AGN and Galaxy evolution from Deep X-ray surveys

0.19 MB·English

by  P. Tozzi

Mem.S.A.It.Vol.,1
(cid:13)c SAIt 2006
AGN and Galaxy evolution from Deep X-ray
surveys
P. Tozzi1,R.Gilli2,andtheCDFSTeam
1 INAF–OsservatorioAstronomicodiTrieste,viaG.B.Tiepolo11,I–34131Trieste,Italy
6
e-mail:[email protected]
0
2 INAF– OsservatorioAstronomico di Bologna, viaRanzani 1, I–40127, Bologna, Italy
0
e-mail:[email protected]
2

n
a
J Abstract.DeepX–raysurveysareprovidingcrucialinformationontheevolutionofAGN
andgalaxies.WereviewsomeofthelatestresultsbasedontheX–rayspectralanalysisof
9
thesourcesdetectedintheChandraDeepFieldSouth,namely:i)constraintsonobscured
1
accretion;ii)constraintsonthemissingfractionoftheX–raybackground;iii)theredshift

distributionofCompton–thicksourcesandTypeIIQSO;iv)thedetectionofstarformation
1
activityinhigh–zgalaxiesthroughstackingtechniques;v)thedetectionoflargescalestruc-
v
tureintheAGNdistributionanditseffectonnuclearactivity.Suchobservationalfindings
2
areconsistentwithascenariowherenuclearactivityandstarformationprocessesdevelop
5
togetherinananti–hierarchicalfashion.
4
1
Key words. X-rays: diffuse background – surveys – cosmology: observations – X–rays:
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galaxies–galaxies:active
6
0
/
h
1. Introduction tween 80% and 90% (see Bauer et al. 2004
p
andtherecentlyrevisedestimatebyHickox&
-
o In the last years deep X–ray surveys with the Markevitch 2005), providing an almost com-
r ChandraandXMM–Newtonsatellites(Brandt plete census of the accretion history of mat-
t
s etal.2001;Rosatietal.2002;Alexanderetal. ter ontosupermassiveblackholesthroughthe
a 2003;Hasingeretal.2001),paralleledbymul-
cosmic epochs. However, the most interest-
:
v tiwavelength campaigns (see, e.g., GOODS, ingoutcomesgowellbeyondthedemographic
i Giavalisco et al. 2004), provided several cru- characterizationoftheextragalacticX–raysky.
X
cial information on the evolution of the AGN
Indeed, the physical and evolutionary proper-
ar and galaxy populations. The bold result from ties of the AGN populationare now revealing
the two deepest X–ray fields, the Chandra how they formed and how they are linked to
DeepFieldNorth(CDFN,observedfor2Ms)
their host galaxies. For the first time, the lu-
andtheChandraDeepFieldSouth(CDFS,ob-
minosityfunctionofAGNhasbeenmeasured
servedforabout1Ms)isconstitutedbytheres- up to high redshift. A striking feature is the
olution of the X–ray background (XRB) into
downsizing,oranti–hierarchicalbehaviour,of
single sources, mostly AGN, at a level be- the nuclear activity: the space density of the
brightestSeyfertIandQSOispeakingatz≥2,
Sendoffprintrequeststo:P.Tozzi
2 P.Tozzi&R.Gilli:LatestresultsfromtheCDFS
while the less luminous Seyfert II and I peak
at z ≤ 1 (Ueda et al. 2003; Hasinger et al.
2005;LaFrancaetal.2005).Ananalogousbe-
haviourispresentlyobservedinthecosmicstar
formation history: at low redshift star forma-
tion is mostly observed in small objects (see,
e.g., Kauffmannet al. 2004),while at redshift
2orhigher,starformationactivityisobserved
also in massive galaxies(with M ∼ 1011M ,
∗ ⊙
see,Daddietal.2004a).
The global picture, as outlined by the
present data, requiresa tight link between the
formation of the massive spheroids and the
central black holes, as witnessed by the rela-
tion between black hole and stellar masses or
betweenblackholemassandthevelocitydis-
persion of the bulge (Kormendy & Richstone
1995; Magorrian et al. 1998; Ferrarese & Fig.1. Thecolor imageoftheCDFS(1Msexpo-
Merritt2000).Theanti–hierarchicalbehaviour sure, Rosati et al. 2002). TheX–ray color code is:
in both star formation and AGN activity red:0.3–1keV;green:1–2keV;blue:2–7keV.Note
(which reflects in an anti–hierarchical super- that fainter sources appear more absorbed (bluer)
massiveblackholesgrowth,seeMerloni2004; thanbrighterones.
Marconietal.2004;foranalternativeviewsee
Hopkinsetal.2005),isenvisagedbytheoreti-
cal models where energy feedback is invoked 2. PropertiesoffaintAGNfromX–ray
to self–regulate both processes (see Fabian spectralanalysis
1999;Granatoetal.2004).
The resolution of the XRB, the long–awaited
In these Proceedings, we will describe a resultsinceitsdiscoveryin1962,hasbeenob-
fewobservationalresultsobtainedfromthelat- tained simply by counting the point sources
est analysis of the X–ray and optical data in foundinthedeepX–rayimagestakenwiththe
the Chandra Deep Field South and North and ChandraandXMM–Newtonsatellites.Thisre-
which,inourview,areconsistentwiththispic- sultisclearlyshownbythesharpimagesofthe
ture.Indetail,theseresultsconcernthefollow- CDFSandtheCDFNinFigure1and2.These
ingissues: images also show visually the solution of the
so–calledspectralparadox:faintersourcesare
moreabsorbed(andappearbluerinX–raycol-
ors) than brighter ones, so that the total spec-
– the physical properties of AGN from the trum of the XRB, resulting from the summed
X–ray spectral analysis of faint X–ray contributionofthewholeAGNpopulation,has
sources; aslopeΓ ≃ 1.4,flatterthanthattypicalofthe
– themissingfractionoftheXRB; intrinsicnuclearemissionΓ=1.8(asobserved
– the distribution of obscured QSO and in unabsorbed AGN). The resolved fraction
Compton–thick sources and their relation of the XRB has been recently revised slightly
withthecosmicmassaccretionhistory; downwards to be about 80% in both bands
– starformationinhigh–zgalaxiesmeasured (Hickox&Markevitch2005).AGNmakesthe
intheX–raybandthankstostackingtech- 83% and the 95% of the resolved fractions
niques; in the soft and in the hard band respectively.
– theeffectsoflargescalestructureontonu- On the other hand,star forminggalaxiescon-
clearactivity. tributesonly3%and2%(Baueretal.2004).
P.Tozzi&R.Gilli:LatestresultsfromtheCDFS 3
Fig.3. Intrinsic N distribution representative of
H
thewholeAGNpopulationintheCDFS.Errorsare
Fig.2.ThecolorimageoftheCDFN(2Msexpo- obtained from the poissonian uncertainties on the
sure).SamecolorcodeasofFigure1. numberofdetectedsourcesineachbin.Thedashed
curveisalognormal distributionwithhlog(N )i =
H
23.1 and σ = 1.1. All Compton–thick candidates
have been placed in the bin at N = 1024 cm−2
However, quoting the resolved fraction of H
(Tozzietal.2006).
theXRBinthe2–8keVbandissomewhatmis-
leading. Indeed, it has been pointed out that
theresolvedfractionissignificantlydecreasing intrinsic absorptionin termsof equivalenthy-
with increasing energy (Worsley et al. 2005). drogencolumndensityN .
H
In particular, above 5 keV, the resolved frac- Summarizingthemainresultsofouranal-
tioncanbeaslowas50%.Thisfindingopens ysis, we foundthatthe intrinsicspectralslope
againtheissueoftheresolutionoftheXRB,re- Γ is always close to the average value Γ =
quiringthepresenceofa stillundetectedpop- 1.8, without showing any significant depen-
ulationofstronglyabsorbedAGNatmoderate dence on redshift, intrisic luminosity, or in-
redshift, as can be inferred from the spectral trinsicabsorption.Wefindsignificantevidence
shape of the missing XRB (see Worsley et al. of the 6.4 keV Fe line in 12% of the sources
2005). withspectroscopicredshift.Wedetectthepres-
The issue of the missing XRB opens sev- enceofasoftcomponent(possiblyduetopar-
eralquestionsonthephysicalpropertiesofthe tial coveringor to scattered emission) in only
X–ray sources and calls for a detailed X–ray 8 sources. We measured the intrinsic column
spectralanalysisofthefaintAGNpopulation. density N for each source, after freezing the
H
In a recent Paper (Tozzi et al. 2006) we went spectralslopeΓ=1.8forthefaintestones.The
through a detailed X–ray spectral analysis of intrinsic N distributionhasbeenobtainedaf-
H
the largemajorityof the X–raysourcesfound ter correcting for the detection probability as
intheCDFS(321inthe1Mscatalogafterex- a function of the flux (the sky–coverage)and
cludingstarsandlow luminositysourceswith for sourcesbelow the limiting flux of the sur-
L <1041ergs−1,seeGiacconietal.2002).In vey. We find that most of the AGN have high
X
particular,the knowledgeofthe spectroscopic intrinsicabsorptionwithN > 1022cm−2 (see
H
or photometric redshift for almost all the X– Figure 3). A fraction of the AGN (more than
ray sources(Szokolyet al. 2004;Zheng et al. 10%) are Compton–thick sources, defined as
2004),allowedus to measurethe value of the sourceswithN ≥ 1.5×1024 cm−2.Onlyfew
H
4 P.Tozzi&R.Gilli:LatestresultsfromtheCDFS
of these elusive sources are actually detected
(we identify only 14 Compton–thick candi-
dates in the CDFS), buth their actual number
density is expectedto be high. Indeed,due to
theirspectralshape,wellrepresentedbyavery
hard reflection spectrum, only a small part of
their population can be detected by Chandra,
whichismostlysensitiveinthesoftband.We
alsofindthatabout80%ofthesources(among
the 139 AGN for which good optical spectra
areavailable),followaone–to–onecorrespon-
dence between optical Type I (Type II) and
X–rayunabsorbed(X–rayabsorbed,definedas
sourceswithN ≥1022cm−2)sources,aspre-
H
dictedbytheoriginalversionoftheunification
modelforAGN(Antonucci1993).
Fig.4. Red circles: contribution to the resolved
3. ThemissingfractionoftheXRB
XRBintheCDFSindifferentenergybands.Small
The detailed knowledge of the spectral shape greensquaresarefromWorsleyetal.(2005).
allowsustobetterevaluatethedetectionprob-
ability of each source class. This is an im-
portant aspect, since the probability of be- bandisobtainedbyfittingtheX–rayspectrum
ing includedin a surveydependssignificantly ofeachsourcewithapowerlawplusanintrin-
on the spectral shape. Therefore, by a care- sicabsorption.Wefindthatthedecreaseofthe
ful spectral analysis, we are able to estimate resolved fraction as a function of the energy
withhighaccuracythecontributionofthemost rangeisnotaspronouncedasinWorsleyetal.
elusive absorbed sources to the XRB. Indeed, (2005), as shown in Figure 4 (Tozzi et al., in
thedetectionprobabilityislowerformoreab- preparation).Thisimpliesthatweareactually
sorbedsources,sincethemaximumdetectabil- seeingafractionofthepopulationresponsible
ity is achieved in the soft band; therefore,the forthemissingXRB.Duetothestrongabsorp-
contribution to the XRB of strongly absorbed tion,mostofthesesourcescannotbedetected
sourcesfoundintheCDFSishigherthanthat eveninthedeepestChandraorXMM–Newton
of sourceswith thesame energyflux butwith surveys, and their discovery must wait for an
no or little absorption. This aspect was not higherenergy,high–sensitivityX–raymission,
fullyappreciatedinpreviousworks,wherethe or make use of radio or submillimetric data
detectionprobabilitywasonlyafunctionofthe (seeMart`inez–Sansigreetal.2005).
fluxandnotofthespectralshape,withaconse-
quentunderestimateoftheactualnumberden-
sityofthemostabsorbedsources. 4. AGNandGalaxyformation:TypeII
We recompute the resolved XRB in the
QSOandCompton–thicksources
CDFS, and compareit to the total extragalac-
tic XRB spectrum modeled as a power law Another interesting piece of information
with Γ = 1.41 and 1 keV normalization of comes from the redshift distribution of the
11.6 keV cm−2 s−1 sr−1 keV−1 (De Luca & Compton–thick candidates in the CDFS. As
Molendi 2004). The contribution to the XRB showninFigure5,thesesourcesaredistributed
from CDFS sources is obtained directly by in a wide range of redshift, a significant part
summingthecontributedfluxfromeachsource ofthemaroundz ∼ 1.Theirredshiftandtheir
weightedbytheinverseofthedetectionprob- level of absorption match well with the val-
ability.The contributedfluxin a givenenergy uesexpectedforthesourcesresponsibleofthe
P.Tozzi&R.Gilli:LatestresultsfromtheCDFS 5
missing XRB (see Worsley et al. 2005), rein-
forcingthereliabilityofourcandidatesources.
Another interesting class of sources are
the so called Type II QSO. Since we do not
haveanopticalspectralclassificationforallthe
sources,weconsiderhereabsorbedQSO,sim-
ply defined on the basis of the X–ray proper-
ties as brightsources (L > 1044 erg s−1) with
N > 1022 cm−2. Some of them have been
H
shown to correspond to Type II QSO on the
basis of optical and submm data (Norman et
al. 2002,Mainierietal.2005a).Mostofthem
are among the optically faint sources of the
sample (Mainieri et al. 2005b). We select 54
sources with these properties distributed on a
wide range of redshifts (see Figure 6), corre-
spondingto80%ofthesourceswith L > 1044
erg s−1 in our sample. We remarkthat we ex- Fig.5. Normalized redshift distribution of the
ploreherealimitedluminosityrange(L<1045 Compton–thickcandidatesintheCDFS(14sources,
ergs−1),giventhesmallvolumesampled.This solidhistogram)comparedwiththenormalizeddis-
confirmsanywaythatTypeIIQSOconstitutea tributionofthewholesample(321sources,dashed
significantfractionoftheAGNpopulation. histogram).
These findings can be interpreted in the
framework of the anti–hierarchical scenario,
5. StarformationseeninX-rayat
as described in the Introduction. Absorbed
high–z
QSO may be sources associated to massive
spheroidsexperiencingatthesame time rapid X–ray emission witnessing star formation
growthofthecentralblackholeandstrongstar eventsisduetoX–raybinariesandhotgasas-
formationactivity.In thiscase, the absorption sociated with superwinds and SNa remnants.
is not ascribed to circumnuclear matter, as in The main advantage of X–ray studies of the
the simplest version of the unification model, cosmicstarformationrateistoavoidproblems
butto the gasdistributedona muchwiderre- of obscuration, which severely affects optical
gion strongly polluted by star formation pro- observations. The price to pay is that, since
cesses. Indeed, the redshift distribution of the star forming galaxieshave luminosities in the
QSOpopulation,andthereforeoftheabsorbed range 1039–1042 erg s−1, it is difficult to de-
ones which represent a significant fraction of tectthemathighredshiftsevenin thedeepest
it, peaks at a redshift≥ 2, an epochwhen the X–ray surveys. The first normal star–forming
numberdensity of massive and powerfulstar- galaxyX–rayluminosityfunctionhasbeende-
burst galaxies is expected to be interestingly rivedbyNormanetal.(2004)inthecombined
high,asfoundrecentlywith near–IRobserva- ChandraDeepFieldNorthandSouth.There-
tions(Daddietal.2004,see§5). sultsshowanincreasingcosmicstarformation
The strong energetic feedback from both rate proportionalto (1+z)2.7, consistent with
nuclear activity and star formation in such otherstar formationdeterminationindifferent
largeobjects,isexpectedtoinhibitfurtherac- wavebands.However,thegalaxyXLFisdeter-
cretionandstarformationevents.Ontheother mined only at z ≤ 1, still below the expected
hand, smaller objects, where the feedback is maximumofthecosmicstarformationhistory.
lessefficient,areabletoretaingasthatcanbe On the other hand, other selection tech-
accretedsubsequently,allowingforepisodesof niques, like the one using the B–z vs z–K
obscuredaccretionandstarformationatlower color diagram (see Daddi et al. 2004b), are
redshifts. able to identify actively star forming galaxies
6 P.Tozzi&R.Gilli:LatestresultsfromtheCDFS
stackedimagesof22BzKselectedgalaxiesin
theCDFSinthesoftandthehardbands.This
canbeconsideredanimageofabout20effec-
tive Msof a typicalz ∼ 2 massive,star form-
inggalaxy.TheimagesareshowninFigure7.
In the soft band we detect a total 96±23 net
counts,which,foranaveragespectralslopeof
Γ = 2.1, correspondsto an average2–10keV
rest frame luminosityof 9×1041 erg s−1, im-
pliyingastarformationrateofabout∼190M
⊙
yr−1 (see Ranalli et al. 2003), in good agree-
ment with the estimate from the reddening–
correctedUVluminosities.Ontheotherhand,
we have no detection in the observed–frame
hardband,confirmingthesteepspectralslope
(hardnessratioHR<−0.5atthe2σlevel)and
thereforethenon–AGNnatureofthesesources
Fig.6. Normalized redshift distribution of the ab- (Daddietal.2004b).
sorbed QSO in the CDFS (54 sources, solid his- This findings confirm that stacking tech-
togram)comparedwiththenormalizeddistribution niquesonsourcesselectedinotherwavebands,
of all the sources with intrinsic X–ray luminosity are extremely useful in exploring the level of
L>1044ergs−1(67sources,dashedhistogram) X–ray emission from star formation at high–
z,anaspectwhichconstitutesoneofthemost
compellingscientific casesforthe nextgener-
ationX–rayfacilities.
6. LargeScaleStructureindeep
X–raysurveys
With the spectroscopic follow–up of X–ray
Fig.7.StackedX–rayimagesinthesoft(left)and sourcesdetectedintheCDFS,significantlarge
hard(right)bandof22BzKselectedgalaxiesinthe scale structure has been discovered both in
CDFSfield(seeDaddietal.2004b).
CDFSandCDFN,asshowninFigure8.Inpar-
ticular,twoprominentspikeshavebeenfound
intheCDFSatz = 0.67andz = 0.73with19
at z ≥ 1.4, with typical stellar masses larger sourceseach(Gillietal.2003),corresponding
than1011M⊙andaveragestarformationrateof to spikes found in the distribution of galaxies
≃ 200M⊙ yr−1.Theestimatednumberdensity in the K20 survey (an ESO–VLT optical and
of these z ∼ 2 star forming galaxies suggests near–infrared survey down to K ≤ 20 cover-
that we are peering into the formation epoch ingpartoftheCDFS,seeCimattietal.2002).
of massive early–type galaxies (Daddi et al. ComparingtheX–rayandopticalcatalogs,we
2004a). foundthatinthestructureatz=0.73,thefrac-
The expected X–ray emission from each tion of active galaxies is the same as in the
oneofthesehigh–z,starburstgalaxiesisbelow field,whileintheoneatz=0.67itishigherby
the flux limits of the deepest X–ray surveys. afactorof2.Wealsonotethatthestructureat
However, we can add together the images of z = 0.73includesa cluster ofgalaxies,which
theX–rayfieldsinthepositionsofthegalaxies, may have biased downwards the estimate of
to obtain a stacked image of all the optically theAGNfraction.Thisfindingconstitutesone
selected star forminggalaxies.We createdthe ofthefirsttantalizinghints(significantonlyat
P.Tozzi&R.Gilli:LatestresultsfromtheCDFS 7
Fig.8. Redshift distribution of X–ray sources in
CDFS(left)andCDFN(right,Gillietal.2005).
Fig.10. SpatialcorrelationfunctionsintheCDFS
(opencircles)afterremovalofthetwomajorspikes
compared with that measured in the CDFN (filled
circles,Gillietal.2005).
lengths measured between the two fields dis-
appears when the two most prominent spikes
intheCDFSareremoved(seeFigure10).This
shows that larger fields of view are needed to
kill the cosmic varianceand performa proper
investigation of the correlation properties of
Fig.9. Projected correlation functions of X–ray X–ray detected AGN. We mention two main
sources in the CDFS (open circles) and CDFN projects,theCOSMOSsurveywiththeXMM–
(filledcircles,Gillietal.2005). Newtonsatellite (Hasinger et al. in prepara-
tion), and the Extended CDFS with Chandra.
TheExtendedCDFScomplementstheoriginal
the2σlevel)thatlargescalestructurecantrig- 1MsexposurewithfourChandraACIS-Ifields
gernuclearactivity. with250kseach,bringingthelinearsizeofthe
field fromtheformer16arcminto32arcmin.
InvestigationofthespatialclusteringofX–
ray sources in both fields (Gilli et al. 2005) First results have been published by Lehmer
points out a significant difference in the cor- etal. (2005),whilespectroscopicfollowupis
relation lengths. We find r = 8.6 ± 1.2h−1 currently under way. The main goal is to bet-
0
MpcintheCDFS,andr = 4.2±0.4h−1Mpc ter understand the effect of large scale struc-
0
in the CDFN, with similarly flat slope (γ = tureontonuclearactivity,andtheevolutionof
1.33± 0.11 and 1.42± 0.07 respectively), as theclusteringofX–raysources.
shown in Figure 9. If we consideronly AGN,
we obtain higher correlation lenghts, in the
7. Conclusions
range5−10h−1Mpc.Sinceatz∼1late–type
galaxieshaveacorrelationlenghtof∼ 3.2h−1 We presented few selected topics which we
Mpcwhileearly–typegalaxieshave∼ 6.6h−1 find particularly relevant among the latest re-
Mpc (Coil et al. 2004), the high correlation sults from deep X–ray surveys. We can sum-
lengths measured for AGN in the CDFS are marizeourconclusionsasfollows:
consistentwiththeideathatatz∼1AGNwith
Seyfert–like luminosities are hosted by mas- – a population of strongly absorbed, possi-
sivegalaxies.Thedifferenceinthecorrelation bly Compton–thick AGN at z ∼ 1 is still
8 P.Tozzi&R.Gilli:LatestresultsfromtheCDFS
missing to the census of the X–ray sky; Ferrarese,L.,&Merritt,D.2000,ApJL,539,
thedetailedX–rayspectralanalysisoffaint 9
sources shows that we are detecting some Giacconi,R.etal.2002,ApJS,139,369
of them, and help us in obtaining a com- Giavalisco,M.etal.2004,ApJL,600,93
plete reconstruction of the cosmic accre- Gilli,R.,etal.2003,ApJ,592,721
tionhistoryontosupermassiveblackholes; Gilli,R.etal.2005,A&A,430,811
– we find several absorbed sources (the so– Granato,G.L.,etal.2004,ApJ,600,580
called TypeIIQSO) amongthe population Hasinger,G.,etal.2001,A&A365,L45
of bright AGN, possibily witnessing the Hasinger,G.,etal.2005,A&A,441,417
rapid growth of the super massive black Hickox, R.C., & Markevitch, M. 2005, ApJ,
holes associated to strong star formation inpress,astro-ph/0512542
events; Hopkins,P.F.,etal.2005,ApJ,630,716
– thankstostackingtechniques,wedetected Kauffmann, G., et al. 2004, MNRAS, 353,
the X–ray emission associated to massive 713
starforminggalaxiesatredshiftashighas Kormendy, J., & Richstone, D. 1995,
z∼2,thereforepeeringwithX–raysinthe ARA&A,33,581
epochofmassivegalaxyformation; LaFranca,F.,etal.2005,ApJ,635,864
– investigationoflargescalestructureinthe Lehmer,B.D.,etal.2005,ApJS,161,21
X–raydetectedAGNdistributionprovides Magorrian,J.,etal.1998,AJ,115,2285
tantalizinghintsofitseffectonnuclearac- Mainieri,V.,etal.2005a,MNRAS,356,1571
tivity. Studies of spatial correlation of X– Mainieri,V.,etal.2005b,A&A,437,805
ray sources require larger fields of view Marconi,A.etal.2004,MNRAS,351,169
to kill the cosmic variance and therefore Mart`inez–Sansigre, A., et al. 2005, Nature,
evaluateproperlytheevolutionoftheAGN 436,666
clusteringproperties. Merloni,A.2004,MNRAS,353,1035
Norman,C.,etal.2002,ApJ,571,218
Such observational findings are providing Norman,C.,etal.2004,ApJ,607,721
crucial information on the evolution of AGN Ranalli, P., Comastri, A., & Setti, G. 2003,
and galaxies. Present–day data are consistent A&A,399,39
with a scenario where nuclear activity and Rosati,P.etal.2002,ApJ,566,667
starformationprocessesdeveloptogetherinan Szokoly,G.P.,etal.2004,ApJS,155,271
anti–hierarchicalfashion. Tozzi,P.,etal.2006,A&A,submitted
Ueda,Y.,etal.2003,ApJ,598,886
Acknowledgements. P.T. thanks the Organizers for Worsley, M.A., et al. 2005, MNRAS, 357,
providingapleasantandstimulatingscientificenvi- 1281
ronmentduringtheworkshop. Zheng,W.,etal.2004,ApJS,155,73
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