Grand challenges in Milky Way and galaxies

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Extragalactic astronomy is a relatively young science. Its birth may be set at the time of the “greatdebate”betweenHarlowShapleyandHeberCurtisontheextragalacticnatureofthenebulae(whichculminated with a meeting of the two protagonists in April 1920) or, with a more conservativestance,afewyearslateratthetimeofEdwinHubble’sdiscoveryofCepheidvariablesinMessier31,the Andromeda galaxy and the nearest spiral galaxy to our own (Hubble, 1925). In the late 1920sthe Kapteyns Universe (that posed the Sun as the center of the Milky Way) and Shapley’s view (agiantMilkyWayofwhichthespiralnebulaewerepart)hadfallenoutoffavorwiththeappreciationoftheimportanceofinterstellarmatterintheabsorptionoflight.EveniftheUniverse’sconstituentswerestarsandgas,theywerenotconfinedtoasinglegalaxy,ourowngalaxy,theMilkyWay.Inthe1950stherecalibrationoftherelationbetweenperiodandluminosityoftheCepheidstarsprovideda distance scale consistent with the one presently adopted, greatly increasing the distance of eventhe nearest spiral nebulae and settling the issue of the island universes forever (Baade, 1958).Frontiers in extragalacticastronomy—which,unlike other fields, can be literally associatedwithaphysicaldistance—haveprogressedasimprovementsininstrumentalcapabilitiesmadeitpossibleto detect and to study more and more distant objects over an ever broader range of frequencies,from the radio to the γ-ray domain. Many results on distant sources are inferred by analogy withbetter studied sources, which are usually brighter and closer. Until now this approach—which isepistemologically risky (Salmon, 2012)—has not led research into major dead ends. There havebeen fully unexpected, and less unexpected discoveries that proved to be lasting paradigm shifts.The discovery of quasars expanded the cosmic scenario to distances previously unimaginable(Schmidt, 1963). The inference of dark matter in cluster of galaxies (Zwicky, 1933), and of darkmatter influence on the rotation curve of galaxies (Rubin et al., 1980) in the early 1970s providedearly glimpses of the “dark Universe” as we understand it today. The realization of the importanceof obscuration and extinction phenomena that affect even the most powerful extragalactic sourcescamegraduallywiththedevelopmentofX-ray,spectro-polarimetricandIRinstrumentation.Otherdiscoveriesfulfilleddreamsthathadbecomereality,likethefirstplanetarytransitinfrontofaMilkyWay star that was not the Sun (Charbonneau et al., 2000). All of them enriched our view of theUniverse.Forcomprehensivereviews,thereadermayconsiderthehistoricalchaptersofD’Onofrioet al. (2012) and D’Onofrio et al. (2015) that provide first-hand accounts on major discoveries ofthe last 50 years of Galactic and extra-galactic astronomical research. The possibility of a trulyrevolutionary paradigm change was vented perhaps only once, at the time of quasars discovery(Arp, 1987). In the mid 1960s quasar distances derived from the Hubble law looked enormous,and the ensuing quasar power demanded physics that appeared exotic at the time: accretion ontoasupermassiveblackhole(Salpeter,1964;Zel’DovichandNovikov,1965).Theissuewasdefinitelysettled when astronomers could see better at larger distances. With the advent of Hubble SpaceTelescope in the early 1990s quasars were definitely confirmed as nuclei of distant galaxies asimpliedbyHubble’slaw(Bahcalletal.,1997).Activegalacticnuclei(AGN)—whichincludequasarsthat were once believed to be rare—were found to be relatively common and to play an importantrole in the evolution of galaxies (e.g., Kormendy and Ho, 2013, and references therein).