eaa.iop.org DOI: 10.1888/0333750888/1667 Local Group Mario L Mateo From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Tue Feb 07 18:39:28 GMT 2006 [131.215.103.76] Terms and Conditions Local Group E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Local Group of the night sky, lending them a ghostly appearance and making them very difficult to detect, even at close range1. Not long after EDWIN HUBBLE established that galaxies are Much of the recent success in finding new Local ‘island universes’ similar to our home galaxy, the MILKY Group members is due to the availability of the many large- WAY, he realized that a few of these external galaxies are scale photographic surveys of the sky carried out since the considerably closer to us than any others. In 1936 he seminal Palomar Sky Survey of the 1950s. Soon after these first coined the term ‘Local Group’ in his famous book surveys were begun, visual searches of the photographic The Realm of the Nebulae to identify our nearest galactic plates identified new nearby galaxies. Starting in the neighbors. More than 60 yr later, the galaxies of the 1970s, automated measurements and analyses of the plates Local Group remain particularly important to astronomers from these surveys helped uncover nearly all of the most because their proximity allows us to obtain our most recently discovered Local Group members. However, detailed views of the properties of normal galaxies beyond even the most complete optical survey cannot find all of our own. These nearby systems also provide our clearest the galaxies in the sky. For example, searches for galaxies near the bright band of the Milky Way itself are severely views of how galaxies interact with one another in the hindered by the high stellar density in this part of the relatively small volume of space of the Local Group. sky and by the clouds of gas and dust within the plane The brightest members of the Local Group are so close of our Galaxy. This INTERSTELLAR MATTER effectively blocks to us that on a clear, dark night away from city lights it is all optical light from distant objects, making it impossible possible to see them with the unaided eye: in the southern to find galaxies lurking in the background. Ongoing and hemisphere the LARGE MAGELLANIC CLOUD (LMC) and SMALL planned surveys in the infrared and radio wavelengths can MAGELLANIC CLOUD (SMC) shine brightly, while in the north penetrate the haze of the Milky Way by detecting radiation the ANDROMEDA and TRIANGULUM GALAXY can be seen as faint that is unaffected by dust obscuration. These searches are smudges of light in the sky. These two galaxies are, in fact, almost certain to reveal several new Local Group members the most distant objects visible with the naked eye. From in coming years. both hemispheres, the gossamer glow of the Milky Way Another complication in producing a complete census reveals the presence of billions of stars spread throughout of the Local Group is the uncertainty involved with the thin disk of our home galaxy. These five galaxies defining the group’s boundary. The best way to establish constitute the most luminous and massive members of the this is to determine which local galaxies are gravitationally Local Group. bound to one another. Since M31 and the Milky Way Although the existence, if not the true nature, of dominate the mass of all probable Local Group members, the five naked-eye Local Group galaxies and the Milky this process requires a good estimate of the masses of Way has been known to humans since antiquity, the first these two giant galaxies (see below). In addition, we member identified telescopically was Messier 32 (more need accurate information on the distances and motions typically referred to as M32) by G-J Le Gentil in 1749. of individual candidate Local Group galaxies to determine Since then, astronomers have steadily identified additional whether they are physically bound to the M31–Milky Way members of the Local Group. By the time Hubble first system. Table 1 lists the 43 galaxies that appear to be likely introduced the concept of the Local Group in 1936, he was members of the Local Group based on this approach. able to list 11 galaxies that he considered to be members of Although well defined, this method of identifying and the group. At present (1999), 43 galaxies can be catalogued counting members of the Local Group is highly uncertain. as probable members of the Local Group; these systems are For example, apart from a few of the nearest galaxies, we listed in table 1 along with the dates of discovery for each. cannot measure the PROPER MOTION—the angular movement Remarkably, more Local Group members have been found across the sky—of external galaxies. A galaxy that may in the past 30 yr than in all previous human history. Also, be moving towards or away from us at a moderate speed the era of discovery is almost certainly not over as future may be moving very rapidly across our line of sight. Thus, surveys uncover more members or as new nearby galaxies some of the galaxies in the table that we believe are bound are found serendipitously. to the Local Group may actually only be ‘passing through the neighborhood’. A second problem is that distances Why are the galaxies of the Local Group so difficult to to local galaxies are notoriously difficult to determine identify? The principal reason is that, apart from our Milky reliably. Methods that work for the Magellanic Clouds Way Galaxy and the large Andromeda and Triangulum may not be applicable to other nearby galaxies, and vice galaxies (known also as M31 and M33, respectively), the known members of the Local Group are DWARF GALAXIES.By 1 Unlike the apparent brightness of a galaxy, the surface definition, these systems have low intrinsic luminosities. brightness of an extended object does not change as a function They usually also exhibit very low surface brightness of distance—at least for distances up to a few hundred million light-years. This makes low-surface-brightness galaxies difficult (see LOW SURFACE BRIGHTNESSES GALAXIES). This property is a to detect anywhere. Consequently, a very large number of low- measure of how spread out the galaxy’s light is on the sky. surface-brightness galaxies may still remain hidden throughout In the case of nearly every dwarf galaxy member of the the universe, enough possibly to fundamentally change our views Local Group, the surface brightnesses are lower than that of the distribution and numbers of galaxies in the universe. Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Local Group E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Table 1. Galaxies of the Local Group. Year of RA Declination Distance Luminosity Mass Galaxy Other name discovery (2000) (2000) Type Subgroup (Mly) VT (106L) (106M) ◦ M31 NGC 224 – 00h42.7m +41 16 SbI–II M31 2.5 3.4 25 000 700 000 ◦ Milky Way – 17h45.7m −29 01 Sbc MW 0.03 – 8 300 350 000 ◦ M33 NGC 598 – 01h33.9m +30 40 ScII–III M31 2.7 5.9 3 000 30 000 ◦ LMC – 05h23.6m −69 45 IrrIII–IV MW 0.16 0.4 2 100 20 000 ◦ SMC NGC 292 – 00h52.7m −72 50 IrrIV–V MW 0.19 2.0 580 1 000 ◦ WLM DDO 221 1923 00h02.0m −15 28 IrrIV–V LGC 3.0 10.4 500 150 ◦ M32 NGC 221 1749 00h42.7m +40 52 E2 M31 2.6 8.1 380 2 120 ◦ NGC 205 M110 1864 00h40.4m +41 41 E5p/dSph–N M31 2.6 8.1 370 740 ◦ NGC 3109 DDO 236 1864 10h03.1m −26 10 IrrIV–V N3109 4.1 9.9 160 6 550 ◦ IC 10 UGC 192 1895 00h20.4m +59 18 dIrr M31 2.7 11.6 160 1 580 ◦ NGC 185 UGC 396 1864 00h39.0m +48 20 dSph/dE3p M31 2.0 9.1 130 130 ◦ NGC 147 DDO 3 1864 00h33.2m +48 31 dSph/dE5 M31 2.3 9.4 130 110 ◦ NGC 6822 DDO 209 1864 19h44.9m −14 48 IrrIV–V LGC 1.6 9.1 94 1 640 ◦ IC 5152 1895 22h02.7m −51 18 dIrr LGC 5.2 11.2 70 400 ◦ IC 1613 DDO 8 1906 01h04.9m +02 08 IrrV M31 2.3 9.6 64 795 ◦ Sextans A DDO 75 1942 10h11.1m −04 43 dIrr N3109 4.7 11.3 56 395 ◦ Sextans B DDO 70 1955 10h00.0m +05 20 dIrr N3109 4.4 11.4 41 885 ◦ Sagittarius 1994 18h55.1m −30 29 dSph-N MW 0.08 4.0 18 – ◦ Fornax 1938 02h40.0m −34 27 dSph MW 0.45 7.6 16 68 ◦ Pegasus DDO 216 1958 23h28.6m +14 45 dIrr/dSph LGC 3.1 12.0 12 58 ◦ EGB 0427+63 UGCA 92 1984 04h32.0m +63 36 dIrr M31 4.2 13.9 9.1 – ◦ SagDIG UKS1927-177 1977 19h30.0m −17 41 dIrr LGC 3.4 13.5 6.9 9.6 ◦ And VII Cassiopeia 1998 23h26.5m +50 42 dSph M31 2.5 15.2 5.7 – ◦ UKS2323-326 UGCA 438 1978 23h26.5m −32 23 dIrr LGC 4.3 13.8 5.3 – ◦ Leo I DDO 74 1955 10h08.5m +12 19 dSph MW 0.81 10.1 4.8 22 ◦ And I 1972 00h45.7m +38 00 dSph M31 2.6 12.8 4.7 – ◦ GR 8 DDO 155 1956 12h58.7m +14 13 dIrr GR8 4.9 14.4 3.4 7.6 ◦ Leo A DDO 69 1942 09h59.4m +30 45 dIrr MW 2.2 12.8 3.0 11 ◦ And II 1972 01h16.5m +33 26 dSph M31 1.7 12.7 2.4 – ◦ Sculptor 1938 01h00.2m −33 43 dSph MW 0.26 8.5 2.2 6.4 ◦ Antlia 1985 10h04.1m −27 20 dIrr/dSph N3109 4.1 14.8 1.7 12 ◦ And VI Peg dSph 1998 23h51.7m +24 36 dSph M31 2.7 14.1 1.4 – ◦ LGS 3 Pisces 1978 01h03.9m +21 53 dIrr/dSph M31 2.6 14.3 1.3 13 ◦ And III 1972 00h35.3m +36 31 dSph M31 2.5 14.2 1.1 – ◦ And V 1998 01h10.3m +47 38 dSph M31 2.6 15.0 1.0 – ◦ Phoenix 1976 01h51.1m −44 27 dIrr/dSph MW 1.4 13.2 0.9 33 ◦ DDO 210 Aquarius 1959 20h46.8m −12 51 dIrr/dSph LGC 2.6 14.7 0.8 5.4 ◦ Tucana 1985 22h41.8m −64 25 dSph LGC 2.9 15.2 0.6 – ◦ Leo II DDO 93 1950 11h13.5m +22 09 dSph MW 0.66 12.0 0.6 9.7 ◦ Sextans 1990 10h13.1m −01 37 dSph MW 0.28 10.3 0.5 19 ◦ Carina 1977 06h41.6m −50 58 dSph MW 0.33 10.9 0.4 13 ◦ Ursa Minor DDO 199 1955 15h09.2m +67 13 dSph MW 0.21 10.3 0.3 23 ◦ Draco DDO 208 1955 17h20.3m +57 55 dSph MW 0.27 10.9 0.3 22 versa.