A. The Triple System IC 3481, Anon, IC 3483
These widely separated galaxies which were the first ones to be found interconnected by long luminous filaments of intergalactic matter also turned out to be some of the most remarkable in the sense of presenting astrophysics with some unsuspected and very puzzling problems. Two photographic reproductions of plates obtained with the 48-inch and the 200-inch telescopes are shown in the Plates I and II. In the Figure 1 drawing is reproduced emphasizing some of the significant details.
Some of the data on the triple system shown on the two plates I and II as well as in the drawing 1 are as follows.
IC 3481, Type S0 peculiar, R.A. 12h 30m 21s, Decl. +11° 40.8' (Epoch 1950.0). Apparent photographic magnitude mp = +15.0.
Anon = anonymous galaxy, Type S0p, R. A. 12h 30m 26s, Decl. +11° 40.0', mp = +16.0.
IC 3483, Type Sc peculiar, R.A. 12h 30m 38s, Decl. +11° 37.4', mp = +15.6.
All three galaxies appear completely unresolved on a series of photographs made with the 200-inch telescope in various colour ranges. From the near equality of their apparent magnitudes and of their angular diameters one would off hand conclude that the two galaxies IC 3481 and IC 3483 are roughly at the same distance. This first evaluation is of course greatly strengthened because of the apparent connection of the two galaxies with the smaller and fainter central nebula by faintly luminous bands of intergalactic matter. The curved connection between IC 3483 and Anon is particularly impressive, since it appears to be an extension of one of the spiral arms of IC 3483 as indicated in the Fig. 1. Also, on close inspection it is seen that the yatagan like formation has filamentary structure and that it is the widest in the middle. The maximum surface brightness of the connecting bridges is presumably of the order of mp = +24 per square second of arc and the colour seems markedly blue.
Figure 1. Drawing of the essential features of the interconnected triple galaxy IC 3481, Anon, IC 3483 as they can he distinguished on a series of good plates taken with the 200-inch reflector and the use of various emulsions behind different colour filters. Scale indicates one minute of arc.
From the features discussed in the preceding, the writer and his colleagues felt certain that the three galaxies form a physical triple system and that within narrow limits they are located at the same distance. Under these circumstances the results of the spectral studies which Drs. R. MINKOWSKI and M. L. HUMASON undertook at the request of the writer came as a complete surprise. Dr. MINKOWSKI first in 1952 laid the slit of the prime focus spectrograph of the 100-inch telescope along the connection over the two northern nebulae. Dr. HUMASON from this spectrogram deduced for both nebulae an apparent symbolic velocity of recession of the order of Vs = c / = 7250 km/sec.
It should here be stated that Vs has the dimension of a velocity since c is the speed of light, is the measured displacement from their normal position of the characteristic features in the spectrum of a nebula and is the wave length of these features in their normal unshifted positions. It must be emphasized that Vs is under no circumstance exactly equal to any real velocity. Even if the expansion of the universe were real, Vs would be exactly equal to the relative velocity of recession of a nebula from the observer only when / is very small.
The spectrum of IC 3483 was obtained by Dr. HUMASON in the spring of 1953 and showed an astoundingly low value of the apparent velocity of recession of the order of Vs = 100 km/sec. This completely shocking result made it imperative that all measures be repeated in order to eliminate any possibilities of oversight or gross errors of measurement and of interpretation. Dr. HUMASON kindly spent considerable time on this problem and arrived at the following result, for the communication of which I am greatly indebted to him. Dr. HUMASON'S final data are as follows.
IC 3481. Vs = 7304 km/sec. The spectrum shows a strong continuum with the absorption features at the H and K lines and at the G-band normal. There appear to be no conspicuous emission lines.
Anon. Vs = 7278 km/sec. The continuous spectrum is weak with narrow and weak absorption lines at H and K. The forbidden oxygen line 3727 is likewise present and also weak and narrow.
IC 3483. Vs = +108 km/sec. The continuous spectrum is weak, the H and K lines are normal and the 3727 emission is strong but spottily distributed, as it is mostly the case in spiral galaxies.
No spectral record of the faint intergalactic links between the three galaxies could be obtained so far. This is most unfortunate since the displacement of the spectral lines along the various parts of the long bridge would give us a clue as to the cause for the low value of Vs of IC 3483. One most important conclusion which can be drawn from the practical unobservability of the spectrum of the, bridge is that this intergalactic formation appears luminous because of its stellar content. Indeed, if the observed surface brightness were due in any way to the emission from fluorescent gases, emission lines should be readily observable.
The brightness of IC 3483 was subsequently also determined by Dr. W. E. BAUM. Using a diaphragm 28 sec of arc in diameter he found from his photoelectric measure mp = 15.5 in close agreement with our photographic result. BAUM also obtained mpv(photovisual) = + 15.0 corresponding to an international colour index +0.5. IC 3483 is therefore somewhat redder than the nearer spirals.
We here mention that whenever we refer in this study to absolute distances these will be given on the old scale corresponding to an average value of the apparent symbolic velocity of recession of Vs equal to 550 km/sec per million parsecs. This scale is certainly wrong. From the newest data it appears that a value of 200 km/sec per million parsecs may be closer to the truth. Until reliable new measures are available we shall use the old scale throughout this article, on the basis of which IC 3481 and Anon will be placed at a distance of 13.2 million parsecs, that is roughly as far away as the Coma cluster of galaxies. Although the spiral IC 3483 from its own appearance and from its connection with the other two nebulae should be located at this same distance, the very low value of Vs introduces some doubts as to the validity of this conclusion. As a matter of fact, as soon as it became known that Vs is equal to 108 km/sec, all of the writer's colleagues inclined to the idea that IC 3483 is a foreground nebula. Actually, however, three major possibilities of interpretation remain. These are
1. IC 3483 is a foreground galaxy.
2. IC 3483 is at the same distance as the two other galaxies and its low value of Vs indicates a real radial velocity of about 7000 km/sec relative to the two other members of the triple system.
3. IC 3483 is at the same distance as the two other galaxies, but the large value of Vs between IC 3481 and IC 3483 is caused by some differential effect on the frequency of light quanta travelling cosmic distances. The gravitational drag of light which was discussed by the writer (23) long ago as a possibility of explaining the universal redshift of light in a non-expanding universe would in some cases produce large differential shifts in the spectra of little separated galaxies.
In discussing the relative merits of these three interpretations the following considerations may be advanced.
If IC 3483 is a foreground galaxy it most likely belongs to the Virgo cluster which on the old scale lies at an average distance of seven million light years. The values of Vs for all of the members of this cluster lie in the approximate range - 300 km/sec < Vs < 2500 km/sec. The value Vs = 108 km/sec of IC 3481 also lies in this range. If placed at the distance of seven million light years our galaxy would have an absolute photographic magnitude mp = -11.0 and a greatest diameter of the order of 1500 light years. If a member of the Virgo cluster, it therefore follows that IC 3483 is a most unusual dwarf nebula of distinct spiral structure and with a central surface brightness comparable to that of the giant spirals. These conclusions are very uncomfortable when added to the difficulties previously mentioned, which we here reemphasize. Indeed, it is most unlikely that a foreground galaxy should lie at the end point of the yatagan like long extension of the central nebula designated as Anon and that this yatagan should constitute the exact tangential continuation of one of the spiral arms of IC 3483. Also, the colour index +0.5 obtained by BAUM indicates some reddening as compared with the nearby spirals and therefore greater distance. Finally, if IC 3483 were a foreground nebula, the remaining systems of IC 3481 and Anon would be left as one of the most unusual interconnected galaxies in the sense that among thousands of similar double galaxies not a single one can be found where the "countertide", as we propose to call it, does not constitute the exact tangential continuation of the bridge between the two nebulae in question. It is indeed seen from the plates I and II and from the drawing Fig. 1 that the long extension from Anon which points toward IC 3483 starts out from Anon at a sharp angle relative to the bridge between Anon and IC 3481. This angle on closer analysis can be easily accounted for only if the three nebulae are emerging from a relatively recent triple encounter.
The second possibility of interpretation is that all three galaxies are at the same distance and that the tremendous difference in radial velocities is real. This means that the total velocity of IC 3483 relative to its immediate neighbors is greater than 7200 km/sec, a speed far superior to any known at present for any neighboring bodies with masses comparable to those of regular galaxies. It is of course well known that the velocity dispersion in large clusters may be as high as 2000 km/sec. (25). As the size of a group of galaxies decreases the velocity dispersion decreases likewise and for isolated nebulae it is equal to several hundreds of kilometers per second at the most. Because of these considerations the author originally was inclined to disregard the possibility that a peculiar velocity of the order of 7000 km/sec could be real. Some cogent facts, however, have recently come to light which make the occurrence of very high peculiar velocities more probable or even certain. In the first place there are multiple systems whose members possess relative velocities which are undeniably large compared with the velocities which can be generated by the gravitational interactions of galaxies headed for close encounters or for mutual capture. For instance, in STEPHAN'S Quintet consisting of the galaxies designated as NGC 7317, 7318a, 7318b, 7319 and 7320 the values of Vs for the first four members according to HUMASON are 6736, 6638, 5638 and 6657 km/sec. These nebulae which are inextricably inter-related, and this is particularly true for NGC 7318a and b, have therefore relative peculiar velocities greater than 1100 km/sec. Furthermore a considerable number of galaxies has been observed (26) with emission lines in their spectra indicating actual velocities up to 5000 km/sec for internal gas clouds which no doubt constitute a considerable part of the masses of these galaxies. Most significant, however, are the recent combined findings of the radio astronomers and the spectroscopists investigating the nature of the extragalactic radio sources. Perhaps the most important results along this line of attack have been obtained from the observations of NGC 1275, one of the central member galaxies of the rich Perseus cluster. This object is a radio source and on closer inspection by R. MINKOWSKI (27) was shown to possess an emission line spectrum due to a close penetrating collision of two galaxies with a relative radial velocity of about 3000 km/sec. Although velocities of this magnitude are rare it does not now seem entirely out of the question that the large difference in the redshift of IC 3481 and IC 3483 might be due to an actual relative motion of the two objects. On this interpretation there nevertheless arises another great difficulty. It is well known from astrophysical theory that two stellar systems devoid of gas and dust passing through each other in the process of a head-on collision hardly disturb each other, unless their relative velocity is very low. Because of the insignificant transfer of momentum and of energy from star to star it is in fact difficult to understand how under any ordinary circumstances stellar systems on close encounters could disrupt each other sufficiently to give rise to the filamentary intergalactic formations of stars shown on the various photographs reproduced in this article. Since the transfer of momentum between stars is inversely proportional to their relative velocity, the physical characteristics related to the internal "viscosity" of the three galaxies of our triple system would have to be of a nature and magnitude quite unknown to us at the present if they had been capable of ejecting the observed stellar intergalactic formations during a close encounter with a relative speed greater than 7000 km/sec.
We add that if the three nebulae just discussed are at a distance of 13.2 million parsecs (HUBBLE'S old scale) their absolute photographic magnitudes are -16.0, -15.0 and -15.4 respectively, as one should expect it for objects of this kind. The absolute separation of IC 3481 and IC 3483 in the plane normal to the line of sight would be about 72000 light years and the largest diameters of the two brighter galaxies of the order of 8000 light years.
In view of the fact that there are serious difficulties with both of the interpretations just discussed the third possibility should not be lightly discarded. During the past fifteen years serious difficulties have come to light with respect to the concept of an expanding universe (14). If it were found that the universe is not expanding as a whole, the universal redshift will have to be explained on the grounds of long range interaction of light with matter or with other light. It will in this case be quite possible that the redshift is not a function of the distance alone but depends also on the particular constellations of matter surrounding any individual cosmic trajectory of a quantum of light. Relatively large fluctuations in the redshift might occur as a consequence (28).