Published in PASP 71, p. 394, 1959.

Table of Contents

I

II

III

TABLE I

NOTES TO TABLE I

A recent paper has given an outline of a revised system of classification of the forms of galaxies, together with types for 608 systems. ¹ That investigation will be designated Paper I, and the present one Paper II.

The galaxies classified on the revised Yerkes system in Paper I include a large fraction of the northern entries in the Shapley-Ames Catalogue. It seemed advisable to make the classification approximately complete to some limiting apparent magnitude; a program was therefore devised to classify a number of additional galaxies. Through the kindness of Dr. I. S. Bowen and the Committee of the Mount Wilson and Palomar Observatories, I was permitted to make use of the original negatives of the National Geographic Society-Palomar Observatory Sky Survey for the purpose.

The present paper completes the classification of all galaxies brighter than magnitude 13.1 in the Shapley-Ames Catalogue and north of = -25°, with two exceptions: the object NGC 2149 has been omitted from the catalogue, since it appears to be a diffuse nebulosity, rather than a galaxy; NGC 2798 was omitted inadvertently.

The galaxies included in the present paper were classified on the 48-inch Sky Survey plates during a period as guest investigator at Pasadena in the autumn of 1958. A large number of galaxies in Paper I were reclassified without knowledge as to the earlier type assigned; this overlap furnishes material for an accurate determination of the systematic characteristics of the two series.

The 48-inch Sky Survey plates are ideally suited to the classification of the forms of the brighter galaxies; the uniform quality of the plates makes feasible the use of somewhat finer subdivisions than in the case of Paper I. In many cases, much more detail is visible on photographs made with the great reflectors; however, the uniformity of contrast and resolution on the Sky Survey negatives permits intercomparison of different galaxies to be made with high precision.

As a result of this, certain minor modifications were made in the system of Paper I; these are:

1. The definition of Ep has been broadened to include peculiar elliptical-like systems in addition to those showing dust absorption.

2. Certain highly irregular, disorganized systems have been classified in the category a?Ip; these would have been included among the aI systems in Paper I; the distinction between the aI and the a?Ip groups is shown by a comparison of the galaxies NGC 4490 and NGC 3991, both of which are illustrated in Paper I.

3. Intermediate stages in the form families have been expanded in number; for example, galaxies having an appearance intermediate between classes E and D are classified as ED or DE, depending on whether the E or the D characteristic is more apparent.

In cases where the same galaxy has been classified in Paper I and the present paper, the later type is to be preferred.

A catalogue of 642 galaxies is given in Table I. The types were determined on the blue plates of the Palomar Sky Survey, except in the case of one field where the blue plate was not available. The galaxies classified on the red plate are noted in the remarks to the table. The latter are referred to by an asterisk following the NGC number.

A few galaxies inadvertently omitted from the survey were classified on the glass copies of the Sky Atlas at the Lick Observatory; in addition, one system was classified from the paper print at the Yerkes Observatory.

With the addition of the galaxies listed in Table I, an extensive body of material becomes available on the system described in Paper I. At this stage, therefore, the question can be asked: How closely do galaxies classified in the various subdivisions conform to the definitions outlined in Paper I?

In any attempt at classification, three stages can be recognized: (1) The setting up of a system of categories that are defined in an approximate manner - according to certain preconceived ideas; (2) the classification of a body of specimens into the categories outlined; (3) the description of the categories in terms of the specimens classified in each.

In the course of the progression outlined above, developments and modifications in the original scheme result of necessity. An interesting example of this is furnished by the evolution of the Harvard system of spectral classification into the system of the Henry Draper Catalogue. Some categories tend to disappear; others are modified in nature; and, most important of all, the definitions of the surviving groups undergo a certain amount of systematic change.

A dogmatic insistence on the exactness of the preliminary definitions of the classification system is therefore inadvisable; a valid and precise definition of an empirical system of classification can only be in terms of the observed properties of the specimens classified in each category. As the amount of observational evidence increases, the ideas on which the new classification is based are subjected to increasingly critical tests.

The original basis for the Yerkes form system was the spectral classification of galaxies of Morgan and Mayall. ² In the latter, groups of galaxies were classified into spectroscopic categories of A to K. The galaxies of earliest spectral type were described as "A-systems"; there was no category of "B" galaxies included in the classification. The ultraviolet region of the spectrum of these "A" galaxies generally contains strong absorption lines of hydrogen, and it was considered that the principal contributors to the violet spectral region are main sequence stars of type A.

Spectrograms of higher resolving power obtained more recently at the McDonald Observatory by Mayall and Morgan show that galaxies classified in the "A" category vary greatly among themselves: in some cases (NGC 4490) it seems that B8-A5 main sequence stars are probably responsible for the principal contribution to the light in the ultraviolet region of the spectrum; at the other extreme are galaxies similar to NGC 4214, whose spectroscopic characteristics in the ultraviolet resemble rather closely those of the inner parts of the Orion Nebula. ³ At an intermediate position are galaxies similar to NGC 4449, where the ultraviolet line of He I at 3820 is present. ³ In such systems as NGC 4449 it seems likely that most of the ultraviolet light originates in B1-B3 stars; in this case, there seems to be a resemblance to the population of the Large Magellanic Cloud. ⁴

Now, if this wide range in spectroscopic appearance is to be taken account of in a classification system of forms, further subdivisions in the system are called for; however, at the present time, it does not appear to be practicable to distinguish the three above-mentioned categories of galaxies of early spectral type from each other by their form characteristics alone. We therefore, in the form classification procedure, continue to describe all three categories as "a-systems." At this current stage in the classification development, the category "a-systems" refers to galaxies whose stellar populations, as observed in the ultraviolet region, range from a situation approximately similar to that of the Orion Nebula region to a population in which the principal contribution to luminosity originates in A- or even early F-stars.

In view of the modifications outlined above - and of the fact that further modifications are almost certain to be made in the future - the justification for the carrying out of the reclassification of forms of galaxies in Papers I and II might be called into question. The present justification lies in the fact that the new classification does effect an approximate separation of galaxies according to their stellar populations; the "a" galaxies do have populations rich, by luminosity, in early type stars and gas; the "k" galaxies have a population rich, again by luminosity, in yellow-giant stars; and the galaxies of intermediate form type tend to have mixtures of the two extreme categories above mentioned. The final decision will depend on how useful the new classification proves to be in the future.

I wish to express my thanks to Dr. I. S. Bowen, Director, and to the Committee of the Mount Wilson and Palomar Observatories for the opportunity to carry out the classification of galaxies given here.

The investigation was supported by a grant from the Office of Naval Research.

1. W. W. Morgan, Pub. A.S.P., 70, 364, 1958.
2. W. W. Morgan and N. U. Mayall, Pub. A.S.P., 69, 291, 1957.
3. W. W. Morgan and N. U. Mayall, unpublished.
4. G. and A. de Vaucouleurs, Lowell Obs. Bull., 4, 58, 1959 (No. 92), or Pub. A.S.P., 71, 83, 1959.

   45     Low surface brightness.
  151     Arm tilted from plane.
  157     High surface brightness.
  221     Abnormally steep edge gradient.
  278     From Crossley plate.
  337     Strong arms; no nucleus.
   57     Background, D.
New 1     Low surface brightness.
  474     Exceedingly faint outer spiral arms. Limiting case between S
          and E systems.
  681     Heavy dust lane. Total eccentricity of system (4). Edge-on
          system with very strong E 3-4 central part.
  741     Brightest in group of k systems.
  821     Very faint arms?
 1023     Inner lozenge inclined to fainter, outer part. Faint, arm-like
          extension curving south at eastern end of elongated penumbra.
 1035     Or afI.
 1055     Heavy asymmetric absorption.
 1058     Irregular, thready, bright features.
 1084     Or Sp. Disordered S. High surface brightness.
 1140     Or k??I + tail.
 1179     Low surface brightness.
 1199     Several small k systems nearby.
 1209     Or kED6.
 1241     Companion.
 1275     Faint irregular bright filaments. Brightest in nest of around 12
          bright k systems.
 1337     Plane of system distorted.
 1359     Or a?Ip. Disordered appearance.
 1376     Arms building off ring.
 1385     Disordered appearance; high surface brightness.
 1440     Background (D).
 1518     Or a?Ip.
 1600     Brightest in small k cluster?
 1637     Background D.
 2139     Appendage. High surface brightness. H II regions?
 2207     Appendage.
 2545     One-arm spiral starting from inner loop.
 2633     Three hot spots in bar.
 2639     Very faint thin spiral arm.
 2642     Distant fragment.
 2672     Companion 2673, kE1. Faint appendage, on red plate, to 2673, in
          direction opposite to 2672.
 2685     Helix. Spiral arms around major axis.
 2781     Very faint outer ring, 3 x diameter of inner system.
 2793     Companion.
 2832     Small companion, kE1. Brightest in nest of 8 k systems.
 2855     Dust arc around nucleus.
 2865     On red plate: kE4.
 2907     Heavy dust lane.
 2967     High surface brightness.
 2968     On red plate, classified as pec.
 2974     Very faint spiral arms. Superposed star.
 3034     Dust streamers tending to radial direction. Faint, vein-like
          bright filaments extending out normal to major axis in region
          of minor axis.
 3044     Very flat.
 3052     Or afS3.
 3077     Dust streamers tending to radial direction.
 3081     Ring + nucleus.
 3091     Or kD3. Brightest system in k group.
 3124     Or fS2.
 3158     Brightest in nest of around 10 bright k systems.
 3185     Ring + internal bar.
 3190     Heavy, inclined dust lane.
 3226-7   Some similarity to 5194-5. Arm from spiral seems to join elliptical.
 3277     k?E2? + faint S2 arms.
 3294     Or aS4.
 3368     One-arm spiral.
 3351     Nuclear hot spots? Overexposed.
 3403     Arms very faint.
 3412     Two major axes inclined.
 3414     Bar thin and weak.
 3432     Or a?I7.
 3445     Or aSp. 17 appendage.
 3448     Faint outer extensions.
 3504     Inner part g:D5.
 3556     Or a:S:6. Dust clouds; high surface brightness.
 3593     Dust arc around nucleus.
 3610     Very faint outer arms?
 3619     Like 4151?
 3626     Spiral arms in inclined plane. Heavy dust arc around nucleus.
 3630     Small.
 3631     Nuclear hot spots.
 3640     Inner part, E, surrounded by asymmetrical faint penumbra, with
          possible faint traces of spiral arms.
 3646     Outer ring irregular in structure.
 3655     High surface brightness.
 3664     Or a?Ip.
 3690     Two systems in contact; debris outside.
 3705     Bright inner ring.
 3720     High surface brightness; fS3 = 3719.
 3729     Ring.
 3732     Or k??Sp. Very bright nucleus. Faint, close arm.
 3769     Or gD6. Ring. I6p companion.
 3773     Or pec.
 3782     Like M 82?
 3813     High surface brightness.
 3818     Red plate: kE6.
 3900     Ring.
 3949     High surface brightness.
 3955     Smooth, amorphous background on which brilliant features are
          superposed.
 3957     Or kD7: distant. Dark lane across nucleus.
 3995     Or aSp.
 4024     Or k?B.
 4027     a?Ip.
 4032     Small thready (spiral) arms. Like 1275?
 4050     Or fBS3.
 4085     Or aS6.
 4116     From red plate.
 4123     From red plate.
 4129     Or gD7?-gS7?.
 4138     Dust ring.
 4215     From red plate.
 4220     Or gD6.
 4224     Heavy dust lane. From red plate.
 4233     Circular nucleus. From red plate.
 4234     Or aI. From red plate.
 4235     From red plate.
 4274     Ring.
 4324     Ring.
 4342     Southernmost of group of 5.
 4369     Distant, kD1. Hot spots in nucleus.
 4374     Dust visible on high-resolution plates.
 4395     Low surface brightness.
 4414     High surface brightness.
 4454     Ring.
 4457     Faint outer ring.
 4460     Row of hot spots near nucleus?
 4478     Or kE3.
 4485     Satellite and debris near 4490.
 4496     Overlapping afI.
 4517     Or afI.
 4540     Companion.
 4592     Or afI.
 4670     Eccentric envelope.
 4691     Row of 3 hot spots.
 4697     Dust?
 4698     Faint disk-arm system 3 x size of inner part.
 4699     Distant: kD4.
 4700     Or aI. Chain?
 4710     Irregular dust clouds projected.
 4736     Distant, kD3.
 4747     Or I?p. Extended "tidal" arm. Central row of hot spots.
 4750     Bright ring.
 4753     Irregular dust.
 4765     Or Ip.
 4782-3   Two connected, distorted. Nuclei eccentric.
 4800     Or S3p. Distant: kD3. High surface brightness.
 4861     Comet. Multiple hot spots in head. Spray of bright regions in
          tail.
 4866     Ring. Or gkD6, ring.
 4928     Small; bright.
 4933     2 companions.
 4958     Flat, outer envelope.
 5005     gD background, with S arms delineated principally by absorption
          lanes.
 5044     Brightest member of small k cluster.
 5077     Brightest of group.
 5084     Very flat. Flat, large envelope 2-3 x length of main body.
 5147     Projected star.
 5170     Exceedingly flat.
 5313     High surface brightness.
 5377     Faint outer inclined ring.
 5426     Connected with 5427.
 5427     Connected with 5426.
 5474     Low surface brightness gS1 superposed on north edge of D1 system
          which seems to be smooth and featureless.
 5496     Or afI.
 5533     Thready, blue spiral arms.
 5557     Classified on Sky Atlas print.
 5595-7   Similar pair.
 5691     One irregular arm from Ip? nucleus.
 5713     Brilliant, mottled inner part.
 5750     Or fD4(S). Nucleus + ring.
 5792     Arms from ring.
 5820     Very faint curving extension to SE from end of major axis.
 5850     Or gB3.
 5861     No nucleus on blue plate.
 5866     Heavy dust lane.
F 703     Or afS2.
 5907     Very flat.
 5915     Or Sp. Small, bright spiral.
 6503     High surface brightness.
 6643     High surface brightness.
 6822     Magellanic Cloud type.
 6835     Or kS7.
 7184     Ring.
 7252     Four-petaled daisy.
 7585     Faint spiral arm?