Galaxy ecology: groups and low-density environments in the SDSS and 2dFGRS

1.1 MB·English

by  Michael Balogh

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed2February2008 (MNLATEXstylefilev2.2)
Galaxy ecology: groups and low-density environments in
the SDSS and 2dFGRS
Michael Balogh1, Vince Eke1, Chris Miller2, Ian Lewis3, Richard Bower1,
Warrick Couch4, Robert Nichol2, Joss Bland-Hawthorn5, Ivan K. Baldry6,
Carlton Baugh1, Terry Bridges5, Russell Cannon5, Shaun Cole1, Matthew Colless7,
Chris Collins8, Nicholas Cross7,9, Gavin Dalton4, Roberto De Propris4,
4
0 Simon P. Driver9, George Efstathiou10, Richard S. Ellis11, Carlos S. Frenk1,
0
2 Karl Glazebrook6, Percy Gomez2, Alex Gray12, Edward Hawkins13, Carole Jackson7,

n Ofer Lahav10, Stuart Lumsden14, Steve Maddox13, Darren Madgwick10, Peder Norberg15,
a
J John A. Peacock16, Will Percival16, Bruce A. Peterson7, Will Sutherland16,

5 Keith Taylor11

3 1Department of Physics, University of Durham, South Road, Durham DH13LE, UK
v 2Department of Physics, Carnegie Mellon University,5000 Forbes Avenue, Pittsburgh, PA 15213, USA
9 3Astrophysics, Nuclear and Astrophysics Laboratory, KebleRoad, Oxford OX1 3RH, UK
7 4School of Physics, Universityof NewSouth Wales, Sydney 2052, Australia
3 5Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 1710, Australia
1 6Department of Physics and Astronomy, Johns Hopkins University,3400 North Charles Street, Baltimore, MD 21218-2686 USA
1 7Research School of Astronomy & Astrophysics, The Australian National University,Weston Creek, ACT 2611, Australia
3 8Astrophysics Research Institute, Liverpool John Moores University,Twelve Quays House, Egerton Wharf, Birkenhead, L14 1LD, UK
0 9School of Physics and Astronomy, Universityof St. Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
/ 10Institute of Astronomy, Universityof Cambridge, Madingley Road, Cambridge
h 11California Institute of Technology, Pasadena, CA, 91125-2400, U.S.A.
p 12Department of Computer Science, Carnegie Mellon University,5000 Forbes Avenue, Pittsburgh, PA 15213, USA
- 13School of Physicsand Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
o
14Department of Physics & Astronomy, EC Stoner Building, Leeds LS2 9JT, UK
r
t 15ETHZ Institut fu¨r Astronomie, ETH H¨onggerberg, CH-8093, Zu¨rich, Switzerland
s 16Institute forAstronomy, Universityof Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
a
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ABSTRACT
We analyse the observed correlation between galaxy environment and Hα emission
line strength, using volume-limited samples and group catalogues of 24968 galaxies
at 0.05 < z < 0.095, drawn from the 2dF Galaxy Redshift Survey (MbJ < −19.5)
and the Sloan Digital Sky Survey (Mr < −20.6). We characterise the environment
by 1) Σ5, the surface number density of galaxies determined by the projected dis-
tance to the 5th nearest neighbour; and 2) ρ1.1 and ρ5.5, three-dimensional density
estimatesobtainedbyconvolvingthegalaxydistributionwithGaussiankernelsofdis-
persion 1.1 Mpc and 5.5 Mpc, respectively. We find that star-forming and quiescent
galaxiesformtwodistinctpopulations,ascharacterisedbytheirHαequivalentwidth,
W0(Hα). The relative numbers of star-forming and quiescent galaxies varies strongly
andcontinuouslywithlocaldensity.However,thedistributionofW0(Hα)amongstthe
star-forming population is independent of environment. The fraction of star-forming
galaxies shows strong sensitivity to the density on large scales, ρ5.5, which is likely
independentofthetrendwithlocaldensity,ρ1.1.Weusetwodifferently-selectedgroup
catalogues to demonstrate that the correlation with galaxy density is approximately
independent of group velocity dispersion, for σ = 200–1000kms−1. Even in the low-
est density environments,no more than ∼70 per centof galaxiesshow significant Hα
emission.Basedontheseresults,weconcludethatthepresent-daycorrelationbetween
star formation rate and environment is a result of short-timescale mechanisms that
take place preferentiallyat highredshift, such asstarbursts induced by galaxy-galaxy
interactions.
(cid:13)c 0000RAS
Key words: galaxies: clusters: general, galaxies: evolution, galaxies:interactions
2 Balogh et al.
1 INTRODUCTION assumetherateatwhichgasfallsontoagalaxydependson
itsenvironment(e.g.Cole et al.2000;Hernquist & Springel
The galaxy population today can be effectively described 2003). In these models, satellite galaxies do not have their
as a combination of two distinct types. The first are red, own supply of hot gas to replenish gas in the disk used to
morphologically early-type galaxies with little or no cur- form stars. Thus, star formation will gradually decline in
rentstarformation;theremainderareblue,late-typegalax- anygalaxywhichispartofalargerhalo(Balogh et al.2000;
ies with active star formation. This segregation has been Diaferio et al.2001);groupswillbethefirstenvironmentto
known for a long time; however the superb data from the demonstrate this effect.
Sloan Digital SkySurveyhasrevealed howsurprisingly dis-
tinct these two populations are, at least in terms of their
colours (Stratevaet al. 2001; Baldry et al. 2003). This seg- 1.1 Previous Work
regation is known to be strongly mass-dependent, with the
most massivegalaxies beingpredominantlyred,early-types Our understanding of galaxies in groups and lower density
(Kauffmann et al. 2003). It is currently unclear whether environmentshasbeenhinderedbythedifficultiesinobtain-
galaxy evolution at the present day consists of galaxies ing large, unbiased samples. Most of the work has been re-
changing from one type to another in a short time, or of stricted to compact groups (e.g. Hickson 1982; Rubin et al.
evolution in properties within a given class. 1991; Iglesias-P´aramo & V´ılchez 1999; Coziol et al. 2000;
Nishiura et al. 2000; Verdes-Montenegro et al. 2001;
Direct evidence of galaxy evolution comes from obser-
dela Rosa et al. 2001; Kelm & Focardi 2003), which are
vations of galaxies at different redshifts; this shows that, in
most likely to be physically bound systems. However, these
the past, the average star formation rate (SFR) was much
may be a special class of group, not representative of a
higher (Lilly et al. 1996; Madau et al. 1996; Wilson et al.
typical stage of the hierarchy through which most galaxies
2002), and the typical star-forming galaxy was more lumi-
evolve.
nous (Cowie et al. 1999). Given the near-constancy of the
Postman & Geller (1984) analysed the morphology-
SFR in the Milky Way (e.g. Rocha-Pintoet al. 2000), it
density relation in groups selected from the CfA redshift
seems likely that this downsizing effect is the main char-
survey (Geller & Huchra 1983) and demonstrated that the
acteristic of the decline in global star formation: the most
original relation found in clusters by Dressler (1980) ex-
massive galaxies have recently stopped forming stars alto-
tendscontinuouslytolowerdensities.Belowadensityof∼1
gether, while less massive galaxies continueunhindered.
Mpc−3(M <−17.5)nofurthercorrelationisseen.Simi-
A more indirect form of evolution is observed as the B(0)
lartrendshavesincebeenconfirmedinnumerousotherstud-
changeingalaxy populationsasafunctionoftheirenviron-
ies (e.g. Zabludoff & Mulchaey 1998; Hashimoto & Oemler
mentatagivenepoch.Inparticular,galaxiesindenseenvi-
1999; Tran et al. 2001; Treu et al. 2003) . Interestingly,
ronments(i.e.clusters)tendtohaveearly-typemorphologies
Postman & Geller(1984)foundthetrendwithlocaldensity
(e.g.Dressler1980;Dom´inguez et al.2001;Treu et al.2003)
is the same in both rich clusters and poor groups; the only
andlowSFRs(Balogh et al.1997;Balogh et al.1998,1999;
difference is that poor groups typically sample lower den-
Poggianti et al. 1999). One interpretation of this trend has
sityregionsand,thus,havepopulationsmoredominatedby
been that the cluster environment causes galaxies to trans-
late-typegalaxies.Ithasbeenarguedbasedonthisevidence
form their properties as they move, pulled by gravity, from
that galaxy-galaxy interactions within the group environ-
low density regions into the cluster centre. However, recent
ment are the mechanism responsible for the high fractions
work(e.g.Pimbblet et al.2002;O’Hely2000;Kodama et al.
ofearly-typegalaxiesinclusters(e.g.Zabludoff & Mulchaey
2001),especiallythatbasedonthe2dFGalaxyRedshiftSur-
1998, 2000; Hashimoto & Oemler 2000).
vey (2dFGRS, Mateus & Sodr´e 2003; Lewis et al. 2002a,
Less work has been done on the stellar populations of
hereafter Paper I) and Sloan Digital Sky Survey (SDSS,
group galaxies, though there is good evidence that they
Gomez et al. 2003, hereafter Paper II) has shown that the
are intermediate between those of the field and rich clus-
correlationbetweengalaxytypeandlocaldensityextendsto
ters (Allington-Smith et al. 1993; Hashimoto et al. 1998;
verylow densities,well beyondtheregion wherethecluster
Girardi et al. 2002; Tran et al. 2001; Carlberg et al. 2001).
isexpectedtohavemuchinfluence.Theseworksshowedthat
More recently, analysis of SFRs in galaxy groups selected
thereisasmoothdependenceofSFRonlocalgalaxydensity,
fromthepartially-completed2dFGRSsurveyhasbeendone
andidentifieda“critical”surfacedensityof1Mpc−2,where
by Merch´an & Zandivarez (2002). Based on this group cat-
SFRcorrelations with environmentfirst occur.This critical
alogue, Mart´ınez et al. (2002) found that even the lowest-
density is quite low, and corresponds to regions well out- mass groups, with masses ∼ 1013M , show reduced total
⊙
side thevirialised cluster region; this suggests that galaxies
SFRs relative to the field. Dom´ınguez et al. (2002) claim
maybepre-processed ingroups,beforetheyendupinclus-
that the spectral-type dependence on local density is only
ters.This hadbeen anticipated (e.g. Zabludoff & Mulchaey observedingroupsmoremassivethan3×1013M ;however,
⊙
1998, 2000), since galaxy-galaxy interactions are known to
this claim appears to arise mostly from the fact that their
inducestarformation(e.g.Barton et al.2000;Lambas et al.
low-massgroupsdonotsampledensitiesashighasfoundin
2003) and such interactions should be common in groups.
thehigher-mass groups.
There is evidence that these interactions lead to the build-
upofellipticalgalaxieswithhotX-rayhaloswhichwilllater
beincorporatedintoclusters(e.g.Ponman & Bertram1993;
1.2 The purpose of this paper
Mulchaey & Zabludoff1999;Smith et al.2003).Groupsare
alsoexpectedtobeimportantbecausetheyarethefirstlevel We analyse the local correlation between star-formation
of the super-galactic hierarchy. Models of galaxy formation activity and environment using data obtained from the
(cid:13)c 0000RAS,MNRAS000,000–000
Galaxy ecology in SDSS and 2dFGRS 3
two largest galaxy surveys ever conducted: the 2dFGRS 2dF instrument (Lewis et al. 2002b). The accuracy of each
(Colless et al.2001,2003)andtheSDSS(York et al.2000). individualredshift is ∼85kms−1 (Colless et al. 2001).
In particular, we have selected a sample of galaxy groups
from each survey, but in significantly different ways. Since
the definition of a group is partly subjective, and a variety 2.1.2 The group catalogue
ofalgorithmshavebeendevelopedtofindgroupsinredshift
The group catalogue is based on a friends-of-friends perco-
surveys, comparing results from the two catalogues allows
lationalgorithm,whichlinksneighbouringgalaxiestogether
ustoinvestigatethesensitivityofourresultstothewaythe
if they lie within a specified linking-length of each other
group catalogue is constructed.
(Ekeet al. 2003). The linking-length is scaled with redshift
Thepurposeofthispaperistoestablishhowstarforma-
in order to obtain groups of a constant overdensity in the
tioninthegalaxypopulation,ascharacterisedbythedistri-
magnitudelimitedsurvey.Thisisdonebyscalingthelength
butionofHαemission,dependsonenvironment.Specifically,
accordington(z)−1/3,wheren(z)isthemeangalaxydensity
wewillinvestigatewhetherthemostimportantvariableis1)
atredshift z.Furthermore,thelinkinglengthalong theline
velocitydispersion oftheembeddinggrouporcluster;2)lo-
ofsight(ℓ )isallowedtobelargerthanthatintheplaneof
calgalaxydensity,onscales<∼1Mpc;or3)large-scalestruc- thesky(ℓ||),toaccountfortheeffectsofpeculiarvelocities.
ture,asparameterisedbythedensityon∼5Mpcscales.We ⊥
The algorithm is extensively tested on mock catalogues de-
will show that the number of actively star-forming galaxies
rivedfromnumericalsimulationswithσ =0.9,sothecom-
depends on both the local and large-scale density, but that 8
pletenessandcontaminationofthecatalogueisunderstood.
the properties of the star-forming galaxies themselves do
Fordetailswereferthereaderto(Eke et al.2003);webriefly
not.
review the salient points here. The mock galaxy catalogues
A summary of the paper follows. In Section 2 we de-
are created using the semi-analytic models of Cole et al.
scribethedetailsofthetwodatasetsanalysedherein,focus-
(2000),withthe2dFGRSselection function.Groupsidenti-
ing on the construction of group catalogues, measurement
fied from this mock catalogue are then compared with the
ofemission linestrengths,andsampleselection.Ourresults
corresponding “true” group, identified as galaxies populat-
arepresentedin Section 3.Wediscuss thephysicalimplica-
ing dark matter halos identified using a friends-of-friends
tionsofourresultsinSection4,anddrawsomeconclusions
algorithm with a linking length of b = 0.2 times the mean
in Section 5. Throughout the paper we use a cosmology of
particleseparation. Theparametersofthegroup-findingal-
Ω = 0.3, Ω = 0.7 and H = 100hkms−1Mpc−1, with
m Λ 0
gorithm are tuned to provide the best match between the
h=0.7. Alldistances are proper lengths in units of Mpc.
median properties (size and mass) of theobserved and true
groups.Thesebest-fitparametersareb=ℓ n(z)1/3 =0.13,
⊥
L =2h−1 MpcandR=11, whereL isthemax-
⊥,max ⊥,max
2 DATA imum linking-length permitted across the line of sight and
R = ℓ /ℓ . These values are insensitive to σ for reason-
Wewill uselocal datagleaned from thetwolargest redshift || ⊥ 8
able values of the normalisation. Small, parameterised per-
surveys available: the 2dFGRS (Colless et al. 2001, 2003)
turbations of b and R are allowed to remove small differ-
and the SDSS (Yorket al. 2000). Below we summarise the
ences in the recovered properties that correlate with halo
dataineachsurvey,anddescribeourgroupcatalogues,emis-
mass. The median recovered velocity dispersions are accu-
sion line measurements, and sample selection criteria.
ratetobetterthan∼10%, independentofhalomass, when
compared with the velocity dispersion of the parent dark
matter halo; however, the scatter in this accuracy is large.
2.1 The 2dF Galaxy Redshift Survey
The catalogue is highly complete, recovering > 95% of ha-
2.1.1 Summary of the data los with dark matter mass M <∼ 4×1014 M⊙; the price
to be paid is some contamination from unphysicalsystems.
The 2dFGRS has obtained over 220000 spectra of galaxies
For the most massive haloes, the algorithm is susceptible
selectedinthephotographicb band,fromtheAPMgalaxy
J to a small amount of fragmentation; approximately 10% of
ctiagtuaolougsude.ecTlinhaetitoanrgsettreipds,gapllauxsie9s9arraendloocmaltyedlocinatetwdofieclodns-. haloes with M >∼ 1014M⊙ are fragmented into more than
onegroup with mass at least 20 percent that of theparent
One strip is in the southern Galactic hemisphere and cov-
dark matter halo.
ers approximately 80◦×15◦ centred close to the SGP. The
Foreachgroup,wecalculatethevelocitydispersion us-
other strip is in the northern Galactic hemisphere and cov-
ingthegapperestimateofWainer & Thissen(1976),asdis-
ers 75◦ ×10◦. The 99 random fields are located over the
cussedinBeers et al.(1990),whichisinsensitivetooutliers.
entire region of the APM galaxy catalogue in the southern
The group centre is computed by iteratively rejecting the
Galactic hemisphere outside of the main survey strip. We
mostdistantgalaxyuntilonlytwogalaxiesremain;thecen-
only use the contiguous fields for this work. Full details of
tre is taken to be the position of the brighter of these two
thesurvey strategy are given in Colless et al. (2001).
galaxies.
The survey spectra were obtained through ∼2′′ fibres,
and cover the wavelength range 3600–8000˚A at 9˚A resolu-
tion.Onlythewavelengthrangeof3600–7700˚A isuseddur-
2.1.3 Hα as a star formation tracer in the 2dFGRS
ing the line fitting procedure due to poor signal to noise
and strong sky emission in the red part of the spectrum. The SFR is directly related to the Hα emission luminosity
The wide wavelength range is made possible by the use of (e.g. Kennicutt 1983), and we will use this emission line as
an atmospheric dispersion compensator (ADC) within the ourtracerofstarformation.Toprovideareliabledescription
(cid:13)c 0000RAS,MNRAS000,000–000
4 Balogh et al.
of SFR, however, Hα luminosities need to be corrected for toak-corrected luminosityof M ≈−19.However,forcon-
b
underlyingabsorption,dustextinction,andapertureeffects, sistency with the SDSS sample (see Appendix B), we limit
all of which are important (e.g. Charlot & Longhetti 2001; this sample to M = −19.5, using the average k-correction
b
Hopkinset al. 2001, 2003; Afonso et al. 2003). When these of Norberg et al. (2002). Finally, we exclude galaxies that
correctionsaremade,thereisgoodagreementbetweenHα– arewithin2Mpcofasurveyboundary;morestringentcuts
derived SFRs, and those derived from the far infrared, ra- are made when computing densities on various scales, as
dio or ultra–violet continuum (Hopkinset al. 2003). In this appropriate.
paper, however, we will focus only on the rest-frame equiv- Ourfinal,volume-limitedsamplecontains20154 galax-
alent width of the Hα line, W (Hα),corrected for underly- ies. For computations of W (Hα), we exclude galaxies in
0 0
ingabsorption(seebelow).Uniformdustextinctionwillnot which the continuum was negative, or a Gaussian was a
affecttheW (Hα),thoughselectiveextinctionaroundmas- poor fit to the line (see Paper I). Furthermore, we restrict
0
sivestarswill(Charlot & Longhetti2001).Aperturecorrec- our analysis to data taken after August 1999, since the ex-
tions to the flux can be substantial, typically ranging from treme ends of spectra obtained earlier are severely affected
a factor ∼ 2 to ∼ 6 (Hopkinset al. 2003); however, the ef- by problems with the ADC (Lewis et al. 2002b). These re-
fect of this missing flux on W (Hα) depends on the spatial strictions reduce the usable sample to 12683 galaxies. Of
0
distribution of the Hα emission. As long as the size distri- these, 846 (6.7%) are identified as AGN (see Section 2.1.3)
butionofgalaxiesisindependentofenvironment,neglecting andexcludedfromthesample.Thisleaves11837galaxies,of
this correction does not affect the results of this paper. We which 7012 (59 per cent) are in the friends-of-friends group
will demonstrate that this assumption is justified, in Ap- catalogue;1577(13percent)areingroupswithatleastten
pendix A. members above the luminosity limit, which we use for our
As described in Paper I, all of the measurements of analysis.Thislatterrequirementismadesothatmeaningful
line equivalent widths have been performed using a com- estimates of local density can be made. We use thefriends-
pletelyautomatic procedure.Insummary,upto20individ- of-friendslinkingalgorithmtodesignategroupmembership,
ual absorption and emission lines are fitted simultaneously rather than selecting all galaxies within some specified ra-
with Gaussian profiles. The Hα emission line is accurately dius of a chosen centre. No further selection of the clusters
deblended from the adjacent [Nii]λ6548˚A and [Nii]λ6583˚A ismade,andsomeofthesystemsarethereforedynamically
lines; the [Nii]lines are constrained to be in emission while unrelaxedgroups,forwhichthevelocitydistributionisnota
the Hα line may be either emission or absorption (but not reliableindicatorofdynamicalmass.Thevelocitydispersion
both); measurements are made whether or not the line is distribution of groups with more than ten bright members
detected in emission. We will add 1˚A to the Hα equivalent is shown in Fig. 1. Asdescribed in Section 2.1.2, this is de-
widths to approximately account for the effects of stellar signed tobeaveryhighly complete catalogue that will still
absorption (Hopkinset al. 2003). This correction is not im- have some contamination from unphysical systems, though
portant for galaxies with significant emission, which are of thiscontamination issubstantially reducedbyourselection
interest here, but ensures that the mean W (Hα) is never of groups with at least ten members (Ekeet al. 2003).
0
much less than zero. Because of the uncertain flux calibra-
tion ofthe2dFGRSspectra, wedonotderiveHαluminosi-
ties, or SFRs, but restrict the analysis to the observable 2.2 The first SDSS data release
quantityW0(Hα). 2.2.1 Summary of the data
We also wish to exclude from the analysis galaxies in
whichmostoftheemissioncomesfromanactivegalacticnu- The SDSS (http://www.sdss.org) is a joint, 5 passband (u,
clei(AGN).Sincethefluxcalibrationisnotreliableoverlong g, r, i, z), imaging and medium–resolution (R ≃ 1800)
wavelengths,wejustusethecriterion[Nii]λ6583/Hα>0.63 spectroscopic survey of the Northern Galactic Hemisphere
to identify AGN-dominated spectra, when the equivalent (see York et al. 2000, for details). In May 2003, the SDSS
widths of both lines are greater than 2 ˚A (e.g. Miller et al. publicly released the first official set of data, named DR1,
2003).Sincethe AGNfraction does not appear to correlate which comprises 186,240 spectra of galaxies, stars and
with environment, however (Miller et al. 2003), the correc- QSOs over 1360 deg2 of sky. This release is fully described
tion does not affect any of theconclusions in this paper. in Abazajian et al. (2003), and the reader is referred to
Strauss et al. (2002) for a detailed description of the spec-
troscopic target selection for the SDSSmain galaxy survey.
2.1.4 Sample selection Thespectraareobtainedfrom3′′diameterfibres,larger
thanthoseofthe2dFGRS.Thespectrographsproducedata
We select a volume-limited subset of the contiguous fields, covering 3800-9200 ˚A, with the beam split at 6150 ˚A by a
wherethelower redshift limit, z>0.05, is chosen toreduce dichroic. The spectral resolution at λ ∼ 5000˚A is ∼ 2.5˚A,
oursensitivity toapertureeffects. Theupperredshift limit, and redshift uncertainties are ∼30kms−1.
z < 0.095, is chosen because measurement of Hα emission
becomes difficult when it is redshifted beyond ∼ 7200 ˚A
and night sky emission lines are strong1. At this redshift, 2.2.2 The group catalogue
the magnitude limit of the survey (b =19.45) corresponds
J
Groups are selected from the SDSS in a fundamentally dif-
ferent way from 2dFGRS groups (cf. Section 2.1.2), which
1 InPaperIweusedanupperlimitofz=0.1;thesmallchange allows us to test the sensitivity of our results to the group-
hereismadeforconsistency withthe SDSSsampleas presented findingalgorithm.TheSDSSalgorithm(Nichol,Milleretal.
inPaperIIandthispaper. in prep) is a semi-parametric, high dimensional technique
(cid:13)c 0000RAS,MNRAS000,000–000
Galaxy ecology in SDSS and 2dFGRS 5
greater than 90% pure (Nichol, Miller et al., in prep.). We
excludegroupsinwhichthevelocitydistribution(measured
using all galaxies, not only those identified as clustered in
colour space) is significantly different from a Gaussian, so
that we preferentially select dynamically isolated, relaxed
systems.Thefinalcataloguecontains104bona-fideclusters
withreliablevelocitydispersionsthatarelikelyindicativeof
system mass. This catalogue is a good complement to the
2dFGRScatalogue,asitisahighlypurecatalogue(i.e.,with
littlecontamination),attheexpenseofbeingincompletefor
low-mass groups. Allgalaxies within 1000kms−1 and twice
the virial radius (defined by the galaxy number overden-
sityrelativetothefield)of theclusterareconsidered group
members; theexact radius chosen is not important because
we present our data as a function of local density, which
correlates well with radius within thevirialised region.
2.2.3 Hα as a star formation tracer in the SDSS
We will again use the Hα equivalent width as a tracer
of star formation, as we have done for the 2dFGRS sam-
Figure1.Thehistogramshowstheabundanceofgroupswithat ple (see Section 2.1.3). Emission and absorption lines are
leasttenmembersinthe2dFGRSsampleasafunctionofvelocity
measured automatically from the spectra by fitting multi-
dispersion,correspondingtotheleftaxis.Thesolidpointsarethe
pleGaussians where required.Again ameasurement is pro-
averagenumberofgalaxies(brighterthanMb=−19)pergroup, videdforeveryline,whetherornotitwasdetectedinemis-
withvalues correspondingto the rightaxis. Theerrorbars span
sion, and we make a 1˚A correction for underlying stellar
therangefromthe10thto90thpercentileofthedistribution.
absorption (Hopkinset al. 2003). The effect of converting
W (Hα) to SFR using various conversions (e.g., Kennicutt
0
1998;Hopkins et al. 2001;Charlot & Longhetti2001),were
developed to find galaxies clustered in both position and
discussed in Paper II and will not beconsidered here.
colour. The premise is that galaxies within clusters evolved
For the SDSS spectra we can more effectively ex-
similarly,andthusgalaxyclusterscontainsubsetsofgalaxies
clude AGN (relative to the 2dFGRS), since the spectra
thathavesimilar spectral energydistributions(SED).Note
are accurately flux calibrated. Here, we will consider the
that this is not a red-sequence finder (e.g. Gladders & Yee [Nii]λ6583/Hαand[Oiii]/Hβ ratios, usingtheclassification
2000), and will detect groups of blue galaxies if they have
ofMiller et al.(2003).Wherepossible,allfourlinesareused
similar colours. Galaxy colours are used as a proxy for the
to identify AGN; if only one of the ratios is available, then
shapeoftheSEDsincetheycoveralargerwavelengthrange
onlythatoneisused.Most(87%)oftheAGNareidentified
thanthespectraandbecausethedimensionalityoftheprob- based on the[Nii]/Hαratio alone.
lem isreducedtoamanageable 7-dspace(twospatial posi-
tions,redshift,u−g,g−r,r−i,andi−z).Inpractice,we
expectclusteringincolour-spacetobeasignalfromthered
2.2.4 Sample selection
sequence (e.g. Gladders & Yee 2000). Galaxy overdensities
in this 7-d space are found by comparing to random loca- Wewill usethesame volume-limited sampleas inPaperII.
tions in the SDSS DR1 survey. By design, the algorithm is The main criteria for selection are 0.05 < z < 0.095 and
extremely pure(i.e. few false positives). This high purity is M < −20.6 (k–corrected, for H = 70kms−1Mpc−1). As
r 0
theresult ofthelackofprojection in7-dspace,andtheuse for the 2dFGRS data, the lower redshift limit is imposed
oftheFalseDiscoveryRatestatisticalmethodinchoosinga to minimise aperture bias due to large nearby galaxies (see
thresholdabovewhichgalaxiesareconsideredclustered(see AppendixAformoredetail).Theupperredshiftlimitisthat
Miller et al. 2001). whereourluminositylimitequalsthemagnitudelimitofthe
Thealgorithmhasbeentestedagainstmockcatalogues SDSS(r=17.7;Strauss et al.2002).Fromavolume–limited
(Wechsler et al., in prep), in which galaxies are assigned sample of 19287 galaxies we exclude 4237 that are within 2
r-band magnitudes and placed onto dark matter particles Mpcofasurveyboundary,andanother64galaxiesforwhich
in such a way that the luminosity-dependent clustering of no reliable measurement of Hα is available, leaving 14986
the SDSS seen in Blanton et al. (2003a) is matched. The galaxies. Finally, removing a ∼ 12% AGN contribution we
remaining magnitudes are then added in so that the corre- have13131galaxiesinthefinalsample.Ofthese,1939(15%)
lation between colour and local density, as observed in the are associated with a group in thecatalogue.
SDSS, is also matched. The same cluster-finding algorithm In Fig. 2 we show the velocity dispersion distribution
used on the real data is then run on the mock catalogues. for the selected subset of SDSS groups. The distribution is
Preliminary tests show that the cluster catalogue is >90% differentfromthatofthe2dFGRSgroups(Fig.1),likelydue
complete for clusters with mass M > 2.5×1014M ; the to the fact that the former catalogue is incomplete at the
⊙
completenessdecreases toward lowermasses. Thecatalogue lowest velocity dispersions, while the latter has more con-
is100% pureforsystemswithM >5×1014M andalways tamination.Furthermore,theSDSScatalogueincludesafew
⊙
(cid:13)c 0000RAS,MNRAS000,000–000
6 Balogh et al.
prior knowledge of the nature of the relevant physical ef-
fects,wecannotsaywhichisthemoremeaningfulestimate.
Forexample,ifwebelievegalaxy-galaxyinteractionsareim-
portant, then perhaps it is only the distance to the nearest
neighbour that is relevant (Lambas et al. 2003). It is easy
to imagine scenarios where either the distance to the Nth
neighbour,ordensitymeasured onaphysicalscale,islikely
to bemore meaningful.
In this paper, we will adopt two density estimators,
which are described in Appendix C. The first is a tradi-
tional projected density estimate, Σ , which is based on
5
theprojected distancetothefifth-nearestneighbourwithin
±1000kms−1. Inasmuch as we are willing to believe that
galaxies canbefound indiscrete,relatively isolated groups,
wemustbecautiousaboutinterpretingourdensityestimate
in groups where the number of members is less five. For a
group with only four members, the fifth nearest neighbour
will clearly not lie in the same group and, therefore, the
density computed from this distance may not be what is
wanted. To ensure that we maintain our intuitive idea of a
local environment within groups, we only consider groups
which have at least ten galaxies brighter than our luminos-
Figure2.AsFig.1,butforgroupsintheSDSSsample,restricted
togroupswithatleasttenmembergalaxiesbrighterthan Mr = itylimit.Furthermore,weexcludeanygalaxiesinwhichthe
−20.6. fifth–nearestneighbourismoredistantthantheclosest sur-
vey boundary, or within 1000kms−1 of our redshift limits,
toensureaccuratemeasurements. This maybias usagainst
clusterswithveryhighvelocitydispersions,σ>800kms−1,
findinglow-densityregions,butdoesnotaffecttheobserved
while suchlarge systems tendtobefragmented in the2dF-
trendsof galaxy property with Σ (e.g. Miller et al. 2003).
5
GRS catalogue.
Thesecondestimatorisathree-dimensionaldensityρ ,
θ
obtained by convolving the galaxy field with a Gaussian of
dispersionθ.Inparticular,wewillconsiderthedensitymea-
2.3 Homogeneity of the samples
sured on scales θ = 1.1 and 5.5 Mpc (see Appendix C).
In using the 2dFGRS and SDSS data together, we need to These measurements underestimate the density in clusters
ensure that the samples are comparable in terms of selec- with large velocity dispersions, but are particularly useful
tionandderivedmeasurements.Thisdetailedcomparisonis forprobinglowdensityregimeswherepeculiarvelocitiesare
madeinAppendixB.Theonlydifferencerelevanttoourre- small. Themain disadvantageisthatthesignal-to-noise ra-
sultsisthatthebJ-selectionofthe2dFGRSresultsinasam- tio varies with density, and is low at low densities, when
ple that is more biased toward galaxies with large W0(Hα) there are few galaxies within the aperture. In our analysis
than the r−selected SDSS. Because of this difference, we we will therefore indicate the density at which there is less
willshowmostofourresultsforthetwosurveysseparately. than one galaxy within the Gaussian dispersion θ, and will
Thefinalcombinedsample contains24968 galaxies and204 exclude any galaxies which are located less than 2θ from a
groupswithatleasttenmembers.Approximately14%ofall surveyboundary,to avoid biassing thedensity estimate.
galaxies in thesample are associated with a group.
2.4 Density Estimates 3 RESULTS
The local galaxy number density around a given galaxy is 3.1 Dependence of W0(Hα) on environment
not in itself a well-defined quantity. As can be anticipated
InPapersIand IIweshowed thatthereisastrongcorrela-
fromtheshapeofthecorrelation function(e.g.Baugh et al.
tionbetweenW (Hα)distributionandlocalgalaxydensity.
1999),thedensityofneighbouringgalaxiestendstoincrease 0
In this section we will first define precisely what property
asyouprobeclosertothetargetgalaxy(seeAppendixC2).
ofthegalaxydistributioncorrelateswithenvironment;then
Wearethereforefacedwithtwochoiceswhencharacterising
proceed toexplorehowthiscorrelation dependson thedef-
the density around a galaxy. One is to measure the density
inition of local environment.
within a fixed distance scale. This ensures that both high-
andlow-densityregionsaremeasuredatthesamescale;how-
ever,themeasurementisnoisyandoflimiteddynamicrange
3.1.1 Bimodality in the W (Hα) distribution
due to the arbitrarily small, and finite, number of galaxies 0
within the chosen distance. On the other hand, we can use InFig.3wepresentthecorrelationbetweenW (Hα)andΣ
0 5
a systematically larger scale in lower density regions. This forthe2dFGRSandSDSSsamples.Onthetopaxisweshow
improvessensitivityandprecisionat lowdensities,butmay thedistancetothefifth–nearestneighbourcorrespondingto
bedifficult tointerpret because high- and low-density envi- Σ , which shows that at the lowest densities we are mea-
5
ronmentsaremeasuredatdifferentphysicalscales.Without suring the galaxy distribution on >∼ 3 Mpc scales, while in
(cid:13)c 0000RAS,MNRAS000,000–000
Galaxy ecology in SDSS and 2dFGRS 7
Figure3.ThedependenceofW0(Hα)onprojected,localdensity Figure 4. The W0(Hα) distribution for galaxies in the 2dF-
forthe2dFGRS(leftpanel)andSDSS(rightpanel)samples.The GRS (left panel) and SDSS (right panel) with W0(Hα)> 4˚A,
top axis shows the distance to the 5th nearest neighbour, from in low-density environments (Σ5 < 0.2 Mpc−2, dotted line) and
whichΣ5iscomputed.Thesolidlinesshowthemedianand75th high-densityenvironments (Σ5>2Mpc−2,solid line).Weshow
percentileof W0(Hα), inbinsof 100galaxies. Althoughweonly Poisson-distributed error bars on the high-density subsample,
plotthedataforW0(Hα)<40˚A,theyextendtoW0(Hα)∼100˚A; whichisthesmallerofthetwo.
seeFig.B2forthefullW0(Hα)distributionsashistograms.
the highest and lowest Σ densities. The distributions are
5
very similar; a Kolmogorov–Smirnov test cannot reject the
thedensest regions themeasurement is made at <∼300 kpc. hypothesis that the low- and high-density distributions are
Recall (Section 2.4) that we exclude any galaxy for which
drawn from the same population with more than 1σ confi-
thefifth–nearestneighbourismoredistantthanthenearest
dence. Any difference, no matter how significant, must be
surveyboundary,or within 1000kms−1 of ourredshift lim- small:themeanW (Hα)is∼21.5˚Aand∼20˚Aforthelow-
0
its. The solid lines show the median and 75th percentile of
andhigh-densitypopulations,respectively,inboththe2dF-
the W (Hα) distribution. In both cases, we reproduce the
0 GRS and SDSS samples. Thus, the observed trend of mean
resultsofPaperIandPaperII;thedistributionchangesdis-
W (Hα) with density (Fig. 3) is due almost entirely to the
tinctly,atadensityΣ5 ∼2Mpc−2.Thisischaracterisedby rel0ative proportion of galaxies with W (Hα)>4 ˚A; there is
0
the near-total lack of galaxies with large W (Hα) at densi-
0 atmost weaksensitivitytoenvironmentwithintheactively
tiesgreaterthanthisvalue.Atlowerdensities,thereremains star forming population alone2.
a correlation between W (Hα) and Σ ; however it is weak,
0 5
with the average W (Hα) increasing by only ∼25 per cent
0
over an order of magnitude in local density. The difference 3.1.2 The abundance of star-forming galaxies
in normalisation between theSDSSand 2dFGRSsurveysis
Motivatedbytheresultsoftheprevioussubsection,weshow,
due to the different selection criteria for the spectroscopic
in Fig. 5, how the fraction of galaxies with W (Hα)> 4
sample, as discussed in AppendixB. 0
˚A depends on Σ . It is evident that the fraction decreases
The W (Hα) distribution shown in Fig. 3 reveals the 5
0 steadilywithincreasingdensity;thebreakatΣ ∼2Mpc−2
presence of two distinct galaxy populations: those with sig- 5
is still there, butless apparent than in Fig. 3.
nificant, ongoing star formation covering a broad range in
Hα strength (from ∼ 4˚A to > 40˚A), and those with no Since the identification of a characteristic density can
haveimportant implications, it isuseful tounderstandwhy
ongoing star formation, which form a conspicuous horizon-
it appears so strongly in Fig. 3 (and Papers I and II) but
tal ridge line about W (Hα)= 0. (This is more clearly
0
ismuchweakerinFig.5.Uponcloseexamination,achange
seen in Fig. B2, where we show the W (Hα) distribu-
0
in slope is most evident in the median of the 2dFGRS, at
tionsashistograms).Thisrecallsthebimodalityobservedin
Σ ∼ 1 Mpc−2, and in the 75th percentile of the SDSS, at
the colour distribution (Stratevaet al. 2001; Blanton et al. 5
Σ ∼3 Mpc−2. From Fig. 5 we see that these two densities
2003b); Baldry et al. (2003) find that the red peak distri- 5
correspondtothepointatwhichthefractionofgalaxieswith
bution can be explained as the result of mergers between
galaxies in the blue peak. We will therefore focus our anal-
ysis on the star-forming population, and its variation with 2 We will not deal directly with the distribution of W0(Hα) in
environment. galaxieswithW0(Hα)<4˚A,sincethisdistributionismostlikely
In Fig. 4 we show the W0(Hα) distribution only for dominated by measurement uncertainties, including systematic
those galaxies with W0(Hα)> 4 ˚A, in environments with effectslikestellarabsorption,ratherthanstarformationactivity.
(cid:13)c 0000RAS,MNRAS000,000–000
8 Balogh et al.
Figure5.ThefractionofgalaxieswithW0(Hα)>4˚Aasafunc- Figure 6. The fraction of galaxies with W0(Hα)> 4˚A in the
tion of local density for the 2dFGRS (bottom panels) and SDSS SDSS (dashed line) and 2dFGRS (solid line), as a function of
(top panels) samples. The solid lines represent the full galaxy three-dimensionaldensityρ1.1,estimatedwithaGaussiankernel
sample, in bins each containing 250 galaxies. The dashed lines with 1.1 Mpc standard deviation. Each bin contains 250 galax-
arerestrictedtogalaxieswhichlieingroupsorclusterswiththe ies. The vertical, dotted line corresponds to the density where
indicatedvelocitydispersion,inbinsof50galaxieseach.Poisson- no galaxy lies within the Gaussian filtering scale θ, making the
distributed uncertainties are typically ∼0.1 on the dashed lines densityestimateparticularlynoisy.Onthetopaxisweshowthe
and∼0.05onthesolid lines. estimatedtwo-dimensionalprojecteddensitywithinacylinderof
length2000kms−1,forcomparisonwithΣ5.
W (Hα)> 4˚A drops below 50 (2dFGRS) or 25 (SDSS) per
0
cent;athigherdensitiesthemedianand75thpercentile,re-
(particularly in the 2dFGRS sample) that galaxies in the
spectively,aretracingthenon-starformingpopulationwith
highest-velocity dispersion clusters have a low fraction of
W0(Hα)<4˚A,and thetrend with Σ5 largely disappears. galaxies with W (Hα)> 4˚A for their local density, relative
0
Itis worthstressing that thetrendwith Σ isfairly in-
5 to galaxies in lower-dispersion groups. Projection will have
sensitive to projection effects, despite the fact that we are
a more complex effect on this result, since the physical size
projectingovera±14Mpccylinder,anddespitethenonuni-
ofthegroupdependsonσ;inparticular,weexpecttheover-
form density of the background, as we demonstrate in Ap-
estimation of both Σ and W (Hα) to be greater in high-σ
5 0
pendixC1.1.TosummarizetheresultsofthatAppendix,at
clusters thanin low-σ clusters. This meansthehigh-σ clus-
Σ ∼0.1Mpc−2,wheretheprojectedcontaminationis∼50
5 terW (Hα)distributionshouldexceedthatoflow-σ groups
0
percent,weonly overestimatetheemission-line fraction by
at a fixed Σ ; this is opposite to what we find, and cannot
5
about 5 per cent. This is because the emission-line fraction
therefore be the explanation for the small observed differ-
variesweaklywithΣ atlowdensities,sothereislittlephys-
5 ence.Finallywenotetheremarkablesimilarity betweenthe
ical difference between the properties of the target galax- resultsforσ<400kms−1 groupsinbothsamples.Sincethe
ies and the projected galaxies. At high Σ , the projected
5 2dFGRS catalogue is highly complete, and the SDSS cata-
fraction drops strongly, both because the contrast with the
logueishighlypure,thepersistenceofastrongW (Hα)–Σ
0 5
field is increasing, and because the radius of the projected
relation in groups from both catalogues is not likely due to
cylinderisdecreasing.Thus,theobservedemission-linefrac-
a selection effect.
tion remains within ∼ 5 per cent of the true value, despite
We now consider the three dimensional Gaussian ker-
thefact that thecontrast between thetarget and projected
nel density estimator with a 1.1 Mpc standard deviation,
galaxy populationsbecomeslarge.Theobservedtrendwith
ρ (the motivation for the choice of filtering size is given
1.1
Σ is, therefore, a physical one.
5 in AppendixC). The ρ density measure allows us tofind
1.1
Fig. 5 also shows the correlation for galaxies in groups
galaxies in the very lowest density environments, which are
of different velocity dispersion. We note that, although ve-
not easily measured from Σ since that quantity projects
5
locity dispersion gives a good indication of the group mass thegalaxy distribution overa 2000kms−1 cylinder3.Inour
on average,thereisasignificant scatterarising becauseour
analysis, we exclude galaxies that are within 2.2 Mpc of a
groupsarenotallrelaxed,sphericalsystems.Therefore,the
survey boundary, for which ρ cannot be reliably deter-
1.1
velocity dispersion in some cases will be a better indicator
mined;wealso notethatρ becomesnoisywhenthereare
1.1
of substructure or dynamic state than of mass. The cor-
relation of emission-line fraction with density is present in
bothhigh-andlow-velocitydispersiongroups,selectedfrom 3 Thisprojectioncorrespondsto∼25Mpc,muchlargerthanthe
either the SDSS or 2dFGRS. There is marginal evidence linearsizeoftypicalvoidregions(Bensonetal.2003).
(cid:13)c 0000RAS,MNRAS000,000–000
Galaxy ecology in SDSS and 2dFGRS 9
scales, to determine whether W (Hα) shows any indepen-
0
dent sensitivity to structureon large-scales.
InFig.7wecomparetheGaussian-filtereddensitiesρ
1.1
and ρ for galaxies in the SDSS and 2dFGRS samples.
5.5
Galaxiesareonlyconsiderediftheyareatleast11Mpcfrom
asurveyboundary.InAppendixC2weshowthattheseden-
sityestimatorsaregoodtracersofintuitivelydenseandlow-
densityregions,despitethecomplicationsofpeculiarveloci-
tiesinclusters(Fig.C4).Notethatthe2dFGRSdataextend
tolargervaluesofρ thantheSDSS;thesedataarisefrom
5.5
a large supercluster region at z ∼ 0.0839 and α ∼ 195.5,
δ∼−2.9(J2000degrees).Fig.7showsaclearseparationbe-
tweengalaxiesingroups(200kms−1 <σ<400kms−1)and
clusters (σ > 600kms−1). Galaxies in both environments
span a similar range in local environment, characterised by
ρ . However, galaxies in the high-velocity dispersion clus-
1.1
ters lie at higher densities on 5.5 Mpc scales, as expected
0.01 0.1 0.01 0.1 sincetheyarephysicallylargersystems.Thisisparticularly
true for theSDSS sample, where the groups are selected to
haveapproximately Gaussian distributed velocities, so that
Figure 7.Galaxiesinthe2dFGRS(left panel)andSDSS(right there is likely to be less scatter in the relation between σ
panel) are shown as a function of density computed on 1.1 Mpc andvirialmass.Notethatthehighervelocitydispersionsof
and5.5Mpcscales,usingaGaussianfilteringkernel(smalldots). clustersmeansthatthispseudo-three-dimensionalmeasure-
Galaxies in groups with σ =200–400kms−1 are shown as open mentwillunderestimatethedensity,moresothaningroups.
circles, while those with σ > 600kms−1 are represented as
Thus the real difference between the clusters and groups is
crosses. Only galaxies that are at least 11 Mpc from a survey
even larger than shown here.
boundary are considered. The dashed line shows the minimum
reliabledensity;belowthistherearefewerthanonegalaxywithin
thefiltersizeθ(thecorrespondinglimitonρ5.5 isoffthescaleof In Fig. 8, we illustrate how the fraction of emission-
theplot). linegalaxies(measuredfromthenearest500galaxiesinthis
plane) depends on these two very different density scales.
Interestingly, the contours are not parallel with either axis,
no galaxies within the filtering scale 1.1 Mpc, ρ1.1 < 0.01 which indicates that the population composition shows a
Mpc−3. dependance on large (> 5 Mpc) scales, in addition to the
Fig.6showshowthefractionofgalaxieswithW0(Hα)> more local density measured at 1.1 Mpc. That is, the frac-
4˚Adependsonρ1.1.Toapproximatelycomparethiswiththe tion of emission line galaxies is lower in regions that are
corresponding Σ5 value, we compute the projected density overdenseon5.5Mpcscales, evenwhenthelocal(1.1Mpc)
within a cylinder of length 2×1000kms−1 along the line of overdensity is the same. We have checked that this is not
sight, to obtain an equivalent surface density, shown along anartifact of thesmoothing orthecorrelation between ρ
1.1
the top axis. The emission-line fraction shows a strong de- and ρ , in the following way. For each galaxy we assign a
5.5
pendenceonρ1.1,andgetssignificantlysteeperatρ1.1 >∼0.05 value of ρ5.5, chosen at random from among galaxies that
Mpc−3. However, the correlation is still present at the low- havesimilar (within 10%) values of ρ .This preserves the
1.1
est densities observed.It is not clear whetherthechangein correlations between ρ and ρ , as well as between ρ
1.1 5.5 1.1
slope at ρ1.1 >∼ 0.05 Mpc−3 highlights an interesting physi- andW0(Hα),butremovesanyresidualcorrelation between
calscale, orifρ1.1 simplylosessensitivitytotheunderlying ρ5.5 andW0(Hα).Inthiscase,thecontourslienearlyparal-
density distribution at low densities, an effect that would leltotheρ axis,confirmingthatthecorrelation weseein
5.5
be exaggerated by the logarithmic scale. Furthermore, the Fig.8isreal.Thereissomeevidencethat,athighdensities,
highestdensities(whichtendtocorrespondtoclusterswith the fraction of Hα–emitting galaxies is mostly dependent
high velocity dispersions) will generally be underestimated on ρ , and the dependence on larger scales becomes more
1.1
asaresultoflargepeculiarvelocities,andthismaybepartly importantatlowerdensities.Thedependenceonlarge-scale
responsible for thechange in slope. densitiesisevidentlystrongerthanthedependenceonveloc-
itydispersion(Fig.5),despitethecorrelationbetweenthem
(Fig. 7).
3.2 Local or Global Density?
We have shown that the fraction of star-forming galaxies A cautionary note needs to be added, however. Since
depends strongly on the local density, measured on scales ρ andρ areintrinsicallycorrelated,errorsonthesemea-
1.1 5.5
<∼ 1 Mpc, and that this correlation is approximately inde- surementscangiverisetothetrendsshowninFig.8,evenif
pendent of the velocity dispersion of theembedding cluster thegalaxypopulationonlydependsononeparameter.How-
or group. However, velocity dispersion may not be thebest ever, a comparison with mock catalogues strongly suggests
measurement of large-scale structure;inthissubsection, we that theobserved dependenceon both densityscales is real
will compare densities measured on 1.1 Mpc and 5.5 Mpc (Balogh et al., in prep.).
(cid:13)c 0000RAS,MNRAS000,000–000
10 Balogh et al.
Figure 8. Galaxies in the 2dFGRS (left panel) and SDSS (right panel) are shown as a function of density computed on 1.1 Mpc and
5.5 Mpc scales, using a Gaussian filtering kernel. Only galaxies that are at least 11 Mpc from a survey boundary are considered. The
contourstracethefractionofgalaxieswithW0(Hα)>4˚A,computedforthenearest500galaxiesateachpointinthisplane.Thecontours
are spaced in steps of 0.05, and increase toward lower densities. For the SDSS sample the contours span fractions 0.25–0.45, while for
the2dFGRStheyspan0.35–0.65.
4 DISCUSSION muchlargerscales,∼5.5Mpc.Ifthistrendisreal,itmeans
thegalaxy population must be only indirectly related to its
4.1 Overview present-dayenvironment.Wediscusstheimplicationsofthis
in Section 4.4.
Thankstotheuniquesizeand homogeneityof the2dFGRS
andSDSSdatasets,wehavebeenabletotracetheHαdistri- Thesethreecluesallow ustomakeadvancesinourun-
butionofgalaxiesoverthefullrangeofenvironmentsatthe derstanding of the evolution of star formation in the Uni-
presentday.Thisanalysishasrevealedthreenew,important verse (Section 4.5), and of any additional physics that may
clues about theprocess of galaxy formation: takeplace in dense environments(Section 4.6).
1. The change in W (Hα) distribution as a function of
0
environment is predominantly due to a change in the rela-
4.2 Star-forming galaxies in dense environments
tive numberof star–forming and non-star-forming galaxies;
the W0(Hα) distribution of actively star-forming galaxies If galaxies at the present day are evolving as they move
themselves do not appear to depend strongly on environ- from low-density regions to high-density regions, we should
ment.Thisisasurprisingresultwhichchallengestheoriesin seeasignatureofthistransformation whichdependsonthe
which the environment at the present day induces a trans- relevant timescale. In particular, if all galaxies experience
formation in galaxy properties, as we discuss in detail in a gradual (>∼ 2 Gyr) decline in star formation when they
Section 4.2. become bound to groups or clusters (Balogh et al. 2000;
2. The fraction of star–forming galaxies increases con- Bekkiet al. 2002),weexpectstar-forming galaxies indense
tinuously with decreasing density. However, even at the regions to show systematically lower W (Hα).Fig. 4 shows
0
lowest densities there is a substantial fraction of galax- thatthisisnotthecase,implyingthatsuchagradualdecline
ies with negligible Hα emission; we will discuss the con- is not a common phenomenon at thepresent time.
sequences of this in Section 4.3. There is evidence that the Ontheotherhand,iftheSFRdeclines rapidlytozero,
correlation with environment becomes steeper at Σ5 >∼ 1 this might not be reflected in Fig. 4, since galaxies will
Mpc−2, an effect that is also seen in the morphology– quickly movefrom the star-forming distribution to the qui-
density relation (Postman & Geller 1984; Treu et al. 2003). escentonewithW (Hα)<4˚A.Instead,weneedtolookata
0
This may represent an interesting scale at which differ- longer-livedtracerof starformation (i.e. onewhich changes
ent physics becomes important; for example, as noted by slowlyafterstarformationceases)toobservethiseffect.For
Postman & Geller (1984), at this density the typical dy- example, the Hδ absorption line, which remains strong for
namical time of the group is longer than a Hubble time, ∼1Gyrafterstarformationceases,isacommondiagnostic
so galaxy-galaxy interactions may start to play a role (see of recent activity. The extreme rarity (< 0.1 per cent) of
also Zabludoff & Mulchaey 1998, 2000). brightgalaxieswithstrongHδ butnegligiblestarformation
3. Thefraction of galaxies with Hαemission may depend atlowredshiftprovidesevidenceagainstthepossibilitythat
not only on the local density, but also on the density on SFR has recently declined on a short timescale for a signif-
(cid:13)c 0000RAS,MNRAS000,000–000