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CALTECH AY 21 - Local Group

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eaa.iop.orgDOI: 10.1888/0333750888/1667 Local GroupMario L Mateo FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Tue Feb 07 18:39:28 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsLocal GroupENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSLocal GroupNot long after EDWIN HUBBLE established that galaxies are‘island universes’ similar to our home galaxy, theMILKYWAY, he realized that a few of these external galaxies areconsiderably closer to us than any others. In 1936 hefirst coined the term ‘Local Group’ in his famous bookThe Realm of the Nebulae to identify our nearest galacticneighbors. More than 60 yr later, the galaxies of theLocal Group remain particularly important to astronomersbecause their proximity allows us to obtain our mostdetailed views of the properties of normal galaxies beyondour own. These nearby systems also provide our clearestviews of how galaxies interact with one another in therelatively small volume of space of the Local Group.The brightest members of the Local Group are so closeto us that on a clear, dark night away from city lights it ispossible to see them with the unaided eye: in the southernhemisphere theLARGE MAGELLANIC CLOUD (LMC) and SMALLMAGELLANIC CLOUD(SMC) shine brightly, while in the norththeANDROMEDA and TRIANGULUM GALAXY can be seen as faintsmudges of light in the sky. These two galaxies are, in fact,the most distant objects visible with the naked eye. Fromboth hemispheres, the gossamer glow of the Milky Wayreveals the presence of billions of stars spread throughoutthe thin disk of our home galaxy. These five galaxiesconstitute the most luminous and massive members of theLocal Group.Although the existence, if not the true nature, ofthe five naked-eye Local Group galaxies and the MilkyWay has been known to humans since antiquity, the firstmember identified telescopically was Messier 32 (moretypically referred to asM32) by G-J Le Gentil in 1749.Since then, astronomers have steadily identified additionalmembers of the Local Group. By the time Hubble firstintroduced the concept of the Local Group in 1936, he wasable to list 11 galaxies that he considered to be members ofthe group. At present (1999), 43 galaxies can be cataloguedas probable members of the Local Group; these systems arelisted in table 1 along with the dates of discovery for each.Remarkably, more Local Group members have been foundin the past 30 yr than in all previous human history. Also,the era of discovery is almost certainly not over as futuresurveys uncover more members or as new nearby galaxiesare found serendipitously.Why are the galaxies of the Local Group so difficult toidentify? The principal reason is that, apart from our MilkyWay Galaxy and the large Andromeda and Triangulumgalaxies (known also asM31 and M33, respectively), theknown members of the Local Group areDWARF GALAXIES.Bydefinition, these systems have low intrinsic luminosities.They usually also exhibit very low surface brightness(seeLOW SURFACE BRIGHTNESSES GALAXIES). This property is ameasure of how spread out the galaxy’s light is on the sky.In the case of nearly every dwarf galaxy member of theLocal Group, the surface brightnesses are lower than thatof the night sky, lending them a ghostly appearance andmaking them very difficult to detect, even at close range1.Much of the recent success in finding new LocalGroupmembers is due to the availability of the many large-scale photographic surveys of the sky carried out since theseminal Palomar Sky Survey of the 1950s. Soon after thesesurveys were begun, visual searches of the photographicplates identified new nearby galaxies. Starting in the1970s, automated measurements and analyses of the platesfrom these surveys helped uncover nearly all of the mostrecently discovered Local Group members. However,even the most complete optical survey cannot find all ofthe galaxies in the sky. For example, searches for galaxiesnear the bright band of the Milky Way itself are severelyhindered by the high stellar density in this part of thesky and by the clouds of gas and dust within the planeof our Galaxy. ThisINTERSTELLAR MATTER effectively blocksall optical light from distant objects, making it impossibleto find galaxies lurking in the background. Ongoing andplanned surveys in the infrared and radio wavelengths canpenetrate the haze of the Milky Way by detecting radiationthat is unaffected by dust obscuration. These searches arealmost certain to reveal several new Local Group membersin coming years.Another complication in producing a complete censusof the Local Group is the uncertainty involved withdefining the group’s boundary. The best way to establishthis is to determine which local galaxies are gravitationallybound to one another. Since M31 and the Milky Waydominate the mass of all probable Local Group members,this process requires a good estimate of the masses ofthese two giant galaxies (see below). In addition, weneed accurate information on the distances and motionsof individual candidate Local Group galaxies to determinewhether they are physically bound to the M31–Milky Waysystem. Table 1 lists the 43 galaxies that appear to be likelymembers of the Local Group based on this approach.Although well defined, this method of identifying andcounting members of the Local Group is highly uncertain.For example, apart from a few of the nearest galaxies, wecannot measure thePROPER MOTION—the angular movementacross the sky—of external galaxies. A galaxy that maybe moving towards or away from us at a moderate speedmay be moving very rapidly across our line of sight. Thus,some of the galaxies in the table that we believe are boundto the Local Group may actually only be ‘passing throughthe neighborhood’. A second problem is that distancesto local galaxies are notoriously difficult to determinereliably. Methods that work for the Magellanic Cloudsmay not be applicable to other nearby galaxies, and vice1Unlike the apparent brightness of a galaxy, the surfacebrightness of an extended object does not change as a functionof distance—at least for distances up to a few hundred millionlight-years. This makes low-surface-brightness galaxies difficultto detect anywhere. Consequently, a very large number of low-surface-brightness galaxies may still remain hidden throughoutthe universe, enough possibly to fundamentally change our viewsof the distribution and


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