Wide-Area Studies of 18-cm OH Emission: A New View of ... · Wide-Area Studies of 18-cm OH...

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.

RonAllen- STScI/JHU TransformativeScience- 10/16/17 2

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.

RonAllen- STScI/JHU TransformativeScience- 10/16/1722/17 3

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

4RonAllen- STScI/JHU TransformativeScience- 10/16/1722/17

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


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.


Allen,Hogg,&Engelke (2015)


NotethesmallOHerrorbars...



ICO ∞IOH ?



ICO ≈0


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)


CurrentBlindGBTOHSurveys


ComparingBlindSurveys

OH1667MHzProfileIntegralsfromGBTBlindSurvey

CO(1-0)P

rofileIntegrals(Dam

eetal.(200

1)


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


PerseusArm– ThicknessandRollingMotions

GBTOHZ-distributionat8’

FCRAOCOZ-distributionat8’

PreliminaryFitstothe“staticatmosphere”law:


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

!"#$

!"%$

&"#$

!'("%$ !'("#$ !')"%$ !')"#$ !'*"%$

+,-,./.$0,/

1234

$

+,-,./.$056781234$

!'($ !')$

!$

&$

9$ :$Pointings forGBTOHsurveyoverW5.- Backgroundissmoothedversionof1420MHzcontinuumfromTayloretal.(2003).

Fig.2fromKarr&Martin(2003),showingW5.- Grayscaleis1420MHzcontinuum,- ContoursareCO(1-0)between-31and-49km/s,- DiamondsareOandBstars.


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.


Notethevaryingradiocontinuumbackground…

!"#$

!"%$

&"#$

!'("%$ !'("#$ !')"%$ !')"#$ !'*"%$

+,-,./.$0,/

1234

$

+,-,./.$056781234$

!'($ !')$

!$

&$

9$ :$Pointings forGBTOHsurveyoverW5.- Backgroundissmoothedversionof1420MHzcontinuumfromTayloretal.(2003).

Fig.2fromKarr&Martin(2003),showingW5.- Grayscaleis1420MHzcontinuum,- ContoursareCO(1-0)between-31and-49km/s,- DiamondsareOandBstars.


T_C4.1K


1667

1665

T_C5.0K


T_C5.7K


8.4K

T_C


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)


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.



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


StepstoGalacticSurveysinOHEmission- II

• NorthernHemisphereSurveysandTemplates:– 8’OuterGalaxyOHEmissionSurvey(“OGOES-8”)– NominalRequirements:

• GBTatL-bandwithPhased-ArrayFeed• Tsys <20Konallbeamsatall4OHFrequencies(esp.1720?)• Fullyautomatedtelescopeanddatareduction• Real-timecalibrationofthespectralbaselineintegratedintothereductionsoftware

– RoughobservingtimeestimateforsuchanOHsurvey(example:theFCRAOOGSinCObyHeyer etal1998):

• ≈3yearsonthetelescope


BACKUPMATERIALFOLLOWS


Wide-Area Studies of 18-cm OH Emission: A New View of ... · Wide-Area Studies of 18-cm OH...

Documents

Transcript of Wide-Area Studies of 18-cm OH Emission: A New View of ... · Wide-Area Studies of 18-cm OH...