The Persistent Problem of SpiralGalaxies

The current explanation forspiral galaxies isthatdensity wavesinaspiral formrotate through thedisks ofthese galaxies, con- tinually forming newarmsofhotbright stars andexcited gas.The discussion hereshowsthatmanyobserved properties ofspiral galaxies contradict this assumed density wavemechanism. Alternatively, ithasbeenclear since theearly 1950's thatgalaxies characteristically eject material fromtheir nuclei. A numberofas- tronomers havepresented evidence thatitisthose ejections fromthe central regions ofrotating galaxies thatareresponsible forthespiral arms. Theevidence isreviewed andevaluated here, anditisconcluded thattheformandnature ofthearms, their magnetic fields androta- tional velocity characteristics, canbestbeexplained byejections of material, including plasma, fromwhichthespiral armstarsare formed. Thisconclusion furnishes ananswertothelong-standing problem ofhowthemagnetic fields arise intheouter regions ofspirals. Perhaps mostimportantly, theformation andrenewal ofspiral arms byejection ofplasma doesnotrequire themtobeinrotation onlyun- derthepull ofgravitational forces. Ifrotational energy istransferred toouterregions byejections, thecurrent interpretation ofrotation curves mayoverestimate massesofspiral galaxies. Iftheproblem of "'missing mass"isdiminished, soisthenecessity forexotic suggestions toaccount forthis undetected matter. I.INTRODUCTION r HEEarl ofRosse wasoneofthefirst observers toturn abigtelescope ongalaxies. Around 1850hesketched thesymmetrical spiral pattern ofthe"Whirlpool Ne- bula," nowknownasMessier 51.Astronomical photog- raphy later showed howubiquitous this spiral formwas inouruniverse ofgalaxies. Butstill today, itmaybethat little moreisunderstood about thecauseofspiral struc- turethanwhenthefirst ofthese forms wereglimpsed so long ago. Uponphotographic study withlarge telescopes, itbe- cameapparent that thespiral armswerecomposed prin- cipally ofbright youngstars. Thesehotstars also excited radiation fromassociated clouds ofgas.AsBaadechar- acterized it, theluminous stars andnebulosities outline thearmslike beads onastring. Butthere aroseadifficult paradox. Itbecameknowninthe1940's fromworkby Oort, Lindblad, andothers that galaxies rotated differen- tially, with theinner parts going faster thantheouter. This meantthat inafewrotations aspiral armshould bewound upinacircle. Withtheassumed agesofthegalaxies, however, spirals should haveperformed almost 100rev- olutions! Whywasnottheuniverse filled withring gal- axies?