Last updated: Feb-29-01

Stellar and Gaseous Content of Normal Galaxies

Reviews are given by [Sandage, A. in Galaxies and the Universe (Sandage, A., Sandage, M.; Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 1] and [Spinrad, H., Peimbert, M. in Galaxies and the Universe (Sandage, A., Sandage, M.; Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 37]

Populations in Local Group Galaxies:

Cooling Flows in Clusters and Galaxies (A.C. Fabian. ed.), NATO ASI Series C 229, Dordrecht, Kluwer (1988).
The World of Galaxies (H.C. Corwin Jr., L. Bottinelli, eds.), Berlin, Springer (1989).
The Interstellar Medium in External Galaxies: Summaries of Contributed Papers (D.J. Hollenbach, H.A. Thronson, eds.), NASA Conf. Publ. 3084 (1990).
Galactic and Intergalactic Magnetic Fields (R. Beck, P.P. Kronberg, R. Wielebinski, eds.), Int. Astron. Union Symp. 140, Dordrecht, Kluwer (1990).
The Magellanic Clouds (R. Haynes, D. Milne, eds.), Int. Astron. Union Symp. 148, Dordrecht, Kluwer (1991).
Dynamics of Disk Galaxies (B. Sundelius, ed.), Proceedings of a Conference at Varberg Castle, Goeteborg, Dept. of Astron. & Astrophys. (1991).
Panchromatic View of Galaxies - Their Evolutionary Puzzle (G. Hensler, C. Theis, J. Gallagher, eds.), Gif-sur-Yvette, Editions Frontieres (1994).
Hodge, P., Annu. Rev. Astron. Astrophys. 27 (1989) 139.

The stellar and gaseous content of galaxies varies systematically along the morphological sequence from E to Sm. Variations along the spirals from Sa to Sd [ Sandage, A. in Galaxies and the Universe (Sandage, A., Sandage, M.; Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 1.]:

1) increasing absolute luminosity of the brightest stars in regions of spiral arms,

2) increasing percentage of mass in form of gas and dust,

3) increasing size and number of HII regions in the spiral arms,

4) progressively bluer (B - V) and (U - B) colors, indicating progressively earlier stars that contribute most of the light.

Physical Parameters along the Hubble Sequence: [Roberts, M.S. & Haynes, M.P., Annu. Rev. Astron. Astrophys. 32 (1994) 115.]

Elliptical Galaxies: [de Zeeuw, T. & Franx, M., Annu. Rev. Astron. Astrophys. 29 (1991) 239.]

On the Understanding of the Hubble Sequence: [Lake, G., Sky Tel. 83 (1992) 515.]

On Dust and Gas Properties of Galaxies: [Sakamoto, K., Ishizuki, S., Kawabe, R. & Ishiguro, M., Astrophys. J. 397 (1992) L27.]

Already broad-band photographic or photoelectric colors show that hot stars are important contributors to the blue light in the centers of Irr and Sc systems and in the outer spiral-arm regions of many Sc, Sb and possibly Sa systems. No early-type stars are needed to explain the colors of centers of most Sa, Sb and E systems.

A more quantitative approach to galaxy population studies are 1) photoelectric narrow-band measurements of colors (continuum) and line indices sensitive to stellar temperature, luminosity and abundance differences, and 2) equivalent widths from low- or medium-dispersion slit spectra.

The spectroscopic categories of the low-dispersion approach to the determination of the stellar content are summarized in Table 1.

**Table 1.** Low-Dispersion Spectroscopic Categories for Galaxies [Morgan, W.W. & Osterbrock, D.E.: Astron. J. 74 (1969) 515], extended by. [Spinrad, H., Peimbert, M. in Galaxies and the Universe (Sandage, A.R., Sandage, M., & Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 37.]

Category	Description	Spectroscopic identification	Typical galaxies

Orion	HII regions, blue stars; often irregularly shaped galaxy	Strong emission lines like the Orion Nebula, a hot-star continuum He I absorption line, other indicators of types B...F in the blue ( 3820 of He is a good indicator of early B stars)	NGC 4214, NGC 4449, LMC bar, M82 core
Intermediate	Nuclear regions of Sc galaxies, main bodies of giant spirals. Yields types f and fg	Blue spectral-type, near F8, very composite spectrum. 3727 [O II] emission common	NGC 5194, NGC 4321
Amorphous	Centers of big Sb, Sa systems. Main bodies of giant E galaxies	The type K0 in most cases, type closer to M0 in the deep red. Emission lines weak	M31, M81, NGC 4472
Weak-lined	Metal-poor population of dwarf E system	Globular-cluster-like; H lines intermediate, metals weak, no emission lines	Dwarf E's like NGC 205, possibly NGC 5195

Infrared studies of stellar content of galaxies: [Aaronson, M. in Infrared Astronomy, Int. Astron. Union Symp. 96 (Wynn-Williams, C.E., & Cruitshank, D.P., eds.), Reidel, Dordrecht (1981) p. 297].

The IR is important as most of the radiation of normal galaxies is emitted in the region > 1 µm.

Review of integrated energy distribution of galaxies: [Whiteford, A.E. in Galaxies and the Universe (Sandage, A.R., Sandage, M., & Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 159.]

Helium abundance [Spinrad, H., Peimbert, M. in Galaxies and the Universe (Sandage, A.R., Sandage, M., & Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 37] :

5 Sc systems	N(He) / N(H) = 0.120,
5 Irr systems	= 0.095.

The Ellipticity of Galaxies

The observed apparent ellipticities (axial ratios) of galaxies are used together with certain plausible assumptions on their intrinsic flattening (for spiral and irregular galaxies) or they are compared with frequency functions of ellipticities of spherical systems in order to derive their true ellipticities. However, for elliptical galaxies the situation is more complex. Triaxial configurations have been assumed in order to explain the observations [e.g. Kormendy p. 115] in [Morphology and Dynamics of Galaxies (J. Binney, J. Kormendy, S.D.M. & White), Sauverny, Geneva Observatory (1982).]

The frequency function of true ellipticities of spheroidal systems

e = 1 - c / a = 1 - q₀

can be derived from the observed frequency function of apparent ellipticities

= 1 - b / a = 1 - q

under the assumption of random orientation of the spin axis, [de Vaucouleurs, G. in Stellar Systems (Figure, S., ed.), Hdb. Physik 59, Springer, Berlin (1959). p. 275 ch. II, b], with a = true major axis, b = apparent minor axis (projection effect), c = true minor axis of the flattened spheroid.

An investigation of isophotal diameters (µ_B = 25.0 mag/arcsec²) of more than 2000 galaxies gives the following conclusions:

(1) Elliptical Galaxies: The distribution of true ellipticities among E galaxies is definitely not uniform (as was supposed in earlier papers) up to 2/3, i.e. q₀ > 1/3. In particular spherical galaxies (q₀ = 1) are rare. On the other hand the sharp cutoff of the observed (apparent) ellipticity at type E 7 indicates that no elliptical galaxies with a high degree of flattening (like the spirals) exist. The best-fit Gaussian model gives <e> 0.36 with a small cosmic dispersion = 0.1.
See also

Radford, S.J.E., Astron. Astrophys. 262 (1992) 13.
Nakai, N., Kuno, N., Handa, T., & Sofue, Y., Pub. Astron. Soc. Japan. 46 (1994) 527.

(2) Lenticular Galaxies tend to be more flattened than ellipticals but the analysis suggest that two groups are present:

a major group ( 90% on of the sample) with <e> = 0.65
a minor group ( 10%) with <e> = 0.35

(3) Spiral Galaxies from S0 to Sm have ellipticity functions similar to the lenticulars with

a major group ( 70%) with <e> = 0.7...0.8
a minor group ( 30%) with <e> = 0.4
i.e., spirals having small bulges or large bulges respectively.

q₀ decreases smoothly along the Hubble sequence from E (morphological parameter t = - 5) to Sd (t = 7).

Beyond stage Sd the ellipticity decreases rapidly so that Magellanic spirals have typically q₀ = 0.

Examples are given in [de Vaucouleurs, G.. Freeman, K.C. Vistas in Astron. 14 (1972) 163].

The existence of thick disks [Burstein, D. in Photometry, Kinematics and Dynamics of Galaxies (Evans, D.S., ed.), University of Texas (1979). p. 81] and extremely thin disks [Goad, J.W., & Roberts, M.S. Astrophys. J. 250 (1981) 79] has been shown observationally.

Intrinsic flattening of 168 E, 267 S0 and SB0, and 254 ordinary spirals are investigated in [Sandage, A.R., & Freeman, K.C. Astrophys. J. 160 (1970) 83].

The ratio of the two components (spheroidal and flat) of the luminosity distribution varies smoothly as a function of the morphological parameter t along the spiral sequence and it suggests that the Hubble sequence from Sa to Sd is basically an angular momentum sequence. The low velocity of rotation found even in flat systems, as compared with the velocity dispersion, ruled out models attributing the flattening only on rotation.

Flattening is a dynamical property which cannot change significantly in times less than the relaxation time (about 10¹² ... 10¹⁴ years). Therefore the difference in intrinsic flattening between E and S galaxies shows that one type cannot evolve into the other. A basic difference must have existed between these groups already at the time of their formation [Sandage, A.R. in Galaxies and the Universe (Sandage, A.R., Sandage, M.; & Kristian, J., eds.) = Stars and stellar Systems Vol. IX, Univ. Chicago Press (1975). p. 1].

The isophotes within one elliptical galaxy are not necessarily concentric, the ellipticity can change from the central to the outer parts. Five classes can be distinguished [Bertola, F., & Galletta, G. Astron. Astrophys. 77 (1979) 363]: increasing ellipticity, decreasing, with maximum, with minimum and constant. Furthermore, the systems can be twisted, i.e. the direction of the major axis of the isophote can change. Table 2 gives some examples.

Bertola, F., & Galletta, G. [Astrophys. J. 226 (1978) L115] describe 5 galaxies with dust lanes crossing a luminous elliptical-like body along the minor axis (NGC 1947, NGC 5128, NGC 5363, and the galaxies associated with Cyg A and PKS 1934-63). They suggest a new class of galaxies: prolate stellar structures cut equatorially by gaseous planes. The dynamics of these systems is complicated and not yet understood.

**Table 2.** Examples for ellipticity trend and twisting.
Bertola, F., & Galletta, G. Astron. Astrophys. 77 (1979) 363.

Galaxy	Classification	Ellipticity Trend	Twisting

NGC 0205	E5 pec	maximum	present
NGC 1265	S0	increasing	suspected
NGC 1270	E3	maximum
NGC 1278	E2 pec	minimum
NGC 1281	E5	maximum
CR 32	E1	increasing	present
IC 0312	E6	maximum
NGC 4125	E6 pec	maximum	present
NGC 4486	E0 pec	increasing	present
NGC 4494	E1	constant

On the True Shape of Galaxies: [Wiklind, T., Combes, F., & Henkel, C., Astron. Astrophys. 297 (1995) 643].