Everything we know about motions within galaxies has been learned in the20th century. One hundred years ago, galaxies were an enigma. But by1899, technology made it possible to replace the human eye with thephotographic emulsion, so spectroscopes becamespectrographs. J. Scheiner, an astronomer at the Potsdam Observatory,obtained the first successful spectrum of a galaxy, M31(Scheiner 1899),and identified M31 as an assemblage of solar-type stars. However,it was not untilSlipher (1914)and Wolf (1914)detected inclined lines in M31 and NGC 4594, the Sombrero galaxy, thatastronomers had observational evidence indicating that stars and gas ina galaxy rotate about its center. With heroic effort,Pease (1918)used the Mount Wilson 60 inch telescope to obtain higher resolutionspectra of M31 (minor axis, 84 hours exposed 1916 August,September, and October; major axis, 79 hours during 1917 August, September, andOctober), covering the inner 2% of the major axis. The gradient in thestellar absorption lines on the major axis and the lack of gradient onthe minor axis confirmed that the motion was rotation. Astronomers knewthat galaxies rotated before they knew what galaxies were.
Observations of M31's rotation(Babcock 1939;Mayall 1951)showed no Keplerian velocity decrease for the outer regions. Withstartling insight, Mayall compared the M31 rotation curve with theorbital velocities of stars in the solar vicinity and questioned whetherthe M31 adopted distance was too small. (It was, by afactor of about 3.) Earlier,Opik (1922)had raised a similar concern based on Pease's minimal "rotation curve."Without fanfare, Opik and Mayall had used the new tool of Doppler shiftspectroscopy to study galaxy kinematics and galaxy masses.
Most astronomers in the mid-fifties grew up believing that disk galaxieshad Keplerian velocities at moderate nuclear distances. Some of thisbelief may have come from Slipher. More at home with the planets thanwith galaxies, Slipher used Saturn as a radial velocity standard for hisM31 spectra; he characterized the spectrum of theSombrero galaxy as "planetary."De Vaucouleurs (1959)concluded from the eight available rotation curves, "In all cases therotation curve consists of a straight inner region ... beyond which therotational velocity decreases with increasing distance to the center andtends asymptotically toward Kepler's third law." With the 20/20 visionof hindsight, plots of the data reveal only a scatter of points, fromwhich no certain conclusions can be drawn.
I had long been interested in how galaxies "ended" and, with my graduatestudents at Georgetown, made a study of the velocities of ~ 1000 O and Bstars beyond the solar circle(Rubin et al. 1962;see also Rubin 1965)in our Galaxy. Our 1962 conclusion, "For R > 8.5 kpc, thestellar curve is flat, and does not decrease as is expected forKeplerian orbits," apparently influenced no one and was ignored even bythe senior author when she returned to the problem of galaxy rotation adecade later.
Margaret and Geoffrey Burbidge(1975and references therein, submitted 1969; followingPage 1952)introduced the modern era of kinematic studies of galaxies when theyexploited the new red-sensitive photographic plates to observeH
emission frominterstellar gas along the major axes of nearby spirals. But nearbygalaxies are large, so only inner parts were observed with the "longslit" of the McDonald 82 inch telescope plus spectrograph. If theBurbidges had placed the spectrograph slit at large nuclear distances,they might have discovered that rotation velocities remain high at largenuclear distances; many galaxies have bright H IIregions at large radii. The 1963-1964 academic year I spent in La Jollaworking with Margaret and Geoff was an invaluable experience for me, andI happily acknowledge here the lasting importance of their encouragementand support.
During the second half of the 20th century, extragalactic astronomy madeenormous strides, due principally to advances in instrumentation:electro-optical devices (initially image tubes; later CCD detectors) foroptical telescopes and large radio telescope arrays for observing the 21cm line of neutral hydrogen. Hundreds of extended rotation curves wereacquired in 1978-1988(;Bosma 1978;;Rubin et al. 1985;;Guhathakurta et al. 1988),and more than 2000 are now available(;Prugniel et al. 1998).In general, optical data have higher spatial resolution, andH I velocities extend farther.
Few galaxies exhibit the Keplerian velocity fall expected at largenuclear distances for a mass distribution which follows the lightdistribution within the galaxy. Instead, rotation velocities generallyremain high at large nuclear distance; occasionally, rotation velocitiesdecrease slightly at the edge of the optical disk, then flatten. Theseobservations have played a major role in convincing scientists that atleast 90% of the total spiral mass, and hence the total mass in theuniverse, is dominated by nonluminous (i.e., dark) matter. It took 50years for the discoveries ofZwicky (1933)and Smith (1936),that clusters of galaxies contained unseen matter, to make it tomainstream astronomy.
Yet not all astronomers had been blind to the contradiction posed by theexponentially falling galaxy disk luminosity and the constant rotationalvelocities.Oort (1940!)wrote of NGC 3115, "It may be concluded that the distributionof mass in the system must be considerably different from the distribution of light... The strongly condensed luminous system appears embedded in a largemore or less homogeneous mass of great density."Freeman (1970:M33, NGC 300) andShostak (1973:NGC 2405) were similarly impressed by thecontradiction; see also Schwarzschild(1954:M31).
We enter the 21st century knowing that galaxy dark halos exist, thatthey contain an order of magnitude more mass than the visible galaxy,and that they are of great extent. In 15 hours of observation, less thatone-tenth the time it took Slipher to obtain the major- and minor-axisspectra of M31, the Sloan Digital Sky Survey(Fischer et al. 2000)detected distortions in over one million background galaxies whose lightwas gravitationally deflected as it passed through the large dark haloof one of 28,000 foreground galaxies. These two observations, early andlate in the 20th century, define the progress that many of us have livedthrough.
Kinematics of galaxies tell us more than the distribution of mass inspirals; they teach us about galaxy evolution. Twenty-five years ago, wecould only dream of obtaining rotation curves for galaxies at distancescorresponding to z
1, whose diameterssubtend only a few arcseconds. Large telescopes and subarcsecond opticsnow make possible observations of moderately distant spirals, z 0.2-0.4(Bershady 1997;Bershady et al. 1999;;Kelson et al. 2000).These have been surpassed with Keck spectrographic observations reachingz 1(Vogt et al. 1996,1997,1999;Koo 1999).Regularly rotating spiral disks were in place when the universe was lessthan half of its present age. The Keck rotation velocities define aTully-Fisher relation (i.e., the correlation of rotation velocity withblue magnitude) which matches to within
0.5 mag that for nearbyspirals. Spiral galaxy evolution, over the last half of the age of theuniverse, has not significantly altered the Tully-Fisher correlation.
The significance lies not in these initial details, but in therealization that galaxy kinematics becomes now a viable parameter forcosmological studies. Fifteen years ago, I predicted(Rubin 1986),"Rotation curves can be obtained for galaxies with redshifts as great asz = 0.05 and probably even z = 0.1. (I hope that some dayI will be amused at the conservative nature of this prediction)." I amnot only amused, I am delighted. Now we struggle for rotation velocitiesat z 5 andperhaps even higher. These too will come; high-resolution millimeterobservations may get there first. If the inferences from the Hubble DeepField images are correct, spheroidal galaxies were in place at z= 3, but well-formed disks were rare. Perhaps the early universe was aninhospitable time and place for large, cold disks.
Studies of even nearby galaxies, especially pathological specimens(Baade's term), also reveal galaxy evolution. Often, their kinematiccomplexity involves more than one spin axis. For example, polar ringgalaxies()permit us to observe mass tracers in two orthogonal planes and offervisible evidence that such objects could not form in a singleevent. Other equally wonderful galaxies confirm the importance ofgravitational acquisitions and mergers in driving galaxy evolution.NGC 4550, an E7/SO disk galaxy near the center ofthe Virgo galaxy, contains a single disk in which intermingled stars orbit, one-half clockwise,one-half counterclockwise().Stars in NGC 4826 (the Black Eye or Sleeping Beauty) orbitwith a single sense, but the gas interior to the outer radius of the prominent dustlane rotates counter to the stars().The bulge in NGC 7331 may(Prada et al. 1996)or may not(Mediavilla et al. 1997)counterrotate with respect to the disk. Water vapor masers observed withmilliarcsecond resolution near the center of NGC 4258 define a warpeddisk in Keplerian rotation about a black hole(Herrnstein et al. 1999).Numerous observations of interacting and merging galaxies, coupled withrealistic computer simulations of stars, gas, and dark matter particles,have identified the paths followed by such galaxies. "Pathological" canoften be understood as one stage in a merger sequence(Schweizer 1998).
Velocities within elliptical galaxies are harder to observe and moredifficult to interpret owing to the uncertain geometry, the lack of gas,and the noncircular orbits within ellipticals. Yet sophisticatedreduction techniques (e.g.,Cretton 1999)now extract from complex integrated stellar spectral line profiles thefull line-of-sight velocity distributions.
Few extended rotation curves for ellipticals exist, yet continuing studywill likely confirm the need for dark matter. For NGC 2434, velocity information(Rix et al. 1997)extends to large nuclear distances (3 or 4re,re = half-light radius). One-half of the masswithin re is dark; dark matter dominatesbeyond. IC 2006 is an elliptical with a large externalcounterrotating gas ring().Interior to the ring, the dark matter exceeds the luminous matter by afactor of 2.
A small nuclear disk of stars and/or gas is not uncommon in studiedellipticals(;Franx et al. 1989;).Some disks are skew, some polar, some counterrotating, and hence theyoffer evidence of a complex evolutionary history. For some, nuclearvelocities show evidence of Keplerian rotation about black holes().
Like the Tully-Fisher relation for spirals, the three-parameter(half-light radius, surface brightness, and central velocity dispersion)fundamental plane relation offers a tool for studying elliptical galaxyevolution. Keck spectra and HST images of 53 galaxies in clusterCL 1358+62 (z = 0.33) define a fundamentalplane similar to that of nearby ellipticals(Kelson et al. 2000).Ellipticals at z = 0.33 are structurally mature; we await datafrom more distant ellipticals.
Throughout this century, knowledge of galaxies has come from aninteresting interplay of what we can discern nearby while living insidea spiral galaxy and what we learn by observing galaxies at adistance. Kinematic studies help. It is likely that this interplay willcontinue to be a valuable tool. We observe that our Galaxy is capturinga dwarf spheroidal in Sagittarius(Ibata et al. 1997)and that the Carina dwarf is losing stars to the Milky Way(Majewski et al. 1999).Velocity streams in the Milky Way halo and globular cluster motions fromearlier captures help convince us that galaxy evolution is a continuousprocess, not a single event. Rather than the Island Universes evolvingin splendid isolation imagined byHubble (1936),a galaxy is a continuously evolving structure which will acquire starsor lose stars through gravitational interactions, will acquire gas orlose gas through infall or galactic winds, and will be actively formingstars or quiescent depending upon its recent history.
Details of galaxy kinematics are unlikely to be a source of majorexcitement in the next century (recall that my record for predicting isnot good). But discoveries which reveal the composition, distribution,and amount of the dark matter will be exciting. New large telescopes,enormous surveys, Fabry-Perot and integral field spectroscopy, andespecially detectors which provide a spectrum at each pixel are alreadychanging the way astronomers observe and analyze data.
Our knowledge of transverse motions of stars, formerly limited to starsin the solar neighborhood, now extends to rapidly orbiting circumnuclearstars(;Ghez et al. 1998)at the Galactic center. For galaxies too, perhaps as far away as Virgo(is this another too conservative prediction?), transverse motions willoffer a new parameter with which to study the universe. Astronomers werestartled by the discoveries which arose from determining radialvelocities; I hope the surprises will be no less from the transversecomponents.
One hundred years ago, galaxies were an enigma. They still are. It isfolly to believe that we know what a galaxy is, while the extent, thedensity distribution, and the composition of
90% of its mass are still adark mystery. Models of enormous complexity exist, which assume thatluminous disks form embedded in cold dark matter structures originatingearly in the universe. In order to make the models fit, adjectivesmodify cold dark matter models: open, mixed, tilted,
. I hope that newobservations and new insights will soon impose tighter constraints uponthese models, as well as tighter constraints upon the dark matter/brightmatter components which produce the observed rotation curves. Some ofthe current complexity must arise from our ignorance.
Happily, distant observations push back still farther the era of diskformation; I like a very old universe. Surprisingly, we cannot yet ruleout a modified gravitational potential, rather than dark matter, as theexplanation of our observations. We have learned much about galaxies inthe last 100 years. I think that we still have major surprises touncover. I hope all of our discoveries will be dwarfed by what will belearned in the next century. This century, we have learned aboutrotating galaxies. But in understanding their role in the evolution ofthe universe, it may be earlier than we think.
