Southern Clusters and Galaxies

By Harlow Shapley and John S. Paraskevopoulos

The present note on interesting southern objects reports studies, frequently preliminary, that are based on photographs made with various instruments at the Boyden Station, but depend chiefly on the plates (SB series) made with the 60-inch reflector. We are indebted to several members of the observing and computing staffs for assistance in making and measuring the plates. The drawing is by Miss Virginia McKibben. The magnitudes are photographic, unless otherwise specified; the galactic coordinates are on the basis of the conventional pole (Trans. I.A.U., 5, 375, 1935).

1. New Globular Cluster, Possibly Intergalactic. — In the course of the systematic examination of long-exposure Bruce plates on regions south of declination −60°, the object N.G.C. 1841, originally observed by John Herschel and described by him as pF, L, iR, vsbM, r, was noted by Mrs Seyfert as a globular star cluster. The position for 1940 is 4^h 54^m, −84° 7′, with galactic coordinates 264°, −31°. The diameter is 2′.4 on the long-exposure A plates.

The new globular cluster is essentially identical in appearance, magnitude, and diameter with the remote northern cluster, N.G.C. 2419, which has been thoroughly studied by Dr. Baade (Mt. W. Contr. 529, 1935). Its latitude is three degrees higher, and, like N.G.C. 2419, it is in a region showing a distribution of external galaxies normal for the galactic latitude.

Baade gives the distance of N.G.C. 2419 as 69.8 kiloparsecs, or 55.8 kiloparsecs if a general space absorption of half a magnitude were adopted. This northern cluster lies far outside the galactic system, and should probably be considered an intergalactic object; but it may well be, as Dr. Baade has pointed out, a member of the local supergalaxy and not an independent metagalactic member. If N.G.C. 1841 is comparably distant, it too is well beyond the accepted confines of the galactic system – a conclusion that must, however, be tentative until the inferred distance can be checked by means of Cepheid variable stars, or, with less certainty, through an accurately standardized photometry of the brightest stars of the cluster.

On small-scale plates Miss Mohr finds that the integrated magnitudes of N.G.C. 1841 and 2419 are 12.2 and 12.1, respectively; the Mount Wilson value of the integrated brightness of N.G.C. 2419 is 11.5 (Christie, Mt. W. Contr. 620, 1940). A number of globular clusters in low galactic latitude, such as N.G.C. 6342 and 6426, are of comparable integrated photographic magnitude; but they lie in regions that are probably much affected by general and local absorption, and therefore probably lie not much, if at all, beyond the bounds of the galactic system.

2. Large New Spiral Without Nucleus. — An elongated nebulous object of faint surface brightness (R.A. = 2^h 55^m.1, Dec. = −54° 51′ (1935); β = −54°) appears on three long-exposure Bruce plates, and is found upon study with the 60-inch reflector to be a peculiar spiral nebula, with no discernible nucleus and with unusual distribution of light. The dimensions are 6′.0 x 0′.8, position angle of major axis 35°. The southern third of this elongated nebulosity is exceedingly faint. At the northern extremity is a group of condensations.

From small scale plates we estimate the total magnitude to be 12.0. The object is perhaps the largest and brightest external galaxy as yet uncatalogued. The peculiarities of its structure and surface brightness have helped to prevent its detection in earlier surveys. The accompanying sketch is from a reflector plate.

3. Compact Group of Bright Galaxies in Corvus. — A two-hour photograph (SB 652), centered on the double galaxy N.G.C. 4724, 7, shows more than one hundred and sixty nebulae in an area of half a square degree. Several are relatively bright and undoubtedly comprise a physical system. The region is not far from the main axis of the metagalactic cloud which we have called the Virgo Extension. The position for 1900 is: R.A. 12^h 45^m.7, Dec. −13° 48′.

The magnitudes and dimensions of the individual objects will be determined, in the course of the fifteenth and eighteenth magnitude surveys, from plates made with the Bruce telescope and with other instruments. The fifteen brightest objects are described in the accompanying tabulations. Most of them have long been known; the double object was recorded by John Herschel, and the surrounding objects bearing Index Catalogue numbers were discovered by Howe in 1898 and 1899, working with the refractor of the Denver Observatory.

COMPACT CLUSTER OF BRIGHT GALAXIES

The following five uncatalogued objects, with differential coordinates measured from the nucleus of N.G.C. 4727, are conspicuous galaxies which probably also belong to this physical group.

4. Nine Bright Open Spirals. — The classifications below are from reflector plates and for some of the objects only revise or extend the classifications from Bruce plates given in Harvard Annals, 88, No. 2, 1932.

A forty-minute exposure shows that N.G.C. 1483 is an oval nebulosity with indistinct spiral arms in which are four hazy condensations of the seventeenth magnitude, nearly as bright as the elongated diffuse central nucleus.

Faint surface brightness, non-symmetrical arms, and a very elongated bright nucleus characterize the large object I 5201.

Several of these open spirals are richly nucleated and all of them might be profitably watched for the appearance of supernovae.

5. Seven Bright Barred Spirals. — In the following tabulation the classifications are based on reflector plates; and the magnitudes, when given, are from Harvard Annals, 88, No. 2, 1932. The angular dimensions, which have been estimated directly from the photographs, would probably be increased appreciably if taken from densitometer tracings.

N.G.C. 6699 might be classed as a "plate spiral," as defined in Harvard Reprint 184, note 13, pp. 34-35, 1940; and N.G.C. 7418 of the preceding list may also be of this special class. N.G.C. 6744 is found to be, in angular measure, one of the largest spirals known.

The object N.G.C. 6300 is a conspicuous barred spiral with rather closely wound arms, inclined forty degrees to the line of sight. The magnitude is estimated with difficulty because of the superposition of several stars with magnitudes 12 and 14.5 (H.A., 88, 60, 62, 1932). The magnitude of the nucleus is approximately 15.5. The cross-bar is diffuse, and the lunettes (areas between bar and surrounding nebulous ring) are irregularly cloudy – that is, occupied by nebulosity.

6. Bright String Nebula in Vela. — An exceptional streak or string of emission nebulosity in Vela has been photographed on long-exposure plates of various series made at the Boyden Station. The chief peculiarity of the nebula is its length and narrowness. On the Bruce plates (1′ = 1 mm) it can be traced for about four degrees, and throughout most of its winding course it is between 1′ and 2′ in width. On the plates (2′.8 = 1 mm) made with the Metcalf triplet, the nebula can be followed continuously through an irregular wavy arc for 4°.5, from a point 15′ north of e Velorum (8^h 34^m.2, −42° 38′, sp. A5) southwestward to a point half a degree south of H.D. 72127 (8^h 26^m.0, −44° 23′, sp. B5).

There are a few naked-eye stars in the vicinity of this emission nebula with spectral class B3 and later, but no conspicuous blue star has been found near the center of the arc, which has perhaps a radius of two or three degrees. Halfway along the nebula there are some faint branches on the concave side. The whole nebulosity is faint, and it would be reasonable to expect that further extensions of the faint emission might be detected on a photograph taken with a suitable Schmidt-type instrument. The near-by Milky Way star clouds are strongly marked with absorption nebulosity.

The coordinates (1900) for the middle of the string-nebula arc may be taken as: