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Vera Rubin's Excellent 1983 Science Article on Dark Matter

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Vera Rubin's Excellent 1983 Science Article on Dark Matter 24 June 1983, Volume 220, Number 4604 SCIENCE the galaxy carries the stars toward the observer, spectral lines are shifted to- ward the blue region of the spectrum with respect to the central velocity. On the opposite side, where rotation carries The Rotation of Spiral Gala3Kies the stars away from the observer, lines are shifted toward the red spectral re- Vera C. ]Rubin gion.Several years later, Pease (8) convinc- ingly illustrated that rotation was respon- sible for the inclined lines. Using the 60- inch telescope on Mount Wilson during Historians of astronomy may some um, Scheiner correctly concluded that the months of August, September, and day call the mid-20th century the era of the nucleus of M31 w;as a stellar system October 1917, he patiently acquited a 79- Downloaded from galaxies. During those years, astrono- composed of stars li1 ce the sun, rather hour exposure. of the spectrum of the mers made significant progress in under- than a gaseous object. nuclear region of M3 1 with the slit standing the structure, formation, and Even earlier, LordIand Lady Huggins aligned along the apparent major axis of evolution of galaxies. In this article, I (4) had attempted at tlheir private obser- the galaxy. Inclined lines appeared. A will discuss a few selected early steps in vatory to obtain a spectrum of M3 1, but second exposure made during these 3 determining the internal dynamics of gal- their labors were frauight with disaster. months in 1918, with the slit placed per- http://science.sciencemag.org/ axies, as well as current research con- Exposure times were so long that each pendicular- to the major axis along the cerning the rotation and mass distribu- fall when M31 was overhead they would apparent minor axis, showed no inclina- tion within galaxies. These studies con- expose consecutively on one plate dur- tion of the lines. Along the minor axis, stars move at right angles to the line-of- sight of the observer, so no Doppler shift Summary. There is accumulating evidence that as much as 90 percent of the mass appears. The rotation of galaxies was of the universe is nonluminous and is clumped, halo-like, around irndividual galaxies. established even before astronomers un- The gravitational force of this dark matter is presumed to be responsible for the high derstood what a galaxy was. rotational velocities of stars and gas in the disks of spiral galaxie:s. At present, the By the mid-1920's, astronomers knew form of the dark matter is unknown. Possible candidates span a range in mass of that we lived in a Galaxy composed of on March 24, 2021 1070, from non-zero-mass neutrinos to massive black holes. billions of stars distributed principally in a disk, all rotating about a distant center. Each spiral viewed in a telescope, locat- tribute to the present view that much of ing the early evening hours, until they ed at an enormous distance beyond our the mass of the universe is dark. could build up the requisite exposure Galaxy, is itself a gigantic gravitationally In 1845, Lord Rosse (1) constructed a (perhaps 100 hours). They would then bound system of billions of stars all mammoth 72-inch reflecting telescope, superpose a solar spe:ctrum adjacent to orbiting in concert about a common cen- which remained the largest telescope in the galaxy spectrum to calibrate the ter. This knowledge came from a variety the world until the construction of the wavelength scale. Yea?r after year, their of observational discoveries: Henrietta "modern" 100-inch telescope on Mount single exposure was a failure, sometimes Leavitt's (9) discovery of Cepheid varia- Wilson in 1917. Unfortunately, the showing no galaxy spe,ctrum, sometimes bles, stars whose periods of light fluctua- Rosse telescope did not automatically contaminated by the solar spectrum; tion reveal their true brightnesses and compensate for the rotation of the earth, once the plate was a(ccidentally placed hence distances; Shapley's (10) demon- so an object set in the field of the tele- emulsion side down, wrhere it dried stuck stration that the globular clusters sur- scope would race across the field. Nev- to the laboratory tabl4e. A copy of their round our own Galaxy like a halo, with a ertheless, Lord Rosse made the major 1888 failure is publishced along with their geometric center located not at the sun discovery that some nebulous objects remarkable stellar spectra (4). but at a great distance in the dense star show a spiral structure (2). Slipher (5), workinjg with the 24-inch clouds of Sagittarius; and Hubble's (11) Insight into the nature of these enig- reflector at Lowell Otbservatory, initiat- discovery of Cepheids in M31 and M33. matic objects was slow in coming, but by ed in 1912 systematic spectral observa- In a brilliant study, Opik (12) used the 1899 the combination of good tracking tions of the bright in ner regions of the rotational velocities in M31 to estimate telescopes and photographic recording nearest galaxies. FoIr NGC 4594, the its distance. His result, 450,000 parsecs made it possible for Scheiner (3) to ob- Sombrero galaxy, he detected inclined he correct- tain at the Potsdam Observatory a spec- lines [(6); see also (7)],, which Vera C. Rubin is a staff member of the Depart- trum of the nucleus of the giant spiral in ly attributed to stars c(ollectively orbiting ment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, and an Andromeda, M3 1. From the prominence' about the center of tbiat galaxy. On the adjunct staff member of the Mount Wilson and Las of the H and K absorption lines of calci- side of the nucleus whLere the rotation of Campanas Observatories. 24 JUNE 1983 1339 (1 pc = 3 x 10i3 km), comes closer to measurements may be made from stellar tory (near La Serena, Chile). A few the distance in use today, 750,000 pc, absorption lines. One way around this spectra come from the 2.5-m du Pont than the distance of 230,000 pc which difficulty is to measure velocities from Telescope of the Carnegie Institution of Hubble derived from the Cepheids and the emission lines arising in the ionized Washington at Las Campanas (also near which he was still using in 1950 (13). gas clouds surrounding the hot young La Serena). Optical observations are Opik's distance was based on the proce- blue stars which delineate the spiral made with spectrographs which incorpo-' dure used currently to determine the structure. Because the light from these rate an RCA C33063 image tube (20). masses ofgalaxies, and I will discuss it in clouds is emitted principally in a few The image is photographed from the final some detail. For a particle of mass m lines of abundant elements (hydrogen, phosphor of this tube. Use of this elec- moving in a circular orbit with velocity V ionized oxygen, ionized nitrogen, and tronic enhancement device makes it pos- about a spherical distribution of mass M ionized sulfur), a measurable exposure is sible to obtain spectra at a high spatial at distance r from the center, the equality obtained in a fraction of the time re- scale and of high velocity accuracy with of gravitational and centrifugal forces quired for the stellar exposure. This is exposure times of about 3 hours, on gives the method we employ in our optical Kodak IIIa-J plates which have been (1) observations. A third procedure is to baked in forming gas and preflashed to GM(r)m = mV2(r) observe in the radio spectral region, at enhance their speed. r2 r the wavelength of 21 cm emitted by the We choose galaxies that are relatively where G is the constant of gravitation hydrogen atom. Hydrogen gas, a major isolated, that are not strongly barred, and M(r) is the total mass contained out constituent of the prominent dust lanes that subtend an angular size at the tele- to a distance r from the center. It follows seen in spiral galaxies and hence opaque scope which approximately matches the that the mass interior to r is given by for an optical astronomer, becomes the length of the spectrograph slit, and that object of observation for the radio as- are viewed at relatively high inclination = M(r) rV2(r)/G (2) tronomer. to minimize uncertainties in transform- Downloaded from or Following the pioneering work dis- ing line-of-sight velocities to orbital ve- in the Distances are cal- = (3) cussed above, observations of the rota- locities galaxy. V(r) [GM(r)lr]"2 tion of galaxies proceeded very slowly. culated from velocities arising from the Today we adopt a distance to a galaxy A major addition to our knowledge came cosmological expansion, adopting a (generally from its Hubble velocity), de- frotn the extensive work of Margaret and Hubble constant of 50 km sec- Mpc . termine the rotational velocity V at each Geoffrey Burbidge (14). Due to the long A sample of Sc program galaxies and http://science.sciencemag.org/ r, and then calculate the variation of M observing times, velocities were ob- spectra is shown in Fig. 1; the spectra with r. Opik assumed that the ratio of tained only for the brightest inner re- were taken with the spectrograph slit mass to luminosity was the same for M3 1 gions. Only for a few of the nearest aligned along the galaxy major axis. The as for our Galaxy, and used M and V to galaxies was it possible to observe indi- galaxies are arranged by increasing lumi- determine r. Both the dynamics and the vidual regions and map the rotation to nosity.
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