey oflhe SOulhern Sky. (a) inner structure; (b) sl10wing ouler arms. Pl1olographic work by H. Zodel.

ping it in the two CO lines (1-0) and NGC 1808 is a spectacular barred sion comes from the nucleus. Further (2-1). On the photograph in Figure 1 is galaxy, where dust filaments seem to observations (in particular in CO (2-1)) Superimposed a map of CO (1-0) spec­ emerge from the plane. It was mapped are needed. tra obtained so far. An interesting result in CO by M. Dahlem, U. Mebold, U. IS that spectra in the centre present two Klein (MPlfR, Bonn, FRG) and R. Booth 4, IRAS Galaxies and Mergers v~locity components, revealing a defi­ (Onsala, Sweden). The central area c1ency of CO emission at the systemic shows ring-like rotation. The velocity Far-infrared observations by IRAS velocity. This behaviour suggests the peaks correspond to the maximum of have revealed that galaxy mergers can presence of a ring of molecular clouds the rotation curve. The optical filaments trigger huge starbursts: most of the ul­ Inside the bar, supporting the results of have a molecular gas counterpart: CO traluminous IRAS galaxies, that radiate hYdrodynamical models in barred sys­ outflows are normally observed to the 90 % or more of their total luminosity in tems (Schwarz 1984, Combes and Gerin major axis. Is the central starburst the the infrared, are interacting galaxies and 1985). cause of this gas ejection, as is pro­ mergers. In collaboration with M. Gerin (Paris). posed in M 82? One problem in these actively star­ N. Nakai (Nobeyama) and J. M. van der forming objects is to determine the gas Hulst (Groningen) we have mapped the excitation. Indeed, it is likely that the gas 3. Interacting Ring Galaxies barred galaxy NGC 1097, wh ich is the is heated by the starburst, and becomes prototype of the nuclear ring barred spi­ Another kind of ring morphology is less optically thick in the CO lines. This ral. From previous observations with the obtained with nearly head-on collisions would yield an overestimation of the to­ 45-m of Nobeyama (Japan), and the between galaxies. The prototype of tal molecular mass, if standard 30-m of IRAM, we determined that these objects is the Cartwheel. The tidal N(H2)/I(CO) conversion ratios were molecular clouds are tracing the nuclear phenomenon has been simulated with used. Such a phenomenon occurs in nng (Gerin et al., 1988). With the SEST great success by Lynds and Toomre the central part of Messier 82, where 15-m we mapped the whole extended (1976) and Theys and Spiegel (1976). the CO (2-1 )/CO (1-0) integrated emis­ dlSk, and discovered that most of the The main observational difference with sion ratio R between the two rotation mass has accumulated towards the previous rings is that stars participate in lines of CO, reaches 4 in some regions, centre: about 50 % of the mass is found the ring structure, and not only the gas. indicating a large fraction of hot optically in the central beam, i. e. within a radius Also the radii of these rings are much thin gas. of 1.7 kpc, and we know that it is evenly larger than those of the gaseous nuclear NGC 3256 has been observed by C. dlluted in the central beam, according to rings. Is the gas compressed in the den­ Dupraz, F. Casoli and M. Gerin in the the higher resolution observations. Bars sity wave that corresponds to the ring? two CO lines with the SEST 15-m tele­ are indeed able to drive the gas inwards, Sometimes the ring can be decom­ scope. The ratio R is about 1, but the as predicted by theoretical models. posed in knots: do tlley correspond to surprise was in the weakness of the We have also undertaken a survey of star formation? We have discovered CO 13CO (1-0) emission with respect to the galaxies with Ha rings, to check the emission in two of these ring galaxies, 12CO (1-0) one. The integrated emission prediction that most molecular clouds IC 4448 and AM 064-741 (F. Casoli, F. ratio between the two isotopic mole­ may be accumulating in these features Combes, C. Dupraz from the Paris cules is about 30, while it is around 10 in (Combes, Gerin and Buta, 1989). CO group). The 45" resolution does not en­ most ordinary galaxies. This high ratio nngs cannot be seen directly but they able us to distinguish between nuclear indicates that the gas is optically thinner are revealed by typical two-horn-pro­ or ring emission, but in AM 064-741 the than usual. The very peculiar line ratios files (such as seen in NGC 1365). kinematics suggest that the CO emis- in this object may be due to it being a 23 0.1 ratios are still found in galaxies without higher in the past. The ultraluminous nuclear activity. IRAS objects have luminosities larger The life time of the star burst can be than 10'2 .. Mirabel et al. (1988) have extrapolated from these efficiencies. In detected four of these monsters, 6 10 0.05 time scales of a few 10 yrs, the merger possessing 1-6 10 MG of molecular remnants should become devoid of gas. Their L1R/M(H2) ratio is between 20 molecular gas. This result supports the and 80, much larger than in classic star­ currently well-developed idea that the burst galaxies, like Messier 82. The merging of two spirals will form an ellip­ highest systemic velocity among these o tical galaxy, devoid of cold gas. An ideal objects is 27,500 km/s, wh ich demon­ object to test this hypothesis is the strates tl1e ability of the SEST 15-m southern merger remnant NGC 7252, telescope to detect faint and broad one of the pet galaxies of FranQois emission lines. 4500 5000 Schweizer (1982). This object is con­ This brief survey, far from exhaustive, Veloclly (km/s) spicuous by its two tidal tails, that repre­ already shows how exciting extragalac­ Figure 3: NGC 7252: CO (1-0) and HI profiles sent the "smoking gun" evidence of the tic work can be with the SEST 15-m towards this newly-born elliptical galaxy. merging of two spiral galaxies (Fig. 2). telescopel Numerous loops, shells and ripples add to the evidence. The luminosity profile is 1 4 References surprisingly regular and follows the r / merger between two equal-mass spiral law, characteristic of ellipticals, until a Combes, F., Gerin, M. (1985) Astron. Astro­ galaxies, as suggested by its very per­ large distance. Yet this object was seen phys. 150,327. turbed appearance, with two tidal tails. to be very rich in molecular gas (Dupraz Combes, F., Gerin, M., Buta, R. (1989) As­ tron., Astrophys., in prep. If the infrared luminosity L is re-radi­ et al. 1989): about 3 109 MG, within 1R Dupraz, C., Casoli, F., Combes, F. (1989) in ated by dust heated by the recent star 7 kpc. The observed line shape sug­ prep. formation, the ratio L1R/M(H2) is an indi­ gests that the CO emission comes from Gerin, M., Nakai, N., Combes, F., (1988) As­ cator of star-formation efficiency. These matter confined to a disk, which is also tron. Astrophys. 203, 44. interacting and merging galaxies have observed in Ha. This surprising result Lynds, R., Toomre, A. (1976) Astrophys. J. the highest known ratios: L1R/M(H2) of indicates that not all of the molecular 209,382. the order of 50 or greater, while it is of gas is consumed in the star burst, as Mlrabel, I. F., Booth, R. S., Garay, G., Johans­ the order 1-3 in normal galaxies. There previously thought, or that matter con­ son, L. E. B., Sanders, D. B. (1988) Astron. is also the possibility that a significant tinues to fall down onto the disk, long Astrophys. 206, L20. Sandqvist, Aa., Elfhag, T., Jörsäter, S., Lind­ part of L comes from dust heated by after the merging event. 1R blad, P. O. (1989) in prep. an active nucleus (in that case the emis­ At higher redshifts, the galaxies that Schwartz, M. P. (1984) Monthly Notices Roy. sion region is highly confined towards can be detected in CO are all monsters: Astron. Soc. 209, 93. the centre), so that the ratio L1R/M(H2) is huge starburst galaxies, corresponding Schweizer, F. (1982) Astrophys. J. 252, 455. not a good indicator of star-formation to interacting or merging objects, the Theys, J.C., Spiegel, E.A. (1976) As/rophys. efficiency. However, high L1R/M(H2) frequency of mergers being probably J. 208, 650.

Extragalactic Continuum Sources

E. VALTAOJA, Metsähovi Radio Research Station, Espoo, Finland

sites of energy generation and channel­ Introduction ing in active galaxies. Long wavelengths As with most other high-frequency ("Iong" in the case of quasars meaning radio telescopes, continuum work everything longer than one centimetre) While SEST opens up completely new occupies only a small fraction - current­ show only evolved structures, such as southern vistas, its location also pre­ Iy about 5 %- of SEST's total time. The old, ejected knots; the millimetre regime sents some problems in continuum importance of these observations in in­ is where the real action iso work. The continuum observer dreams creasing our understanding of quasars Most events seen at centimetre of uninterrupted multifrequency light­ and other extragalactic sources is, how­ wavelengths have their precursors on curves revealing the various con­ ever, large. higher frequencies. This forewarning stituents and processes found in Active The millimetre-to-IR observations capacity is especially useful for space Galaxy Nuclei (AGN), but the reality usu­ probe the innermost parts of the radio­ VLBI purposes in choosing the best ally shrinks to a scatter of isolated flux emitting regions of active galactic nu­ "targets of opportunity" for observa­ measurements. In most cases, one clei: the radio cores, possibly the begin­ tions. The millimetre spectrum and its would greatly benefit from supporting nings of radio jets, become optically thin variations can also tell if compact struc­ data on other frequencies, but there are on mm-wavelengths, where also the ture is present in the source, and not many Southern telescopes available outbursts reach their maximum stages. whether it will be a good candidate for for that purpose. As these regions remain below the re­ VLBI observations; with sufficient flux solving power and above the standard data it may even be possible to produce SEST Measurements frequencies of VLBI, high frequency flux model maps of the sources. Clues to the measurements give us our only glimp­ nature of different radio sources must During its first year SEST has been ses of the very cores, the still mysterious also be searched at high frequencies. used for most of the purposes outlined

24