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Galaxy Types Galaxy Types The galaxies which Hubble studied were often spiral in form, like M31, but not all galaxies are spirals. Our nearest neighbours, the Magellanic Clouds, have little organised structure and are prototype irregular galaxies. The other main variety, or `morphological type', of large galaxy are the ellipticals, which are regular in shape and free of substructure (e.g. M87 in Virgo). The appearance of elliptical galaxies depends just on the population of stars they contain; as they emit most light at longer wavelengths they must contain mostly red stars. Spirals contain gas and dust between the stars and the spiral patterns are delineated by both dark dust lanes and bright regions where new stars are forming from the gas. As a population of young stars will contain bright blue ones, spirals are also bluer in colour than ellipticals. Hubble (again!) came up with the system which serves as the starting point for all more complex classifications; the famous `tuning fork' diagram. The handle comprises the elliptical galaxies, running from circular (as projected on the sky) E0s to more and more flattened E1s to E7s. Here the numeral (0 to 7) represents the elongation of the galaxy's image via the quantity 10(1 b=a), where b=a is the ratio of the short (minor) to long (major) axis lengths, e.g. an axis ratio of 0.7:1− corresponds to E3. The prongs contain spiral galaxies. While elliptical galaxies are ellipsoidal in their 3-D shape, spiral galaxies are basically flat discs. They can look very elongated if seen edge-on, but they often also have a central spheroidal component known as the `bulge'. The characteristic spiral pattern, produced by the bright spiral arms, can be more or less tightly wound. Sa galaxies have the most tightly wound arms, while the pattern in Sb, Sc and Sd galaxies becomes progressively more open (that is, the `pitch angle' of the spiral becomes larger). In step with this, spirals show a decrease in the importance of their spheroidal components, i.e. Sa s have large bulges while Sc s have small bulges and bulges are almost non-existent in Sd galaxies. At the end of the sequence, Hubble placed the Irregulars, which are again ‘flat’ but with chaotic patterns of bright regions. Intermediate are the Sm s, with fragmentary arm-like structures, and Irregulars are nowadays often referred to as Im galaxies (the `m' in each case standing for Magellanic). The reason for there being two prongs to the fork is that spirals can also be separated into two sequences depending on whether the spiral arms start from the central bulge or from the ends of a further component, a central `bar' { the types SBa, SBb, etc. Roughly half of all spirals are barred. Hubble later added another category, the S0 (or lenticular) galaxies, where the prongs join the handle. They possess large bulges and disc components with no sign of any spiral pattern, and can be viewed as intermediate between the ellipticals and spirals. Elliptical and S0 galaxies are often referred to jointly as `early type' galaxies, and Sa s as early type spirals, while Sc s etc. are late type spirals, because Hubble conjectured that they might form an evolutionary sequence. More elaborate classification schemes for spirals exist, e.g. filling in intermediate types such as Sab or Sbc, and the intermediate family of weakly barred systems, SABa and so on. (In this system the original unbarred galaxies are SAa etc.). de Vaucouleurs introduced a numerical sequence to represent the main morphological types. These run from T = 5 up to 1 for ellipticals through to S0s, 0 for S0/a galaxies, 1 for Sa galaxies and so on up to 5 for− Sc and−9 for Sm, i.e. one numerical class for each Hubble sub-class. Class 10 is used for irregulars. van den Bergh's classsification system attempts to indicate the intrinsic luminosity of a galaxy. He found that the luminosity of a spiral galaxy correlated roughly with the structure of its arms. More luminous spirals had better developed, well defined continuous arms, while low luminosity ones had weak, patchy arms. The former are sometimes called `grand design' spirals, the latter ‘flocculent' spirals. A luminous Sc galaxy with very clearly defined arms would then be an ScI and a less bright one with indistinct arms an ScIII, the Roman numerals denoting the `luminosity class'. Luminosities and Sizes If we know the distance to a galaxy and measure its apparent brightness, then we can use the inverse square law to deduce its intrinsic luminosity (power output). When discussing galaxies (at least at optical wavelengths) astronomers seldom work in conventional `physics' units such as Watts, usually preferring Solar luminosities, L , or working in magnitudes (see Note). 11 10 Our Galaxy contains 10 stars and has a luminosity around 2 10 L . Given the Sun's (blue) absolute magnitude of 5.48,∼ this implies a total absolute magnitude×for the Galaxy of M 20:3. B ' − 1 Our giant neighbour M31 has MB 20:8 (i.e. it is about 1.5 times as luminous). It was originally thought that the luminosity range of'galaxies− was quite small, with the Magellanic Clouds representing the faint tail of the distribution. (They have MB 18 and 16:5.) However, this is due to what is called a `selection'effect’,− not−the true state of affairs. Very luminous galaxies may really be rare, but we will be biassed against including low intrinsic luminosity systems in our samples, since they will need to be very nearby in order to look bright to us. On the other hand, very luminous galaxies will be seen easily throughout some large volume of space. Exactly this problem is encountered with stars. The apparently bright stars like Vega and Rigel are mostly (quite distant) stars intrinsically much brighter than the Sun. However, in a representative volume of space most stars are much less luminous than the Sun. The companions to the Andromeda spiral, M32 and NGC 205 are prototypes of the class of dwarf elliptical (dE) galaxies. They are 100 times less luminous than M31 itself and look like small versions of a normal (giant) elliptical galaxy∼, ellipsoidal in shape and with little or no internal structure (though some, denoted dE,N for `nucleated', contain bright central star clusters. Dwarf irregular (dI) galaxies also exist, with similar luminosities, e.g. NGC 6882, one of the galaxies in which Hubble first found Cepheids. None of these dwarf types is included properly in the tuning fork classification. Evidence for a very wide range of galaxy luminosities came in the late 1930s with Shapley's discov- ery of two `dwarf spheroidal' galaxies; companions of our Galaxy known as the Fornax and Sculptor Dwarfs. Dwarf spheroidals (dSph) are the extension to yet lower luminosities of the dEs, with Sculptor only 1/100 of the brightness of previously known systems. Thorough surveys of the sky, such as the Palomar Observatory Sky Survey (POSS) in the 1950s and the UK Schmidt Telescope (UKST) sky surveys undertaken from the 1970s, revealed even fainter examples. The lowest luminosity currently known companions to our Galaxy are the Draco and Ursa Minor dwarfs, while M31's companion And IX, discovered in 2004, is marginally fainter and thus the least luminous galaxy known. Each is −5 5 about 10 of the luminosity of M31 (i.e. a few 10 L ). At MB 8, these are similar to the magnitudes of very bright single stars. The brigh×test giant ellipticals'in− galaxy clusters, called cD galaxies, can range up to about MB 24, i.e. 25 times brighter than our Galaxy. Determining L is straightforward' once− we know the distance to a galaxy, but sizes are more problematic { galaxies do not have obvious `edges', they fade away gradually. To describe the physical size of a galaxy, we need to consider how the stars (or other constituents) are distributed as a function of distance from the galaxy's centre. For stars, this is reflected in the radial distribution of the intensity, the light emitted per unit area, or surface brightness (SB). The different galaxy types each have characteristic intensity profiles, I(R), but a general measure of the size of a galaxy is its `half- light' or ‘effective' radius. This is the radius of the circle (projected on the sky) which encloses half the total light of a galaxy. A large spiral galaxy will have an effective radius, Re, up to 10 kpc. Large E galaxies are of similar dimensions, while cD galaxies, which are surrounded by large extended envelopes, have R up to 100 kpc. Dwarf galaxies are much smaller, dSph ranging down to 200 pc. e ∼ ∼ Surface Brightness High L galaxies also have high SB (in their central regions). Dwarf galaxies are typically of low SB { they look rather diffuse and and can be hard to see against the faint sky foreground glow from our atmosphere. (This was the main reason for their relatively recent discovery). Physically the low SB is due to a low surface density of stars; whether this corresponds to a low volume density depends on how `thick' the galaxy is along the line of sight. −2 2 SB is not usually quoted in Wm , but in L /pc or magnitudes per square arc second (µ). To 26 16 translate between these, 1 L = 3:86 10 W and 1 parsec = 3:086 10 m so × × 2 −7 −2 1 L =pc = 4:05 10 Wm : (1) × For the Sun MB = 5:48, and as 1 pc subtends 0.1 radians (20626 arc sec) at the standard distance of 2 10 pc used to calculate absolute magnitudes, 1 L /pc corresponds to µB = MB + 5 log(20626) = 27:05 B magnitudes per sq arc sec (Bµ): (2) 2 Characterizing the overall SB by the mean inside the effective radius (i.e.
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