Pub. Astron. Soc. Pacific, Volume 86, February 1974 SPIRAL and IRREGULAR GALAXIES ASSOCIATED with RICH REGULAR CLUSTERS HERBERT

Pub. Astron. Soc. Pacific, Volume 86, February 1974

SPIRAL AND IRREGULAR GALAXIES ASSOCIATED WITH RICH REGULAR CLUSTERS

HERBERT J. ROOD Institute for Advanced Study, Princeton, New Jersey Astromony Department, Michigan State University Received 6 August 1973

Thirty galaxies brighter than nip = IS"1? in a 23 square-degree region centered on the Coma cluster are classified as spirals and irregulars on 48-inch Schmidt plates. The radial surface density distribution of these galaxies, corrected for contamination by field galaxies, indicates that about 15% of the galaxies associated with the cluster in the brightest 2.7-magnitude interval are spirals and irregulars. The homogeneous radial distribution of spirals and irregulars in a 2-square-degree region centered on Abell 2199 suggests that all or nearly all of these galaxies are members of the field. The above conclusions are provisional because (1) they are derived from relatively small samples of galaxies and (2) galaxy classifications from 48-inch Schmidt plates could contain significant uncertainties. Key words: galaxies — clusters of galaxies

I. Introduction The names of these galaxies with their magni- tudes mp (Zwicky and Herzog 1963) and radial It is estimated that more than half of the velocities V0 corrected for solar motion relative brightest galaxies in the irregular Virgo and to the local group (Rood, Page, Kintner, King Hercules clusters are spirals and irregulars (e.g., (RPKK) 1972; Tifft 1972; Tifft and Gregory 1973) see Burbidge and Burbidge 1959; Abell 1968). are listed in Table I. The mean velocity and The situation for rich regular clusters is less velocity dispersion of the Coma cluster are 6888 clear. Abell writes that they are "nearly or com- km sec-1 and 900 km sec-1, respectively (RPKK). pletely devoid of spiral and irregular galaxies." If we use the Omer, Page, and Wilson (1965) The purpose of the present paper is to make estimate of 48.5 field galaxies per square degree quantitative estimates of the fraction of spiral to mv = 17.8, and assume mp — mv = 1.0, then and irregular galaxies associated with rich the expected number of uniformly distributed regular clusters on the basis of the radial surface- field galaxies with mp á 15.7 in the 0.30-square- density distribution of galaxies classified with degree survey region is only one galaxy. There- 48-inch Schmidt plates. The Coma and Abell fore, most of the five spiral and irregular galaxies 2199 clusters are studied. It is found that about could be associated with the cluster, although a 15% of the brightest galaxies associated with the conclusion based on only five galaxies is, of Coma cluster could be spirals and irregulars course, very uncertain. (section II), while no spirals and irregulars are Mayall (1960) presented a homogeneous detected in Abell 2199 (section III). A discussion sample of the 82 galaxies most suited for spec- of uncertainties in the analyses is given in troscopy (high surface brightness and central section IV. concentration) within a 6-degree by 6-degree area centered on the Coma cluster. Seventy-one II. Coma Cluster of these galaxies are associated with the cluster The center of the Coma cluster is between two on the basis of their radial velocities. RPKK supergiant galaxies (Heidmann 1965; Bahcall classified these galaxies with 48-inch Schmidt 1973a). By classifying galaxies with a set of plates. They found 20 spirals and irregulars and 200-inch plates in a 0.30 square-degree central concluded from radial velocity data that 13 are region of the Coma cluster. Rood and Baum associated with the cluster. These data are re- (1967) found five spiral and irregular galaxies. produced in Table II. The radial distributions 99

TABLE I TABLE II Spiral and Irregular Galaxies in the Sample Spiral and Irregular Galaxies of Rood and Baum (1967) for the Core in the Mayall Sample of the Coma Cluster of the Coma Cluster Mayall R Type Type Type Galaxy (') (RPKK) (Present Study) RB Type (Present Vo Cluster Members: Galaxy (RB 200-inch) Study) (km sec-1) 1821 195 SB N4858 SBb I 15.5 9398 N4735 130 SB SB RB82 + 81 SBa + DW -+- 15.6 5136 N4848 28 I S 14040 I/SO 15.1 7759 A12 51 I I RB219 SBa so 15.1 5366 A13 21 SB SO N4921 SBb SB 13.7 5472 N4907 17 S SB0/SB N4911 17 S S/SB N4922 83 Ip E/S0 + I of the Mayall sample of spirals and irregulars N4921 22 SB SB are given in Table III. The observed radial dis- A17 141 I S/I tributions are compared with a homogeneous A18 173 I surface density distribution and the overall dis- N4966 107 S SO + Ε tribution of cluster members (King 1972). It is N5000 143 SB SB seen that the uncorrected sample of 20 galaxies Nonmembers: has a radial distribution which lies between the N4670 200 Haro 9 homogeneous and King distributions. The cor- N4712 203 S rected sample of 13 galaxies, however, has a ra- N4725 195 S/SBp dial distribution which agrees with the King N4747 170 SB S distribution at the 70% significance level; that is, N4793 90 S/SB S/SB 70% of the samples of galaxies selected at random N4961 80 SB S from a King distribution have a X2 larger than A20 182 S that observed. Assuming that the 13 spirals and irregulars are associated with the cluster, we estimate that 13/71 = 18% of the Mayall galaxies TABLE Illa associated with the cluster are spirals and ir- Radial Distribution of Mayall Spiral regulars. and Irregular Galaxies Associated with In order to increase the sample of spirals and the Coma Cluster irregulars, I have classified all galaxies listed in fí(') N{hom) N(obs) N(King) the catalog of Zwicky and Herzog (1963) (i.e., 0-40 0.4 5 4.9 all galaxies with mp ^ 15.7) on 10-inch by 10- 40-130 4.0 4 5.2 inch (4? 75 X 4? 75) 48-inch Schmidt plates, 130-223 8.6 4 2.9 2 2 kindly loaned to me by C. T. Kowal and G. A. Pr(X > X ,obs ) <0.5% - Welch. Five well-exposed plates taken with good seeing were used. They include two 103a-O TABLE IXIb plates (one with a GG-13 filter), two 103a-D plates with Wrl2 filter, and one IIIa-J plate with Radial Distribution of all Wr2C filter. No obvious dependence of the Mayall Spirals and Irregulars classification type on emulsion was noticed. (Members Plus Nonmembers) in the Tables I and II compare types from the present Field Centered on the Coma Cluster study with types given in the literature. Of the HO Mhom) Mobs) MKing) 215 galaxies classified in the present study, 30, 0-40 0.6 5 7.5 that is 14%, are spirals and irregulars. 40-130 6.2 6 8.0 Properties of the 30 identified spiral and ir- 130-223 13.2 9 4.5 regular galaxies are given in Table IV. Their Pr(X2 > X2obs) < 0.5% 5%

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SPIRAL AND IRREGULAR GALAXIES 101 radial distributions, uncorrected and corrected velocities; that is, if (say) 2/5 of the galaxies in a for superimposed field galaxies, are given in ring are without known radial velocities, then Table V. The correction for field galaxies was 2/5 of the geometrical area of the ring is used to estimated as follows; Five out of the 18 galaxies estimate the number of field galaxies in the ring with known radial velocities are considered to with unknown radial velocities. The total num- be field galaxies because their radial velocities ber of field galaxies in a ring is the sum of the are more than 3000 km sec-1 (3.3σ) from the number with, plus the number without, known cluster mean. It is then assumed that 5/18 of the radial velocities. 12 galaxies without known radial velocities are Table Vb shows that the radial distribution of field galaxies. The 5/18 X 12 = 10/3 field the spiral and irregular galaxies, corrected for galaxies are then assumed to be distributed in field galaxies, is more centrally concentrated the annular rings in proportion to the area than a homogeneous distribution. The observed "occupied by" galaxies with unknown radial distribution agrees with the King distribution in

TABLE IV Spiral and Irregular Galaxies of the Coma Cluster with Types Derived from 48-inch Schmidt Plates in the Present Study NGC Vo R(') Type ΙΟ (km/secr1) (Present Study) 12h48m5 27° 39' 15.5 122 SB 4735 12 48 .6 29 12 15.1 6687 130 SB 4747 12 49 .3 26 02 13.2 1197 170 S 12 51 .4 29 51 15.3 126 SB 12 51.6 27 25 14.9 91 I 4793 12 52 .2 29 12 12.3 2544 90 SB 12 55 .4 29 55 15.2 105 S 4858 12 56.6 28 23.2 15.5 9398 14 I 4848 12 55 .7 28 31 14.2 7221 28 s A12 12 56.2 27 26 15.4 7666 51 I 12 57 .2 28 54 15.6 5342 40 I 842* 12 58 .2 29 17 14.6 66 s 4911 12 58 .5 28 03.5 13.7 8018 17 s 4921 12 59 .0 28 08 13.7 5472 22 SB 4922 12 59 .0 29 35 14.2 7376 83 Ε: + I 12 59 .1 28 57 15.3 48 SB 12 59 .7 27 55 15.1 7188 36 SB 12 59 .8 28 29 15.5 8323 36 SB 13 01.3 26 21 15.7 125 I 13 02 .0 27 34 15.5 74 I 13 02 .0 29 05 15.3 8047 79 I 4961 13 03 .3 28 00 13.5 2574 80 S 13 04 .2 29 06 15.0 105 SBp 13 05 .0 26 59 15.5 11517 128 Ε + I 13 05 .6 27 01 15.6 10352 131 SB 13 06.5 28 27 14.3 120 SB 13 06.6 29 18 15.7 135 I A19 13 06.9 29 38 15.1 6450 150 SB 5000 13 07 .4 29 10 14.0 5673 143 SB 13h07ni6 30°18' 15.4 182 I

TABLE Va 843 km sec-1 is the velocity dispersion of the Radial Distribution of Coma Spiral and cluster), but 67% of all field galaxies occupy this Irregular Galaxies Typed in the range. We conclude that all or nearly all of the Present Study, spirals and irregulars in the field of the Abell Uncorrected for Field Galaxies 2199 cluster are field galaxies. R(') N(hom) Mobs) N(King) IV. Uncertainties 0-40 1.9 7 13.0 The conclusions derived in sections II and III 40-75 4.7 4 6.5 must be considered provisional for several 75-110 7.6 7 5.1 reasons. 110-130 5.6 6 2.3 1. The apparent association of spiral and 130-160 10.3 6 3.1 2 2 irregular galaxies with the Coma cluster un- Pr(X > X ) < 0.5¾ — 1¾ obs covered in section II conflicts with a result by Abell (1968). From a study of the areal distribu- TABLE Vb tion of 47 galaxies that could be definitely Radial Distribution of Goma Spiral and classified as spirals on 48-inch Schmidt photo- Irregular Galaxies Typed in the graphs covering a 70-square-degree region in- Present Study, Corrected for Field Galaxies cluding the center of the Coma cluster, Abell ñ(') Mhom) Mobs) MKing) found that these galaxies are not significantly 0-40 1.3 7.0 9.2 concentrated to the cluster center. 40-75 3.3 3.0 4.6 2. The analyses in the present paper often 75-110 5.3 4.1 3.6 only marginally satisfy the rule-of-thumb that 110-130 3.9 4.0 1.6 the chi-squared test is valid "if at least some of 130-160 7.2 3.0 2.2 the classes have expected frequency 5 or more" 2 2 Pr(X > X obs) <0.5% (Brunk 1960). 3. The assignment of morphological types to galaxies from 48-inch Schmidt plates are subject the sense that 30% of the random samples from to large uncertainties. This is demonstrated by the King distribution have values of X2 greater the comparison of 48-inch Schmidt classifications than that observed. with the much more certain 200-inch classifications given in Table I and the comparison be- III. Abell 2199 tween independent 48-inch Schmidt classifica- Abell 2199 is a rich regular cluster centered on tions given in Table II. Moreover, of the 30 the supergiant galaxy NGC 6166 (Bahcall 1973b). classifications given in Table IV, there are a Classifications for 170 galaxies with Β < 17^ in surprisingly large number of irregular galaxies a two-square-degree field centered on the cluster (11). It is possible that elliptical and SO galaxies are given by Rood and Sastry (1972). The types with structural peculiarities are sometimes mis- were derived from a 48-inch Schmidt Illa-J taken for spirals or irregulars. This is especially plate. Fifty-two of the galaxies are spirals and the case for the Coma cluster, where several irregulars. In Table VI, the observed radial dis- peculiar galaxies in the cluster core have been tribution of the spirals and irregulars are com- identified on 200-inch plates (Rood 1968). pared with a homogeneous distribution, the Classifications based on plates with the resolu- distribution for the Ε -l· SO + SB0 galaxies in tion of well-exposed 200-inch plates are probably the Rood-Sastry survey, and the distribution for necessary for a definitive study of the distribution all galaxies with Β = 17^5 given by Bahcall of spiral and irregular galaxies in the region of (1973b). A X2-test shows that the observed spiral the Coma cluster. and irregular distribution agrees with the homo- It is a pleasure to thank Drs. John and Neta geneous distribution on the 30% significance Bahcall for encouragement and helpful dis- level. Only four of the 52 spiral and irregular cussions, Dr. George Abell for reading the manu- galaxies have known radial velocities. These lie script and offering useful suggestions, and Drs. within ±2σ of the cluster mean, (where σ = G. T. Kowal and G. A. Welch for the loan of

TABLE VI Radial Distribution of Spiral and Irregular Galaxies in the Field of Cluster Abell 2199 /'EΕ + SO \ RC) ]V(hom) iV(obs) Ν \ + SB0/ N(Bahcall) 0-10 2.3 4 10.6 10.7 10-20 6.9 5 11.6 8.4 20-30 11.5 14 11.6 10.0 30-40 16.1 15 12.6 15.2 40-47.5 15.1 14 5.6 7.7 Pr(x2 > x2obs) 30% < 0.5% 2%

48-inch Schmidt plates. I wish to thank Dr. Carl Burbidge, G. F., and Burbidge, Ε. M. 1959, 130, Kaysen for his hospitality at the Institute for 629. Advanced Study. This research was sponsored Heidmann, J. 1965, Ann. d'Ap.28, 380. King, I. R. 1972, Ap. J. {Letters) 174, L123. by National Science Foundation grant No. Mayall, N. U. 1960, Ann. d'Ap23, 344. GP-16147A#1. Omer, G. C., Page, T. L., and Wilson, A. G. 1965, A./. 70, 440. Rood, H. J. 1968, Pub. A.S.P. 80, 424. references Rood, H. J., and Baum, W. A. 1967, A./. 72, 398. Rood, H. J., and Sastry, G. N. 1972, A./. 77, 451. Abell, G. Ο. 19Θ8, In Stars vol. 9, Rood, H. J., Page, T. L., Kintner, E. C., and King, I. R. A. Sandage and M. Sandage, eds. (Chicago; Univer- 1972, Ap. J.175,627. sity of Chicago Press). Tifft, W. G. 1972, Ap. J. 175, 613. Bahcall, Ν. Α. 1973ο, Ap. J. 183,7 Tifft, W. G., and Gregory, S. A. 1973, 181, 15. 1973¾. Ap./.186, 1179. Zwicky, F., and Herzog, Ε. 1963, Brunk, H. D. 1960, An Intro and of Clusters of Galaxies,vol. 2 (Zurich; L. Speich, Statistics (Boston: Ginn and Company). for California Institute of Technology).