CLASS B0827+525: `Dark lens' or binary radio-loud quasar?

(willbeinsertedbyhandlater)

Your thesaurus odes are:

12.07.1;12.04.1;11.17.4 B0827+525

ASTRONOMY

AND

ASTROPHYSICS

July29,2000

CLASS B0827+525: `Dark lens' or binary radio-loud quasar?

L.V.E.Koopmans 1; 5 ,A.G.deBruyn 2;1 ,C.D.Fassna ht 3;4 ,D.R.Marlow 5;6 ,D.Rusin 6 ,R.D.Blandford 3 , I.W.A. Browne 5 , P. Helbig 5;1 , N. Ja kson 5 , S.T. Myers 6;4 , T.J. Pearson 3 , A.C.S. Readhead 3 , P.N. Wilkinson 5 ,E. Xanthopoulos 5 , H. Hoekstra 1 1

KapteynAstronomi alInstitute,P.O.Box800,9700AVGroningen,TheNetherlands

2

NFRA,P.O.Box2,7990AADwingeloo,TheNetherlands

3

CaliforniaInstituteofTe hnology,Pasadena,CA91125,USA

4

NRAO,P.O.BoxO,So orro,NM87801

5

UniversityofMan hester,NRALJodrellBank,Ma leseld,Cheshire SK119DL,England

6

DepartmentofPhysi s,UniversityofPennsylvania,Philadelphia,PA19104,USA

Re eived;a epted

Abstra t. We present radio, opti al, near-infrared and

spe tros opi observationsof thesour e B0827+525. We

onsider this sour e asthebest andidate from the

Cos-mi Lens All-Sky Survey (CLASS) for a `dark-lens'

sys-tem orbinary radio-loud quasar. The system onsists of

tworadio omponentswithsomewhatdierentspe tral

in-di es,separatedby2.815ar se .VLBAobservationsshow

thatea h omponenthassubstru tureonas aleofafew

mas. A deep K{band exposure with the W.M. Ke k{II

Teles ope reveals emission near both radio omponents.

TheK{bandemissionoftheweakerradio omponent

ap-pears extended, whereas the emission from the brighter

radio omponentis onsistent with apoint sour e.

Hub-ble Spa e Teles ope F160W{band observations with the

NICMOS instrument onrms this. A redshift of 2.064

is found for the brighter omponent, using the LRIS

in-strumentontheW.M.Ke k{IITeles ope.Theprobability

that B0827+525 onsists of two unrelated ompa t

at-spe trumradio sour esis3%,although thepresen eof

similarsubstru tureinboth omponentmightredu ethis.

We dis uss two alternative s enarios to explain this

system:(i) CLASSB0827+525is a`dark-lens'systemor

(ii)B0827+525isabinaryradio-loudquasar.B0827+525

hasmetall riteriathatthusfarhavein100%ofthe ases

onrmed asour e asan indisputable gravitationallens

system. Despite this, no lens galaxy has been dete ted

with m

F160W

23 mag. Hen e, we might have found the

rst binaryradio-loudquasar. At thismoment, however,

wefeelthatthe`dark-lens'hypothesis annotyetbefully

ex luded.

Key words:Cosmology:gravitationallensing{

Cosmol-ogy:darkmatter{quasars:individual:B0827+525

Sendoprintrequeststo:L.V.E.Koopmans

1.Introdu tion

There are severalindire t waysto dete t thepresen eof

dark matter in galaxies at relatively low redshifts (e.g.

mi rolensing,rotation urvesofspiralgalaxies,polar-ring

galaxies, et .). However, at intermediate and high

red-shifts, weak and strong gravitational lensing is the only

methodofdete tingthepresen eofdarkmatterongalaxy

s ales.

Gravitational lensing does not depend on the

lumi-nosity or olor of the lensing mass distribution. One

an therefore expe t to nd `dark-lens' galaxies in large

gravitational-lens surveys, if they make up a signi ant

fra tion of massivegalaxies (e.g. Hawkins1997). On the

basisof onrmedand andidatelarge-separation( >

 3

ar -se )gravitational-lenssystems,whi hwereallopti ally

se-le ted, Hawkins(1997) on ludes that around75%of all

galaxies ould be `dark' (i.e. extremely underluminous).

This highfra tion wasshownto bein onsistentwith

ob-servationalresults(Ja ksonet al.1998)from theCosmi

LensAll-SkySurvey(CLASS;Browne1999;Myers1999)

andalsoveryunlikelyonthebasisofanumberof

statisti- alarguments(e.g.Ko hanek,Fal o&Mu~noz1999;Peng

etal.1999).Averyhighfra tionofdarkgalaxiestherefore

seems to be ex luded, although a mu h smaller fra tion

(10%) annotyet beruled out.Thedete tionofonlya

single dark-lensgalaxywould alreadyhave severe

onse-quen esforthestandardpi tureofgalaxyformation(e.g.

White & Rees 1978). Su h systems would have allowed

dark matter to ollapse in stru tures massive enoughto

reate multipleimages throughgravitationallensing,but

atthesametimeprohibitbaryoni mattertosettleinthe

dark-matterhaloandinitiatestarformation.Clearly,one

has to be areful in dismissing any andidate dark-lens

galaxyasbeingeither abinary quasarora han e

undoubt-A

B

A

MERLIN 5-GHz

VLA 15-GHz

VLA 5-GHz

VLA 1.4-GHz

VLA 8.5-GHz

VLBA 5-GHz

VLBA 5-GHz

B

1

2

2

1

Fig.1.SummaryoftheVLA,MERLIN andVLBAradioimagesof B0827+525(see textformoredetails). Contoursstartat

the3{levelandin reasebyfa torsof2.Therst ontoursare0.19 mJybeam 1 ,0.37 mJybeam 1 ,0.37mJybeam 1 ,1.15 mJy beam 1 , 0.88 mJybeam 1 ,0.40 mJybeam 1 ,0.77 mJy beam 1

, for the MERLIN 5{GHz,VLBA 5{GHz (A),VLBA

5{GHz(B),VLA15{GHz,8.5{GHz(dis overyimage), 5{GHZand1.4{GHzimages,respe tively.

Inthispaper,wepresentthestrongest`dark-lens'

an-didatefoundin theCLASSsurvey:CLASSB0827+525 1

.

InSe tion 2,wepresentradio,opti al,near-infraredand

spe tros opi observationsofthissystem.InSe tion3,we

examineboththe`dark-lens'andbinaryradio-loudquasar

hypotheses to explain this system. In parti ular, we

de-rive onstraintsonthelens-galaxymass-to-lightratioand

omparethosewithobservationsof`luminous-lens'

galax-1

Not to be onfused with the gravitational-lens system

ies. In Se tion 4, we summarize our results and suggest

future workonthissystem.

2.Observations

TheCLASSsurveyaimstondallmultiply-imaged

at-spe trum(0.5forS

 /

)radiosour esinthe

north-ernhemispherewithatotal uxdensityofS

5GHz

30mJy,

a ux-densityratio betweenlens images 10,a Gala ti

latitudejbj>10 Æ

gravitationallens(GL)systems,that anbeusedtostudy

thestru ture andevolutionoflens galaxiesat

intermedi-ateredshiftsand onstrainthe osmologi alparameters,in

parti ulartheHubbleparameter(H

0

).AtpresentCLASS

hasdis overedatleast17newGLsystems(Browne1999;

Myers1999).

2.1. Radio observations

Ifnotspe iedotherwise,theradiodatapresentedinthis

se tion are ux- alibrated in the NRAO data-redu tion

pa kageAIPSandself- alibrated,imagedandmodel-tted

in DIFMAP (Shepherd 1997). To obtain ux-densities of

thetworadio omponentsofB0827+525,theuv-datawas

ttedbytypi allytwoGaussian omponents.

As part of the CLASS sample of around 15200

at-spe trum radio sour es observed during the 1994, 1995

and1998Very-Large-Array(VLA)A{arrayseasons

(My-ers1999), a30{se snapshotof B0827+525was madeat

8.5GHz on 1995 August 13. The alibrated VLA image

(Fig.1) showstwo unresolved omponents that are

sepa-ratedby2.8ar se andhavea ux-densityratioof2.7[i.e.

S

A

24mJyandS

B

9mJy℄.

Withanaveragesour edensityintheCLASSsurveyof

1persq.deg,theprobabilityof han ealignmentoftwo

unrelated ompa tradio-loudquasarswithin2.8ar se

isaround2
10 6

,whereasthelensingrateisaround10 3

.

Inasampleof15200sour estheo urren eofsu ha lose

alignmenthas aprobability ofonly 3%. B0827+525 was

thereforeregardedasa andidateGLsystem.

Toimprovetheresolutionandobtainatwo-point

spe -tral indexbetweentheradio omponents,asnapshot

im-age of B0827+525 was made on 1997 January 3 with

theMulti-ElementRadioLinked Interferometer Network

(MERLIN)at5GHz.A ux-densityratioof2.5wasfound

[i.e. S

A

34mJyand S

B

14mJy℄ and a omponent

sep-aration of 2.8 ar se ,very similar to the VLA 8.5{GHz

observations.Subsequentlong-tra kMERLIN5{GHz

ob-servationsweremadeon1997De ember8,withatotal

in-tegrationtimeonthesour eof10hours.The alibrated

image(Fig.1)hasanrms-noiselevelof60Jybeam 1

and

aresolution of 50mas. Both omponents remain

unre-solved.The ux-densitiesareS

A

34.0mJyandS

B 14.8

mJy,respe tively.Nosignofextendedemissionisdete ted

intheimageabovealevelof0.6%.Thesimilarityin

ux-density ratioat 8.5 and 5GHzstrengthened the asefor

alensingexplanationof thissystem.

To improve the resolution by another order of

mag-nitude, B0827+525 was observed with the V

ery-Long-Baseline-Array (VLBA) at 5GHz on 1997 August 2.

Snapshots were obtained over a range of hour angles

to improve the uv- overage. Phasereferen ing was done

by rapid swit hing between thenearbystrong alibrator

B0828+493 (2min) and B0827+525 (5{7min). The

to-tal integration time on B0827+525 was 35 min. The

-10

100

1

10

Flux (mJy)

Freq(GHz)

B0827+525

15GHz

5/98

6/99

6/99

5/98

A

B

Fig.2.Radiospe traofB0827+525AandB,determinedwith

theVLAinA{arrayat1.4,4.9,8.5and14.9GHzon1999June

27.

and an rms-noise level of 0.1mJybeam 1

. The two

ra-dio omponentswereea hmodeledbytwoGaussian

sub- omponents. Component A shows a NE{SW extension,

whereas omponent B shows an area of weak extended

emissionin theNW{dire tion.ThelowSNRs oftheNE{

omponentinimageAandtheNW{extensionofimageB

doesnotwarrantmodeling them bymorethan two

sub- omponents. Thepositions of thebest-t Gaussians and

theirrespe tive uxdensitiesarelistedinTable1.

Both radio omponents show fairly at spe tra from

4.9to8.5GHz.Theintegrated uxdensityofB0827+525

(A+B) in the WENSS survey, (705 mJy at 325 MHz,

mean epo h 1992.2), is about equal to the ux density

at 1.4GHzintheNVSS,FIRSTandWSRTobservations

takeninthesummerof1997.Thissuggeststhatthe

indi-vidual omponentspe tramust attentoaspe tralindex

of zero below 1.4GHz. Alternatively, one of the

ompo-nent spe tra ould be ome inverted leavingroomfor the

other to remain straight. High resolution low frequen y

dataareneededtodistinguishbetweenthesepossibilities.

A2{monthmonitoring ampaign (8 epo hs)with the

WSRTat 1.4GHz(with15{ar se resolution)in the

pe-riodJuly-September1997indi atesvariabilityinthetotal

uxdensity:thesour e uxdensityslowlyde reasedfrom

71to58mJywithatypi alerrorofabout1mJyin

indi-vidual measurements.It isnot leartowhi h omponent

these variationsshouldbeattributed.Thevariationtime

s ale is rather short even though both omponents are

ompa t.Itisperhapsmorelikelythatthe1.4{GHz

vari-ations are due to refra tiveinterstellar s attering (RISS;

e.g. Ri kett et al. 1984), an explanation requiring a less

extremesize forthe omponents.

To examinethe radio spe traof both omponents in

moredetail,near-simultaneousVLA1.4,4.9,8.5and14.9{

Table 1. VLBA 5{GHz data on B0827+525 (1997 Aug. 2) The VLBA phase- enter was RA 08 31 5:36200, DEC 52 Æ 25 0 20: 00

24100. Positions (
x,
y) are given with respe t to to this phase- enter.The 4.9{GHz ux-density (S

4:9 ) of the

sub- omponentsandtheir de onvolvedmajor axis,axialratioand positionangle(N!E) arealsogiven.Thesub- omponents

of omponentAareunresolved.Thepositionalerroris2.5masdividedbytwi etheSNRofthe omponents.Theerrorsonthe

majorandminoraxesofthesub omponentsinBareroughly25%.

A1 A2 B1 B2
x(mas) 1064.02 1063.18 1074.16 1076.43
y(mas) 930.74 929.89 900.40 899.78 maj.(mas) { { 1.75 2.23 min/maj { { 0.65 0.55  ( Æ ) { { -31.40 5.80 S4:9 (mJy) 24.80.1 6.80.1 9.30.1 3.60.1

Table 2. VLA multi-frequen y data on B0827+525, taken

near-simultaneouslyon1999 June27. Theinternal errorsare

indi ated. (GHz) SA(mJy) SB(mJy) 14.9 29.10.4 8.00.4 8.5 29.70.1 10.70.1 4.9 33.10.1 14.00.1 1.4 48.20.3 27.40.3

both omponentsareshownin Fig.2.ComponentsAand

Bhavefairlydierentradiospe trawithspe tralindi es

 8:5 1:4 =0:270:01(A)and 8:5 1:4 =0:520:01(B)(where S  / 

). Variability in omponent A at 14.9 GHz was

dete ted from the 1998 May 15 and 1999 June 27 VLA

observations,whi his morelikelytobeintrinsi and not

RISS, be ausethe rms variability due to RISSde reases

rapidlytowardshigherfrequen ies(e.g.Ri kettetal.1995;

Walkeretal.1998;seealsoKoopmans&deBruyn2000).

Variability at both lower (1.4GHz) and higher

(15GHz) frequen ies makes a dire t omparison of the

spe tralindi esdiÆ ult.Theformaldieren einspe tral

indexdoesnotdisqualifythesour easpossibleGL

andi-date, but ompli ates theargumentationin a `dark-lens'

hypothesis.WewillreturntothisissueinSe t.4.

2.2. Opti al andnear-infraredobservations

HubbleSpa eTeles ope(HST)observationsweremadeon

1998April18with theNear-InfraredCameraand

Multi-Obje t Spe trometer (NICMOS) at 1.6 m (F160W),

roughly orresponding to ground-based H{band. The

NIC1 amera was used whi h provides a dete tor s ale

of 43 mas pixel 1 and a eld-of-view of 11 00 11 00 . The

total exposure time was 2624 se . The data were

sub-je ted to the standard NICMOS alibration pipeline in

IRAF 1

whi h (i) orre ts for known instrumental ee ts

(i.e. at eld and dark urrent), (ii) performs gain and

ux alibration and (iii) removes osmi rays and ags

degraded and suspe t data values 2

. The image was ut

around B0827+525 and rotated to orre t to standard

(middle). The primary northern omponent (A) is

un-resolved and learly dete ted (m

F160W

=19.60.2 mag).

Component B is just barelydete ted as a region of

ex-tended emission (m

F160W

=22.60.2 mag). No sign of a

lensing galaxy is seen in the image to m

F160W

=23 mag,

althoughtheextendedemissionnearimageB ouldpartly

befromapossiblelensgalaxy.Noopti al orre tion (i.e.

de onvolution)wasappliedtotheimage,be ausethePSF

distortion 2

isnotexpe tedto hangeappre iablyoverthe

image separationofonly2.8ar se .

Similarly,B0827+525wasobservedinK{bandon1998

May 9with theNear InfraRedCamera(NIRC)at Ke k.

A total of 25 frames, ea h having a 60{se integration

time,wereobtainedoftheeld.Theteles opewasmoved

by approximately 10{15 ar se between individual

inte-grations. Theseeing at thetime of the observations was

0.6{0.75ar se (FWHM).The exposures were ombined

and rotated to orre t the orientation and the resulting

imageisshowninFig.3(right).Theobservations onrm

the on lusions drawn from the HST exposure,that

im-ageAis onsistentwithapointsour eandimageBseems

somewhatextended.

An R{band image wastaken on1998 May19 on the

WHT. Two600-se exposures were obtainedwith a

see-ing of 1.4{1.5 ar se (FWHM). It was re-observed on

1998 May 20 with two exposures of 600{se and one of

300{se exposure,respe tively.Theseeingwasaround1.0

ar se (FWHM). All exposures were ombined and

re-du ed in the standard way in the data-redu tion

pa k-ageIRAF.Theresultingimage (Fig.3,left)onlyshow

im-age A, whereas image B is notdete ted abovethe noise

level. Unfortunately no useful alibration was obtained,

but basedontheexposuretime andexpe tednoise-level,

1

IRAF (Image Redu tion and Analysis Fa ility) is

dis-tributed by the National Opti al Astronomy Observatories,

whi hare operatedbythe Asso iationofUniversities for

Re-sear h in Astronomy under ooperative agreement with the

NationalS ien eFoundation.

2

Adetaileddes riptionoftheNICMOS alibrationpipeline

pro ess,zero-point uxes and PSF an befound inthe Near

NICMOS/F160W

B

A

B

A

10"

N

E

A

Keck NIRC K-band

WHT R-band

Fig.3.Left:R{bandimageofB0827+525,observed1998May19withtheWHTonLaPalma.Middle:HSTF160WNICMOS

image of B0827+525, observed on1998 April 18. The separation between omponentsA and B is 2.8 ar se . Right: NIRC

K{bandimageofB0827+525,observedwiththeW.MKe k{IITeles opeon1998May9.

weroughlyestimatefor omponentAanapparent

magni-tudeofm

R

22.0{22.5magandfor omponentBalower

limitm R >  24.0mag. 2.3. Spe tros opi observations

B0827+525wasobservedwiththeLowResolution

Imag-ing Spe trograph (LRIS; Oke et al. 1995)on the W. M.

Ke kIITeles opeonthenightof1998April21.Three

ex-posuresweretakeninlongslitmodewithatotalexposure

timeof4500se .The300groovesmm 1

gratingwasused,

givingadispersionof2.44 

Aperpixel.Theslitwaspla ed

alongboth omponents,withapositionangleof50 Æ

.The

nalspe trum oversawavelengthrangeof4024{9012 

A.

The spe trawere redu ed using standard IRAF tasks. A

orre tion for the response of the CCD was determined

from observations of the Oke standard star BD332642

(Oke 1990).The orre ted spe trawere weighted by the

squares oftheir SNRs and o-added to produ e thenal

B0827+525spe trum.Fig.4 showsthespe trumof

om-ponentA,whi hhasbeensmoothedusingabox arkernel

withawidthofvepixels.Emissionlinesasso iatedwith

Si-IV1397,C-IV1549,He-II1641,andpossiblyC-III℄

1909areseen,establishingasour eredshiftof2.064.No

emission atthe position of omponent Bisdete ted. We

alsondnoeviden einthespe trumof omponentAfor

ase ondredshift,possiblyfromalensgalaxy.

3. Analysis

Beforepro eedingwithouranalysis,letussummarizethe

mostimportantobservationalresults:Wehavefoundthat

B0827+525 onsists of two radio omponents, separated

both omponents have substru ture at a s ale of a few

mas. The integrated radio spe tra of both omponents

from 1.4 to 14.9 GHz are somewhat dierent. The

red-shift forthebrightestradio omponent(A) is2.064.

Op-ti al ounterparts for both radio omponents are found

in H and K{band observations. The emission asso iated

withthefainterradio omponentappearsextended,whi h

ouldsuggestthatitisveryfaintgalaxyemission.

3.1. The`dark-lens' hypothesis

To omparetheobservedlimitsonthemass-to-lightratio

ofapossiblelensgalaxywiththosefoundfortypi allens

galaxies, we rst al ulate the velo ity dispersionof the

lensing mass,using theSingular-Isothermal-Sphere(SIS)

mass model (Binney & Tremaine 1987). This model

re-latesthelens-imageseparation(
)to aline-of-sight

ve-lo ity dispersion (), given the osmologi al model and

thesour e-andlensredshifts:

=8
   2  D ds D s  ; (1)

where is the velo ity of light, and D

ds

and D

s are

the angular-diameter distan es between lens-sour e and

observer-sour e, respe tively (e.g. S hneider, Ehlers and

Fal o1992).The angular-diameterdistan eis afun tion

of redshifts and osmologi al model. Using the

ompo-nent separation, we an furthermore al ulate the mass

ontainedinsidetheEinstein radius

M(
)=3:1
10 10  
 1 00  2  D Gp  M  ; (2) whereDD d D s =D ds andD d

dis-Fig.4.LRISspe trumofB0827+525,observedwiththeW.MKe kIITeles opeon1998April21.

lensgalaxy,usingthelterband(e.g.U,B, V,:::),we

subsequentlyusetherelation

L  =L ; =10 0:4
(M ; m  +DM+K  ) ; (3)

whereDMisthedistan emodulusofthelensgalaxy,M

;

istheabsolutemagnitude ofthesun,m



istheapparent

magnitudeof thelensgalaxyandK



istheK{ orre tion

forthelterband.Weusethelowerlimitofm

F160W >23

mag on the lens galaxy, found from the HST exposure

(Fig.3)to onstrainthemass-to-lightratioinH{band.We

use the H{band observations, be ause K orre tions are

relativelysmall(<0.2magforz1and<0.5magforz2;

Poggianti 1997). Wedo not use evolutionary orre tions

andassumeM

H;

=3.46mag.

For a sour e redshift of 2.064, we have plotted the

H{band mass-to-lightratioin Fig.5for twodierent

os-mologies(A: m =1 and  =0;B: m =0.3and  =0.7) and H 0 =65 km s 1 Mp 1 . We assume a

Friedmann-Robertson-Walkeruniverse.Aminimummass-to-light

ra-tioaround100isfound,using osmologyB.This

mass-to-light ratio implies a lens-galaxyredshift around 1.5 and

a high velo ity dispersionof 400{500km/s. For

osmol-ogyA,theminimummass-to-lightratiowouldin reaseto

mass-to-light ratio in H{band for B0827+525is a fa tor

15{100largerthantheH{bandmass-to-lightratiosof

typ-i allensgalaxiesintheCLASSsurvey.

InFig.6, wehaveplotted theexpe ted H{band

mag-nitude of the lens galaxy as fun tion of redshift, galaxy

type and osmology.We used (i) the velo ity dispersion

determined from eqn.1,(ii)the relationbetweenvelo ity

dispersionand B{bandmagnitude ofellipti aland spiral

galaxies from Fukugita et al. (1991) and (iii) the olors

and K{ orre tions from Poggianti(1997).FromFig.6 an

upper limit on m

H

of 17 mag is found, nearly

indepen-dent of galaxy type. If evolutionary orre tions are

ap-plied (Poggianti 1997), we nd no signi ant dieren es

intheseresults.Theupperlimitisatleast6magbrighter

thanthelowerlimitontheH{bandmagnitudeofthe

pos-sible lens galaxy, whereas for all onrmed GL systems

(Ja ksonet al. 1998),for whi h the lensand sour e

red-shiftsareknown,dieren esbetweentheobservationsand

themodel are <

 1mag.

Hen e,ifB0827+525isalenssystemthentheobje tis

indeed a`darklens'system.However,alsoimage Bmust

Fig.5.EstimatedlowerlimitontheH{bandmass-to-light

ra-tio of the lens galaxy, as fun tionof redshift(see text). The

solid line is for a (m=1, =0) osmology, the dot-dashed

line for a (m=0.3, =0.7) osmology. We assume H0=65

kms 1

Mp 1

.

3.2. Thebinary quasarhypothesis

As argued in Se t.2.1 the probability of a han e

align-ment oftworadio-loudquasarsis rather small.Hen e, if

thesour eisnotalenssystemsitismostlikelyapairof

physi ally lose quasars.We allthis the `binaryquasar'

hypothesis.However,weemphasizethatsofarwehaveno

eviden eforthequasarnatureof omponentBinthe

op-ti al!

The ompa t at-spe trumradiosour epopulation

se-le ted viaCLASS is generallybelieved to emit its radio

emission in a highly non-isotropi manner (e.g. Orr &

Browne 1982) with the radio axis pointed in our

dire -tion within a small angle. As rst argued by S heuer &

Readhead (1979) there should then be manyradio-quiet

quasars (QSO) for every radio-loud quasar (QSR). It is

then obvious that the probability of nding tworelated

radio-loud quasars pointed at us is rather small. In fa t

we anprobablystatethatforeveryQSR{QSRpairthere

should bearound20 QSR-QSOpairsin the CLASS

sur-vey,if5%ofallQSOsarealsoradio-loud,i.e.30mJyat

8.5GHz(e.g.Hooperet al.1995;Bis hof&Be ker1997).

Under the assumption that B0827+525 is the only

QSR{QSRpair(i.e.binaryradio-loudquasar)inCLASS,

theprobabilitythata at-spe trumradiosour eispartof

aQSR{QSR pairis around1in 15000.This meansthat

the probability that aCLASS sour e is part of a QSR{

QSOpairisaround1in750.IntheLargeBrightQuasar

Survey(LBQS;Hewett etal. 1998)twoQSO{QSOpairs

were found from a sample of around 1000 opti ally

se-le ted quasars,hen e1in 500.Thisnumberis very lose

to thatfoundfrom theCLASSsurvey,whi hmeansthat

is onsistent with therate of QSO{QSO pairsin opti al

quasarsurveys(seealsoKo haneket al.1999).

However,ifB0827+525 is abinaryradio-loud quasar

andwe ompareitwiththelistofwideseparationquasar

pairs in Ko hanek et al. (1999), we noti e two things:

First, B0827+525, has the smallest separation (2.8

ar -se or 23 kp /h

50

) of all quasar pairs. Se ond, only 2

outof13non-lens quasarpairshavehigherredshifts (i.e.

LBQS 1429-008 and Q2345+007). On the other hand,

B0827+525 would also be one of the largest separation

lens systems (e.g. Browne 1999; Myers 1999; CASTLES

survey 2

).B0827+525thusappearstobeatthe

parameter-spa eborderdelineated bytheopti alquasarpairslisted

byKo haneketal.(1999)andthe onrmedGLsystems.

It isdiÆ ulttoseehowsigni antthese issuesarein the

ontext of binary quasars and how it possibly relatesto

thefa t thatbothquasarsareradio-loud.

4.Dis ussion& Con lusions

Iftakenat fa evalue,mostoftheobservationaleviden e

seemsto beinfavorof B0827+525beingabinary

radio-loudquasar:(i)somewhatdierentradiospe trafrom1.4

to14.9GHzforthetwo omponents,(ii)adierentradio

andopti albrightnessratio,(iii)theweakerradio

ompo-nentappearsslightlyresolved,(iv)theextendednatureof

theopti alandnear-infraredemissionnear omponentB,

whereasthatofimageAis ompa tand(v)theagreement

withthenumberstatisti sofbinaryradio-loudquasarsin

theCLASSsurveywiththoseofopti alsurveys.

However, the most interesting explanation of

B0827+525, a `dark-lens' system, annot onvin ingly

be ex luded and in view of the osmologi alimportan e

of the presen e of galaxy-sized on entrations of dark

matter {eventhoughtheymustberare(e.g. Ja ksonet

al.1998){wenowdis usswhywebelievethe`dark-lens'

hypothesis an not yet be dis arded: (1) We have seen

that at least oneof the radio omponents is variable. If

theyarelensimages,atimedelaybetweenthem ombined

with variabilityofthesour e anresultinadieren ein

simultaneouslymeasuredspe tralindi es,espe iallyifthe

timedelayisoftheorderofthevariabilitytimes ale.(2)

If wepla e amassive`dark-lens' galaxynear omponent

B, ontaining alarge amountof dust, this galaxywould

obs ure most opti al and a signi ant fra tion of the

near-infrared emission from omponent B. In the se ure

lens systemB0218+357asimilarly largedis repan y has

been observed between the opti al and radio brightness

ratios of the lens images (e.g. CASTLES survey), whi h

an also be explained in terms of obs uration by the

high- olumn density of the ri h ISM in the lens galaxy

(e.g. Wiklind & Combes 1995; Menten & Reid 1996;

CombesWiklind&Nakai1997;Combes&Wiklind 1997;

Gerin et al. 1997; Combes & Wiklind 1998). This dust,

Fig.6.TheestimatedH{bandmagnitudeofthelensinggalaxyfor4galaxytypes{E(solid),S0(dash),Sa(dot-dash)andS

(dot){,plottedas fun tionof redshift(seetext). Thehorizontaldashedline indi atestheobservedlowerlimit of23mag on

theH{bandmagnitude.Two osmologiesareshown.

however,wouldprobably notblo k allof thelens-galaxy

emission. In fa t, most of the extended emission seen

near radio omponent B ould be omingfrom the lens

galaxy itself and not from the quasar. (3) The opti al

brightnessofimageBissurprisinglylowforaradio-loud

quasar,whi hmightsuggestsomeform ofextin tion.(4)

A high s attering measure, asso iated with the ionized

omponentoftheISMinthelensgalaxy(seeforexample

Marlowet al.1999;Joneset al.1996), ould furthermore

s atter-broaden radio omponent B, explaining why it

appearssomewhatresolved.

So far in the `dark-lens' hypothesis wehaveassumed

thatthelensingmassisasinglegalaxydominatedby

non-baryoni matterinitsinnerpartsandthereforeextremely

underluminous.Ifthelensingmassdistributionisa

high-redshift (z >



1) group or luster, the onstraints on the

mass-to-light ratio, whi h is usually an order of

magni-tudelargerin lustersand groupsofgalaxies thanin the

innerpartsofgalaxies(e.g.vanderMarel1991;Carlberg,

Yee &Ellingson1997), issomewhat alleviated.However,

formostofthelensgalaxiesfrom theCLASSsurvey,the

H{bandmass-to-lightratiosarearoundunity(Ja ksonet

al. 1998). This is two orders of magnitude smaller than

theminimumH{bandmass-to-lightratiofoundinSe t.3.1

and anthereforenotentirelya ountforthehigh

mass-to-lightratiowithinthe ontextofa`normal'galaxy

evo-lution. Hen e also in the ase that the lensing mass is

agroupor luster,it mustbeunderluminous.Apossible

andidateforthistypeofmass on entrationwasre ently

foundbyErbenet al.(2000).

Thus, although the binary quasar hypothesis seems

more likely at fa e value, the `dark-lens' hypothesis an

notberuledout onvin ingly.However,thelatter

hypoth-esisdoesgive on retepredi tions:First,thesubstru ture

tortedbytheinterveninggravitationalpotential.Wehave

obtainedmulti-frequen yglobalVLBIdataonB0827+525

atthreefrequen iestotestthis.(2)Ifthevariabilityofthe

radio omponentsis intrinsi , oneshould beable to

or-relate their light- urves to nd a time-delay. This ould

denitively prove or disprove whether B0827+525 is a

`dark-lens' system. (3) If the extended H and K{band

emission seen near omponent B is partly from a lens

galaxy,itshouldhavearedshiftsmallerthan2.064.

Near-infraredspe tros opyofthisemissionshould thereforebe

attempted.

A knowledgments

The authors would like to thank Lee Armus and David

Hogg for obtaining aKe k NIRC K{band image. LVEK

andAGdeBa knowledgethesupportfrom anNWO

pro-gram subsidy (grant number 781-76-101). This resear h

wassupportedinpartbytheEuropeanCommission,TMR

Program, Resear h Network Contra t

ERBFMRXCT96-0034`CERES'. TheNationalRadioAstronomy

Observa-tory is a fa ility of theNational S ien e Foundation

op-erated under ooperative agreement by Asso iated

Uni-versities,In . TheWesterbork SynthesisRadioTeles ope

(WSRT) is operated by the Netherlands Foundation for

Resear hinAstronomy(ASTRON)withthenan ial

sup-portfromtheNetherlandsOrganizationforS ienti

Re-sear h (NWO).MERLIN is anational UKfa ility

oper-atedbytheUniversityofMan hesteronbehalfofPPARC.

This resear h used observations with the Hubble Spa e

Teles ope, obtained at the Spa e Teles ope S ien e

In-stitute, whi h is operated by Asso iated Universities for

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