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Evading the Zone of Avoidance Included Six Normal Spirals, Four Assorted Dwarfs and One Probable Magellanic Virginia Trimble Irregular

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Evading the Zone of Avoidance Included Six Normal Spirals, Four Assorted Dwarfs and One Probable Magellanic Virginia Trimble Irregular ~2t~2------------~----------------------N8NSAN0~8NS----------------NA_TU__ R_E_v_o_L_ . 3_~__ 19_N_o_v_E_M_BE~R~t9~87 into the eyepiece of a normal microscope, phase contrast or DIC allow the accurate measures the amount of HI present. Emis­ is made to scan a particular plane of the location of the fluorescent components sion lines from irregular and dwarf gal­ specimen. A digitized image is built up within the cell. As for CCDs, video rates axies are more nearly gaussian in shape point by point, by collecting either the are not practical , but the available rates of and rather narrower. Thus Kerr and fluorescent signal or simply the reflected 1-5 frames per second are fast enough to Henning can say a good deal about their light that travels back along the same compete with any other method of record­ 16 galaxies, even though only one was path. Thus, successive levels of fluores­ ing fluorescent images. 0 previously known (as NGC2377) and one cent staining of cell cytoplasm can be other visible on the Palomar Sky Survey Linda Amos is at the MRC Laboratory of imaged independently. Simultaneously Molecular Biology, Hills Road, Cambridge prints. Nine have characteristic spiral line scanned (non-confocal) images using CB22QH, UK. profiles and three more may, while the other four are probably dwarf or irregular Radioastronomy galaxies. For comparison, the 11 un­ obscured sources were all previously known and/or identifiable as galaxies and Evading the zone of avoidance included six normal spirals, four assorted dwarfs and one probable Magellanic Virginia Trimble irregular. Thus, blind 21-cm searching can pick out a wider range of galaxy types than NEARLY one-fifth of the extragalactic sky be a bridge of galaxies with velocities near does the infrared-driven method. In addi­ 1 lurks out of sight, obscured by dust and 5,000 km s- • Searches for 21-cm emission tion, galaxies can be found reliably right crowded by images of stars in the disk of from neutral hydrogen (H 1) in the 250 down to the galactic equator, leaving no our own Milky Way. Astronomers, sus­ sources (using the Arecibo 1 ,000-foot zone of avoidance at all. Once the galaxies pecting with Jane Austen's Catherine, telescope) revealed 63 spiral galaxies with have been identified in this way , IRAS 1 that the locked drawer contains more velocities less than 8,000 km s- , 29 of data for the relevant locations can be co­ interesting objects than the open ones, them previously unknown. And the pre­ added. In the pilot programme, co-adding have long wished to penetrate this zone dicted bridge is there. The average velo­ yielded plausible counterpart sources for of avoidance with infrared and radio city of galaxies in that part of the sky is most of the obscured galaxies. 1 observations. The desire has become 4,950 km s- • Could one probe the entire three more acute with the recognition of very This method has the advantage of con­ steradian zone of avoidance this way? In large clumps and holes in the distribution siderable efficiency- one knows exactly principle, yes , for there is a suitable of galaxies and clusters of galaxiesu . Such where to point the radio telescope. It has southern counterpart to the NRAO 91-m large structures are a prime test of galaxy­ the disadvantage that IRAS galaxies are in the Parkes (Australia) 64-m. But the formation models'. But to trace them out largely a somewhat special subset of spirals, beam width of such telescopes is narrow, properly, we must be able to map the rich in gas and young stars. In addition, about 10', versus for instance 6° for the 48- whole sky. Indeed, at least two known there still remains a narrower zone of inch Schmidt optical telescope that did the sheet-like features disappear into the zone avoidance, because the infrared sources Palomar survey. Thus one requires of avoidance and do not seem to come out are crowded so close together within 3.5° several hundred thousand telescope the other side'. of the galactic plane that the colours pointings (of about five minutes each) Technology is, at long last, catching up necessary for selection of candidate compared to a few hundred half-hour with wishful thinking, and two publica­ galaxies cannot be measured. Searches of optical exposures. And only a limited tions'·" now report the results of 21-cm additional sky area outside this narrower range of velocities can be covered at a radio searches for extragalactic objects at zone can, nevertheless, provide informa­ time, though one would clearly like to go previously inaccessible low galactic lati­ tion on many hundreds of currently out to the 12,000 km s- 1 limit of existing tudes. So far, at least, there be here no unknown galaxies and their distribution in optical samples. dragons. But there are a few dozen newly space, tracing structure over nearly the A complete programme would, there­ discovered galaxies- apparently normal whole sky. fore, necessarily involve many years and spirals, irregulars, and dwarfs, all well Kerr and Henning' adopted a different many observers. But, of course, it need not outside the Local Group - with the approach, looking for 21-cm emission be complete to be useful. In particular, promise of many more to come. No without guidance from observations at now that both the infrared-driven and dragons had really been expected, other wavelengths. In a pilot programme, blind 21-cm techniques are known to although earlier chance discoveries in the the 91-m transit radio telescope at the work, deliberate searches can be mounted zone of avoidance had included the two National Radio Astronomy Observatory for continuations and bridges from known large galaxies Maffei I and II immediately (NRAO), West Virginia, scanned a clusters and filaments into the zone of outside the Local Group7 and, just pos­ random 0.5 per cent of the northern avoidance. And we can reasonably expect sibly, a tiny galaxy' almost inside the avoidance zone hunting for H 1 with that at least a few dragon tails will turn 1 Milky Way. velocities between 300 and 7,500 km s- • ~ - 0 The two investigations were rather dif­ In more than 100 hours of observing, they I. Kirshner, R.P., Oemler, A .. Schechter, P.L. & Shectman, ferent in conception. Dow et aU began found 16 interesting sources. A smaller S. Astrophys. f. 248 , L5 7 (1981). 2. Batusky, D.J. & Burns, 1. 0. Astr. f. 90, 1413 (1 985) . with a catalogue of infrared sources pin­ region of unobscured sky, examined simi­ 3. White, S.D .M. et a/. Astrophys. f . 313. 505 (1987) . pointed by the Infrared Astronomy larly for comparison, yielded 11 . 4. de Lapparent. V., Geller, M.J . & Huchra, J.P. Astrophys. f . 302 , L1 (1986). Satellite (IRAS). They selected 250 Though the method may not seem very 5. Dow, M.W., Lu, N.Y., Salpeter, E.E. & Lewis, B.M. sources in two areas between 3.5° and 15° efficient, an H 1 line profile is remarkably Bull. Am. astr. Soc. 18 , 1034 (1986). informative once you find it. Spiral 6. Kerr, F.J. & Henning, P.A . A srrophys. f. 320, L99 (1987). from the galactic plane, distinguished by 7. Maffei, P. Pub/. Astr. Soc. Pac. 80,618 (1968). particular infrared colours and fluxes to galaxies reveal themselves through steep­ 8. Simonson, S.C. Astrophys. f. 201, LI03 (1975). exclude most galactic stars and bits of inter­ sided, flat or dimpled-top spectral line 9. Giovanelli , R. & Haynes, M . Astr. f. 90, 2445 (1985). stellar cirrus from their list. One area was shapes, whose central velocity tells you 10. Tully. R .B. & Fisher , J.R. Astr. Astrophys. S4 , 061 (1 977). chosen deliberately to fall between the the distance to the galaxy (via Hubble's Virginia Trimble is Professor of Phyics at the Perseus-Pisces and Lynx-Ursa Major law). The line width is proportional to University of California, Irvine, California 92717 and is Visiting Professor of Astronomy at superclusters of galaxies, where Giovanelli the optical luminosity and mass of the University of Maryland, College Park, and Haynes• had predicted there should the galaxy"'. And the intensity of the line Maryland 20742, USA . © 1987 Nature Publishing Group.
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