A. The Milky Way System and the Magellanic Clouds
On his visit here in 1952, I discussed some of the problems related to interconnected multiple galaxies with Professor K. LUNDMARK who called my attention to various aspects of the past work in this field. Dr. LUNDMARK suggested in particular that I study the observations which Sir J. F. W. HERSCHEL had made during his stay from 1834 until 1838 in South Africa concerning a possible connection between the Milky Way and the Large Magellanic Cloud. HERSCHEL makes the following significant statement (32).
"Entirely without telescopic aid, when seated at a table in the open air, in the absence of the moon" HERSCHEL scanned the southern skies from a South African Observatory at the Cape of Good Hope and found that no branch of the Milky Way whatsoever forms "any certain and conspicuous junction with the Nubecula Major; though on very clear nights I have sometimes fancied, a feeble extension of the nearer portion of the Milky Way in Argo (where it is not above 15° or 20° distinct) in the direction of the nubecula".
The problem of the existence of a possible intergalactic bridge between our own galaxy and the Large Magellanic Cloud was apparently later on forgotten and only revitalized recently after an interval of 120 years. In view of the fact that thousands of faint luminous formations between many neighboring galaxies had been found during the past decade it seemed to me that a reexamination of the surroundings of the Milky Way would be definitely worthwhile. I consequently contacted Dr. R. v. d. R. WOOLLEY, Director of the Observatory at Mount Stromlo as well as Dr. B. Y. MILLS and suggested to them that visual, photographic and photoelectric explorations of the regions between the Milky Way and the Magellanic Clouds should be undertaken to check up on HERSHEL'S original attempts. I also felt that in some instances the methods of radio astronomy applying both to the general radiation and the 21 cm waves might give more clean cut results, especially in those regions of the sky where dense formations of foreground stars mar the appearance of faint clouds in the background. All of these suggestions were kindly acted upon and significant preliminary results have been obtained by Dr. G. DE VAUCOULEURS as well as by the Australian radio astronomers. Without going at the present time into any details a few of the aspects of the problem may here be discussed as they were kindly transmitted to the writer by Dr. DE VAUCOULEURS on his visit to Pasadena in August 1955. Some of these aspects are as follows.
Although several indications have been found, the existence of the
suspected bridge between the Milky Way and the Large Magellanic
Cloud is as yet unproven. On the other hand a long filament-like
extension was found by DE VAUCOULEURS to emerge from the Large Cloud on
the side opposite to the Milky Way. This filament which is similar to the
countertides shown in the Plates III,
IV and
V seems to be of blue
colour. It is entirely unresolved on photographs which reach stars of the
apparent photographic magnitude +17.0 or the absolute magnitude -1
to -2. There is no indication that this long spur is a particularly strong
emitter of the
H
line
or of the 21 cm wave and of the general radio
emission (at for instance 3.5 m wave length). The existence of this spur,
if it can be interpreted as some sort of a countertide, would of course
indicate that a direct tidal connection between the Milky Way and the
Large Cloud is also probable.
As far as the space between the Large and the Small Cloud is concerned there exists a spur of the latter pointing toward the former. This spur is well resolved and shows super-giant stars perhaps as bright as mp = -7. It is also relatively strong emitter of the 21 cm wave. According to the information transmitted to me by Dr. DE VAUCOULEURS the total emission in this wave-length from the spur and from a semispherical and apparently empty area surrounding it, is equal to the emission from the main body of the Small Cloud, where the Cloud and the area mentioned are of about the same size but of entirely different material content. As an interesting feature it should be added that if the spur on the Small Cloud is due to tidal action because of the proximity of the Large Cloud, no countertide on the Small Cloud is visible.
The explorations of the whole field surrounding the Magellanic Clouds in terms of the continuous radio waves is not yet completed.
As Dr. DE VAUCOULEURS has suggested, a possible extension of the Milky Way in the constellation of Herculis might be looked for since such an extension would constitute the countertide to the suspected bridge between the Milky Way and the Large Cloud. The writer attempted to approach this problem through relative counts of stars and of distant galaxies in Herculis, but the task proved too herculean indeed. I suggested therefore to the British radio astronomers to explore the regions involved in an attempt to prove or disprove the existence of the looked for extension through the analysis of radio wave intensity contours. In the meantime, during his stay in July 1955 at the Lick Observatory, Dr. DE VAUCOULEURS has photographed the regions involved with lenses of two and of five inches aperture respectively and has not found any evidence for a countertide of surface brightness greater than about +25/square second of arc.