GALACTIC AND EXTRAGALACTIC STUDIES, XXIII. OPACITY OF THE SOUTHERN MILKY WAY DUST CLOUDS BY HARLOW SHAPLEY AND JACQUELINE SWEENEY HARVARD COLLEGE OBSERVATORY Communicated June 3, 1955 The greater richness of the southern celestial hemisphere when compared with the northern is illustrated by its brightest constellations, Scorpius, Sagittarius, Centaurus, and Crux, and in such stellar giants of brightness and size as Sirius, Antares, Canopus, and Achernar. It is the hemisphere of the nearest external galaxies (the Magellanic Clouds) and of the central nucleus of our Milky Way. A consequence of the latter is that more than four-fifths of the known globular star clusters, including the two brightest, Omega Centauri and 47 Tucanae, are also southern, as is the heavily obscured Messier 4, probably the nearest of all globular clusters. But perhaps the most outstanding features of the southern sky are the brilliance of the gaseous nebulosities in Orion, Carina, and Sagittarius and the darkness of the large obscurations among the Milky Way star clouds, especially the darkness of the Coalsack and of the complex of obscurities around Rho Ophi- uchi. An examination of the opacity of these discrete dark nebulosities, and of the general cosmic dust that obscures the distant parts of the southern Milky Way, is reported in this communication. 1. On the basis of galaxy counts on photographs made with the Mount Wilson reflectors, E. P. Hubble published in 1934 his well-known picture of the distribution of faint galaxies. He was able to take his sampling survey southward only to dec- lination -30°. Hubble's work on northern galaxies is now being reinforced, or actually supplanted, by the full-coverage atlas of the northern sky by C. D. Shane and his co-workers at the Lick Observatory and by the Schmidt camera atlas pre- pared on Mount Palomar with the collaboration of the National Geographic Society. The Lick survey extends into the southern sky as far as declination -23°; the National Geographic Society-Palomar Observatory Sky Atlas, to declination -27 °. Farther south the only general coverage is that provided by the Bruce telescope at the Boyden Station, with some support provided in recent years by the Armagh- Dunsink-Harvard Baker-Schmidt reflector at that station. The principal value of Hubble's survey was its outlining of the Region or Zone of "Avoidance." The term, incidentally, is a misnomer. It was used by the senior author in pointing out the apparent absence of globular clusters in galactic latitudes lower than + 5° and was later adopted by Hubble in describing the similar but wider "avoidance" of the Milky Way by the galaxies. Properly speaking, in both cases, we are dealing with a Region of Obscuration. Behind the dust clouds of the Milky Way there is undoubtedly a more or less uniform population of external galaxies, and probably in the direction of the galactic center there are un- discovered low-latitude globular star clusters. 2. The plates available at the Harvard Observatory provide for a thorough check on the Region of Obscuration, both for the northern sky, where, however, the work can now best be furthered through the Lick and Palomar surveys, and for the southern sky, where the Bruce plates provide complete coverage.' The 837 Downloaded by guest on September 28, 2021 838 ASTRONOMY: SHAPLEY AND SWEENEY PROC. N. A. S. present report deals with the galaxy distribution, and the derivation of dust-cloud opacity, for a region that covers one-fifth of the whole sky. The survey extends in galactic longitude from 2000 to 3400, in galactic latitude from +30° to -30°. More than four hundred long-exposure photographs have been used and approxi- mately sixty thousand galaxies marked. The plates record stars fainter than the eighteenth magnitude and galaxies to magnitudes between 17.5 and 18.0, the limits depending on the speed of the photographic emulsions and the sky condi- tions. We adopt in this survey 17;7 as the average photographic magnitude limit for galaxies. In latitudes lower than 4 20' we cannot satisfactorily use the star-count method to set up magnitude sequences or to determine on each plate magnitudes for the faintest stars or galaxies. We have therefore in this survey assumed that all plates are equally penetrating when proper corrections are made for exposure time and plate quality. In this respect our method follows that used by Hubble in his sampling surveys. For 9 per cent of the plates exposure lengths deviate appreciably from the standard 3 hours, and corrections have been made to the galaxy counts, as follows: Exposure Time No. of Correction (Min.) Plates Factor 100-120- 2 1.5 120-150 10 1:.25 >210 26 0.8 The results of the survey are best shown in the -diagrams' of Figures 1 and 3, de- scribed below. They extend, in a sense, Hubble's northern survey in low lati- tudes to the remainder of the Milky Way, but with a much fuller coverage. The tabular presentation of the data used for the diagrams will soon be published in the Harvard Observatory Annals, 106, No. l2. 3. In addition to the 415 Bruce plates represented in the diagrams, a large num- ber of additional long-exposure plates, and some plates made with other telescopes, have been used'for checking. The earliest of the plates was made in 1898, but only eighteen were made prior to 1910, and therefore the work depends chiefly on modern emulsions-Cramer Hi-Speed and Eastman 103a-O. Most of the photo- graphic work was done under the supervision of Dr. John S. Paraskevopoulos, late superintendent of the Boyden Station. 'The examination of the plates has been carried through by a number of assistants all of whom have had extensive experience in the detecting of galaxy images; but the principal workers in the examination of plates and. the construction of the diagrams have-.been Miss Jacqueline Sweeney, Pedro Kokaras, Miss Ann B. Hearn, and Miss Catherine M. Hanley. The senior author has examined and qualified all'plates and examined also nearly all the galaxy images. The qualification of the plates has been made on an arbitrary scale that considers stellar image form and size and the nature of the background. Qualities have been reduced to "good" by the following scheme: No. of Correction No. of Correction Quality Plates Factor Quality Plates Factor 10 44 1.0 6 34 1.8 9 104 1.0 5 11 2.2 8 141 1.0 4 1 2.6 7 80 1.4 The correction factors are applied to the galaxy counts. Downloaded by guest on September 28, 2021 VOL. 41, 1955 ASTRONOMY: SHAPLEY AND SWEENEY 839 "0 kk i 0s cu XTTr 3 W .1. .X GO 0.. C) w! * 0 \*- bo a) le -D 0 0 0 05 A N~ 'Ua) m 0 a) ,C)~~~~~~~~~~~~~~ ,* 0 105 05 0 ce a) 0L .) as L- 0 (I)Q) W.__! a- ._ ,G *e CV ...L- ,_,.F'1_ _,_ ss S . = 4 Qf) 5 w I '*t' ,@*-qB_-o,. * w S X2 [email protected] * * . o .z o o o .s se O o rnt,.[--t.cx .m_..r1 - _ Downloaded by guest on September 28, 2021 fii....... 840 ASTRONOMY: SHAPLEY AND SWEENEY PROC. N. A. S. 4. For the construction of Figure 1 we have used the adjusted numbers of galaxies per square degree, R9, for the central nine square degrees on each plate (a square three degrees on a side). In this central area the magnitudes and nebular counts need no correction for image deformation. For eighty-seven of the plates the coverage is increased by using the adjusted numbers of galaxies per square degree for the central twenty-one square degrees (see Fig. 2). This greater coverage was derived only for plates of the higher qualities centered at declinations south of -27°. North of that declination the fuller survey is left to northern telescopes. S 13 14 15 12 I 2 3 16 I1 4 5 6 17 E I0 7 8 9 18 I -I -- I 21 20 19 FIG. 2.-Diagram of the numbering in the central twenty-one square-degree area of the Bruce plates. The counts for the outer twelve degrees in these enlarged areas were adjusted for distance from the center before they were combined with the Ng data for the construction of Figure 1. The appropriate mean correction for distance from plate center for the twelve outlying square degrees was found by simply deriving (for the thirty-four most populous plates) the ratios of the number of galaxies in each of these outlying square degrees to the number per square degree in the central nine square degrees. The ratios are so nearly alike that we have used the mean ratio and applied it inversely to the total numbers for the outer twelve square degrees of each plate. The factor does vary slightly from square to square, as follows: ~~~-SQUARE ---- 10 11 12 13 14 15 16 17 18 19 20 21 Mean ratio 0.70 0.81 0.77 0.73 0.79 0.63 0.67 0.84 0.81 0.79 0.81 0.74 Mean error 0.06 0.06 0.06 0.09 0.07 0.04 0.06 0.05 0.07 0.08 0.07 0.05 Downloaded by guest on September 28, 2021 VOL. 41, 1955 ASTRONOMY: SHAPLEY AND SWEENEY 841 The average mean ratio for all twelve squares is 0.76; it corresponds to an average magnitude loss in these outer regions of 5m = 0.20. The correction has been ap- plied only to those plates (higher latitudes) where N9 > 16.