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Home , Andromeda Galaxy, Magellanic spiral, New General Catalogue, NGC 1569, NGC 2859, NGC 2997

218 Galactic Taxonomy Ultra Deep Field Eleven days of time reaches mV ~ 29. It reveals roughly 10,000 3’ 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 . On April 26, 1920, a pair of lectures were delivered to the National Academy of Sciences by (1885 - 1972) and Heber Curtis (1872 - 1942) on “The Scale of the ”. Shapley argued that Universe had only one large , 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 ) 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 “”, 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 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 “” S Andromedae observed in 1885, which we now know was a . 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. ’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 variables (Type II Cepheids), leading to a factor of 2 underestimate. Messier & NGC The has several aliases, most importantly Messier 31 (M31) and NGC 224. These indicate membership in 2 of the great astronomical catalogs. In 1771, 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 of Nebulae and Clusters of (NGC) using observations from William & and others. It ultimately contained 7840 objects in the northern and southern sky. The Coma cluster of galaxies, >1000 strong, has a mean distance of 100 Mpc, in the direction of the . 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 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 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 , 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 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 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 , these show rudimentary spiral arms with very active 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 , 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 , 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 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, 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 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 lacks the young stars responsible for the spiral’s emission nebulae and 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. 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). Thus spirals and irregular galaxies show strong IR emission. M51 (NGC 5194) Elliptical galaxies are much weaker sources in the infrared. Radio The bulk of radio emission in normal galaxies comes from gas clouds, hence spirals are brighter than ellipticals.

This 21cm 2008 map of local galaxies shows the dominance of the spiral structure in the radio. X-rays X-ray emission from galaxies is dominated by point sources, typically X-ray binaries and young supernova remnants, −7 with LX ~ 10 LB. Hot coronal gas, the result of long vanished supernova remnants, also contributes significantly in spirals. M101 The little gas seen in ellipticals is also hot, T ~ 106 K, and is thus visible in X-rays. It is likely the result of stellar mass −1 loss with �rms ~ 200 km s . Multi- Galaxy A multi-wavelength view of the Sb spiral M81.

X-ray: ROSAT UV: Astro-1 BVR: KPNO

near-IR: Spitzer mid-IR: Spitzer Radio: VLA As in stars, the (or ) of spectral lines, z = Δλ/λ, allows us to measure the radial velocity.

�r = cz in the non-relativistic limit. Unlike most stars, galaxies are extended objects, thus we can measure the radial velocity as a function of position in the galaxy and determine the orbits of the stars. The image we collect is a two-dimensional projection of the three-dimensional galaxy, limiting our ability to determine the true kinematics. For a spiral galaxy, the inherently circular disk simplifies the analysis. The inclination of a galaxy, i, can be determined from the aspect ratio of the 2D projection, q = cos i (i = 0° for face-on, i = 90° for edge-on). Rotation Curve Measuring allows the determination of the radial velocity along the apparent long axis of a galaxy, �r(R), which is related the the orbital velocity, �c.

�r(R) = �c(R) sin i + �r,0

�r,0 is the radial velocity of the center of the galaxy and the average of the galaxy as a whole. Rearrangement reveals

Galactic Mass Mapping the orbital velocities in the Andromeda galaxy reveals a nearly flat rotation curve out to nearly 3°. At the distance of M31, d = 700 kpc, this is R ≈ 36 kpc, −1 �c(R = 36 kpc) ≈ 230 km s . 41 11 ≈ 9 × 10 kg ≈ 4 × 10 M☉ All spiral galaxies show similar rotation curves, with no sign of Keplerian falloff, indicating that missing mass is a universal problem. 66-90% of galactic matter is dark. Correlations While wide dispersions are seen in the rotation curves of spiral galaxies, certain trends are evident.

The maximum velocity, Vmax , increases with increasing galactic B-band luminosity (LB).

For galaxies of similar LB, Vmax tends to be higher in Sa type −1 −1 galaxies than in Sc, 〈Vmax〉 = 299 km s for Sa, 222 km s and 175 km s−1 for Sc. However, the ranges are considerable, 167–367 km s−1 for Sa, 144–330 km s−1 for Sb, 99–304 km s−1 for Sc. −1 Irr exhibit much smaller Vmax = 50–70 km s .

There is also a correlation between a galaxy’s LB and its radius, measured at constant surface brightness. Motions in Ellipticals Spectra reveal that the kinematics of ellipticals are very different that spirals, with relatively slow rotational speeds, �r(R), but large random velocity components characterized by a , σ(R). For example, in NGC 4365, �r < 80 km s−1 but σ < 270 km s−1. NGC 4365 NGC 4365 evidences a “kinematically decoupled core” (rotating around a different axis), which is fairly common in ellipticals. It also shows evidence of some younger stars (2-8 Gyr old). Stellar Pressure As in the halo & bulge of a spiral galaxy, it is random velocities that keep an elliptical galaxy from collapsing. By analogy to a thermal gas, we can refer to such a system as pressure-supported, to differentiate them from rotationally-supported disks. We can extend the analogy and think of 〈�2〉 like a “temperature”. Unfortunately, we measure only �r. If the velocity dispersion is isotropic, the same in all 2 2 directions (for example, �rot << σ), 〈� 〉 = 3 〈σ 〉. Next Time Observing Galaxies