V. COLLIDING GALAXIES AS RADIO SOURCES

7. During the past decade the methods of radio astronomy have opened up new vistas on the field of multiple galaxies, and a few most significant results have been achieved already. Data have been given in the literature [19], [20] on the following extraordinary galaxies: NGC 1275, a double galaxy in Cygnus [Cygnus A radio source], NGC 2623, NGC 4486, NGC 4038 to 4039 and NGC 5128. From direct photographs, as well as from spectral analyses in the photographic range, the first two mentioned systems seem to be definitely-galaxies in collision. NGC 2623 and NGC 4038 to 4039 also have the appearance of colliding galaxies, while the nature of NGC 4486 and NGC 5128 is more uncertain.

Among the few abnormally powerful extragalactic radio sources there seems to exist a bewildering variety of physical conditions, so that no systematic classification is as yet possible. For instance, the ratio of the luminosities of these objects in the radio regions and the optical regions covers a most astounding range, as is seen from the entries in Table 1, which were kindly supplied me by my colleague at the California Institute of Technology, Professor J. G. BOLTON.

Dr. BOLTON also submits that the sources Hydra A and Hercules A have values m_r of about + 6.0 and photographic magnitudes of about +17. These are extragalactic objects with double nuclei and, if the identifications can be accepted, are objects intermediate between Cygnus A and NGC 1275.

NGC 1275. This double galaxy, which is shown in Fig. 11, is in the center of the large Perseus Cluster and is located at R.A. 3^h16^m30^S and Decl. + 41° 19'.6 (1950).

Figure 11. NGC 1275. Photograph taken with the 200-inch telescope, emulsion Eastman 103 a-O, exposure time 10 mm. Scale indicates one minute of arc.

For a description of some of the structural and spectroscopic characteristics of NGC 1275 we quote from the director's report for the Mount Wilson and Palomar Mountain Observatories in the Year Book No. 54 (1954-1955) of the Carnegie Institution of Washington, where we read on p. 25:

"Although colliding galaxies are safely established as radio sources, we have at present little information about the details of such collisions. The strongest radio source of this kind, Cygnus A, is not very suitable for detailed investigation because of its great distance. A much better subject is the colliding pair NGC 1275 in the Perseus cluster of galaxies. Here an early-type spiral is in collision with a late-type spiral and the pair is close enough to show much structural detail on plates taken at the 200-inch telescope. An examination of the existing plates convinced Baade that it should be possible to localize the regions of the colliding gases on exposures with suitable plate-filter combinations if in such 'hot spots' the total light emitted by the colliding gases were of the same order as the continuum of the unresolved stars. Attempts thus to localize the hot spots by their H, O III, and other emissions were successful, and a detailed picture of the emission regions was obtained.

The spectroscopic investigation of these spots by MINKOWSKI led to the following results. In the northern part of the object the emission lines are double, showing a velocity of + 5200 km/sec for the early-type spiral and of + 8200 km/sec for the late-type spiral. The emission spectra of both nebulae consist of strong 0 II lines and weak lines of hydrogen. In the southern part an entirely different type of spectrum appears, showing one set of asymmetrical lines of considerable width, indicating a velocity of + 5200 km/sec. H lines and forbidden lines of 0 I, 0 II, 0 III, Ne III, and S II appear. This spectrum appears superposed on the nucleus of the early-type spiral where it was first observed by HUMASON many years ago. But it is not restricted to the nuclear region, as happens in certain spirals with bright semistellar nuclei, and it appears with somewhat less intensity farther to the south.

The appearance of the object and the spectroscopic observations suggest the interpretation that the late-type spiral is inclined against the early-type spiral in such a way that in the north the late-type spiral is in front. Moving toward the early-type spiral, it has penetrated the other system in the center and south of it, where now highly excited gas with large internal motions shows the aftermath of the collision. The velocity of the mixed gas is close to the velocity of the early-type system; this requires that the early-type system, be the more massive one, a conclusion which is supported by the general appearance of the two galaxies. The actual collision is in progress to the north, where the gas masses of the two spirals can still be seen separately. A more detailed discussion of the collision suggests a duration of the order of a million years; more than half this time has passed, and the effects of the collision should now be past their maximum."

One of the most important results of this investigation is [5] that the two spirals involved in the collision have a relative radial velocity of 3000 km/sec. This is the highest value so far known, except for the difference of 7000 km/sec found by ZWICKY for the apparently interconnected galaxies IC 3481 and IC 3483. This large velocity difference led many astronomers to doubt that the IC 3481 and IC 3483 could really be members of a triple system of galaxies in the process of a mutual encounter [4]. The large velocity difference observed in NGC 1275, however, make this assumption appear less improbable than it originally was.

Cygnus A. This radio source seems to be related to two late type galaxies located at R.A. 9^h57^m45^S and Decl. + 40° 35' 46" whose redshift corresponds to a symbolic velocity of recession of approximately V_s = 16830 km/sec. The distance of the system, on HUBBLE'S old distance scale is 3.3 X 10⁷ parsecs. For a description of the photographic appearance of the double galaxy and some of its spectroscopic properties we refer to the article by W. BAADE and R. MINKOWSKI [19]. We only mention here that some high excitation lines of Ne III, Ne V and 0 III are observed in high intensity, indicating the violence and the effectiveness of the collision of the gas clouds involved, although their radial velocity differences are only of the order of 500 km/sec, much smaller than in NGC 1275. The Cygnus A double galaxy seems to be a giant system with a total optical emission estimated at 5.6 x 10⁴² ergs/sec 2 x 10⁹ times the luminosity of the Sun and an even greater total emission in the radio region of 8 x 10⁴² ergs/sec. BAADE and MINKOWSKI remark that "the source of energy for the radio emission may be the relative kinetic energy of the colliding galaxies, which is of the order of 10⁵⁹ ergs for a relative velocity of 500 km/sec". At the present rate of radio emission the total energy radiated in the radio region in a period of one million years would be of the order of 2 x 10⁵⁶ ergs. The collision may last a few million years. We again mention that all estimates made refer to HUBBLE'S old distance scale, assuming an average value V_s 550 km/sec per million parsecs.

NGC 4486 (Messier 87). This globular galaxy, which is a member of the Virgo cluster and is located at R.A. 12^h28^m18^S and Decl. + 12° 40.1', shows a jet like extension originating in its center. According to spectra obtained by HUMASON, the jet is rather blue in color but has a continuous spectrum. Recently some of the prominent condensations in the jet were found by BAADE to be partially polarized. The globular galaxy itself has a normal spectrum of type G, but superposed on the nucleus appears a strong emission line of O II at 3727 which is shifted relative to the nuclear G-type spectrum by - 295 ± 100 km/sec. Contrary to the cases of Cygnus A and NGC 1275, where the radio emission may be ascribed to the gas clouds within two colliding galaxies impinging on one another, the origin of the radio emission in NGC 4486 is not cleared up. This origin becomes still more puzzling if the observations, recently announced [21] by BROWN and SMITH, can be confirmed that a sizeable part of the radio emission comes from a disk 50' of arc in diameter, as contrasted with the visual disk of NGC 4486 which is only a few minutes of arc across.

NGC 5128. The origin of the radio emission from NGC 5128, located at R.A. 13^h22^m28^S and Decl. -42° 45'.6 (1950), is as little understood as that from NGC 4486. BAADE and MINKOWSKI [19] have suggested that NGC 5128 is a double galaxy consisting of an unresolved E₀ galaxy and a second stellar system of much later type in front of it or interpenetrating it, thus accounting for the unusually strong and wide absorption lane. The apparent radial velocity of the system is 450 km/sec, with superposed velocity differences which are not very conspicuous. Because of this fact, the interaction of the two assumed galaxies would be of an entirely different type from that in the NGC 1275 and Cygnus A radio sources. ZWICKY, with the 48-inch Schmidt telescope has recently found some evidence of polarized light being emitted from certain condensations of NGC 5128. These observations, which must be checked with the 200-inch telescope, are not necessarily related to the radio emission, which seems to come from two regions, namely, a central ellipsoidal part a few minutes of arc in extent and a very much larger area about two degrees in diameter [19].

8. Conclusions. From the very scanty data available so far, it is clear that most interesting results may be expected from the study of multiple galaxies. For the present we cannot be sure, however, about very many of the structural and physical features of these systems. For instance, for the groups shown in the Figs. 8, 9 and 10, no spectroscopic data of any kind are available on the main bodies constituting these groups, not to speak about the spectra of the connecting links. The fact that from photographic observations in the visual range the radio emissive properties can in no way be predicted, suggests that it would be of the greatest importance, as in many other fields of astronomy, to extend, if possible, the spectral explorations into the ranges from one micron to one centimeter wavelength, as well as to the far ultraviolet. Efforts which are now being made in these directions will have to make use both of entirely novel recording instruments and of high flying rockets to get away from the absorption in the atmosphere.

As to the dynamic interpretation of the structural features of multiple galaxies, one aspect clearly begins to stand out which indicates that some of the views by present day astronomers about stellar systems may be in need of a radical revision. These views are for instance maintained by BAADE and MINKOWSKI in their discussion [19] of the radio source Cygnus A, where they say:

"This suggests that we are dealing with the exceedingly rare case of two galaxies which are in actual collision. The main features of such a collision have been discussed by SPITZER and BAADE [22]. On the cosmical time scale, collisions of galaxies are a rather frequent phenomenon in the rich clusters of galaxies. As far as the stars of the colliding systems are concerned, such a collision is an absolutely harmless affair. The average distance between two stars is so large that the two galaxies penetrate each other without any stellar collision"

The study of the features of multiple galaxies shows immediately that there must be something radically wrong in the above conclusions by SPITZER and BAADE. Indeed, it is at once obvious from the observations that it is clouds, filaments and jets of stars which are ejected massively from galaxies in collision. As far as the very extended bridges and filaments between largely separated galaxies are concerned, we have not, in fact, succeeded so far to prove that they contain gases and dust, although this is undoubtedly the case. The conclusions by SPITZER and BAADE, as well as by others, concerning the consequences of the encounters of galaxies, are erroneous because of their neglect to evaluate the effects of large scale tidal effects from system to system, as well as the interactions between gas and dust clouds with individual stars and large groups of stars. Furthermore, the electromagnetic actions within the dispersed clouds may materially contribute to the internal viscosity of stellar systems which, by the testimony of the structural features of the connecting links of galaxies, must be very much greater than is usually admitted.

Unfortunately, because of the present relatively high intensity of the night sky, caused by prolonged solar activity, it will be some time before we can hope to explore efficiently the very faint extensions in multiple galaxies. As soon as feasible, it is proposed, however, to pursue such studies with the utmost vigor. This is important not only because of the information to be gained on the structure and evolution of the galaxies themselves. The analysis of the extended projections into intergalactic space will ultimately give us new clues on the population of intergalactic space and on the average density in the universe, which, from other lines of reasoning recently given, seems to be so great as to rule out any of the cosmological theories so far proposed which are based on the general theory of relativity and which assume that the universal redshift observed in the spectra of distant galaxies corresponds to an actual expansion of the universe [9], [23].

¹ Includes an allowance for extended envelope. r₀ is defined by the relation m_p - m_r = 2.5 Log₁₀ r₀ and therefore represents the ratio of the radio intensity of the abnormal galaxies to that of normal S_b or S_c galaxies, where the intensities are measured in terms of unit radiations in the photographic range. Back.