Physical Parameters Along the Hubble Sequence

Annu. Rev. Astron. Astrophys. 1994. 32: 115-52
Copyright © 1994 by Annual Reviews. All rights reserved

5. SUMMARY

In this review we have discussed quantifiable properties of galaxies and their dependence on morphological type. Why the present-day galaxy population follows these trends is a major challenge of theories of galaxy formation and evolution. Here, we conclude with the following summary of the behavior of the ``typical'' galaxies found in today's large catalogs:

1. There are three general categories of galaxies: elliptical, classical spiral, and dwarf. This last category includes Sd and Sm. The nature of S0's remains uncertain, perhaps because they are indeed a transition type.

2. There is a pronounced spatial segregation of morphologies in the sense that early-type galaxies are more clustered.

3. For a volume-limited sample, we find that the classical spirals - Sa, Sb, Sc - show a small systematic variation in median size and in median luminosity becoming larger and brighter as they become later. There is a suggestion of a similar trend for total mass. We do not find these dependences in the flux-limited sample.

4. The late-type dwarfs are indeed different from the classical galaxies. The dwarfs become smaller, fainter, and less massive over the range Scd-Im.

5. The mass-to-luminosity ratio is essentially constant over the entire sequence S0-Im. Although this ratio is distance dependent, it shows essentially no Malmquist bias.

6. The total mass surface density sigma _T decreases systematically over the sequence S0/a-Im.

7. There are a number of systematic trends with type in these data that are clearly identifiable with star formation processes. Thus color is a measure of past and present star formation activity, as are X-ray and radio radiation. The H II region parameters relate to current activity, while the cool gas components measure the potential for future star formation. The trend of abundances is also consistent with the above picture but FIR measurements are not.

8. In all of this we stress the wide range to be found for any parameter within any type. The range is always larger than even the most pessimistic error estimates. Much of this wide range is intrinsic. The only instance where overlap does not occur at least over the interquartile interval is for Im's versus classical spirals for the three basic parameters of R_lin, L_B, and M_T.

9. The wide range in any of the parameters described above must not be interpreted to mean that parameter space is unbounded. There is an upper limit to L_B as displayed in Figure 1. But similarly there are lower bounds to at least the classical spirals. In particular, there are no very faint or very small Sb galaxies.

10. In such a summary it seems worthwhile to restate the morphological distinctions for the sequence elliptical - classical spiral - dwarf. The sequence is one of bulge-to-disk ratio, largest for the ellipticals and zero for the dwarfs. It is also the development of spiral arms, ending with what might be thought of as a limiting case, an Im as just an arm.
ACKNOWLEDGEMENTS

We thank Riccardo Giovanelli for his collaboration in developing the database used herein and both he and David Hogg for many discussions relevant to our review. Thanks are also given to Harold Corwin for suppling us with a digital copy of the RC3 and to Fran Verter, Trinh Thuan, Jim Condon and Joel Bregman for answering inquiries and supplying data. M.P.H. receives support through NSF grants AST-9014850 and AST-9023450.

1.	There are three general categories of galaxies: elliptical, classical spiral, and dwarf. This last category includes Sd and Sm. The nature of S0's remains uncertain, perhaps because they are indeed a transition type.

2.	There is a pronounced spatial segregation of morphologies in the sense that early-type galaxies are more clustered.

3.	For a volume-limited sample, we find that the classical spirals - Sa, Sb, Sc - show a small systematic variation in median size and in median luminosity becoming larger and brighter as they become later. There is a suggestion of a similar trend for total mass. We do not find these dependences in the flux-limited sample.

4.	The late-type dwarfs are indeed different from the classical galaxies. The dwarfs become smaller, fainter, and less massive over the range Scd-Im.

5.	The mass-to-luminosity ratio is essentially constant over the entire sequence S0-Im. Although this ratio is distance dependent, it shows essentially no Malmquist bias.

6.	The total mass surface density _T decreases systematically over the sequence S0/a-Im.

7.	There are a number of systematic trends with type in these data that are clearly identifiable with star formation processes. Thus color is a measure of past and present star formation activity, as are X-ray and radio radiation. The H II region parameters relate to current activity, while the cool gas components measure the potential for future star formation. The trend of abundances is also consistent with the above picture but FIR measurements are not.

8.	In all of this we stress the wide range to be found for any parameter within any type. The range is always larger than even the most pessimistic error estimates. Much of this wide range is intrinsic. The only instance where overlap does not occur at least over the interquartile interval is for Im's versus classical spirals for the three basic parameters of R_lin, L_B, and M_T.

9.	The wide range in any of the parameters described above must not be interpreted to mean that parameter space is unbounded. There is an upper limit to L_B as displayed in Figure 1. But similarly there are lower bounds to at least the classical spirals. In particular, there are no very faint or very small Sb galaxies.

10.	In such a summary it seems worthwhile to restate the morphological distinctions for the sequence elliptical - classical spiral - dwarf. The sequence is one of bulge-to-disk ratio, largest for the ellipticals and zero for the dwarfs. It is also the development of spiral arms, ending with what might be thought of as a limiting case, an Im as just an arm.