Abstract:Theionizingphotonsproducedbymassivestarsarekeyactorsingalaxyevolu9on.Ionizingphotonproduc9onandescapeispoorlyunderstood.Improvedspace-based,spa9ally-resolved,mul9plexedspectroscopiccapabili9escoveringλobs=1000–3000Å,inconcertwithspectroscopyfromtheELTsandJWST,wouldleadtodefini9veanswersastohowionizingphotonsareproducedandleaked,whatpopula9onsofgalaxiesareresponsibleforionizingphotonleakage,whatdetermineswhetherescapeispossible,andhowionizinggalaxypopula9onsevolveovercosmic9me.In-handUVspectraofbrightgravita9onallylensedgalaxiesdemonstratethisscience.Thisposterisanadapta1onofourAstro2020whitepaper(Rigbyetal.)

Theproduc9onandescapeofionizingphotonsfromgalaxiesovercosmic9me:Demonstra9ngscienceforfuturemissionsusingin-handUVspectraoflensedgalaxiesJaneRigby(NASAGoddard),MaOBayliss(MIT),DanielleBerg(OSU),RongmonBordoloi(NCSU),JohnChisholm(UCSC),MichaelFlorian(NASAGoddard),MaOhewHayes(StockholmU.),MichaelGladders(U.Chicago),BethanJames(STScI),SangeetaMalhotra(NASAGoddard),SallyOey(U.Michigan),JohnO’Meara(KeckObservatory),EmilRivera-Thorsen(U.Oslo),K.Sharon(U.Michigan)

KeyScienceQues;ons:• Howdogalaxiesproduceionizingphotons?

• Howdoionizingphotonsescapefromgalaxies?

• Whichgalaxiesleakionizingphotons?WhichgalaxiesreionizedtheUniverse?

• WhataboutthosegalaxiespermiOedionizingphotonstoescape?

1.Ionizingphotonsareimportant. TheUniversewasre-ionized;thereforetheremusthavebeenenoughionizingphotonstodothejob,andtokeeptheUniverseionizedatlaterepochs.Studyinggalaxiesintheepochofreioniza9onisakeysciencegoalforJWST,whichwillcharacterizethegalaxyluminosityfunc9onandmeasurephysicalproper9eslikedensi9esandmetallici9es.However,JWSTwillbeunabletomeasuretheionizingfluxesofgalaxiesathighredshie,forasimplereason:theopacityoftheintergalac9cmediumpreventsionizingphotonsfromz⪅4fromreachingEarth.Onlyatredshiesz⪅4isitpossibletomeasureionizingfluxfromgalaxies(seeFig.9ofSteideletal.2018).

Ionizingphotonsareproducedbymainsequenceandevolvedmassivestars.TheionizingphotonsionizetheHIIregions,powerthestrongnebularemissionlinesthatarekeydiagnos9csofgalaxyevolu9on,andifthereareclearchannels,escapetheirparentgalaxy.Themassivestarsthatproducetheionizingphotonsalsoproducethenon-ionizingultravioletcon9nuum,nucleosynthesizetheheavierelements,anddistributethoseelementsthroughacombina9onofstellarwindsandsupernovaewinds. Aeerheroiceffort,36galaxieshavebeenspectroscopicallyiden9fiedto"leak"ionizingphotons,spanningredshiesfromz~0toz~3(Leitetetal.2011;Borthakuretal.2014;Vanzellaetal.2015,

2016,2018;Izotovetal.2016a,b;Leithereretal.2016;Shapleyetal.2016;Bianetal.2017;Izotovetal.2018a,b;Steideletal.2018;Rivera-Thorsenetal.2019;Figure1).WearejustbeginningtolearnwhatkindsofgalaxiesmighthavereionizedtheUniverse.

3.Ionizingphotonscanescapegalaxies.How?NeutralHatomsabsorbionizingphotons,whichblockstheirescape.Toescape,ionizingphotonscantravelthroughclearchannels(orhighlyionizedchannels)inthegas,orcanstreamoutofdensity-boundedHIIregionsthataresmallerthantheirStrömgrenradii(Figure3).Ionizingphotonsfromgalaxyoutskirtsshouldhaveaneasier9meescaping(Keenanetal.2017),andcouldplayanoutsizedrole.Itisnotunderstoodwhetherescapeisdeterminedbylocalfactors(star

forma9onhistory,nebulargeometryandioniza9on)orlarge-scalefactors(halomass,galaxymergers,environment),noristheroleoffeedbackunderstood.Dothemassivestarsbreakopentheirownneutralgasshells,likehatchlings,oristheneutralgastornopenbylarge-scalegalac9cevents?HowimportantisthehigherUVbackgroundathigherredshieatweakeningtheshells?Donumeroussmall“eggs”dominateionizingphotonescape,orthemostluminousones? Lymanalphaisanindirectprobeoftheneutralgas(Verhammeetal.2006;Jaskot&Oey2014;Rivera-Thorsenetal.2017b,Yangetal.2016,2017).Severalotherindirectpredictorsofescapehavebeensuggested.

4.Thesescienceques;onscanbesolved,defini;vely,overcosmic;me.1)RedshiO0.1<z<1.5:TheionizingUVandnon-ionizingUVcanonlybecapturedfromspace.Crucialmeasurementsaresub-arcsspa9allyresolvedspectra,coveringλrest=1000–3000ÅatR=5000andSNR>15perresoln.element,andtheionizingλrest=900ÅregionatR~100s.Cri9calgoalsaretospa9allyresolvewhichstar-formingregionsareresponsiblefortheionizingphotons,andmeasureproper9esofstar-formingregions(Z,stellarage,SFR).Spa9allyresolvingionizingemissionistheonlywaytodirectlydeterminethephysicalprocessesofescapeandtheroleoffeedback.Stretchingthewavelengthcoverageasblueaspossible,tomeasuretheionizingspectralslope,wouldconstrainkeyunknownsaboutmassivestarevolu9on.

2)RedshiO1.5<z<2.5:Mul9-objectspectroscopicsurveyswith20–30mground-basedELTswillobtaintherest-framenon-ionizingUVspectraoflargenumbersofgalaxies,atseeing-limitedresolu9on,andthuscharacterizetheirmassivestellarpopula9ons,aswellasdiagnos9csofnebularcondi9onsandgasinflowandouxlows(e.g.Shapleyetal.2003;Peznietal.2000;Rigbyetal.2019b).TheELTSwillalsocoverLyɑovermostofthisredshierange.However,theELTscannotmeasuretheionizingcon9nuaforanyofthesegalaxies—thismustbedonefromspace,andwouldrequiresensi9ve,lowspectralresolu9on,spa9allyresolvedspectroscopyofthesegalaxieswithaspacetelescopeatλrest=900Å,whichattheseredshiesisλobs=2200–3150Å.

3)RedshiO2.5<z<3:TheELTswillcapturetheionizingfluxoflargenumbersofgalaxiesintheveryblueop9cal(λobs=3200–3600Å),andtheirnon-ionizingrest-frameUVspectra(λrest=1200–3000Å)intheop9calandnear-IR(λobs=4200–12000Å).Thismeasurementwouldproveespeciallydifficultiffaintgalaxiesareresponsibleforthebulkofionizingphotonproduc9on.Thesemeasurementswouldbeseeing-limited,andthereforewouldlackspa9alinforma9onastowhichpartsofgalaxiesleakionizingphotons.

4)RedshiOs7<z<9:Inlongintegra9ons,theELTsandJWSTwillobtainnon-ionizingrest-frameUVandrest-frameop9calspectraldiagnos9csofgalaxiesatthereioniza9onepoch,includingindirectpredictorsofionizingescape.Theinves9ga9onsdiscussedintheprevious3pointswouldprovideamuchdeeperunderstandingofionizingphotonproduc9onandescapeforredshies0<z<3.ItwouldthenbepossibletoapplythisknowledgetotheELTandJWSTobserva9onsoftheepochofreioniza9on,tocomprehensivelydeterminewhichgalaxiesreionizedtheUniverse.

Tosummarize,theproduc9onofionizingphotonsbymassivestars,andtheirescapefromgalaxies,arekeyunsolvedissuesinourunderstandingofgalaxyevolu9onandhowtheUniversewasreionized.Onlyasystema9capproach,usingJWSTandtheELTsatthehighestredshies,usingtheELTsat1.5<z<3,andafutureUVspaceobservatory,cancomprehensivelymeasurethemassivestellarpopula9onswithinstar-forminggalaxies,determinewhichkindsofgalaxiesleakionizingphotonsandhowtopredictthatescape,andapplythatknowledgetofinallyunderstandwhichgalaxiesendedthedarkagesbyreionizingtheUniverse.

Facilities needed Capabilities

JWST Deep NIRSpec and MIRI spectra of rest-frame optical diagnostics of galaxies at reionization

ELTs Multiplexed spectroscopic surveys covering λobs = 3200-12000Å

Large space UV telescope (LUVOIR, HabEx) Spatially-resolved spectroscopy that covers the ionizing λrest = 900Å, λobs = 1000–3000Å, at low spectral resolution (R~100s)

Large space UV telescope (LUVOIR, HabEx) Spatially-resolved spectroscopy that covers λobs = 1000–3000Å at R~5000 and SNR > 20 per resolution element

Figure 1:  Left panel: The gravitationally lensed Sunburst Arc at z=2.4.  Right panel: Ionizing photons escape from a compact, rapidly star-forming region.  Adapted from Rivera-Thorsen et al. (2019)

2.Massivestarsmakeionizingphotons. Thenon-ionizingUVspectraofgalaxiesfeatures

arichsuiteofdiagnos9csofthehotstars,namelybroadHeIIemission,thePCygniprofilesofCIV,SiIV,andNV,andphotosphericabsorp9onlines(deMelloetal.2000;Crowtheretal.2016).

Wehavedemonstrated(Chisholmetal.2019)thatwecanmeasuretheageandmetallicityofhotstellarpopula9onsbyfiznghigh-qualityrest-frameUVspectraofz~2lensedgalaxies(Rigbyetal.2018). Differentstellarpopula9onsynthesismodels,whenfizngthesamehigh-qualityspectra,differbyfactorsofseveralinthepredictedionizingphotonproduc9onrateandtheionizingspectralslopeλrest<900Å(Chisholmetal.2019;Figure2).Inotherwords,thecurrentunknownsaboutmassivestellarpopula9onstranslatetofactors-of-severaluncertaintyintheexpectedionizingphotonproduc9onratesofgalaxies.WeneedtobeOercharacterizethemassivestellarpopula9onswithinstar-forminggalaxies,theirintrinsicionizingproduc9onrate,andwhatdetermineswhetherthoseionizingphotonsaretrappedorcanescape.

Figure 2:   λrest= 1000–2000Å galaxy spectra reveal the age and metallicity of the massive stellar population, by fitting stellar population synthesis (SPS) models.  Current SPS models disagree by factors-of-several in the amount of ionizing radiation that should be produced by a given young stellar population. This disagreement is due to fundamental uncertainties about massive stellar evolution and spectra. From Chisholm et al. (2019), using z~2 lensed galaxy spectra from Megasaura (Rigby et al. 2018a).

Figure 3: Two different scenarios for how ionizing photons could escape.  Left: while most sight-lines are blocked by neutral gas along, narrow channels permit escape.  Right: the ionizing photons overwhelm the small amount of gas available, such that the H II region is smaller than its Strömgren sphere (“density-bounded”), and ionizing photons can freely stream out.