Peculiar early-type galaxies in the SDSS Stripe82

14 Pages·2010·2.17 MB·English

Mon.Not.R.Astron.Soc.000,1–??(2009) Printed22April2010 (MNLATEXstylefilev2.2)
Peculiar early-type galaxies in the SDSS Stripe82
Sugata Kaviraj⋆1,2,3
0
1Blackett Laboratory, Imperial College London, London SW7 2AZ, UK
1
2Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT UK
0
3Department of Physics, University of Oxford, KebleRoad, Oxford OX1 3RH, UK
2

r
p
A 22April2010

1
2 ABSTRACT
We explore the properties of ‘peculiar’ early-type galaxies (ETGs) in the local Uni-

] verse, that show (faint) morphological signatures of recent interactions such as tidal
O
tails, shells and dust lanes. Standard-depth (∼ 51s exposure) multi-colour galaxy
C images from the Sloan Digital Sky Survey (SDSS) are combined with the signif-
. icantly (∼2 mags) deeper monochromatic images from the public SDSS Stripe82
h
(−50◦ < α < 59◦,−1.25◦ < δ < 1.25◦) to extract, through careful visual inspec-
p
- tion, a robust sample of nearby (z < 0.05), luminous (Mr < −20.5) ETGs, includ-
o ing a subset of ∼ 70 peculiar systems. ∼ 18% of ETGs exhibit signs of disturbed
r morphologies (e.g. shells), while ∼ 7% show evidence of dust lanes and patches. An
t
s analysis of optical emission-line ratios indicates that the fraction of peculiar ETGs
a
that are Seyferts or LINERs (19.4%) is twice the corresponding values in their re-
[
laxed counterparts (10.1%). LINER-like emission is the dominant type of nebular

1 activityinallETGclasses,plausiblydrivenbystellarphotoionisationassociatedwith
v recent star formation. An analysis of UV-optical colours indicates that, regardless
5 of the luminosity range being considered, the fraction of peculiar ETGs that have
7 experienced star formation in the last Gyr is a factor of ∼ 1.5 higher than that
7
in their relaxed counterparts. The spectro-photometric results strongly suggest that
3
the interactions that produce the morphological peculiarities also induce low-level
.
4 recent star formation which, based on the recent literature, are likely to contribute
0 a few percent of the stellar mass over the last ∼ 1 Gyr. Peculiar ETGs preferen-
0
tially inhabit low-density environments (outskirts of clusters, groups or the field),
1
either due to high peculiar velocities in clusters making merging unlikely or because
:
v shell systems are disrupted through frequent interactions within a cluster crossing
i time. The catalogue of galaxies that forms the basis of this paper can be obtained
X
at:http://www.mssl.ucl.ac.uk/∼ska/stripe82/skavirajstripe82.datoronrequestfrom
r the author.
a
Key words: galaxies: elliptical and lenticular, cD – galaxies: peculiar – galaxies:
interactions – galaxies: evolution – galaxies: formation
1 INTRODUCTION lar populations in these galaxies has formed at high red-
shift (z > 1), possibly over short timescales of ∼ 1 Gyr
Given their dominance of the stellar mass density in the
(Thomas et al. 1999). The mass assembly at late epochs
local Universe (Bernardi et al. 2003a), understanding the
(z < 1) is better probed using rest-frame UV data (Yi et
formation and subsequent evolution of early-type galax-
al. 2005; Kaviraj et al. 2007a, 2007b, 2008; Schawinski et
ies (ETGs) is a key element of the modern galaxy evolu-
al. 2007a), which is more sensitive to the small fractions of
tion effort. Their obedience to simple optical scaling rela-
young stars that are expected to form in early-types in the
tions, such as the Fundamental Plane (e.g. Jorgensen et al.
standard model (e.g. Cole et al. 2000; Hatton et al. 2003;
1996; van Dokkum& Franx 1996; Saglia et al. 1997) and
Kaviraj et al. 2005; DeLucia et al. 2006; 2007), plausibly
the colour-magnitude relation (e.g. Bower et al. 1992;
throughrepeatedminormergeractivity(Kaviraj et al.2009;
Ellis et al. 1997; Stanford et al. 1998; Gladders et al. 1998;
2010).
van Dokkumet al.2000),indicatesthatthebulkofthestel-
In the context of the standard model, in which merg-
ers play an important role in the buildup of stellar mass
⋆ E-mail:[email protected] in present-day ETGs (e.g. Cole et al. 2000; Hatton et al.
2 Sugata Kaviraj
2003; De Lucia et al. 2006), there is a need to study the nitudesandaspartoftheSDSSSupernovaSurvey(Frieman
role of galaxy interactions and to explore the star for- etal.2008).ThepublicStripe82imaging(released withthe
mation that accompanies these events. The role of merg- SDSS DR7) offers an r-band co-addition of 122 runs, with
ers in the nearby Universe has recently been explored individual areas of the ∼ 300 deg2 stripe area having been
through studies of close pairs (e.g. Patton et al. 2000; covered between 20 and 40 times (Abazajian et al. 2009).
Le F`evreet al. 2000; Patton et al. 2002; Lin et al. 2004; The Stripe82 imaging reaches ∼2 mags deeper than single
Bell et al. 2006; McIntosh et al. 2008; Bundy et al. 2009; SDSSscans (which havemagnitude limits of 22.2, 22.2 and
Darg et al. 2010a,b) and quantitative estimates of struc- 21.3magsintheg,r andi-bandsrespectively).Ther-band
tural morphology (e.g. Conselice et al. 2003). The tell-tale coaddsusedinthispaperthushaveanominaldepthof∼24
signatures of recent interactions and their impact on the mags.
ETG population can be most efficiently studied by ex- The novelties of this paper are two-fold. First, we si-
ploring ETGs that exhibit morphological peculiarities and multaneouslyinspectstandardmulti-colourimagesfromthe
many important studies have contributed to a sizeable lit- SDSS DR7 and their deeper Stripe82 counterparts to both
erature on peculiar ETGs. Early-type galaxies which ex- identify ETGs and flag members of the ETG population
hibit systems of shells (see e.g. Malin & Carter 1980; 1983; that show morphological signatures of recent interactions
Malin et al.1983;Schweizer et al.1990;Schweizer & Seitzer (e.g. shells, tidal tails and dust lanes). Second, we explore
1992; Sikkemaet al. 2007) have long been recognised as thespectro-photometricpropertiesofthepeculiarETGpop-
remnants of recent merging, most probably through minor ulation (exploiting optical emission lines and UV-optical
mergers with various orbital confiurations (see e.g. Quinn colours) and studytheir local environments.
1984; Prieur 1988; Hernquist& Spergel 1992). The pres- Thecataloguepresentedhereisexpectedtohavehigher
ence of dust lanes in spheroidal systems - NGC 5128 be- purityinthemorphologicalclassificationsthanitspredeces-
ing the archetype in the local Universe (see e.g. Graham sors, with the added advantage of identifying ETGs that
1979)-isthoughttobetheresultofaccretionofcoldmate- exhibit morphological signatures of recent interactions. It
rial from gas-rich companions within the last few Gyrs (see provides a useful reference sample of peculiar ETGs from
Hawarden et al. 1981;and references therein). the SDSS, with a homogeneous set of spectro-photometric
WiththeadventoftheSloanDigitalSkySurvey(SDSS; productsavailablefromtheSDSSpipeline,whichissuitable
Abazajianetal.2009),representativesamplesoflocalETGs for follow up using current and future instruments. Forth-
have been culled and their star formation histories (SFHs) coming companion papers will explore the structural pecu-
exploredusingphotometricandspectroscopicdiagnosticsof liarities such as shell systems and dust lanes in more detail
star formation activity (e.g. Bernardi et al. 2003a; Kaviraj and correlate them with the star formation and chemical
et al. 2007b, Kaviraj 2008). A key aspect of such work is enrichment histories of theindividual galaxies.
the identification of galaxy morphologies from survey im-
ages. Given the prodigious output of modern observational
surveys such as the SDSS, a useful strategy to tackle the
2 GALAXY SAMPLE AND VISUAL
largevolumesofdataisoftentouseproxiesforgalaxymor-
INSPECTION
phology, such as colour (since early-typesdominate the red
sequence) or pipeline parameters such as the concentration Webeginbydescribingtheprocessofvisualinspectioncar-
index(sinceearly-typeshavemorestronglypeakedlightpro- riedoutonthesampleofgalaxiesexploredinthisstudy.To
files than spirals). Significant efforts havebeen made to ex- maximise the accuracy of the morphological classifications,
ploit the SDSS pipeline to separate the galaxy population werestrict ourselvestobrightgalaxies (Mr <−20.5) in the
intoitsconstituentmorphologicaltypesandstudytheprop- nearby Universe (z <0.05).The total numberof galaxies in
erties of early-type systems (see e.g. Eisenstein et al. 2001; theparent sample is 902.
Bernardi et al. 2003a,b,c; Park & Choi 2005). Each galaxy is morphologically classified by simulta-
While pipeline classification of morphologies is reason- neous inspection of its multi-colour standard-depth image
ably efficient, there has been a realisation that the most and its deeper monochromatic Stripe82 counterpart. Note
accurate method of estimating galaxy morphology remains that the images used in this study are those provided by
direct visual inspection of galaxy images (e.g. Fukugita et the public SDSS DR7 release. The objects are carefully
al. 2007; Schawinski et al. 2007; Lintott et al. 2008). While assigned to three main morphological classes: early-type
inspection of standard SDSS images does allow us to iden- galaxies (ETGs), late-type galaxies (LTGs) and ‘Sa-like’
tify early-typeand late-type systems with reasonable accu- systems which are bulge-dominated galaxies with faint
racy, at least in the redshift range z <0.1 (see e.g. Lintott spiral features that are visible in the Stripe82 imaging but
et al. 2008), a drawback is the relatively low exposure time invisible in the standard-depth images. The ETG popula-
(∼51seconds)andtheresultantlackofdepth.Deepimages tion is furthersub-dividedinto thefollowing categories:
inevitably showrichermorphological detail and,in particu-
lar for early-type galaxies, often reveal signatures of recent (1) Relaxed ETGs
merging such as fans, shells and tidal debris (van Dokkum Early-type galaxies that do not exhibit any signs of mor-
2005). phologicaldisturbancesatthedepthoftheStripe82images.
InthisstudyweaimtocombinestandardSDSSimaging
with that in the SDSS ‘Stripe82’, a stripe along the celes- (2) ETGs with tidal features
tial equator in the Southern Galactic Cap (−50◦ < α < Early-type galaxies that exhibit shells, fans and tidal tails
59◦,−1.25◦ < δ < 1.25◦) that has been imaged multiple (typically in the Stripe82 images). Note that this category
times, both to allow the stacked data to reach fainter mag- includes a small number of systems that are in the early
Peculiar early-type galaxies 3
Figure 1.Examplesofrelaxedearly-types(topthreerows)andgalaxiesclassifiedaslate-type(bottom threerows).Weshowboth
themulti-colourstandard-depth image(left-handcolumn)anditsdeeperStripe82counterpart(right-handcolumn).
4 Sugata Kaviraj
Figure2.Examplesofearly-typeswithmorphologicaldisturbances(shellsandtidaldebris).Notethatthefeaturesareonlyvisible
intheStripe82images(right-handcolumn)andareinvisibleinthestandard-depthimages(left-handcolumn).
Peculiar early-type galaxies 5
Figure 3. Moreexamples of early-types with morphological disturbances (shells and tidal debris). Note that the features areonly
visibleintheStripe82images(right-handcolumn)andareinvisibleinthestandard-depthimages(left-handcolumn).
6 Sugata Kaviraj
Figure4.Examplesofearly-typeswithdustlanes,visibleinthemulti-colourSDSSimaging.Typicallytheseobjectsalsoshowtidal
featuresinthedeepStripe82imaging.
Peculiar early-type galaxies 7
Figure 5. Examples of systems that appear to be the progenitors of early-type galaxies, in the early stages of relaxation after a
recentmergerorinteraction.
8 Sugata Kaviraj
stages of relaxation after a merger and could be early-type 2.1 Comparison to previous studies of
progenitors. morphologically disturbed early-types
Itisusefultocompareourresultstoprevioussurveysofmor-
(3) ETGs with dust features
phologically disturbed ETGs in the local Universe. The de-
Galaxies that show the presence of dust patches and
tectionoftidalfeaturesisclearlydependentonthedepthof
dust lanes (typically in the multi-colour standard-depth
theimaging in question or,morespecifically, on thesurface
imaging).
brightness (SB) limits reached by the survey. In the 1980s
and 1990s two deep studies of early-type galaxies were per-
Note that, in what follows, we refer to categories (2)
formed by Malin & Carter (1983) and Schweizer & Seitzer
and (3) collectively as ‘peculiar ETGs’. The classification
(1992). Both studies reached surface brightness (SB) limits
processisrepeateduntilthenumbersintheETG,LTGand of µB ∼ 26.5 mag arcsec−2, which translates to µr ∼ 26
Sa categories begin to converge and the numbers in these mag arcsec−2 for a purely old stellar population. The Ma-
categories betweenclassification runsarewithin 1% ofeach
lin&Carterstudydetectedtidalfeatures,shellsandripples
other.Inourcase,suchconvergencewasachievedafterfour
in roughly 17% of their elliptical galaxy sample (which was
such runs. Note also that between runs 3 and 4 the sample
comprisedof137objects).TheSBlimit ofthestandardex-
of peculiar ETGs and Sa-like systems did not change. posure SDSS r-band images is µr ∼ 24 mag arcsec−2 (see
We find that ∼34% of the Stripe82 galaxy population theSDSSprojectbook1)whichyieldsanestimateofµr ∼26
areclassifiedasETGs.AtthedepthoftheStripe82andDR7 mag arcsec−2 for the deeper SDSS Stripe82 images used in
images,∼18%ofETGsshowtidalfeatures,while∼7%show
this study. Given the similar SB limits, it is not surpris-
evidencefor dust lanes and patches.
ingthatthefractionofETGslabelledasmorphologicaldis-
In Figure 1 we present examples of galaxies classified
turbedinthispaperaresimilartothefractionsreportedby
as relaxed ETGs (top three rows). For comparison we also
Malin & Carter (1983).
show objects classified as LTGs (bottom three rows). The
The deepest survey of ETGs in the local Universe has
standard-depthmulti-colourimageisshownintheleft-hand
been performed by van Dokkum (2005), who studied the
columnanditsdeeperStripe82counterpartisshownonthe
redsequencepopulation observedbytheMUSYC(Gawiser
right. Figures 2 and 3 present typical examples of peculiar
et al. 2006) and NOAO Deep Wide-Field (Jannuzi et al.
ETGs that exhibit tidal features such as shells. Notice that 2004) surveys down to SB limits of µ ∼ 29 mag arcsec−2
the features are invisible in the standard DR7 images. Fig-
(thepointsourcelimitswere∼27mags).Thestrikingresult
ure4presentsexamplesofgalaxieswhichexhibitdustlanes.
from this study was that ∼ 70% of the ETGs on the op-
Thepresenceofdustfeatures isoften accompanied bymor-
tical red sequence showed signs of disturbed morphologies.
phological disturbances, indicating that the dust may have
In other words, it seems likely that morphological distur-
been deposited by a recent interaction event.
bancesmaybeubiquitousintheETGpopulationandtheir
InFigure 5wepresentasubsetof theETGs with tidal
successful detection is driven simply by the depth of the
features that appear to be in the early stages of relaxation
imagingavailable.ThepeculiarETGpopulationcatalogued
intoanearly-typeremnant.Notethat,whilegalaxiesinthis
hereistherefore biased towards disturbedETGs with more
morphological subclass have clear bulges, the ‘early-stage’
prominent morphological disturbances.
nature of the merger remnant can often make it difficult
to pin down the morphology of the final remnant robustly.
For example, the galaxy in the bottom row of Figure 5, in
addition to a strong bulge component, may also be in the 3 EMISSION LINE DIAGNOSTICS: STAR
process of building a disk. Note that the bulge-dominated FORMATION AND AGN ACTIVITY
natureofthisobject isstrongly supportedbypipelinemea-
sures of morphology such as the SDSS fracdev parameter, Webeginourspectro-photometricanalysisbyexploringthe
which measures the contribution of a bulge profile to the emissionlinediagnosticsofourgalaxysample.Opticalemis-
overalllightprofileofthegalaxy;inprinciplefracdev=0in- sion line ratios are commonly used to determine the pres-
dicatesapuredisk,whilefracdev=1indicatesapurebulge, ence of (Type II) AGN and separate the AGN population
althoughpriorexperienceindicatesthathighfracdevvalues into its various types using a ‘BPT’ diagram (e.g. Baldwin
do not exclude the possibility of a (visually detected) disk- et al. 1981; Kauffmann et al. 2003a). Figure 6 shows the
likecomponent.Thegalaxyinquestionhasfracdev=1inall BPT diagram for the Stripe82 galaxies. Note that galaxies
SDSSfilters. only appear in this plot if the S/N in all emission lines is
Recent work (e.g. Springel & Hernquist 2005; Robert- > 3. Grey points indicate ETGs that are relaxed. ETGs
son et al. 2006; Hopkins et al. 2009) has shown that disks with tidal distortions are shown using blue points, ETGs
canberebuiltin(major) mergerswheretheinitialgasfrac- with dust features are shown using red points and ETGs
tions are high (≫ 0.1). Feedback from star formation and which show both dust features and tidal distortions are in-
supernovae act to pressurise the interstellar medium, redis- dicated using green points. The dark contours indicate the
tributing gas spatially and to larger radii where it does not distribution of peculiar ETGs while the grey contours indi-
feel theinternal torquesinduced by themerger. Asa result catethedistributionofrelaxedETGs.Objectsthatliebelow
thisgasdoesnotloseangularmomentumandcontributeto thecurvedline are classified as‘Star-forming’. Seyferts and
fuellingthestarburst,allowingittothenreformadisk.Itis LINERs occupy separate sections on the right-hand side of
possiblethatthegalaxyinthebottomrowofFigure5isan
example of such a system that is the product of a gas-rich
major merger. 1 http://www.astro.princeton.edu/PBOOK/strategy/strategy.htm
Peculiar early-type galaxies 9
Table1.AGNpropertiesoftheStripe82early-typegalaxysam-
pleinthemagnituderangeM(r)<−20.5.TheAGNsinboththe
relaxedandpeculiarETGpopulationsareshown(columns1and
2) as well as the change in the fractions of AGN types between
peculiarandrelaxedETGs(column3).
RelaxedETGs PeculiarETGs f(pec)/f(rel)
Star-forming 1.3% 4.5% 3.5
Composites 1.3% 1.5% 1.1
Seyferts 1.6% 3.0% 1.9
LINERs 9.2% 16.4% 1.8
of activity can be driven by low-intensity emission from
AGN,stellarphotoionisationorshockspropagatingthrough
the gas. Recent work by Stasin´ska et al. (2008) has shown
that the presence of diffuse LINER-like emission from stel-
Figure6.A‘BPT’plotfortheStripe82ETGsample.Notethat lar photoionisation can be indistinguishable from AGN ac-
galaxies onlyappear on this plot ifthe S/N inall emissionlines tivity. The source of the emission is better probed using
is > 3. Grey points indicate ETGs that are relaxed. Peculiar integral-field-spectroscopy (IFS) data which can be used to
ETGs with tidal distortions are shown using blue points, those
explore the spatial extent of the emission and discriminate
with dust features are shown using red points and those which
betweencentralemission(indicativeofanAGN)orspatially
show both dust features and tidal distortions are indicated us-
extended (diffuse) emission from the stellar population in
ing green points. The dark contours indicate the distribution of
thegalaxy.UsingIFSdataSarzietal.(2010) haveexplored
peculiarETGs,whilethegreycontours indicatethedistribution
of relaxed ETGs. The distribution of AGN types is similar be- thenatureoftheionisedgasemissionintheSAURONsam-
tweentherelaxedandpeculiarETGpopulationsanddominated pleofgalaxies,originallystudiedbySarzietal.(2006).Their
byLINER-likeactivity. resultsindicate that onlyin a small fraction of objects that
have radio and X-ray cores is the nebular emission consis-
tent with a central AGN. In particular, the source of the
the plot while ‘Composites’ (which host both star forma- emission-line activity in galaxies that carry signatures of
tion and AGN activity) occupy the intermediate regions of RSF is likely to be OB stars or a PAGB phase associated
thediagram2.Table1summariestheAGNpropertiesofthe withtheyoungstellarcomponent.Inlightoftheseresults,it
Stripe82 sample. seemsplausiblethattheincreasedLINERactivityobserved
It is worth noting that the fraction of ETGs that are in peculiar ETGs is primarily driven by stellar photoioni-
actively star-forming is typically very small. Only 1.3% of sation associated with the RSF that is induced by recent
relaxedETGsfallonthestar-forminglocus,whiletheequiv- interactions that give rise to these objects. The enhanced
alentvalueforthepeculiarETGpopulationis4.5%.Ourre- star formation in peculiar ETGs is indicated both by the
sultsareconsistentwithSchawinskietal.(2007),whofinda higherfractionofpeculiarETGsthatareclassifiedas‘Star-
similar value for the fraction of star-forming ETGs (4.3%). forming’ in the BPT diagnostics above and by an excess of
Table 1 indicates that, not unexpectedly, the star-forming UV-blue colours in the population, as discussed in Section
fraction in the peculiar ETG population is a few factors 4 below.
higher than that in their relaxed counterparts, presumably
duetostarformation associated withtheinteractionevents
thatproducetheobservedmorphological disturbances.The 4 COLOURS AND RECENT STAR
small fraction of actively star-forming ETGs suggests that FORMATION
the peak star formation rates in these events is low, con-
We now turn to the photometric properties of the Stripe82
sistent with what might be expected from gas-poor major
galaxy population. We explore both the optical colours of
mergers or, more plausibly, minor mergers, that are pre-
oursample(usingphotometryfromtheSDSS)andtheirUV
dicted to be the principal driver of star formation in ETGs
colours (using photometry from the recent GALEX space
atlateepochs(Kavirajetal.2009; Kavirajetal.2010). We
telescope, see Martin et al. 2005). Recent work has shown
revisit this issue in Section 4, where the UV colour is used
that the UV spectrum is very sensitive to small amounts
asamoredetailedprobeoftherecentstarformation(RSF)
of RSF. This makes the UV especially useful in identifying
in theETG sample.
ETGs that may havehad small amountsof RSFcontribut-
Wefindthatthefraction of objects classified as ‘AGN’
ing a few percent of their stellar mass over the last ∼ 1
(i.e.Seyferts+LINERs)doublesinthepeculiarETGscom-
Gyr (Kaviraj et al. 2007b; Kaviraj et al. 2008) and makes
pared to that in their relaxed counterparts. However, it is
it particularly pertinent to the study of the peculiar ETG
worthnotingthattheoverwhelmingbulkoftheemission in
population in our sample.
both relaxed and peculiar ETGs is LINER-like. This type
In Figure 7 we present the optical (u − r) colour-
magnituderelation(CMR;top)oftheStripe82galaxysam-
2 NotethatCompositesarealsosometimescalled‘TransitionRe- ple and the (u−r) colour histograms (bottom) for the re-
gion’objects. laxedandpeculiarETGpopulations.TherelaxedETGsare
10 Sugata Kaviraj
shown using small black circles while the various classes of
peculiar ETGs are indicated using colours. Galaxies with
Seyfert or LINER-like emission are indicated using boxes.
Late-type galaxies are indicated in grey. Median values of
the histograms are indicated using the red lines. We find
that the relaxed and peculiar ETG populations have very
similar optical colour distributions with the peculiar ETGs
bluer, on average, by less than 0.05 mags. We use a Mann-
Whitney(MW; Walpole & Myers 1985) U-test toquantita-
tively compare the colour distributions of the relaxed and
peculiar ETGs and test the hypothesis that they have the
same median. For the (u−r) colour, the MW test yields
a nearly-normal test statistic Z ∼ -0.5 and a probability
p ∼ 0.31 of achieving a value greater than Z. Thus, at the
5% significance level we cannot reject the hypothesis that
thetwo distributions havethe same median.
Figure 8 shows the UV CMR for the Stripe82 galaxy
population (top) and (NUV −r) colour histograms for the
ETG population, split into its relaxed and peculiar sub-
sets(bottom).NotethattheGALEXNUV filteriscentred
around 2300˚A. In contrast to the optical analysis above,
several differences become visible between the UV colours
of the two ETG sub-populations. The top panel of this fig-
ure indicates that, in the magnitude range M(r) < −22,
the (NUV −r) colour distribution of the relaxed ETGs is
virtually indistinguishable from that of theirpeculiar coun-
terparts. However, in the range −22 < M(r) < −21 the
peculiar ETGs become distinctly bluer in the (NUV −r)
colour,which drivestheoverall offsetinthetwohistograms
in the bottom panel of this figure. This difference in be-
haviour can be quantified by noting that the slope of the
(NUV −r) CMR for the relaxed ETGs is -0.27 while that
forthepeculiarETGsis-0.61i.e.twicethevaluefortheirre-
laxedcounterparts.Thedifferenceinthemedian(NUV −r)
ofthetwoETGsubclasses,drivenbythebluewardtrendof Figure7.TOP:Theoptical(u−r)colour-magnituderelationof
theless luminous galaxies, is ∼0.6 mags. theStripe82galaxysample.K-correctionsfromtheSDSSpipeline
An MW test in the (NUV −r) colour distributions of have been applied to the photometry. The relaxed ETGs are
shown using small black circles while the various classes of pe-
the relaxed and peculiar ETG populations yields a p-value
culiarETGsareindicatedusingcolours.GalaxieswithSeyfertor
of 0.02, indicating that, unlike in the case for (u−r), the
LINER-likeemissionareindicatedusingboxes.Late-typegalaxies
medians of these two distributions are different at the 5% areindicatedusingsmallgreycircles.BOTTOM:The(u−r)his-
significancelevel.Tofurtherconfirmthisresultweestimate togram forthe Stripe82 ETGs. Median values of the histograms
the standard deviation in the medians using bootstrapping areindicatedusingtheredlines.Thesolidlinerepresentsrelaxed
and compare these to the difference in the medians them- ETGswhilethedottedlineindicatespeculiarETGs.Therelaxed
selves.Thestandarddeviationofthe(NUV −r)medianfor andpeculiarETGpopulations haveverysimilardistributionsin
relaxedETGsis0.05mags,whilethecorrespondingvaluefor the (u−r) colour, indicating that any star formation accompa-
their peculiar counterparts is 0.09 mags. The sum of these nying the interaction events is weak and does not perturb the
opticalcolours.
standarddeviations(0.14mags)isafewfactorssmallerthan
the difference in the medians themselves (0.6 mags), which
supports the results of the MW test and indicates that the z∼0.1(includingthoseontheredsequence),whileemission
(NUV −r) colour distributions of the relaxed and peculiar linesaretypicallyusefulonlyiftheS/Nissatisfactorilylarge
ETGs are indeed different. (≥3),whichmakesitdifficulttostudyweaklystarforming
ETGs using SDSSemission lines such as Hα.
At low redshift care needs to be taken in interpreting
4.1 Tracing recent star formation using the UV
theUV flux from early-typegalaxies because there is a po-
colour
tential contribution from old, horizontal branch stars that
The UVoffers an alternative route to exploring theRSFin leadstothe‘UVupturn’phenomenonaround1500˚Ainsome
early-type galaxies. While emission lines such as Hα trace nearby giant ellipticals (see e.g. Yi et al. 1997; 1999). How-
‘instantaneous’ star formation activity, theUV spectrum is ever, using the (NUV −r) colour of one of the strongest
anintegraloffluxfromthehot,massive,mainsequencestel- UV upturn galaxies as a guide, Kaviraj et al. (2007b) have
larpopulationingalaxieswhichsurvivefor∼1Gyr.Further- argued that objects bluerthan(NUV −r)∼5.4 arehighly
more,thephotometricnatureoftheindicatormeansthata likely to harboursome RSFi.e. theirUVcolours cannot be
robustdetectionofUVfluxcanbemadeinall ETGswithin satisfied simply byinvoking UVupturnflux.