ASPConferen e Series,Vol.**VOLUME**,**PUBLICATIONYEAR**
**EDITORS**
Photometri and Spe tros opi Observations of the
CLASS Parent Population
J.P.M Kean,I. W. A.Browne,N.J. Ja kson, P.N. Wilkinson
University of Man hester, Jodrell Bank Observatory,Ma les eld,
Cheshire, SK11 9DL
P.Helbig
Kapteyn Astronomi al Institute,P.O. Box 800, 9700 AV Groningen,
TheNetherlands
Abstra t.
We present photometri and spe tros opi observations of a sample
of 36 at-spe trum radio sour es between 5 and 12 mJy. These
obser-vations willbe usedto determinethe radioluminosity fun tionforfaint
at-spe trum radio sour es whi h is a vital part of the CLASS
gravita-tionallensstatisti sused to onstrainthe osmologi alparameters.
1. Introdu tion
Usingthegravitational lensstatisti sfrom theJVAS lenssurvey(S
5GHz >200
mJy; Patnaik et al. 1992) Helbig et al. (1999) al ulated for a at-universe,
0
< 0:84 at the 95% on den e level. The 5 times larger CLASS lens survey
(S
5GHz
> 30 mJy; Browne et al. 2000) will onstrain
0
further, providing
we know the redshift distribution of the lenses and the parent population of
unlensed sour es. The latter of these two will potentially produ e the largest
un ertainties sin e our knowledge of the redshift distribution of at-spe trum
radiosour es is limited to S
5GHz
> 25 mJy (Marlow et al. 2000). Sin e
grav-itational lensingmagni esa sour e's ux-density, obje ts with S
5GHz
5 mJy
ouldpotentiallybegravitationallylensedintotheCLASSsampleandtherefore
need to be in luded in the redshift distribution of the parent population. For
this reason we observed a sample of 36 at-spe trum radio sour es between 5
and12mJywiththegoalofredu ingtheun ertaintiesintrodu edbytheparent
populationluminosityfun tion.
2. Sample Sele tion and Observations
The5to12mJysamplewassele tedinexa tlythesamemannerastheCLASS
sample. Around 800 sour es were sele ted from the NVSS 1.4 GHz atalogue
above 2.5mJyandobservedwiththeVLAat5 GHzinorderto ndonlythose
at-spe trumsour eswith <0:5whereS 5GHz
1:4GHz /
. Thisleft100sour es
whi h werethenobservedwiththeVLAat 8.4GHz,thefrequen yat whi h all
12 mJyremained. Spe tros opy of the opti ally bright obje ts was attempted
usingthe WHT. The BR IJHK photometry of the opti allyfaint obje ts was
obtainedusingtheNOT, WHT, and UKIRT teles opes inJuly1999.
3. Photometri Redshifts and Preliminary Results
Photometri redshifts were al ulated using the spe tral energy distribution
(SED) ttingte hnique. This method ompares theobserved SEDwitha
tem-platespe trumofaStarburstandEllipti algalaxylookingfor ommonspe tral
featuressu hastheLy- and4000
A breaks. The photometri redshiftpa kage
we useis hyperz (Bolzonella etal. 2000). Problemsusingphotometri redshifts
ano urwhenthereis ontaminationoftheSEDfromAGN light. AGNswere
identi edfromtheirmorphology,blue olourex ess,spe tros opy,andfromthe
2
model ts
Ofthe36sour es inthe ompletesample,27 weredete ted intwoormore
olours. Those sour es dete tedinless thanfour olourswillhave large
photo-metri redshifterrors. Theobje tsaregenerallyredandfaint(meanR K 4,
mean m
K
19). The preliminary mean redshift of the sample based on the
photometri redshifts obtained with hyperz and the limited redshifts obtained
spe tros opi ally is z 1:5, at 30% ompletness. This will learly hange
on eall thephotometri and spe tros opi datahasbeenfullyanalysed.
4. Future Work
Weare urrentlyattemptingtoobtainspe tros opi redshiftsforasub-sampleof
obje ts withinthesample inorderto verifythephotometri redshiftte hnique.
Wealsohavea ompletesampleof46 at-spe trumradiosour esbetween25and
50mJy. Spe tros opyhasyieldedredshiftsfor37%ofthissample(Marlowetal.
2000). Mostoftheremainderareopti allyvery faint. We havealready beguna
programof obtainingtheremaining redshiftsfromthissamplephotometri ally.
A knowledgments. This resear h was supported in part by the
Euro-pean Commission, TMR Programme, Resear h Network Contra t
ERBFM-RXCT960034 'CERES'.
Referen es
Bolzonella,M., Miralles,J., PelloR., 2000, A&AS,a epted
Browne, I. W.B., &Myers,S.2000, these pro eedings
Helbig,P., Marlow,D. R.,QuastR., Wilkinson,P.N., Browne I. W.A.,
Koop-mans,L. V.E.1999, A&AS,136, 297
Marlow, D. R., Rusin D., Ja kson N. J., Wilkinson, P. N., Browne I. W. A.,
Koopmans, L.V.E. 2000 AJ,119, 2629
