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Galaxy Classification Much of modern extragalactic astronomy deals with studying how galaxies evolve with time. If galactic evolution can be understood, then parameters such as q0 , Ω0 , and Λ will follow, along with data on the initial conditions for the formation of the universe. Before we can study evolution, however, we have to define what a galaxy is, and how it is structured. Hence, the first step to understanding galaxies is to classify them. The most famous classification scheme, of course, is the Hubble tuning-fork system, which starts out with spherical ellipticals, evolves to the transitional S0 (lenticular) galaxies, and then branches out into Sa, Sb, and Sc spirals and barred spirals. Over the years, this system has been built up, so that lenticulars are now subdivided into S01 , S02 , and S03 (depending on the smoothness of the luminosity profile, and the amount of gas present in the galaxy). In addition, there are now the additional symbols “(r),” which indicates the presence of an inner ring, “R,” which signifies the presence of an outer ring, and “(s),” which says that the spiral arms begin at the end of a bar or are traced to the galaxy’s center, rather than the galaxy’s inner ring. Needless to say, this scheme isn’t very elegant. A complementary classification system, which seeks to make a parallel with stellar evolution, is the DDO (David-Dunlap Observatory) Luminosity Classification system of van den Bergh. In this method, supergiant galaxies with well-developed bright spiral arms and bars have the Roman numeral I (like supergiant stars), and small, low-surface-brightness, irregular galaxies have the roman numeral V. Of course, since one doesn’t usually know a galaxy’s distance, it is somewhat difficult to estimate its true luminosity. The system therefore assumes that the galaxies with the most well-developed arms are also the most luminous. In the Revised Shapley-Ames Galaxy Catalog (which lists the ∼ 1300) brightest galaxies in the sky, Sandage adds the DDO luminosity classification onto his Hubble classification, so, for example, the galaxy NGC 1097 is given the type RSBbc(rs)I-II. (In other words, NGC 1097 is a very large barred Sbc spiral with an outer ring, an inner ring, and arms that begin at the end of the galaxy’s bar.) A more computer-friendly system was devised for the 2nd Reference Catalog of Bright Galaxies by de Vaucouleurs. In his system, galaxies are given a numerical T designation based on compactness. The most compact elliptical galaxies are assigned T = −6; normal ellipticals have T = −5, and lenticular galaxies have negative numbers near zero. Spiral galaxies start at T = +1 for Sa, and proceed to T = +11 for those blue, irregular galaxies that are essentially extragalactic H II regions. In de Vaucouleurs’ scheme, there is no difference between a normal spiral and a barred-spiral galaxy. In the 1970’s van den Bergh noticed that spiral galaxies in clusters seemed different from those in the field. In particular, many cluster spirals seemed to have less gas and less star-formation than their counterparts in low-density environments. (The S0 galaxies, which are spiral disks without arms or gas, are the extreme example of this phenomena.) Van den Bergh therefore defined a system where Anemic Spirals occupied the transition between regular spirals and lenticulars. In this system, sequences would be Sa → Aa → S0a, Sb → Ab → S0b, etc. For these Anemic galaxies, it is as if something is quenching their active star formation. An interesting system that is still (partially) with us and has (some) interesting uses is the classification system of Morgan. The system has two components, a “concentration” component, and a “form” component. The concentration part of the system is the observed correlation between the types of stars present in a galaxy, and how compact the galaxy is. Elliptical galaxies have mostly old stars, and their integrated light is dominated by K supergiants. These objects are also the most condensed systems; i.e., they are highly concentrated. Irregular galaxies with no central mass condensation tend to have younger stars and a corresponding earlier spectral type. Thus, Morgan defined an “a-f-g-k” concentration index based on the spectral classification of stars. For the form index, Morgan chose the (capital) letters “S” for spiral, “B” for barred spiral, “E” for elliptical, “I” for irregular, “Ep ” for elliptical peculiar (with dust), “D” for a rotationally symmetry without elliptical structure (i.e., a diffuse system), “L” for low-surface brightness, and “N” for any system with a small, brilliant nucleus (like a Seyfert galaxy). On top of this, Morgan then added a number from 1 to 7 based on apparent inclination: face-on spirals were S1, while highly elongated systems could be S7. Thus, a Morgan class might be kS4, fS1, fgB1, or gkS7. The Morgan system lives on today principally in the designation of “N” galaxies, which are sometimes used to refer to small galaxies with an active galactic nucleus, and through the identification of some galaxies as “cD”. The cD classification (which was actually defined about 5 years after the original Morgan paper) refers to galaxies in the centers of clusters which have an elliptical galaxy-like core surrounded by a huge amorphous envelope of stars. These systems are probably the largest collections of stars in the universe; since some cD galaxies have multiple nuclei, they have sometimes been described as “galaxies at lunch.” Properties of Elliptical Galaxies The first step in investigating the evolution of galaxies is to understand the properties of those galaxies today. We’ll start with the elliptical galaxies. We can summarize their properties as follows: 1) In general, elliptical galaxies, as projected on the sky, have complete 2-dimensional symmetry. The question of whether these objects are symmetric in practice, or are tri-axial is open (though dynamical modeling suggests that triaxiality can only last for a short time). Some ellipticals have “fine structure,” such as very weak ripples, shells, and boxy (not elliptical) isophotes. These signatures are weak, but real. In general, those ellipticals with fine structure are slightly bluer than equivalent galaxies without fine structure. 2) The apparent flattening of elliptical galaxies is given in the Hubble classification scheme as En, where n is defined in terms of the apparent semi-major axis, a, and semi-minor axis, b, as n = 10(a − b)/a (23.01) Ellipticals range in flattening from E0 (round) to E7. No elliptical is flatter than E7. The data are not consistent with the hypothesis that all ellipticals are E7 and appear flattened by the effects of random viewing angles. Most likely there is a spread of flattenings centered around E3, or thereabouts. 3) Rotation is not important in most elliptical galaxies. That is, E7 galaxies are not flattened due to rapid rotation. 4) There is little or no star formation in elliptical galaxies. However, the spectral energy distribution of some ellipticals turns up in the ultraviolet. (In other words, since elliptical galaxies are made up of old stars, the composite spectrum of an elliptical should look like that of a ∼ 4, 000◦ K star. However, many ellipticals are brighter at 1500 Å than they are at 2000 Å.) 5) There is very little cold interstellar medium in elliptical galaxies. However, there is x-ray gas at a temperature of about T ∼ 106 K. One can easily see where this gas comes from. The stars in an elliptical galaxy must be losing mass. If the stars are moving isotropically at σ ∼ 200 km s−1 , then the atoms of lost material, if thermalized, will have a temperature of 1 3 mH σ 2 ∼ kT =⇒ T ∼ 106 degrees K 2 2 (23.02) 6) Ellipticals are almost always found in dense environments. The few field ellipticals that exist may have swallowed their neighbors. 7) Most ellipticals have weak radial color gradients: they are redder on the inside than they are on the outside. This may be due to age (older stars have a redder turn-off mass), or metallicity (metal-rich stars are intrinsically redder than their metal-poor counterparts). 8) Elliptical galaxies populate a “fundamental plane” in luminositysurface brightness-velocity dispersion space. But there are many other reflections of this plane. For example, • Elliptical galaxy luminosity correlates with color. Large ellipticals are redder than small ellipticals. • Elliptical galaxy color correlates with absorption line strength. Redder galaxies have stronger absorption features. • Elliptical galaxy absorption features correlate with the UV upturn. Galaxies with strong absorption features have larger UV excesses. • Elliptical galaxy UV excess correlates with the number of planetary nebulae in the galaxy. Galaxies with large UV excesses have, relatively speaking, fewer planetary nebulae. 9) There are several laws that re-produce the observed luminosity profile of an elliptical galaxy. The most famous is the de Vaucouleur r1/4 -law. Under the law, the surface brightness of an elliptical galaxy, I (units of ergs cm−2 s−1 arcsec−2 ), as a function of the distance from the galaxy center, r, is given by µ log I Ie (µ ¶ = −3.33071 r re ¶1/4 ) −1 (23.03) The key scaling variables in this equation are re and Ie . The scaling length re is called the effective radius: it is the radius that encloses half the total light from the galaxy. The variable Ie is the surface brightness of the galaxy at radius re . Note that with a little math, (23.03) can be transformed to the form m = a + b r1/4 (23.04) where m is the surface brightness of the galaxy (in magnitudes per square arcsec), and a is the central surface brightness.