An evolution of the IR-Radio correlation?

0.54 MB·English

by  R. J. Beswick

An evolution of the IR-Radio correlation?
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R. J.Beswick
0 ∗
2 JodrellBankCentreforAstrophysics,TheUniversityofManchester
E-mail: [email protected]
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T.W.B. Muxlow
J
7 JodrellBankCentreforAstrophysics,TheUniversityofManchester
E-mail: [email protected]

]
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H. Thrall
p
JodrellBankCentreforAstrophysics,TheUniversityofManchester
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o
E-mail: [email protected]
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t
s A. M.S.Richards
a
[ JodrellBankCentreforAstrophysics,TheUniversityofManchester

E-mail: [email protected]
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v
S. T.Garrington
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3 JodrellBankCentreforAstrophysics,TheUniversityofManchester
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E-mail: [email protected]
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.
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Usingextremelydeep(rms 3.3m Jy/bm)1.4GHzsub-arcsecondresolutionMERLIN+VLAra-
0
∼
8 dio observationsof a 8.5 8.5 field centred upon the Hubble Deep Field North, in conjunction
′ ′
0 ×
withSpitzer24m mdatawepresentaninvestigationoftheradio-MIRcorrelationatverylowflux
:
v
densities. BystackingindividualsourceswithinthesedataweareabletoextendtheMIR-radio
i
X
correlationtotheextremelyfaint( microJyandevensub-microJy)radiosourcepopulation.Ten-
r ∼
a tativelywedemonstrateasmalldeviationfromthecorrelationforthefaintestMIRsources. We
suggestthatthissmallobservedchangeinthegradientofthecorrelationistheresultofasuppres-
sionoftheMIRemissioninfaintstar-forminggalaxies.Thisdeviationpotentiallyhassignificant
implicationsforusingeithertheMIRornon-thermalradioemissionasastar-formationtracerat
lowluminosities.
Fromplanetstodarkenergy: themodernradiouniverse
October1-52007
UniversityofManchester,Manchester,UK
Speaker.
∗
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
AnevolutionoftheIR-Radiocorrelation? R.J.Beswick
1. Introduction
Radioand infrared emission from galaxies inboth thenearby and distant Universe isthought
to arise from processes related to star-formation, hence resulting in the correlation between these
twoobserving bands. Theinfrared emission isproduced from dustheated byphotons from young
stars and the radio emission predominately arises from synchrotron radiation produced by the ac-
celerationofchargedparticlesfromsupernovaeexplosions. Ithashoweverrecentlybeensuggested
that at low flux density and luminosities there may be some deviation from the tight well-known
radio-IR correlation seenforbrightergalaxies[1,2].
[1] argue that while the IR emission from luminous galaxies will trace the majority of the
star-formation in these sources, in low luminosity galaxies the IR emission will be less luminous
thanexpectedconsideringtherateofstar-formationwithinthesource(i.e. theIRemissionwillnot
fully trace the star-formation). In this scenario the reduced efficiency of IR production relative to
thesourcestar-formation rate(SFR)wouldbetheresultofinherentlylowerdustopacitiesinlower
luminosity sources and consequently less efficient reprocessing of UV photons from hot young
stars intoIRemission. Thesimpleconsequence ofthisisthatatlowerluminosities thenearlinear
radio-IR correlation L (cid:181) Lg ,with g >1(e.g. [3, 4])willbe deviated from. Ofcourse such an
radio IR
assertionisdependentupontheradioemissionprovidingareliabletracerofstar-formation atlow
luminosities whichmaybeequallyinvalid.
Recently[2]havepresentedastatisticalanalysisofAustraliaTelescopeCompactArray(ATCA)
20cm observations of the 24m m sources within the Spitzer Wide Field Survey (SWIRE). In this
work [2] have co-added sensitive (rms 30m Jy) radio data at the locations of several thousand
∼
24m m sources. Using this method they have statistically detected the microJy radio counterparts
of faint 24m m sources. At low flux densities (S24m m =100m Jy) they confirm the IR-radio cor-
relation but find it to have a lower coefficient (S1.4GHz=0.039S24m m) than had previously been
reported at higher flux densities. This coefficient is significantly different from results previously
derivedfromdetections ofindividual objects(e.g. [5])andisspeculated by[2]tobetheresultofa
change intheslopeoftheradio-IR correlation atlowfluxdensities.
In this work (which is descirbed in more detail in [6, 7]) we have utilised very deep, high
resolution 20cm observations of the Hubble Deep Field North and surrounding area made using
MERLINandtheVLA[8]incombinationwithpubliclyavailable24m mSpitzersourcecatalogues
from GOODS to study the MIR-Radio correlation for microjansky radio sources. This study ex-
tends the flux density limits of the radio-IR correlation by more than an order of magnitude for
individual sources and overlaps the flux density regime studied using statistical stacking methods
byotherauthors.
2. Observations
2.1 RadioData
Extremely deep radio observations of the HDF-N region were made in 1996-97 at 1.4GHz
usingbothMERLINandtheVLA.Theseobservationswereinitiallypresentedin[8],[9]and[10].
Theresultsfromthecombined18dayMERLINand42hrVLAobservationsaredescribedindetail
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AnevolutionoftheIR-Radiocorrelation? R.J.Beswick
3 3
IR flux density = 247.27 to 309.84microJy IR flux density = 197.27 to 247.27microJy IR flux density = 247.27 to 309.84microJy IR flux density = 197.27 to 247.27microJy
2 2
1 1
0 0
-1 -1
-2 -2
Peak flux = 18.820 microJy/bm Peak flux = 14.740 microJy/bm Peak flux = 8.4092microJy/bm Peak flux = 5.5563 microJy/bm
Bottom contour = 2.111microJy/bm, N=22 Bottom contour = 1.414microJy/bm, N=49 Bottom contour = 2.111microJy/bm, N=22 Bottom contour = 1.414microJy/bm, N=49
-3 -3
3 3
IR flux density = 157.44 to 197.27microJy IR flux density = 125.63 to 157.44microJy IR flux density = 157.44 to 197.27microJy IR flux density = 125.63 to 157.44microJy
2 2
Relative position (arcsec)-110 Relative position (arcsec)-110
-2 -2
Peak flux = 4.7807 microJy/bm Peak flux = 4.1161 microJy/bm Peak flux = 4.4681 microJy/bm Peak flux = 5.1166 microJy
Bottom contour = 1.492microJy/bm, N=44 Bottom contour = 1.347microJy/bm, N=54 Bottom contour = 1.492microJy/bm, N=44 Bottom contour = 1.347microJy/bm, N=54
-3 -3
3 3
IR flux density = 100.25 to 125.63microJy IR flux density = 80.0 to 100.25microJy IR flux density = 100.25 to 125.63microJy IR flux density = 80.0 to 100.25microJy
2 2
1 1
0 0
-1 -1
-2 -2
Peak flux = 2.9846 microJy/bm Peak flux = 2.2793 microJy/bm Peak flux = 3.4989 microJy Peak flux = 2.5482 microJy/bm
Bottom contour = 1.183 microJy/bm, N=70 Bottom contour = 1.228microJy/bm, N=65 Bottom contour = 1.183 microJy/bm, N=70 Bottom contour = 1.228microJy/bm, N=65
-3 -3
3 2 1 0 -1 -2 -33 2 1 0 -1 -2 -3 3 2 1 0 -1 -2 -33 2 1 0 -1 -2 -3
Figure1: Mean(left)Raelnatidve pmositioen d(aricsaecn) (right)imagesofthe1.4GHzradioemissionfRoelrativae plolsitsiono (aurcsrecc)eswithinthesix
faintest24m mfluxdensitylogarithmicbinsplottedinFig.1indescendingfluxdensityorderfromtop-leftto
bottom-right. Therangeof24m mfluxdensityoverwhicheachimagehasbeenstackedisshownatthetop
ofindividualpanels. Eachimageiscontouredwithlevelsof 2, 1.414, 1,1,1.414,2,2.828,4,5.657,8,
− − −
11.31,16,22.63and32times3 (3.3/√N)m Jybm 1,whereNequalsthenumberof24m msourcepositions
−
×
averagedin the map. Thepeakflux density,lowestplottedcontourandnumberof IRsourceswhichhave
beenaveragedover(N)islistedatthebottomofeachimagepanel.
in [8]. The combined MERLIN+VLA image has an rms noise level of 3.3m Jy per 0.2 circular
′′
beammakingitamongstthemostsensitive, high-resolution radioimagesmadetodate.
2.2 GOODS-NSpitzer24m mobservations
As part of the GOODSenhanced data release1 (DR1+ February 2005) a catalogue of Spitzer
24m m source positions and flux densities for the GOODS-N field were released. This source
catalogue islimitedtofluxdensities >80m Jyproviding ahighlycompleteandreliablesample. At
the time of writing this 24m m source catalogue represents the most complete and accurate mid-
infrared sourcelistpublicly availablefortheGOODS-N/HDF-Nfield.
All24m msourceswhicharedetectedopticallyinGOODSHSTACSimagesshowanaccurate
astrometric alignment with their optical counterparts implying that the astrometry between these
two data-sets and their subsequent catalogues is self-consistent. However, a comparison of the
astrometric alignment of the positions of sources catalogued by GOODS derived from their HST
ACSimages[11,7]showsthere tobeasystematic offset indeclination of 0.342fromtheradio
′′
−
referenceframe. Thislineardeclinationcorrection,althoughsmallrelativetotheSpitzerresolution
1http://www.stsci.edu/science/goods/DataProducts/
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AnevolutionoftheIR-Radiocorrelation? R.J.Beswick
at 24m m is significant when compared to these high resolution radio data. This linear correction
hasbeenappliedtotheSpitzersourcepositions priortoallcomparisons betweenthetwodatasets.
Figure2: Left: Radio1.4GHzversusthe24m mfluxdensity. Sourcesfromthe8.5 8.5HDF-Nfieldare
′ ′
×
plottedindividually(smallblacktriangles).Themedianradiofluxdensitylogarithmicallybinnedby24m m
fluxdensitiesareplottedaslargefilledredcircles. Thesolidredlinerepresentsthebest-fitlinetothebinned
HDF-Ndataalone.SourceswithintheCDFS-SWIREfielddetectedatboth24m mand1.4GHzfromNorris
et al. [12] are plotted as either green open stars (identified as AGN) or green triangles (not identified as
AGN).Allsourcesdetectedatboth1.4GHzand24m mintheSpitzerFirstLookSurvey(FLS)withsource
positionseparationsof<1.5areplottedinorange[13].NotethequantisationoftheSWIREandFLSpoints,
′′
in thisandsubsequentplotsisa resultoftheaccuracyofthefluxdensitiestabulatedin theliterature. The
blueplusesandfittedlinesinthelowportionoftheplotshowtheIR-radiocorrelationderivedfromstacking
radioemissionatthepositionsof24m msourcesintheCDFSandELAISfieldbyBoyleetal. [2].
Right: Intheupperpanelthefluxdensityratio(S24m m/S1.4GHz)versus24m mfluxdensityisshown. The
individual sources from the HDF-N field are plotted as small black triangles, the median values of these
HDF-N source binnedas a functionof S24m m as filled circles, green stars show the median binnedvalues
forsourceslistedasnon-AGNwithintheCDFS-SWIREsampleofNorrisetal. [12]andtheblue‘crosses’
and‘pluses’showthefluxdensityratiosderivedfromthestackinganalysisoftheSWIRE-CDFSandELAIS
fieldsrespectivelyfromBoyleetal.[2].Theoverlaidblackdottedlineisthemeanvaluefor(S24m m/S1.4GHz)
derivedbyAppletonetal.[5]. Thislineisequivalenttoq =0.84 0.28withthegrayfilledboxrepresenting
24
±
theareaenclosedbytheseerrorsandthefluxdensityrangeinvestigatedbyAppletonetal..Thevaluesofq
24
against24m mfluxdensityareplottedinthelowerpanel.Thesymbolswithinthisplotareidenticaltothose
in the upper panel, individual HDF-N sources are not included for clarity. The additional diagonal solid
greenline representsa line of constantradiofluxdensity of100m Jy, the lowestfluxdensity ofsourcesin
theSWIREsampleplottedhere(sourcesabovethislineareexcludedbythislimitfromtheSWIREsample).
ThisfluxdensitylimitwillsignificantlyeffectthevaluesofthebinnedSWIREdatapoints(greencrosses)
negativelybiasingthefourlowestfluxdensitybins. ThisbiasonlyeffectstheSWIREsample.
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AnevolutionoftheIR-Radiocorrelation? R.J.Beswick
3. Results & Discussion
Using these two highly sensitive data sets the radio emission from a sample of faint 24m m
galaxieshasbeeninvesitigated. Ofthe377Spitzersourceswithintheradiofield303werefoundto
have total radio flux denisties in excess of 3 times the local rms in our deep radio imaging. Many
ofthesesources,however,havepeakfluxdensitiescomparableorbelowthepointsourcedetection
threshold oftheseradiodata.
Byusingbothstatisticallystackedimagesandradiofluxdensitymeasurementsatthepositions
ofindividual Spitzer 24m msources wehave investigated theradio structures andfluxdensities of
faint IRsources. Infigure 1thestatistically averaged radio emission from faint 24m mgalaxies in
the GOODS-Nfield are shown. Each of these images is the statistically combined radio emission
from the location of several tens of Spitzer 24m m sources and has been contoured at multiples of
threetimes3.3m Jybeam 1 dividedby√NwhereNequalsthenumberofSpitzersourcepositions
−
thathavebeenstackedtogether. Ascanbeseenintheseimagestheoff-sourcenoiselevelsachieved
approaches the value expected when co-adding multiple images with near-Gaussian noise proper-
ties. Theco-added imagermsachieved inthefaintest 24m mfluxdensity bin(80.0to100.25m Jy)
is0.45and0.56m Jybeam 1 inthemeanandmedianco-added imagesrespectively.
−
TheGaussianfittedsizesoftheradioemission inthestacked images(Fig1)provideanupper
limit on the average size of the radio counterparts of these faint IR sources. The largest angular
sizes of the radio emission in the median stacked images created from this sample range between
1.4and2 . Thisisapproximatelyequivalenttoalinearsizeof10kpcatredshiftsbeyond1. These
′′ ′′
upperlimitsontheradiosourcesizesareconsistentwithradioemissionongalacticandsub-galactic
scalesandoriginating withinkpc-scale starburst systems.
Bycombining the Spitzer 24m m fluxdensities and the extracted 1.4GHzradio fluxdensities
for these sources itis possible to begin to characterise the radio emission of the faintest IR galax-
ies. Shown in the left-hand portion of figure2 is 1.4GHz total flux density of sources within the
8.5 8.5MERLIN+VLAfieldplottedagainsttheir24m mfluxdensity. Asadirectcomparisonalso
′ ′
×
overlaidonthisdiagramaresourcefluxdensitiesfromvariousotherdeepmulti-wavelength obser-
vational campaigns ofdifferent regions ofsky. Ascanbeseen these newHDF-N/GOODS-Ndata
significantly extendtheobservedIR-radiocorrelationforgalaxiesbetweenthesetwowavelengths.
Binningandre-plottingthesedataintermsofS24m m/S1.4GHz andq24 (log(S24m m/S1.4GHz)(see
figure2right)clearlyshowtheslopeandtightnessofthiscorrelation. However,itcanbeseenfrom
figure2thatthesedataappeartoshowthatthiscorrelationbeginstodeviateforthefaintestsources.
This deviation is small (and tentative) but implies that the faintest galaxies are under-luminous
at MIR wavelengths relative to their observed radio wavelengths. This deviate, whilst small, has
potentially direct implications on the use of either of these bands to quantify star-formation rates
in faint galaxies. A morecomplete discussion ofthese results and their implications can befound
in[6,7].
4. Conclusion
Using one of the deepest high-resolution 1.4GHz observations made to date, in conjunction
withdeep 24m mSpitzer source catalogues from GOODS,wehave investigated themicroJy radio
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AnevolutionoftheIR-Radiocorrelation? R.J.Beswick
counterparts offaintMIRsources. Theseobservations confirmthatthemicroJyradiosourcepopu-
lation follow theMIR-radiocorrelation andextend thiscorrelation byseveralorders ofmagnitude
to very low flux densities and luminosities, and out to moderate redshifts. This extension of the
MIR-radiocorrelation confirmsthatthemajorityoftheseextremelyfaintradioand24m msources
arepredominantly poweredbystar-formation withlittleAGNcontamination.
Statistically stackingtheradioemissionfrommanytensoffaint24m msourceshasbeenused
tocharacterise thesizeandnatureoftheradioemissionfromveryfaintIRgalaxieswellbelowthe
nominal radio sensitivity of these data. Using these methods the MIR-radio correlation has been
further extended and a tentative deviation in this correlation at very low 24m m flux densities has
beenidentified.
References
[1] Bell,Eric,F.2003,ApJ,586,794
[2] Boyle,B.J.,Cornwell,T.J.,Middelberg,E.,Norris,R.P.,Appleton,P.N.,Smail,Ian,2007,
MNRAS,376,1182
[3] Cox,M.J.,Eales,S.A.E.,Alexander,P.,Fitt,A.J.,1988,MNRAS,235,1227
[4] Price,Rob.,Duric,Nebojsa.,1992,ApJ,401,81
[5] Appleton,P.N.,Fadda,D.T.,Marleau,F.R.,Frayer,D.T.,Helou,G.,Condon,J.J.,Choi,P.I.,Yan,
L.,Lacy,M.,Wilson,G.,Armus,L.,Chapman,S.C.,Fang,F.,Heinrichson,I.,Im,M.,Jannuzi,B.T.,
Storrie-Lombardi,L.J.,Shupe,D.,Soifer,B.T.,Squires,G.&Teplitz,H.I.,2004ApJS,154,147
[6] Beswick,R.J.,Muxlow,T.W.B.,Thrall,H.,Richards,A.M.S.,2006,Toappearintheproceedings
of’AttheEdgeoftheUniverse’(9-13October2006,Sintra,Portugal)Eds.Alfonsoetal
(astro-ph/0612077)
[7] Beswick,R.J.,Muxlow,T.W.B.,Thrall,H.,Richards,A.M.S.,2008,AcceptedtoappearinMNRAS
[8] Muxlow,T.W.B.,Richards,A.M.S.,Garrington,S.T.,Wilkinson,P.N.,Anderson,B.,Richards,E.
A.,Axon,D.J.,Fomalont,E.B.,Kellermann,K.I.,Partridge,R.B.,Windhorst,R.A.2005MNRAS,
358,1159
[9] Richards,E.A.,Kellermann,K.I.,Fomalont,E.B.,Windhorst,R.A.,Partridge,R.B.,1998,AJ,116,
1039
[10] Richards,E.A.,2000,ApJ,533,611
[11] Richards,A.M.S.,Muxlow,T.W.B.,Beswick,R.J.,Allen,M.G.,Benson,K.,Garrett,M.A.,
Garrington,S.T,Gonzalez-Solarez,E.,Harrison,P.A.,Kettenis,M.,Laing,R.A.,Richards,E.A.,
Thrall,H.,vanLangevelde,H.J.,Walton,N.A.,Wilkinson,P.N.,Winstanley,N.,2007A&A472,
805(astro-ph/0706.3777)
[12] Norris,RayP.,Afonso,José,Appleton,PhilN.,Boyle,BrianJ.,Ciliegi,Paolo,Croom,ScottM.,
Huynh,MinhT.,Jackson,CaroleA.,Koekemoer,AntonM.,Lonsdale,CarolJ.,Middelberg,Enno,
Mobasher,Bahram,Oliver,SebJ.,Polletta,Mari,Siana,BrianD.,Smail,Ian,Voronkov,MaximA.,
2006,AJ,132,2409
[13] Fadda,Dario,Marleau,FrancineR.,Storrie-Lombardi,LisaJ.,Makovoz,David,Frayer,DavidT.,
Appleton,P.N.,Armus,L.,Chapman,S.C.,Choi,P.I.,Fang,F.,Heinrichsen,I.,Helou,G.,Im,M.,
Lacy,M.,Shupe,D.L.,Soifer,B.T.,Squires,G.K.,Surace,J.,Teplitz,H.I.,Wilson,G.,Yan,L.,
2006,AJ,131,2859
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