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MAGNITUDES and COLORS of 176 EXTRAGALACTIC NEBULAE Joel Stebbins and Albert E

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MAGNITUDES and COLORS of 176 EXTRAGALACTIC NEBULAE Joel Stebbins and Albert E MAGNITUDES AND COLORS OF 176 EXTRAGALACTIC NEBULAE Joel Stebbins and Albert E. Whiteord Mount Wilson and Palomar Observatories - Carnegie Institution of Washington California Institute of Technology and Washburn Observatory Received November 6, 1951 ABSTRACT The magnitudes of 176 extragalactic nebulae from 9.2 to 18.2 mag. have been determined with a photo- multiplier; of these, 97 have also been measured for color, and both magnitudes and colors are reduced to the International System. Together with a previous list, published in 1937, the results give magnitudes for 310 and colors for 187 different nebulae. Relations for reducing all results to a common system are derived. The new data indicate a correction of —0.2 mag. to the photographic magnitudes between 10 and 13 of the Harvard Survey. As before, the nebulae of Hubble’s types E, SO, Sa, and Sb average about the same color at C = +0.85 mag., International; the Sc’s are bluer at Cp = +0.47 mag. Owing to the high latitudes of the objects in the present list, they are not suitable for detection of space reddening in the galaxy. The excess reddening of the distant nebulae has been discussed in a previous paper. The present extension of the earlier photoelectric photometry of extragalactic nebulae with the reflectors at Mount Wilson1 was undertaken at the suggestion of Edwin Hubble and Walter Baade to provide additional standards of magnitude and color for faint nebulae. The observations were obtained in the spring of 1947, Whitford going to Mount Wilson for March and April, Stebbins for May and June. The installation and the method of observation have already been described in a paper on some of the fainter nebulae of the present program.2 The principal improvements on the work of 1937 have been the substitution of a 1P21 photomultiplier for a Kunz photocell; the rebuilding of the photometer to take focal diaphragms up to 19 mm instead of 10 mm in diameter, thus increasing the field from 2Í7 to 5Í1 on the 100-inch; and the reference of the meas- ures to polar standards directly instead of by way of the Harvard visual magnitudes. At the outset we planned to get the magnitudes of nebulae correct to 0.1, but, after the work was well started, the suggestion was made that we might also measure some stars in Selected Areas to 0.01 or 0.02 mag. for additional standards. This expansion of the program, with another decimal place of accuracy thrown in, added some difficul- ties, but the enlarged program was carried through. The results for stars in Selected Areas 57, 61, and 68 have since been published.3 The present report includes the magni- tudes of 176 nebulae, with the colors of 97 of them,'all reduced to the International scale. The observations were taken with the series of focal diaphragms listed in Table 1, where the corresponding fields are given for each telescope. When a diaphragm is men- tioned in the tables which follow, these field diameters serve to identify the telescope. We prefer to use fixed known openings rather than a variable-iris diaphragm. The results of the observations are in Table 2. The first column contains the NGC number. In the next two columns the galactic co-ordinates l and b are from the Lund 1 Stebbins and Whitford, Mt. W. Contr., No. 577; Ap. /., 86, 247, 1937. 2 Stebbins and Whitford, Mt. W. Contr., No. 753; Ap. /., 108, 413, 1948. 3 Stebbins, Whitford, and Johnson, Ap. /., 112, 469, 1950. 284 © American Astronomical Society • Provided by the NASA Astrophysics Data System MAGNITUDES AND COLORS OF NEBULAE 285 tables, interpolated from the NGC positions brought up to 1900. The type is by Hubble or Baade. In the next two columns the Harvard photographic magnitude BP g and the diameters are from the Shapley and Ames catalogue.4 The first diameter in the dia- phragm column applies to the magnitude measurement; the second applies to the color. In the few cases where a rectangular diaphragm was used, the two dimensions are given, e.g., 6'8 X 2.'3 for NGC 3556; here there was no change for the color. When a dia- phragm includes the whole nebula, the sky often contributes 80 per cent of the total light. Since the color is in most cases rather insensitive to the size of the diaphragm, a smaller hole for this part of the measurement gives a better nebula-to-sky ratio and higher accuracy. In spirals, however, the outer regions are always bluer than the inner TABLE 1 Focal Diaphragms Diameter Diameter (Mm) 60-Inch 100-Inch (Mm) 60-Inch 100-Inch 0.7. 0Í3 O'. 2 9.1. 4'. 1 2'.4 1.5. 0.7 0.4 12.6. 5.7 3.4 2.7. 1.2 0.7 19.0. 8.6 5.1 5.0 2.3 1.3 ones;5 but we could not stop to make a detailed study of every nebula, even if that were possible. The magnitudes Pgv and colors Cp on the International scale were derived from the photoelectric measures by the relations m Cp= +0 96+ 1.00C47, (i) Pgv=Pe +0.29Q, where C47 is the photoelectric color index with blue and yellow filters and Pe is the photo- electric magnitude measured in the clear, both corrected for atmospheric extinction and referred to standards of the North Polar Sequence. Relations (1) differ little from the revised ones later derived for the stars observed in 1947.6 When the color was not meas- ured, Cp was assumed to be +0.86 mag. for types E, SO, Sa, and Sb, and equal to +0.47 mag. for Sc. The adoption of mean colors seldom leads to an error of 0.1 mag. in Pgp. When the field of the diaphragm is smaller than the major diameter of the nebula, Pgp is in parentheses; the difference Pgp—HPg is given only when Pgp takes in the whole nebula. Additional data are in the “Remarks” column. For instance, the first object, NGC 147, was observed on two nights in 1944 with average deviations ± 0.15 mag. and ± 0.00 mag. for magnitude and color, respectively. When an additional magnitude with a smaller diaphragm was obtained, the difference is, of course, always in the direction of numerical- ly greater magnitudes; e.g., for NGC 2685 the magnitude for 2Í3 was 0.24 fainter than for 4Í1. An interfering near-by star was either excluded (“Star out”), or if it was in- cluded with the nebula, its light was determined separately and subtracted from the total (“Star elim.”). Other remarks are self-explanatory. 4 Harvard Ann., 88, 43, 1932. 5 F. H. Scares, Proc. Nat. Acad. Sei., 2, 553, 1916; E. F. Carpenter, Pub. A.S.P., 43, 294, 1931; C. K. Seyfert, Ap. 91, 529, 1940; also unpublished photoelectric measures by the authors. 6 Stebbins, Whitford, and Johnson, op. cit., p. 471. © American Astronomical Society • Provided by the NASA Astrophysics Data System .284S TABLE 2 .115. Magnitudes and Colors of Nebulae NGC Type EPg Diam. Diaph. Pgp 'P Pgp- Remarks 1952ApJ. EPg 147*. 87 ?9 —14?0 12“1 6Í5 3 !8 8:6 8:6 11“2 0m90 .Qm9 ±0.15, ±0.00 (2), 1944 147*. 87.9 -14.0 |gPp 12.1 6.5 3.8 2.4 2.4 (12.7) 0.87 1947 185*. 88.9 -14.2 gp 11.8 3.5 2.8 4.1 4.1 11.4 0.96 -0 + ±0.02, ±0.04 (2), 1944 185*. 88.9 -14.2 11.8 3.5 2.8 2.4 2.4 (11.6) O'. Ç6 1947 205. 89.0 -20.9 EP 10.8 8.0 3.0 4.1 4.1 (10.1) 0.77 ±0.06, ±0.02 (2), 1944 205. 89.0 -20.9 Ep 10.8 8.0 3.0 2.4 2.4 (10.6) 0.67 1947 206. 89.0 -21.8 4.1 4.1 11.0 0.66 1944 221. 89.4 -21.7 E2 9.5 2.6 2.1 4.1 4.1 9.4 0.90 -0.1 ±0.06, ±0.00 (2), 1944 221. 89.4 -21.7 E2 9.5 2.6 2.1 0.7 0.7 (10.2) 0.88 1947 224. 89.5 -21.3 Sb (4.5) 160 40 4.1 4.1 (7.2) 1.00 ±0.02, ±0.01 (2), 1944 224. 89.5 -21.3 Sb (4.5) 160 40 2.0 2.0 (8.0) 1.01 ±0.08, ±0.01 (3), 1944 224. 89.5 -21.3 Sb (4.5) 160 40 0\7 0.7 (9.5) 0.98 1947 598. 102.1 -30.7 Sc 7.8 60 40 4.1 0.63 C only, 1944 2146, 102.7 + 25.5 Sap 11.6 5.0 2.5 5.7 11.6 0.0 p 2300. 94.6 + 28.2 El 12.2 1.0 0.7 2.3 2.3 12.5 0.97 + 0.3 2314. 106.6 +28.1 E3 12.9 1.2 1.2 13.3 0.93 + 0.4 2379. 152.5 + 23.2 El 1.2 14.7 2403. 117.6 +30.2 Sc 10.2 16 10 2.3 0.50 C only 2460. 123.7 + 32.4 Sb 12.7 2.3 12.7 0.0 p 2532. 155.1 + 31.7 Sc 2.3 13.0 2535. 164.9 + 29.4 Sc 2.3 13.4 2536.
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