eaa.iop.orgDOI: 10.1888/0333750888/2618 Disk GalaxiesJay Gallagher, Linda Sparke, Lynn Matthews FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 22:39:44 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsDisk GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSStellar disks are the main structural features of diskgalaxies, which we divide into the spiral, S0, and irregu-lar morphological classes. The disks are highly flattened,with approximate circular symmetry. In spiral and irreg-ular galaxies, the disk contains gas as well as stars, andfine structure is common, including the spiral armswhich define that class. Our nearest disk galaxy is ourown MILKY WAY, a spiral galaxy; the Sun lies in the planeof the disk, about 8 kpc from its center.The stars and gas of a galaxy disk follow near-circu-lar orbits around the center, in the plane of the disk. Thedisk does not rotate rigidly like a turntable, but differen-tially: stars in the outer regions take longer to completean orbit than those close to the center. The orbital motionof the stars and gas supports the disk against collapseunder gravity. Organized rotation dominates all othermotions; random velocities account for very little of thekinetic energy, so that disks are dynamically ‘cold’, or atleast ‘cool’. In the Milky Way, disk stars near the Sunmove at about 200 km s–1, taking about 250 Myr to com-plete an orbit, but their random motions are typicallyonly 30 km s–1. Because the disks are cool, they tend to beunstable to forming internal substructures such as barsand spiral arms.Giant disk galaxies, with luminosities more thanabout 6109Lo.(our Milky Way is roughly three timesbrighter), are composite systems. The round innerbulge is much denser than the disk, and ‘hotter’; thestars have large random motions. Within the bulge is ayet denser star cluster, the galactic nucleus; somenuclei contain massive black holes at their centers.Giant galaxies follow a morphological sequence,named for its originator, Edwin Hubble; see GALAXIES:CLASSIFICATION. The S0 galaxies have large centralbulges or spheroids, and a smooth stellar disk; spiralarms, gas and star formation are normally absent.Along the sequence from Sa through Sb to Sc, the cen-tral bulge becomes smaller, while the prominence ofthe spiral arms increases, along with the fraction of gasand newly formed stars.Dwarf galaxies are smaller, less luminous, and lessmassive than the giants. They are also more diffuse,with reduced densities of stars and gas, and they lackthe dense round central bulges. At the boundarybetween dwarfs and giants are the Sd galaxies, withvery thin stellar disks, and only rudimentary spiralstructure. The trend towards less organized opticalstructure continues through the Sm Magellanic irregu-lar class to the dwarf irregulars (dIrr), which are theleast massive and least luminous, with blue luminosi-ties below 108Lo.. All of these classes of dwarf diskgalaxies are rich in gas, and have relatively large contri-butions from young stars.Structures of galaxy disksStellar disksThe Milky Way is the only galaxy where the three-dimensional structure is well known; elsewhere, we seeonly two-dimensional projections of the galaxies. Byobserving many galaxies, randomly distributed over allpossible viewing angles, we deduce the average three-dimensional structures of the various classes. In diskgalaxies that we view nearly face-on, perpendicular tothe plane containing the disk, the projected brightness ofstarlight declines smoothly with radius. At a given wave-length λ, the run of intensity Iλ(r) with radius r from thedisk center, is roughly exponential: Iλ(r) ~~I(0) e–r/hr. Theradial scale length hris the e-folding length for the starlightintegrated vertically through the disk. In most spiralgalaxies, hris 1–5 kpc; dwarfs can have hr<1 kpc while insome peculiar giant galaxies hr>10 kpc.In most disk galaxies, near the center where r<hr, theluminosity rises above the inward extrapolation of theexponential law describing the main disk. We can definethe disk central brightness Iλ(0) by extending the diskmodel to r = 0; the total luminosity of the disk is simplyLλ=2πhr2Iλ(0). Measured values of the central disk bright-ness are commonly around Iλ*(0)~~;150 Lo.pc–2in bluelight; this is the Freeman peak. It is now recognized thatthe Freeman value reflects an approximate upper boundto disk surface brightness. While few galaxy disks havebrighter centers, disks exist with lower surface bright-ness, ranging down to a few percent of the Freemanluminosity density. These are the low-surface-brightnessgalaxies. One caution: although it is usual to extrapolatethe exponential law in this way, it is not clear if the stel-lar disks really extend into the central galaxy.We see our own galaxy, the Milky Way, as a lumi-nous path across the sky on a dark night; the Sun is nearthe mid-plane of our disk, so we observe the stellar diskin projection from within. The disks of other galaxies alsoappear as narrow bands of light when we see them edge-on. If an edge-on disk contains only stars, with no inter-stellar matter to block light, we can use brightness mapsto derive the distribution of stellar density ‘above’ themid-plane in the z-direction.Stellar disks have complicated vertical structures. Inour own Milky Way, the youngest stars are concentratednear the disk mid-plane, with exponential scale heightshz<0.1–0.2 kpc (see GALACTIC THIN DISC). Older stars forma ‘thick disk’ (see GALACTIC THICK DISC) with a scale heighthz~~1 kpc. The vertical scale height of each component ofa stellar disk depends on its velocity dispersion perpen-dicular to the disk; higher speeds yield thicker disks,since the stars can travel further from the midplane.Young stars are born from clouds of gas that are dynam-ically ‘cold’; they have little vertical kinetic energy, andare confined close to the mid-plane. As they orbit, starsfeel the gravitational forces from large lumps of gas andDisk GalaxiesCopyright © Nature Publishing Group 2002Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2002Dirac House, Temple Back, Bristol, BS21 6BE, UK1Disk GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSstars that are present in the spiral arms. The repeatedtugs ‘heat’ them, increasing their random