The Stellar Epoch in the Evolution of the Galaxy A
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Astronomy Reports, Vol. 44, No. 11, 2000, pp. 711–718. Translated from Astronomicheskiœ Zhurnal, Vol. 77, No. 11, 2000, pp. 803–812. Original Russian Text Copyright © 2000 by Tutukov, Shustov, Wiebe. The Stellar Epoch in the Evolution of the Galaxy A. V. Tutukov, B. M. Shustov, and D. S. Wiebe Institute of Astronomy, Russian Academy of Sciences, ul. Pyatnitskaya 48, Moscow, 109017 Russia Received May 14, 1995 Abstract—We consider the astrophysical evolution of the Galaxy over large time scales, from early stages (an age of ~108 yrs) to the end of traditional stellar evolution (~1011 yrs). Despite the fact that the basic parameters of our stellar system (such as its size, mass, and general structure) have varied little over this time, variations in the characteristics of stars (their total luminosity, color, mass function, and chemical composition) are rather substantial. The interaction of the Galaxy with other stellar systems becomes an important factor in its evolution 100–1000 Gyr after its origin; however, we take the Galaxy to be isolated. In the model considered, the basic stages of Galactic evolution are as follows. The Galaxy forms as the result of the contraction (collapse) of a protogalactic cloud. The beginning of the Milky Way’s life—the relaxation period, which lasts about 1−2 Gyr—is characterized by active star formation and final structurization. The luminosity and colors of the Galaxy are correlated to the star formation rate (SFR). The young Galaxy intensely radiates high-energy pho- tons, which are mostly absorbed by dust and re-emitted at IR wavelengths. In the subsequent period of steady- state evolution, the gas content in the Galactic disk gradually decreases; accordingly, the SFR decreases, reach- ing 3–5M( /yr at the present epoch and decreasing to 0.03M(/yr by an age of 100 Gyr. Essentially all other basic parameters of the Galaxy vary little. Later, the decrease in the SFR accelerates, since the evolution of stars with masses exceeding 0.4M( (i.e., those able to lose matter and renew the supply of interstellar gas) comes to an end. The Galaxy enters a period of “dying”, and becomes fainter and redder. The variation of its chemical composition is manifested most appreciably in a dramatic enrichment of the interstellar gas in iron. The final “stellar epoch” in the life of the Galaxy is completed ~1013 yrs after its formation, when the evolution of the least massive stars comes to an end. By this time, the supplies of interstellar and intergalactic gas are exhausted, the remaining stars become dark, compact remnants, there is no further formation of new stars, and the Galactic disk no longer radiates. Eventually, infrequent outbursts originating from collisions of stellar remnants in the densest central regions of the Galaxy will remain the only source of emission. © 2000 MAIK “Nauka/Interpe- riodica”. 1. INTRODUCTION Observations of objects at large redshifts provide evidence for an open and infinitely expanding Universe The study of the origin and evolution of the Uni- (see, for example, [4] and the recent estimates in [5]). verse is a strategic problem in astronomy. The history Hence, when developing an evolutionary model for the of our own stellar system—the Sun and solar system— future of the Galaxy, we are limited only by the time constitutes an important part of this problem. These when fundamental physical processes (for example, the questions are closely related to the origin and develop- decay of protons and electrons) considerably change ment of life, which are vitally important for mankind. the structure of matter itself. For the Galaxy, however, Until recently, as a rule, natural science (astrophysics, this timescale is very long, and its parameters will vary in this instance) was concerned only with the prior evo- radically long before such ages. Let us consider briefly lution of the Universe; recently, however, serious inter- the basic evolutionary landmarks in the history of a typ- est has also been expressed in the astrophysical aspects ical gaseous–stellar system. of the future. One example is the large international conference “The Future of the Universe” held in 1999 The first stage in the history of the Galaxy is an in Hungary. epoch of large-scale star formation; i.e., the period over which the bulk of the gas from the protogalactic cloud The information stored in stellar physics and chem- is transformed into stars. It is commonly assumed that istry not only makes it possible to consider the future the star formation rate (SFR) in the Galaxy has development of our stellar aggregate qualitatively, but remained essentially constant, and has differed from –1 –2 also to study many aspects of its evolution quantita- the present value (0.8–13M( (Gyr) pc [6]) by no tively [1, 2]. Moreover, the luminosity function of gal- more than a factor of a few. For example, in their study axies in clusters indicates that Milky Way-type galaxies of the evolution of the solar neighborhood, Pardi and contain most of the light-emitting matter in the Uni- Ferrini [7] derived the ratio of the present star forma- verse [3], suggesting that predictions made for the Gal- tion rate and the SFR averaged over the lifetime of the ψ ψ ≅ axy should be applicable to all “normal” galaxies. Galaxy to be now/ ave 0.2. The maximum SFR in this 1063-7729/00/4411-0711 $20.00 © 2000 MAIK “Nauka/Interperiodica” 712 TUTUKOV et al. –1 –2 model is 20M( (Gyr) pc , which is reached 1 Gyr after inside the star to be Mlow = 0.077M( [14], we obtain the formation of the Galaxy. Similar values are obtained τ∗ ~ 2 × 1013 yrs. This time determines the total dura- in the model of [8]. tion of the stellar stage in the life of the Galaxy, when Until recently, these results seemed to be consistent its composition and the properties of its populations with the available observational data. Madau et al. [9] evolve. After completion of this stage, the Galaxy con- discovered that the global SFR derived from optical sists exclusively of cold, compact stellar remnants: observations is about 30% of the present value at z = 4, white (or, to be more exact, by that time already black) reaches its maximum—which exceeds the present dwarfs, neutron stars, and black holes. value by an order of magnitude—at z = 1–2, and then The large-scale structure of the Galaxy in this time gradually decreases. This means that large-scale star interval is essentially constant. An upper limit for the formation in the Universe occurred when it was around lifetime of the Galaxy as a whole is determined by the 4 Gyr old (in the standard cosmological model). Some laws of stellar dynamics. An isolated system of gravi- authors (for example, Pettini et al. [10]) have stressed tating particles (stars) dynamically relaxes on a times- that the “optical” SFR could be underestimated by a clae [15, 16] substantial factor due to absorption by dust. ν However, small values for the maximum SFR in the τ R N r = ----------------------, (2) Universe’s past came into conflict with recent observa- V 2 4πln N tions of galaxies at large redshifts at infrared and sub- ν millimeter wavelengths [11]. According to the data of where R is the size of the system, a typical random [12], there are a large number of galaxies with high velocity, V the “regular” velocity, and N the total num- ber of particles. Assuming for a spherical subsystem SFRs exceeding 100M(/yr at z > 5. Taking into account ν 11 the large numbers of such sources, it was concluded = V = 200 km/s, N = 10 , and R = 10 kpc, we obtain τ 17 that they must be ordinary galaxies at early stages of r ~ 10 yrs. The time for evaporation of particles from τ τ their evolution, rather than some exotic objects. the system e is 100 r to order of magnitude. For the ν τ 16 Observations of these primordial galaxies suggest disk population, ~ 40 km/s and r ~ 10 yrs. The that they all experienced a short burst of star formation interaction of stars with clouds of interstellar gas can lasting no longer than 1 Gyr. The short duration of this reduce this time by several orders of magnitude [17]. At interval is reflected in the chemical composition of old the current age of the Galaxy, the structure of the stellar objects in the Galaxy, which display enhanced abun- disk is determined to a considerable extent precisely by dances of oxygen compared to iron. This is usually this relatively efficient process. Later, however, the interpreted as evidence that these objects formed over a density of the gas component decreases and the role of timescale shorter than the lifetimes for the progenitors clouds becomes less important. It has been suggested of type Ia supernovae (the main producers of iron); i.e., that the general structure of the stellar disk will not shorter than ~5 × 108 yrs. This value is also close to the undergo substantial changes for at least 1013–1014 yrs gravitational timescale for a protogalactic cloud. Thus, [18]. Then, close approaches of stars (or, more pre- we can distinguish an initial evolutionary timescale for cisely, their remnants) will “divide” the system into an the Galaxy of the order of 1 Gyr—the epoch for the ini- extended halo and a dense inner core, possibly contain- tial formation of stars and basic Galactic structure. ing a supermassive black hole. The further global evolution of the Galaxy up to the These estimates were obtained for an isolated gal- present time apparently proceeded quite quietly.