A STUDY OF CENTAURUS A AT 31 CENTIMETERS J. G. Bolton and B. G. Clark California Institute of Technology Radio Observatory Owens Valley, California The radio source Centaurus A was one of the first to be discovered, and shortly after its discovery it was identified with the peculiar galaxy NGC 5128. Interferometry by Mills 1 showed that there was a central source of dimensions 6' X 3', and Bolton, Westfold, Stanley, and Slee 2 found that there was an associated extended source of dimensions of the order of 2. The extended object was also observed at 400 Mc/s by McGee, Slee, and Stanley, with a pencil-beam antenna, 3 and by Piddington and Trent at 600 Mc/s. 4 The latter investigators suggested that the extension might form a bridge between NGC 5128 and our own galaxy. The true extent of this object has only really become apparent since the studies by Sheridan 5 with the 85 Mc/s Mills cross (beam width SO 7 ) and by Shain 6 with the 19.7 Mc/s Mills cross (beam width 1?4). These observations show an elongated object nearly 10 in length lying almost along a line of constant right ascension. The present study was carried out with a 90-foot equatorially mounted radio telescope at a frequency of 960 Mc/s. At this wavelength the beam width is about 50 minutes of arc. The receiver used had a conventional crystal mixer with an excess noise temperature of about 300 K. No image rejection was employed ; thus the signal was received in two bands at 930 and 990 Mc/s, each 10 Mc/s wide. The receiver input was switched at 400 cycles per second with a diode switch between the main horn feed pointed at the reflector and a reference horn directed away from the reflector. The 400 cycles per second difference signal was recorded. The observations consisted of a series of drift curves taken with the telescope fixed on a particular declination. Due to the low altitude of this object, only three observations (at different hour angles), each lasting about 40 minutes, were made on each night. One of these was for calibration purposes at declination 42 40'. The maximum signal received from the direction of the central source corresponded to an antenna temperature of 50 K, but the 29
30 J. G. BOLTON AND B. G. CLARK receiver stability was such that complete reliance could be placed on signals of only 0?5 K from the outer regions of the source. The results are shown on the equal-area chart of Figure 1, where the contours are in units of 0?5 K. The contours are dashed in the extended feature around 44 and toward larger right ascensions, as these depend on a rather long extrapolation of the zero baseline. It is believed that this feature is indeed part of the source complex, but the possibility that it may belong to our galaxy cannot be entirely excluded. This feature is one of the new Fig. 1. Observed brightness distribution of 960 Mc/s radiation from NGC 5128/Centaurus A on an equal-area chart. Contour lines are steered through observed points shown as dots. Contours are in units of approximately 0 5 K; antenna temperature intervals were chosen to avoid undue crowding. Dashed lines in the left hand part of the diagram are in regions where the observations are somewhat uncertain. Dashed lines in the central region are contours that result from the subtraction of a point source of 92 units. Crosses mark the highest points in the observed drift curves.
CENTAURUS A AT 31 CENTIMETERS 31 results of the present work. Another is that a distinct trough exists between the central concentration and the southern ex- tended region. This does not show as distinctly in the 85 Mc/s contour map, possibly because the antenna pattern of the Mills cross has somewhat broader skirts than that of the 90-foot re- flector. It has already been established by Mills at lower frequencies that there is a source of relatively small dimensions coincident with the optical center of NGC 5128. 1 An inspection of the 960 Mc/s contours suggests that the major contribution to them in the vicinity of a = 13 h 23 m, ô = 42 40' is due to the same small source. As a first step in the analysis, an attempt was made to determine the contribution of the point source. This was done by trial subtractions of contours representing the known antenna pattern of sources of various flux densities. The upper limit to the correct value was obviously set by the fact that negative residuals could not be permitted. The most plausible residual contours con- sistent with no negative values were obtained by subtracting a source of flux density corresponding to 92 units, or 460 X 10-26 watts m -2 (c/s) -1. These residual contours are shown by the dashed lines in Figure 1. Separate integrations of the residual contours were then car- ried out for the three regions: (a) north of ô = 42 43'; (b) south of ô = 42 43 /, but west of a = 13 h 38 5 ; and (c) the re- mainder, being the long southeast extension. The values of flux density are as follows : Flux Density Percentage Source [lo -26 watts m~ 2 (c/s)- 1 ] of Total Center 460 23 North 535 27 South 770 39 East 220 11 If the east source is omitted, the central source contributes 26% of the total. This value is to be compared with 25% at 85 Mc/s, estimated by Sheridan, and 11% at 19.7 Mc/s, estimated by Shain.
32 J. G. BOLTON AND B. G. CLARK Again neglecting the east source for comparison with other observers, the total flux densities are 19.7 Mc/s 2.8x10-22 85 Mc/s 8.7 X10-23 960 Mc/s 1.8 X10-23 These values fit closely a simple power spectrum of flux density oc À 0-7, and thus it seems unlikely that Shain s relatively lower percentage for the central source is due to an intrinsically higher value for the extended region. Shain finds on comparison of the profiles along lines of constant declination that the extended source is somewhat wider in declination at 19.7 Mc/s than at 85 Mc/s. A similar comparison of the profiles at 85 Mc/s and 960 Mc/s shows practically no difference. As there is probably some uncertainty in the form of the antenna beam of the 19.7 Mc/s cross and as ionospheric effects would produce additional smearing, it is suggested that the differences in the profiles and in the percentage contribution of the central source could both be due to observational uncertainties. In any event, there appears to be no variation of the spectrum with position over an elevento-one frequency range between 960 and 85 Mc/s and possibly very little over a forty-to-one range.* In general, contours in the neighborhood of extended sources indicate a width of the source greater than that of the antenna pattern. Consequently the actual brightness distribution differs only slightly from the observed distribution. Figure 2 represents the result of an attempt to partially remove the effect of the antenna pattern by the method described by Bolton and Westfold. 7 For this purpose a rather simple overlying grid was used to represent the antenna pattern ; the grid consisted of an inner circle, two intermediate annuli, each divided into two parts, all of equal weight, and an outer annulus of weight one-half. While such a process * Note added in proof : Mr. C. A. Shain has kindly informed the writers that further observations have substantiated his previous results, and that he believes the ionospheric influence on them to be relatively small. He suggests that the lower ratio of the intensity of the central source to the integrated intensity at 19.7 Mc/s may be due to absorption in H n regions.
CENTAURUS A AT 31 CENTIMETERS 33 Fig. 2. The basic data of Figure 1 with the effects of the central source removed and with partial corrections made for the effects of the antenna pattern. cannot recover certain detail that is irretrievably lost in the original observations, a comparison of the observed and deduced contours does at least reveal the areas in which the observational smoothing is serious. The most appreciable changes between the observed and the recovered contours appear in the intensification of the southern source and in the sharpening of low-level contours along the western edge of the southern source, suggesting that this edge is sharply bounded. Edges that are just as sharply defined occur at the north edge of the south source and south edge of the north source, but they could be produced as a consequence of the removal of the central source. The contour pattern shown in Figure 2 suggests that both of the extended sources may be double. The north source could consist of a sharply defined but still somewhat extended object at a = 13 h 25 m, b = 41 45' and a more diffuse object farther
34 J. G. BOLTON AND B. G. CLARK north. The southern source may split into a sharply defined object at a = 13 h 20 m, ô = 44 30' and a diffuse object to the southeast of this. The sharp and diffuse objects may possibly be pairs centered on NGC 5128. No observations were made at lesser right ascensions than 13 h 10 m, in the region where an object symmetrically opposite to the east source could exist. Unfortunately, the possibility of this was not realized until after the region had passed into the daytime sky. Neglecting the east source, the brightness distribution in Figure 2 bears a remarkable resemblance to that of the Cygnus A source deduced from interferometry by Jennison 8 and his coworkers. Jennison finds that the Cygnus source consists of two objects of relative flux density 1 and 1.2, each about 50" by less than 30" in extent; the centers of the two objects are separated by 82". The observations would admit complications within the individual objects and possibly up to 10% of the total radiation in a point source at the center. If NGC 5128 were studied in the same manner, the results would probably indicate, in addition to a central point source, two extended objects of relative flux density 1 and 1.4, each about 3 by less than 2 in extent, whose centers are separated by 4. This remarkable similarity between the two radio distributions is very surprising in view of the disparity between the optical counterparts. The radio dimensions of NGC 5128 are about 200 times those of Cygnus A. The sizes of the optical counterparts out to the faintest outer regions are 25' X 25' for NGC 5128 and 18" X 30" for Cygnus A. Thus the radio size/optical size ratio is about three times as large in the case of NGC 5128 as in the case of the Cygnus A nebulosity. A similar result is obtained from consideration of the apparent magnitudes of the two systems. The true extent of the radio source is difficult to determine, as the distance of NGC 5128 is not known. Probably the most reliable estimate has been made recently by Sersic (private communication via Minkowski) from a study of the H n regions. Sersic s value is 4 X 10 6 parsecs. Thus the over-all dimensions of the north-south source complex cannot be less than 7 X 10 5 parsecs, as corrections for projection effects could only increase the size. The distance from the center to the extreme edge of the
CENTAURUS A AT 31 CENTIMETERS 35 southeast source is at least 5 X 10 5 parsecs. These dimensions are much greater than the separation of individual galaxies in some clusters. Similar dimensions for a radio galaxy are also demanded for the source Hercules A, if its identification with a nineteenth-magnitude elliptical suggested by Roberts, Bolton, and Harris 9 is correct. There is evidence that other galaxies that are abnormal radio emitters, such as M 87, NGC 1316, NGC 1275, and Hydra A, have extensions much larger than the visible galaxies ; however, the ratios are comparable to that of Cygnus A. It may be significant that these last five are members of clusters, whereas NGC 5128 and the proposed optical counterpart for Hercules A are field galaxies. 1 B. Y. Mills, Aust. J. Phys., 6, 452, 1953. 2 J. G. Bolton, K. C. Westfold, G. J. Stanley, and O. B. Slee, Aust. J. Phys., 7, 96,1954. 3 R. X. McGee, O. B. Slee, and G. J. Stanley, Aust. J. Phys., 8, 347, 1955. 4 J. H. Piddington and G. H. Trent, Aust. J. Phys., 9, 74, 1956. 5 K. V. Sheridan, Aust. J. Phys., 11, 400, 1958. 6 C. A. Shain, Aust. J. Phys., 11, 517, 1958. 7 J. G. Bolton and K. C. Westfold, Aust. J. Sei. Res., Ser. A, 3, 19, 1950. 8 R. C. Jennison, in Paris Symposium on Radio Astronomy, R. N. Bracewell, ed. (Stanford, Calif. : Stanford University Press, 1959), p. 309. 9 J. A. Roberts, J. G. Bolton, and D. E. Harris, Pub. A.S.P., 72, 5, 1960.