Large-Scale Coherent Dipole Anisotropy ?

7 Pages·0.36 MB·English

by  S. Basilakos

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed1February2008 (MNLATEXstylefilev1.4)
Large-Scale Coherent Dipole Anisotropy ?
S.Basilakos1,2 & M. Plionis1
1 National Observatory of Athens, Lofos Nimfon, Thesio, 18110 Athens, Greece
2 Physics Dept., Universityof Athens, Panepistimiopolis, Greece
8 1February2008
9
9
1 ABSTRACT
n Wehavereanalyzedandcomparedthedipolesofthe1.2Jyand0.6Jy(QDOT)
a IRAS galaxy samples. We find strong indications from both samples for (a)
J significantcontributionstothegravitationalfieldthatshapestheLocalGroup

8 motionfromdepthsupto∼170h−1Mpcand(b)alarge-scalecoherenceofthe
2
dipoleanisotropy,indicationsprovidedmainlybythe factthatthedifferential

dipoles of large equal volume shells are aligned with the CMB dipole and
1
v exhibit significant dipole signals. The two IRAS dipoles are indistinguishable
1 within 50 h−1 Mpc and beyond ∼ 130 h−1 Mpc while the QDOT dipole,
7 havinga lowerflux limit, continuesgrowingwith respectto the 1.2Jy sample
2 up to ∼100 h−1 Mpc in agreement with Rowan-Robinsonet al (1990).
1
0
Keywords: cosmology: observations - galaxies: distances and redshifts -
8
9 infrared: galaxies - large scale structure of Universe
/
h 1 INTRODUCTION probably implies that the apparent dipole convergence
p is spurious, duetolack of adequately sampling thedis-
-
o ThepeculiarvelocityoftheLocalGroupofgalaxieswith tant density fluctuations. Only the optical Abell/ACO
r respect to the Cosmic Microwave Background (CMB) cluster sample is volume limited out to a large enough
st with u =622 km/sec towards (l,b)=(277◦,30◦) is a depth ( 240h−1 Mpc) to allow a more reliable deter-
:a well esLtaGblished fact (cf. Kogut et al. 1993). The most minatio≃n of Rconv which was found to be ≃ 160 h−1
v probablecauseforthismotionaswellasfortheobserved Mpc(Scaramella et al1991; Plionis &Valdarnini1991;
i
X peculiarmotionsofothergalaxiesandclusters(cf.Dekel Branchini&Plionis1996).Recently,thisresulthasbeen
r 1997 and references therein) is gravitational instability confirmedusingX-rayclustersamples,whicharefreeof
a (cf. Peebles 1980). This is supported by the fact that the various systematic effects from which the optical
thegravitationaldipole(acceleration) ofmanydifferent catalogues suffer (Plionis & Kolokotronis 1998).
samples of extra-galactic mass tracers is well aligned Ifthereisalinearbiasrelationbetweenthecluster,
withthegeneraldirectionoftheCMBdipole(cf.Yahil, the galaxy and the underlying matter density fluctua-
Walker & Rowan-Robinson 1986; Lahav 1987; Lynden- tions, as usually assumed (cf. Kaiser 1984), then the
Bell et al 1988; Miyaji & Boldt 1990; Rowan-Robinson galaxy dipole should also have similarly deep contribu-
etal1990; Straussetal1992; Hudson1993; Scaramella tions. In this study we reanalyse the 1.2 Jy and the
etal1991;Plionis&Valdarnini1991;Branchini&Plio- deeper QDOT 0.6 Jy IRAS galaxy dipoles, initially in-
nis1996).However,whatstillseemstobeunderdiscus- vestigatedbyStraussetal(1992) andRowan-Robinson
sion is from which depths do density fluctuations con- et al (1990) respectively, with the aim of investigating
tribute to the gravitational field that shapes the Local whetherthere are any such indications.
Groupmotion.Thelargest suchdepthisdefinedbythe
dipole convergence depth, R , which is that depth
conv
where the true gravitational acceleration converges to
2 IRAS GALAXY SAMPLES AND
its final value.The outcome of many studies, using dif-
SELECTION FUNCTIONS
ferentfluxormagnitudelimitedgalaxysamples,isthat
theapparentvalueofR differsfromsampletosam- Weuseinouranalysisthetwoavailableflux-limited60-
conv
ple,intherangefrom40to100h−1 Mpc,withastrong µmIRASsamples;onelimitedatS =1.2Jy(Fisher
lim
dependence to the sample’s characteristic depth. This et al 1995) and the other at S = 0.6 Jy (Rowan-
lim
(cid:13)c 0000RAS
2 S.Basilakos & M.Plionis
in-6 sampling rate (PSCz). It is evident that although
theQDOTsample is deeper,it samples thegalaxy dis-
tribution more sparsely than the1.2 Jy sample.
3 DIPOLE CALCULATION
Wedetermine the peculiar acceleration of Local Group
galaxiesbymeasuringmomentsoftheIRASgalaxydis-
tribution. The dipole moment: D = φ−1(r)r−2rˆ, is
calculatedbyweighingtheunitdirectioPnalvectorpoint-
ing to the position of each galaxy, with the gravita-
tionalweightofthatgalaxyandsummingoverallavail-
able galaxies with distances greater than 5 h−1Mpc
(on smaller scales the observed galaxies do not ad-
equately represent the true distribution; cf. Rowan-
Robinson et al 1990). Similarly the monopole term is:
M = φ−1(r)r−2.
Figure1.TheexpectedN(r)distribution,accordingtothe
IRASgalaxyluminosityfunction,forthe1.2Jy,theQDOT WPethenestimatethegravitational acceleration in-
0.6JyandthePSCz0.6JyIRASsamples. ducedontheLGfromthedistributionofIRASgalaxies
by:
Robinson etal 1990), which hasa1in 6sampling rate. V (r)= H◦Rconv D(r) (2)
g M( R )
The IRAS 1.2 Jy contains 5763 galaxies with b > 5◦ ≤ conv
| |
while theQDOT contains 2086 galaxies with b >10◦. (cf.Miyaji &Boldt 1990; Plionis et al1993). Usinglin-
| |
Note that although the two catalogues are not totally earperturbationtheory(cf.Peebles1980)andequation
independent,acrosscorrelation revealedonly105com- (2)wecanrelatetheLocalGrouppeculiarvelocitywith
mon galaxies (with δθ 0.6◦ and δcz 800 km/sec). theestimated acceleration by:
≤ ≤
To estimate the local acceleration field it is neces-
u (r)=β V (r) (3)
LG I g
sary to recover the true galaxy density field from the
observed flux-limited samples. This is done by weight- where βI = Ω0.6/bI and bI is the IRAS galaxy to un-
ingeachgalaxybyφ−1(r),wheretheselectionfunction, derlyingmass bias factor.
φ(r), is defined as the fraction of the galaxy number
density that is observed above the flux limit at some
3.1 Treatment of the IRAS galaxy data
distance r. Therefore
1 Lmax Duetosystematiceffectsandbiasespresentinthedata
φ(r)= Φ(L)dL (1) wehavetoperformvariouscorrectionstotherawdipole
n Z
h gi Lmin(r) estimates.Firstlyweneedtotreattheexcluded,dueto
where L (r) = 4πr2νS is the luminosity of a cirrusemission, galactic plane.Wedosobyextrapolat-
min lim
sourceatdistancercorrespondingtothefluxlimitS , ing to these regions the data from the rest of the unit
lim
ν = 60-µm and n is the mean galaxy number den- spherewith thehelp of a spherical harmonic expansion
g
h i
sity, given by integrating the luminosity function over ofthegalaxysurfacedensityfieldandasharpmask(cf.
thewholeluminosityrange,withLmin =7.5 107 h2L⊙ Yahil et al 1986; Lahav 1987). Secondly, about 4% of
×
sincelower luminosity galaxies arenot representedwell the sky is not covered by the catalogue and we apply
intheavailablesamples(cf.Rowan-Robinsonetal1990; to these areas a homogeneous distribution of galaxies
Fisher et al 1995), and Lmax = 1013 h2L⊙. Obvi- havingthemean weight, estimated from therest ofthe
ously, φ(r) is a decreasing function of distance because sky.Thirdly,duetodiscretenesseffectsandthesteepse-
asmallerfraction oftheluminosityfunctionfalls above lection function with depth we have an additive dipole
theflux limit at greater distances. term, the shot-noise dipole, for which we have to cor-
For the QDOT sample we used the Saunders et al rect our raw dipole estimates. Assuming Gaussianity,
(1990) luminosity function while for the IRAS 1.2 Jy the Cartesian components of the shot noise dipole are
weusedtheparameterisedselectionfunctionofYahilet equal (σ =σ =σ ) and thus σ2 =3σ2 (cf. Hud-
x y z 3D i,1D
al (1991). We have verified, however, that the two se- son 1993). Taking the coordinate system such that one
lection functions are indistinguishable from each other of the shot-noise dipole components is parallel to the
when applied to the same flux limit. In figure 1 we z-axis of the true dipole and we can attempt an ap-
present the φ(r) of the IRAS 1.2 and 0.6 Jy samples, proximatecorrection oftherawdipoleaccordingtothe
for the 1-in-6 (QDOT) as well as for the unavailable 6- following model:
(cid:13)c 0000RAS,MNRAS000,000–000
Large-Scale Anisotropy 3
Figure 2. Difference between the Monte-Carlo and the
Straussetal(1992) 1-dshot-noiseestimates,forbothIRAS
1.2JyandQDOTsamples.
D =D σ /√3 (4)
cor raw 3D
−
Note that this correction model although more severe
than the usual D2 = D2 σ2 model, it provides
cor raw− 3D
qualitativelysimilardipolecorrections.Wechoose,how-
ever, to use this model in order to be conservative and
toobtainasortoflowerlimittotheresultingdipole,as
far as the shot-noise correction is concerned, and thus
via eq.(3) an upperlimit to the estimated cosmological Figure3.Comparisonoflocal1.2JyIRASgalaxypeculiar
β parameter (see section 4.4). velocities provided by our model and by the full iterative
To calculate σ we use two methods; a Monte- algorithmwithincz∼3600km/sec(seetext). Thediagonal
3D
Carlo simulation approach in which we randomise the linecorrespondstovrecons·ˆr=Vbulk·ˆr.
angular coordinates of all galaxies while keeping their
distance, and thus their selection function, unchanged
v(r) ˆr d3r 0 and the last term of eq.(5) is domi-
while the second method is the analytic estimation of · ≈
Rnated by the LG term; we thus obtain that in the LG
Straussetal(1992):σ2 φ−1r−4(φ−1+1).Figure2
3D ≃ i i i frame (ie., when using cz =H◦r), structures in the di-
showsthedifference(inveloPcityunits)betweenthetwo
rection of our motion appear at aredshift smaller than
shot-noiseestimates.Itisevidentthatthetwomethods
theirtruedistanceintheCMBframeandthustheywill
give equivalent results although due to the statistical
artificially enhance the amplitude of the gravitational
nature of the first method we believe that it performs
dipole.
better on large depths, where the number density of
However, many studies indicate that local galax-
IRASgalaxies is verylow.
ies have peculiar velocities not randomly oriented but
ratherparticipatinginacoherentflow(bulkmotion)to-
3.2 z to 3d frame correction getherwiththeLocalGroup(ie.,v(r) v(0))withinat
≈
leastavolumeofradius 5000km/sec(cf.Lynden-Bell
The final but essential correction is to transform red- ∼
etal1988,Dekel1994;1997,Strauss&Willick1995).If
shiftsto3-ddistancesinordertominimisethesocalled
so, it would be reasonable to evaluate the IRAS dipole
‘Kaiser’ effect (Kaiser 1987). This effect can be under-
in theLG frame, since in this case cz H◦r. However,
stoodbynotingthatthedistributionofgalaxies inred- ≈
this is not absolutely true since there should exist also
shift space differs from that in real comoving space by
a velocity component due to the local, non-linear, dy-
a non-linear term:
namics acting between nearby galaxies and/or clusters
cz =H◦r+(v(r) v(0)) rˆ (5) of galaxies. We can therefore view the galaxy peculiar
− ·
velocitiesasconsistingoftwovectorcomponents;abulk
where v(0)( u ) is the peculiar velocity of the Lo-
≡ LG flow and a local non-linear term:
cal Group and v(r) the peculiar velocity of a galaxy
at position r. If v(r) had random orientation, then v(r)=V (r)+v (r) (6)
bulk nl
(cid:13)c 0000RAS,MNRAS000,000–000
4 S.Basilakos & M.Plionis
Inserting eq.(6) in eq.(5) and assuming that v(r) ˆr and lower dipole bounds respectively, differs very little
· ≈
V (r) ˆr , ie., that the dominant component is that and thus the z to 3d frame correction does not have a
bulk
· ⋆
of the bulk flow we can use the observed bulk flow major consequence in our main dipole results. We do
profile, as a function of distance, given by Dekel (1994; however investigate, in section 4.2, possible systematic
1997) and combined with that of Branchini, Plionis & effects that could be introduced by the frame transfor-
Sciama(1996)tocorrectthegalaxyredshifts.Thezero- mation procedure in our IRAS-CMB dipole alignment
point, V (0), and the direction of the bulk flow is results.
bulk
estimated applying eq.(6) at r = 0 and assuming, due
to the “coldness” of the local velocity field (cf. Peebles
1988), that vnl(0) vinf (where vinf is the LG infall 4 MAIN RESULTS
≃
velocity to the Virgo Supercluster). Using the average
Infigure4awepresentthetwoIRASdipolesinredshift
valuefrom theliterature,ie.v 170km/sec,weob-
inf
tain V (0) 500 km/sec toward≃s (l,b) (276◦,15◦). space. We observe that they are consistent, although
bulk ≃ ≃ up to 50 h−1 Mpc the 1.2 Jy dipole is systemati-
Wetestourmodelbycomparingpeculiarvelocities ∼
cally higher than the QDOT dipole. However, once we
that it provides with those resulting from the full dy-
correctforredshiftspacedistortions(figure4b)thetwo
namical algorithm (kindly provided by Dr. Enzo Bran-
correcteddipolesalmostcoincidewithin50h−1Mpc,as
chini)which estimates, usinglinear theory,thegravita-
theyshouldsincebothsamples,duetotheirrelativelow
tional acceleration at the position of each galaxy and
flux limits, are good tracers of the Local Universe. Up
thenrecoversthereal-spacegalaxydistancesbysolving
to 130h−1 Mpctheamplitudesofthetworeal-space
iteratively thegeneralised Hubblelaw of eq.5(cf. Yahil ∼
dipoles deviate with the QDOT being larger than the
et al 1991; Strauss et al 1992). In figure 3 we present
IRAS 1.2Jy, which is to be expected since the QDOT
this comparison for relatively local galaxies in regions
sample has a lower flux limit and thus it can ‘see’ the
of δρ/ρ<1 (since at dense regions thenon-linear com-
distantmatter fluctuationsbetterthanthe1.2 Jy sam-
ponent that we neglect in our model will dominate the
ple. Beyond 130 h−1 Mpc, however, the two IRAS
galaxy peculiar velocity). We find a good correlation ∼
dipoles coincide again. As can be seen from figure 4,
within cz 4000 km/sec which is in fact the region
∼ redshiftspacedistortionsenhancesignificantlythereal-
where such corrections can affect the dipole. The cor-
space dipole (by 12%) only within 50 h−1 Mpc.
relation, at larger distances, progressively fades away ∼ ∼
Theuncertaintiesofthevelocityfieldmodel,probedby
since the bulk flow amplitude is low at such distances
theMonte-Carlosimulationsdiscussedpreviously,intro-
andthegalaxypeculiarvelocitiesaredominatedbythe
duceasmallscatterinthereal-spacedipoleasindicated
distant local dynamics. In any case at such distances
by the errorbars in figure 3b. Using the Lauer & Post-
we do have v(r) ˆr d3r 0 and thus redshift space
· ≈ man (1994) bulk-flow leaves unaltered our main dipole
distortions aRre dominated by the LG term in eq.(5) for
results with only a small ( 40 km/sec) amplitude de-
which we do indeed correct the galaxy redshifts. Note ∼
crease.
thatwehaveverifiedthattheamountofscatterseenin
figure3iswellreproducedfromourmodelifweinclude
arandomlyorientednon-linearvelocitycomponentwith 4.1 Evidence for >100 h−1 Mpc dipole
(vnl ˆr)2 1/2 320 km/sec. contributions
h · i ≈
Wehavefurthertestedtherobustnessoftherecov-
Between 150 180 h−1 Mpc there is an apparent
ered real-space distribution by performing 200 Monte- ∼ −
amplitude bump, seen in both redshift and real-space
Carlo simulations in which we vary v (and there-
inf IRASdipoles.Thisbumpisaccompanied bya 5◦ de-
fore also the amplitude and slightly the direction of ∼
creaseofthemisalignmentanglebetweenthetwoIRAS
V (0)) as well as the amplitude of V (r) for all
bulk bulk
dipoles and that of the CMB, the overall misalignment
r’s, by randomly sampling a Gaussian having as mean
angles at r = 200 h−1 Mpc being 23◦ and 35◦
(µ) thenominal velocity valuesandσ=2µ/3. Further- ∼ ∼
for the 1.2 Jy and QDOT samples, respectively. These
more,weinvestigatehowourresultschangewhenusing
factssuggestthatthisdipoleamplitudebumpisnotdue
thebulk-flowdirection of Lauer& Postman (1994); ie.,
(l,b) (343◦,52◦) with V (r) = 650 km/sec for to shot-noise uncertainties but rather it is an intrinsic
bulk
r 13≃0 h−1 Mpc. | | effect, indicating the existence of contributions to the
≤ Local Group motion from such large scales. Such con-
Finally, we would like to point out that it so hap-
tributions cannot be accurately determined, however,
pensthattheIRASdipole,estimatedineithertheLGor
from the present flux-limited samples and deeper sam-
the CMB frames, which should provide a sort of upper
plesarerequiredfor sucha task(see Kolokotronis etal
1996). TofurtherinvestigatetheseprobabledeepIRAS
⋆ in a sense we assume that the vector average of vnl(r)· dipole contributions we estimate the differential dipole
ˆr over a whole sky distribution of galaxies is ≃ 0; not an in large equalvolumeshells (seePlionis, Coles & Cate-
unreasonableassumptioninthelimitofdensesampling. lan 1993 for an earlier attempt in z-space). Weinvesti-
(cid:13)c 0000RAS,MNRAS000,000–000
Large-Scale Anisotropy 5
Table 2.QDOTdipoleresults(asinTable1).
h−1Mpc Ngal S/N l◦ b◦ δθcmb pmc pf
5-110 1267 2.3±0.2 232.5 30.3 38±3 0.016 0.011
110-139 210 -0.1±0.2 98.8 36.6 113±26 0.89 0.71
139-159 96 1.2±0.2 271.1 -12.7 43±9 0.116 0.134
159-175 66 1.2±0.1 272.1 31.6 5±2 0.004 0.002
175-188 53 -0.9 164.4 -73.7 125 0.55 0.78
188-200 31 0.7 318.4 -15.6 60 0.55 0.25
S D
= raw cos(δθ ). (7)
N σ cmb
3D
for the case of δV 5.5 106 h−3 Mpc3 (six shells).
≃ ×
We observe that for the QDOT sample there are 3
shells with relatively small misalignment angles and
dipolesignaltonoiseratios>1,thedeepestshellbeing
[159 175]h−1Mpc,inwhichδθ 5◦.Forthe1.2Jy
cmb
− ∼
sample,whichalthoughshallowerhasabettersampling,
wehavesmall misalignment angles(δθ < 27◦)inthe
cmb
same shells but also in a deeper shell (18∼8 200 h−1
−
Mpc). Out of these four aligned shells there are signifi-
cantdipolecontributions(S/N >1)onlyinthreewhile
theprobability thatthesealignmentsarerandomisex-
tremely low. The formal probability that two vectors
are aligned within δθ is given by the ratio of the solid
anglewhich corresponds toδθ,tothesolid angle ofthe
whole sphere, ie., p (δθ)=sin2(δθ/2).
f
Wecannowestimatethejointprobabilityofalign-
ment,withintheobservedδθ ,ofN independentvec-
cmb
tors, which is given by:
Figure 4. (a) IRAS 1.2 Jy (continuous line) and QDOT
(dashed line) dipoles in redshift-space. (b) the correspond- N
ing dipoles inreal-space. The errorbars(shown only forthe PN p (δθ)/p (90◦) (8)
i i
≈
QDOT dipole)are estimated fromMonte-Carlo simulations Y
i=1
ofthevelocityfieldmodel(seetext).
Between three shells (first, fourth and sixth) the IRAS
galaxycorrelation functioniszero,duetothelargedis-
tancesinvolved,andconsequentlytheshellscanbecon-
Table1.Differential1.2JyIRASdipoledirections,thecor-
sidered independent. Due, however, to the vicinity and
responding misalignment angles with respect to the CMB
dipole, the dipole signal to noise ratio and probabilities of thereforethepossiblecorrelationbetweenthethirdand
alignment within δθ (see text for complete definition). The fourthQDOTshellwewillconsidertheirjointprobabil-
errorbarsareestimatedfromMonte-Carlosimulationsofthe ities as limits. Therefore, we havethat the joint proba-
velocity field model, with the last two shells having no cor-
bility of alignment between the CMB and the differen-
responding uncertainties because we take Vbulk(r) = 0 for
r>170h−1 Mpc(seetext). tialIRASequal-volumedipoledirections(forthosewith
significant dipole signal S/N >1) is:
h−1Mpc Ngal S/N l◦ b◦ δθcmb pmc pf
2 10−4< P2,3 < 8 10−4
5-110 4329 4.7±0.2 246.0 35.5 26±3 0.003 0.051 × ∼ QDOT∼ ×
110-139 390 0.6±0.2 311.8 84.1 55±17 0.522 0.213 P3 2 10−4 .
139-159 220 1.7±0.2 268.9 8.5 22±4 0.026 0.036 1.2Jy ≃ ×
159-175 128 0.6±0.2 307.1 31.5 26±7 0.073 0.051 for theQDOT and IRAS1.2 Jy samples, respectively.
175-188 72 0.0 96.6 61.4 89 0.655 0.492
188-200 59 1.6 282.8 43.2 14 0.029 0.016
4.2 Test for systematic alignment errors
gate shell sizes ranging from 2.5 106 to 8.2 106 h−3 The observed differential dipole alignments could in
× ×
Mpc3 which give qualitatively similar results. In tables principle be due to errors in the correction used to re-
1 & 2 we present the differential dipole directions and cover the 3-d frame in which we measure the dipole.
misalignmentangleswithrespecttotheCMBdipoleas For example, if redshift errors especially at large dis-
wellasameasureofthesignificanceofthedipoleofeach tances, were the sampling rate is low, were comparable
individual shell, given by: to a significant fraction of the LG velocity, then using
(cid:13)c 0000RAS,MNRAS000,000–000
6 S.Basilakos & M.Plionis
the3-dgalaxydistancesestimatedfromeqs(5),(6)and
v(0)=622km/sec, wecould artificially produceafalse
alignment of the distant shells differential dipole with
that of the CMB. We therefore address the question
of which are the dipole alignments induced totally due
to our frame transformation procedure ie., for the ex-
treme case where there is no intrinsic dipole and no
LG peculiar velocity. We run 5000 Monte-Carlo simu-
lations in which we destroy the intrinsic IRAS galaxy
dipoleaswellasredshiftspacedistortionsbyrandomis-
ing the angular coordinates of the galaxies while keep-
ingtheirdistancesandthereforetheirselectionfunction
unchanged. On this intrinsically random galaxy distri-
bution we apply our z to 3d space transformation and
then measure the artificially induced differential dipole
alignments due to the frame transformation itself. The
couplingbetweenthespacedistortion andtheselection
function results in a non-trivial alignment behaviour.
In the first shell we observe anti-alignments while at
more distant shells the artificial alignment effect does
Figure5.Contourplotofthesmoothed1.2JyIRASgalaxy
appear. For example at the last shell the median δθ is
distributioninrealspaceandonthesupergalacticplane.
67◦ insteadof90◦.However,itisimpossibletocreate
∼
theobservedIRASdipolealignments ifthereisnotrue
(Shapley 1930; Scaramella et al 1989; Raychaudhury
signalpresent.Wequantifythisbymeasuringtheprob-
1989).
ability, p (δθ), of observing in our Monte-Carlo simu-
mc
To investigate further the possible cause of the
lationsdipolealignmentsaslargeastheobservedIRAS
presentIRASdipoleresultswehavesmoothedtheIRAS
differential dipole alignments. For the shells of interest
1.2 Jygalaxy distribution in a403 cubewith acell size
we find that this probability is low and comparable to
of10h−1 MpcusingaGaussian with smoothingradius
theexpectedp ,whichimpliesthattheframe transfor-
f equal to one cell and weighting each galaxy by φ−1.
mation uncertainties cannot induce the observed IRAS
Duetothecouplingbetween theselection functionand
dipole alignments (see the corresponding values of p
mc
theconstant radiussmoothing, wecorrecttheresulting
and p in thetables).
f
smoothed distribution for a distance dependent effect,
We conclude that the differential dipole directions
which we quantified using N-body simulations (details
arenotrandomlyorientedwithrespecttotheCMBand
willbepresentedinaforthcomingpaper).Infigure5we
thereforewedonotonlyhaveindicationsforsignificant
present the smoothed IRAS 1.2 Jy galaxy distribution
dipole contributions from large depths but also for a
onthesupergalacticplane(of10h−1Mpcwidth)within
coherent anisotropy extendingtothese large scales.
170 h−1 Mpc. The contour step is 0.4 in overdensity
while the δρ/ρ = 0 level appears as a thick continuous
line.
Well known structures appear in this plot; the
4.3 Possible cause of the large-scale IRAS
largest and most evident is the Shapley concentration
dipole contributions
locatedat(X ,Y ) ( 120,60),thePerseus-Pisces
sup sup
≈ −
It is interesting that stronge evidence exist for deep supercluster at (X ,Y ) (60, 40), the Coma su-
sup sup
≈ −
dipole contributions from the available galaxy cluster percluster at (X ,Y ) ( 20,70), the Ursa-Major
sup sup
≈ −
data. Contributions up to 20% 30% of the total supercluster at (X ,Y ) (100,100), the Pisces-
sup sup
∼ − ≈
cumulativeopticalandX-rayclusterdipole,werefound Cetus supercluster at (X ,Y ) (50, 140) while
sup sup
≈ −
from 140 160 h−1 Mpc depths (Scaramella et al the Great Attractor (Hydra-Centaurus complex ?), at
∼ −
1991; Plionis & Valdarnini 1991; Branchini & Plionis (X ,Y ) ( 30,30) appears in the foreground of
sup sup
≈ −
1996; Plionis & Kolokotronis 1998). Similar coherence theShapleyconcentration.Furthermore,figure5isvery
of the differential dipoles in equal volume shells was similartothecorrespondingAbell/ACOclusterdensity
also found in galaxy cluster case (Plionis & Valdarnini field(cf.Tullyetal1992;Branchini&Plionis1996,their
1991;Plionisetal1993).Thesestudieshaveshownthat figure 3), and we therefore obtain a consistent picture,
thecauseofthedeepdipolecontributionsshouldbeat- frombothIRASgalaxyandAbell/ACOclusterdata,in
tributed mostly to the Shapley concentration, a huge which the Shapley concentration is the most probable
mass overdensity located at 140 h−1 Mpc in the causeofthedeepdipolecontributionswhilethegeneral
∼
general direction of the Hydra-Centaurus supercluster alignment of the Great Attractor, Perseus-Pisces and
(cid:13)c 0000RAS,MNRAS000,000–000
Large-Scale Anisotropy 7
Shapleysuperclustersismostprobablythecauseofthe BranchiniE.,PlionisM.,SciamaD.W.,1996,ApJ,461,L17
apparent coherence of theIRASgalaxy dipole. DekelA.,1994,ARA&A,32,371
DekelA.,1997,inL.daCostaed.‘GalaxyScalingRelations:
Origins,Evolution&Applications’,Springer,inpress
4.4 Ω◦0.6/bI from the IRAS dipoles Fisher,K.B.,Huchra, J.P.,Strauss, M.A.,Davis,M.,Yahil,
A.&Schlegel,D.,1995,ApJS,100,69
Usingthereal-spacedipoleresultsandequation(3),we
HudsonM.J.,1993,MNRAS,265,72
can estimate the density parameter β . However,
IRAS KaiserN.,1984,ApJ,284,L9
the value obtained should be considered rather as an
KaiserN.,1987,MNRAS,227,1
upper limit since the deep contributions to the dipole, Kogutetal,1993,ApJ,419,1
for which we do have strong indications, are probably Kolokotronis,V.,Plionis,M.,Coles,P.,Borgani,S.,Moscar-
notfullyrevealedbythepresentsamples(seeKolokotro- dini,L.,1996,MNRAS,280,186
nis et al 1996). Taking into account the scatter among LahavO.,1987,MNRAS,225,213
thetwoIRASsamples,theamplitudevariationsatlarge LauerT.R.&Postman,M.,1994,ApJ,425,418
depthsandtheuncertaintiesofthevelocitymodelused Lynden-Bell, D., Faber, S.M., Burstein, D., Davies, R.L.,
Dressler, A., Terlevich, R.J. & Wegner, G. 1988, ApJ,
to recover thereal-space galaxy distances, we find:
326,19
β < 0.78( 0.1), (9) Lynden-Bell, D., Lahav O. & Burstein, D., 1989, MNRAS,
IRAS
∼ ± 241,325
in agreement with the QDOT analysis of Rowan-
MiyajiT.,BoldtE.,1990, ApJ,353,L3
Robinson et al (1990) but slightly larger, although Peebles,P.J.E.,1988,ApJ,332,17
within 1σ, than the 1.2 Jy results of Strauss et al Peebles P.J.E.,1980. The LargeScale Structure of the Uni-
(1992). This value of βI implies either that Ω◦< 0.66 verse,PrincetonUniversityPress,PrincetonNewJersey
for bIRAS =1 or Ω◦ 1 for bIRAS> 1.28. ∼ Plionis,M.&Valdarnini,R.,1991, MNRAS,249,46
≃ ∼ PlionisM.,ColesP.,CatelanP.,1993,MNRAS,262,465
Plionis M. & Kolokotronis, V., 1998, ApJ, 500, June 10th
issue.
5 CONCLUSIONS
Rowan-Robinson M., Lawrence A., Saunders W., Craw-
Using a consistent analysis procedure we find that ford J., Ellis R.S., Frenk C.S., Parry I., Xiaoyang X.,
within 50h−1 Mpc, both the1.2Jy and 0.6Jy (QDOT) Allington-SmithJ.,EfstathiouG.,KaiserN.,1990, MN-
RAS,247,1
IRASsamples,giveidenticaldipoleresultswhilebeyond
Raychaudhury, S.1990,Nat.,342,251
this depth the QDOT dipole increases substantially up
SaundersW.,Rowan-RobinsonM.,LawrenceA.,Efstathiou
to 100 h−1 Mpc, in agreement with Rowan-Robinson
G., Kaiser N., Ellis R.S. & Frenk, C.S., 1990, MNRAS,
et al (1990). Furthermore there are significant indica-
242,318
tionsfor(a)dipolecontributionsfromdepths 170h−1
∼ Scaramella,R.,Vettolani,G.,Zamorani,G.,1991,ApJ,376,
Mpc,inagreement withotherrecentlarge-scale studies L1
(cf.Plionis&Kolokotronis1998)and(b)acoherenceof Scaramella, R., Baiesi-Pillastrini, G., Chincarini, G., Vet-
the dipole anisotropy extending to similar depths. The tolani,G.&Zamorani,G.1989,Nat.,338,562
most probable cause of these deep dipole contributions Shapley,H.1930,HarvardObs.Bull.,874,9
is the Shapley mass concentration, while of the dipole StraussM.A.&Willick,J.A.1995,Phys.Rep.,261,271
coherence is the general alignment, on the supergalac- Strauss M.A., Yahil A., Davis M., Huchra J.P., Fisher K.,
1992ApJ,397,395
tic plane, of the Perseus-Pisces supercluster, the Great
Tully, R.B., Scaramella, R., Vettolani, G. & Zamorani, G.,
Attractor and theShapley concentration, which span a
1992, ApJ,388,9
range of 200h−1 Mpc.
∼ YahilA.,Walker, D.,Rowan-Robinson,M.,1986, ApJ,301,
A similar study of the complete (6 in 6) IRAS
L1
0.6 Jy sample (PSCz) should give better indications of YahilA.,StraussM.,DavisM.,HuchraJ.P.,1991,ApJ,372,
these results, although the overall amplitude of the ef- 380
fect couldbeprobablyestimated byadeepercatalogue
(limited at a lower fluxlimit).
ACKNOWLEDGEMENTS
Wethank Enzo Branchini for providing uswith the re-
constructed IRAS 1.2 Jy galaxy peculiar velocities.
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