SPIRAL AND IRREGULAR GALAXIES ASSOCIATED WITH RICH REGULAR CLUSTERS

Transcription

1 Pub. Atron. oc. Pacific, Volume 8, February 1974 PRAL AND RREGULAR GALAXE AOCATED WTH RCH REGULAR CLUTER HERBERT J. ROOD ntitute for Advanced tudy, Princeton, New Jerey Atromony Department, Michigan tate Univerity Received Augut 1973 Thirty galaxie brighter than m p = 15 7 in a 23 quare-degree region centered on the Coma cluter are claified a piral and irregular on 48-inch chmidt plate. The radial urface denity ditribution of thee galaxie, corrected for contamination by field galaxie, indicate that about 15% of the galaxie aociated with the cluter in the brightet 2.7-magnitude interval are piral and irregular. The homogeneou radial ditribution of piral and irregular in a 2-quare-degree region centered on Abell 2199 ugget that all or nearly all of thee galaxie are member of the field. The above concluion are proviional becaue (1) they are derived from relatively mall ample of galaxie and (2) galaxy claification from 48-inch chmidt plate could contain ignificant uncertaintie. Key word: galaxie cluter of galaxie. ntroduction t i etimated that more than half of the brightet galaxie in the irregular Virgo and Hercule cluter are piral and irregular (e.g., ee Burbidge and Burbidge 1959; Abell 198). The ituation for rich regular cluter i le clear. Abell write that they are nearly or completely devoid of piral and irregular galaxie. The purpoe of the preent paper i to make quantitative etimate of the fraction of piral and irregular galaxie aociated with rich regular cluter on the bai of the radial urfacedenity ditribution of galaxie claified with 48-inch chmidt plate. The Coma and Abell 2199 cluter are tudied. t i found that about 15% of the brightet galaxie aociated with the Coma cluter could be piral and irregular (ection ), while no piral and irregular are detected in Abell 2199 (ection ). A dicuion of uncertaintie in the analye i given in ection V.. Coma Cluter The center of the Coma cluter i between two upergiant galaxie (Heidmann 195; Bahcall 1973a). By claifying galaxie with a et of 200-inch plate in a 0.30 quare-degree central region of the Coma cluter, Rood and Baum (197) found five piral and irregular galaxie. The name of thee galaxie with their magnitude m v (Zwicky and Herzog 193) and radial velocitie V 0 corrected for olar motion relative to the local group (Rood, Page, Kintner, King (RPKK) 1972; Tifft 1972; Tifft and Gregory 1973) are lited in Table. The mean velocity and velocity diperion of the Coma cluter are 888 km ec -1 and 900 km ec -1, repectively (RPKK). f we ue the Omer, Page, and Wilon (195) etimate of 48.5 field galaxie per quare degree to m v = 17.8, and aume m p m v = 1.0, then the expected number of uniformly ditributed field galaxie with m p ^ 15.7 in the 0.30-quaredegree urvey region i only one galaxy. Therefore, mot of the five piral and irregular galaxie could be aociated with the cluter, although a concluion baed on only five galaxie i, of coure, very uncertain. May all (190) preented a homogeneou ample of the 82 galaxie mot uited for pectrocopy (high urface brightne and central concentration) within a -degree by -degree area centered on the Coma cluter. eventy-one of thee galaxie are aociated with the cluter on the bai of their radial velocitie. RPKK claified thee galaxie with 48-inch chmidt plate. They found 20 piral and irregular and concluded from radial velocity data that 13 are aociated with the cluter. Thee data are reproduced in Table. The radial ditribution 99

2 100 HERBERT J. ROOD TABLE piral and rregular Galaxie in the ample of Rood and Baum (197) for the Core of the Coma Cluter RB Galaxy T yp e (RB 200-inch) N4858 b RB a + DW 040 /c RB219 a N4921 b Type (Preent tudy) -+- /O o Vo (km ec -1 ) of the Mayall ample of piral and irregular are given in Table. The oberved radial ditribution are compared with a homogeneou urface denity ditribution and the overall ditribution of cluter member (King 1972). t i een that the uncorrected ample of 20 galaxie ha a radial ditribution which lie between the homogeneou and King ditribution. The corrected ample of 13 galaxie, however, ha a radial ditribution which agree with the King ditribution at the 70% ignificance level; that i, 70% of the ample of galaxie elected at random from a King ditribution have a X 2 larger than that oberved. Auming that the 13 piral and irregular are aociated with the cluter, we etimate that 13/71 = 18% of the Mayall galaxie aociated with the cluter are piral and irregular. n order to increae the ample of piral and irregular, have claified all galaxie lited in the catalog of Zwicky and Herzog (193) (i.e., all galaxie with m v = 15.7) on 10-inch by 10- inch (4? 75 X 4? 75) 48-inch chmidt plate, kindly loaned to me by C. T. Kowal and G. A. Welch. Five well-expoed plate taken with good eeing were ued. They include two 103a-O plate (one with a GG-13 filter), two 103a-D plate with Wrl2 filter, and one lia J plate with Wr2C filter. No obviou dependence of the claification type on emulion wa noticed. Table and compare type from the preent tudy with type given in the literature. Of the 215 galaxie claified in the preent tudy, 30, that i %, are piral and irregular. Propertie of the 30 identified piral and irregular galaxie are given in Table V. Their TABLE piral and rregular Galaxie in the Mayall ample of the Coma Cluter Mayall R Galaxy (') Cluter Member: N N A12 51 A13 21 N N N N A17 1 A N N Nonmember: N N N N N N A Type Type (RPKK) (Preent tudy) p Haro 9 / Bp / O 0/ / E/0 + / O + E / TABLE lia Radial Ditribution of Mayall piral and rregular Galaxie Aociated with the Coma Cluter R(') Mhom) Mob) MKing) Pr(X 2 > X 2 ob) <0.5% - 70% TABLE llb Radial Ditribution of all Mayall piral and rregular (Member Plu Nonmember) in the Field Centered on the Coma Cluter Pr(X 2 > X 2 ob) Mhom) Mob) 5 9 MKing) %

3 PRAL AND RREGULAR GALAXE 101 radial ditribution, uncorrected and corrected for uperimpoed field galaxie, are given in Table V. The correction for field galaxie wa etimated a follow: Five out of the 18 galaxie with known radial velocitie are conidered to be field galaxie becaue their radial velocitie are more than 3000 km ec -1 (3.3a) from the cluter mean. t i then aumed that 5/18 of the 12 galaxie without known radial velocitie are field galaxie. The 5/18 X 12 = 10/3 field galaxie are then aumed to be ditributed in the annular ring in proportion to the area occupied by galaxie with unknown radial velocitie; that i, if (ay) 2/5 of the galaxie in a ring are without known radial velocitie, then 2/5 of the geometrical area of the ring i ued to etimate the number of field galaxie in the ring with unknown radial velocitie. The total number of field galaxie in a ring i the um of the number with, plu the number without, known radial velocitie. Table Vb how that the radial ditribution of the piral and irregular galaxie, corrected for field galaxie, i more centrally concentrated than a homogeneou ditribution. The oberved ditribution agree with the King ditribution in NGC C* A12 842* A TABLE V piral and rregular Galaxie of the Coma Cluter with Type Derived from 48-inch chmidt Plate in the Preent tudy 12 h 48 m lw^ 27 39' ' Vo (km/ec -1 ) K(') Type (Preent tudy) E: + p E +

4 102 HERBERT J. ROOD TABLE Va Radial Ditribution of Coma piral and rregular Galaxie Typed in the Preent tudy, Uncorrected for Field Galaxie Pr(X 2 > X 2 ob) N(hom) N( ob) N(King) % TABLE Vb Radial Ditribution of Coma piral and rregular Galaxie Typed in the Preent tudy, Corrected for Field Galaxie Pr(X 2 > X 2 ob) Yi'liom) <0.5% N(ob) MKing) % the ene that 30% of the random ample from the King ditribution have value of X 2 greater than that oberved.. Abell 2199 Abell 2199 i a rich regular cluter centered on the upergiant galaxy NGC 1 (Bahcall 1973h). Claification for 170 galaxie with B < 17^ in a two-quare-degree field centered on the cluter are given by Rood and atry (1972). The type were derived from a 48-inch chmidt Ha-J plate. Fifty-two of the galaxie are piral and irregular. n Table V, the oberved radial ditribution of the piral and irregular are compared with a homogeneou ditribution, the ditribution for the E + O 4* 0 galaxie in the Rood-atry urvey, and the ditribution for all galaxie with B = 17^5 given by Bahcall (1973h). A X 2 -tet how that the oberved piral and irregular ditribution agree with the homogeneou ditribution on the 30% ignificance level. Only four of the 52 piral and irregular galaxie have known radial velocitie. Thee lie within ±2a of the cluter mean, (where a = 843 km ec -1 i the velocity diperion of the cluter), but 7% of all field galaxie occupy thi range. We conclude that all or nearly all of the piral and irregular in the field of the Abell 2199 cluter are field galaxie. V. Uncertaintie The concluion derived in ection and mut be conidered proviional for everal reaon. 1. The apparent aociation of piral and irregular galaxie with the Coma cluter uncovered in ection conflict with a reult by Abell (198). From a tudy of the areal ditribution of 47 galaxie that could be definitely claified a piral on 48-inch chmidt photograph covering a 70-quare-degree region including the center of the Coma cluter, Abell found that thee galaxie are not ignificantly concentrated to the cluter center. 2. The analye in the preent paper often only marginally atify the rule-of-thumb that the chi-quared tet i valid "if at leat ome of the clae have expected frequency 5 or more (Brunk 190). 3. The aignment of morphological type to galaxie from 48-inch chmidt plate are ubject to large uncertaintie. Thi i demontrated by the comparion of 48-inch chmidt claification with the much more certain 200-inch claification given in Table and the comparion between independent 48-inch chmidt claification given in Table. Moreover, of the 30 claification given in Table V, there are a urpriingly large number of irregular galaxie (11). t i poible that elliptical and O galaxie with tructural peculiaritie are ometime mitaken for piral or irregular. Thi i epecially the cae for the Coma cluter, where everal peculiar galaxie in the cluter core have been identified on 200-inch plate (Rood 198). Claification baed on plate with the reolution of well-expoed 200-inch plate are probably neceary for a definitive tudy of the ditribution of piral and irregular galaxie in the region of the Coma cluter. t i a pleaure to thank Dr. John and Neta Bahcall for encouragement and helpful dicuion, Dr. George Abell for reading the manucript and offering ueful uggetion, and Dr. C. T. Kowal and G. A. Welch for the loan of

5 HO Pr(X 2 > X 2, ob/ PRAL AND RREGULAR GALAXE 103 TABLE V Radial Ditribution of piral and rregular Galaxie in the Field of Cluter Abell 2199 /E 'E + O \ N(hom) iv(ob) N \ + 0/ N(Bahcall) % inch chmidt plate. wih to thank Dr. Carl Kayen for hi hopitality at the ntitute for Advanced tudy. Thi reearch wa ponored by National cience Foundation grant No. GP-17A#1. REFERENCE Abell, G. O. 198, in tar and tellar ytem, vol. 9, A. andage and M. andage, ed. (Chicago: Univerity of Chicago Pre). Bahcall, N. A. 1973o, Ap.J. 183, b, Ap. J. 18, Brunk, H. D. 190, An ntroduction to Mathematical tatitic (Boton: Ginn and Company). Burbidge, G. F., and Burbidge, E. M. 1959, Ap. J. 130, 29. Heidmann, J. 195, Ann. d Ap. 28, 380. King,. R. 1972, Ap. J. (Letter) 174, L123. Mayall, N. U. 190, Ann. d Ap. 23, 344. Orner, G. C., Page, T. L., and Wilon, A. G. 195, A.]. 70, 440. Rood, H. J. 198, Pub. A..P. 80, 424. Rood, H. J., and Baum, W. A. 197, AJ. 72,398. Rood, H. J., and atry, G. N. 1972, AJ. 77, 451. Rood, H. J., Page, T. L., Kintner, E. C., and King,. R. 1972, Ap. J. 175, 27. Tifft, W. G. 1972, Ap.J. 175,13. Tifft, W. G., and Gregory,. A. 1973, Ap.J. 181, 15. Zwicky, F., and Herzog, E. 193, Catalog of Galaxie and of Cluter of Galaxie, vol. 2 (Zurich: L. peich, for California ntitute of Technology).