Broad, weak 21 cm absorption in an early type galaxy: spectral-line finding and parametrization for future surveys

0.56 MB·English

by  J. R. Allison

Mon.Not.R.Astron.Soc.000,1–6(2012) Printed25January2013 (MNLATEXstylefilev2.2)
Broad, weak 21cm absorption in an early type galaxy: spectral
line finding and parametrization for future surveys
J. R. Allison1(cid:63), S. J. Curran1,2, E. M. Sadler1,2 and S. N. Reeves1,2,3
1SydneyInstituteforAstronomy,SchoolofPhysicsA28,UniversityofSydney,NSW2006,Australia
2ARCCentreofExcellenceforAll-skyAstrophysics(CAASTRO)
3CSIROAstronomy&SpaceScience,P.O.Box76,EppingNSW1710,Australia
ABSTRACT
WereportconclusiveverificationofthedetectionofassociatedHI 21cmabsorptionin
3
theearly-typehostgalaxyofthecompactradiosourcePMNJ2054–4242.Weestimatean
1
effectivespectrallinevelocitywidthof418 20kms 1andobservedpeakopticaldepth
0 ± −
of2.5 0.2percent,makingthisoneofthebroadestandweakest21cmabsorptionlines
2
yetdet±ected.ForTspin/f >100KtheatomicneutralhydrogencolumndensityisNHI (cid:38)
n 2 1021cm 2.Theobservedspectrallineprofileisredshiftedby187 46kms 1,with
a × − ± −
J respecttotheopticalspectroscopicmeasurement,perhapsindicatingthattheHI gasis
infallingtowardsthecentralactivegalacticnucleus.Ourinitialtentativedetectionwould
3
likely have been dismissed by visual inspection, and hence its verification here is an
2
excellent test of our spectral line detection technique, currently under development in

anticipationoffuturenext-generation21cmabsorption-linesurveys.
]
O
Key words: methods: data analysis – galaxies: active – galaxies: ISM – radio lines:
C
galaxies
.
h
p
-
o
r 1 INTRODUCTION Inpreviouswork,Allisonetal.(2012a)reportedthetenta-
st Thecoldatomicneutralhydrogencontentofgalaxies(HI)plays tive detection of broad (∆vFWHM ∼ 500kms−1) HI 21cm
a absorption towards the compact flat-spectrum radio source,
[ a vital role in the evolution of galaxies, through formation of PMNJ2054–4242(Griffith&Wright1993;Healeyetal.2007),
stars and efficient accretion of matter on to active galactic nu-
usingaspectrallinedetectionmethodbasedonaMonteCarlo
3 clei (AGNs). By using the 21cm hyperfine transition of HI,
v Bayesiantechnique.FromobservationsusingtheAustraliaTele-
observed as absorption against a background radio source of
2 scopeCompactArrayBroadbandBackend(CABB;Wilsonetal.
known brightness, we can measure the distribution and kine-
5 2011),andbyconsideringtheratioofthemarginallikelihoods,
matics of line-of-sight cold gas (T ∼ 100K). Previous ob-
4 thedatawerefoundtowarranttheinclusionofthespectralline
servations of the 21cm line in associated absorption often ex-
3 hypothesisoverthecontinuum-onlyhypothesis.Herewereport
1. hniebnitt(s∼pe1c0trkaml lsin−e1)praonfidlebsrowadithwbinogths farodmeepa,snhaarlrlooww ccoommppoo-- furtherobservationstoconclusivelyverifythepresenceofthis
1 nent (∼100kms−1) (e.g. Mirabel 1989; Morganti et al. 2001, broad and shallow absorption line. This is one of the broadest
2 and weakest 21cm absorption lines yet detected in isolation,
2005a;Salteretal.2010;Allisonetal.2012a).Fromobserva-
1 withnopreviousindicationofabsorptionfromanexistingdeep,
tions at high spatial resolution there, is evidence that in some
: narrowcomponent.Conclusiveverificationofthisdetectionpro-
v casesthesebroad,shallowcomponentsmightbeassociatedwith
vides an excellent test of our spectral line finding method, a
Xi a circumnuclear torus (e.g. Struve & Conway 2010; Morganti techniquethatcanbeusedfordetectingandparametrizingsuch
etal.2011).Additionally,Morgantietal.(2005b)havesuccess-
r spectrallinesinfuturelarge-scaleabsorptionsurveys.
a fullydetectedextremelybroadabsorptionwithingas-richhosts, ThroughoutthisPaperweadoptaflatΛCDMcosmology
eachofwhichhasanexistingdeep,narrowabsorption-linecom-
withΩ =0.27,Ω =0.73andH =71kms−1Mpc−1.Quan-
ponent.Thesesystemsareassociatedwithhigh-velocitygasout- M Λ 0
tities estimated from the data are given by their mean and 1σ
flows,providingaprobeofthefeedbackbetweenthecentralra-
uncertaintyunlessotherwisestated.
diosourceandthestar-forminginterstellarmediumofitshost
galaxy.Otherexamplesofextremebroadening includethede-
tectionofHIabsorptiontowardsthecandidatebinaryblack-hole
4C37.11(Manessetal.2004;Morgantietal.2009),withafull
2 OBSERVATIONSANDDATAREDUCTION
width at zero-intensity of 1600kms−1 and dominated by two
narrow,deeppeaks.However,therearecurrentlyveryfewde- In addition to the observations in 2011 April reported by Al-
tectionsthataredominatedbybroadabsorption,withno(orvery lison etal. (2012a),a further 12h observationof PMNJ2054–
little)accompanyingnarrowcomponent(seee.g.Jaffe1990). 4242, at RA(J2000) = 20h54m01s.79 and Dec.(J2000) =
−42◦42(cid:48)38(cid:48).(cid:48)7,wascarriedoutin2012June17–18.TheCABB
system provides a zoom band of 64MHz across 2048 chan-
(cid:63) E-mail:[email protected] nels, which at the centre frequency of 1.342GHz has a ve-
(cid:13)c 2012RAS
2 J.R.Allisonetal.
Figure1.Thesynthesized-beamweightedspectrum,combiningATCA21cmdatafromobservationsin2011Apriland2012June.Thedatawere
binnedtoaresolutionof21kms−1,whichisapproximatelythreetimesthatoftheindividualspectra.Theopticalspectroscopicredshiftisindicatedby
theverticalarrow(zopt=0.042893±0.000150;Jonesetal.2009).
locity resolution ∼ 7kms−1. The six-element ATCA was ar- observingprogramin2012April,weobtainedfurtherlowspec-
ranged in the 6D East-West configuration, with baseline dis- tral resolution (230kms−1) CABB data. While not useful for
tancesintherange0.08–5.88km.Withthisconfigurationwe parametrizing the absorption, the 21cm spectral line profile is
obtained an angular scale sensitivity range of approximately 8 wideenoughtobeseenacrossthreechannelsinthedataandso
– 600arcsec, and a primary beam full width at half-maximum providedadditionalverificationofourdetection.
(FWHM)of35arcmin.Weobservedthetargetsourcein20min
scans,whichwereinterleavedwith1.5minobservationsofthe
nearby secondary calibrator, PKSB2211–388 (S =
1.384GHz
1.86±0.01Jy),1forgainandband-passcalibration.Forthecal- 3 ANALYSIS
ibrationoftheabsolutefluxscale,weobservedPKSB1934–638
Hereweprovideabriefsummaryofthemethodusedfor21cm
(S = 14.94±0.01Jy)1 atregularintervalsseparated
1.384GHz spectrallinedetectionandparametrization.Foramoredetailed
by2h.Forthe2012Juneepoch,weachievedatotalintegration
description please refer to work by Allison et al. (2012a,b),
timeof8.6honthetargetsource.
andreferencestherein.Inordertoverifywhether21cmHIab-
Weimplementedthesamedatareductionprocedureasre-
sorption has been detected with significance, we estimated the
portedbyAllisonetal.(2012a),againsubtractingthebaseline
conditional probability of a spectral line model (parametrized
ripple signal from other continuum sources within the field of
byasumofspectrallinecomponentsandafirst-orderpolyno-
view.Thedatawereflagged,calibratedandimagedusingtasks
mial)andcomparedittothatofacontinuum-onlymodel(also
fromtheMIRIAD2package(Saultetal.1995),andimplemented parametrized by a first-order polynomial). We also included a
usingapurpose-builtCABBHIdatareductionpipeline.Weup- nuisanceparameter,whichrepresentssystematicerrorduetoim-
dated our original procedure to include some minor improve-
perfectdatareduction.Themodelspectrumismultipliedbythis
mentstothecalibrationandbaselineripplesubtraction,andhave
parameter,withanon-uniformpriorprobabilitygivenbyanor-
appliedtheseheretothedataobtainedbothin2011Apriland
mal distribution of µ = 1.0 and σ = 0.1, which represents a
2012June.AssumingthatthefluxdensityofPMNJ2054–4242
10percentsystematicerror.Notethatwehaverevisedouresti-
hasnotvariedsignificantlybetweenthetwoepochs(whichwas
mate of the systematic error in the ATCA data with respect to
notevidentfromourobservations),weconstructedasingle,nat-
thatgivenbyAllisonetal.(2012a).
uralweighteddatacubefromthecombinedvisibilitydata.Since
Wehaveassumednopriorinformationabouttheprobabil-
theATCAdoesnotobserveatconstantradialvelocity(known
ity of detection of a spectral-line, since it is difficult to draw
asDopplertracking),theobservedfrequencychannelsforeach
strongstatisticalconclusionsfromtheexistingsmallsampleof
epochcorrespondtoslightlydifferentvelocities.Toobtainap-
detectedassociatedHIabsorptionsystems(e.g.Curran&Whit-
proximately uniform variance in flux across the velocity spec-
ing2010;Curranetal.2011a;Allisonetal.2012a).Therefore,
trum(whilestillretainingspectralinformation),webinnedthe
the ratio of probabilites for each model hypothesis, given the
combined data to a resolution of 21kms−1, which is approxi-
data,isequaltotheratioofprobabilitiesforthedatagiventhe
mately three times that of the individual spectra. Fig.1 shows
model hypotheses (also known as the ratio of marginal likeli-
the resulting synthesized-beam weighted spectrum centred on
hoodsorBayesianevidence).WedefinethestatisticRby
PMNJ2054–4242.Basedonthemedianabsolutedeviationfrom
(cid:18) (cid:19)
the median (e.g. Whiting 2012), the estimated per spectral- R≡ln EHI , (1)
channel uncertainty is 1.05mJy, which is consistent with the E
cont
uncertainties in the spectra for each epoch and the subsequent
whereE andE aretheBayesianevidenceforthespectral
velocity binning (see Table1). The 21cm HI absorption can HI cont
lineandcontinuum-onlymodelsrespectively.IfthevalueofRis
clearlybeseenuponvisualinspection,slightlyredshiftedwith
greaterthanzero,thenthisgivesthelevelofsignificanceforthe
respect to the systemic velocity, given by optical spectroscopy
spectrallinehypothesis.However,ifthevalueofRislessthan
from the 6dF Galaxy Survey (6dFGS; Jones et al. 2009). This
zero,thenthedatadonotwarranttheinclusionofaspectralline
feature is seen throughout observations in both epochs, and in
componentinthemodelandsowerejectthishypothesis.
bothpolarizations,stronglyindicatingthatitisnotgeneratedby
In order to calculate the Bayesian evidence, we used an
atransientartefactsuchasradiofrequencyinterference.Wealso
efficient Monte Carlo algorithm provided by Feroz & Hobson
donotseethisfeatureinspectrathatarerandomlyselectedat
(2008)andFerozetal.(2009),called MULTINEST,whichim-
positionsawayfromthesource.Atathirdepoch,duringanother
plementsthenestedsamplingalgorithmbySkilling(2004).We
optimizedthesignificancestatisticRbyincreasingthenumber
1 http://www.narrabri.atnf.csiro.au/calibrators/ ofspectrallinecomponents,thusgivingthebest-fittingspectral
2 http://www.atnf.csiro.au/computing/software/miriad/ linemodel.
(cid:13)c 2012RAS,MNRAS000,1–6
Broad,weak21cmabsorptioninanearlytypegalaxy 3
Figure2.Resultsoffittingtospectrafrom21cmobservationsin2011Apriland2012JunewiththeATCA.Theradialvelocityaxisisgivenintherest
framedefinedbytheopticalspectroscopicredshift(verticaldashedline).Thedatahavebeensimultaneouslyfittedbyacombinedcontinuumpolynomial
andaspectrallinemodel.Thesolidblacklinerepresentsthedataaftersubtractionofthebest-fitting(maximum-likelihood)continuumcomponent.The
redlinerepresentsthebest-fittingspectrallinemodel(seetextfordetails).Thebluelinerepresentsthebest-fittingresidual,includingavelocity-axis
offsetforclarity.
4 RESULTSANDDISCUSSION whereτ(v)istheopticaldepthoftheabsorbinggasasafunction
of the rest-frame radial velocity. If we assume that the 21cm
4.1 Parametrizationofthe21cmabsorption absorptionisopticallythin(∆S /fS (cid:46) 0.3),theabove
line cont
expressionreducesto
WeimplementedtheanalysismethodoutlinedinSection3,ini-
tiallyrepresentingthe21cmspectrallinebymultipleGaussian
∆S τ (v)
components.Thedatainbothepochswarranttheinclusionofthe τ(v)≈ line ≈ obs , (3)
fS f
spectrallinemodeloverthecontinuum-onlymodeland,bycom- cont
paringtheBayesianevidencefordifferentnumbersofspectral
whereτ (v)istheobservedopticaldepth.Sincemany21cm
linecomponents,thesingle-componentmodelisfavoured.How- obs
absorption lines in the literature are far from being well mod-
ever,whileasingleGaussiandoesprovideagoodfittothecom-
elledbyasingleGaussiancomponent,forthepurposeofcom-
bineddata(atmaximumlikelihoodχ2 /d.o.f =0.94±0.06),
ml parison,wedefineaneffectivewidthby
upon visual inspection the spectral line profile appears more
steepsidedthanpredictedbythismodel.Totestthishypothe-
(cid:82)
τ (v)dv
sis,wetriedanempiricallymotivatedspectrallinemodel,which ∆v ≡ obs , (4)
providesforasimultaneouslybroadandsteep-sidedtrough-like eff τobs,peak
profile,andisgivenbytheproductoftwoGausserrorfunctions
(R.Jurek andT.Westmeier, privatecommunication).Thisis a whereτobs,peak isthepeakobservedopticaldepth.Thisdefini-
simplified version of the 21cm spectral line model developed tion of the spectral line width avoids missing the contribution
byWestmeier,Jurek&Obreschkow(inpreparation).Sincethis of broad,shallow components (aswhen usingthe FWHM), or
modelcontainsonemoreparameterthanasingleGaussianpro- the width parameter being strongly dependent on the spectral
file,whichdeterminesthesteepnessoftheprofilesides,itmust uncertainty(aswhenusingthefullwidthatzero-intensity).For
provideasignificantlybetterrepresentationofthedatainorder the special case of a single Gaussian component, the effective
tobepreferred. widthisjustafactorof1.06timestheFWHM.InTable1,we
summarize the derived spectral line parameters from the best-
Again using Bayesian inference, we found that the 2011
fittingmodels.Parametervaluesestimatedfromthe2011April
April data favour the single Gaussian model, while the 2012
and 2012 June data are mostly consistent with each other and
Juneandcombineddatafavourthetrough-likemodel.Thisre-
anysignificantdifferencesaretheresultofthechoiceofmodel
sult is perhaps unsurprising given the relatively low signal-to-
parametrization(thesingleGaussianisintrinsicallymorepeaked
noiseratio(S/N)ofthe2011Aprildata.Inthecaseofthecom-
than the trough-like model). The small difference between the
bineddata,thetrough-likemodelisfoundtobeapproximately
valueofRforourre-analysisofthe2011Aprildata,withthat
35timesmorelikelythanthesingleGaussianmodel,withasig-
nificanceabovethecontinuum-onlymodelofR=109.5±0.1. fromthepreviousanalysisbyAllisonetal.(2012a),istheresult
ofimprovementstoourdatareductionmethod.
Whenwereplacedtheuniformpriorfortheredshiftofthe21cm
spectral-linewithanormalpriorof12859±500kms−1,given Thecolumndensityofatomicneutralhydrogen(incm−2)
by the 6dFGS optical redshift (Jones et al. 2009) and typical asafunctionoftheintegratedopticaldepth(inkms−1)isgiven
infall/outflow gas velocities, the spectral line significance in- by(Wolfe&Burbidge1975)
creasestoR=111.8±0.1.Fig.2showsthebest-fittingspectral (cid:90)
line model for each of the individual spectra and the velocity- NHI = 1.823×1018Tspin τ(v)dv
binnedcombineddata.
T (cid:90)
Forasufficientlybrightbackgroundsource,theactualop- ≈ 1.823×1018 spin τ (v)dv, (5)
f obs
ticaldepthisrelatedtothecontinuumfluxdensity(S ),the
cont
fractionoffluxthatiscoveredbytheforegroundHIgas(f)and whereT isthemeanharmonicspintemperatureofthegas(in
spin
the observed spectral-line depth (∆S ). This relationship is
line K).Basedonthisrelation,weestimatetheHI columndensity
givenby
towardsPMNJ2054–4242tobe
(cid:18) (cid:19) (cid:18) (cid:19)(cid:18) (cid:19)
∆S T 1.0
τ(v)=−ln 1− line , (2) N ≈1.91±0.14×1021 spin cm−2, (6)
fS HI 100K f
cont
(cid:13)c 2012RAS,MNRAS000,1–6
4 J.R.Allisonetal.
Table1.AsummaryofthederivedpropertiesofHIabsorptionfrommodelfittingtotheATCA21cmdata,whereσchanistheestimateduncertainty
perchannel;czpeakistheredshiftatpeakspectral-linedepth;∆Sline,peakisthepeakspectrallinedepth;Scont,peakisthecontinuumfluxatpeak
(cid:82)
spectrallinedepth; τobs(v)dv isthevelocityintegratedobservedopticaldepth;∆veff istheeffectivevelocitywidth(asdefinedbyequation4);
Risthenaturallogarithmoftheratioofprobabilitiesforthesingle-componentspectrallinemodelversusthecontinuum-onlymodel;χ2 /d.o.f is
ml
thereducedchi-squaredstatisticforthebest-fitting(maximumlikelihood)modelparameters(undertheassumptionofmodellinearity).Notethatthe
uncertaintiesincludethesystematicerrorinthedatarepresentedbyanuisanceparameterwithanormalprior.
Epoch Resolution σchan czpeak ∆Sline,peak Scont,peak (cid:82) τobs(v)dv ∆veff R χ2ml/d.o.f
(kms−1) (mJy) (kms−1) (mJy) (mJy) (kms−1) (kms−1)
2011April 7 3.83 13073±38 4.8±0.8 160±11 15.8±2.8 540±120 24.2±0.1 0.98±0.03
2012June 7 1.91 13048±10 3.9±0.4 163±10 10.1±0.7 418±20 95.0±0.1 1.02±0.03
Combined 21 1.05 13046±10 4.1±0.4 163±10 10.5±0.7 418±20 109.5±0.1 0.91±0.06
which for values of T /f (cid:38) 10K is equivalent to that of a windowfunctionwithawidthof5)andappliedathresholdcut-
spin
dampedLyman-αabsorber3(N >2×1020cm−2). off of three times the S/N. However, by using a Bayesian ap-
HI
proachtospectrallinefindingandfitting,Allisonetal.(2012a)
foundthatthefeaturewassignificantabovethenoiseandthatit
occupiedaparameterspacethatwasdifferenttothatofthefalse
4.2 HighvelocitycoldgastowardsPMNJ2054–4242
detectionsfromcontinuumartefacts.Theredshiftisclosetothat
By applying Bayesian inference to our 21cm ATCA data we oftheopticalspectroscopicmeasurement,whichstronglysug-
have detected cold, high column density HI gas towards the geststhatitarisesfromassociatedHI absorption.Allofthese
compactflat-spectrumradiosourcePMNJ2054–4242.Theob- indicators prompted further observations in 2012 June, with a
served total integrated optical depth for the absorption is rel- longerintegrationtime,leadingtoconfirmationthatthefeature
atively large (see Table1), however, this is distributed over a isindeeda21cmabsorpionline.
broadspectrallinewidth,producingamodestpeakopticaldepth Suchbroad,shallow21cmabsorptionlinesphysicallycor-
ofτobs,peak =2.5±0.2percent.The21cmspectrallineisred- respond to a large spread in the velocity of HI gas along the
shiftedwithrespecttotheopticalspectroscopicredshift(z = line of sight. Existingdetections of broad absorption are often
opt
0.042893±0.000150;Jonesetal.2009)by187±46kms−1, associatedwithanacompanyingnarrow,deepercomponent.For
possibly indicating that the gas is infalling towards the central example, Morganti et al. (2005b) conducted a targeted survey
radio source. The strong broadening of the line perhaps sug- of 11 bright radio-loud AGNs, with evidence of past star for-
gests that the cold gas is rotating with very high velocity, or mationandarichinter-stellarmedium,andmostwithexisting
that the infall is accelerated along the line of sight. A high detectionsofdeep,narrowHIabsorption.Thebroadbandwidth
spatial resolution 21cm study of the similarly compact radio available(±2000kms−1)andthehighsensitivityachieved(rms
sourcePKSB1814–637,byMorgantietal.(2011),showedthat ∼0.4 – 0.8mJybeam−1), using the Westerbork Synthesis Ra-
thebroadenedabsorptioncomponentinthatsystemwaslikely dioTelescope,alloweddetectionofbroadandshallowerabsorp-
located within a circumnuclear disc, while the deeper narrow tion with τ (cid:46) 0.01. Morganti et al. detected broad, shal-
peak
componentarisesfromabsorptioninanextendeddiscofgason lowabsorptiontowards6radiogalaxies,allofwhichwereas-
thekpcscale.Theabsenceofanarrowcomponentinthespec- sociatedwithexistingdeep,narrowcomponents,andhadbulk
trum of PMNJ2054–4242 is perhaps indicative of either low blueshiftedmotioninterpretedasline-of-sightcoldgasoutflows.
column density (or missing) large-scale cold gas, or that any However,ourdetectionofasinglebroadabsorptionlinetowards
extended structure is orientated away from the line of sight to PMNJ2054-4242raisesthequestionastowhetherthereexista
theradiosource.Thesehypothesesareconsistentwiththeopti- populationofveryshallowandbroadHIabsorptionlines,which
calclassificationsofthehostgalaxiesforeachofthesesources: havenoassociatednarrow,deepcomponent,andasyetremain
PKSB1814–637 is hosted by a gas-rich regular edge-on disc, largelyundetected.
while for PMNJ2054-4242 the host is an early type (Loveday Toaddressthisquestion,weshowinthetoppanelofFig.3
1996).However,withoutfurtherinformation,wecanonlyspec- thepeakobservedopticaldepth,versustheeffectivespectralline
ulatehereastothephysicalnatureofthespectrallinebroaden- width,forarepresentativesampleofdetectionsof21cmasso-
ing.Wearethereforepursuingfollow-upmultiwavelengthand ciatedHI absorptionintheliterature4 (Curranetal.2013also
highspatialresolutionobservationstotesttheseinterpretations present a similar plot of the FWHM for a literature sample of
ofthe21cmATCAdata. redshifted absorbers). We have treated multiple spectral com-
ponentsasasingledetection,sothataprofileexhibitingasin-
glebroadcomponentwillhavealargereffectivewidththanthat
with both broad and strong narrow components. Upon inspec-
4.3 Thescarcityofbroad,weak21cmabsorptionlines
tionofFig.3,itisevidentthatthesedataexhibitaweakanticor-
Theleft-handpanelofFig.2showstheinitialdetectionofbroad
21cmabsorptiontowardsPMNJ2054–4242(2011April).From
visualinspectionofthisspectrum,onemightrejectthefeature
4 References for the literature sample shown in Fig.3: Shostak et al.
asbeinganartefact,duetoitswideprofileandlowpeakS/N.
(1983), van Gorkom et al. (1986, 1989), Mirabel (1989, 1990), Jaffe
Indeed by using the automated source-finding tool DUCHAMP (1990),Usonetal.(1991),Carillietal.(1992,1998),Morgantietal.
(Whiting2012),thisoriginalfeaturewasonlydetectedwithsig- (1998),Mooreetal.(1999),Pecketal.(1999,2000),Morgantietal.
nificanceoncewehadsmoothedthespectraldata(usingaHann (2001),Ishwara-Chandraetal.(2003),Vermeulenetal.(2003),Maness
etal.(2004),Morgantietal.(2005a),Gupta&Saikia(2006),Curran
etal.(2006),Orientietal.(2006),Guptaetal.(2006),Emontsetal.
(2008), Struve & Conway (2010), Salter et al. (2010), Emonts et al.
3 ThelowestvalueofTspin/f foundsofar,foradampedLyman-α (2010),Curranetal.(2011a,b),Chandolaetal.(2011),Emontsetal.
absorber,is60K(Curranetal.2007). (2012)andAllisonetal.(2012a).
(cid:13)c 2012RAS,MNRAS000,1–6
Broad,weak21cmabsorptioninanearlytypegalaxy 5
theCommonwealthofAustraliaforoperationasaNationalFa-
cilitymanagedbyCSIRO.Computingfacilitieswereprovided
100
bytheHighPerformanceComputingFacilityattheUniversity
k
ea10(cid:239)1 ofSydney.ThisresearchhasmadeuseoftheNASA/IPACEx-
p
bs, tragalacticDatabase(NED)whichisoperatedbytheJetPropul-
τo10(cid:239)2 sionLaboratory,CaliforniaInstituteofTechnology,undercon-
tract with the National Aeronautics and Space Administration.
10(cid:239)3 ThisresearchhasalsomadeuseofNASA’sAstrophysicsData
SystemBibliographicServices.
∆veff (kms−1)
)
Jy100
(
nt
Sco10(cid:239)1 REFERENCES
×
k AllisonJ.R.etal.,2012a,MNRAS,423,2601
pea10(cid:239)2 AllisonJ.R.,SadlerE.M.,WhitingM.T.,2012b,PASA,29,
bs, 221
τo10(cid:239)3
AltayG.,TheunsT.,SchayeJ.,CrightonN.H.M.,DallaVec-
0 100 200 300 400 500 600 chiaC.,2011,ApJL,737,L37
∆veff (kms−1) Carilli C. L., Menten K. M., Reid M. J., Rupen M. P., Yun
M.S.,1998,ApJ,494,175
Figure3.Theobserved21cmpeakopticaldepth(top),andinfluxunits
CarilliC.L.,PerlmanE.S.,StockeJ.T.,1992,ApJL,400,L13
(bottom),versusthespectrallinewidth(asdefinedbyequation4),for
detectionsofassociatedHIabsorption.Theemptystarsrepresentasam- ChandolaY.,SirothiaS.K.,SaikiaD.J.,2011,MNRAS,418,
plefromtheliterature(seetextfordetailsandreferencestherein)and 1787
thefilledstarrepresentsourdetectiontowardsPMNJ2054–4242.The Curran S. J., Tzanavaris P., Murphy M. T., Webb J. K.,
linesrepresentHIabsorptionforthemaximumobservedcolumndensity Pihlstro¨mY.M.,2007,MNRAS,381,L6
∼1022cm−2,andconstantTspin/f of100K(solid),300K(dashed) CurranS.J.,WhitingM.T.,2010,ApJ,712,303
and1000K(dot-dashed). CurranS.J.etal.,2011a,MNRAS,413,1165
CurranS.J.,WhitingM.T.,MurphyM.T.,WebbJ.K.,Long-
relation between the peak observed optical depth and spectral moreS.N.,Pihlstro¨mY.M.,AthreyaR.,BlakeC.,2006,MN-
linewidth(thePearsonproductmomentcorrelationcoefficient RAS,371,431
r ≈ −0.3),consistentwiththemaximumcolumndensitylimit Curran S. J., Whiting M. T., Sadler E. M., Bignell C., 2013,
∼ 1022cm−2, seen in observations of HI gas both at z ≈ 0 MNRAS,428,2053
(Zwaanetal.2005)andz ≈ 3(Altayetal.2011).However,it Curran S. J., Whiting M. T., Webb J. K., Athreya R., 2011b,
shouldbenotedthatthisfigureispurelyindicativeoftheoverall MNRAS,414,L26
trend,sincewehavenotaccountedforthemyriaduncertainties Emonts B. H. C., Burnett C., Morganti R., Struve C., 2012,
intheseindividualestimates. MNRAS,421,1421
The large majority of detections in the literature sample EmontsB.H.C.,MorgantiR.,OosterlooT.A.,HoltJ.,Tad-
have spectral line widths smaller than our detection towards hunterC.N.,vanderHulstJ.M.,OjhaR.,SadlerE.M.,2008,
PMNJ2054–4242 making this one of the broadest 21cm ab- MNRAS,387,197
sorption lines yet detected. The widest spectral line shown in EmontsB.H.C.etal.,2010,MNRAS,406,987
Fig.3arisesfromabsorptioninthecentralgalaxyNGC1275of FerozF.,HobsonM.P.,2008,MNRAS,384,449
thePerseusCluster(∼550kms−1;Jaffe1990),whichsimilarly FerozF.,HobsonM.P.,BridgesM.,2009,MNRAS,398,1601
hasaprofiledominatedbybroadabsorption.However,theback- GriffithM.R.,WrightA.E.,1993,AJ,105,1666
groundradiosourceinthisdetection(3C84)hasafluxdensityof GuptaN.,SaikiaD.J.,2006,MNRAS,370,L80
19.7Jy,makingitasubstantiallymorepowerfulprobeoflowop- Gupta N., Salter C. J., Saikia D. J., Ghosh T., Jeyakumar S.,
ticaldepthgasthanPMNJ2054-4242.Therefore,inthebottom 2006,MNRAS,373,972
panelofFig.3,wehaveplottedtheproductofthepeakobserved HealeyS.E.,RomaniR.W.,TaylorG.B.,SadlerE.M.,Ricci
opticaldepthandcontinuumfluxdensityversusthespectralline R.,MurphyT.,UlvestadJ.S.,WinnJ.N.,2007,ApJS,171,
width.Ourdetectionoccupiesaregionofparameterspacewith 61
lowspectrallinedepthandbroadwidth,whichhasnotyetbeen Ishwara-Chandra C. H., Dwarakanath K. S., Anantharamaiah
largelyexploredbyexistingsearchesforassociatedHIabsorp- K.R.,2003,J.Astrophys.Astron.,24,37
tion.Findingsuchararelydetectedsystem,indatawitharela- JaffeW.,1990,A&A,240,254
tivelylowS/N,demonstratesthepowerofouranalysismethod. JonesD.H.etal.,2009,MNRAS,399,683
Thedevelopmentofsuchsophisticatedautomatedspectralline LovedayJ.,1996,MNRAS,278,1025
findingtechniqueswillbevitallyimportantfordetectionofthese ManessH.L.,TaylorG.B.,ZavalaR.T.,PeckA.B.,Pollack
broad,weakabsorptionlinesinfuturelarge-scale21cmabsorp- L.K.,2004,ApJ,602,123
tionsurveys. MirabelI.F.,1989,ApJL,340,L13
MirabelI.F.,1990,ApJL,352,L37
Moore C. B., Carilli C. L., Menten K. M., 1999, ApJL, 510,
L87
ACKNOWLEDGMENTS
MorgantiR.,EmontsB.,OosterlooT.,2009,A&A,496,L9
WethankRussellJurekandTobiasWestmeierforusefuldiscus- MorgantiR.,HoltJ.,TadhunterC.,RamosAlmeidaC.,Dicken
sionson21cmspectral-lineparametrization.JRAacknowledges D., Inskip K., Oosterloo T., Tzioumis T., 2011, A&A, 535,
supportfromanARCSuperScienceFellowship.TheCentrefor A97
All-skyAstrophysicsisanAustralianResearchCouncilCentre MorgantiR.,OosterlooT.,TsvetanovZ.,1998,AJ,115,915
ofExcellence,fundedbygrantCE110001020.TheATCAispart MorgantiR.,OosterlooT.A.,TadhunterC.N.,vanMoorselG.,
oftheAustraliaTelescopeNationalFacilitywhichisfundedby EmontsB.,2005a,A&A,439,521
(cid:13)c 2012RAS,MNRAS000,1–6
6 J.R.Allisonetal.
MorgantiR.,OosterlooT.A.,TadhunterC.N.,vanMoorselG.,
KilleenN.,WillsK.A.,2001,MNRAS,323,331
MorgantiR.,TadhunterC.N.,OosterlooT.A.,2005b,A&A,
444,L9
OrientiM.,MorgantiR.,DallacasaD.,2006,A&A,457,531
PeckA.B.,TaylorG.B.,ConwayJ.E.,1999,ApJ,521,103
PeckA.B.,TaylorG.B.,FassnachtC.D.,ReadheadA.C.S.,
VermeulenR.C.,2000,ApJ,534,104
SalterC.J.,SaikiaD.J.,MinchinR.,GhoshT.,ChandolaY.,
2010,ApJL,715,L117
SaultR.J.,TeubenP.J.,WrightM.C.H.,1995,inASPConf.
Ser.,Vol.77,AstronomicalDataAnalysisSoftwareandSys-
temsIV,ShawR.A.,PayneH.E.,&HayesJ.J.E.,ed.,As-
tron.Soc.Pac.,SanFrancisco,p.433
ShostakG.S.,vanGorkomJ.H.,EkersR.D.,SandersR.H.,
GossW.M.,CornwellT.J.,1983,A&A,119,L3
SkillingJ.,2004,inAIPConf.Ser.,Vol.735,BayesianInfer-
ence and Maximum Entropy methods in Science and Engi-
neering, Fischer R., Preuss R. & Toussaint U. V., ed., Am.
Inst.Phys.,NewYork,p.395
StruveC.,ConwayJ.E.,2010,A&A,513,A10
UsonJ.M.,BagriD.S.,CornwellT.J.,1991,Phys.Rev.Lett.,
67,3328
van Gorkom J. H., Knapp G. R., Ekers R. D., Ekers D. D.,
LaingR.A.,PolkK.S.,1989,AJ,97,708
van Gorkom J. H., Knapp G. R., Raimond E., Faber S. M.,
GallagherJ.S.,1986,AJ,91,791
VermeulenR.C.etal.,2003,A&A,404,861
WhitingM.T.,2012,MNRAS,421,3242
WilsonW.E.etal.,2011,MNRAS,416,832
WolfeA.M.,BurbidgeG.R.,1975,ApJ,200,548
Zwaan M. A., van der Hulst J. M., Briggs F. H., Verheijen
M.A.W.,Ryan-WeberE.V.,2005,MNRAS,364,1467
ThispaperhasbeentypesetfromaTEX/LATEXfilepreparedby
theauthor.
(cid:13)c 2012RAS,MNRAS000,1–6