An evolution of the IR-Radio correlation at very low flux densities?

1.1 MB·English

by  R. J. Beswick

Mon.Not.R.Astron.Soc.000,1–14(2007) Printed2February2008 (MNLATEXstylefilev2.2)
An evolution of the IR-Radio correlation at very low flux
densities?
R.J.Beswick,1⋆ T.W.B.Muxlow,1 H.Thrall,2 A.M.S.Richards,1 S.T.Garrington1
8 1MERLIN/VLBI National Facility, Jodrell Bank Observatory,The Universityof Manchester, Macclesfield, Cheshire, SK11 9DL
0 2Jodrell Bank Observatory, The Universityof Manchester, Macclesfield, Cheshire, SK11 9DL
0
2
Accepted 2008January6.Received2008January3;inoriginalform2007November 6
n
a
J
ABSTRACT
7
Inthis paperwe investigatethe radio-MIRcorrelationatverylowflux densities using

] extremely deep 1.4GHz sub-arcsecond angular resolution MERLIN+VLA observa-
h tions of a 8′.5×8′.5 field centred upon the Hubble Deep Field North, in conjunction
p with Spitzer 24µm data. From these results the MIR-radio correlation is extended
- to the very faint (∼microJy) radio source population. Tentatively we detect a small
o
r deviation from the correlation at the faintest IR flux densities. We suggest that this
t small observed change in the gradient of the correlation is the result of a suppres-
s
a sionofthe MIRemissioninfaintstar-forminggalaxies.Thisdeviationpotentially has
[ significant implications for using either the MIR or non-thermal radio emission as a

star-formationtracer of very low luminosity galaxies.
1
v
Key words: galaxies:starburstgalaxies:high-redshiftinfrared:galaxiesradiocontin-
5
uum: galaxies
3
0
1
.
1
0 1 INTRODUCTION radio correlation holds for relatively bright star-forming
8 galaxies (S20cm > 115µJy) out to at least a redshift of 1
Since 1970s and 1980s studies of the radio and far-infrared
0 (Appleton et al. 2004).
(FIR) properties of galaxies have shown there to be a
: Radio and infrared emission from galaxies in both the
v tight correlation between their emission in these two ob-
nearby and distant Universe is thought to arise from pro-
i serving bands which extends over several orders of mag-
X cesses related to star-formation, hence resulting in the cor-
nitude in luminosity (van derKruit 1973; Condon et al.
r relation between these two observing bands. The infrared
a 1982). The advent of the Infrared Astronomical Satel- emission is produced from dust heated by photons from
lite (IRAS) All Sky Survey in 1983 (Neugebauer et al.
young stars and the radio emission predominately arises
1984; Soifer et al. 1987) enabled much larger system-
from synchrotron radiation produced bythe acceleration of
atic samples of galaxies to be studied at infrared wave-
charged particles from supernovae explosions. It has how-
lengths and hence further demonstrated the consistency
ever recently been suggested that at low flux density and
and tightness of this correlation, albeit for relatively
luminosities there may be some deviation from the tight
nearby sources (Helou et al. 1985; de Jong et al. 1985;
well-known radio-IR correlation seen for brighter galaxies
Condon & Broderick 1986; Condon et al. 1991; Yunet al.
(Bell 2003; Boyle et al. 2007).
2001).FollowingIRAS,deepobservationsusingtheInfrared
Space Observatory (ISO) allowed fainter and higher red-
shift galaxies to be observed at mid-infrared (MIR) wave-
lengths.TheseISOobservationsshowedthattheMIRemis- Bell(2003)arguethatwhiletheIRemissionfromlumi-
sion from galaxies is loosely correlated with radio emission nous galaxies will trace the majority of the star-formation
across a wide range of redshifts, tentatively extending to in these sources, in low luminosity galaxies the IR emis-
z 4 (Cohen et al. 2000; Elbaz et al. 2002; Garrett 2002). sion will be less luminous than expected considering the
∼
More recently the launch of the Spitzer Space Tele- rate of star-formation within the source (i.e. the IR emis-
scopeinAugust2003hasgreatlyincreasedthesensitivityof sionwillnotfullytracethestar-formation).Inthisscenario
MIR observations and hence our ability to study the MIR- thereducedefficiencyofIRproductionrelativetothesource
radio correlation. Early results, such as from the Spitzer star-formation rate (SFR)would betheresult of inherently
First Look Survey (FLS), have confirmed that the MIR- lower dust opacities in lower luminosity sources and conse-
quently less efficient reprocessing of UV photons from hot
young stars into IR emission. The simple consequence of
⋆ [email protected] this is that at lower luminosities the near linear radio-IR
2 Beswick et al.
Figure 1.Left-handpanel:Radio1.4GHzversustheMIR24µmfluxdensityofall377individualsources(smalltriangle),andmedian
radioflux density logarithmicallybinned by their 24µm flux density (filled circles)within the 8′.5 8′.5 field. Right-hand panel: Control
×
plotof1.4GHzradiofluxdensitiesplotted againstthe24µmsourcefluxdensities ofthesample.Theradiofluxdensitiesofthecontrol
samplehavederivedinexactlythesamemannerasthefluxdensitiesplottedintheleft-hand,panelbuthavebeenmeasuredatrandomly
assignedskypositionsinthe8′.5 8′.5radioimagewherenoknownsourcesatanywavelength arelocated.
×
correlation Lradio ∝LγIR, with γ > 1 (e.g Cox et al. 1988; 2 DATA AND ANALYSIS
Price & Duric 1992) will be deviated from. Of course such
2.1 MERLIN+VLA observations
anassertion isdependent upon the radioemissionproviding
a reliable tracer of star-formation at low luminosities which Extremely deep radio observations of the HDF-N region
may be equally invalid. were made in 1996-97 at 1.4GHz using both MERLIN and
the VLA. These observations were initially presented in
Recently Boyle et al. (2007) have presented a statisti-
Muxlow et al. (2005), Richardset al. (1998) and Richards
calanalysisofAustraliaTelescopeCompact Array(ATCA)
(2000). The results from the combined 18 day MERLIN
20cm observations of the 24µm sources within two regions
and 42hr VLA observations are described in detail in
(the Chandra Deep Field South (CDFS) and the European
Muxlow et al.(2005).ThecombinedMERLIN+VLAimage
Large Area ISO Survey S1 (ELAIS)) of the Spitzer Wide hasanrmsnoiselevelof3.3µJyper0′.′2circularbeammak-
FieldSurvey(SWIRE).InthisworkBoyle et al.(2007)have
ingit amongst themost sensitive, high-resolution radio im-
co-addedsensitive(rms 30µJy)radiodataatthelocations
∼ ages madeto date.
of several thousand 24µm sources. Using this method they
Using the same methods as described in Muxlow et al.
havestatisticallydetectedthemicroJyradiocounterpartsof
(2005) these combined MERLIN and VLA observations
faint24µmsources.Atlowfluxdensities(S24µm =100µJy)
have been used to image the entire unaberrated field of
they confirm the IR-radio correlation but find it to have view, 8.5 8.5 arcmin2 in size, centred upon the original
a lower coefficient (S1.4GHz=0.039S24µm) than had previ- MERLIN×pointing position (α = 12h36m49s.4000, δ =
ouslybeenreportedathigherfluxdensities.Thiscoefficient +62◦12′58′.′000(J2000))1.Thisimagehasanrmsnoiselevel
issignificantlydifferentfromresultspreviouslyderivedfrom of 3.6µJybeam−1 and has been convolved with a 0′.′4 cir-
detections of individual objects (e.g. Appleton et al. 2004)
cular beam. These observations have been shown to align
and is speculated by Boyle et al. (2007) to be the result of
withtheInternationalCoordinate ReferenceFrame(ICRF)
achangein theslopeof theradio-IR correlation at low flux
to betterthan 15mas (Muxlow et al. 2005).
densities.
Inthispaper,weutiliseverydeep,highresolution20cm
observationsoftheHubbleDeepFieldNorthandsurround- 2.2 GOODS-N Spitzer 24µm observations
ingareamadeusingMERLINandtheVLA(Muxlow et al.
As part of the GOODS enhanced data release2 (DR1+
2005) in combination with publicly available 24µm Spitzer
February 2005) a catalogue of Spitzer 24µm source posi-
source catalogues from GOODS to study the MIR-Radio
tions and flux densities for the GOODS-N field were re-
correlation for microjansky radio sources. This study ex-
leased. This source catalogue is limited to flux densities
tends the flux density limits of the radio-IR correlation by
>80µJy providing a highly complete and reliable sample.
morethananorderofmagnitudeforindividualsourcesand
At the time of writing this 24µm source catalogue repre-
overlaps the flux density regime studied using statistical
stacking methods by Boyle et al. (2007). Additionally we
employ statistical stacking methods, similar to those used
by Boyle et al. (2007), to extend the correlation further to 1 Public access to these radio data will be made available via
still lower flux densities. anon-demandradioimagingtooldevelopedbytheauthorsusing
VirtualObservatorytools.A detailedexplanation ofthis service
WeadoptH0 =75kms−1Mpc−1,Ωm =0.3andΩΛ=0.7 canbefoundinRichardset al.(2007).
throughout this paper. 2 http://www.stsci.edu/science/goods/DataProducts/
An evolution of the IR-Radio correlation? 3
2.3 Measurement of source radio flux densities
ThefieldcoveredbytheseGOODS-NSpitzer24µmobserva-
tions contains 1199 24-µm sources identified with flux den-
sities>80µJy.The8.5 8.5arcmin2 radiofieldcontains377
×
of these Spitzer sources with 24µm flux densities ranging
from 80.1 to 1480µJy.
RadioemissionwiththedeepMERLIN+VLAdatahas
been measured at the corrected position of each individual
24µm source within the radio image. The radio flux den-
sity was measured within aseries of concentric ringswith a
maximum radius of 4arcsec centred on each 24µm source.
Statistical analysis of these measurements and a sample of
stronger radio sources within the same field shows that the
radio flux density increases within progressively larger an-
nuliouttoaradiusof1′.′5.Forstronger(>5σ)radiosources
the total flux density recovered using this method within a
radiusof1′.′5isbetween90%and100% ofthesource’stotal
flux density as measured by other means, such as Gaussian
fitting.Additionallytheaverage(eithermeanormedian)ra-
dialprofileofallof the24µmsources showsthatalmost all
of the radio flux density is recovered within radii less than
1′.′5(Beswick et al. 2006;Beswick et al.in prep).Through-
out the rest of this paper the radio flux density recovered
within a radius of 1′.′5 of each IR Spitzer source will be as-
Figure 2.Radio1.4GHzversus the24µmflux density. Sources
from the 8′.5 8′.5 HDF-N field are plotted individually (small sumed to be equalto thetotal radio source flux density.
×
black triangles). The median radio flux density logarithmically
binned by24µm flux densities areplotted as largefilledredcir-
cles. The solid red line represents the best-fit line to the binned Asaconfirmationofthismethodseveralthousandran-
HDF-N data alone. Sources within the CDFS-SWIRE field de- dom sky positions within the radio field were selected and
tected at both 24µm and 1.4GHz from Norris et al. (2006)
cross-referenced to excludeany which were at the positions
are plotted as either green open stars (identified as AGN) or
ofknownsourcesatanywavelengths.Ateachoftheseblank
green triangles (not identified as AGN). All sources detected at
positionstheradiofluxdensitywasmeasuredusinganiden-
both1.4GHzand24µminthe SpitzerFirstLookSurvey(FLS)
with source position separations of <1′.′5 are plotted in orange tical method as outlined above. The fluxdensities recorded
(Faddaet al. 2006). Note the quantisation of the SWIRE and within each of these annuli showed no positive bias within
FLS points, in this and subsequent plots is a result of the ac- anyringandfollowed thesameGaussiandistributionasde-
curacy of theflux densities tabulated inthe literature. The blue rived from the pixel-by-pixel noise statistics of the whole
pluses and fitted lines in the low portion of the plot show the image once all radio sources greater than 40µJy have been
IR-radio correlation derived from stacking radio emission at the subtracted. As further test of these methods 90 fake, faint
positionsof24µmsourcesintheCDFSandELAISfieldbyBoyle (< 3σ) and partially-resolved radio sources were injected
etal.(2007).Abriefsummaryofthebasiccharacteristicsofeach
intotheu-vdata.ThesedatawerethenFourierinvertedand
ofthedatasetsplottedincludedinTable1.
cleanedinanidenticalmannertotherealdataandthenthe
fluxdensitiesof thesefake sources wereextracted using the
methods outlined above. Whilst, as expected,these sources
were not detected individually their radio emission, when
averagedtogether,wasconsistentwiththeaveragefluxden-
sents the most complete and accurate mid-infrared source
sityofthepopulation offakesourcesinjectedintothedata.
list publicly available for theGOODS-N/HDF-Nfield.
Amore detailed description of thismethodand thera-
All 24µm sources which are detected optically in
dio characteristics of this sample of 24µm Spitzer source,
GOODS HST ACS images show an accurate astrometric
includingsourcecataloguesandsizeinformation,willbepre-
alignment withtheiropticalcounterpartsimplyingthatthe
sented in a forthcoming paper.
astrometry between these two data-sets and their subse-
quent catalogues is self-consistent. However, a comparison
oftheastrometricalignment ofthepositionsofsourcescat-
alogued by GOODS derived from their HST ACS images
3 RESULTS
(Richards et al. 2007; Muxlow et al. in prep) shows there
to be a systematic offset in declination of 0′.′342 from the Of the 377 24µm Spitzer sources within the radio field 303
−
radioreferenceframe. Thislineardeclination correction,al- werefound,bythismethod,tohavetotalradiofluxdensities
though small relative to the Spitzer resolution at 24µm is greaterthan3timesthelocalmaprms.Manyofthesesource
significant when compared to these high resolution radio have peak flux densities below 3-σ and hence were not se-
data. This linear correction has been applied to the Spitzer lected using the radio-only detection methods employed by
source positions prior to all comparisons between the two Muxlow et al.(2005).The1.4GHzversus24µm fluxdensi-
data sets. ties of all 377 of these source are plotted in left-hand panel
4 Beswick et al.
Table 1.Summaryofdeep1.4GHzradioand24µmsurveysrefereedtointhispaper.
Survey Radio Instrument Radiosenitivity Angular Reference
SurveyArea (µJybeam−1) Resolution
HDF-N/GOODS-N 72.25arcmin2 VLA+MERLIN 3.3 0′.′2 0′.′2 Muxlowet al.2005, thispaper
ATLAS(CDFS/ELAIS) 3.7deg2 ATCA 20 60 11′′×5′′ Norriset al.2006
SpitzerFLS 5deg2 VLA →23 5×′′ Condonet al.2003,Faddaet al. 2006
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
n (arcsec)1 n (arcsec)1
o o
ositi0 ositi0
p p
Relative -1 Relative -1
-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
Relative position (arcsec) Relative position (arcsec)
Figure3.Mean(left)andmedian(right)imagesofthe1.4GHzradioemissionforallsourceswithinthesixfaintest24µmfluxdensity
logarithmic bins plotted in Fig.1 in descending flux density order from top-left to bottom-right. The range of 24µm flux density over
which each image has been stacked is shown at the top of individual panels. Each image is contoured with levels of 2, 1.414, 1,1,
1.414, 2,2.828, 4,5.657, 8, 11.31, 16,22.63 and32times3 (3.3/√N)µJybm−1,whereNequals the numberof 24µm−sou−rceposi−tions
×
averagedinthemap.Thepeakfluxdensity,lowestplottedcontourandnumberofIRsourceswhichhavebeenaveragedover(N)islisted
atthebottom ofeachimagepanel.
ofFig1.Thecontrolsample,plottedintheright-handpanel half of this figure and show an expected radio excess when
of Fig1, is derived using an identical radio flux extraction compared to their 24µm flux densities. Source which were
methods and from the same radio data but at randomly notidentifiedas AGNbyNorris et al.(2006) areplotted as
assigned locations coincident with no known source posi- trianglesinthisfigure.Itshouldbenotedthatthesesources
tion. The flux densities of these sources are also plotted in have not been classified as containing an AGN but may in
Fig2,alongwithsourcesdetectedintheshallowersurveysof fact be either AGN, star-forming galaxies or a combination
theCDFS-SWIREfieldbyNorris et al.(2006)usingATCA, of thetwo.
and theSpitzer FLS using theVLAbyFadda et al. (2006).
Overlaid in blue in the lower half of Fig.2 are the flux
Sources from the southern CDFS-SWIRE field which have
densities of groups of median stacked infrared sources from
been categorised byNorris et al. (2006) as containing AGN
the ELAIS and CDFS field from Boyle et al. (2007) along
(plotted as stars) are generally situated toward the upper
with fitstothesedata.Alsooverlaid,as filledred circles, in
An evolution of the IR-Radio correlation? 5
thisfigurearethemedianvaluesofalloftheHDF-Nsources 0.73 derived from non-AGN sources catalogued in observa-
logarithmicallybinnedbytheir24µmfluxdensities.Thereis tions of the SWIRE field by Norris et al. (2006). A similar
acleardiscrepancybetweentheaveragedradiofluxdensities studyofbrightersourcesintheSpitzerFLS(Appleton et al.
derived from these HDF-Nobservations and theflux densi- 2004) show the mean value to be (q24=0.84 0.28) slightly
±
ties from the ELAIS and CDFS fields (Boyle et al. 2007) higher than average value derived here. The distribution of
which will bediscussed further in the following sections. q24 values for all sources in this HDF-N study is shown in
UsingananalysismethodsimilartoBoyle et al.(2007) Fig.4.
the mean and median images of the radio emission from InFigs.5,6,7and8theparameterq24isplottedagainst
HDF-N field of the 6 lowest 24µm flux density logarith- S24µm, S1.4GHz and redshift. The monochromatic value of
mic bins (as plotted in Figs.1 and 2) have been derived q24 represents the slope of the IR-radiocorrelation and the
and are shown in Fig.3. Each of these images is the sta- size of dispersion in q24 is related to the strength of the
tistically combined radio emission from the location of sev- correlation.Ineachofthesefiguresnok-correctionhasbeen
eral tens of Spitzer 24µm sources and has been contoured applied to either the radio or IR luminosities since we have
at multiples of three times 3.3µJybeam−1 divided by √N incompleteredshiftsforthesample,andinadequatespectral
where N equals thenumberof Spitzer source positions that information is known about the majority of the individual
havebeen stacked together. As can be seen in these images sources in either the IR or radio bands. A similar study by
theoff-sourcenoiselevelsachievedapproachesthevalueex- Appleton et al.(2004)hasshownthatk-correctionsmadeto
pectedwhen co-addingmultipleimages with near-Gaussian individualsourcesresultinonlyaminimalsystematiceffect
noise properties. The co-added image rms achieved in the on the average valueof q24 below redshift 1.
∼
faintest 24µm flux density bin (80.0 to 100.25µJy) is 0.45 InFig.5thevaluesofq24forsourcesfromtheseHDF-N
and0.56µJybeam−1 inthemeanandmedianco-addedim- data, the SWIRE field (Norris et al. 2006) and the Spitzer
ages respectively.Thefluxdensitiesmeasured directly from FLSfield(Faddaet al.2006)areplottedagainst 24µmflux
thesestackedradioimagesareconsistentwiththestatistical density. In the upper panel both the AGN identified and
mean and median flux densities within the equivalent bins non-AGN sources of Norris et al. (2006) are shown whilst
derived from the individual source flux extraction method in the lower panel the AGN sources are excluded for clar-
describedinSection2.3(Table2)andplottedinFigs.1and ity. In the upper panel the AGN sources from the SWIRE
2. In each stacked image the largest angular size of the re- field, as expected, deviate from the correlation generally
gions showing elevated radio emission within Fig.3 is en- showing an excess of 1.4GHz radio emission compared to
closed within the 1′.′5 radius circular area from which the their 24µm emission. The HDF-N field covers a consider-
individualsource fluxdensities havebeen extracted. ably smaller area and was originally chosen to have few
The sizes of the composite radio sources in these co- brightradio-AGN,whichisconfirmedbytheminimalnum-
added images (Fig.3) represent a combination of the true berofHDF-Nsourcesdisplayinganexcessof1.4GHzradio
radiosourcesize,anyoffsetsbetweentheIRandradioemit- emission. Amongst the HDF-N sources and the non-AGN
ting regions within the sources and any random errors in sourcesfromtheSWIREfield(Norris et al.2006,andFig.5
the Spitzer source positions, all of which will be convolved here)q24showsasmalltrendtowardlowervaluesatsmaller
with the synthesised beam applied to these radio data (0′.′4 24µm fluxdensities.TheSWIREsources plottedherehave
circular beam in this case). No systematic offset is seen be- been selected to have both radio and IR flux density above
tween the emission in these composite radio images from acertainthreshold.Thelowest radiofluxdensityofsources
their nominal image centres implying that no significant within the SWIRE field catalogue (Norris et al. 2006) is
non-random errors between the astrometric alignment of 100µJy. A line of constant radio flux density (100µJy, the
these data-sets remain. Furtheranalysis of theradio source lowestindividualsourcedetectionsintheSWIREcatelogue)
sizes and structures will bethe subject of a future paper. is overlaid on Fig.5 above which sources from the SWIRE
In Fig.3 the mean (left) images of the radio emis- studyareexcluded.TheHDF-Nradiodataplottedherehas
sion for the two highest 24µm flux density bins are dom- asensitivitylimit 10timeslower thanthatoftheSWIRE
∼
inated by emission from a few bright sources. Several of data,thuspotentiallyonlyexcludingsourcesintheextreme
these brighter radio sources can be seen in Fig.1 scattered top left-hand part of this plot. Furthermore, the sample of
above S1.4GHz =200µJy. The presence of these brighter ra- sources plotted here has been subject to no radio detec-
diosourcesandthelownumbersofimagesstackedtogether tionthresholdlimithenceremovingthispotentialbiascom-
is clearly evident when the mean and median averaged im- pletely.
agesarecompared.Inparticularinthesecondbrightestbin Theq24valuesfortheHDF-Ndataandthesourcesfrom
(197.27to247.27µJy)twomJyradiosourcesarepresentand Norris et al. (2006) binned as a function of S24µm, along
offset from their equivalent 24µm position. It is likely that with the ratio of S24µm and S1.4GHz are shown in Fig6.
these bright radio sources, which deviate from the radio-IR Especially within thebinnedpoints, thesetwo plotsshow a
correlation,maycontainradioAGNwhichwouldfeasiblybe tentativetrendforlowq24and SS12.44GµmHz withdeclining24µm
spatially offset from any ongoing star-formation and 24µm fluxdensity.Thegrayareasanddottedlineinthesetwopan-
position. elsshowthevalueoftheq24asderivedfromtheSpitzerFLS
An alternative method of projecting the correlation (Appleton et al. 2004). The HDF-N observations presented
between these 24µm and 1.4GHz data is as a function here are consistent with those of Appleton et al. (2004) at
of the commonly defined parameter q24, where q24 = high 24µm fluxdensities.
log(S24µm/S1.4GHz).The mean and median valueof q24 for The effect of the 24µm sensitivity cut-off (80.1µJy) in
alloftheHDF-Nsourcespresentedinthisstudyis0.52and the GOODS-Ndata and the equivalent limit in the data of
0.48 respectively, compared to the value of 0.69 0.36 and Norris et al. (2006) will, however, positively bias measure-
±
6 Beswick et al.
Table2.Fluxmeasurementsfromstackedimagesandbinneddata.Thefluxdensityinthestackedimageshasbeenmeasuredbyfitting
a single Gaussian to the radio emission. The errors attached to measurements made from the stacked images are the formal errors of
these Gaussian fits, rather than the image rms which in each case is comparable to σ. Sources within 3arcsec of the edge of the radio
fieldhavebeenexcludedfromthestackedimages,henceslightlyreducingthenumberofsourcescombinedineachimagerelativetothose
includedinthebinneddata.
Binneddata Stackedimages
Median Mean Median Mean
S24µm Number S1.4GHz S1.4GHz σ1.4GHz Number S1.4GHz S1.4GHz
(µJy) (µJy) (µJy) (µJy) (µJy) (µJy)
88.8 65 28.9 22.6 0.68 65 26.7 8 27.8 6
± ±
112.5 72 34.1 32.0 0.62 70 34.0 7 32.9 5
± ±
138.9 57 49.5 47.7 0.81 54 63.1 9 58.1 7
± ±
174.7 46 58.8 56.1 1.21 44 64.8 10 60.4 8
± ±
219.4 49 69.4 90.7 1.68 49 76.1 8 90.4 6
± ±
275.0 22 72.5 79.0 3.01 22 81.9 13 53.3 4
± ±
mentsofq24atlowradiofluxdensities.Thisisclearlyshown
inFig.7.ThelowerfluxdensitylimitimposedbytheSpitzer
sensitivityresultsinanexclusionofsourcesinthelowerpor-
tionofFig.7.Thisconsequentlybiasesthedeterminationof
thevalueofq24 foranycompletesampleofsourceswithlow
radio flux densities.
Ofthe377Spitzersourcewithinthe8′.5 8′.5HDF-Nra-
×
dio field,259 of thesegalaxies havepublished spectroscopic
or photometric redshifts. The q24 values for this subset of
sources, along with 50 non-AGN SWIRE sources, are plot-
tedwith respect totheirredshift inFig8.Withinthesetwo
data-sets q24 is seen to slightly reduce at higher redshifts
although this effect is small and significantly less than the
Figure4.Histogramofthedistributionofq24forthe377sources
scatter. These values of q24 have not been k-corrected. The withinthe8′.5 8′.5fieldcentredupontheHDF-N.
application of a k-correction to these data will result in a ×
smallincreaseinq24whichwillincreaseasafunctionofz.It
isinterestingtoalsonotethatrecentstudiesofdiscreteareas
within 4 very nearby star-forming galaxies (Murphyet al.
2006a)haveshownq24tovaryacrosstheextentofindividual ThemajorityoftheIRselectedsourceswithintheHDF-
sources.However,Murphyet al.(2006a)deriveameanvalue N region have radio flux densities less than 100µJy, with
ofq24 1whenintegratingoverentiregalaxiesintheirsam- onlyafewsourceswhichshowasignificantexcessof1.4GHz
≈
ple, at z 0, which is consistent with many of the source in flux density, when compared to the IR-radio correlation.
≈
boththeSWIREandHDF-Nsamplesbutsomewhathigher Thisimplies that thevast majority of theseIR-selectedmi-
than theaverage valuederived at higher redshifts. croJy radio sources are primarily driven by star-formation
Using the available redshifts the observed luminosity withlittlesignificantcontaminationfromAGN.Thisiscon-
has been calculated and is plotted in Fig.9, including both sistent with the results of Muxlow et al. (2005) who find
thesources in theHDF-Nregion and thenon-AGNsources thatbelow100µJy greaterthan70 percentofradiosources
in the SWIRE field. As can be seen in Fig.9 the 24µm-to- arestarburstsystems.However,thedatapresentedhereare
1.4GHzcorrelation,forindividualsources,extendsdownto basedonanIRselected sampleratherthanaradioselected
L1.4GHz 1021WHz−1 (L24µm 1021.7WHz−1). sample.Thiswillinherentlyresultinthesamplebeingdomi-
≈ ≈
natedbystar-formingsystemsbutwillnotexcludethepres-
enceof a population of weak microJy AGN sources.
Atthepositionsofthemajorityofthelowest24µmflux
4 DISCUSSION densitiessourcesthepeakradioemissionisnotsignificantly
greaterthantheimagerms.Asaconsequencemostofthese
4.1 Radio emission from faint 24µm sources:-
sources were not formally identified in the previous radio
Extending the IR-Radio correlation
study(Muxlow et al.2005).Inmanycase, especially atlow
Theprimaryresultsofthisworkdemonstratethatthissam- 24µm flux densities, the extracted total radio flux density
pleofindividual24µmselectedsourcesfollowtheMIR-radio ofindividualsourcesisclosetotheradiosensitivityofthese
correlation down to radio flux densities of a few microJy. data(Fig1).Whendirectlycomparedwiththecontrolsam-
These results are consistent with shallower independentra- ple(Fig1,Right),whichhasbeencompiledbyextractingthe
dio surveys(such as theATCA observations of theSWIRE radiofluxdensityfromblankareaswithinthesamedataus-
field) and seamlessly extend the IR-radio correlation down ing identical methods, a significant correlation between the
to lower fluxdensities (see Fig.2). strength of the total radio and 24µm emission is evident.
An evolution of the IR-Radio correlation? 7
Figure5.The24µmfluxdensityversusq24.All377sourcefrom Figure 6. In the upper panel the flux density ratio
the8′.5 8′.5HDF-Nfieldareplottedasindividualsources(small (S24µm/S1.4GHz) versus 24µm flux density is shown. The in-
×
blacktriangle).Intheupperpanelsourcesdetectedatboth24µm dividualsourcesfromtheHDF-Nfieldareplottedassmallblack
and1.4GHzintheSWIRE-CDFSfieldfromNorriset al.(2006) triangles, the median values of these HDF-N source binned as a
areplotted ingreen(smalltrianglesaresourcesnotidentifiedas function of S24µm as filled circles, green stars show the median
AGNandopenstarshavebeenidentifiedasAGN).Sourcesfrom binned values for sources listed as non-AGN within the CDFS-
theSpitzerFLSareplottedinorangefromFaddaetal.(2006).In SWIRE sampleofNorriset al.(2006) andtheblue‘crosses’and
the lower panel all of the Spitzer identified sources in the HDF- ‘pluses’ show the flux density ratios derived from the stacking
N and the only non-AGN sources from Norris et al. (2006) are analysisoftheSWIRE-CDFSandELAISfieldsrespectivelyfrom
plotted.Additionallyplottedas‘pluses’and’crosses’inthelower Boyle et al. (2007). The overlaid black dotted line is the mean
panel arethevalues ofq24 as derivedfromthe stacking analysis value for (S24µm/S1.4GHz) derived by Appleton et al. (2004).
by Boyle et al. (2007) of the SWIRE-CDFS and ELAIS fields Thislineisequivalenttoq24=0.84 0.28withthegrayfilledbox
±
respectively.Thesolidgreenlineoverlaidonbothpanelsisaline representing the area enclosed by these errors and the flux den-
of constant radio flux density of 100µJy, the lowest radio flux sity range investigated by Appleton et al. (2004). The values of
densityofsourcesintheSWIREsampleplottedhere. q24 against 24µm flux density are plotted in the lower panel.
The symbols withinthis plot areidentical to those inthe upper
panel,individualHDF-Nsourcesarenotincludedforclarity.The
additional diagonal solid green line represents a line of constant
However, the radio sensitivity limit of these data precludes
radio flux density of 100µJy, the lowest flux density of sources
thedetaileddiscussionofindividuallowfluxdensitysources.
in the SWIRE sample plotted here (sources above this line are
excluded by this limitfrom the SWIRE sample). This flux den-
sitylimitwillsignificantlyeffectthevaluesofthebinnedSWIRE
4.1.1 The average radio emission from the faintest IR
datapoints(greencrosses)negativelybiasingthefourlowestflux
sources
densitybins.Thisbiasonlyeffects theSWIRE sample.
The radio emission from many of the individual low flux
density 24µm sources is too faint to be imaged with high
signal-to-noiseintheseradiodata.However,itispossibleto lowest emission levels the radio flux densities of individ-
characterise their average radio emission by either binning ual sources will be subject to moderate or large errors the
the measured radio excess at their locations or by forming binned ensemble of these points will robustly represent the
composite radio images from many sources. With the rela- flux distribution of sample as a whole. Using the radio flux
tivelylimitedsampleofIRsourceswhichareco-spatialwith densities extracted at the positions of the individual 24µm
theseradioobservations(377intotal)itisonlystatistically sources, significant radio emission isdetected in everyaver-
viable to use either binning or image stacking methods at aged 24µm flux density bin.
the lowest IR flux densities where the number of sources To further test the viability of measuring the statisti-
that can be combined becomes large. calradio fluxdensityfrom sources in thissample below the
Overlaid in Figs.1 and 2 are the median flux densities formal radio detection limit of these MERLIN+VLA data,
ofIRsources in theHDF-Nregion imaged here.These bins the images have been stacked at the positions of multiple
have been logarithmically sampled as a function of 24µm IRsources, binnedas afunction of their24µm fluxdensity
flux density and include all sources within these bins re- (Fig.3).Thesepostage-stampimageshavebeenaveragedon
gardless of their measured radio flux density. Whilst at the apixel-by-pixelbasis,whereeachpixelinthestackedimage
8 Beswick et al.
Figure 8. The q24 versus redshift. All 259 source from the
8′.5 8′.5 HDF-N field are plotted as individual sources (black
×
triangle) with known redshifts. Fifty sources from the CDFS-
SWIRE field detected at both 24µm and 1.4GHz, and redshift
information from Norris et al. (2006) are plotted as green tri-
angle. Only sources not identified as AGN by Norris et al. 2006
areplotted. Nok-correctionhas been appliedto the data points
plotted.
structures of the radio emission from both these composite
images (Fig.3) and theindividual sources themselves is be-
Figure 7. The 1.4GHz flux density versus q24. The samples yondthescopeofthispaperandwillbepresentedinalatter
plotted and symbols used are the same as those plotted in publication.
Fig5. In both plots the solid line represents a line of constant
S24µm=80.1µJywhichisequaltothedetectionthresholdofthe
GOODS-NSpitzersampleused. 4.1.2 Comparison with other results
Two directly comparable studies of the 1.4GHz radio and
24µm MIR emission from sources in the Spitzer FLS
is either the mean or median value of the pixels within the and the SWIRE fields have been made by Appleton et al.
individual 24µm source image stacks. Each of the 6 low- (2004) and Boyle et al. (2007) respectively. Appleton et al.
est bins, for both the mean and median averaged images, (2004) compared observations of the Spitzer FLS made by
show significant levels of radio emission. The 1.4GHz flux Condon et al. (2003) using the VLA in its B-configuration
density in these images, extracted via Gaussian fitting, is (5′′restoring beam) with 24µm and 70µm Spitzer observa-
consistent with the binned level of emission derived from tions.Theseobservationshadadetectionthresholdof90µJy
measuring individual sources, demonstrating thevalidity of and 500µJy at 1.4GHz and 24µm respectively. Whereas
either method (Table2). Boyle et al. (2007) co-added deep (σ 30µJy/bm) 1.4GHz
∼
The sub-arcsecond angular resolution of these MER- ATCAobservationsofthesouthernSWIREfieldattheposi-
LIN+VLAobservations,inconjunctionwiththeirrelatively tionsofmanythousandsofSpitzer24µmsources,regardless
goodu-vcoverage,allowtheaverageradiostructuresoffaint of their detection at radio wavelengths, in order to statisti-
IRsourcestobeinvestigated.Themedianandmeanimages callydetectradioemissionatthefewµJylevel.Asacompar-
inFig.3,especiallyinthetwohighestfluxdensitybinsshow isontotheHDF-Nobservationspresentedhere,datapoints
some distinct differences. These differences arise due to in- from the SWIRE field (Norris et al. 2006), the Spitzer FLS
clusion of a few bright radio sources which dominate the (Faddaet al. 2006) and the co-added flux densities derived
averageemission includedinthesebins.Inparticularinthe byBoyle et al. (2007) are plotted in Figs.2, 5, 6 and 7.
second highest flux density mean image several ‘bright’ ra- Using the Spitzer FLS Appleton et al. (2004) studied
dio sources are offset from the map centre. The positional MIR to radio correlation for 508 sources with redshift in-
offset of radio emission from these sources is real and in- formation,confirmingresultsfrom similarprevioussurveys.
dicates that, at least in these cases, that the origins of the Following this work Fadda et al. (2006) have published a
radio and IR emission are different. more comprehensive catalogue of 17,000 24µm sources
∼
The Gaussian fitted sizes of the radio emission in the from the Spitzer FLS of which 2415 have radio counter-
′′
stacked images (Fig3) provide an upper limit on the aver- parts within 4 . Just over 2000 sources with radio counter-
age size of the radio counterparts of these faint IR sources. parts within 1′.′5 are plotted in Figs.2, 5 and 7. In general
Thelargestangularsizesoftheradioemissioninthemedian the sources plotted follow the MIR to radio correlation but
stackedimages created from thissamplerange between1′.′4 these data contain many outliers with elevated radio emis-
and 2′′. This is approximately equivalent to a linear size of sion, characteristic of galaxies containing AGN.
10kpc at redshifts beyond 1. These upper limits on the ra- The study of Appleton et al. (2004) included sources
diosourcesizesareconsistentwith radioemission ongalac- withS24µm ∼>0.5mJyandS1.4GHz >90µJy.Themajorityof
tic and sub-galactic scales and originating within kpc-scale thissamplehaveredshiftsof∼<1.Themean(non-k-corrected)
starburstsystems.Amoredetailedanalysisofthesizesand valueof q24 derived byAppleton et al.(2004) is 0.84 0.28,
±
An evolution of the IR-Radio correlation? 9
which is shown in Fig6. The non-AGN sources detected 1400MHz(ATESP,Australia Telescope ESO Slice Project)
at both 1.4GHz and 24µm within the SWIRE field cata- Prandoni et al. (2000) found, upon comparison with NVSS
logued by Norris et al. (2006) are also plotted in Figs.2, 5 fluxdensitiesforthesamefield,thatwhilstforbrightsources
and 7. These sources are limited to >0.1mJy and ∼>170µJy the ATCA and NVSS flux density values were consistent,
at 1.4GHz and 24µm respectively. Using only the sources for faint sources the ATCA results underestimated the ra-
within the SWIRE field identified as not being AGN by dio flux densities by up to a factor of 2 compared with the
Norris et al.(2006)themean(non-k-corrected)valuesofq24 VLANVSS.Thisunderestimation ofradiofluxdensitywas
is 0.69 0.39. The radio flux density threshold of both of seen by Prandoni et al. (2000) to increase with diminishing
±
these surveys overlaps with the brighter radio sources in radio brightness. Whilst it remains unclear as to the cause,
this MERLIN+VLAstudy. orreality,ofthiseffect,ifapplicabletotheELAISandCDFS
These deep HDF-NMERLIN+VLAobservations trace fields, it could result in the elevated values of q24 seen by
individual sources with considerably fainter flux density Boyle et al. (2007) at very low radio fluxdensities.
than those observed in the SWIRE and Spitzer FLS sur-
veys. These observations (Figs.2, 5 and 7) are consistent
4.2 An evolution in the IR-radio correlation?
with the extrapolation to lower flux densities of results
found previously (e.g. Appleton et al. 2004; Norris et al. It has been suggested, both theoretically (Bell 2003) and
2006; Fadda et al. 2006). However both the SWIRE and observationally (Boyle et al. 2007),that theIR-radiocorre-
Spitzer FLS surveys which sample sources with somewhat lationmayevolveatlowerfluxdensities.Oneoftheaimsof
higher flux densities sources have mean values of q24 which thisstudyistoinvestigate thiscorrelation forverylow flux
areconsistentbutslightlyhigherthanthevalueof0.52 0.37 density radio sources.
±
derived from these HDF-Ndata. DerivedfromtheHDF-Ndataalonetheaveragevalueof
Averaging together the radio emission from multi- q24 showsasmalltrendtoreduceasafunctionoflowerval-
ple 24µm Spitzer source within the CDFS and ELAIS uesofS24µm (Fig.5&6).Thiscanbefurtherdemonstrated
fields Boyle et al. (2007) have traced the radio emission by splitting these data into two approximately equal sam-
against MIR sources with flux densities comparable to ples, above and below S24µm=150µJy. The mean and me-
thosedetectedinthesemoresensitiveHDF-Nobservations. dianvaluesofq24 forthe187sourcesbelow150µJyare0.45
Boyle et al. (2007) haveexploited thelarge numberof MIR and 0.36 respectively, compared with 0.56 and 0.54 derived
sourcesdetectedinthesefieldstoaveragetogethermultiple from the 190 sources with S24µm >150µJy. These HDF-N
radioimagesmadeattheMIRsourcepositionsandstatisti- results along with results from samples of brighter galaxies
callydetectradioemissionsignificantlybelowtheirradiode- (Appleton et al. 2004; Norris et al. 2006) are broadly con-
tectionthreshold.Thismethodisdirectlycomparabletothe sistentwithaverageq24 reducingasafunctionofdecreasing
methods applied to these radio observations of the HDF-N S24µm (see Fig.5 & 6).
andisidenticaltothemethodsusedtoproducethestacked There are several contributing factors which will affect
images presented inFig.3and Table2.Themedian stacked theseresultsandthosefrom previousstudies.Thesefactors
radio flux densities of MIR sources derived by Boyle et al. includebiasesintroducedbyfluxdensitythresholdsofobser-
(2007)showasignificantlyreducedS1.4GHzrelativetoS24µm vations, the application or non-application of k-corrections
(i.e. higherq24 value)compared with thedistributionof in- and possible AGN contamination of sample galaxies.
dividual sources detected in the Spitzer FLS (Faddaet al. Previous studies, such as Appleton et al. (2004), have
2006;Appleton et al.2004),thecatalogued SWIREsources usedsamplesofsourcesdetectedatbothradioandinfrared
(Norris et al. 2006) or the HDF-N results presented here. wavelengths. The implications of a radio detection limit on
Whilst there are several differences between the studies of theseresultswillbetoexcludesources withhighq24 atlow
Boyle et al. (2007) and those presented here, such as the S24µm.ThelineoverlaidonFig.5representstheupperlimit
sensitivity(rms 30µJyversus 3µJy)andangularresolu- imposed on the ATCA observations of the SWIRE survey
tion (6′′versus 0∼′.′4) of the two∼surveys, it remains hard to bytheradio detection threshold ofthesedata (Norris et al.
simply reconcile theseresults. 2006). In a manner similar to that used by Boyle et al.
Boyle et al.(2007)recognisedtheinconsistencyoftheir (2007)noradiodetectionlimithasbeenappliedduringanal-
calculated values of q24(=1.39) when compared with 0.84 ysisoftheseMERLIN+VLAdatawiththeradiofluxdensity
derivedbyAppleton et al.(2004)andhaveextensivelysim- beingrecordedforallMIRsourcesregardlessofsignificance
ulated and tested the correctness of their analysis meth- oftheirradiodetection.Thismethodwill resultinerrorsin
ods.Sincetheresults ofBoyle et al.(2007) arebased onan the measured radio flux densities for very weak individual
infrared-selected sample with no radio detection limit, this radiosourcesbutwill notdirectlybiasagainst largervalues
discrepancy could possibly be explained by the presence of of q24. However when many sources are statistically aver-
apreviouslyunknownpopulationoffaintradiosourceswith aged togetherthemean andmedian valueofdetectedradio
a low radio to infrared flux density ratio. However the val- emission and hence the derived value of q24 will be robust
ues derived by Boyle et al. (2007) are not confirmed in the evenforsourceconsiderablyfainterthantheradiodetection
study presented here. threshold of these data.
A possible alternative explanation may be that the No direct k-correction has been applied to these HDF-
ATCAobservationsareunderestimatingtheradiofluxden- N flux density results because in many cases the spectral
sity of sources at the lowest flux densities when compared shapes and redshifts of objects are not known adequately
to these other surveys which have used radio data ob- enough to apply any correction with confidence. Addition-
tained using the VLA and/or MERLIN. It is interesting allypreviousstudies,suchasbyAppleton et al.(2004),have
to note that an earlier deep survey using the ATCA at shown that the application of k-corrections to individual
10 Beswick et al.
Figure 9. The luminosities at 1.4GHz and 24µm for 224 24µm Spitzer sources with redshifts and significant radio flux density from
the HDF-N (black triangles) and 50 non-AGN 24µm sources with redshifts catalogued by Norris et al. (2006) (green triangles). The
luminosities shown have not been k-corrected. All 1.4GHz luminosities have measured within an aperture of radius 1′.′5 centred upon
theSpitzerMIPS24µmsourceposition.NotethefewcaseswhereHDF-Nsourceswithapparentlylowradioluminositywithrespectto
MIRluminosityarelargenearbyspiralswithanopticalextentlargerthanthe3′′.
sources below z=1 only introduces a small change to the of the individual sources. In particular, for sources at red-
averagevalueofq24 asafunctionofredshift.Ofthesources shifts > 1 significant spectral features from PAHs and sili-
withknownredshiftsinoursample78percenthaveredshifts cates observed in local starburst galaxies (e.g. Brandl et al.
less than 1, and thus will be minimally affected. Within 2006) are shifted intotheSpitzer 24µm bandwhich can re-
Figs.5and7thevaluesofq24forindividualsourcesareplot- sultinlargeadditionalk-correctionfactors.Theinclusionof
ted against their flux densities at each wavelength. These these line features can result in an increase of a factor of 2
sources sample a wide range of luminosities and redshifts or more in S24µm compared with commonly used starburst
and hence the systematic effects resulting from not apply- spectraltemplates(Faddaet al.2006;Yan et al.2007).This
ingak-correctionwilleffectindividualsourcesbutwillcause is equivalent to a change in q24 of approximately 0.3. The
only minimal changes in any overall trend of the sample as generaleffectoftheMIRk-correction,resultingfromaspec-
a function of flux density. However, the effects of the ap- tralindexofastarburstgalaxyexcludinglineemission,will
plication of a k-correction to these data do warrant further be to boost S24µm as a function of redshift by an amount
discussion. thatwilldependuponspectralslopeofthegalaxytemplate
used. For a luminous starburst this will result in a boost-
Assumingthatthe24µmsourcesidentifiedinthissam-
ple are predominately powered by star-formation, at an ing with a MIR spectral slope α515 ∼ 2 (Yan et al. 2007).
As such the effect of this k-correction applied to the 24µm
observed radio frequency of 1.4GHz the emission will be
data will be larger than that applied to the 1.4GHz data,
dominated by non-thermal synchrotron emission from su-
pernovaeandsupernovaremnants(Muxlow et al.2007).As hence the overall effect will be to increase the value of q24
as a function of redshift.
suchthenon-thermalradiosignalwillbeboostedbyafactor
of (1+z)0.7, assuming a synchrotron power law (S ν−0.7). Uncorrected q24 is plotted against redshift for the
∝ sources with known spectroscopic or photometric redshifts
This k-correction will become incorrect for the highest red-
inFig8.Applyingk-correctionstothesedatapointswillre-
shift sources as the rest-frame radiation will be shifted
to higher radio frequencies where the emission from other sultinasmallincreaseinq24forthehighestredshiftsources
plotted. Within the scatter of the small sample of sources
mechanisms such as thermal free-free emission will become
plottedinFig8nosignificanttrendisobservedasafunction
increasingly important. For example a source at redshift of
∼>2.5 the radio emission observed at 1.4GHz will have been of redshift to z≈3. This is consistent with previous studies
(e.g. Garrett 2002; Appleton et al. 2004).
emittedatarest-framefrequencyof∼>5GHzatwhichpoint
an increasing proportion of the radio emission from a star-
forminggalaxywillarisefromthermalprocessesresultingin
areductionintheradiospectralslopeandhencek-correction 4.2.1 Potential contamination from AGN
that should berequired.
In addition to instrumental and analysis effects outlined
Accuratelyk-correctingtheSpitzer24µmfluxdensities aboveitispossiblethatthefluxdensitiesofsourcesrecorded
requiresdetailedknowledgeofthemid-IRspectraltemplate in this study may include some proportion of contamina-