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Galaxy Disks Galaxy disks P.C. van der Kruit Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, the Netherlands ([email protected]) K.C. Freeman Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road, ACT 2611, Australia ([email protected]) Keywords: disks in galaxies: surveys, luminosity distributions, mass distributions, disk sta- bility, warps and truncations, scaling laws, thick disks, chemical evolution, abundance gradients, formation, S0 galaxies Abstract The disks of disk galaxies contain a substantial fraction of their baryonic matter 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 is therefore particularly important for understanding how galaxies form and evolve, and the cause of the variety in which they appear to us. Ongoing large surveys, made possible by new instrumentation at wavelengths from the ultraviolet (GALEX), via optical (HST and large groundbased telescopes) and infrared (Spitzer) 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 have a range in surface brightness from canonical disks with a central surface brightness of about 21.5 B-mag arcsec−2 down to very low surface brightnesses. 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 radii. Stellar disks display abundance gradients which flatten at larger radii and sometimes even reverse. The existence of a well-defined baryonic (stellar + HI) Tully-Fisher relation hints at an approximately uniform baryonic to dark 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 is still not fully understood. The final version of this chapter as it will appear in Annual Reviews of Astronomy & Astrophysics, vol. 49 (2011), will have fewer figures and a somewhat shortened text. 1 Contents 1 INTRODUCTION AND OVERVIEW3 1.1 Historical Background......................................3 1.2 Setting the Scene.........................................6 2 SURVEYS 6 3 STELLAR DISKS 8 3.1 Luminosity Distributions.....................................8 3.1.1 Exponential disks.....................................8 3.1.2 Three-dimensional distributions............................. 11 3.2 Stellar Kinematics, Stability and Mass............................. 12 3.2.1 Vertical dynamics.................................... 12 3.2.2 Stellar velocity dispersions in the plane........................ 14 3.2.3 Origin of the constant scaleheight............................ 15 3.2.4 Mass distributions from stellar dynamics........................ 17 3.3 Age Gradients and Photometric M=L Ratios......................... 18 3.4 Global Stability, Bars and Spiral Structure.......................... 20 3.5 The Flatness of Disks...................................... 21 3.6 `Superthin' Galaxies....................................... 22 3.7 Warps in Stellar Disks...................................... 22 3.8 Truncations............................................ 23 3.9 Nuclei of Pure-Disk Galaxies.................................. 25 4 HI DISKS 26 4.1 HI Distributions, Kinematics and Dynamics.......................... 26 4.2 Dark Matter Halos........................................ 28 4.3 Outer HI and Warps....................................... 30 4.4 Dustlanes in Disks........................................ 33 5 CHEMICAL EVOLUTION AND ABUNDANCE GRADIENTS 34 5.1 Gas-phase Abundance Gradients................................ 35 5.2 Stellar Abundance Gradients.................................. 35 6 SCALING LAWS FOR DISK GALAXIES 37 6.1 The Tully-Fisher Law...................................... 37 6.2 Scaling Laws involving the Galaxy Diameter.......................... 39 6.3 The Mass-Metallicity Relation.................................. 39 6.4 The Surface Density-Mass Relation............................... 40 6.5 Scaling Laws for Dark Matter Halos.............................. 40 7 THICK DISKS 40 7.1 Statistics of Incidence...................................... 41 7.2 Structure of Thick Disks..................................... 41 7.3 Kinematics and Chemical Properties.............................. 41 7.4 Relation of Thick Disk to the other Galactic Components.................. 42 7.5 Thick Disk Formation Scenarios................................. 43 8 FORMATION OF DISKS 43 8.1 Disk Formation Scenarios.................................... 43 8.2 Disks at High Redshift...................................... 45 8.3 Baryon Acquisition by Disk Galaxies.............................. 46 9 S0 GALAXIES 48 References 50 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◦. Comparison to modern star counts shows that Herschel counted stars consistently down to magnitude V ∼ 15 (van der Kruit, 1986). It was the first description of the flattened nature of galactic disks. 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) has 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 was 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 working picture for the formation and evolution of the Galaxy and by implication of galaxies in general. It was modified by Searle & Zinn(1978) to include extended evolution, whereby the outer globular clusters originated and underwent chemical evolution 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
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