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Wide-AreaStudiesof18-cmOHEmission:ANewViewofGalacticStructure
asSeeninLow-DensityMolecularGas
RonAllen
ScienceMissionOfficeSpaceTelescopeScienceInstitute
andPhysics/AstronomyDepartment
JohnsHopkinsUniversity
WhyOH?- I
• OHisubiquitous intheGalacticISM:– TheL-bandradiolinesofOHwereinitiallydetectedinabsorption (Weinreb
etal1963),andOHabsorptionwaslaterfoundtobewidelyobservableintheGalaxy(e.g.Goss1968;Turner1979).
– OHemission withanomalouspropertieswassubsequentlyfoundnearstar-formingregions(Weaveretal1965)andlateridentifiedasamaserphenomenon(seee.g.Reid&Moran1981forareview).
– SteadyadvancesinL-bandreceiversensitivityhaverecentlyshownthatnon-maserOHemissioncanalsobewidelyobservedintheGalaxyinregionslargelydevoidofbrightradiocontinuumemissionandactivestarformation(Allenetal2012,2013;Dawsonetal2014).
– Thisfaint,ubiquitousOHemissioncanbedetectedatboth1665and1667MHz(Allenetal2015)andappearstobeapproximatelyinLTE,therebysimplifyingtheinterpretationoftheobservations.
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WhyOH?- II• OHisamoleculartracer:
– OHprovidesaviewoftheISMthatiscomplementarytothatseeninCO(1-0)emission.
– WithsufficientS/N,OHemissioncanbeseeninregionsthatarecurrentlyfaintorevenabsentinCOemission,henceOHhasbecomeatracerforthe“(CO-)DarkGas”(Allen,Hogg,&Engelke 2015).
• TheL-bandOHlinesareopticallythin.– Theusualtracerweusetostudythedistributionandmotionsofthe
ISMingalaxiesisHI,butHIisknowntobeopticallythickinthediskofourGalaxy,inseveralothernearbygalaxies(e.g.M31:Braun2009),andinallotheredge-ongalaxiessofarstudied(Petersetal.2017).
– CO(1-0)emissioniswidelyknowntobeopticallythick,anditsquantitativerelationtoH2iscontroversial(the“X-factor”).
– TheconnectiontoN(OH)andultimatelytoN(H2)isdependentonhowwellwecanunderstandthephysicsofOHlineformation,atopicthatisdirectlyopentotheoreticalstudyandtoobservationalconfirmation.
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GBT OH “First Blind Pilot Survey”(Allen, Hogg, & Engelke 2015).
3X9gridcenteredat:L=105.0°,B=+1.0°
ΔL=ΔB=0.5°(undersampled -
GBTFWHM≈8’@18cm)1’≈1pcatPerseusArm
GalacticrotationseparatesfeaturesinValongtheLOS:- Localgas:VLSR ≈0km/s- Perseus:VLSR ≈-65km/s- Outer:VLSR ≈-100km/s
Reidetal2014,ApJ 783,130
DistancesalongLOSknowninparsecbytriangulation:
NRAOGreenBankTelescope
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Whatdidwedo?
104.0104.5105.0105.5106.0Galactic Longitude
−1
0
1
2
3
Gal
actic
Lat
itude
• 3X9gridofGBTpointings nearL=105,B=+1,on0.5° spacing,straddlingtheGalacticPlane.
– 66hoursrequested.
– L-band:1420/1665/1667/1720MHz
– frequency-switchingmode
– 2-hourintegrationsatOH,finalsensitivity≤3.5mK rms in0.55km/schannels
– 5-minintegrationsatHI
– GBTFWHM:8.9’atHI,7.6’atOH• GBTresolutionatOHisthesameasCfA atCO
• COdataavailableat8.4’FWHM– CfA archives– Dameetal.(2001)
– observeatsamepointingpositions
– regionchosentobefaintinCO
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ComparisonofCOandOHProfileIntegrals
OH1667MHzProfileIntegralsfromGBTBlindSurvey
CO(1-0)P
rofileIntegrals(Dam
eetal.(200
1)
Outof27pointings,- 78%haveOHemissiondetections- ManyspectrahavemultipleOH
featuresatdifferentradialvelocities
- OutofthedetectedOHfeatures,lessthanhalfhaveCOcounterpartsinDameetal.(2001)
- 27CO“non-detections”vs.20COdetections,1.35:1ratio
Thereisroughlytwiceasmuchmoleculargasinthisregionasisusuallyassumed.
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Allen,Hogg,&Engelke (2015)
Allen,Hogg,&Engelke (2015)
NotethesmallOHerrorbars...
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Allen,Hogg,&Engelke (2015)
ICO ∞IOH ?
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Allen,Hogg,&Engelke (2015)
ICO ≈0
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ThesepointshaveOHS/N� 7,butarestatistically=0inCO.
Thisbothconfirmedtheexistenceofthe“DarkGas”intheISMfirstdescribedbyGrenier etal(2005),andsuggestedthatOHisanewtracerforit.
OurNewGBTSurveys:
• The“OneSquareDegree”survey(Busch,Allen,&Hogg):– Thisisa“densified”survey,coveringaone-square-degreeregionpartoftheoriginal
blindsurvey,butnowwithagridspacingofoneGBTFWHMat1666.4MHz(≈0.125°)insteadof0.5°.Also,theOSDobservationsarelimitedtotheradialvelocityrangeofthePerseusArm,whichisataknowndistancefromtheSun.ThisexploratoryprogramisaimedeventuallyatstudyingthespatialstructureofthemoleculargasseeninOHemission.
• The“PerseusArm”Survey(Allen,Hogg,&Engelke):– This“sparse”studyofthePerseusArmhastwomainaims:tomeasurethe“Rolling
Motions”ofmoleculargas,andtodeterminethescaleheightandoffsetofmoleculargasfromtheGalacticPlane.
• The“W5”Survey(Engelke,Allen,&Hogg):– Thisisa“dense”surveyofW5withthegoalofcomparingthemoleculargascontentof
thisstar-formingregionasdeterminedinOHwiththevaluesfromCOobservations.
• ThesenewOHsurveyresultsarenowbeingcomparedwiththegreatly-improvedCOsensitivityoftheFCRAOOuterGalaxySurvey(Heyer etal1998)
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CurrentBlindGBTOHSurveys
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ComparingBlindSurveys
OH1667MHzProfileIntegralsfromGBTBlindSurvey
CO(1-0)P
rofileIntegrals(Dam
eetal.(200
1)
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RegionofthefirstGBT“BlindSurvey”Allen,Hogg,&Engelke (2015),Fig10.
COdatafromDameetal(2001)SPARSESURVEY27pointings ,Area4! °
PreliminarydatafromtheGBT“OneSquareDegreeSurvey”
COdatafromFCRAO/OGS(Heyer etal1998)DENSESURVEY:81pointings,Area1!°
FromapaperinpreparationbyJHUPhDstudentMichaelBusch
Results areconsistent:ImprovedsensitivityinCOhasnoteliminated
the“CO-DarkGas”
CO#Profile#Integrals#by#Galac4c#La4tude#
Galac4c#La4tude#(Degrees)#
CO#Profile#Integral#(K
*km/s)#
@4#
@2#
0#
2#
4#
6#
8#
10#
12#
14#
@6# @4# @2# 0# 2# 4# 6#
OH#Profile#Integrals#by#Galac4c#La4tude#
Galac4c#La4tude#(Degrees)#
OH#Profi
le#Integral#(K
*km/s)#
0#
0.1#
0.2#
0.3#
0.4#
0.5#
0.6#
0.7#
I6# I4# I2# 0# 2# 4# 6#
ContourmapisHIfromBurtonetal.(1985).ReddotsareournewOHobservations.WorkbyR.Allen,P.Engelke,andD.Hogg
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PerseusArm– ThicknessandRollingMotions
GBTOHZ-distributionat8’
FCRAOCOZ-distributionat8’
PreliminaryFitstothe“staticatmosphere”law:
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Fitsto:Y=A•sech2[(B– B0)/C]
GBTOH(1667):A=0.15±0.03K•km/s
B0 =0.40± 0.91°C=3.45± 1.19°
HWHM=3.0°± 1.0°
FCRAOCO(1-0):A=5.27±1.42K•km/s
B0 =-0.94± 2.8°C=2.82± 2.02°
HWHM=2.5°± 1.8°
PreliminaryResults- W5OHSurvey
!"#$
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9$ :$Pointings forGBTOHsurveyoverW5.- Backgroundissmoothedversionof1420MHzcontinuumfromTayloretal.(2003).
Fig.2fromKarr&Martin(2003),showingW5.- Grayscaleis1420MHzcontinuum,- ContoursareCO(1-0)between-31and-49km/s,- DiamondsareOandBstars.
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WorkbyNRAOReber FellowP.Engelke
CalculatingOHColumnDensities
Recallthat
N OH( ) = 2.3×1014 TexTex −TC
φ v( )dv∫
N OH( ) = 2.3×1014Tex τ v( )dv∫
€
nunl
=gugle−hνkBTex
Weneedtoknow:1)Integralunderspectrallines2)ContinuumtemperatureTCincidentfrombehindOHthecloud3)ExcitationtemperatureoftheOHtransition
- ThishasledtoanefforttomeasureTex directlyinW5,andtoanewpuzzle:thecaseofthethedisappearingOHemission.
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Notethevaryingradiocontinuumbackground…
!"#$
!"%$
&"#$
!'("%$ !'("#$ !')"%$ !')"#$ !'*"%$
+,-,./.$0,/
1234
$
+,-,./.$056781234$
!'($ !')$
!$
&$
9$ :$Pointings forGBTOHsurveyoverW5.- Backgroundissmoothedversionof1420MHzcontinuumfromTayloretal.(2003).
Fig.2fromKarr&Martin(2003),showingW5.- Grayscaleis1420MHzcontinuum,- ContoursareCO(1-0)between-31and-49km/s,- DiamondsareOandBstars.
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T_C4.1K
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1667
1665
T_C5.0K
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T_C5.7K
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8.4K
T_C
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PreliminaryExcitationTemperatures fromtheW5emissiondataalone
Tex_1667 =5.1+/- 0.5K
Tex_1665 =5.9+/- 0.2K
- Notethattheexcitationtemperatureforthe1667MHzlineappearstobealmost1Klowerthantheexcitationtemperatureforthe1665MHzemissionlineinW5.- Thisresultispreliminary,butthedifferenceisconsistentwithanold
discoverybyCrutcher (1979)aswellasrecentdata(Tangetal.2017).
- Wearenotawareofadetailedexplanationforthisdifference.
(PaperinpreparationbyNRAOReber Pre-doctoralFellowPhilipEngelke)
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WhyOH?- III• TheCons:OHisnotaseasytoobserveasCO(1-0):
– thesamereasonfortheeasyexcitationoftheOHlines(verysmallEinsteinA)coupledwiththelowOHabundancemeansthatthe18-cmemissionisveryfaint,andlongintegrationtimesarerequired.
– ThefaintlevelsoftheOHemission(TB�10-20milliKelvin)placestringentrequirementsonthestabilityofthereceivers andontheobservingmethodsanddatareductionprocedures.
– LowvolumedensitymeanthatOHisalmostcertainly“sub-thermal”,reducingthelinebrightnessevenfurtherandmakingitevenhardertodetecttheemission,especiallyifTex ≈TC .
• ThePros:OHisanewtracerforthelarge-scalemolecularISMwithmarkedlydifferentphysicalpropertiesfromCO:– OHisalreadyrevealingamoreextensivedistributionofmoleculargas inthe
GalaxythanwesuspectedfromCOsurveys.– OHholdspromiseforbecomingareliablequantitativetracerofthe
molecularISMbasedonknownphysics,anddirectlyobservablelinkstoN(H2)viaUVabsorptionlinesofbothH2 andOHtonearbyUV-brightstars.
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TheFuture
StepstoGalacticSurveysinOHEmission- I
• NorthernHemisphereSurveysandTemplates:– 30’OuterGalaxyOHEmissionSurvey(“OGOES-30”)– NominalRequirements:
• dedicated25mtelescope• Fullyautomatedtelescopeanddatareduction• Lowestpossiblesingle-pixelTsys (<20K)• All4OHlines(mainly1665+1667;also1720forlarge-scaleshocksintheISM– mighthavetomiss1612owingtoRFI)
– ObservingtimeestimateforsuchanOHsurvey:(template:initialLRDSHIsurveywiththePenticton85’telescope):
• 10monthsonan85’telescope
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StepstoGalacticSurveysinOHEmission- II
• NorthernHemisphereSurveysandTemplates:– 8’OuterGalaxyOHEmissionSurvey(“OGOES-8”)– NominalRequirements:
• GBTatL-bandwithPhased-ArrayFeed• Tsys <20Konallbeamsatall4OHFrequencies(esp.1720?)• Fullyautomatedtelescopeanddatareduction• Real-timecalibrationofthespectralbaselineintegratedintothereductionsoftware
– RoughobservingtimeestimateforsuchanOHsurvey(example:theFCRAOOGSinCObyHeyer etal1998):
• ≈3yearsonthetelescope
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BACKUPMATERIALFOLLOWS
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