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1. INTRODUCTION

This meeting is a tribute to Ken Freeman, his research and that with his students. Ken's most cited paper is undoubtedly "On the disks of spiral and S0 galaxies" (Freeman, 1970). Although it was de Vaucouleurs (e.g. 1959), who first showed observationally that the surface brightness distributions can be described by an exponential law, it was Ken in his seminal paper who collected the material available and studied the distribution of parameters and the dynamical relations. He derived the rotation curve and angular momentum distribution for an exponential disk in centrifugal equilibrium, found that 28 of the 36 galaxies have approximately the same face-on central surface brightness (known as "Freeman's law") and on the basis of the rotation curves of NGC300 and M33 concluded that there must be undetected matter of at least the mass of the detected galaxy.

Table 1. ADS Citations up to end 2000 of important papers

Author(s) Year Reference Subject ADS citations
        Tot. 9/0 yr-1

Schönberg & 1942 ApJ 96, 161 H-burning and 57 5 1.0
Chandrasekhar     S-C core      
Baade 1944 ApJ 100, 137 Stellar populations 129 5 2.3
de Vaucouleurs 1948 An'Ap 11, 247 R1/4-law 388 42 7.5
Spitzer & 1951 ApJ 114, 385 Secular evolution of 111 5 2.3
Schwarzschild     stellar motions      
Sandage & 1952 ApJ 116, 463 Giant-branch evolu- 55 6 1.1
Schwarzschild     tion; MS turn-off      
Hoyle & 1955 ApJSuppl 2, 1 Giant-branch evolu- 87 5 1.9
Schwarzschild     tion; HR-diagrams      
Salpeter 1955 ApJ 121, 161 Salpeter-function 922 154 20.5
Burbidges, 1957 RMP 29, 547 Stellar nucleosyn- 343 15 8.0
Fowler & Hoyle     thesis      
Oort, Kerr & 1958 MN 118, 379 HI structure of the 40 2 2.0
Westerhout     Galaxy      
Schmidt 1959 ApJ 129, 243 SF; ``Schmidt-law'' 376 39 9.2
King 1962 AJ 67, 471 King law 440 42 11.6
Sandage 1962 ApJ 135, 333 NGC188 and chem- 98 - 2.6
      ical evolution      
Eggen, Lynden- 1962 ApJ 136, 748 Collapse of the 670 61 17.6
Bell & Sandage     Galaxy      
Schmidt 1963 ApJ 137, 758 G-dwarf problem 227 10 6.1
Toomre 1964 ApJ 139, 1217 Local disk stability 607 83 16.9
Lin & Shu 1964 ApJ 140, 646 Density wave theory 232 18 6.4
Goldreich & 1965 MN 130, 97 Disk instability and 128 14 3.7
Lynden-Bell 1965 MN 130, 125 spiral structure 195 27 5.6
King 1966 AJ 71, 64 King models 803 46 23.6
Lynden-Bell 1967 MN 136, 101 Violent relaxation 347 25 10.5
Schmidt 1968 ApJ 151, 393 Quasars; V / V-test 504 34 15.8
Tinsley 1968 ApJ 151, 547 Photometric evolu- 66 5 2.1
      tion      
Freeman 1970 ApJ 160, 811 Exponential disks 820 84 27.3
Freeman, San- 1970 ApJ 160, 831 Origin of Hubble 232 11 7.7
dage & Stokes     types      
Searle 1971 ApJ 168, 327 Disk abundance 349 10 12.0
      gradients      
Peebles 1971 A&A 11, 377 Origin of angular 54 5 1.9
      momentum      
Toomre & Toomre 1972 ApJ 178, 623 Interacting galaxies 785 90 28.0
Searle, Sargent & 1973 ApJ 179, 427 Photometric evol- 337 20 12.5
Bagnuolo     ution of galaxies      
Ostriker & 1973 ApJ 186, 467 Disk stability and 439 29 16.3
Peebles     dark halos      
Tully & Fisher 1977 A&A 54, 66 TF-relation 547 63 23.8
Wielen 1977 A&A 60, 263 Secular evolution of 220 29 9.6
      stellar motions      
Larson & Tinsley 1978 ApJ 219, 46 Photometric evolu- 554 29 25.2
      tion      
Searle & Zinn 1978 ApJ 225, 357 Globular clusters 539 58 24.5
      and halo formation      
Tinsley 1980 FCP 5, 287 Photometric and 449 57 15.0
      chemical evolution      
van der Kruit & 1981 A&A 95, 105 3-D galaxy disk 280 20 14.8
Searle     model      
Gilmore & Reid 1983 MN 202, 1025 Galactic thick disk 291 16 17.1

It is of some interest to see how Ken's 1970-paper compares in citation rate to others. For this purpose I collected such information using the NASA Astrophysics Data System (ADS). The citation scores in ADS are not complete, but certainly indicative and internally consistent. The first exercise was to draw up a list of what I feel are the most important papers related to studies of structure of galaxies. In Table 1 I give the total citation score up to the end of 2000, that for 1999+2000 ("9/0") and the average number of citations per year. In this listing I find only a few papers that are comparable or exceed Ken's 1970-paper in citations. These are Ed Salpeter's 1955 paper on star formation in the Galactic disk in which he defines the "Salpeter function", Ivan King's 1966 paper on the dynamics of globular clusters and the "King models" and the study by the Toomre & Toomre in 1973 on models for interacting galaxies.

To complete the search I also checked in ADS which papers between 1945 and 1975 were annually the most highly cited papers in Ap.J. (main journal), A.J., M.N.R.A.S. and A&A. One that comes close is the galaxy redshift survey of Humason et al. (1956) (754 citations, 16 in 1999+2000). Outside the field of galaxies there are two papers that clearly exceed it, namely Shakura & Sunyaev (1973; A&A 24, 337; 1823 citations and 313 in 1999+2000) on the appearance of black holes in binary systems and Nino Panagia (1973, AJ 78, 929; 1026 and 46) on parameters of early type stars, while Brocklehurst (1971, MN 153, 471; 787 and 28) on hydrogen population levels in gaseous nebulae comes close.

In any case, Ken's 1970 paper is well within the absolute top ten of citation scores of papers in astronomy and deservedly so.

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