AA49CH09-vanderKruit ARI 5 August 2011 9:18 ANNUAL REVIEWS Further Click here for quick links to Galaxy Disks Annual Reviews content online, including: 1 2 • Other articles in this volume P.C. van der Kruit and K.C. Freeman • Top cited articles 1 • Top downloaded articles Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, • Our comprehensive search The Netherlands; email: [email protected] 2Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, ACT 2611, Australia; email: [email protected] Annu. Rev. Astron. Astrophys. 2011. 49:301–71 Keywords The Annual Review of Astronomy and Astrophysics is disks in galaxies: abundance gradients, chemical evolution, disk stability, online at astro.annualreviews.org formation, luminosity distributions, mass distributions, S0 galaxies, scaling This article’s doi: laws, thick disks, surveys, warps and truncations 10.1146/annurev-astro-083109-153241 Copyright c 2011 by Annual Reviews. Abstract ! All rights reserved The disks of disk galaxies contain a substantial fraction of their baryonic mat- 0066-4146/11/0922-0301$20.00 ter and angular momentum, and much of the evolutionary activity in these galaxies, such as the formation of stars, spiral arms, bars and rings, and the various forms of secular evolution, takes place in their disks. The formation and evolution of galactic disks are therefore particularly important for under- standing how galaxies form and evolve and the cause of the variety in which they appear to us. Ongoing large surveys, made possible by new instrumen- tation at wavelengths from the UV (Galaxy Evolution Explorer), via optical (Hubble Space Telescope and large groundbased telescopes) and IR (Spitzer Space Telescope), to the radio are providing much new information about disk galaxies over a wide range of redshift. Although progress has been made, the dynamics and structure of stellar disks, including their truncations, are still not well understood. We do now have plausible estimates of disk mass-to- light ratios, and estimates of Toomre’s Q parameter show that they are just locally stable. Disks are mostly very flat and sometimes very thin, and they have a range in surface brightness from canonical disks with a central surface 2 brightness of about 21.5 B-mag arcsec− down to very low surface bright- by California Institute of Technology on 01/21/14. For personal use only. nesses. It appears that galaxy disks are not maximal, except possibly in the largest systems. Their HI layers display warps whenever HI can be detected beyond the stellar disk, with low-level star formation going on out to large Annu. Rev. Astro. Astrophys. 2011.49:301-371. Downloaded from www.annualreviews.org radii. Stellar disks display abundance gradients that flatten at larger radii and sometimes even reverse. The existence of a well-defined baryonic (stellar + HI)Tully-Fisherrelationhintsatanapproximatelyuniformbaryonictodark matter ratio. Thick disks are common in disk galaxies, and their existence appears unrelated to the presence of a bulge component; they are old, but their formation is not yet understood. Disk formation was already advanced at redshifts of 2, but at that epoch disks were not yet quiescent and in full ∼ rotational equilibrium. Downsizing (the gradual reduction with time in the mass of the most actively star-forming galaxies) is now well-established. The formation and history of star formation in S0s are still not fully understood. 301 AA49CH09-vanderKruit ARI 5 August 2011 9:18 1. INTRODUCTION AND OVERVIEW 1.1. Historical Background Disks are the most prominent parts of late-type spiral galaxies. The disk of our own Milky Way Galaxy stretches as a magnificent band of light from horizon to horizon, particularly from a dark site at southern latitudes, as in the Astronomy Picture of the Day for January 27, 2009 (Pacholka 2009). Its appearance with the Galactic Center high in the sky is reminiscent of the beautiful edge-on spiral NGC 891 (Hawaiian Starlight 1999), which can be regarded as a close twin to our Galaxy (van der Kruit 1984). What might in hindsight be called the first study of the distribution of stars in a galaxy disk is William Herschel’s famous cross-cut of the sidereal system based on his star gauges (On the Construction of the Heavens, Herschel 1785), from which he concluded that the distribution of stars in space is in the form of a flattened system with the Sun near its center. His counts, on which he based this section of the system, were performed along a great circle on the sky almost perpendicular to the Galactic equator, crossing it at longitudes 45◦ and 225◦ and missing the poles by 5◦.ComparisontomodernstarcountsshowsthatHerschelcountedstars consistently down to magnitude V 15 (van der Kruit 1986). ∼ The next major step in the study of the distribution of stars in the Milky Way was that of Jacobus Kapteyn (Kapteyn & van Rhijn 1920, Kapteyn 1922). In spite of his deduction that interstellar extinction must have the effect of reddening of starlight with increasing distance, Kapteyn (1909a,b, 1914) was unable to establish the existence of interstellar absorption in a convincing way and was led to ignore it. As a result, he ended up with a model for what we now know to be the Galactic disk that erroneously had the Sun located near its center. The proceedings, The Legacy of J.C. Kapteyn (van der Kruit & van Berkel 2000), have a number of interesting studies of Kapteyn and astronomy in his time. Even before galaxies were established to be island universes, spiral structure was discovered in 1845 by William Parsons, the Third Earl of Rosse. His famous drawing of M51 appears in many textbooks, popular literature, and books on the history of astronomy. The importance of the disk and the development of spiral structure were the basis for the classification scheme that John Reynolds and Edwin Hubble developed; the background and early development of galaxy classification were described by Sandage (2005). Eventually the concept of stellar populations, first proposed by Baade (1944), led to the famous Vatican Symposium of 1957 (O’Connell 1958), where a consistent picture was defined to interpret the presence of disk and halo populations in the context of the structure and formation of galaxies. Not long after that, the collapse picture of Eggen, Lynden-Bell & Sandage (1962) provided a by California Institute of Technology on 01/21/14. For personal use only. working picture for the formation and evolution of the Galaxy and, by implication, of galaxies in general (Sandage, Freeman & Stokes 1970). It was modified by Searle & Zinn (1978) to include Annu. Rev. Astro. Astrophys. 2011.49:301-371. Downloaded from www.annualreviews.org extended evolution, whereby the outer globular clusters originated and underwent chemical evo- lution in separate fragments that fell into the Galaxy after the collapse of the central halo had been completed. The basic discrete two-component structure of the edge-on galaxy NGC 7814 led van der Kruit & Searle (1982b) to deduce that there are two discrete epochs of star formation, one before and the other after virialization of the spheroid and the formation of the disk. Two related important developments in understanding the properties of galaxies and their formation were the discovery of dark matter halos, and the appreciation of the role of hierarchical assembly of galaxies. The concepts of hierarchical assembly were already around in the early 1970s and became widely accepted at a landmark symposium on the Evolution of Galaxies and Stellar Populations at Yale University in 1977 (Tinsley & Larson 1977). The discovery of dark matter halos (see below) led to the two-stage galaxy formation model of White & Rees (1978), in 302 van der Kruit Freeman · AA49CH09-vanderKruit ARI 5 August 2011 9:18 which hierarchical clustering of the dark matter took place under the influence of gravity, followed by collapse and cooling of the gas in the resulting potential wells. The Hubble Space Telescope (HST) made possible the high-resolution imaging of galaxies at high redshift and showed directly that merging and hierarchical assembly are significant in the formation of massive galaxies. The significance of internal secular evolution for the evolution of disks has become clear in recent years. The presence of oval distortions, bars, and spiral structure can have a profound effect on the changing structure of disks, as has been extensively reviewed by Kormendy & Kennicutt (2004) and Kormendy (2007). The rotation of galaxies was discovered early in the twentieth century. For a historical intro- duction, see van der Kruit & Allen (1978), Gilmore, King & van der Kruit (1990, chapter 10), and Sofue & Rubin (2001), documenting the first derivation of rotation velocities as a function of radius using optical absorption lines (Pease 1914), emission lines (Babcock 1939), and HI (van de Hulst, Raimond & van Woerden 1957; Argyle 1965), all in the Andromeda Nebula M31. The subject developed into the extensive mapping of the velocity fields of disk galaxies in the optical and HI, which eventually led to the discovery of flat rotation curves and the existence of dark matter (see, e.g., reviews by van der Kruit & Allen 1978, Faber & Gallagher 1979, Roberts 2008). Quantitative surface photometry of disk galaxies to study their structure and luminosity distri- butions began with the work of Reynolds (1913) for the bulge of M31 (for a review, see Gilmore, King & van der Kruit 1990, chapter 5). Photometry of the much fainter disks came later and re- vealed the exponential nature of the radial surface brightness distributions. This was first described in a Harvard thesis, using observations of M33 (Patterson 1940): the data appear as figure 10 in the review by de Vaucouleurs (1959a). He had undertaken in the late fifties a systematic survey of the light distributions in nearby spirals, particularly in M31 and M33 (de Vaucouleurs 1958, 1959b), and established the universal exponential disk description of the radial light distribution in galactic disks.