Classification and Morphology of External Galaxies

II. MORPHOLOGY

The main classification criteria are as follows:

SA (r) : has a small, sharp, very bright and round nucleus isolated in the center of a circular ring at the edge of which emerge many tightly wound filamentary spiral arms or arcs; the nucleus and ring often merge and disappear in the over-exposed image of the central bulge (as illustrated), but the high multiplicity of the spiral pattern is usually sufficient for identification; a weak outer ring (R) made up of many closely coiled spiral arcs is often present in early stages of this sequence. Examples are NGC 488 (illustrated from P 200"), NGC 7217, NGC 6753 (22); other examples: NGC 4736 (M94) (21), Sab and NGC 5055 (M63), Sbc.

5. NGC 7475 (W 100"): E ⁺5. The merest traces of differentiation between nucleus and lens appear at the first transition stage between E and S0. Compare with 1, 2 and 6. 6. NGC 7166 (S 74"): SA (r) 0 ^-. The lens and envelope still simulate an elliptical, but a small nucleus appears in the lens. Note size of nucleus; compare with 2 and 5.
7. NGC 1553 (S 30"): SA (r) 0 ^o. A weak trace of ring appears at the edge of the lens. Note small, distinct nucleus in lens and faint, outer envelope. Compare with 3, 6 and 8. 8. NGC 7702 (S 74"): SA (r) 0 ⁺. A definite ring, well separated from the small nucleus appears at the edge of the lens. The very faint envelope does not show up on this image of short exposure. Note asymmetry of nucleus and ring near minor axis indicating the presence of dark matter in the equatorial plane. Compare with 4 and 7.
45. NGC 4215 (W 100"): SA (r) 0 ^o: (sp). This short exposure shows only the small nucleus and the weak ansae, being the edge-on view of the incipient ring structure in the lens. Compare with 7, 8 and 44. 21. NGC 4736 = M94 (P 200"): (R) SA (r) ab. Only the complex spiral pattern of high multiplicity in the lens appears in this reproduction. The small nucleus and inner ring (r) surrounding it are lost in the overexposed central bulge; the outer, smooth ring structure (R) extending beyond the limits of the figure is not shown either. Compare with 8, 17 and 22.
22. NGC 6753 (S 74"): (R) SA (r) b. The inset shows the nucleus and bright inner ring (r) from which emerge the complex spiral pattern in the lens, only partly resolved here; the outer ring (R), which begins to break up into spiral arcs with bright knots is faintly connected to the inner spiral pattern in two diametrically opposite points. Compare with 8, 21 and 23 (allowing for differences in scale and resolution). 23. NGC 5962 (W 100"): SA (r) c. The inset shows the nucleus and inner ring which begins to break up at this stage and from which extend the many knotty and filamentary arms; only weak traces remain of the outer ring structure. Compare with 19, 22 and 24.
24. NGC 6643 (W 60"): SA (r) d. The inset shows the small nucleus separated from broken remnants of the inner ring extending into the complex, irregular spiral pattern. Compare with 20, 23 and 33.

SA (rs) : has a fairly small, fairly sharp, bright and round nucleus in the center of a diffuse lens or bulge out of which two main arms and two or more additional, weaker arms emerge tangentially; the two main arms simulate an incomplete ring around the lens; weak outer arms simulate an (R) structure in the early stages. Examples are NGC 1068 (M77), NGC 3147, NGC 4237, possibly NGC 3521, NGC 4800, NGC 7079, NGC 7590. NGC 5194 (M51) (illustrated from IR 20") is not a very good example as it is more nearly Sbc and intermediate between SA (rs) and SA (s).

44. NGC 4710 (W 100"): SA (rs?) 0 ⁺ (sp). This short exposure shows only the lens and fairly small nucleus surrounded by a heavy ring of dark matter and bright ansae,being the edge-on view of an incomplete ring or incipient spiral arms.

SA (s) : This is the typical, regular logarithmic spiral; it has a fairly large, diffuse, round nucleus extending smoothly into the circular or little elongated lens which shows some spiral pattern of dark matter; two main spiral arms with occasional branching or weaker secondary arms start at the rim of the lens. Examples are NGC 3031 (M81), Sab (illustrated from IR 20"), NGC 4569, NGC 7205 (18) NGC 7331. One diameter of the lens is occasionally slightly brighter and longer and the outer arms may have a tendency to ``return'' to the lens, indicating a transition toward the SAB (s) type; examples are NGC 224 (M31), Sb and NGC 4321 (M100), Sbc.

1. NGC 7144 (S 74"): E0. The appearance of a distinct nucleus is due to a photographic effect; there is actually a smooth, continuous decrease of luminosity from the center outwards. 2. NGC 5273 (P 200"): SA (s) 0 ^-. Traces of spiral structure in the smooth lens and envelope surrounding the large nucleus; little or no dark matter. With lower resolution lens and envelope would simulate a late elliptical; compare with 1 and 5.
3. NGC 4459 (W 100"): SA (s) 0 ^o. A weak ring or whorl of dark matter appears in the lens close to the large nucleus. The envelope does not show up on this view taken with a short exposure. Compare with 4 and 7. 4. NGC 2855 (P 200"): SA (s) 0 ⁺. The ring of dark matter outlines a weak smooth spiral structure around the large and bright nucleus. Compare with 3 and 8.
17. NGC 4569 = M90 (P 200"): SA (s) ab. A large central bulge from which emerge two smooth and massive regular arms which are closely wound and separated by dark lanes. Faint outer spiral extensions showing tendencies to return towards central lens. Compare with 4, 21 and 39. 18. NGC 7205 (S 74"): SA (s) bc. Smaller central bulge with inner nucleus from which emerge two main massive and knotty regular arms; these are less closely wound and show some branching. Compare with 17, 19 and 40 or 41.
19. NGC 1084 (W 100"): SA (s) c. A small central nucleus from which start two main arms with much irregular structure, bright knots and emission objects. Only the brighter, inner regions are shown in this short-exposure image. Compare with 18, 20, 23 and 42. 20. NGC 7793 (S 30"): SA (s) d. A very small central nucleus, resembling a globular cluster, is surrounded by several irregular and broken spiral arms with very many knots. Note that the arms still start from the nucleus. The mean surface brightness decreases steadily from Sa to Sd. Compare with 19, 24 and 33.
33. NGC 300 (S 30"): SA (s) d. A very small, round nucleus, similar to a globular cluster, lies in the middle of the central region from which emerge several, highly resolved and branching arms of low surface brightness. Compare with 20 and 34. NGC 300 is very similar to M33 but of slightly later type. 34. NGC 45 (P 200"): SA (s) dm. Only the central regions of this object of very low surface brightness are visible on this print; the nucleus is very small and faint in a central, amorphous mass from which emerge very faint, irregular and branching arms. Compare with 33 and 35.
35. NGC 5204 (P 200"): SA (s) m. Only a weak spiral pattern remains in the highly resolved, irregular, but symmetrical distribution of supergiant stars and emission knots surrounding the central condensation. Compare with 34 and 37. 39. NGC 4594 (P 200"): SA (s:) a. The very bright and large spheroidal component, made up of high-velocity stars, emits most of the light of the object; the thin and weak flat component includes only very bright Population I supergiants; it is surrounded by a heavy peripheral ring of dark matter; the spiral pattern is faint and smooth. Compare with 17 and 21.

40. NGC 5746 (W 60"): SA (s:) b. The spheroidal component is very much reduced, but still contributes a large fraction of the light; the spiral pattern of bright and dark arms is well defined (save for the projection foreshortening), regular and still fairly smooth, although it begins to show some clumpiness. Compare with 18 and 22.
41. NGC 4565 (P 200"): SA (s:) bc. The still smaller spheroidal component is surrounded by an extensive flat component in which bright and dark spiral arms show much clumpiness and branching. Compare with 18, 19 and 23.

42. NGC 4244 (P 200"): SA (s:) cd. The spheroidal component is now inconspicuous in the center of a very extensive flat component in which the spiral pattern of bright and dark matter shows much irregularity and very many well resolved knots. Compare with 20 and 24. 43. NGC 5866 (P 200"): SA (s) 0 ^o: (sp). The outer envelope simulates an elliptical, but the lens (inset) shows a narrow ring of dark matter surrounding the large nucleus. Compare with 3, 44 and 45.
46. NGC 891 (P 200"): SA (s:) b (sp). This edge-on view of a typical ordinary spiral is for comparison with 43 and 44. Note how the dark matter, limited to a ring close to the nucleus in the lenticulars, spreads to the whole of the intermediate and outer parts of the spiral pattern. Compare also with 39 and 42.

5. NGC 7475 (W 100"): E ⁺5. The merest traces of differentiation between nucleus and lens appear at the first transition stage between E and S0. Compare with 1, 2 and 6.	6. NGC 7166 (S 74"): SA (r) 0 ^-. The lens and envelope still simulate an elliptical, but a small nucleus appears in the lens. Note size of nucleus; compare with 2 and 5.
7. NGC 1553 (S 30"): SA (r) 0 ^o. A weak trace of ring appears at the edge of the lens. Note small, distinct nucleus in lens and faint, outer envelope. Compare with 3, 6 and 8.	8. NGC 7702 (S 74"): SA (r) 0 ⁺. A definite ring, well separated from the small nucleus appears at the edge of the lens. The very faint envelope does not show up on this image of short exposure. Note asymmetry of nucleus and ring near minor axis indicating the presence of dark matter in the equatorial plane. Compare with 4 and 7.
45. NGC 4215 (W 100"): SA (r) 0 ^o: (sp). This short exposure shows only the small nucleus and the weak ansae, being the edge-on view of the incipient ring structure in the lens. Compare with 7, 8 and 44.	21. NGC 4736 = M94 (P 200"): (R) SA (r) ab. Only the complex spiral pattern of high multiplicity in the lens appears in this reproduction. The small nucleus and inner ring (r) surrounding it are lost in the overexposed central bulge; the outer, smooth ring structure (R) extending beyond the limits of the figure is not shown either. Compare with 8, 17 and 22.
22. NGC 6753 (S 74"): (R) SA (r) b. The inset shows the nucleus and bright inner ring (r) from which emerge the complex spiral pattern in the lens, only partly resolved here; the outer ring (R), which begins to break up into spiral arcs with bright knots is faintly connected to the inner spiral pattern in two diametrically opposite points. Compare with 8, 21 and 23 (allowing for differences in scale and resolution).	23. NGC 5962 (W 100"): SA (r) c. The inset shows the nucleus and inner ring which begins to break up at this stage and from which extend the many knotty and filamentary arms; only weak traces remain of the outer ring structure. Compare with 19, 22 and 24.
24. NGC 6643 (W 60"): SA (r) d. The inset shows the small nucleus separated from broken remnants of the inner ring extending into the complex, irregular spiral pattern. Compare with 20, 23 and 33.

1. NGC 7144 (S 74"): E0. The appearance of a distinct nucleus is due to a photographic effect; there is actually a smooth, continuous decrease of luminosity from the center outwards.	2. NGC 5273 (P 200"): SA (s) 0 ^-. Traces of spiral structure in the smooth lens and envelope surrounding the large nucleus; little or no dark matter. With lower resolution lens and envelope would simulate a late elliptical; compare with 1 and 5.
3. NGC 4459 (W 100"): SA (s) 0 ^o. A weak ring or whorl of dark matter appears in the lens close to the large nucleus. The envelope does not show up on this view taken with a short exposure. Compare with 4 and 7.	4. NGC 2855 (P 200"): SA (s) 0 ⁺. The ring of dark matter outlines a weak smooth spiral structure around the large and bright nucleus. Compare with 3 and 8.
17. NGC 4569 = M90 (P 200"): SA (s) ab. A large central bulge from which emerge two smooth and massive regular arms which are closely wound and separated by dark lanes. Faint outer spiral extensions showing tendencies to return towards central lens. Compare with 4, 21 and 39.	18. NGC 7205 (S 74"): SA (s) bc. Smaller central bulge with inner nucleus from which emerge two main massive and knotty regular arms; these are less closely wound and show some branching. Compare with 17, 19 and 40 or 41.
19. NGC 1084 (W 100"): SA (s) c. A small central nucleus from which start two main arms with much irregular structure, bright knots and emission objects. Only the brighter, inner regions are shown in this short-exposure image. Compare with 18, 20, 23 and 42.	20. NGC 7793 (S 30"): SA (s) d. A very small central nucleus, resembling a globular cluster, is surrounded by several irregular and broken spiral arms with very many knots. Note that the arms still start from the nucleus. The mean surface brightness decreases steadily from Sa to Sd. Compare with 19, 24 and 33.
33. NGC 300 (S 30"): SA (s) d. A very small, round nucleus, similar to a globular cluster, lies in the middle of the central region from which emerge several, highly resolved and branching arms of low surface brightness. Compare with 20 and 34. NGC 300 is very similar to M33 but of slightly later type.	34. NGC 45 (P 200"): SA (s) dm. Only the central regions of this object of very low surface brightness are visible on this print; the nucleus is very small and faint in a central, amorphous mass from which emerge very faint, irregular and branching arms. Compare with 33 and 35.
35. NGC 5204 (P 200"): SA (s) m. Only a weak spiral pattern remains in the highly resolved, irregular, but symmetrical distribution of supergiant stars and emission knots surrounding the central condensation. Compare with 34 and 37.	39. NGC 4594 (P 200"): SA (s:) a. The very bright and large spheroidal component, made up of high-velocity stars, emits most of the light of the object; the thin and weak flat component includes only very bright Population I supergiants; it is surrounded by a heavy peripheral ring of dark matter; the spiral pattern is faint and smooth. Compare with 17 and 21.
40. NGC 5746 (W 60"): SA (s:) b. The spheroidal component is very much reduced, but still contributes a large fraction of the light; the spiral pattern of bright and dark arms is well defined (save for the projection foreshortening), regular and still fairly smooth, although it begins to show some clumpiness. Compare with 18 and 22.	41. NGC 4565 (P 200"): SA (s:) bc. The still smaller spheroidal component is surrounded by an extensive flat component in which bright and dark spiral arms show much clumpiness and branching. Compare with 18, 19 and 23.
42. NGC 4244 (P 200"): SA (s:) cd. The spheroidal component is now inconspicuous in the center of a very extensive flat component in which the spiral pattern of bright and dark matter shows much irregularity and very many well resolved knots. Compare with 20 and 24.	43. NGC 5866 (P 200"): SA (s) 0 ^o: (sp). The outer envelope simulates an elliptical, but the lens (inset) shows a narrow ring of dark matter surrounding the large nucleus. Compare with 3, 44 and 45.
46. NGC 891 (P 200"): SA (s:) b (sp). This edge-on view of a typical ordinary spiral is for comparison with 43 and 44. Note how the dark matter, limited to a ring close to the nucleus in the lenticulars, spreads to the whole of the intermediate and outer parts of the spiral pattern. Compare also with 39 and 42.