Astronomy 218 Galactic Taxonomy Ultra Deep Field Eleven days of Hubble Space Telescope time reaches mV ~ 29. It reveals roughly 10,000 3’ galaxies packed in a 1/20° × 1/20° field of view. A wide variety of shapes are evident. Shapley & Curtis As modern telescopes revealed the diversity of galaxies in the early part of the 20th Century, it remained an open question whether these were part of the Milky Way. On April 26, 1920, a pair of lectures were delivered to the National Academy of Sciences by Harlow Shapley (1885 - 1972) and Heber Curtis (1872 - 1942) on “The Scale of the Universe”. Shapley argued that Universe had only one large galaxy, with a diameter of 100 kpc, with the Sun offset from the center and that spiral nebulae were nearby gas clouds. Curtis argued that Universe had many galaxies (island universes) like the Milky Way, which has a diameter <10 kpc and is centered near the Sun. Winning the Debate Curtis was generally counted the winner of the “Great Debate”, largely on the strength of his public speaking. On the basis of evidence, the debate was a draw. Shapley’s argument was dominated by his determination of the distribution of globular clusters. Curtis used several arguments based on stellar properties. One of his primary lines of evidence was observations of 11 novae in the Andromeda Galaxy which were on average 10 magnitudes fainter than those in our Galaxy, implying a distance > 150 kpc. Shapley countered this argument with the observed brightness of the “nova” S Andromedae observed in 1885, which we now know was a supernova. The Distance to Andromeda With an angular size of 4°, a size comparable to the Milky Way (R ~ 25 kpc) would imply a distance of ~ 778 kpc. Thus the Island Universe theory required a much larger Universe. Edwin Hubble’s (1889-1953) observations of Cepheids in Andromeda settled the debate, yielding d ~ 275 kpc. Hubble mistook the metal-rich Cepheids in Andromeda for dimmer metal-poor W Virginis variables (Type II Cepheids), leading to a factor of 2 underestimate. Messier & NGC The Andromeda Galaxy has several aliases, most importantly Messier 31 (M31) and NGC 224. These indicate membership in 2 of the great astronomical catalogs. In 1771, Charles Messier compiled a list of northern objects that were often mistaken for comets. His final list, published in 1784, had 103 members, 34 of them galaxies. From 1888 to 1905, J. L. E. Dreyer published the New General Catalogue of Nebulae and Clusters of Stars (NGC) using observations from William & John Herschel and others. It ultimately contained 7840 objects in the northern and southern sky. Coma Cluster The Coma cluster of galaxies, >1000 strong, has a mean distance of 100 Mpc, in the direction of the constellation Coma Berenices. Almost every object visible is a galaxy. Zooming in reveals a myriad of galactic shapes. Hubble’s Classification In 1926 Edwin Hubble introduced a morphological classification scheme for galaxies which is still used today. Hubble’s “tuning fork” was originally envisioned as a evolutionary path, from left to right. It remains a convenient way to remember the galaxy classifications, although it has no physical meaning. Hubble Type E Ellipticals are classified according to their apparent shape from E0 (spherical) to E7 (the most elongated) M49 E2 M84 E3 M110 E5 The classification is based on the ratio of the projected semi- major and semi-minor axes, q = b/a. The familiar eccentricity is expressed e = (1−q2)½. In Hubble’s scheme, an En classification is derived from the ellipticity ε = 1 − q, with n = 10 ε. Elliptical Galaxies Elliptical galaxies appear as smooth glowing elliptical blobs. They have no spiral arms and no disk. They come in many sizes, from giant ellipticals with 1012 stars (L ~ 1011 L⊙), down to dwarf ellipticals < 106 stars. Ellipticals in general contain little, if any, cool gas and dust, and little evidence of ongoing star formation, but there are exceptions to this rule. The surface brightness I(r) (brightness per unit area) M87 E0p of brighter ellipticals, like M87, generally exhibits a log I ∝ −r¼ radial dependence. Lenticular Galaxies Intermediate between the elliptical and spiral galaxies in Hubble’s scheme, S0 galaxies have a disk and large bulge, but no spiral arms (SB0s exhibit a central bar). They have little interstellar gas, but considerable dust may be present. Lenticular refers to their similarity in shape to a convex lens NGC 1201 S0 NGC 2859 SB0 Hubble Type S Hubble’s classification for spiral galaxies, Sa-Sb-Sc(-Sd), are not determined by the nature of the spiral structure, but by the relative size of their central bulge, Sa has the largest bulge, Sb is smaller, and Sc is the smallest. M81 Sa M51 Sb NGC 2997 Sc Spiral Galaxies Approximately ¾ of large galaxies are classified as spiral, either type S or SB. NGC 5679 Spiral structure was first noted in 1845, in M51, the Whirlpool Galaxy, using a 1.8 m reflector. The surface brightness I(r) of their disks generally falls off exponentially (log I ∝ −r) with radius, while the bulge follows log I ∝ −r¼. Sa galaxies tend to have less gas & dust, but more tightly wound spiral arms with Sb and Sc galaxies showing progressively looser spirals, but more gas and dust. These correlations are, however, not perfect. Seeing Spirals The rational for classifying spirals by their bulge size is the difficulty in identifying spiral structure in edge- on galaxies. The Sombrero galaxy exemplifies this problem, with no visible spirals but a large central bulge marking it as type Sa. Hubble Type SB Many, perhaps even most, spiral galaxies exhibit a strong central bar, like the Milky Way. The Hubble classification SBa-SBb-SBc is also determined by the relative size of the central bulge. NGC 1300 SBa NGC 1365 SBb NGC 6872 SBc The Milky Way (probably Hubble type SBbc) and Andromeda Galaxy (Sb) are fairly typical of large, bright spiral galaxies, with luminosities L ~ 1010 L⊙. Magellanic Spiral Gerard De Vaucouleurs (1918-1995) extended the Hubble classification. He added type Sd and well as type Sm & SBm, the Magellanic spirals. Named for the Milky Way’s satellite Large Magellanic Cloud, these show rudimentary spiral arms with very active star formation and, in the case of the LMC and other SBm, strong central bars. The LMC, and presumably other members of this type, are 10 lower in mass, with MLMC ~ 10 M⊙ ~ 1/10 MMW. Hubble Type Irr Many of De Vaucouleurs new classifications were originally members of Hubble’s Irregular classification, Type Ir. Our neighboring Small Magellanic Cloud, shown here with the Large Magellanic Cloud, remains type Ir. The SMC has a mass MSMC ~ 7 × 109 M⊙. The SMC is somewhat bar shaped, leading some to classify it as SBm pec. Irregular Galaxies The irregular galaxies have a wide variety of shapes. AM 0644-741 Irr I NGC 1569 Irr II Most have considerable gas and dust, with copious star formation present. Type Irr I (e.g. AM 0644-741) show some structure, but too distorted to fit type S or E, while Type Irr II (e.g. NGC 1569) are truly structure-less. Dwarf Galaxies Hubble’s original classification scheme was limited to luminous galaxies with high surface brightness, a result of the photographic technology available in the 1920s. The lowest luminosity galaxies (L < 109 L⊙), called dwarf galaxies, tend to be diffuse and spread over a wider area of the sky, further reducing their surface brightness. Dwarf spirals (R < 3 kpc) are NGC 1705 very rare, with most dwarf galaxies being dwarf irregular (dI) or dwarf ellipticals (dE). The smallest are called dwarf spheroidals (L < 3 × 107 L⊙). Nearby Dwarves The closest galaxies to the Milky Way, Canis Major Dwarf (8 kpc) and Sagittarius Dwarf Elliptical (20 kpc), also called SagDEG, are both dwarfs. Low surface brightness makes them difficult to find (SagDEG was discovered in 1994, CMa Dwarf in 2003 and is disputed), but dwarfs are estimated to make up 80-90% of local galaxies. Galactic Taxonomy Like stellar type and luminosity class, Hubble’s morphological classification scheme allows us to find the common elements among the many galaxies. Also like stellar type it does not tell us how galaxies are related. Visible Light The visible light from a spiral galaxy is dominated by the stellar bulge and disk components. While the halo has a similar luminosity to the bulge, it is much more diffuse. The disk is populated by newborn massive O & B stars, each M31 with the luminosity of 108 M dwarfs, giving it a blue cast, with Sc and Sb bluer than Sa. The average over the galaxy is similar to stars of F or G spectral type (Teff ~ 7000 K) including strong absorption lines. Seeing Spiral Arms The HII regions associated with newly formed stars add a significant emission line contribution to the visible galactic light, concentrated in the spiral arms. M31 The majority of the UV radiation from a galaxy is due to these same O & B stars, thus a UV image is also dominated by the spiral arms. Light from Ellipticals An elliptical galaxy lacks the young stars responsible for the spiral’s emission nebulae and ultraviolet light. The elliptical’s older stellar population is redder than that of a spiral, similar to an orange K star (Teff ~ 4000 K) with strong NGC 5195 M51 (NGC 5194) absorption lines. Some ellipticals show a second spectral peak toward the UV, believed to result from blue horizontal branch stars. Infrared While M dwarf stars contribute significant luminosity in the near infrared, galactic light in the middle and far infrared (10-100 μm) is dominated by warm dust (T ~100 K).