Table Of ContentMon.Not.R.Astron.Soc.000,1–21(2008) Printed22October2008 (MNLATEXstylefilev2.2)
A Survey for Redshifted Molecular and Atomic Absorption Lines
II.AssociatedHI, OHand millimetrelinesinthe z ∼> 3Parkes Quarter-Jansky Flat-spectrum
Sample
8
0 S. J. Curran1⋆, M. T. Whiting1,2, T. Wiklind3,4, J. K. Webb1, M. T. Murphy5,7, C. R. Purcell1,6
0
1SchoolofPhysics,UniversityofNewSouthWales,SydneyNSW2052,Australia
2
2CSIROAustraliaTelescopeNationalFacility,POBox76,EppingNSW1710,Australia
t 3OnsalaSpaceObservatory,S-43992Onsala,Sweden
c
4SpaceTelescopeScienceInstitute,Baltimore,Maryland21218,USA
O
5InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA,UK
6JodrellBankCentreforAstrophysics,UniversityofManchester,AlanTuringBuilding,OxfordRoad,ManchesterM139PL,UK
2
7CentreforAstrophysicsandSupercomputing,SwinburneUniversityofTechnology,POBox218,Hawthorn,VIC3122,Australia
2
]
h Accepted—.Received—;inoriginalform—
p
-
o
r ABSTRACT
t
s
a We present the results of a z > 2.9 survey for HI 21-cm and molecular absorption in
[ thehostsofradioquasarsusingtheGiantMetrewaveRadioTelescopeandtheTidbinbilla70-
mtelescope.Althoughtheatomicgashasbeensearchedtolimitscapableofdetectingmost
4
v knownabsorptionsystems,noHIwasdetectedinanyofthetensources.Previouslypublished
1 searches, which are overwhelminglyat redshiftsof z ∼< 1, exhibita 42%detection rate (31
8 outof73sources),whereastheinclusionofoursurveyyieldsa17%detectionrate(2outof
5 12 sources) at z > 2.5. We therefore believe that our high redshift selection is responsible
1 forourexclusivenon-detections,andfindthatatultra-violetluminositiesofLUV ∼> 1023 W
. Hz−1, 21-cmabsorptionhasneverbeendetected.We also find thisto notonlyapplyto our
5
0 targets, but also those at low redshiftexhibitingsimilar luminosities, givingzero detections
8 outof a total of16 sourcesoverz = 0.24 to 3.8. Thisis in contrastto the LUV ∼< 1023 W
0 Hz−1sourceswherethereisanear50%detectionrateof21-cmabsorption.
: Themixof21-cmdetectionsandnon-detectionsiscurrentlyattributedtoorientationef-
v
fects,whereaccordingtounifiedschemesofactivegalacticnuclei,21-cmabsorptionismore
i
X likely to occur in sourcesdesignatedas radiogalaxies(type-2objects,where the nucleusis
r viewedthroughdenseobscuringcircumnucleargas)thaninquasars(type-1objects,wherewe
a haveadirectviewtothenucleus).However,duetotheexclusivelyhighultra-violetluminosi-
tiesofourtargetsitisnotclearwhetherorientationeffectsalonecanwhollyaccountforthe
distribution,althoughthereexiststhepossibilitythatthelargeluminositiesareindicativeofa
changingdemographicofgalaxytypes.WealsofindthatbelowluminositiesofLUV ∼ 1023
WHz−1,bothtype-1andtype-2objectshavea50%likelihoodofexhibiting21-cmabsorp-
tion.
Finally, we do not detect molecular gas in any of the sources. The lack of HI absorp-
tion,combinedwiththeresultsfromPaperI,suggestthesesourcesarenotconducivetohigh
molecularabundances.
Keywords: radiolines:galaxies–galaxies:active–quasars:absorptionlines–cosmology:
observations–galaxies:abundances–galaxies:highredshift
1 INTRODUCTION investigate the gaseous content and large-scale structure as well
anypossiblevariationsinthevaluesofthefundamentalconstants
Redshiftedmolecularandatomicabsorptionlinescanprovideex-
at large look-back times. However, these are currently rare, with
cellent probes of the contents and nature of the early Universe.
In particular, with redshifted radio and microwave lines we can only 67 HI 21-cm absorption systems at z ∼> 0.1 known, com-
prisingof37associatedsystemsand30intervening(Table1).For
molecularabsorptionintheradiobandthesituationisconsiderably
⋆ E-mail:[email protected] worsewithonlyfiveredshiftedOH18-cmsystemscurrentlyknown
2 S. J.Curranet al.
Table 1. The known redshifted (zabs ∼> 0.1) HI 21-cm absorbers (updated from Paper I,
Curranetal.2006).Absorbertypesare:BLRG–broadlineradiogalaxy,CSO–compactsymmetric
object,CSS–compactsteepspectrumsource,DLA–dampedLyman-αabsorptionsystem(MgII–
DLAcandidate), GPS–gigahertzpeakedspectrumsource,HFP–highfrequencypeakergalaxy,
Lens–gravitationallens,OHM–OHmegamaser,Red–redquasar,RG–radiogalaxy.Thenumberof
eachtypeisgiven,aswellastheabsorptionredshiftandcolumndensityranges.
Reference Type No. zabs NHI[cm−2]
ASSOCIATEDABSORBERS
Mirabel(1989) RG 2 0.10&0.12 2&6×1018.(Ts/f)
vanGorkometal.(1989) RG 2 0.06&0.10 11&8×1018.(Ts/f)
Usonetal.(1991) RG 1 3.40 3×1018.(Ts/f)
Carillietal.(1992) Red 1 0.25 1×1019.(Ts/f)
Carillietal.(1998) Red 3 0.58–0.67 0.8−8×1019.(Ts/f)
Mooreetal.(1999) Red 1 2.64 8×1018.(Ts/f)
Pecketal.(1999) CSO 1 0.10 3×1019.(Ts/f)
Pecketal.(2000) CSO 1 0.25 5×1018.(Ts/f)
Ishwara-Chandraetal.(2003) CSS/Red 1 1.19 4×1019.(Ts/f)
Vermeulenetal.(2003) BLRG 1 0.22 7×1017.(Ts/f)
... CSS 7 0.19–0.80 0.1−2×1018.(Ts/f)
... GPS 10 0.08–0.65 0.07−3×1019.(Ts/f)
... RG 1 0.24 1×1018.(Ts/f)
Pihlstro¨metal.(2005) OHM 1 0.22 6×1018.(Ts/f)
Curranetal.(2006) RG 1 0.10 4×1019.(Ts/f)
Gupta&Saikia(2006a) RG 1 0.08 6×1018.(Ts/f)
Guptaetal.(2006) CSS 1 0.17 5×1018.(Ts/f)
Orientietal.(2006)∗ HFP 1 0.67 8×1019.(Ts/f)
INTERVENINGABSORBERS
Carillietal.(1993) Lens 1 0.69 1×1019.(Ts/f)
Lovelletal.(1996) Lens 1 0.19 ≈2×1018.(Ts/f)
Chengaluretal.(1999) Lens 1 0.89 1×1019.(Ts/f)
Kanekar&Briggs(2003) Lens 1 0.76 1×1019.(Ts/f)
Kanekar&Chengalur(2003)† DLA 15 0.09–2.04 0.02−6×1019.(Ts/f)
Darlingetal.(2004) DLA 1 0.78 2×1019.(Ts/f)
Kanekaretal.(2006) DLA 1 2.35 4×1017.(Ts/f)
Kanekaretal.(2007) DLA 1 3.39 1×1018.(Ts/f)
Guptaetal.(2007) MgII 3 1.17–1.37 0.4−2×1018.(Ts/f)
Curranetal.(2007a) Lens 1 0.96 2×1018.(Ts/f)
Curranetal.(2007b) DLA 1 0.66 4×1018.(Ts/f)
Yorketal.(2007) DLA 1 2.29 2×1018.(Ts/f)
Zwaanetal.(inprep.) MgII 2 ∼0.6 –
Notes:∗AlsoJ1407+2827,whichiscountedasoneofthe10GPSsofVermeulenetal.(2003).
†See the paper for the full reference list and Curranetal. (2005) for the calculated column
densities.NotethatsincePKS1413+135isanassociatedsystem,ithasbeenincludedinthetop
panel.
(Chengaluretal.1999;Kanekar&Chengalur2002;Kanekaretal. todust,theamountofwhichiscorrelatedwiththemolecularabun-
2003),fourofwhichalsoexhibitaplethoraofmolecularabsorption dance.Furthermore,forthemolecularabsorbers:
linesinthemillimetreregime(seeCombes&Wiklind1998).
– Alloftheabsorptionlineswerefoundatredshiftswereab-
From our previous survey for radio absorption lines in the
sorption(usuallyHI)wasalreadyknowntooccur1.
hostsofthesourcesintheParkesHalf-JanskyFlat-spectrumSam-
– Inallcasestheabsorptionoccurstowardsflatspectrumra-
ple(PHFS),oneHIabsorptionsystemwasclearlydetected(outof
diosources,suggesting compactradiosourcesandthusalarge
fivesearched) and one OH systemtentatively detected (of the 13
effectivecoveragebytheabsorber.
searched),bothatz ∼ 0.1(PaperI).Uponexaminationofthe
abs
previousdetections,weconcluded: Unfortunately, prior to theanalysis undertaken inPaper I, not all
of the above criteria were fully formulated, and so we have also
• For the HI 21-cm absorption there isno overwhelming cor-
relation between the line strength and the optical–near-infrared
1 ExceptinthecaseofPKS1830–211whereagravitationallensofunde-
(V −K)colour.
terminedredshiftwaspreviouslyknown(Subrahmanyanetal.1990).The
• However, for the OH 18-cm absorption thereisaclear rela- redshiftwasfinallydeterminedthrougha14GHzwidespectralscanofthe
tionship,thussuggestingthatthereddeningofthesequasarsisdue 3-mmband(Wiklind&Combes1996a).
RedshiftedRadioAbsorptionLines II 3
targettedsourcesforwhichonlythelastcriterionissatisfied.Inthis 348.65MHz.Aspikewasalsodominantover348.1to348.3MHz
paperwepresenttheresultsofasurveyforredshiftedatomicand andthefullbandbefore18:00UTwasswampedwithRFI,leaving
molecularabsorptionwithinthehostsofsourcesselectedfromthe only40minutesonsourceoverafragmentedband(347.5to347.9
ParkesQuarter-JanskyFlat-spectrumSample(PQFS,Jacksonetal. MHz).Also,sincethebandpasswasobservedat17:00UT,nogain
2002). calibrationispossibleforthissource.
Duetothesearchforcoincidentmolecularabsorption,weini- 1026–084 was observed for 1.9 hours at a centre frequency of
tially prioritised sources in which the HCO+ 0 → 1 transition 316.03MHzand250goodantennapairswereretained.Againse-
would be redshifted into the 12-mm band of the Tidbinbilla 70- vere RFI was present over 315.3 to 315.6 MHz and frequencies
mtelescope.Thisgave70targetsatz > 2.3outofthe878PQFS above316.7MHz.
sources,andlimitingthesamplefurthertosourceswithfluxdensi- 1228–113wassearchedforHIina1.4hourobservationcentered
tiesinexcessof0.5Jyat≈0.4GHz,gaveatotalof19sourcesat on313.69MHz.Inthe210goodantennapairssevereRFIremained
z>2.9. from313.0to313.3MHz,leavinglittleremainingbandbelowthese
frequencies.SincetheOH2Π J = 1/2(4751MHz)transition
1/2
attheredshiftofthissourcefellintothe20-cmbandoftheGMRT,
thislinewassearchedatacentrefrequencyof1049MHzfor4.4
2 OBSERVATIONSANDRESULTS
hours.RFIwasminimaland350goodantennapairswereretained.
2.1 Theredshifteddecimetrewaveobservations 1251–407 was observed for 1.4 hours at a centre frequency of
305.38MHz.Thisbandwasfairlycleanand285goodantennapair-
Theredshifteddecimetrewaveobservationswhereperformedwith
ingswereretained.
the Giant Metrewave Radio Telescope (GMRT)2 in March 2004,
1351–018wassearchedforHIina1.0hourobservationcentered
duringthefirstrunasdescribedinPaperI.Again,asperPaperI,
on301.76MHz.Althoughtherewasnooverwhelminglyapparent
wesearchedforabsorptionwithinthehostattheemissionredshift
RFIover300.8to302.7MHz,thereferenceantennawasaffected,
of the quasar. Our target sample therefore consisted (primarily)
andsonoreliableimagecould beproduced. Theoptical depthis
of objects inthe PQFSwhere either the HI 21-cm or OH 18-cm
therefore derived from the averaged visibilities of the 260 good
(2Π J = 3/2)transitionwasredshiftedintothe90-cm(≈ 327
3/2 antennapairings. OHwasalsosearched atafrequency of 354.23
MHz) band. Due to time constraints during the observations, we
MHzfor1.4hours.Noparticularfrequencywasespeciallysubject
prioritisedthetargetsinwhichB ∼> 19,inordertopreferentially toRFI,althoughflaggingofsomeaffectedbaselineswasrequired,
select sources where the presence of dust, associated with dense
leaving310goodantennapairs.
gas,woulddimthevisibleandultra-violetlight.Thisleft13outof
1535+004 was searched for HI in a 1.4 hour observation cen-
theoriginal19targets(seeTable2).
teredon315.86MHz.VerylittleRFIwaspresentgivingexcellent
Weusedall30antennaeandthe90-cmreceiverovera2MHz
data over the 400 good antenna pairs used. Again, since the OH
bandwidth(givingacoverageof≈ ±900kms−1).Overthe128
2Π J =1/2(4751MHz)transitionattheredshiftofthissource
channels(2polarisations)thisgaveachannelwidthof16kHz,or 1/2
fellintothe20-cmbandoftheGMRT,thistransitionwassearched
avelocityresolutionof≈ 15kms−1.Foralloftherunsweused
at a centre frequency of 1056 MHz for 3.1 hours. One polarisa-
3C48,3C147and3C286forbandpasscalibrationandusedsepa-
tion(LL)wasseverelyaffectedbyRFI,andsubsequentlyremoved,
ratephasecalibratorsforallofthesources,asheavyflaggingofthe
whilefortheremainingpolarisationRFIwasminimaland370good
targetdatacouldresultinpoorselfcalibration.Theflaggingandall
antennapairswereretained.
ofthereductionwasdonebytheMIRIADinterferometryreduction
1630–004wasobservedfor1.0houratacentrefrequencyof321.07
package.Synthesisedbeamsizesweretypically∼> 10′′ forthe90- MHz.UnfortunatelyRFItotallyswampedthisband.
cmand≈6′′forthe21-cmobservations(OH2Π J =1/2red-
1/2 1937–101wassearchedforHIina1.4hourobservationcentered
shiftedfrom6-cm).Aspertheotherobservations(PaperI),channel
on296.72MHz.NomajorRFIwaspresentand300goodantenna
117oftheRRpolarisationandthetelescopepairingbetweenan-
pairswereretained.OHwasalsosearchedatafrequencyof348.31
tennaeE02andE03(15and16inAIPS/MIRIADconvention),were
MHzfor1.8hours.RFIwasminimaland370goodantennapairs
removed.Phasestabilityonallbuttheshortestbaselineswasexcel-
whereretained.
lent.Inthisbandradiofrequencyinterference(RFI)wasconsider-
2215+02wassearchedforHIina1.4hourobservationcenteredon
ablymoreseverethanat21-cm&50-cm(PaperI).Regardingeach
310.67MHz.Althoughnoparticularfrequencieswhereaffectedby
source:
RFI,only190antennapairsprovedtobeofgoodquality.OHwas
0335-122 was observed for 1.5 hours at a centre frequency of
alsosearched at a frequency of 364.69 MHzfor 3.54 hours. Due
319.77 MHz. While there were no overwhelmingly bad frequen-
toRFImanybaselinepairshadtoberemoved,leaving210pairs.
cies,afterflagginginferiordataonly135goodantennapairswere
Note that the bandpass calibrator, 3C295, used for this source is
retained.
unknown to MIRIAD and so this could not be used to correct the
0347–211 was observed for 1.4 hours at a centre frequency of
gainsfor2215+02. Thefluxdensity scaleforthissource(Fig.1)
360.14MHz.ThisbandwasrelativelyRFIfreeand300goodan-
should therefore not be considered as absolute. Furthermore, un-
tennapairswereretained.
likethelower frequency HI observations, thephasestabilitywas
0537–286 was observed for 1.4 hours at a centre frequency of
very poor during this observation, preventing the extraction of a
346.10MHz,with210goodantennapairs.Highamplitudespikes
highqualitycube.
from345.7to345.9MHz,requiredtheremovalofthesefrequen-
2245–059 was observed for 1.4 hours at a centre frequency of
cies.
330.71MHz.Thisbandwassodominated byRFI,thatnouseful
0913+003 was observed for 1.4 hours at a centre frequency of
datacouldberetained.
2 TheGMRTisrunbytheNationalCentreforRadioAstrophysicsofthe
TataInstituteofFundamentalResearch.
4 S. J.Curranet al.
Figure1.TheGMRTspectra.Theordinateineachspectrumshowsthefluxdensity[Jy]andtheabscissatheDopplercorrectedfrequency[MHz].Allspectra
havebeenextractedfromthespectralcube,withtheexception1351–018at302MHzand2215+02at365MHz,wherethevisibilitiesareaveragedtogether.
Forthelatterand2215+02at311MHzthefluxscaleisnotabsolute.
RedshiftedRadioAbsorptionLines II 5
HCN at z = 3.442 HCN at z = 2.702 HCO+ at z = 2.702
HCN at z = 2.711 HCO+ at z = 2.711 CO at z = 4.276
Figure2.TheTidbinbillaspectra.Theordinateineachspectrumshowsthefluxdensity[Jy]andtheabscissatheDopplercorrectedfrequency[GHz].Each
spectrumhasbeensmoothedtoachannelwidthof4kms−1.
2.2 Theredshiftedmillimetrewaveobservations positions and the resulting spectra were averaged and weighted
by T /T2 . Lastly, the data were corrected onto the T tem-
Theredshiftedmillimetrewaveobservationswereperformedwith int sys MB
perature scale by multiplying by the beam efficiency at 22GHz
the Australia Telescope’s Tidbinbilla 70-m telescope3, over sev-
(η≈0.48,Greenhilletal.2003).
eral sessions between November 2003 and March 2005. Again,
the sources were selected according to those in which a strong
transition4 fell into the 1-cm (≈ 22 GHz) band. For the back-
end we used the das xxyy 64 2048 configuration (dual polarisa- 2.3 Results
tionwith2048channelsovera64MHzband),givingaresolution
InTable2wesummariseourresults.Thecolumndensityisderived
of ≈ 0.5 kms−1 over a range of ≈ ±500 kms−1: Such high fromthevelocityintegrateddepth,τ ≡−ln 1− σ ,whereσis
resolution was required, since although optically thick, the line- fS
r.m.s.noiseinthecaseofourexclusivenon-d(cid:0)etections(cid:1)andSandf
widths in the four known millimetre absorption only span a few
thefluxdensityandcoveringfactorofthebackground continuum
kms−1 (Wiklind&Combes 1994, 1995, 1996b, 1998), cf. up to
240kms−1forthe,opticallythin,OHabsorption(seefigure5of source,respectively.Intheopticallythinregime(whereσ/f.S ∼<
0.3),whichappliestothevastmajorityoftheknownHIandallof
Curranetal.2007a).
theOHabsorbers,the1σcolumndensitylimitisgivenby
The half power beam-width (HPBW) of the single dish at
thesefrequenciesis≈ 50′′ and,inordertoensuregoodbaselines, T σ
N =X× dv, (1)
we used position switching with 1 minute per position over a 4- f Z S
point position switching cycle, each source being observed for a
total of 4 hours. In general, the baselines were excellent with no whereforHI21-cm,X =1.823×1018(Wolfe&Burbidge1975)
baseline subtraction of the spectra (Fig. 2) being required. Flag- andT isthespintemperature(Ts)andforOH,X =2.38×1014for
gingofdatawasonlyrequiredwhencommunicationwiththesub- the2Π3/2J = 3/2(1667MHz)transitionandX = 1.61×1014
reflectorhadbeenlost.Duringtheobservations,theT∗ fluxscale forthe2Π1/2J =1/2(4751MHz)transition(Henkeletal.1987),
A
was periodically calibrated against a thermal load provided by a withT beingtheexcitationtemperature(Tx).
noisediode. Anadditional correction forthevariationof antenna For OH, the values of X are derived from the total column
gain with elevation was applied to each spectrum off-line. The densityobtainedfromarotationaltransition,whichisgivenby
datawerereducedusingtheDFM5,graphicalinterfacetotheSPC6 8π ν3 QeEJ/kTx
package.Aquotientwasformedbetweenthesourceandreference N = τdv, (2)
c3 gJ+1AJ+1→J 1−e−hν/kTx Z
whereνistherestfrequencyoftheJ →J+1transition,g and
J+1
3 TheAustralia Telescope isfunded bytheCommonwealth ofAustralia AJ+1→J arethestatisticalweightandtheEinsteinA-coefficient7
foroperationsasaNationalFacilitymanagedbyCSIRO. ofthetransition,respectively,andQ= ∞J=0gJ e−EJ/kTx,with
4 Thatis,transitionsofCO,HCO+andHCN,whichareopticallythickin P
thefourknownredshiftedmillimetreabsorptionsystems.
5 Data From Mopra, available from
http://www.phys.unsw.edu.au/astro/mopra/software.php 7 Theseare taken from Chandraetal. (1995, 1996)or derived from the
6 http://www.atnf.csiro.edu.au/software dipolemoment(e.g.Rohlfs&Wilson2000).
6 S. J.Curranet al.
Table2.Summaryofthesearchfordecimetreabsorptionlinesinthehostsofz ∼> 3PQFSsources.νobsistheobservedfrequencyoftheline[MHz],σrmsis
ther.m.s.noise[mJy]reachedper∆vchannel[kms−1],Scontisthecontinuumfluxdensity(uncalibratedfor2215+02,Sect.2)[Jy],τistheopticaldepthof
thelinecalculatedperchannel,whereτ = 3σrms/Scont isquotedforthesenon-detections,N istheresultingcolumndensity[cm−2],whereTsisthespin
temperatureoftheHI21-cmline,TxistheexcitationtemperatureofthecorrespondingOHlineandf therespectivecoveringfactor.InallcasesOHrefersto
the2Π J =3/2(1667MHz)transition,withtheexceptionof1228–113and1534+004forwhichweobservedthe2Π J =1/2(4751MHz)transition.
3/2 1/2
Finally,inlightoftheresultsofPaperI,welisttheV,B&Kmagnitudes(whereavailable)withtheirrespectivereferences.
PKS zem Line νobs σrms ∆v Scont τ N z-range B V K Ref
0335–122 3.442 HI 319.77 23 15 0.90 <0.077 <2.1×1018.(Ts/f) 3.428–3.456 21.018 20.110 17.510 1,2
0347–211 2.944 HI 360.14 17 13 0.51 <0.10 <2.4×1018.(Ts/f) 2.935–2.953 20.476 – 17.900 2
0537–286 3.104 HI 346.10 54 14 1.61 <0.10 <2.6×1018.(Ts/f) 3.092–3.106 19.290 – 16.770 2,3
... ... ... ... RFIrequiredremovalofz=3.1064−3.1088 3.109–3.115 ... ... ...
0913+003 3.074 HI 348.65 – 13 – RFIdominant 20.998 20.775 – 2,4
1026–084 4.276 OH 316.03 53 15 0.65 <0.24 <1.5×1015.(Tx/f) 4.265–4.283 21.070 – – 2
1228–113 3.528 HI 313.69 26 15 0.49 <0.16 <4.3×1018.(Ts/f) 3.516–3.532 22.010 – 16.370 5,3
... ... OH 1049.17 3.7 18 0.64 <0.017 <4.9×1013.(Tx/f) 3.513–3.543 ... ... ...
1251–407 4.460 OH 305.38 12 15 0.25 <0.14 <5.0×1014.(Tx/f) 4.442–4.478 – 23.7 – 6
1351–018 3.707 HI 301.76 – 16 – <0.090 <2.6×1018.(Ts/f) 3.693–3.722 21.030 19.696 17.070 7,4,3
... ... OH 354.23 39 13 0.75 <0.052 <1.6×1014.(Tx/f) 3.695–3.719 ... ...
1535+004 3.497 HI 315.86 8.6 15 0.37 <0.070 <1.9×1018.(Ts/f) 3.485–3.509 22.500 – 19.54 3
... ... OH 1056.41 4.0 18 0.47 <0.032 <9.4×1013.(Tx/f) 3.486–3.514 ... ...
1630–004 3.424 HI 321.07 – 15 – RFIdominant – 21.81 – 8
1937–101 3.787 HI 296.72 9.4 16 0.61 <0.046 <1.3×1018.(Ts/f) 3.773–3.802 18.800 – 13.816 2,9
... ... OH 348.31 3.5 14 0.51 <0.021 <7.0×1013.(Tx/f) 3.775–3.795 ... ... ...
2215+02 3.572 HI 310.67 14 15 3.07 <0.014 <3.7×1017.(Ts/f) 3.558–3.585 21.840 20.420 19.340 10
... ... OH 364.69 22 13 – <0.089 <2.8×1014.(Tx/f) 3.561–3.583 ... ... ... ...
2245–059 3.295 HI 330.71 – 14 – RFIdominant 19.523 – – 2
References:(1)Ellisonetal.(2005),(2)SuperCOSMOSSkySurvey,Hamblyetal.(2001),(3)P.Francis(priv.comm),(4)SDSSDR6,Adelman-McCarthyetal.
(2008),(5)Chunetal.(2006),(6)Jacksonetal.(2002),(7)Drinkwateretal.(1997),(8)Winnetal.(2002),(9)Smith&Heckman(1989),(10)Francisetal.
(2000).
g = 2J + 1, is the partition function8. Since kT ≫ hν for earlierstudyofsourcesfromthePHFS(PaperI),wedetected21-
J x
Tx ≈ 10 K and ν ∼< 5 GHz, Equ. 2 can be simplified to the cm absorption in one out of four sources, although the only real
aboveexpression(Equ.1)9.Forthemillimetre-wavecolumnden- differencebetweenthePHFSandPQFSsamplesisthatthePQFS
sities,basedonthefourknownsystems,thecoveringfactorisex- hasalowerfluxlimit(0.25Jy,cf.0.5Jy)11.However,duetoour
pectedtobeclosetounity(Wiklind&Combes1994,1995,1996b, restrictionthatthesourcesareatredshiftswheretheHCO+0→1
1998) so, as inthe optically thinregime, thiscan be writtenout- transition is redshifted into the 12-mm band, unlike the previous
sideoftheintegral.However,forthesameexcitationtemperatures searchwhichspannedz≈0.1−3.5,ourtargetsareallatz∼>2.9,
thehigher frequenciesgivekT ∼ E ∼ hν andsothecolumn wherepublishedsearcheshavebeenveryrare(Fig.3):Inthefigure
x J
densitycannotbeapproximatedviaalineardependenceontheex- all of the sources in the hatched region are from our search (Ta-
citationtemperature.Wethereforeadoptthecanonicalvalueof10 ble2),inadditiontothez = 3.497quasar1535+004fromPa-
em
Katz=010. perI.However,thereisone21-cmdetectioninthisband(z =3.4
in0902+343,Usonetal.1991)andsothehighredshiftselectionon
itsowncannotberesponsibleforournon-detections,allofwhich
havebeensearchedtolimitscomparabletothedetections.
3 POSSIBLEEFFECTSINTHENON-DETECTIONOF Since we are searching for associated absorption, although
ATOMICABSORPTION covering factors may be an issue, these will not be subject
to the same geometrical effects found for intervening systems:
FromTable2,itisseenthatwedonotdetect21-cmabsorptionin
SpecificallydampedLyman-αabsorptionsystems(DLAs),where
anyofthetensources forwhichgooddatawereobtained. Inour
Curran&Webb(2006)showastrongcorrelationbetweenlowab-
sorber/quasar angular diameter distance ratios and 21-cm detec-
tions, while high ratios are correlated with non-detections. This
8 Theenergyofeachlevel, EJ,isobtained fromtheJPLSpectral Line suggests that the absorbers located “closer” to us (in an angular
Catalog (Pickettetal. 1998).Anon-linecolumndensity calculator based sense) havesignificantlylargercovering factorsthanthosewhich
onEqu.2isavailableathttp://www.phys.unsw.edu.au/∼sjc/column/ shareasimilarangulardiameterdistancetothebackgroundquasar.
9 Where the values for X are derived using a weighting factor Therefore our lack of 21-cm detections must be due to an-
of gJ/gJ = 16/5 and AJ+1→J = 7.71 × 10−11 s−1 other effect –either low neutral hydrogen column densities, high
(CarPrington&Miller1967)forthe1667MHztransition.Forthe4751MHz spin temperatures or low covering factors (see Equ. 1), although,
transition, gJ/gJ = 8/3 and AJ+1→J = 7.7597×10−10 s−1
(http://wwwP.strw.leidenuniv.nl/∼moldata/datafiles/[email protected]).
10 Since we are analysing high redshift sources, we scale this with the
temperatureofthecosmicmicrowavebackground,TCMB=2.73(1+z), 11 Alsoaslightlystricterlowerlimittothespectralindex(α>−0.4,cf.
givingTx=17−21Koverz=2.702−4.276. α>−0.5,whereSν ∝να).
RedshiftedRadioAbsorptionLines II 7
Table3.Summaryofthesearchformillimetre absorption linesinthehostsofz ∼> 3PQFSsources.νobs isthe
observedfrequencyoftheline[GHz],σrms isther.m.s.noise[mJy]reachedper4kms−1 channel, Scont isthe
continuumfluxdensity[Jy]andτistheopticaldepthofthelinecalculatedperchannel.Thecolumndensity[cm−2]
ofeachmoleculeiscalculatedforf = 1andanexcitationtemperatureofTx = 10Katz = 0.Themagnitudes,
whereavailable,aregiveninTable2.For0434–188,B=20.86(Ellisonetal.2005)&K=16.21(Drinkwateretal.
1997)andfor0601–17,B=20.445&R=20.172(Hamblyetal.2001).
PKS zem Line νobs σrms Scont τ N z-range
0335–122 3.442 HCN0→1 19.953 4.5 0.245 <0.055 <9.7×1012/f 3.435–3.449
0434–188 2.702 HCN0→1 23.942 8.7 0.220 <0.12 <1.7×1013/f 2.698–2.707
... ... HCO+0→1 24.092 4.6 0.202 <0.068 <4.3×1012/f 2.698–2.707
0601–17 2.711 HCN0→1 23.884 7.0 0.202 <0.10 <1.4×1013/f 2.706–2.716
... ... HCO+0→1 24.034 6.3 0.207 <0.091 <5.7×1012/f 2.705–2.716
1026–084 4.276 CO0→1 21.848 3.9 0.137 <0.085 <7.8×1015/f 4.268–4.284
3.1 Incidentfluxes
3.1.1 21-cmluminosities
Althoughoursamplecouldbehighlyheterogeneous,sinceouronly
criterion for our own targets is that they are high redshift PQFS
sources (Sect. 1), there must be some reason why 21-cm absorp-
tionisdetectedinsomequasarsandradiogalaxies,whilenotbe-
ingdetectedinothers,particularlyathighredshift.Curran&Webb
(2006)previouslyinvestigateda“proximityeffect”inDLAs,where
ahigh21-cmfluxmaymaintainahighpopulationintheupperhy-
perfine level, thus having relatively few anti-parallel spin atoms
available to absorb the 1420 MHz radiation (Wolfe&Burbidge
1975).Although therewereafew non-detections and zerodetec-
tions at incident flux densities of ∼> 104 Jy in this sample, there
wasnooverwhelmingtrend,withtheaforementionedgeometrical
effectsbeingapparentlymuchmoresignificantwithregardtothe
detectionof21-mabsorption.
Whenweconsideroursample,wecannotdetermineincident
Figure 3. The scaled velocity integrated optical depth of the HI line fluxesforthenon-detections, aswehaveno knowledge of where
(1.823×1018. τdv)versusthehostredshiftforthepublishedsearches
the neutral gas would be located relative to the emission region.
forassociated21R-cmabsorption(seeTable1andAppendixA).Thefilled However,sincewearejustlookingforstatisticaldifferencesinthis
symbolsrepresentthe21-cmdetectionsandtheunfilledsymbolsthenon-
sample, we can still investigate this effect through the luminosi-
detections, with stars designating quasars and circles galaxies (see Sect.
ties.Thespecificluminosityof thequasar attherestframeemis-
3.2.2).ThehatchedregionshowstherangeofourHIsearches(Table2).
TheotherresultsarefromthereferencesquotedinTable1(seeAppendix sion frequency, νem, is Lν = 4πDQ2SOSobs/(zem +1), where
A),withtheaddition ofthetwozem > 5non-detections ofCarillietal. DQSO is the luminosity distance to the quasar12, Sobs is the ob-
(2007). served flux density13 and zem +1 corrects for the redshifting of
thefrequencyincrement.Usingthisexpression,inFig.4weshow
the derived quasar frame luminosities in relation to the velocity
integrated optical depth of the 21-cm absorption. From this, the
vertical histograms show that, on the whole, the non-detections
have been searched sufficiently deeply, many to a higher sensi-
asstated,thelatterwouldhavetobeduetosmallintrinsicabsorp- tivityrequiredforthedetections.Thehorizontalhistogramsshow
tion cross sections, since for both the 21-cm detections and non- considerable overlap, and, while the average logL is higher
radio
detections in associated systems, the absorber/quasar angular di- forthenon-detections,thedifferenceissmall(logL =27.06
radio
ameterdistanceratiosareclosetounity.Intheabsenceofmeasure- cf.26.71),andthedistributionsofluminositiesfordetectionsand
mentsoftheneutralhydrogencolumndensitiesfromtheLyman-α non-detectionsarenotstatisticallydifferent(theprobabilityofthe
line, unlike DLAs, we cannot compare how NHI varies between nullhypothesisfortheKolmogorov-Smirnovtestis33.7%).There-
the detections and non-detections nor speculate on spin tempera- fore,althoughahighincident21-cmfluxmaymakeadetectionless
tureeffects,althoughCurranetal.(2005,2007c)haveshownthat likelythroughahighlypopulatedupper hyperfinelevel,thisdoes
spintemperaturesinDLAsmaynotvarybynearlyasmuchaswas notappeartobeanoverwhelmingcauseofthenon-detections.
previously believed (Kanekar&Chengalur 2003), and that these
maygenerallybebelowT ≈ 2000K(atleastuptoz ≈ 3.4).
s abs
Again, however, for these sources wehave adegeneracy of three 12 ThroughoutthispaperweuseH0 =75kms−1Mpc−1,Ωmatter =
variables,allofwhichmaybemutuallycoupled(seeCurranetal.
0.27andΩΛ=0.73.
2007c),thusmakingtherelativecontributionofeachverydifficult 13 Sincewearesearching forabsorption atthehostredshift, thequasar
toascertain. frame21-cmfluxdensityisgivenbytheobservedvalue.
8 S. J.Curranet al.
Figure4.ThescaledvelocityintegratedopticaldepthoftheHIlineversusthequasarframe21-cmluminosityforthequasarssearchedforassociatedHI
absorption.Again,thehatchedregionshowstherangeofourHIsearches.Throughoutthispaperthefilledsymbols/hatchedhistogramrepresentthe21-cm
detectionsandtheunfilledsymbols/colouredhistogramthenon-detections.
3.1.2 Ultra-violetluminosities plyingthetransformationsofFukugitaetal.(1996)toensurecon-
sistency between the bands. From these measurements, we esti-
Althougha21-cmproximityeffectisnotapparentforDLAsoras-
mate the λ ≈ 1216(1 +z)A˚ flux according to the prescription
sociatedabsorbers,intheultra-violetsuchaneffectiswellknown,
inAppendixB,andthencetheluminosityintherest-frameofthe
where a high ionising flux from the quasar is believed to be re-
galaxy/quasar.
sponsibleforthedecreaseinthenumberdensityoftheultra-violet
Since the sources cover a range of redshifts, the interpola-
Lyman-α lines as z approaches z (Weymannetal. 1981;
abs em tions/extrapolations necessary to estimate L involve different
Bajtliketal. 1988). To excite the hydrogen beyond the realm of UV
bands for different sources. We can check the validity of the es-
21-cm absorption does not require ionisation of the gas (by 912
timatesbycomparingtheinterpolatedvalueofthefluxatthenom-
A˚ photons), but “merely” excitation above the ground state by a
inal 1216A˚ wavelength (from BVR) withthat extrapolated from
Lyman-α (1216 A˚) photon, although the lifetime in this state is
theJHK bands. Thelatter situationissimilar tothecase of ex-
only∼ 10−8 s.Inanycase,sinceboththeionisingandLyman-α
trapolatingfromopticalbandsforlow-redshiftsources.Inoursam-
photonsare∼ 106 timesasenergeticasthespin-fliptransition,if
ple, there are only three sources in the high (z > 2.5) redshift
thegasisexcitedbyLyman-αabsorption,muchofitwillalsobe
groupforwhichwehavemorethanonenear-infraredbandavail-
ionised.
able:J0414+0534,1937−101(bothofwhichhave2MASS15pho-
Therefore,inordertodeterminethe∼1000A˚ fluxes,wehave
tometry)and2215+020(Francisetal.2000)andforthese,wecan
exhaustivelysearchedtheliteratureandon-linearchivestoobtain
compare the extrapolation of the JH measurements to that ob-
opticalandnear-infraredphotometryofasmuchofthesampleas
tained fromBR. 2215+020 has the same extrapolation for both,
possible (see Appendix A). In some cases, we use the photome-
1937−101 has JH overestimating the flux by a factor of ∼ 5,
try of the Sloan Digital Sky Survey (SDSS, Yorketal. 2000)14,
andJ0414+0534 overestimatesby∼ 50. Theanalysisofthelat-
taken from Data Release 6 (Adelman-McCarthyetal. 2008), ap-
tersourcecouldbeaffectedbytheextremeoptical–near-IRcolour
of V − K = 10.26 (Lawrenceetal. 1995) [see Sect. 4], most
likelydue tothe presence of dust inthe intervening gravitational
14 The SDSS is managed by the Astrophysical Research Consortium lensor host galaxy16.Theother twosources, however, show that
(ARC)fortheParticipating Institutions. TheParticipating Institutions are theextrapolationfromlongerwavelengths(rest-frameoptical)can
theAmericanMuseumofNaturalHistory,AstrophysicalInstitutePotsdam, giveavalueoftheUVfluxinbroadagreementwiththeextrapola-
UniversityofBasel,UniversityofCambridge,CaseWesternReserveUni-
versity,TheUniversityofChicago,DrexelUniversity,Fermilab,theInsti-
tuteforAdvancedStudy,theJapanParticipationGroup,TheJohnsHopkins
University,theJointInstituteforNuclearAstrophysics,theKavliInstitute 15 TheTwoMicronAllSkySurvey,isajointprojectoftheUniversityof
forParticleAstrophysicsandCosmology,theKoreanScientistGroup,the MassachusettsandtheInfraredProcessingandAnalysisCenter/California
Chinese Academy of Sciences (LAMOST), Los Alamos National Labo- InstituteofTechnology,fundedbytheNationalAeronauticsandSpaceAd-
ratory,theMax-Planck-Institute forAstronomy(MPIA),theMax-Planck- ministrationandtheNationalScienceFoundation.
InstituteforAstrophysics(MPA),NewMexicoStateUniversity,OhioState 16 NotethatonthebasisoftheirdetectionofHIinthelensinconjunction
University, University ofPittsburgh, University ofPortsmouth, Princeton withastronglimitontheOHcolumndensity,Curranetal.(2007a)[and
University, the United States Naval Observatory, and the University of referencestherein]suggestthatthereddeningisoccurringinthez=2.64
Washington. hostgalaxy.
RedshiftedRadioAbsorptionLines II 9
Figure5.AsperFig.4,butwiththescaledvelocityintegratedopticaldepthoftheHIlineversusthequasarframeultra-violetluminosity.
tionfromrest-frameUV.WenotethatanextrapolationusingJH 3.2 Orientationeffects
forthesesourcesisroughlyequivalenttousingBRforsourcesat
3.2.1 UnifiedschemesofAGN
0.6∼<z∼<1.0.
Plottingour results,inFig.5weseeanequal mixof 21-cm Previously, the mix of 21-cm detections and non-detections has
detections and non-detections below the median value of the UV beenattributedtounifiedmodelsofactivegalacticnuclei(AGN).
luminosity range 18.04+24.91 =21.48 . However, at higher lu- This is an orientation effect due to the presence of a dense sub-
2
minosities the dist(cid:0)ribution is dominated(cid:1)by non-detections which pc circumnuclear disk or TORUS19 of gas which obscures the
become exclusive at LUV ∼> 1023 W Hz−1, the range occupied UV/opticallightfromtheAGN:Intype-1objects,theobscuration
byourobservedsample(Table2)17.Investigatingthelikelihoodof hasitsrotationaxisdirectedtowardsusandtheAGNandthecen-
suchadistributionoccurringbychance,definingapartitionatthe tralised broad-line region are viewed directly, whereas in type-2
median of the sample gives 52 objects (26 detections & 26 non- objects the AGN is hidden, and only the more extended narrow-
detections)inthelowluminositybinand33objects(7detections lineregionisvisible(seeAntonucci1993;Urry&Padovani1995).
& 26 non-detections) in the logLUV > 21.48 bin. For an unbi- Itishypothesisedthatthe21-cmabsorptionoccursinthisobscu-
asedsample,i.e.thereisanequallikelihood ofeitheradetection rationandsoisonlyvisibleintype-2objects,wherethegasinter-
ornon-detection(asmaybeexpectedfromorientationeffects,see ceptstheline-of-sighttotheAGN(e.g.Jaffe&McNamara1994;
nextsection),thebinomialprobabilityof26ormoredetectionsout Conway&Blanco1995).
of52objectsoccurringis55%.However,theprobabilityof25non- For instance, of four 21-cm detections in a sample of 23
detectionsormoreoutof33objectsintheotherbinisjust0.23%. z < 0.7radiogalaxies(Morgantietal.2001),twoarenarrow-
em
MovingthepartitiontologLUV = 23.0,thuscoveringtherange lineradiogalaxies,indicatingtype-2objects,andtheothertwoare
of our targets(Fig.5),inthelow luminosity binthereare33 de- weak-line radio galaxies. Therefore, superficially at least, this is
tectionsoutof69objects(againanear50%detectionrate)andin consistent with the 21-cm absorption occurring in an intervening
the high luminosity bin, 0 detections out of 16. Again, assuming torusordisk(Morgantietal.2001).However,thefactthatthereare
anunbiasedsample,thebinomialprobabilityofthislatterdistribu- significantredshift(velocity)offsetsbetweenthe21-cmandoptical
tionis0.0015%, asignificanceof4.3σ assumingGaussianstatis- lines,leadstheauthorstoconcludethatsomeoftheHIresponsi-
tics18,againsttheprobabilityofa21-cmabsorptiondetectionbeing blefortheabsorptionislocatedfartheroutthanthecentralsub-pc
unrelated to the ultra-violet luminosity. Applying a Kolmogorov- occupiedbythedenseobscuration.Thepresenceofsignificantgas
Smirnovtest,thereisa99.25%confidencethattheUVluminosity motions,duetoinfall,outflows,jetinteractionsaswellasgeneral
distributionofthedetectionsdiffersfromthatofthenon-detections. galactic rotation20 is also suggested by the large velocity offsets
Thereforethereisarelationshipbetweentheultra-violetluminosity foundinasampleof19detectionsbyVermeulenetal.(2003)[see
andthelikelihoodofdetecting21-cmabsorption. Fig.6].
Furthermore,Pihlstro¨metal.(2003)notethat21-cmabsorp-
tionismorelikelytobedetectedinradiogalaxiesthaninquasars,
17 ThemeanluminosityofthedetectionsislogLUV = 20.5±1.2,cf.
logLUV = 21.8±1.7 for all of the non-detections and logLUV =
20.8±1.1forthelowluminositynon-detections. 19 e.g.ThickObscurationRequiredbyUnifiedSchemes(Conway1999).
18 Shouldtherebeanoverwhelming errorinthebandextrapolations, to 20 Although gasinthegalactic diskmaybeexpected tosharethesame
thepointwherethez >1luminositiescannotbeusedinthecomparison, orientationasthesub-pcobscuration(Curranetal.1998,cf.Greenhilletal.
forthez<1sourcesalonetheresultremainssignificantat2.9σ. 2003,seealsoCurran2000a).
10 S. J.Curranet al.
3.2.2 Quasar–galaxyclassifications
Theclassification schemes used by these authors do not apply to
all the sources under consideration in this work. We have there-
foreappliedourownschemeinordertoexaminetheeffectofthe
relativestrengthof thequasar/AGNnucleusover thehost galaxy.
Theaimistodistinguishsourceswhoseappearanceisdominated
by the nuclear source (the “quasars”) from those whose appear-
anceisdominatedbytheextendedstellarlightofthehostgalaxy
(the “galaxies”). The latter sources are those with either intrinsi-
callyweakerAGNorthetype-2AGNthatareobscuredfromthe
line-of-sight.Theclassificationwasmostlydoneonamorpholog-
icalbasis, usingSuperCOSMOSSkySurveyimagesand classifi-
cation(whichdoesastar/galaxyclassificationbasedonthesource
profile), DSS2 images, as well as detailed imaging and spectro-
scopic analysis (where the spectra are examined for evidence of
galaxylightorquasaremissionlines)availablefromtheliterature.
Eachsourcewasexaminedindividuallytoensuretheclassifications
wereconsistent.Therearesomeobviouscaveatswiththisprocess.
Figure6.The21-cmabsorptionstrengththeversusthevelocityoffsetof
the21-cmabsorptionfromthehost.Herewehaveplottedeachindividually By looking at the morphology, we are biasing ourselves some-
resolvedabsorption componentatitslinestrength andvelocity offsetfor what against high-redshift galaxies, since we lose the resolution
eachofthedetections (Table1).Thesolidsymbolsrepresentblueshifted and surface-brightness sensitivity for detecting the galaxy light.
(approaching)HIandthehollowsymbolsredshifted(receding)HI.Inas- However,manyofthehigh-redshiftsourceshavehigher-resolution
sociated systems these offsets represent peculiar motions rather than the imaging or spectroscopic observations available that allow us to
Hubbleflow.Furthermore,thereareoftenvelocitydifferencesbetweendif- determinetheimportanceofthehostgalaxyinthesource’sappear-
ferentopticallinesandsothesevelocities cannotbereliablyconvertedto
ance.
distanceoffsetstoyieldtheincidentfluxontheabsorbinggas.
In order to compare the proportions of detections and non-
detections for galaxies and quasars, we use the following statis-
tic. If we have two measured proportions pˆ = X /N and
1 1 1
pˆ =X /N ,withthetotalproportionpˆ=(X +X )/(N +N ),
suggesting that thequasars areviewed at lower inclinations, thus 2 2 2 1 2 1 2
then
havingtheobservedradioemissionbypasstheabsorbinggaswhich
obscures theoptical/UV light.Theorientations foraverylimited T = pˆ1−pˆ2 , (3)
sample (Pihlstro¨metal. 2003) are quantified by the core promi- [pˆ(1−pˆ)(N−1+N−1)]1/2
1 2
nence, where the galaxies have core emission fractions of f <
c whichhasastandardnormaldistribution(meanzero,varianceone)
0.03,cf.f >0.04forthequasars,thelargercoreprominencebe-
c under thenull hypothesis, beingthat thetwoproportions (pˆ and
ingattributedtoamoredirectviewingangletotheAGN.Froma 1
pˆ ) are the same. We examined the proportion of sources that
largersample,Gupta&Saikia(2006b)findthatthegalaxieshave 2
we classified as galaxies (as opposed to quasars), and found that
a median value of f = 0.011, compared to f = 0.028 for
c c sourceswithdetectedabsorptionhadamuchhighergalaxypropor-
the quasars. However, the distribution becomes more mixed with
tion(28outof34)thansourceswithoutabsorption(28outof56).
nearlyhalfof thequasarslocatedbelow themedianvalueforthe
These fractions are different at 99.67% confidence. Additionally,
galaxies (fc ∼ 0.01) and withfivegalaxies at fc ∼> 0.03. Addi- almostallthehigh-L sourcesare“quasars”–only1outofthe
tionally, although none of the quasars have been detected in 21- UV
24sourceswithL >1022.5WHz−1isagalaxy.
cm absorption, all of these five galaxies exhibit strong (N > UV
HI
1.6×1018.T /f cm−2)absorption,thusweakeningtheargument
s
that core dominance indicates lower inclinations, or at least, the
3.2.3 Ourresultsinthecontextoftheunifiedschemes
amountofcoldneutralgaslyingalongthesight-linetotheAGN.
Manyofthesecorefractionshavehoweverbeenestimated21, All of the above discussion on orientation effects applies to the
andoverestimatescouldexplainthefive21-cmabsorbinggalaxies low redshift sample, where searches have previously been con-
exhibitingalargecoreprominence. Nevertheless, fromtheirCSS centrated (Fig. 3)22. Of our own low redshift searches (Paper I),
andGPSsample(Gupta&Saikia2006b)thereisa21-cmdetection one target out of three was detected in 21-cm absorption, i.e. as
rateof1outof9forquasars,cf.15outof23forgalaxies,support- perthe33%detectionrateofVermeulenetal.(2003),therateex-
ingtheexistenceofabias.Quantifyingthis,thebinomialprobabil- pectedfromunifiedschemes23.Forthetwoundetected,1450−338
ityof15ormoreoutof23detectionsoccurringinoneclass,while (z =0.368)hasanestimatedUVluminosityofLUV = 4×1018
8 or more non-detections occur in another class is 0.20%, again WHz−1 and2300−189(z = 0.129)LUV = 1×1020 WHz−1,
consistent withquasars beingtheresultof lower inclinedobscur- cf. LUV = 5 × 1019 W Hz−1 for the detection in 1555−140
ingtori.
22 Thelargegapinredshiftspacebetweentheseandthehighredshifttar-
gets (Fig. 3)is duetoalack of(RFIfree) coverage over≈ 350to700
MHz.
21 Usingassumedspectralindicesandatafrequencyof8GHz,whichis 23 Thedetectionrateforagivenhalf-openingangle,α,is1−cosα,giv-
anorderofmagnitude higherthanthetypical observedredshifted 21-cm ingadetectionrateof29%forarandomlyorientedpopulationwitha90◦
frequency. openingangle.
