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Proc. Natl. Acad. Sci. USA Vol. 74, No. 5, pp. 1767-1774, May 1977 : Outstanding problems and instrumental prospects for the coming decade (A Review)* t (///space) JEREMIAH P. OSTRIKER Princeton University Observatory, Peyton Hall, Princeton, New Jersey 08540 Contributed by Jeremiah P. Ostriker, February 3, 1977

ABSTRACT After a review of the discovery of external It is more than a little astonishing that in the early part of this galaxies and the early classification of these enormous aggre- century it was the prevailing scientific view that man lived in gates of into visually recognizable types, a new classifi- a universe centered about himself; by 1920 the 's newly cation scheme is suggested based on a measurable physical quantity, the of the spheroidal component. It is ar- discovered, slightly off-center position was still controversial. gued that the new one-parameter scheme may correlate well The discovery was, ironically, due to Shapley. both with existing descriptive labels and with underlying A little background to this debate may be useful. The Island physical reality. Universe hypothesis was not new, having been proposed 170 Two particular problems in extragalactic research are isolated years previously by the English , Thomas Wright as currently most fundamental. (i) A significant fraction of the (2), and developed with remarkably prescience by Emmanuel energy emitted by active galaxies (approximately 1% of all galaxies) is emitted by very small central regions largely in parts Kant, who wrote (see refs. 10 and 23 in ref. 3) in 1755 (in a of the (microwave, , and x-ray translation from ). wavelengths) that were previously inaccessible to observation. The physical processes by which regions with <10-9 the volume of the luminous stellar parts of galaxies produce such enormous I come now to another part of my system, and because it quantities of energy are currently the subject of much specu- suggests a lofty idea of the plan of creation, it appears to me as the lative debate. (il) It appears that most of the mass of ordinary most seductive. The sequence of ideas that led us to it is very sim- galaxies resides far from the central luminous region, with the ple and natural. They are as follows: let us imagine a system of volume containing most of this mass > 103 times the volume stars gathered together in a common plane, like those of the Milky containing most of the -emitting stars; the nature, amount, Way, but situated so far away from us that even with the and extent of this mass are quite unknown. we cannot distinguish the stars composing it; let us assume that its New instruments that will be operating in the next decade and distance, compared to that separating us from the stars of the that may be helpful in solving these two problems are briefly , is in the same proportion as the distance of the Milky mentioned with particular emphasis on the advances expected Way is to the distance from the earth to the sun; such a stellar in angular resolution at wavelengths for which picture-taking world will appear to the observer, who contemplates it at so enor- ability has historically been poor or nonexistent. mous a distance, only as a little spot feebly illumined and subtend- ing a very small angle; its shape will be circular, if its plane is per- pendicular to the line of sight, elliptical if it is seen obliquely. The HISTORICAL INTRODUCTION faintness of its light, its form, and its appreciable diameter will ob- Exactly 56 years ago in this Academy, on April 26, 1920, there viously distinguish such a phenomenon from the isolated stars was an event, perhaps common in the public imagination, but around it. uncommon in real science: there was a great debate (1). It was between Heber Curtis from and Harlow Shapley of Harvard, and was held in the hope of resolving, or But these ideas were almost pure speculation. They were at least illuminating, a scientific issue then of great moment. put on a quantitative foundation by , a Ger- What was the real nature of the spiral nebulae (compare Fig. man-English musician, instrument builder, and astronomer, 1)? Although several ancillary issues were discussed as well who was delighted to find in 1784f with his own 1/2-m reflector (including, especially, the size of our own Milky Way), the that the Milky Way could be resolved into innumerable stars. conflict might be summarized in the following two quotes. "The glorious multitude of stars of all possible sizes was truly astonishing." We shall see again and again in extragalactic re- Shapley: The evidence is opposed to the view that the spirals are search an increase in angular resolving power, made possible galaxies comparable with our own. In fact there appears by technological progress, leading directly to a significant sci- as yet no reason for modifying the tentative hypothesis that entific breakthrough. the spirals are not composed of stars at all but are truly Herschel, the first systematic investigator of our stellar sys- nebulous objects." tem, conceived of a plan to count stars in certain selected 15° areas as a function of the apparent stellar brightness. From this, by assuming the inverse square law and that all stars were ap- Curtis: The spirals are not intragalactic objects but island universes proximately the same absolute brightness as the nearby like our own . Sirius, he deduced the dimensions of our own system to be ap- X Abbreviation: pc, = 3.26 light years = 3.09 X 1018 cm. proximately 300 parsec 1500 parsec (pc). (The numerical * From a lecture given at the National Academy of Sciences, Wash- values are derived with the now known distance to Sirius which ington, DC, Spring 1976. Herschel had used as a unit.) This was not a bad first estimate. t By invitation. From time to time, reviews on scientific and techno- logical matters of broad interest are published in the PROCEED- t This was the same year, incidentally, when Messier's famous catalog INGS. of nebulous objects to be avoided by comet hunters was published. 1767 Downloaded by guest on September 29, 2021 1768 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977)

FIG. 1. NGC 5457 or M101, 101 in Messier's 1784 catalog of nebulous objects. A bright (M, = 8.2), relatively nearby (distance 4107 light years), giant, face-on spiral; an excellent example of Hubble type Sc (compare Fig. 3). Photograph courtesy of Hale Observatories. Believing the Island Universe hypothesis, he concluded that if brightest red stars in the central bulge of Andromeda, M32, and Andromeda (no. 31 in Messier's catalog) were the same size as NGC 205 (Astrophy. J. 100,137, 1944). Then Cepheid variable the Milky Way and made of the same kind of stars, it would be stars were also found (4) and the case was clinched. After that nebulous in appearance but that somewhat better techniques there was a great deal of hard work to be done but the outlook, should be able to resolve it also into individual stars. Herschel crystallized in Hubble's book, Realm ofthe Nebulae (5, 6), was clearly realized that the nebulae were not a homogeneous group fixed. We enter the modern era of extragalactic studies. of objects but that some, Orion for example, were comprised of "a shining fluid of nature totally unknown to us"; that is, he CLASSICAL VIEW OF GALAXIES recognized the division into gaseous and extragalactic nebu- Absolute, quantitative measurements had to await subsequent lae. decades when the photographic plate would be replaced by The analogous appearance between our own Milky Way and photoelectric detectors, but Hubble rapidly took the first es- certain edge-on systems made the Island Universe hypothesis sential steps. Following the early lead of the Irishman, Lord specially plausible (compare Fig. 2 upper and lower), but the Rosse, Hubble discovered certain regularities, the taxonomy concentration of spiral nebulae to the north and south galactic of extragalactic objects. He based his classification scheme on poles could not be reconciled with that picture before astron- dimensionless features which could be easily seen on optical omers knew that the apparent rifts in spiral galaxies, including photographic images. our own, were caused by dust obscuration rather than a defi- His classification scheme (compare Fig. 3) and its extensions ciency of stars. were dependent on three parameters: (i) for featureless (ellip- The Curtis-Shapley debate left the issue, as debates will, tical) systems, the ratio of axes of the flattened optical image; unresolved. But new instruments, particularly the 2.5-m Mt. (ii) for spirals, the character of the arms; and (iii) for spirals, the Wilson reflector, settled the question within 5 years. Edwin ratio of spheroidal to flat components. The most important Hubble was soon able to resolve the disc of M31 and its com- extensions and modifications were made individually by W. panion M33 into stars. These were of about the expected lu- W. Morgan, A. Sandage, G. de Vaucouleurs, and S. van den minosity. That is, if M31 is the same size and brightness as our Bergh (7-10). It is important to note that none of these systems galaxy and its apparent size gives its distance, then the resolved was based on basic physical properties of galaxies, although the stars are as bright as the brightest blue stars in our galaxy. classified properties might correlate with them. My own belief Somewhat (18 years) later, with new red-sensitive plates used is that the original system has been extraordinarily useful and on the 2.5-m Mt. Wilson Telescope, Walter Baade resolved the that the extensions, which may permit a much more accurate Downloaded by guest on September 29, 2021 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977) 1769

FIG. 2. (Upper) The nearby Sbc-Sc spiral galaxy NGC 891 seen edge-on (Hale Observatories photograph) compared with (Lower) our own Milky Way (Yerkes Observatory photograph) which from our vantage is, of course, also seen edge-on.

description of the optical images, have had a marginal utility. vided galactic material into two categories (neglecting three The more comprehensive classification systems have the virtue intermediate types): of allowing the easy isolation of the peculiar objects that do not Population I: The subsystem of dust and gas in galax- fit the simple Hubble scheme. The study of these odd-ball ies and the young stars recently born from same. This is systems has, in turn, been quite rewarding because they often the spiral arm population. show interesting behavior at radio, infrared, or x-ray wave- Population II: The old low-mass stars having chemical lengths. composition closer to the presumably primeval, nearly Gradually some theoretical understanding of the significance uncontaminated H + He. (Note that for of Hubble's scheme developed after Baade's early theory of anything else is called a "metal.") This is the spheroidal stellar populations. Extended, summarized, and codified in the distribution. 1957 Vatican Symposium (11), this theory, very roughly, di- The ratio of Population I to Population II in a system corre-

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FIG. 3. The standard descriptive classification scheme for galaxies from Edwin Hubble's monograph, Realm of the Nebulae (1936). First journal publication was in 1926 (5, 6). Downloaded by guest on September 29, 2021 1770 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977)

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2.4 FIG. 5. Edge-on giant spiral galaxies. (Upper) NGC 4594 "the Sombrero" type Sa. (Lower) NGC 4565. type Sb, (Hale Observatories

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bulge component. Thus, I would propose that when any quantity is plotted versus Lo - (Ior02) (in erg/s) a statistically will occur. This is not the to discuss 6- significant relation place -16 -18 - 20 -22 theories of galaxy formation, but clearly Lspheroidal K (Ioro2) is MB a function of the mass of the initial . To show that FIG. 4. (Upper) Relation between color of elliptical galaxies, this is plausible, let me note that several convincing correlations (U - B)o = constant X log (ultraviolet light/blue light), and lumi- have already been found: (i) color vs (14); (ii) color nosity V26 = constant X log (luminosity) + constant [Sandage (15)]. vs luminosity (15) (compare Fig. 4 upper); (iii) scale size ro vs Brighter galaxies emit relatively more red and less blue light. (-) luminosity (12); (iv) velocity dispersion of stars vs luminosity Virgo; (0) Coma. (Lower) Relation between the velocity dispersion (16) (compare Fig. 4 lower). Thus, one could construct a new of stars (measured in km/s) in the central parts of elliptical galaxies on a property, and the of the and Jackson (16)]. Stellar classification scheme based physical the visual luminosity galaxies [Faber component. velocities are larger in more luminous galaxies. (or photographic) luminosity of the bulge Ellip- ticity and character of spiral patterns would be irrelevant. At one end of the sequence would be giant ellipticals like M87 lates well with the Hubble type and, from this fact, ideas of (NGC 4486) with very large luminosity in a spheroidal com- galactic evolution naturally were formulated. ponent; one would progress to smaller velocity dispersion, scale Now we are about to enter the era of systematic quantitative size, and metal content to typical ellipticals like NGC 3379 that investigation of galaxies, the 1970s (although there was some have a luminosity characteristic of ellipticals in clusters. Then pioneer work by de Vaucouleurs and others as far back as the would follow Sa spirals with very large spheroidal components, mid-1950s). Photometric studies showed that spiral galaxies e.g., NGC 4594, the Sombrero (Fig. 5 upper) which is an ex- could be decomposed into two parts: (i) a spheroidal component treme example of an edge-on system of this type. Progression whose was described by the law, I(r) = Io(l would continue through Sb systems, e.g., NGC 4565 (Fig. 5 + r/ro)-2 where r is the projected distance from the center of lower), and Sc systems as M101 and NGC 891 (Figs. 1 and 2 the galaxy; (ii) a disc component, the axisymmetric part of upper) which have smaller bulge components and smaller ve- which could be described by the law M(P) = Io exp (-W/-o), locity dispersions. Next come Magellanic type irregular systems, where ZJ is the distance from the rotation axis and 2(W) is the which in fact do contain a spheroidal bulge population, al- surface brightness corrected to the perspective of a viewer though it is difficult to detect (17). Finally, one would reach the normal to the rotation axis. Thus, four parameters specify the dwarf spheroidals like Sculptor and , which have very axisymmetric part of the light distribution. But correlations low velocity dispersion and metallicity. have been found between ro and Io (12) and between Zo and Spirals exist only in a moderate-sized band in such a se- coo (see Eq. 22, ref. 13). quence. One could speculate that in very massive, dense sys- Thus, disc and spheroidal components each form a one- tems, star formation is sufficiently efficient to use up all the gas parameter family. I believe, although it has not been shown before it collects in a plane, and that in very low-mass systems, quantitatively, that the following hypothesis may be pro- with small escape velocities, supernovae are able to drive out posed: gas so it again cannot collect in a central plane; the spirals having significant (partially secondary) (18) gas and stars in a disc are 2o = ZO(Io) for spirals. only possible in an intermediate range. In any case, it is an ob- servational fact that the central potentials (ca velocity disper- If this is true, then the properties of all field galaxies may be sions) of spirals are intermediate between those of field ellip- predicted from any one quantity, say the luminosity of the ticals and dwarf ellipticals. Downloaded by guest on September 29, 2021 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977) 1771

4r- GALAXY NGCi275 or 3C84 (Per A) I 351

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° -2 0h X-RAY \ 0 -41 I 8 9 10 11 12 13 14 15 16 17 18 19 20 log v (Hz) FIG. 6. (Left) The NGC 1275 with emission line nuclear spectrum; barely visible wisps indicate explosive activity. (Right) The emitted by NGC 1275 covers an enormous range; gaps in microwave and ultraviolet bands indicate parts ofspectrum for which earth's atmosphere is opaque and no satellite experiments have been performed yet. Flux f, observed at earth versus emission frequency v.

By the mid-1970s a new consensus had been reached. One velocities. Since then, radio studies have shown that many of conceived of a typical galaxy as these contain very small (<0.1 pc), variable, nonthermal components in their nuclei. In the last few years, it has been 019- 10L

GALAXY NGC5128 (CEN A)

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FIG. 7. (Left) The peculiar NGC 5128 which is a very bright radio source Cen A. At times, observers have thought the galaxy showed collision of galaxies of different types (E + S). (Right) emitted by NGC 5128. Downloaded by guest on September 29, 2021 1772 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977)

QUASAR 3C273 5-

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cb0 -2 _ -3_ X -RAN\ II I 6 7 8 9 10 11 12 13 14 15 16 17 18 log v (Hz) FIG. 8. (Left) Optical picture of the brightest quasar 3C273 (center). Object at left is a foreground star. Note wisp emanating from 3C273 (at four o'clock). (Right) Emission spectrum of 3C273; note similarities between this figure and Figs. 6 (Right) and 7 (Right) implying similarity of physical processes in active galaxies and quasars.

to the thermal output from stars and the average volume outer parts, indicating that the mass is either in low mass, low emissivity in x-rays is probably within a factor of 10 of the in- efficiency stars, or is in some nonstellar form. frared emissivity. Because the density of QSOs increases sharply Various other dynamical methods can be used with less se- (25) as we look away in space and back in time, it is quite pos- curity to measure the mass at still larger distances. The evidence, sible that the total luminosity produced by these mysterious put together by P. J. E. Peebles, E. L. Turner, A. Yahil, and mechanisms active in the central regions, r < 10-1 pc, of gal- myself, for giant spiral galaxies (29) (Fig. 10) indicates that the axies, may, when integrated over time and space, exceed that mass may increase nearly linearly with radius to > 105 pc where from the optically prominent volume 109 times larger. The M(r) 10123 MO more than 10 times the mass contained within mechanism today is quite unknown; the problem is still the the orbit of the sun. The nature of this mass is totally unknown foremost totally unsolved question in . and all attempts to see it optically have been fruitless. This is probably the second outstanding problem in astrophysics Mass today. The second problem arises when dynamical questions are asked, including the basic one of whether the Universe will expand THE NEXT DECADE forever or whether gravitation can ultimately reverse the ex- pansion. One asks here primarily about the mass density in all Where can we turn? The problems mentioned have been de- forms rather than the light output. Most of the mass that we bated in the journals and even, like the Shapley-Curtis debate, know about is associated with galaxies, and we have discovered at open scientific meetings. Pure thought and polemics have in the last several years to our amazement that most of this as- not been helpful and we, as ever, must turn to observations. As sociated mass lies outside the visible discs. The most persuasive good luck will have it, the prospects for new instrumentation evidence comes from galactic rotation curves. One can show in the next decade are extraordinarily exciting. Angular reso- from simple Newtonian theory that the mass interior to a cir- lution at all observing wavelengths will be dramatically im- cular orbit is proportional to the radius of the orbit times the proved. square of the rotation velocity multiplied by Newton's universal X-ray astronomy which, in the past decade, has made ex- constant G. The numerical proportionality constant depends tremely important discoveries in , probably on the geometrical distribution of the interior mass but is never finding and helping measure the mass of neutron stars and far from unity. Thus, v(r) observed may be used to calculate possibly discovering the first truly collapsed objects-black M(r). Optical observations (26), which typically extend to <104 holes-will have instruments aloft capable of imaging extra- kpc, had shown that the masses of spirals were 1010-1011 MO galactic objects on a scale comparable to present ground-based and typically less efficient than the sun by a factor of 10 in optical instruments. HEAO B, an 0.2-meter, 5-inch imaging producing light [i.e., (MIL) = 10 in solar units]. In the solar x-ray telescope, is the first of this new generation. system v(r) K r12, the mass is nearly all in the central object, In a comparable leap forward is underway the sun, and it was anticipated that beyond 5-10 kpc, galactic with the building of the very large array (VLA) in New Mexico. rotation curves would similarly become Keplerian. This did not Ultimately 27 25-meter discs are to be arranged in a Y. Two are turn out to be the case when radio measurements became in place already and planning for the others is far advanced. available from the 21-cm line emitted by cold gas. Again, angular resolutions comparable to ground-based optical These showed that the rotation curves leveled out when fol- are expected. In order to formulate some idea of the lowed to 30-60 kpc (27, 28; M. J. Roberts and R. N. Whitehurst, advance expected from these x-ray and radio instruments, we personal communication). The best studied case is our neighbor can ask the question in reverse. How much information would Andromeda (compare Fig. 9 upper and lower). Its rotation optical astronomers lose in going from seconds of arc resolution curve is nearly flat at large distances v(r) a r indicating the available now to minutes of arc available to the naked eye and startling fact that M(r) K r, quite contrary to the light distri- most present-day nonoptical instruments? bution L(r). The estimated value of (M/L) is >100 (M/L)o in Finally, the optical (including infrared) astronomers will not Downloaded by guest on September 29, 2021 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977) 1773

10 12 14 16 18 2 RADIAL DISTANCE (kpc) FIG. 9. (Upper) Our companion galaxy, the Andromeda (type Sb). (Lower) On the same scale, the rotation velocity of gas orbiting its center and, derived from that, the total mass within spheres of increasing radii (27, 28). Note that the mass in the outer parts continues to increase in regions from which very little light is emitted implying most of the mass is not in ordinary (solar) type stars, but some other dark form.

be outdone. There is a telescope in an advanced stage of plan- importance which I have not described for lack of space) will ning and currently due to be launched within the next decade certainly contribute significantly to our knowledge of the or- by the space shuttle called the Space Telescope (ST). With 2.4-m dinary stellar parts of galaxies (102 < rp < 104). Whether or nearly diffraction-limited optics capable of operating from not they will tell us what is happening at rpc < 102 and in what nearly 10-1 to 102 microns it may be expected to produce an resides the mass at rp > 104 pc no one now can know. We do advance in optical astronomy comparable to that produced by know (if we let history be our guide) that our picture of the the large optical telescopes in the first half of this century. Universe is likely to be again significantly altered and en- This group of instruments (and several others of comparable larged.

1.0 1. Shapley, H. & Curtis, H. (1921) Bull. Natl. Res. Counc. 2, 171-217. TO ZD 2. Wright, T. (1750) New Hypotheses of the Universe (London). _ ~~~~~~T 3. Kant, E. (1755) Algemeine Naturgeshichte und Theorie des Himmells, translation by E. Hubble. 4. Hubble, E. (1926) Astrophy. J. 63, 236-274. 0.0 -vdBT 5. Hubble, E. (1936) Realm of the Nebulae (Yale University Press, F~0-< New Haven, CT). c'J 6. Hubble, E. (1926) Astrophy. J. 63,281-341. : =' .~~ CD, 7. Morgan, W. (1962) Astrophy. J. 135, 1-10. 0 8. Sandage, A. (1961) The Hubble Atlas of Galaxies (Carnegie Inst., ' -1.U4nt .~~~~~~~~~~- Washington, DC). 9. Vaucouleurs, G. de (1949) Handbuch de Physik (Springer Verlag, Berlin) Vol. 53, p. 274. 10. van den Bergh, S. (1960) Astrophy. J. 131, 215-223. 11. Blaauw, A., ed. (1958) "Stellar populations," Ric. Astr. Specola -2.0 -1.0 -0.0 Vaticana 5,333. LOG 12. Oemler, G. (1976) Astrophy. J., in press. (RmPC) 13. Freeman, K. (1970) Astrophy. J. 160,811-830. FIG. 10. The mass of giant spiral galaxies, M, in units of 1012 14. Faber, S. (1973) Astrophy. J. 179, 731-754. solar masses, within a distance R in units of 106 pc of the center as 15. Sandage, A. (1972) Astrophy. J. 176,21-30. measured by various dynamical methods (see ref. 29 and E. L. Turner 16. Faber, S. M. & Jackson, R. E. (1976) Astrophy. J. 204, 668- and J. P. Ostriker, unpublished data, for details). Note the general 683. trend indicating a total mass greater than 10 times the normally ac- 17. van den Bergh, S. (1975) Lectures presented at Princeton Uni- cepted mass of -1011 Me residing at distances greater than 10 times versity Observatory, spring 1975. the normally assumed sizes of 10-2 Mpc = 10,000 pc. Rotation (0); 18. Ostriker, J. P. & Thuan, T. X. (1975) Astrophy. J. 202, 353- tidal (0); binary (0); timing (0). 364. Downloaded by guest on September 29, 2021 1774 Astronomy: Ostriker Proc. NatI. Acad. Sci. USA 74 (1977)

19. Larson, R. & Tinsley, B. (1974) Astrophy. J. 192,293-310. 26. Burbidge, E. M. & Burbidge, G. R. (1975) in Galaxies and the 20. Seyfert, C. K. (1943) Astrophy. J. 97,28-40. Universe, Vol. IX of Stars and Stellar Systems, eds. Sandage, A., Sandage, M. & Kristian, J. (Univ. of Chicago Press, London), 21. Hoyle, F. & Fowler, W. A. (1963) Monthly Notices of Royal p. 81-121. Astronomical Society 125, 169-176. 27. Roberts, M. (1975) in International Astronomical Union Sym- 22. Colgate, S. A. (1967) Astrophy. J. 150, 163-192. posium No. 69, ed. Hayli, A. (D. Reidel Publishing Co., Dor- 23. Lynden-Bell, D. & Rees, M. J. (1971) Monthly Notices ofRoyal drecht, Neth.), pp. 331-340. Astronomical Society 152,461-475. 28. Roberts, M. & Rots, A. H. (1973) Astron. Astroph. 26, 483- 24. Arons, J., Kulsrud, R. M. & Ostriker, J. P. (1975) Astrophy. J. 198, 485. 687-707. 29. Ostriker, J. P., Peebles, P. J. E. & Yahil, A. (1974) Astrophy. J. 25. Schmidt, M. (1968) Astrophy. J. 151, 393-409. 193, L1-L4. Downloaded by guest on September 29, 2021