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Two Kinds of Halo Gas Identified Small Companions for Early Dinosaurs

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Two Kinds of Halo Gas Identified Small Companions for Early Dinosaurs N~A~ru~~~v~0=L.~m~12~JMru~~AA~Y~I~~~ ___________________ NEWSANDVI8NS ____________________________________ 1=11 stars is observed are considerably different Galaxy structure from these expected for corotating gas. Evidently, these gas clouds have large peculiar velocities with respect to a co­ Two kinds of halo gas identified rotating halo. The data are qualitatively from Joel Bregman consistent with features of a corona that rotates more slowly than the disk and of ALTHOUGH most of the cool gas in our beyond a height of 3,000 light years. galactic fountain models. More data are galaxy lies in the galactic disk, it is not However, little neutral gas exists further needed before the various models can be entirely confined there and the study of gas than 6,000 light years from the disk. Albert tested critically and more studies along the above and below the disk (halo gas) has has added to our understanding by com­ lines of Albert's would be valuable. 0 been an especially vigorous area of research paring the difference in absorption for during the past few years. Unfortunately, it pairs of stars, one of which is just above the Joel Bregman is in the department ofphysics at has frequently been difficult to galactic disk and the other in the halo. She New York University. discriminate between disk and halo gas, for shows that the halo is rich in gas and it is not possible to determine distances to distinguishes between two types of neutral I. Mhort, C,E. Aslrophys. J. 271,509(1983). gas clouds and we can only look at the halo halo gas: a layer of gas similar to disk gas 2. York, D. G., Ann. Rev. Aslr. Ap. 20,221 (1982). gas through the disk. In a recent paper C. that has a scale height of about 3,000 light 3. Webster, A, Mon. Nol, R. /ISIr. Soc. 115, S07 (1978). 4. HiJdon, J.C. Aslrophys. J. 232, 113(1979). E. Albert has used a new approachl, a years, and higher velocity gas richer in cer­ ~ . Nousek, J. A., Fried, P. M., Sanders, W. T. Kraushaar, study of low ionization gas within 8,000 tain heavy elements (Ti, Ca, and Na) that is W.L. Aslrophys. I . 251,83 (1982). light years of the disk (the lower galactic 6. Giovanelli, R. ASlr. J, 15, Jl~~ (1980). found only above the disk. Whether or not 7. Schwarz, U. J. '" Oort, J .H. Aslrophys. 101, 30~ halo). She has conclusively shown that a there is a true difference between the (1981). 8. Sonsaila, A. '" York, D.G. Aslrophys. I . 2042, 976 considerable amount of neutral (cool) gas distribution of warm and neutral gas and (1980). exists in the lower galactic halo and that how these components are related to the 9. Savage, B.D., '" de Boer, K.S. Aslrophys. I . 243, there are two kinematically distinct types of million degree gas and the cosmic rays re­ 0460 (1981), 10. Pettini, M. '" West, K. A. Aslrophys. I . 260, 561 halo gas. mains to be seen. (1982). Gas is believed to exist in the halo in The absorption-line gas is rotating, but II. York, D. G., Blade., J. C., Cowie, L. L .. Morton, 2 D.C., Sonaaila, A. '" Wu, C.C. Aslrophys. I . 155, several different forms • Weak radio con­ not precisely with the galactic disk. This 467 (1982). tinuum emission has been attributed to has been clearly shown by Albert and 12. Hobbs, L.M .• Moraan, W.W .• Mhort, C.E. '" Lockman, l3 F.J., Aslrophys. I . 243, 690(1982). synchrotron emission from cosmic ray others who demonstrated that the 13. deBoer, K.S. " SavaSe, B.D. Aslrophys. I . 265, 210 electrons that may extend up to 30,000 light velocities at which absorption against halo (1983). years from the disk3.4 • Plasma at temperatures of millions of degrees may Palaentology also permeate the halo and be responsible for part of the diffuse X-ray background (0.1-2 keVp . Virtually nothing is known Small companions for early about the vertical distribution of this gas except that it is unlikely to extend beyond 30,000 light years from the disk. Radio dinosaurs telescope surveys have revealed the from Michael Benton presence of neutral hydrogen gas at a few hundred degrees kelvin but with unusual FOSSILIZED cave systems around Bristol, latest new forms to be described have been velocities6,7 ; this gas is presumed to lie in England, have yielded an important fauna Gephyrosaurus5.6, a 4O-cm long lizard-like the halo as well. Finally, absorption line of small reptiles dating from the times of animal, and Planocephalosaurus. a small spectroscopy in the optical and ultraviolet the early dinosaurs, about 200 million years sphenodontid3 • bands has been used to demonstrate the ago. Until recently, these were thought to The dating of the fissures has always presence of halo gas with a temperature in be specialized upland faunas because the been a problem. Some may be late Triassic the range 103-10' K8-1O . animals were rather different from those of (Norian) in age, others Rhaetic, and others Optical and ultraviolet spectroscopy the typical lowland dinosaur beds. Strong early Jurassic (between about 190 and 225 have been powerful tools for analysing the evidence has now, however, been million years ago). Some of the South properties of halo gas. By detecting ab­ presented against this view, and new infor­ Wales fissures have been dated as Rhaetic sorption lines caused by gas intervening mation on the dating and palaeoecology of or early Jurassic on the basis of plants, between the Sun and a background source these remarkable faunas has also been spores and gastropods 9• One of the fissures (a star or quasar), one is able to estimate the givenl~ . near Bristol has yielded more abundant column density, vertical distribution, Fossil mammal teeth were first found in spores which suggest a Rhaetic age I . The chemical abundance and temperature of the Bristol fissures by Charles Moore over main fissure sites, and the reptiles that they the gas. Strong absorption is commonly 100 years ago 7, and the frrst reptile remains have produced are described in references detected for the ultraviolet resonance lines were described 8 in 1939. These consist of 1,4,9-11. of C IV and Si IV. which is interpreted as the bones of a small lizard-like animal At the end of the Triassic period, the evidence for warm gas (l0"-10' K) lying (Clevosaurus), very like the unique living scenery around Bristol consisted of between 1,500 and 10,000 light years above tuatara, Sphenodon. from New Zealand. limestone hills in which deep fissures and the disk9- 1 I . For neutral gas « 103 K), a Many more important mammal and reptile caves formed. The climate was subtropical more precise study of the vertical distribu­ fossils have been collected since then from with wet and dry seasons, and the verte­ tion has been made by comparing the total dozens of fossil caves in South Wales and brate fauna consisted of dinosaurs, small column density (from the 21 cm HI emis­ around Bristol. The reptiles include a lizard-shaped reptiles and early mammals. sion line) with the column density between remarkable small gliding animal called Occasionally, these animals fell into the the Sun and a halo star (using the Ly a line . Kuehneosaurus. isolated bones of the fissures, or were washed in by flash floods of neutral hydrogen) J2. carnivorous crocodile-shaped Rileya and with mud and sand. The habitat was There is a vast amount of gas above the the dinosaur Thecodontosaurus. a croco­ assumed to have been a rocky 'upland', thin disk of the galaxy (half-thickness dile, and various as yet undescribed about 300 light-years) and about one-third sphenodontids and small archosaurs Michael Benton is in the University Museum, of the total column density of neutral gas is (crocodiles/ dinosaurs/thecodontians). The Parkes Road, Oxford, OX1 3PW. 0028.j)836/84/ 0201 I I ~I SOI .OO C 1984 Macmillan Journals Ltd .
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