NUV-IR colours of red sequence galaxies in local clusters

0.72 MB·English

by  Timothy D. Rawle

Mon.Not.R.Astron.Soc.000,1–11(2007) Printed2February2008 (MNLATEXstylefilev2.2)
NUV–IR colours of red sequence galaxies in local clusters
Timothy D. Rawle1⋆, Russell J. Smith1, John R. Lucey1, Michael J. Hudson2,
Gary A. Wegner3
8 1Department of Physics, Durham University,Durham DH1 3LE, UnitedKingdom
0 2Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
0 3Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755
2

n
a 14January2008
J

5
1 ABSTRACT
We present GALEX near-UV (NUV) and 2MASS J band photometry for red se-

] quence galaxies in local clusters. We define quiescent samples according to a strict
h
emissionthreshold,removinggalaxieswithveryrecentstarformation.Weanalysethe
p
NUV–J colour–magnitude relation (CMR) and find that the intrinsic scatter is an
-
o order of magnitude larger than for the analogous optical CMR (∼0.35 rather than
r 0.05 mag), in agreement with previous studies. Comparing the NUV–J colours with
st spectroscopically-derivedstellarpopulationparameters,wefindastrong(>5.5σ)cor-
a relation with metallicity, only a marginaltrend with age, and no correlationwith the
[ α/Fe ratio.We explore the origin of the large scatter and conclude that neither aper-

1 tureeffectsnortheUVupturnphenomenoncontributesignificantly.Weshowthatthe
v scatter could be attributed to simple ‘frosting’ by either a young or a low metallicity
0 subpopulation.
9
3 Key words: galaxies:elliptical and lenticular, cD – galaxies:stellar content – ultra-
2 violet: galaxies.
.
1
0
8
0 1 INTRODUCTION galaxieshasanintrinsicscatteranorderofmagnitudelarger
v: than its optical counterpart; ∼0.5 mag compared to 0.05
The optical colour–magnitude relation (CMR) shows
i mag (e.g. Yi et al. 2005). Hot young stellar populations
X that brighter early-type galaxies are also redder dominate the ultraviolet (UV) flux for ∼100 Myr after an
(Visvanathan & Sandage 1977), and is traditionally
r episode of star formation (ten times longer than Hα emis-
a regarded as arising from the mass–metallicity sequence (cf.
sion after star formation; Leitherer et al. 1999). The large
Dressler 1984, Kodama & Arimoto 1997). Gas loss, caused
intrinsic scatter in the UV CMR is therefore often inter-
by supernova wind, occurs earlier in less massive galaxies.
pretedasdifferingquantitiesofveryrecent,albeitlowlevel,
Therefore,asmallerfractionofgasisprocessedbeforebeing
star formation (Ferreras & Silk 2000).
expelled from the less massive galaxy (Mathews & Baker
1971), resulting in lower average metallicities (Larson In intermediate age populations (∼1–3 Gyr), the near-
UV (NUV; 2000–3000˚A) flux is dominated by hot stars on
1974). Bower, Lucey & Ellis (1992) found a very small
intrinsic scatter in the U–V CMR (∼0.05 mag) and, due the main sequence turn-off (e.g. O’Connell 1999). The sen-
to the sensitivity of the U band to the presence of young sitivity of the turn-off to the epoch of formation empha-
sisestheimportanceoftheUVbandsforagedetermination
stars, interpreted this as a small age dispersion. Age
(Dorman, O’Connell & Rood 2003).
and metallicity are observed to have the same effect on
broadband optical colours, whereas spectral line indices Old (∼10 Gyr) metal-poor populations have a signifi-
can be used to break the degeneracy (Worthey 1994). cant UV flux contribution from very hot (T ∼ 10000K)
eff
Kuntschner& Davies (1998) claimed that the CMR is blue horizontal branch (BHB) stars (Maraston & Thomas
driven by metallicity variations with luminosity, although 2000;Lee, Lee & Gibson2002).Howeverthesetendtoreside
Nelan et al. (2005) found evidence for a strong age–mass in globular clusters or galactic haloes (where Fe/H < −1),
relation in addition to this metallicity–mass trend (see also where they are useful age indicators (Kaviraj et al. 2007),
Caldwell, Rose & Concannon 2003, Thomas et al. 2005). ratherthan in relatively metal-rich elliptical galaxies.
The ultraviolet–optical CMR for non-star-forming The UV picture is further complicated by the pres-
ence of the ultraviolet upturn (or UV excess, UVX) phe-
nomenon. First observed by Code (1969), this unantic-
⋆ E-mail:[email protected] ipated upturn dominates the far-UV (FUV; < 2000˚A)
2 T. D. Rawle et al.
in UVX galaxies. In contrast, the NUV can be decom- NUV K-correction are also discussed. Our conclusions are
posed into two separate components: the blue end of the given in Section 5.
main sequence/subgiantbranch,andtheUVXcontribution
(Dorman 1997). Burstein et al. (1988) found that the UVX
can sometimes be appreciable at wavelengths as long as
2 DATA
2700˚A:forexample,inNGC4649∼75percentoftheNUV
flux can be attributed to the UVX component. However, We use two complementary samples of red sequence galax-
the UVX cannot be explained by the BHB population, as ies in local clusters: the first is explicitly red sequence se-
the temperature required to fit the upturn would be T & lected by optical colour, and is a large sample, containing
eff
20000K, whereas BHBs are usually no hotter than T ∼ ∼10timesthenumberofgalaxies;thesecondhastheadvan-
eff
12000K (O’Connell 1999). tageofhigherqualityspectroscopy,andusesanemissionline
cut (Sec.2.3) toensure ared sequencesample. Smith et al.
Burstein et al. further reported that FUV flux (as-
(2007) Fig. 1 demonstrates that Hα selection efficiently re-
sumed to trace the UVX) is strongly correlated with the
moves all galaxies bluer than the red sequence and is more
Mg line strength (∼metallicity) and also with the velocity
2 restrictive than a cut on colour.
dispersion,whichisaproxyforgalaxymass.However,more
recentstudies(e.g.Rich et al.2005)haveweakenedthecase
for a strong UVX vs metallicity relation. From analysis of 2.1 Galaxy samples
internalcolourgradients, O’Connell et al. (1992) concluded
2.1.1 NFPS sample
that the FUV flux in most early-types originates from old
stellar components. Drawing on these results, the source of TheNOAOFundamentalPlaneSurvey(NFPS;Smith et al.
the UVX is tentatively identified as hot, low mass, helium 2004, Nelan et al. 2005) is a study of X-ray selected clus-
burning stars, such as extreme horizontal branch (EHB) tersdistributedoverthewholeskyandatredshiftsbetween
or ‘failed’ AGB (AGB-manqu´e) stars and their progeny 0.015 < z < 0.072. More than 4500 galaxies lying within 1
(seeYi, Demarque & Oemler1997,orthereviewO’Connell Mpc of the centre of each cluster, and within 0.2 mag of
1999). theclusterredsequenceontheB–RCMR(seeSmith et al.
The Galaxy Evolution Explorer (GALEX; launched in 2004Fig.3),wereobservedspectroscopically.Ofthese,3514
2003; Martin et al. 2005, Morrissey et al. 2007) is revolu- have redshift, velocity dispersion and spectral line strength
tionising UV astronomy, with high resolution imaging in measurements (from 2 arcsec diameter fibres).
twobands:near-ultraviolet(NUV;λ =2310˚A)andfar-
eff
ultraviolet (FUV; λ = 1530 ˚A). Using analysis of both
eff
2.1.2 Shapley Supercluster (SSC) sample
NUV–V andFUV–V vsB–V relations,Donas et al.(2006)
suggest thattheFUV–NUV colourreflectsanextensionof The second sample of galaxies concentrates on the core of
the colour–metallicity relation into the UV, as well as de- theShapleysupercluster(SSC;AbellclustersA3556,A3558,
ducing that ∼10 per cent of ellipticals have residual star A3562 at z ≈ 0.049). This sample consists of 541 galaxies
formation. Using the NUV–r colour, Kaviraj et al. (2006) selectedfrom NFPSimagingbuttoadeeperlimit (R<18;
also find non-negligible young stellar populations in mor- Smith et al.2007).Follow-up spectroscopyforthesetargets
phologically selectedearly-typegalaxies.Salim et al.(2007) were obtained using 2 arcsec diameter fibres, equating to 2
derivestarformationrates(SFRs)frombroadbandphotom- kpcatthedistanceofShapley.Asetofthreenon-redundant
etry dominated by the UV, and find good agreement with line indices were fit to the models of Thomas et al. (2003,
SFRs deduced from spectroscopic indices (predominantly 2004) in order to estimate age, metallicity (Z/H) and α-
using Hα). However, they also confirm that some galaxies abundance (α/Fe) for each galaxy. The primary tracer of
withnoHαemissionshowsignsofstarformationintheUV age in this scheme is HγF; for metallicity Fe5015 is used;
bandsand attributethis to post-starburst galaxies. Mgb5177 is the α-abundance indicator. This method is de-
Here,webuilduponthesepreviousstudiesbyexploring scribed in detail in Smith et al. (in prep.).
the relationship between the NUV–J colour and spectro-
scopicstellarpopulationindicatorsforasampleofquiescent, 2.2 GALEX and 2MASS data
redsequencegalaxiesinnearbyclusters.Thispaperisorgan-
isedasfollows.Section2describesourtworedsequencesam- Galaxy Evolution Explorer (GALEX) near-ultraviolet
ples and associated 2MASS and GALEX datasets. The cri- (NUV)bandimagesareavailablefor26(fromatotalof93)
teriausedtoremovegalaxieswithemissionaredescribed.In NFPS clusters. Due to a detector fault, only some clusters
Section3weshowthatalargeintrinsicscatterisfoundinthe have associated far-ultraviolet (FUV) band images. Most
NUV–J colours of these quiescent cluster galaxies. Metal- of theimages are from ourguest investigator snapshot pro-
licity is shown to be strongly correlated with the NUV–J gramme(GALEXGI1 004)whichtargetedasubsetofNFPS
colour,although thereremainsalargeresidual scatter.Sec- clusterswithlowgalacticextinctionandhavingalargenum-
tion4discussespossibleexplanationsforthisscatter,show- berofspectrafromSmith et al.(2004).Inaddition,asmall
ingthatmorphological abnormalities,aperturebiasandthe number of images of comparable depth from the GR2 GI
UV upturn do not contribute significantly. We investigate archive,mediumimagingsurvey(MIS)andneargalaxysur-
simple‘frosting’modelswithalowmassfractionofyounger vey(NGS)havebeenused.Table1listsalltheimagesanal-
stars(oralternativelyalowmassfractionpopulationoflow ysed along with their centre position and stacked exposure
◦
metallicity, blue horizontal branch stars), and show that time. GALEX images have a 1.25 diameter, but only the
◦
thesecouldaccountforthescatter.Theuncertaintiesinthe central 1.2 field has been analysed due to the poor image
NUV–IR colours of red sequence galaxies 3
Table 1.GALEXNUV images.cz (kms−1)isclusterredshiftintheCMBframe.Asingleexposureisusedunlesscolumnsixindicates
thenumberofcoaddedimages.Total exposuretimeforcoaddedimagesisgivenforNUV and(wheretheimageisavailable)FUV.
GALEXimage CentreRA CentreDec Cluster(s) czCMB Images texp (secs)
(J2000) (J2000) (kms−1) coadded NUV FUV
GI1 004001 A2734 001121.6 –285100 A2734 18249 3 3535
GI1 067001 UGC0568 0003 005508.9 –010247 A0119 12958 1556 3024
MISDR1 16976 0422 011429.3 +150146 A0160A 12794 1444 1444
GI1 004002 A0262 0001 015245.6 +360858 A0262 4464 1698 1698
NGA NGC1058 024326.6 +362539 A0376 14371 1265 1265
GI1 004003 A3104 0001 031421.6 –452512 A3104 21560 1588 1588
GI1 004027 A3158 0001 034257.6 –533748 A3158 17542 1026 887
GI1 004004 A3266 043124.0 –612624 A3266 17713 2 1441 1441
GI1 004005 A0548 054640.0 –253721 A0548A/B 12439 4 3155
GI1 004006 A3376 060143.2 –395924 A3376 14016 2 2176 2176
GI1 004007 A3389 0002 062157.7 –645735 A3389 8075 976
MISDR1 24335 0270 101307.7 –002325 A0957 13849 1703 1703
GI1 004025 A3528 0001 125357.6 –291348 A3528A/B 16764 1697
GI1 004008 A1644 0002 125712.0 –172436 A1644 14478 1163
GI1 009003 HPJ1321m31 0001 132105.8 –313220 A3556 14660 1615
GI1 004010 A3556 132526.1 –313607 A3556,A3558 14660 2 1805
GI1 004011 A3558 0001 132757.6 –313000 A3558 14660 1676
MISDR1 33707 0586 144246.3 +033911 MKW8 8449 1698 1698
GI1 004016 A1991 0001 145431.1 +183823 A1991 17741 967
GI1 004026 A2063 0001 152336.0 +083634 A2063 10444 1512
NGA NGC6166 162839.9 +393324 A2199 8872 1437 1437
GI1 004020 A3716 205157.5 –524648 A3716 13141 2 3179 1689
GI1 004021 A2399 0004 215719.1 –074759 A2399 17046 1322
GI1 004022 A2589 232357.6 +164647 A2589 12001 3 4252
GI1 004023 A4059 0003 235659.9 –344535 A4059 14660 635
quality at the edges. GALEX images have a plate scale of Table 2. Size of the galaxy samples, following the selection cri-
1.5 arcsec pixel−1 and a PSF FWHMof ∼5arcsec. teriaapplied.
We analyse infrared tiles from the J band (λ =
eff
1.25µm) of the Two Micron All Sky Survey (2MASS; NFPS SSC
Skrutskieet al. 2006). We measure directly from the tiles,
Originalsample 4527 541
rather than adopt photometry from the 2MASS extended
...withusableNUV andJ photometry 1493 307
source catalogue (XSC; Jarrett et al. 2000), as some of our
...withczandlogσ data 990 267
target objects are unresolved. Additionally, we require the
...afteremissionlinecut 920 156
PSF to match that of the NUV images (2MASS J PSF ......afteroptical apparentmagnitudecut1 544 101
FWHM ∼3 arcsec) and therefore Gaussian smooth the ......withstellarpopulationparameters2 – 87
2MASStilesbeforeanalysis.2MASStileshaveaplatescale ......withFUV andHγFdata3 222 –
of 1 arcsec pixel−1.
NOTE–see1(Sec3.1),2(Sec3.2)or3(Sec4.2)
SExtractor (Bertin & Arnouts 1996) was employed
indualimagemodetodetectallobjectsintheJ band.Pho-
tometry was measured for all sample targets in both bands
using a range of matched apertures: a Kron-type aperture
(SExtractor’s MAG AUTO) and seven apertures 3–34.5
arcsec in diameter (MAG APER). Throughout this work,
J band Kron apertures are used for total luminosity, and AB system (J = J + 0.91; Blanton et al. 2005),
AB Vega
matched 12 arcsec diameter apertures for NUV–J colours. and have been corrected for galactic extinction using
OurJ band photometry is in good agreement with 2MASS the reddening maps of Schlegel, Finkbeiner & Davies
(∼0.2 mag RMS) for objects in theXSC. (1998); AFUV=8.29×E(B–V), ANUV=8.87×E(B–V),
Targets with SExtractor apertures flagged as trun- AJ=0.902×E(B–V) (Cardelli, Clayton & Mathis 1989;
cated, or with a deblending error, have been removed from Schlegel et al.).
the sample, and only confirmed cluster members with red- EstimatesoftheK-correctionintheUVbandsarecur-
shiftandlogσ measurementswithintheSmith et al.(2004) rently derived empirically from poorly constrained spectra,
orSmith et al.(2007)datasets(NFPSandSSCrespectively) and therefore subject to large uncertainties. Kaviraj et al.
areusedintheanalysis.Table2liststhenumberofgalaxies (2006, fig. 22) estimate that the NUV–i correction would
in the samples at this stage, and after subsequent restric- be ∼0.1 mag throughout the range in redshift considered
tions. here. We do not apply a K-correction in this study. This
All colours and magnitudes are measured in the issue is addressed further in Section 4.4.
4 T. D. Rawle et al.
the galaxies removed by the emission cut from Section 2.3.
Priortotheemissioncut,SSChasalargercolourrangethan
NFPS.ThisisbecauseNFPSwasexplicitlyselectedonB–R
colourwhileSSConlyontotalRbandapparentmagnitude.
The Hαcut efficiently removes thevery blueobjects.
All target galaxies were within the 2MASS J band de-
tectionlimit,buttheNUV bandhasa5σ detectionlimitof
22.5–23.5 mag, depending on the coadded image exposure
time.Figure1(upperpanel)showsthatfortheNFPS,these
limits result in a bias against faint red galaxies. Assuming
the brightest NUV detection limit, a sample cut is applied
totheJ bandapparentmagnitudeat15.0mag.FortheSSC
sample, all of the targets appear on just two NUV images
with similar exposure times, and a (5σ) detection limit of
23.2 mag. Although in practice only two SSC targets have
a non-positive flux in the NUV band (compared to ∼8 per
cent in NFPS), for consistency, SSC has been treated in a
similar manner, with a cut applied at 15.8 mag. These cuts
areshownasdashedlinesinFigure1andensureacomplete
sampling of the colour range over the selected luminosity
interval.
Thecolour–(absolute)magnitudediagramsforthelow-
emissiongalaxiesarepresentedinFigure2.Thereisacorre-
lation between the luminosity and colour; brighter galaxies
tendtoberedder.However,thereisalargescatter;rmsdis-
Figure 1. Colour–magnitude diagram for the samples (NFPS persions of 0.37 and 0.30 mag for NFPS and SSC samples
upperpanel;SSClowerpanel).Greypointsarethoseremovedby respectively. The smaller scatter within the SSC sample is
emissionline criteriadetailed inSec. 2.3. The larger tail of grey
probably due to the slightly more restrictive Balmer emis-
points for SSC results from the lack of an explicit colour cut on
sion line criteria (see Sec. 2.3). The scatter in each sample
the originalsample. Solidlines are GALEX detection limitsand
does not increase by more than ∼10 per cent unless the
dashed lines are the applied J band apparent magnitude cuts.
Medianerrorbarsare∼0.05maginJ bandand∼0.1maginthe cut criteria are relaxed beyond an equivalent width of -1
colour. ˚A. Only 5 per cent of the scatter can be accounted for by
photometric measurement error. As intrinsic scatter domi-
nates, all correlations in this study are computed without
2.3 Emission-line cuts error weighting.
Table 5 summarises the CMRs for our two sam-
Inordertoconstructasampleofemission-freeredsequence
ples. Additionally, the CMRs measured in three
galaxies, a restriction is made on emission line strengths.
previous studies (Yiet al. 2005, Boselli et al. 2005,
Mindfuloftheeffectofnebularemission ‘fillin’fortheage-
Haines, Gargiulo & Merluzzi 2007) are shown for compari-
sensitive Balmer lines (Hβ, Hγ and Hδ), the preferred cut
son. Different sample selections were used for each of these
is on the Hα line. Unfortunately, Hα was not measured for
studies.Yiet al.usea‘UV-weak’early-typegalaxysample,
most NFPS galaxies, so the selection criteria of the origi-
selected from SDSS by concentration index and luminosity
nal NFPS reduction has been adopted (Nelan et al. 2005).
profiles, and then by the flux ratios F(NUV)/F(r) and
Specifically,thisinvolvesa cuton theHβ equivalentwidth,
EW(Hβ)>−0.6˚A(negativeEWdenotesspectrallineemis- F(FUV)/F(r) both being less than 0.07. The sample
covers a J band luminosity range comparable to our work.
sion)supplementedbyacutonOIIIλ5007,EW(OIIIλ5007)
>−0.8 ˚A. The cut on theSSC sample, which has Hαmea- Boselli et al. use a volume-limited sample of galaxies in
the Virgo cluster, with a subsample defined as elliptical
surements, follows the prescription of Smith et al. (2007),
whichusesEW(Hα)>−0.5˚A(approximatelyequivalentto by visual classification. Haines et al. use a volume limited
EW(Hβ) > −0.2 ˚A). The OIII cut would not remove any sample of local galaxies from SDSS, with the subsample
(labelled ‘passive red sequence galaxies’) restricted by the
additionalgalaxies.Thesecutsensureredsequencesubsam-
emission line criteria EW(Hα) >−2˚A.
ples, free of galaxies with a sizeable star formation compo-
Despitetheseselectiondifferences,ourderivedrelations
nentoranopticallystrongactivegalacticnucleus.Thedata
are in good agreement with the previous studies. The scat-
and photometry for the resulting subsamples are reported
ter is consistent (∼0.3–0.5 mag) given the different sample
in Tables 3 and 4.
definitions, and considerably large in comparison to that of
theoptical CMR (∼0.05 mag; Bower et al. 1992).
Velocity dispersion provides an alternative mass proxy
3 RESULTS to luminosity, and, for optical colours, the σ correlation
appears more fundamental (Bernardi et al. 2005). Figure 3
3.1 NUV–J colour relations
presents the samples in terms of their NUV–J colour and
Figure 1 shows the NUV–J colour–(apparent) magnitude logσ.Thereisaclearcorrelation, with slopesof0.73±0.11
diagram fortheNFPSandSSCsamples. Shownin greyare and0.65±0.17forNFPSandSSCrespectively,buttherms
NUV–IR colours of red sequence galaxies 5
Table 3.Data for galaxies inthe NFPS sample. Galaxy position is encoded inthe ID. czCMB (kms−1)is the mean cluster redshiftin
the CMB frame. czhel (kms−1) is galaxy redshift in the heliocentric frame. Magnitudes (Kron-type apertures) and colours (12 arcsec
diameterapertures)areintheABsystemandhavebeengalacticextinctioncorrected, butnotK-corrected.
GalaxyID cluster galaxy apparent J NUV–J FUV–J
cz cz
CMB hel
NFPJ043305.0–612235 17714 16144 15.750±0.071 6.732±0.142 7.205±0.250
NFPJ043306.7–612614 17714 17702 14.898±0.047 7.091±0.102 7.497±0.184
NFPJ043307.6–611338 17714 16356 14.659±0.042 6.979±0.095 7.659±0.195
NFPJ054415.7–255429 12939 10674 14.693±0.045 6.886±0.077 -
NFPJ054431.6–255550 12939 13247 14.783±0.047 6.767±0.086 -
Fullcontent ofthistableisavailableintheelectronicversion
Table4.DataforgalaxiesintheSSCsample.AsinTable3.SuperclustermeanczCMB=14660kms−1forallgalaxies.Stellarpopulation
parametersgivenwhereavailable(ageinGyr).
GalaxyID czhel apparentJ NUV–J log(age/Gyr) [Z/H] [α/Fe]
NFPJ132418.2–314229 13948 14.197±0.035 7.088±0.085 0.94±0.03 0.27±0.02 0.24±0.02
NFPJ132423.0–313631 14642 14.847±0.048 6.706±0.103 0.89±0.05 0.11±0.03 0.15±0.02
NFPJ132425.9–314117 13888 14.380±0.038 6.604±0.079 – – –
NFPJ132426.5–315153 14922 14.676±0.043 6.575±0.081 0.95±0.04 0.23±0.03 0.24±0.02
2MASXJ13250387–3132449 14266 14.792±0.049 6.626±0.109 – – –
Fullcontent ofthistableisavailableintheelectronicversion
Table 5.NUV–J CMRs.
original transformed rms
x y a b dispersion
NFPS J NUV–J –0.11 4.45 0.37
SSC J NUV–J –0.23 1.93 0.30
Y051 r NUV–r –0.23 1.88 0.58
B052 H NUV–H –0.07 4.89 0.47
H073 r NUV–r –0.18 2.66 0.37
Notes – Original x and y parameters are given for reference.
a and b are for relations in the form NUV–J = aJ +b, as-
suming the following colours: (J–H)AB=0.2, RAB=r–0.21, (J–
R)AB=−0.8. 1from Yietal. (2005). 2from Bosellietal. (2005).
3fromHaines,Gargiulo&Merluzzi(2007).
vironments than our samples. Their blue galaxy fractions
are plotted in Fig. 4 and show a markedly higher fraction
for a given velocity dispersion. For example, our two sam-
ples have a blue fraction of ∼40 per cent only at the low-
est sigma (logσ < 1.8), while the Schawinski et al. sample
reaches this blue fraction at logσ =2.2. At face value, this
result suggests a large difference between field and cluster
Figure2.NUV–J colour–absolutemagnitudediagramforboth
samples, with best fits in black. Relations from previous stud- galaxy populations. However, the differences in sample se-
iesingrey:Yietal.(2005)=dashed,Bosellietal.(2005)=dotted, lectionhavetobeconsidered(Schawinski et al.useasample
Haines,Gargiulo&Merluzzi (2007)=dash-dotted. Median error selected on morphology), which is beyond the scope of this
barsare∼0.05maginJ bandand∼0.1maginNUV–J. study.
scatter (0.36 and 0.32 mags) is indistinguishable from that
3.2 Stellar population parameters in the Shapley
of the CMR.
sample
Figure 4shows thefraction of‘blue’galaxies asafunc-
tion of logσ for our two samples. Blue galaxies are defined In order to investigate the physical origin of the large in-
by NUV–J < 6.4 mag, as used in Schawinski et al. (2006) trinsicscatterfoundin theNUV–J colour, weexaminethe
whostudySDSSgalaxiesprobingtomuchlowerdensityen- relationship between colour and the stellar population pa-
6 T. D. Rawle et al.
Figure 3. NUV–J vs logσ for NFPS (upper panel) and SSC
(lowerpanel).Medianerrorbarsare∼0.02and∼0.01forlogσin
thetwosamplesrespectivelyand∼0.1inthecolour.
Figure 5. NUV–J (upper) and B–R (lower; from
Mercurioetal.) colours vs log(age) for the SSC sample.
SymbolsizerepresentsJ bandluminosity;larger=brighter.Note
that the lower panel has a much smaller range in the vertical
scale.
NUV–J and age for the remaining objects would be corre-
lated. Wefind only a marginal correlation between age and
NUV–J,withaslopeof0.46±0.25andanintrinsicscatter
(after accounting for the measurement error) of 0.33 mag.
FromtheupperpanelofFig.5,itisalsoapparentthatmore
Figure 4.FractionofgalaxieswithNUV–J <6.4asafunction luminous, and by inference larger, red sequence galaxies (J
of logσ. NFPS: squares/solid line; SSC: triangles/dashed line.
bandluminosityisshowninFigs.5–7bythesymbolsize)are
EstimatesfromFigure1ofSchawinskietal.(2006)aregivenfor
not solely confined totheredder NUV–J colours, although
comparison(circles/dottedline)andshowamuchsteeperincrease
they dotend to be theoldest.
inthenumberofbluegalaxieswithdecreasingσ.
Figure 5 (lower panel) shows the relation between age
and B–R colour for the same sample of galaxies (note the
rameters (age, metallicity, α-abundance) for the SSC sam- much smaller range in the vertical scale). There is a strong
ple. correlation;aslopeof0.21±0.04 andrmsdispersionof0.05
In the following analysis, involving only the SSC sam- mag. This figure confirms that the NUV scatter is not due
ple, the derived limit on J band apparent magnitude (Sec. to contamination by optically blue galaxies.
3.1) is not applied, allowing a larger number of galaxies. The analogous NUV–J correlation with the metallic-
(It should be noted that thesubsequent conclusions are ro- ity is given in Figure 6. There is a strong trend between
bust against the use of the J band cut.) The emission line [Z/H] and NUV–J, with a slope of 1.27 ± 0.23 and an
selection criteria are retained, and thesample is further re- rms dispersion of 0.32 mag (∼90 per cent of which is in-
strictedbytheoverlapofthephotometricandspectroscopic trinsic scatter). The lack of galaxies to the top left is not
datasets (see Table 2). the result of a selection effect. Of the two target galaxies
NUV–J vslog(age)isshowninFigure5(upperpanel). undetected in the NUV band (see Sec. 3.1), only one has
Galaxies with nebular emission have been removed via the a low metallicity. Assuming the non-detection is due to a
emission linecriteria.Therefore,iftheUVsourcesalsocon- redder-than-average colour, this would add a single galaxy
tributetotheopticalfluxandhavestrongBalmerlines,the to the upper left of the plot, but would not significantly
NUV–IR colours of red sequence galaxies 7
Figure8.NUV–J residualsfromthe[Z/H]relationvslog(age).
Figure 6. NUV–J vs metallicity [Z/H]. Symbol size represents
SymbolsizerepresentsJ bandluminosity;larger=brighter.
J bandluminosity;larger=brighter.
0.3mag(∼0.25magintrinsicscatter).Thisisincontrastto
the case of the B-R colour where the majority of the scat-
ter can be attributed to age and metallicity (Smith et al.
in prep.).
4 DISCUSSION
This section explores possible causes of the large intrin-
sic scatter observed in the NUV–J colour. We investigate
aperturebias,theUVupturnphenomenonand‘frosting’by
youngormetalpoorsubpopulations. Additionally,wecom-
ment on theuncertaintyin theNUV band K-corrections.
4.1 Aperture bias/morphology
The spectroscopy used to estimate the stellar parameters
is derived from 2 arcsec diameter fibres, whilst the NUV–
Figure 7. NUV–J vs α-abundance [α/Fe]. Symbol size repre-
J coloursarefrom12arcsecdiameteraperturephotometry.
sentsJ bandluminosity;larger=brighter.
Eliminatingaperturebiascompletelywouldrequirematched
apertures for the photometry and spectroscopy, but unfor-
affect the fit. In general, the most luminous galaxies form tunately thePSF of the2MASS and GALEX images is too
a ridgeline at redder colours, although there are significant large for reliable 2 arcsec aperture photometry.
bright outliers to this trend (the most obvious being the Broadband colours and spectroscopic measurements
bright,blue,metal-richgalaxyontheright;NFPJ132729.7– (e.g. Tamura & Ohta 2004, S´anchez-Bl´azquez et al. 2007)
312325). Lower metallicity galaxies tend to be bluer and show that elliptical galaxies generally have flat radial pro-
less luminous. This supports Rampazzo et al. (2007), who files in age, and regular metallicity gradients (decreasing
use simulations to predict a correlation of NUV–IR colour [Z/H]withradius).Theapertureeffectthereforeflattensthe
with metallicity, but little dependence on age in popula- NUV–J vsZ/Hrelation,aslarger,reddergalaxieswilltend
tions greater than 2–3 Gyr after a star formation episode. toexhibithighermetallicitieswithinthefibre.However,the
However, the slope of our observed metallicity trend is 2–3 effectissmall,withonly∼0.1dexchangeinmetallicityover
times weaker than that derived from theoretical spectra by a 1 dex differencein aperture radius.
Dorman, O’Connell & Rood (2003). Inspectionofthegalaxiesinhighresolutionimagescan
Modelevolutionarytrackssuggestthatstellarevolution ascertain whether there are morphological peculiarities, or
dependsonα/Fe(Salasnich et al.2000;Dotteret al.2007). neighbouringobjects, contributingtoanenhancedlargera-
However,wefindnodiscerniblerelationbetweentheNUV– dius NUV flux. Figure 9 shows colour vs age and metallic-
J colourandtheα-abundanceinthissampleofredsequence ity(asinFigs. 5–6),highlightingthefewgalaxies thathave
galaxies (rms scatter of 0.37 mag; Fig. 7). highresolutionHST-ACSimagesavailable.Fortunately,one
Figure 8 shows the residuals from the NUV vs metal- of these objects is the most obvious outlier in the whole
licity relation (Fig. 6) against log(age). Although theresid- sample (NFPJ132729.7–312325); it is the reddest in B–R,
uals are more strongly correlated with age (∼3σ) than the buthasan unusuallyblueNUV–J colour forsuch ametal-
coloursthemselvesare,thermsdispersionisonlyreducedto rich,luminousgalaxy.IntheHST-ACSimage(Fig.10),the
8 T. D. Rawle et al.
low mass, metal-rich, helium burning stars with small en-
velopes currently thought to be the most likely candidate
fortheUVupturn(O’Connell1999).TheUVupturnisone
ofthemostheterogeneousphotometricpropertiesofoldstel-
larpopulationsinearly-typegalaxies,withaspreadofupto
∼4 magnitudes in the FUV (O’Connell 1999), so certainly
seems a plausible explanation for thescatter.
The hot UVX component appears in the spectra as a
smoothcontinuumwithanabsenceofemission andabsorp-
tion lines. Hence, the FUV flux from a galaxy can be used
toestimatetheextentofthecontributionintheNUV band
(Burstein et al.1988).Unfortunately,noneoftheSSCgalax-
ieshaveavailable FUV photometry,soin ordertoestimate
theextentoftheUVXcontribution,thecorrectionsarecal-
culated for all objects in the NFPS sample that have the
necessaryFUV data.However,stellarpopulationageshave
not been derived for individual NFPS galaxies, so instead
Figure9.Colourvsstellarparameters,highlightinggalaxieswith wecomparethecorrectedNUV–J colourstothetraditional
availableHST-ACSimages.SymbolsizereflectsJ bandluminos- ‘age-tracing’ Balmer line HγF. This line does not trace age
ity(larger=brighter).Inallcases,ACSconfirms early-typemor- cleanly, being affected toa small degree by themetallicity.
phology,uncontaminated byneighbours. HγFvsNUV–J for theNFPS sample isshown in Fig-
ure 11. Given the relative strengths of the colour vs. age
and [Z/H] relations (Figures 5 and 6), the correlation seen
here is most likely a reflection of the metallicity, rather
thanage,dependenceofHγF.Forcomparison,theobserva-
tionshavebeenoverlaidbyage/metallicitygridsconstructed
from themodelsofMaraston (2005,M05;upperpanel) and
Bruzual & Charlot (2003, BC03; lower panel). Both grids
lie redward of the observed data, most likely due to nei-
ther model including EHB stars, although the BC03 grid
provides the better description of the data. Previous stud-
ies (e.g. Salim et al. 2007) have noted that while the BC03
models do not explicitly include EHB stars, the UV light
from old stellar populations (primarily post-AGB stars) re-
produce several of the correlations found in observational
data, including the relation between FUV–NUV and B–V
Figure 10. HST-ACS image of the outlier NFPJ132729.7– colours (see Donas et al. 2006,particularly fig. 5).
312325, witha12arcsecdiameteraperturemarked.
Dorman, O’Connell & Rood (2003) introduced correc-
tions to their NUV–V colours by considering the relative
contributions of the hot and cool (i.e. non-UVX) compo-
galaxyappearstobealargeellipticalwithnoabnormalities.
nents at FUV, NUV and V. They assumed a negligible
Therearealsonoobviouscontaminating objectsthatcould
contribution of the cool component to the FUV flux, but
beresponsible for an anomalous bluecolour.
allowed hot stars to contribute at V. Here, we use analo-
Oncloseexamination,theothertargetswithHST-ACS
gous corrections for the NUV–J colours, which are simpler
imagesalsoappeartobe‘normal’ellipticalsofvarioussizes,
becauseitissafetoassumenoUVXcontributionatJ.The
with no obvious peculiarities or large radius NUV contrib-
corrected colours are given by
utors. We conclude that a large scatter in NUV–J colour
ispresent evenin objects with confirmedregular early-type (NUV −J) =−2.5×log[10−0.4(NUV−J)obs
corr
morphologies and no contaminants. Additionally, a scatter
of ∼0.3–0.35 mag is still obtained when smaller matched −α×10−0.4(FUV−J)obs] (1)
apertures (4.5 arcsec diameter) are used for the colour, de-
where α ≈ 0.3 is the ratio of NUV to FUV flux for the
spite the probability of a contaminating neighbour being
hot component (appropriate for a T = 24000K star, see
reduced by ∼85 percent. eff
Dorman et al.).
The resulting corrections are shown as vertical lines in
Figure 12 (where the points indicate the value of the cor-
4.2 UV upturn
rectedcolour).Thecorrectionsmovethecoloursredwardas
Another possible explanation for the scatter in NUV–J expected,butareonlyoftheorderof∼0.2magnitudes,and
colour is NUV contamination by the UV upturn (UVX). do not reduce the scatter in the NUV–J (0.29 mag before
The Thomas et al. (2003) models used in the stellar popu- corrections,0.30magafter).Obviouslytherearenoextreme
lations parameter calculations include low-metallicity blue UVX galaxies with large hot component contributions to
horizontal branch sub-populations and thermal pulsing the NUV flux in this sample, and as such the UVX phe-
asymptotic giant branch stars, but they do not include the nomenonisunlikelytoresponsibleforthescatterinNUV–
NUV–IR colours of red sequence galaxies 9
Figure12.NUV–J (withoutK-correction)againstHγFBalmer
index for galaxies with available FUV data. Points indicate the
corrected colour, with the ‘tail’ showing the correction vector.
BC03 grid as in Fig. 11 (lower panel). Median error bars are
showntopright.
∼30percentofthegalaxies cannotbeaccountedforbythe
SSPmodel.
Vectors of the frosting effect on NUV–J colour and
spectroscopicagehavebeencalculatedusingtheBC03mod-
els. Frosting of a 15 Gyr base population by a 1.5 Gyr
population with a mass fraction µ=0.03, and by a 0.7 Gyr
population of µ=0.01 are shown. The M05 models result in
Figure11.NUV–J (withoutK-correction)againstHγFBalmer marginally steeper frosting vectors, as would be expected
index for NFPS galaxies. Upper panel: M05 grid (age={5,12,15 giventhesteeperSSPtrack.Thevectorsshowthatfrosting,
Gyr}, [Z/H]={–0.33,0.00,+0.35}). Lower panel: BC03 grid even at a modest level of 1–2 per cent for a ∼1 Gyr popu-
(age={5,12,15 Gyr}, [Fe/H]={–0.33,+0.09,+0.55}). (5,–0.33) is lation, could account for a sizeable portion of thescatter in
the bluest (age,[Z/H]) grid point. Median error bars are shown
the NUV–J colour. A young population of this size would
topright.
not be apparent in theB–R colours.
Thespectroscopicageissensitivetofrostingviathein-
creased hot-star contribution to theBalmer lines, and for a
J for NFPS.Wespeculatethat theUVXeffects intheSSC
givenchangetothespectroscopicage,theUVcoloursareaf-
sample are smaller because, on average, the galaxies have
fectedmorestronglybyfrostingthanbyloweringtheageofa
lower luminosities.
single-burstpopulation.ThissupportstheassertionthatUV
coloursarepartlydependentonlowlevelrecentstarforma-
tion (Ferreras & Silk 2000, Kaviraj et al. 2006, Salim et al.
4.3 Stellar population ‘frosting’
2007).However,therearespectroscopicallyoldgalaxieswith
Thesimple modelscanbegeneralised byconstructingcom- blue colours (∼10 per cent of the sample) which cannot be
posite stellar populations. A common invocation of this is accounted for bythefrosting scenario described above.Ad-
residualstarformationintheformof‘frosting’galaxieswith ditionally,RoseCaIIindexresults(Smith et al.in prep.)ap-
a small mass fraction of young stars (Trager et al. 2000), pearnottosupportthepresenceofyoungstellarpopulations
which manifests itself in the observables as a bluer colour (. 1.5 Gyr) in themajority of red sequence galaxies.
andayoungeragethanthebasepopulation.Frostingshould Formost of thisstudy,wehaveneglected thebluehor-
not affect the spectroscopic metallicity or α-abundancesig- izontal branch (BHB) as cluster red sequence galaxies have
nificantly as these are primarily driven by the larger, older [Z/H]∼0.However,itispossiblethatalowmetallicitypop-
population (Serra & Trager 2007). ulation with a BHB morphology may be present in some
Figure 13 shows the extent to which frosting can ac- galaxies (Maraston & Thomas 2000). Therefore, an alter-
countforthescatter,aftertheeffectofmetallicity hasbeen native ‘frosting’ scenario consists of a low mass fraction,
removedusingthecorrelationofFig.6.Simplestellarpopu- old, low metallicity, BHB stellar population embedded in a
lation(SSP)tracksfromboththeM05andBC03modelsare [Z/H]=0galaxy.Estimates from theM05models showthat
shown.TheslopeoftheBC03trackfitstheobservedreden- ‘frosting’ by a 4–5 per cent mass fraction population with
velopewell, whereas M05 predicts amuchsteeper variation [Z/H]=–1.35 could give an NUV–J colour ∼1 mag bluer,
withage.Allowingforthesystematiceffectsofaperturebias whilechangingtheB–Rbyonly∼0.03mag.Theeffectofthe
and the UV upturn(∼0.3 mag bluer in observed NUV–J), low-metallicity frosting on the derived spectroscopic age is
10 T. D. Rawle et al.
Figure 14. CMR residuals for the NFPS sample (Fig 2; upper
Figure 13. Simple stellar population tracks from the M05 and
panel)vsheliocentricgalaxyredshift.Thefilledsquaresandsolid
BC03 models shown in the context of the NUV–J colour (cor-
lineshow the predicted trend if the K-corrections from the M05
rectingto[Z/H]=0usingthecorrelationfromFig.6)vslog(age).
modelswerenecessary,butnotapplied.
Vectors indicate two BC03 frosting scenarios with minor popu-
lationsof{µ,age} ={0.03,1.5Gyr} and{0.01,0.7Gyr} (solidand
dashedvectorrespectively).
This implies the uncorrected residuals should have a steep
correlation with redshift, which is not observed. A flatter
K-correction is preferred by the data, as in Kaviraj et al.
likelysimilartotheeffectoffrostingbyayoungcomponent,
(2006).AlongwiththeEisenstein et al.spectrumdescribed
sincebothcasesaredrivenbyincreasedA-starcontribution
above, this result highlights the uncertainty in the UV K-
to theBalmer lines.
corrections (as well as the problems of stellar population
models in theUV).
4.4 NUV K-corrections
K-corrections for the UV bands are poorly constrained, as 5 CONCLUSIONS
the spectral shape of galaxies at wavelengths shorter than
∼3000˚Aisnotwellknown.PreviousstudiesoftheUVCMR Using GALEX UV and 2MASS J band photometry, we
largely ignore the K-correction, or in the case of Yi et al. have investigated the relationship between UV–IR colours
(2005, who estimate corrections of 0.1−0.2 mag for z = andspectroscopically derivedstellar population parameters
0−0.25) apply correcions based on theluminosity distance (age,metallicityandα-abundance)forredsequencegalaxies
without considering the spectral shape of galaxies. in local clusters.
Kaviraj et al. (2006) compute NUV K-corrections for Weselectgalaxiesusingstrictemissioncriteriatoavoid
best fit model SEDs derived from SDSS and GALEX pho- contaminationfromgalaxieswithveryrecentstarformation.
tometry.Corrections of∼0.1magwerefoundforredshiftsz WeanalysetheNUV colour–magnituderelation(CMR)for
< 0.1. However, corrections for their model of a 9 Gyr old ourtwo samples of quiescent galaxies (920 in NFPS;156 in
simplestellarpopulation aremuchlarger(0.4−1.0magfor SSC),andfindrmsdispersionsof0.37and0.30mag(intrin-
0.04 < z < 0.11). sicscatterof0.36and0.29mag)respectively.Thisissimilar
As an illustration, we calculate K-corrections from the to previously reported values of ∼0.5 mag and is an order
models of Maraston (2005, M05) at various redshifts (0.015 of magnitude larger than the scatter in the optical CMR
<z<0.072),bothforsimplepopulationsandgalaxiesinthe (∼0.05 mag).
‘frosting’scenariodescribedinSec4.3.K-correctionsof0.2– We compared the NUV–J colour to the spectroscopic
1.2mag areobtained,dependingon themetallicity and age stellar population parameters for 87 galaxies in the SSC
(increasingeitherincreasesthecorrection),andonthemass sample and found thefollowing:
fractionoftheyoungstellarcomponent.Thecorrectionsare • ThereisasignificantNUV–J vsmetallicitytrend,with
dominatedbythe2640˚A spectralbreak(cfEisenstein et al. a slope of 1.27±0.23 and an rms dispersion of 0.32 mag.
2003),whichisredshiftedcompletelyoutoftheNUV band • There is only a weak NUV–J vs age trend after the
byz≈0.07.InEisenstein et al.,whostudytheaveragespec-
metallicityeffecthasbeenremoved,andnocorrelationwith
tra of 726 luminous, red, SDSS galaxies at 0.47<z <0.55,
α-abundance.
the break appears less prominent than in the M05 spectra • There is a large intrinsic scatter (∼0.25 mag) in the
(a ∼50 per cent drop in fluxas opposed to ∼80 per cent).
NUV–J colour at fixed age and metallicity which cannot
Fig 14 shows the regression line residuals from the
beeasily accounted for with simple stellar populations.
colour–magnitudediagramfortheNFPSsample(Fig2;up-
per panel) plotted against the galaxy redshift in the he- The unexpected blue colours of at least some objects,
liocentric frame. The solid line shows the expected trend includinganinfluentialoutlier,cannotbeattributedtolarge
if the M05 K-corrections were necessary, but not applied. radiuscontamination from otherobjects, and aperture bias