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Home , Dwingeloo 1, Zone of Avoidance

NEWS AND VIEWS EXTRAGALACTIC ASTRONOMY------ clusters) is squarely bisected by the Unveiling a new galaxy ZOA, leaving us to speculate on how many are really in this region. David Burstein Whereas the disk of the is the to those who study ON a clear, dark, moonless summer (or the ZOA that the Dwingeloo survey is but other galaxies, it is a subject in its own winter) night, look at the sky around one of several being done by astronomers, right to those who study our Galaxy. Over midnight. You will see myriads of as a meeting earlier this year3 discussed. the past 10 years, the advent of near­ pasted on the dome of the sky, bisected by Surveys in the optical, infrared and radio infrared and infrared detectors with high a fuzzy arc that is an amalgam of dark and regions of the spectrum are being done in quantum efficiency that can pierce the light diffuse patches. This arc was called various directions within the ZOA by dust have made observational study of the the 'Milky Way' by the Greeks. We know various groups. Indeed, the paper on page disk plausible. Inevitably, such studies are today that the Milky Way is the disk of our 77 draws together workers from several bound to uncover extragalactic objects Galaxy, seen edge-on by us, residents of groups; Lahav and Lynden-Bell have had lying behind the ZOA; for example, Ibata that disk. Yet what is beautiful to and colleagues 1 discovered the the eye on a dark night is the bane • .. . .. Sagittarius during a of extragalactic astronomers, as -·- • ~ .. .. ! survey that was designed to study J -· the dust and stars within the disk ·.. ~ stars within the disk and bulge of (' • of our Galaxy veil the rest of the "' .. ..- • "' our Galaxy. from our sight. Extra­ !• .. • •.- ~ Study of the stars and dust in the .. 0 galactic astronomers have long -·· .. •• • ••• ,. a: disk of our own Galaxy is impor- called the Milky Way the 'zone • . ·•··. ' .• tant for other cosmological • ·• . .. . - reasons as well. The discovery of of avoidance' for the fact that gal­ ~ : . • . •.. - axies appear to avoid that region ,; ,. . • • .. the faint lumps and bumps of the of the sky. This is an illusion, of " • . • "'. • • early Universe by the COBE course. Only this summer, we first . • • • I • .. satellite first required careful re­ .. ,; realised that the Milky Way had ' . ·.· moval of the much brighter fore­ 0 • • • an attendant dwarf galaxy so close . .. ·. ground infrared emission from that it was in danger of being .. ( . both stars and dust in the disk of 1 .. ·. · \A~: . . pulled apart ; and now, on page • "'! .... our Galaxy (the newly discovered ... .. ~-t · ~· . . 77 of this issue, R. C. Kraan­ .. :•·. ·. . .. ~ ... galaxies, on the other hand, are 2 ·. Korteweg et al. describe their • •• .. .. ·.. • • 0 far too faint to affect the COBE • • . .·.. ;,.. . discovery of a nearby ••• results). COBE, indeed, contri­ • • ... hidden by the murk. .. .· • , . butes considerably to our know­ ·. .. •( 4 If we are truly going to under­ .. ~ . • ledge of the Galactic disk . Separ­ stand the structure of the Uni­ .: . • ._,._ ..- . ately, large-scale surveys that are verse, we must be able to study the .. • 0 designed to find the dark-matter 15 per cent or so that is obscured • • ,: candidates known as MACHOs • • . .. .•· . . by our own Milky Way. The trick ' • ,.. . . (massive compact halo objects; is to search the region at a .... ' .. • •• • really, very low-mass -like ob­ wavelength that is not badly . •0 • ·:• • .. jects) must observe as many stars obscured by dust, and the Dwing­ •• .... as possible in order to have a eloo telescope in The Netherlands . _t ,. -.· ~ . .. reasonable probability of detect­ is, for the time being, entirely Agalaxyon-· our doorstep- image of taken by the ing the light variation caused by dedicated to doing just that. A Isaac Newton Telescope. passage of one MACHO along systematic survey of the zone of our line of sight to one star. So avoidance (ZOA) region is being made at a long-standing cosmological interest in MACHO surveys place high priority on a wavelength of 21 em (the emission what lies behind the ZOA, Henning was relatively dust-free paths within the disk comes from atomic , H I). earlier involved in a more limited H I and bulge of our Galaxy. Analysis of the results is only just begin­ survey aimed at detecting nearby galaxies, Until 10 years ago, most maps of ning, and the first fruits of the survey and to the best of my knowledge, Kraan­ galaxies on the sky placed a veritable include the new spiral galaxy. More can Korteweg herself was the first to start 'celestial incognito' on the ZOA; this surely be expected to emerge as the survey systematically examining optical images region was simply kept blank. Ten years continues. for galaxies within the ZOA, some six or ago our understanding of the distribution The new galaxy, seen glimmering faint­ seven years ago. of stars in the disk more than 2 kpc from us ly behind the foreground stars in the Why this flurry of interest? After all, was limited to a very few lines of sight and photograph, lies about 0.1° below the astronomers have known about the to studies of only the brightest stars. It is and has an angular size of obscuration of the disk of the Milky Way encouraging to see the veil of the Milky about 3 arcminutes as imaged by the for nearly a century. Speaking for myself, Way being slowly lifted; and we await with discoverers. As far as Kraan-Korteweg et my attention was first drawn to the ZOA anticipation the full unveiling. D a!. can guess, it is about 3 megaparsec seven years ago, when it became apparent away (about 107 light years), too distant to that one of the largest components of the David Burstein is in the Department of be a member of our of motion of our Galaxy, relative to the Physics and Astronomy, Arizona State Uni• galaxies. It therefore is not likely to share expansion of the Universe, is due to the versity, Tempe, Arizona 85287, USA. the fate of the dwarf galaxy discovered gravitational attraction of mass that lies earlier this year, which is a mere 24 behind the ZOA. Moreover, the largest 1. lbata, R. A., Gilmore, G. &Irwin, M. J. Nature370, 194-196 (1994). kiloparsec away and is apparently being nearby concentration of visible mass that 2. Kraan-Korteweg, R. C. eta/. Nature 372, 77-79 (1994). pulled apart by the Milky Way. we do know exists (called variously the 3. Unveiling Large-Scale Structures behind the Milky Way (eds Balkowski, C. & Kraan-Korteweg, R. C.) (Astr. Soc. It is indicative of the importance placed Great Attractor or the Hydra-Centaurus­ Pacif. Conf. Series67, 1994). on understanding the Universe beyond Pavo-Indus-Telescopium collection of 4. Arendt, R. et at. Astrophys. J. 425, L85--88 (1994). 38 NATURE · VOL 372 · 3 NOVEMBER 1994