Galaxy and Mass Assembly (GAMA): Fine filaments of galaxies detected within voids

0.51 MB·

by  Mehmet Alpaslan

Mon.Not.R.Astron.Soc.000,1–6(2014) Printed30January2014 (MNLATEXstylefilev2.2)
Galaxy and Mass Assembly (GAMA): Fine filaments of
galaxies detected within voids
Mehmet Alpaslan1,2, Aaron S.G. Robotham2, Danail Obreschkow2, Samantha Penny3,
Simon Driver1,2, Peder Norberg4 Sarah Brough7, Michael Brown3, Michelle Cluver5,
4
Benne Holwerda6, Andrew M. Hopkins7, Eelco van Kampen8, Lee S. Kelvin9,
1
0 Maritza A. Lara-Lopez7, Jochen Liske, 8, Jon Loveday10, Smriti Mahajan11,
2
Kevin Pimbblet3,12,13
n
a 1SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK
J 2International Centre for Radio Astronomy Research, 7 Fairway, The University of Western Australia, Crawley, Perth,
8 Western Australia 6009, Australia
2 3School of Physics, Monash University, Clayton, Victoria 3800, Australia
4Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
] 5Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa
O 6European Space Research and Technology Centre, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
7Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
C
8ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Mu¨nchen, Germany
h. 9Institut fu¨r Astro- und Teilchenphysik, Universita¨t Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
p 10Astronomy Centre, University of Sussex, Falmer, Brighton, BN1 9QH, UK
- 11School of Mathematics and Physics, The University of Queensland, St Lucia 4072, Australia
o 12Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
r 13Department of Physics and Mathematics, University of Hull, Cottingham Road, Hull, HU6 7RX, UK
t
s
a
[

1 30January2014
v
1
3 ABSTRACT
3 BasedondatafromtheGalaxyandMassAssembly(GAMA)survey,wereportonthe
7 discovery of structures that we refer to as ‘tendrils’ of galaxies: coherent, thin chains
1. of galaxies that are rooted in filaments and terminate in neighbouring filaments or
0 voids. On average, tendrils contain 6 galaxies and span 10 h−1 Mpc. We use the so-
4 called line correlation function to prove that tendrils represent real structures rather
1 than accidental alignments. We show that voids found in the SDSS-DR7 survey that
: overlap with GAMA regions contain a large number of galaxies, primarily belonging
v
totendrils.Thisimpliesthatvoidsizesarestronglydependentonthenumberdensity
i
X andsensitivitylimitsofasurvey.Wecautionthatgalaxiesinlowdensityregions,that
r maybedefinedas‘voidgalaxies,’willhavelocalgalaxynumberdensitiesthatdepend
a on such observational limits and are likely higher than can be directly measured.
1 INTRODUCTION shapesofvoidsaredefinedbythebaryonicmatterthatsur-
rounds them, namely filaments and walls. These features
The>1h−1 Mpcenvironmentofagalaxyinavoidissparse form the easily recognised large scale structure (LSS) in
relative to a galaxy in a dense cluster, making such sys- the Universe we see today. Galaxy surveys have conclu-
temsidealforstudyingtheevolutionofgalaxiesindependent sivelyshown,throughavarietyoffilamentfindingmethods
ofenvironmentalprocesses.Beingextremelyunderdensere- (e.g.,Doroshkevichetal.2004;Pimbblet2005;Colberg2007;
gions, voids are an easily recognised feature of the Cosmic Forero-Romero et al. 2009; Arago´n-Calvo et al. 2010; Mur-
Web.Theyspanbetween20to50h−1 Mpcandhaveatyp- phyetal.2011;Smithetal.2012;Alpaslanetal.2013),that
icalgalaxydensitythatis10%lessthanthatsurroundinga galaxies tend to coalesce in large linear structures that can
typicalfieldgalaxy(Panetal.2012).Thefewgalaxiesthat span up to 60 to 100h−1 Mpc or beyond (Gott et al. 2005).
doexistinvoidsaresubjecttodynamicsthatareuniqueto There is little consensus, however, as to an exact quantita-
these underdense regions (Blumenthal et al. 1992; Sheth & tive description of the properties of LSS, be it filaments or
van de Weygaert 2004). Voids serve as tools for constrain- voids.
ing cosmological parameters, or for testing the accuracy of
large cosmological simulations, as shown in Dekel & Rees Early studies of voids (Joeveer et al. 1978; Gregory &
(1994); Lavaux & Wandelt (2010); Park et al. (2012). The Thompson 1978; Kirshner et al. 1981; de Lapparent et al.
(cid:13)c 2014RAS
2 M. Alpaslan et al.
????) have been supplemented by more recent large galaxy regions, with intricate geometries (not just straight linear
redshift surveys (Colless et al. 2001; Abazajian et al. 2009) structures).Thelargestfilamentsareoftenbetterdescribed
that provide comprehensiveand complete pictures of voids, aslarge‘complexes’orconglomerationsoffilamentsthatin-
complemented by numerous void-finding techniques (e.g., tersectatamassivenode.Voidgalaxiesarefoundinunder-
El-Ad & Piran (1997); Hoyle & Vogeley (2002); Aragon- dense regions that surround filaments and show very little
Calvoetal.(2010).TheVoidGalaxySurvey(VGS,Kreckel clusteringsignalatallscalesbeyond5h−1Mpc.Tendrilsare
et al. 2012) is one such effort that has identified galaxies in substructures of galaxies that exhibit coherent linear struc-
voids from the SDSS-DR7 data. Beygu et al. (2013) have ture, but do not have the same levels of overdensity that
recently identified the system VGS 31, which is comprised filamentsdo;theyappeartoberootedinfilaments,branch-
ofthreegalaxiesinavoidthatappeartobealignedlinearly. ingintootherfilamentsorterminatinginvoids.InAlpaslan
ThesystemislinkedtogetherbyasinglemassivecloudofHI et al. (2013) we find that for a sub-sample of galaxies with
gas.Riederetal.(2013)haverecentlyshownthatsuchstruc- M (cid:62)1010.61M and m <19.4 mag, tendril galaxies con-
∗ (cid:12) r
turescanbeaccuratelyreproducedinsimulations,however; tain up to one quarter of the total stellar mass of galaxies
the existence of substructures in voids has been previously in the GLSSC.
predicted (including ‘low-density ridges’ of galaxies within Below, we provide an abridged description of the
voidsvandeWeygaert&vanKampen1993;Gottlo¨beretal. method used to generate the GLSSC but for full details,
2003). The detection of such prominent structure in a void we refer the reader to Alpaslan et al. (2013). The algo-
isanindicationthattheymeritfurtherstudy.Observation- rithm uses a three pass approach, where the first step is
ally, this means designing and conducting surveys that are to identify primary filaments composed of galaxy groups
expansive, deep, and highly spectroscopically complete on (Robotham et al. 2011). This is done by generating a min-
all scales. imal spanning tree (MST) across all groups in a volume-
GAMA (Driver et al. 2009, 2011) is an ongoing spec- limitedsamplewithgalaxycoordinatesconvertedintoaco-
troscopic survey that spans three equatorial fields centred moving Euclidean space. Any links greater than a distance
on the equator at α = 9h (G09), α = 12h, (G12) and α = b are trimmed. This value is chosen such that at least 90%
14.5h(G15)andtwosouthernfieldscentredonα=34deg, of groups with L (cid:62) 1011L are found in filaments. For
∗ (cid:12)
δ =-7deg(G02)andα=345degandδ =-32.5deg(G23). the second pass, all galaxies at a distance r from the near-
Each field measures 12×5 deg2, out to m < 19.8 mag est filament are associated with that filament, and referred
r
and z (cid:54) 0.5. The survey is highly spectroscopically com- to as ‘filament galaxies’ alongside galaxies within filament
plete (> 98%) and probes the galaxy stellar mass function groups.Thispopulationrepresentsthemostprominentand
down to M ≈ 108M , giving us an unprecedented view easily identifiable LSSs. The final pass involves generating
∗ (cid:12)
into the realm of low mass galaxies in the nearby Universe. an MST on all galaxies not in filaments, and trimming any
This, combined with up to 1050 targets per square degree, linksgreaterthanalengthq.Galaxiesleftintheremaining
makes GAMA an ideal body of data for searching for LSS structures are identifed as tendril galaxies, and all isolated
spanning many orders of magnitude in galaxy stellar mass. galaxies are referred to as void galaxies. The parameters r
Alpaslan et al. (2013) have used the GAMA survey to and q are selected such that the two point correlation func-
lookforLSSinthenearby(z(cid:54)0.213)Universe.TheGAMA tionξ(R)ofvoidgalaxiesshowsnosignalatlargedistances
LargeScaleStructureCatalogue(Alpaslanetal.2013;here- (givenbyR);quantitativelythisisdonebyselectingq such
(cid:82)
afterreferredtoastheGLSSC)classifiesasubsetof45,542 that R2ξ(R)dR is minimised. Tendril galaxies are there-
GAMA galaxies as belonging to filaments, voids, and intro- foredefinedsuchthattheymustbeatadistancerawayfrom
duces a third population of galaxies lying in what we refer the nearest filament galaxy, and at no further than q from
to as tendrils. These are described in detail in this paper. the nearest tendril galaxy. The GLSSC is generated with
Tendrilsarecoherentstructurestypicallycontainingupto5 b=5.75h−1 Mpc; r=4.12h−1 Mpc; and q=4.56h−1 Mpc.
or 6 galaxies that span roughly 10 h−1 Mpc and are rooted Thisdefitionofavoidgalaxyisdifferenttotheusualmethod
in filaments, either connecting to other filaments or termi- ofdefiningvoidgalaxiesbasedontheirpresencewitinalow
nating in voids. In Section 2 we briefly discuss the creation densityregion,whichallowsforpossibleclustering.Wealso
of the GLSSC and the data on which it is based. Section 3 generate a set of LSS catalogues for a set of GAMA mock
describes tendrils in detail, focusing on their structure and catalogues, which are described in Robotham et al. (2011)
howtheyarelocatedwithrespecttovoids.Throughoutthis and(Mersonetal.2013).Thesemockcataloguesare9mock
paper, we use H = 100 kms−1 Mpc−1, Ω = 0.25, and lightcones,generatedbypopulatinghaloesfromtheMillen-
0 M
Ω =0.75,andh=H0/100km−1s−1Mpc−1 =1isassumed nium Simulation (Springel et al. 2005) with the Galform
Λ
when calculating absolute magnitudes. semi-analyticmodel(Boweretal.2006)andaredesignedto
match the geometry and r-band survey luminosity function
of the GAMA survey (Loveday et al. 2012). In Alpaslan et
al.(2013)weshowthatthereisexcellentagreementbetween
2 LSS IN GAMA
observedfilamentarystructuresintheGLSSCandfilaments
The GLSSC classifies 45,542 galaxies with 0 (cid:54) z (cid:54) 0.213 extracted from the mock catalogues.
and M (cid:54) −19.77 mag in the G09, G12, and G15 fields as
r
belongingtodifferentkindsofLSSusingamodifiedminimal
spanningtreealgorithm.TheGLSSCestablishesthreehier-
3 TENDRILS
archiesinLSS:filamentsandvoids,andathirdintermediate
populationreferredtoastendrils.Filamentsarelargestruc- In Figure 1, we show the different galaxy populations in
tures of galaxies and groups that exist in highly overdense a 12×11 deg2 slice of the G12 field, with 0.15 (cid:54) z (cid:54) 0.2.
(cid:13)c 2014RAS,MNRAS000,1–6
Tendrils and voids in GAMA 3
Filaments Tendrils Void galaxies All
RA (deg)
174 178 182 186 174 178 182 186 174 178 182 186 174 178 182 186
0.2 0.2 0.2 0.2
0.18 0.18 0.18 0.18
z
0.16 0.16 0.16 0.16
0.15 < z < 0.2 0.15 < z < 0.2 0.15 < z < 0.2 0.15 < z < 0.2
-1 < Dec < 0 -1 < Dec < 0 -1 < Dec < 0 -1 < Dec < 0
Figure 1. A section of the G12 field with different galaxy populations shown in each panel. From left to right the populations shown
are galaxies in filaments with the filament minimal spanning tree (blue and cyan respectively); galaxies in tendrils (green); galaxies in
voids(red);andallthreepopulationsintheirrespectivecolours.
Theleftpaneldisplaysallfilamentgalaxiesinblue,withthe 215h−1 Mpcsidelength.Thereferencefunctionl (r)isthe
0
MSTlinksshownontopincyan.Tendrilgalaxiesareshown linecorrelationofanequalnumberofpoints,randomlydis-
in the next panel, void galaxies in the third, and all four tributedinanequivalentsurveyvolume.Thefactorf isthe
populationsofgalaxiesareshowntogetherintherightmost volume fraction of the GAMA cones within the cubic box.
√
panel. This final panel shows that tendrils generally branch The term f ensures that ∆l(r) is approximately indepen-
off from filaments and penetrate into voids, often bridging dent of the volume of the cubic box (according to Section
between two distinct filaments. Most importantly, tendrils 3.4 of (Obreschkow et al. 2013)).
arcacrossrangesofrightascensionanddeclinationaswellas We calculate ∆l(r) separately for filament, tendril and
redshift, suggesting that they are not statistical alignments voidgalaxies,aswellasallgalaxiescombined.Thiscalcula-
by chance or caused by redshift space distortions, but real tionisperformedindividuallyforeachofthethreeequatorial
structures-aclaimthatwillbesubstantiatedinSection3.1. GAMAfieldsaswellasfortheGAMAmockcatalogues.Fig-
ure 2 shows the functions ∆l(r) of the observed data (solid
lines)withmeasurementuncertainties(1σerrorbars).These
3.1 Filamentarity uncertainties arise from the limited number of independent
Fourier modes in the finite survey volume. They do not in-
We can quantify the filamentarity, defined as the linearity
cludecosmicvarianceduetocorrelatedLSS.Inturn,dashed
ofstructure,ofthefilament,tendrilandvoidgalaxiesusing
linesandtheirerrorbarsrepresentthemedianandstandard
their line correlation l(r). This correlation is an estimator
deviationof∆l(r)ofthenineGAMAmockcatalogs.These
of spatial statistics introduced by Obreschkow et al. (2013)
standarddeviationsnaturallyincludeboth,uncertaintiesin
to characterize the cosmic density field δ((cid:126)r) or its Fourier
the measurements of ∆l(r) and cosmic variance in the sur-
transform (FT) δˆ((cid:126)k). Since the isotropic power-spectrum vey volume. Since these error bars are much larger than
p(k)∝k−2(cid:82) dφdθ|δˆ((cid:126)k)|2 exhaustively exploits the informa- thoseoftheobserveddata,cosmicvarianceisthedominant
tion contained in the amplitudes |δˆ((cid:126)k)| of a homogeneous uncertainty in ∆l(r) across all considered scales.
andisotropicfield,Obreschkowetal.chosetoinvestigatethe Observed and simulated filaments, tendrils and voids
residualinformationinthephasefactors(cid:15)ˆ((cid:126)k)≡δˆ((cid:126)k)/|δˆ((cid:126)k)|. show very similar values for ∆l(r) on all scales across the
The inverse FT (cid:15)((cid:126)r) has a vanishing two-point correlation three GAMA fields. In all cases, filaments alone exhibit a
ξ (r); thus the lowest non-trivial correlations of (cid:15)((cid:126)r) are higher correlation ∆l(r) than all galaxies together. Void
2
three-point correlations. The line-correlation l(r) is defined galaxiesshowverylittleexcesslinecorrelationatallscales,
asasuitablynormalisedversionoftheisotropicthree-point indicating that for the magnitude limited sample in the
correlation of (cid:15)(r) for three points on a straight line, sepa- GLSSC,voidgalaxiesarefreefromstructure.Tendrilsshow
ratedbyr.Geometrically,itturnsoutthatl(r)measuresthe a clear intermediate line correlation, leaving no doubt that
degree of straight filamentarity on length scales r in about thosestructurescontainrealfilamentaritywellbeyondthat
thesamewaythatξ (r)measurestheclusteringonscalesr. ofarandompointset.Thisexampleeffectivelydemonstrates
2
Figure3inObreschkowetal.(2013)displaysl(r)foraseries the usefulness of higher-order statistical estimators such as
ofdensityfieldsandillustrateshowl(r)candistinguishlin- l(r) to characterize LSS.
earover-densitiesfromsphericalones,unlikeξ (r)andmore
2
robustly than the traditional three-point correlation.
Given the non-cartesian volume of the GAMA cones
3.2 Relationship with voids
andthevaryingnumberofgalaxiesineachsample,wechose
to measure the filamentarity via the excess line correlation In the GLSSC, a void galaxy is defined as a galaxy that is
√
∆l(r)=[l(r)−l (r)] f.Here,l(r)isthelinecorrelationof at least 4.56 h−1 Mpc away from the nearest galaxy that
0
thegalaxies,takenaspointsofequalmass,truncatedtodis- belongs to a tendril (and tendril galaxies themselves must
tances larger than 400h−1 Mpc and fitted in a cubic box of be at least 4.12 h−1 Mpc away from the nearest filament),
(cid:13)c 2014RAS,MNRAS000,1–6
4 M. Alpaslan et al.
RA (deg)
0 5 10 15 20 25 30 129 133 137 141 174 178 182 186 211.5 215.5 219.5 223.5
0.05 G09 AFillla gmaelanxti egsalaxies 0.05
0.04 TVeonidd grila glaaxlaiexsies 0.04 0.09 0.09 0.09
0.03 0.03
0.02 0.02
0.01 0.01 0.07 0.07 0.07
0 0
on l(r)Δ 0.040.05 G12 AFTVielolla nigdmda grelialan xgltai aegxlsaaielxasiexises 0.040.05
orrelati 0.03 0.03 0.05 0.05z 0.05
ne c 0.02 0.02
Excess li 00.01 00.01 0.03 0.03 0.03
0.05 G15 AFillla gmaelanxti egsalaxies 0.05
0.04 TVeonidd grila glaaxlaiexsies 0.04
0.03 0.03
0.01 0.01 0.01
0.02 0.02
0.01 0.01 G09 G12 G15
0 0
0 5 Co1r0relation sc15ale r (h-1M2p0c) 25 30 Figure 3. Each circled galaxy in this figure is a galaxy in the
GLSSCthatislocatedinsideavoididentifiedinSDSS-DR7data:
bluecirclesrepresentgalaxiesinfilaments,greencirclesrepresent
Figure2.Excesslinecorrelation∆l(r)shownforfilament(blue),
galaxies in tendrils, and red circles represent galaxies in voids.
tendril(green)andvoidgalaxies(red),aswellasallgalaxiescom- Wenotethatweshowthefull5◦ declinationforeachfieldinthis
bined(black)acrossallthreeequatorialGAMAfields.Thedashed
figure,resultinginincreasingprojectioneffectsathigherredshifts.
linesshowthemedian∆l(r)fortheGAMAmockcatalogues,for
whichweshowboththeformal∆l(r)errorastypicalerrorbars,
andthespreadbetweenmocklightconesassolidlineswithnoar- GAMAfields,exceptingthe−1◦ <δ<0◦regionofG09.We
rowheads.Thefunction∆l(r)roughlymeasurestheprobabilityof
definethevoidsthatareusedtocomparetoGAMAdataus-
findingthreeequidistantpointsseparatedbyronastraightline,
ing the maximal sphere radius parameter of each void from
inadensityfieldthatisanalogoustoGAMAbutstrippedofall
thePanetal.(2012)catalogue,andfind132galaxiesinthe
two-point correlation. Shaded regions show lengths where ∆l(r)
is unreliable due to the limited declination range of the GAMA GLSSC that lie within the inner two thirds of these voids.
cones(∼40h−1 Mpconaverage). These galaxies are displayed in Figure 3, where we show all
galaxies in the GLSSC for the three equatorial fields, and
circlethe132galaxiesgalaxiesthatarematchedtothe1130
so they exist in the most underdense regions of the GAMA GAMAgalaxiesintheSDSSvoids.Thecirclesarecoloured
fields. We do not determine where voids are, or their sizes, accordingtothestructuresthosegalaxiesaredeterminedto
only the galaxies that exist in very underdense regions. beintheGLSSC:bluecirclesaregalaxiesinfilaments,green
Recently, Pan et al. (2012) have released a catalogue circles are galaxies in tendrils, and red circles are galaxies
of void galaxies obtained from the SDSS-DR7 data using in voids. We find that only 25% of GLSSC galaxies with
a voidfinder similar to the one introduced by El-Ad & Pi- Mr < −19.77 mag found in SDSS voids are galaxies that
ran (1997). In Pan et al. (2012), voids are identified within wealsoidentifytobeisolated,withthevastmajority(64%)
a volume limited subsample of 120606 SDSS galaxies with alignedalongfinetendrils,andafurther11%associatedwith
Mr < −20.09 mag and z < 0.107. Galaxies are first deter- filaments.Forasubsampleof79galaxieswithMr <−20.06
mined to be within the field or not, using the third nearest mag, 15% are filament galaxies, 61% in tendrils and 24%
neighbour distance d and the standard deviation of this in voids. Penny et al., in prep, will examine, in detail, the
3
distance σ . Any galaxy with d = d +1.5σ > 6.3h−1 properties of the 1130 GAMA galaxies in SDSS voids.
d3 3 d3
Mpc is considered to belong to a cosmic filament or a clus-
terandisreferredtoasa‘wall’galaxy.Allothergalaxiesare
classifiedasfieldgalaxiesandareremovedfromthesample.
4 DISCUSSION AND SUMMARY
Wall galaxies are gridded into cells of 5 h−1 Mpc, and all
emptycellsareconsideredtobepossiblecentresofvoids.A Using the GLSSC, we have identified a new class of struc-
sphereisgrownfromeachcelluntilitisboundedbyfourwall turesinadditiontogroups,filamentsandvoids.Tendrilsex-
galaxies, and any two spheres with more than 10% overlap hibit a coherent linear structure, but do not have the same
are considered to belong to the same void. A sphere must levelsofoverdensitythatfilamentsdo,andeitherconnectfil-
havearadiusofatleast10h−1Mpctobeconsideredavoid. amentsorarerootedinonefilamentandterminateinavoid.
Using the SDSS-DR7 void catalogue from Pan et al. IntheGLSSCweidentify14150galaxiesthatliewithinsuch
(2012), we identify 1130 galaxies with 0.001 (cid:54) z (cid:54) 0.1 structures (compared to only 2048 void galaxies and 29503
and m <19.8 within voids that lie in the three equatorial galaxies in filaments). Tendril galaxies contain 23.8% of all
r
(cid:13)c 2014RAS,MNRAS000,1–6
Tendrils and voids in GAMA 5
stellarmassforamagnitudeandmasslimitedsampleofthe and the participating institutions. The GAMA website is
GLSSC; however they are tenuous enough to be difficult to http://www.gama-survey.org/.
detect using structure finders that rely on density smooth-
ing,oronlyconsidergalaxiesingroups.Thesemethodswill
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