Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. III. Subpopulations: distributions, shapes, origins

0.92 MB·English

by  Thorsten Lisker

Accepted by TheAstrophysicalJournal
PreprinttypesetusingLATEXstyleemulateapjv.12/14/05
VIRGO CLUSTER EARLY-TYPE DWARF GALAXIES WITH THE SLOAN DIGITAL SKY SURVEY.
III. SUBPOPULATIONS: DISTRIBUTIONS, SHAPES, ORIGINS
Thorsten Lisker, Eva K. Grebel, Bruno Binggeli, and Katharina Glatt
AstronomicalInstitute, Dept.ofPhysicsandAstronomy,UniversityofBasel,Venusstrasse7,CH-4102Binningen,Switzerland
Accepted by The Astrophysical Journal
ABSTRACT
Fromaquantitativeanalysisof413Virgoclusterearly-typedwarfgalaxies(dEs)withSloanDigital
Sky Survey imaging data, we find that the dE class can be divided into multiple subpopulations that
differ significantlyintheir morphologyandclusteringproperties. ThreedE subclassesareshapedlike
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thick disks and show no central clustering: (1) dEs with disk features like spiral arms or bars, (2)
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dEs with central star formation, and (3) ordinary, bright dEs that have no or only a weak nucleus.
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Thesepopulationsprobablyformedfrominfallingprogenitorgalaxies. Incontrast,ordinarynucleated
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dEs follow the picture of classicaldwarf elliptical galaxies in that they are spheroidalobjects and are
n centrallyclusteredlikeE andS0 galaxies,indicating thatthey haveresidedin the cluster since a long
a
time, or were formed along with it. These results define a morphology-densityrelation within the dE
J
class. WefindthatthedifferenceintheclusteringpropertiesofnucleateddEsanddEswithnooronly
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a weak nucleus is not caused by selection biases, as opposed to previously reported suggestions. The
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correlationbetweensurfacebrightnessandobservedaxialratiofavorsoblateshapesfor allsubclasses,

but our derivation of intrinsic axial ratios indicates the presence of at least some triaxiality. We
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discuss possible interrelations and formation mechanisms (ram-pressure stripping, tidally induced
v
star formation, harassment) of these dE subpopulations.
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2 Subject headings: galaxies: dwarf — galaxies: structure — galaxies: evolution — galaxies: statistics
4 — galaxies: fundamental parameters — galaxies: clusters: individual (Virgo)
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7 1. INTRODUCTION disk features in 410 Virgo cluster early-type dwarfs
0 (Lisker et al. 2006a, hereafter Paper I). We thereby in-
As the most numerous type of galaxy in clusters,
/ cluded galaxies classified as dwarf elliptical and as dS0
h early-type dwarf galaxies are ideal probes to study the
to avoid any preselection bias, and assigned them the
p physical processes that govern galaxy formation and
commonabbreviation“dE”,whichweadoptforthis Pa-
- evolution in environments of different density. The
o per as well. We identified disk features in 36 dEs, and
pronounced morphology-density relation (e.g., Dressler
r argued that they constitute an unrelaxed population of
t 1980; Binggeli et al. 1987) suggests that early-type
s disk-shapedgalaxiesdifferentfromtheclassicaldwarfel-
dwarfs were either formed mainly in high-density envi-
a
lipticals (Paper I).
: ronments,ororiginatefromgalaxiesthatfellintoa clus-
v But the dE class shows yet more diversity: nucleated
terandweremorphologicallytransformed. However,the
i and non-nucleated dEs have different clustering prop-
X actual formation mechanisms are still a matter of de-
erties (van den Bergh 1986; Ferguson & Sandage 1989),
bate(seeJerjen & Binggeli2005,andreferencestherein).
r their flattening distributions differ (Binggeli & Cameron
a Most of the proposed scenarios are based on the vigor-
1991; Ryden & Terndrup 1994; Binggeli & Popescu
ous forces acting within a cluster environment,like ram-
1995), and color differences were reported as well
pressure stripping (Gunn & Gott 1972) of dwarf irregu-
(Rakos & Schombert 2004; Lisker et al. 2005). More-
lar (dIrr) galaxies (e.g., van Zee et al. 2004), tidally in-
over,severalofthebrightdEsdisplaybluecentralregions
duced star formationin dIrrs (Davies & Phillipps 1988),
causedby recentor ongoingstarformation(Lisker et al.
or so-called harassment (Moore et al. 1996) of infalling
2006b,hereafterPaperII),andalsodifferintheirspatial
late-type spirals through close encounters with massive
and flattening distributions from the bulk of dEs. Thus,
cluster members.
prior to discussing possible formation mechanisms, we
Early-type dwarfs form a rather heterogeneous class
need to systematically disentangle the various dE sub-
of objects. In addition to the classical dwarf ellipti-
classesobservationally. ThisisthepurposeofthisPaper.
cals, Sandage & Binggeli (1984) introduced the class of
dwarfS0 (dS0) galaxies,which were conjectured to have 2. SAMPLESELECTION
diskcomponents,basedonsignatureslikehighflattening
While our dE sample selection was already described
or a bulge+disk-like profile (Binggeli & Cameron 1991).
in Papers I and II of this series, these studies were still
The identification of spiral substructure then provided
based on the Data Release 4 of the Sloan Digital Sky
the first direct proof for a disk in an early-type dwarf
Survey (SDSS; Adelman-McCarthy et al. 2006). Since
(Jerjen et al. 2000), which, however, had not been clas-
we are now using the full SDSS Data Release 5 (DR5,
sified as dS0 but as dwarf elliptical. Inspired by simi-
Adelman-McCarthy et al.2007)dataset,weprovidehere
lar discoveries (e.g., Barazza et al. 2002; Graham et al.
a detailed, updated description of our selection.
2003; De Rijcke et al. 2003), we performed a search for
2.1. Selection process
Electronicaddress: [email protected],[email protected],[email protected],[email protected]
2 Lisker, Grebel, Binggeli, & Glatt
The Virgo Cluster Catalog (VCC, Binggeli et al. essaryparameterstofluxcalibratethem. Thepixelscale
1985), along with revised classifications from of 0′.′396 corresponds to a physical size of 30 pc at our
Barazza et al. (2002, VCC 1422), Barazza et al. (2003, adopted Virgo cluster distance of d = 15.85Mpc. The
VCC 0850), and Geha et al. (2003, VCC 1488), con- SDSS imaging camera (Gunn et al. 1998) takes data in
tains 1197 galaxies classified “dE” or “dS0”, including drift-scanning mode nearly simultaneously in the five
candidates, that are certain or possible cluster mem- photometric bands, and thus combines very homoge-
bers according to Binggeli et al. (1985), Binggeli et al. neous multicolor photometry with large area coverage
(1993), and Paper II. 552 of these fall within our and sufficient depth to enable a systematic analysis of
chosen limit in apparent B magnitude from the VCC dEs. The images have an absolute astrometric accuracy
of m ≤ 18.0 mag (see Paper I). This is the same of rms ≤ 0′.′1 per coordinate, and a relative accuracy
B
magnitude limit up to which the VCC was found to between the r band and each of the other bands of less
be complete (Binggeli et al. 1985). When adopting a than 0.1 pixels (Pier et al. 2003). They can thus easily
Virgo cluster distance of d = 15.85Mpc, i.e., a distance be aligned using their astrometric calibration and need
modulus m−M = 31.0 mag (see, e.g., Ferrarese et al. not be registered manually.
2000), which we use throughout, this corresponds Thermsofthenoiseperpixelcorrespondstoasurface
roughlyto a limit inabsolute magnitude ofM ≤−13.0 brightness of approximately 24.2 mag arcsec−2 in the u-
B
mag. band, 24.7 in g, 24.4 in r, 23.9 in i, and 22.4 in z. The
SixgalaxiesarenotcoveredbytheSDSS.Whileweini- typical total signal-to-noise ratio (S/N) of a bright dE
tiallyincludedobjects withuncertainclassification(e.g., (m ≈ 14) amounts to about 1000 in the r-band
B,VCC
“dE?”), we then excluded all 50 galaxies that appeared withinanapertureradiusofapproximatelytwohalf-light
to be possible dwarf irregulars from visual inspection of radii. For a faint dE (m ≈ 18) this value is typically
B
thecoaddedSDSSg,r,andiimages(seePaperI),orwere about 50. While the S/N in the g and i-band is similar,
classified as “dE/Im”. Three more objects (VCC 0184, itisseveraltimes lowerinthe z-bandandmorethanten
0211, and 1941) were excluded because they appear to times lower in the u-band.
beprobablebackgroundspirals. Finally,VCC1667could TheSDSSprovidesphotometricmeasurementsforour
notbeclassifiedproperly,sinceitissignificantlyblended galaxies,butwefoundthesetobeincorrectinmanycases
with multiple other galaxies. This leads to a final dE (Lisker et al.2005). The SDSSphotometricpipeline sig-
sample of 492 certain or possible cluster members, con- nificantly overestimates the local sky flux around the
taining 426 certain cluster members on which we focus Virgo dEs due to their large apparent sizes and low sur-
in the present Paper. face brightness outskirts. This affects the derivation of
isophotal and Petrosian radii, the profile fits, and sub-
2.2. Presence of nuclei sequently the calculationoftotalmagnitudes, whichcan
Whileourclassificationofnucleatedandnon-nucleated be wrong by up to 0.5 mag. For this reason, we used B
dEs relies on the VCC, it is known from HST observa- magnitudes from the VCC throughout the first two pa-
tions that many apparently non-nucleated dEs actually pers of this series. In the meantime, we have performed
hostafaintnucleushardlydetectablewithground-based our own structural and photometric measurements (see
imaging (Cˆot´e et al. 2006, also see Lotz et al. 2004b). A Sect. 4), which we shall use here as well as in future pa-
direct comparison of the VCC classification with the re- pers of this series. Still, whenwe refer to B magnitudes,
sults from Cˆot´e et al. (2006) shows that, as a roughrule these were adopted from the VCC.
of thumb, the detection of dE nuclei in the VCC be- Heliocentric velocities for part of the sample are
comesincompletefornucleusmagnitudesthatarefainter provided by the NASA/IPAC Extragalactic Database
than the respective value of the host galaxy’s central (NED; also see Paper II for more detailed references).
surface brightness, measured within a radius r = 0.1′′
(Fig. 1). Our non-nucleated dEs could thus be more ap- 4. IMAGEPREPARATIONANDANALYSIS
propriately termed dEs without a nucleus of significant 4.1. Sky subtraction
relative brightness as compared to the underlying light
of the galaxy’s center. In fact, Grant et al. (2005) sug- The sky level on the SDSS images can vary by some
gestedthatdEsclassifiedasnucleatedandnon-nucleated tenths of the noise level across an image. For a proper
might actually form a continuum of dEs with respect to determinationofPetrosianradiiofthedEs(seeSect.4.3)
relative nucleus brightness. Therefore, the VCC classi- despite their low surface brightness outskirts, it is thus
fication basically translates into probing opposite sides not always sufficient to subtract only a single sky flux
of this continuum — and this is exactly what makes it value from each SDSS image. Therefore, we performed
useful for our study of dE subclasses. If the relative sky subtraction through the following procedure. First,
brightness of a nucleus depends on its hostgalaxy’sevo- we constructed object masks for each SDSS image from
lutionary history, then one might expect nucleated and the so-called segmentation images of the Source Extrac-
“VCC-non-nucleated” dEs to exhibit different popula- torsoftware(Bertin & Arnouts1996)byexpandingthese
tion properties. throughsmoothing with a Gaussianfilter (using IRAF1,
Tody1993). Apreliminaryskylevelwasthendetermined
3. DATA for each image as the median of all unmasked pixels,
The SDSS DR5 covers all VCC galaxies except for an clipped three times iteratively at 3σ. In orderto reacha
approximately 2◦ ×2◦.5 area at α ≈ 186◦.2, δ ≈ +5◦.0.
It provides reduced images taken in the u, g, r, i, and 1 IRAF is distributed by the National Optical Astronomy Ob-
servatories, which are operated by the Association of Universities
z bands with an effective exposure time of 54s in each
forResearchinAstronomy,Inc.,undercooperativeagreementwith
band(see alsoStoughton et al.2002), aswellasthenec- theNationalScienceFoundation.
Virgo early-type dwarfs. III. Subpopulations 3
higher S/N than that of the individual images, we then We perform an iterative process of determining shape
produced a coadded image by summing the (weighted) and total flux for each galaxy, as described below.
g,r, andi-bandimagesasdescribedinPaperI.We then Throughoutthisprocess,wemaskdisturbingforeground
obtainedanimprovedobjectmaskfromthecoaddedim- or background objects, i.e., we do not consider masked
age and used this to refine our sky level measurement. pixels in any calculation. We start with deriving
Finally, the sky flux distribution across the image was the Petrosian radius (Petrosian 1976), as defined by
determined by computing the average flux – clipped Stoughton et al. (2002), on the coadded image. Using a
five times iteratively at 3σ – of all unmasked pixels in circularaperturewithonePetrosianradius,wethenfind
201×201pixelboxes,centeredevery40 pixels. This grid the center of the galaxy’s image by iteratively searching
of values can be stored as a 52×38 pixel “sky image”. for the minimum asymmetry, following Conselice et al.
Pixels in this sky image that did not contain useful val- (2000). The asymmetry A is calculated as
ues due to too many masked pixels in the parent image
were linearly interpolated using IRAF fixpix. We then |f −f |
i i,180
applied a 3×3 pixel median filter to the sky image, ex- X
A= i , (1)
panded it to match its parent SDSS image’s size (using
|f |
IRAFmagnify withlinearinterpolation),andsubtracted i
X
it from the latter. This yields the final u, g, r, i, and z i
images. where f is the flux value of the i-th pixel, and f is
i i,180
We point out that there is, to our knowledge, no gen- thefluxvalueofthecorrespondingpixelinthe180-degree
eral agreement or recipe as to whether to use, e.g., the rotated image.
clipped mean, the median, the clipped median, or the The asymmetryis computedusing aninitially guessed
mode, for determination of the sky level. However, it is central position (from Paper I for objects in the SDSS
advisable that the chosen approach be reconciled with DR4, and from visual examination for objects in DR5,
the image measurements to be performed, which in our using SAOImage DS9, Joye & Mandel 2003), as well as
case is the derivation of Petrosian radii (see Sect. 4.3). forusingthesurroundingeightpositionsina3×3gridas
Sincethelatterisbasedontheaverage fluxwithingiven center. If one ofthe surroundingpositionsyields a lower
annuli, we chose to use the clipped average flux of all asymmetry, it is adopted as new central position. This
unmasked pixels for our sky level measurement. This process is repeated until convergence. We perform two
guarantees that the resulting flux level in each image is of these “asymmetry centerings”: a first one with a step
zero as “seen” by the Petrosianradius calculation.2 size of 1 pixel, and a second one with a step size of 0.3
pixels. The initial and final value typically differ by less
4.2. Calibration and extraction than a pixel.
We thencompute the parametersdefining anelliptical
We calibrated the sky subtracted SDSS images using
aperture (axial ratio and position angle) from the image
the provided flux calibration information (photometric
moments(Abraham et al.1994),andderivea“Petrosian
zeropoint and airmass correction). We also corrected
semimajor axis” a , i.e., we use ellipses instead of cir-
for the reported SDSS zeropoint offsets in the u and p
cles in the calculation of the Petrosian radius (see, e.g.,
z bands from the AB system (Oke & Gunn 1983,
Lotz et al.2004a). Within this elliptical aperturewith a
see http://www.sdss.org/dr5/algorithms/fluxcal.html).
semimajoraxisaof1a ,weperformanotheriterationto
However, before working with the images, it is advan- p
re-derive the elliptical shape parameters from the image
tageous to put together adjacent images: a number of
moments, and also to re-derive a .
galaxiespartlyextendbeyondtheimageedgesandreap- p
Theellipticalshapeisthenappliedtomeasurethetotal
pear on the corresponding neighbouring image. Bright
flux in the r band within an elliptical aperture with a=
dEs typically have apparent diameters of 300 pixels
2a , which also yields a half-light semimajor axis in r
or more, which is rather large compared to the SDSS p
(a ). Using this value for a , we go back to the
image size of 2048×1489 pixels. The SDSS astrometric hl,r hl,r
coadded image and fit an ellipse to the isophotal shape
calibrationallowsus to accuratelyput togetheradjacent
of the galaxy at a = 2a , using IRAF ellipse. The
images, which we did before extracting an 801×801 hl,r
elliptical annulus used for the isophotal fit ranges from
pixelcutout imagefor eachgalaxy. These cutoutimages 20.75a to 21.25a .
were then corrected for Galactic extinction, using one hl,r hl,r
Thisnewellipticalshapeisnowusedtoderivethefinal
value per image, calculatedwith the dust maps andcor-
Petrosian semimajor axis on the coadded image, and to
responding software of Schlegel et al. (1998, provided at
subsequently measure again the total flux in the r band
http://www.astro.princeton.edu/∼schlegel/dust/data/).
within a = 2a , yielding the final value for a . The
From the g, r, and i cutout image we produced a final p hl,r
isophotal shape is then measured again at a = 2a ,
coadded image for each galaxy. hl,r
yieldingthe axialratiothatwe shalluse throughoutthis
Paper.
4.3. Morphology Since we maskeddisturbing foregroundor background
objectsby notconsideringtheir pixels, ourmeasuredto-
2 Thereasonwhysuchconsiderationsareatallnecessaryisthe talfluxforagivengalaxyisalwayslowerthanitwouldbe
sameas that forwhichthe SDSS pipelineoverestimated the local withoutanysuch“holes”inthe galaxy’simage. Inorder
skyflux: theVirgodEsarelargeinapparentsizeandcover104 to to correctfor this effect, we subdivide the final aperture
105 pixels,buttheirlowsurfacebrightnessoutskirtscausealarge of each galaxy into 20 elliptical annuli of equal width,
number of these pixels to have S/N< 1. Thus, a wrong sky level
and assign each masked pixel the average flux value of
estimateoftheorderofjustafewtenthsofthenoiselevelcanhave
alargeeffectintotal. its respective annulus. This yields our finalvalue for the
4 Lisker, Grebel, Binggeli, & Glatt
totalr bandfluxandthecorrespondingmagnitude. The reddens (Paper II). Therefore, and because the dE(bc)s
difference to the uncorrected value is typically less than aredefined throughcolorinstead ofmorphologicalprop-
0.1 mag. erties, we do not consider them a morphological dE sub-
For13ofourdEs,thederivationofthePetrosianradius class.3 Ontheotherhand,theirstarformationandpres-
did not converge, due to the fact that these galaxies sit ence of gas (Paper II) might imply that their formation
within the light of nearby bright sources. While in some process is not completely finished yet. It thus appears
of these cases, it would still be possible to “manually” morecautioustoseparatethemfromtherestofdEs(see
defineanaxialratioforthegalaxy,wedecidedtoexclude Fig. 2) in order to not bias the population properties
these objects from our sample, since no reliable r band of the other subclasses. In the discussion (Sect. 7) we
magnitudes can be derived, which are needed for our try to assess which dE type(s) the dE(bc)s could pos-
definitions of dE subclasses in Sect. 5. This leaves us sibly evolve into. Note that four objects are common
with a working sample of 413 Virgo cluster dEs. to both the dE(di) and the dE(bc) sample. We exclude
5. EARLY-TYPEDWARFSUBCLASSES these from the sample of dE(di)s, which now comprises
33 galaxies. Table 1 lists our dEs along with their sub-
5.1. Subclass definitions
class.
Of our 413 Virgo dEs, 37 display disk features, like A similar subdivision of the dE class into bright
spiral arms, bars, or signs of an edge-on disk (Paper I, and faint (non-)nucleated subsamples was performed by
adding VCC 0751 to the objects listed there in order to Ferguson & Sandage (1989), also with the aim of study-
updatetoSDSSDR5). Wetermtheseobjects“dE(di)s”, ing shapes and spatialdistributions of the resulting sub-
and separate this dE subclass from the ordinary, “fea- samples. Our subdivision is different in two respects:
tureless”dEs(Fig.2). Inordertofurtherexplorethedi- first,Ferguson & SandagedefinedallgalaxieswithmB <
versityofthe latter, we performa secondarysubdivision 17.5magas“bright”,whereasourmagnitude separation
intonucleated(“dE(N)”)andnon-nucleated(“dE(nN)”) (at mr = 15.67 mag) is done at significantly brighter
galaxies,basedontheidentificationofnucleiintheVCC values and divides our full sample into equally sized
as outlined in Sect. 2.2. Since a further subdivision of halves. Second, we have the advantage of excluding
the dE(di)s would lead to statistically insignificant sub- dE(di)s and dE(bc)s from the “normal” dEs, thereby
samples,weshallinsteaddiscusstheirnucleatedfraction obtaining cleaner subsamples, especially for the bright
inthetext. Finally,sinceourgalaxiesspanarangeofal- objects: all but three of the dE(di)s are brighter than
most 5 mag in r, it appears worth performing a tertiary mr =15.67 mag.
subdivision into dEs brighter and fainter than the me- While Ferguson & Sandage (1989) found statistically
dian r brightness of our full sample, namely m =15.67 significantdifferences inthe spatialdistributionsoftheir
r
mag. Moreover,all but three of the dE(di)s are brighter subsamples – with dE(N)s being much more centrally
than this value; thus our subdivision allows us to com- clustered than the bright dE(nN)s – their flattening dis-
pare them to ordinary dEs of similar luminosities. The tributions were only based on eye-estimated axial ratios
percentage of each subsample among our full sample of from photographic plates. These can be uncertain by
413 dEs is given in parentheses in Fig. 2, whereas the ∼20% (Ferguson & Sandage 1989). With our measured
actual number of galaxies contained in each subsample axial ratios from the coadded SDSS images at hand, we
is given in the left column of Fig. 3. therefore present in the following subsection a more de-
The subclasses defined so far are based on struc- tailed and accurate study of the flattening distributions
tural properties only — for morphological classification of the different dE subsamples, and attempt to deduce
of galaxies, it is not advisable to use color informa- their approximate intrinsic shapes.
tion. However, in Paper II we identified a significant
number of dEs with blue centers (17 galaxies, including 5.2. Subclass shapes
VCC 0901 from the SDSS DR5). These objects, termed
From the axial ratio measurements of our galaxies
“dE(bc)s”, exhibit recent or ongoing central star forma-
(Sect. 4.3), we put together the flattening distributions
tion,similartoNGC205intheLocalGroup. Theywere
of each dE subsample. These are presented in the sec-
morphologically classified as dwarf ellipticals or dS0s by
ondcolumnofFig. 3 asrunning histograms,i.e., at each
Sandage & Binggeli(1984),andtheirregular,early-type
samplingpointweconsiderthenumberofobjectswithin
morphology was confirmed in Paper II; thus, they are
a bin of constant width, and normalize the resulting
not possible irregular galaxies, which we have excluded
curve to an area of 1. The bin width is 0.15, which
from our samples here and in previous papers of this
we have chosen to be one fifth of the range in axial ra-
series. The flattening distribution of the dE(bc)s was
tio covered by our galaxies. The sampling step is 0.04
found to be incompatible with intrinsically spheroidal
(one quarter of the bin width). The bright and faint
objects (Paper II), and their distribution with respect
dE(N)s,andalsothe faintdE(nN)s, predominantlyhave
to local projected density suggests that they are an un-
ratherroundapparentshapes,whilethebrightdE(nN)s,
relaxed population. The latter result is similar to the
dE(di)s, anddE(bc)s exhibit a significantfractionof ob-
spatial distribution of Virgo and Fornax dwarfs with
jects with rather flat apparent shapes.
early-typemorphologythataregas-richand/orshowstar
formation(Drinkwater et al.2001;Conselice et al.2003; 3 A morphological peculiarity of several dE(bc)s is that they
Buyle et al. 2005). showcentralirregularities,whicharepresumablyduetogas,dust,
While it is not clear a priori that any of the dE sub- and/orstarformation,similartoNGC205. Thesecanbeseen,e.g.,
classes defined above are evolutionary interrelated, each when constructing unsharp mask images (Paper II). However, an
attempttoquantifytheseweakfeaturesthroughimageparameters
dE(bc) unavoidably evolves into one of the above dE
likeasymmetryorclumpinessyieldednoclearseparationfromthe
types once star formation ceases and the central color bulkofdEs. Moreover,notalldE(bc)s displaysuchfeatures.
Virgo early-type dwarfs. III. Subpopulations 5
Since the division between bright and faint objects at andprolate(blackdash-dottedlines)case. Wealsoshow
m = 15.67 mag is somewhat arbitrary, we test whether 3-D illustrations of the galaxy shapes for each distribu-
r
the difference between the axial ratio distributions of tion (fifth column), using in each case the axial ratio
faintandbrightdE(nN)sbecomesevenmorepronounced of the 25th percentile (left 3-D plot) and the 75th per-
if a wider magnitude separation is adopted. The grey centile (right 3-D plot). These results confirm that the
curves in the respective panels of the second column of bright dE(nN)s, dE(di)s, and dE(bc)s do have lower ax-
Fig. 3 show the distributions for bright dE(nN)s with ial ratios than the bright and faint dE(N)s and the faint
m ≤15.67−0.5mag (23objects)andforfaintdE(nN)s dE(nN)s. Furthermore,wepointoutthatthebrightand
r
with m ≤15.67+0.5 mag (86 objects). While the faint faint dE(N)s span a rather wide range of intrinsic axial
r
dE(nN)sbasicallyremainunchanged,thebrightdE(nN)s ratios, and are, on average, somewhat flatter than what
indeed tend slightly towards flatter shapes, but the dif- was deduced by Binggeli & Popescu (1995): our median
ference is rather small. value (see the 3-D illustrations in Fig. 2) is slightly flat-
Astatisticalcomparisonoftheaxialratiodistributions terthanE3forthe prolatecase,andslightlyflatterthan
ofourdEsubsamplesconfirmswhatisseeninFig.3: aK- E4 for the oblate case.
S test yields very low probabilities that any of the “flat- Can we decide whether the true shapes of our galax-
ter” subsamples (bright dE(nN)s, dE(di)s, and dE(bc)s, ies are more likely to be oblate or to be prolate? For
lower three rows) could stem from the same true dis- this purpose, we make use of the surface brightness test
tribution function as any of the “rounder” subsamples (Marchant & Olson1979; Richstone 1979), againfollow-
(brightandfaintdE(N)s aswellasfaintdE(nN)s, upper ing Binggeli & Popescu (1995). If dEs were intrinsically
three rows). This confirms our findings from Papers I oblate spheroids, galaxies that appear round would be
and II for the dE(di)s and dE(bc)s, respectively. The seen face-on and should thus have a lower mean sur-
resulting probabilities fromthe K-Stest for the pairwise face brightness than galaxies that appear flat; the latter
comparison of the subsamples are given as percentages would be seen edge-on. For the prolate case, the inverse
inFig.4. Interestingly,thelowestprobabilityofallcom- relationshouldbeobserved. However,beforewecanper-
parisons is obtained when matching the distributions of form this test, we need to take into account the strong
bright and faint dE(nN)s: here, the probability of the correlationofdEsurfacebrightnesswithmagnitude(e.g.,
nullhypothesisthattheystemfromthesameunderlying Binggeli & Cameron1991): if,bychance,thefewappar-
distribution function is only 0.10%. Note that the prob- ently round galaxies in one of our smaller subsamples
abilities for the comparison of the “flatter” subsamples would happen to be fainter on average than the appar-
with the “rounder” ones increase slightly with decreas- ently flat ones, this could introduce an artificial relation
ing sample size, going from the bright dE(nN)s to the ofaxialratiowith surface brightness. Therefore,instead
dE(di)sandthentothedE(bc)s. However,theprobabil- of directly using surface brightness like earlier studies
ityfor acommonunderlyingdistributionofdE(bc)s and did, we use the surface brightness offset from the mean
the bright and faint dE(N)s is still only 3.8% and 4.6%, relation of surface brightness and magnitude. We plot
respectively. these values, measured in the r band within a = 2a ,
hl,r
Is it possible to deduce the distributions of intrinsic againstaxialratio(measuredatthesamesemimajoraxis,
axial ratios from those of the apparent ones? As dis- seeSect.4.3)foreachdEsubsample,showninthe right-
cussed in detail by Binggeli & Popescu (1995), the in- most column of Fig. 3. For all subsamples, a positive
trinsic shapes can be deduced when assuming that they correlation of surface brightness offset with axial ratio
are purely oblate or purely prolate. The distribution canbeseen,favoringtheoblatemodelinagreementwith
function Ψ of intrinsic axial ratiosq can then be derived earlier studies (e.g., Marchant & Olson 1979; Richstone
from the distribution function Φ of observedaxialratios 1979;Binggeli & Popescu1995). Forthe “rounder”sub-
p through (Fall & Frenk 1983, eqs. (6) and (9)) samples (top three rows), some additional contribution
by prolate objects might be “hidden” within the rather
2 d q Φ(p)
Ψ(q)= 1−q2 dp (2) large scatter of surface brightness offsets at larger ax-
πp dq Z0 q2−p2 ial ratios. We denote these results in Fig. 3 by the ar-
p rows pointing from the surface brightness test diagram
for the oblate case, and
towards the favored intrinsic galaxy shapes. The arrow
2 1−q2 d q p3Φ(p) sizerepresentstheimpliedcontributionfromintrinsically
Ψ(q)= πp q2 dq Z0 dp q2−p2 (3) psarmolpatleesa(nlodweorbltahtreeeobrojewcst)s,.foArmwohnicghtthhee“oflbalattteerc”asseubis-
p
for the prolate case. Following Binggeli & Popescu favored, the dE(di)s have the lowest axial ratios, with
(1995), we firstdefined adequateanalyticfunctions Φ(p) a median value of 0.33 (bright dE(nN)s: 0.42, dE(bc)s:
that representthe observeddistributions,and then eval- 0.44). The galaxies in these subsamples are thus most
uated the above equations numerically. The analytic likely shaped like thick disks.
“model functions” are shown in the third column of The above considerations needed to be restricted to
Fig. 3; they were constructed from combinations of purelyoblateandpurelyprolateshapes. However,forall
(skewed) Gaussians with each other and, in some cases, subsamples,asmallpartofthededuced(andfavored)in-
with straight lines. Note that, for the dE(bc)s, we de- trinsicoblatedistributionbecomesnegativeatlargeaxial
cidednottofollowtheobserveddistributioninalldetail, ratios,tryingtoaccountforthelownumberofapparently
since it is drawnfrom a rather small sample of 17 galax- round objects. This implies that most of the galaxies
ies, which probably is the cause of the fluctuations seen. might actually have triaxial shapes, in accordance with
The deduced intrinsic distributions are presented in the conclusions of Binggeli & Popescu (1995).
the fourth column of Fig. 3, for the oblate (grey lines)
6 Lisker, Grebel, Binggeli, & Glatt
5.3. Subclass distribution within the cluster the faint dE(nN)s are, among the “lower-density” sub-
samples, closest to the bright and faint dE(N)s, their
While it has been knownfor some time thatnucleated
probability for having the same underlying distribution
and non-nucleated dEs have different clustering prop-
is 0.08 and 0.07%, respectively. These probabilities are
erties (e.g., van den Bergh 1986; Ferguson & Sandage
higher for the dE(di)s and dE(bc)s: although they are
1989), this statement has been challenged by Cˆot´e et al.
located at even lower densities, their rather small sam-
(2006), who conjectured that it might just be the result
ple sizesletthe probabilityincreaseascomparedto that
ofaselectionbiasintheVCC.Itthereforeappearsworth
of the faint dE(nN)s. Finally, the bright dE(nN)s are
toperformaquantitativecomparisonofthedistributions
locatedatsuchlowdensitiesthattheirK-Stestcompar-
ofourdEsubsampleswithinthecluster,andtothenpro-
ison with the dE(N)s yields a probability of 0.00%, and
ceed with testing the issues raised by Cˆot´e et al. (2006)
that even the comparison with the faint dE(nN)s only
in detail.
yields a probability of 3.7% for them having the same
The projected spatial distributions of our subsamples
true distribution. Given the morphological differences
are shown in the middle column of Fig 5. While both
between the subsamples, as deduced in Sect. 5.2, Fig. 6
bright and faint dE(N)s exhibit a rather strong central
basicallyshowsamorphology-densityrelationwithin the
clustering, the faint dE(nN)s appear to be only moder-
dE class.
ately clustered, and the dE(di)s and dE(bc)s show ba-
This view appears to be corroboratedby the distribu-
sically no central clustering. The bright dE(nN)s even
tions of heliocentric velocities (right column of Fig. 5)
seem to be preferentially located in the outskirts of the
of the dE subsamples: that of the bright dE(N)s has
cluster.
a single peak and is fairly symmetric, while especially
To put the above on a more quantitative basis, we
the faint dE(nN)s, dE(di)s, and dE(bc)s display rather
present in Fig. 6 the cumulative distribution of each of
asymmetric distributions with multiple peaks. The lat-
our subsamples with respect to local projected density.
ter could be interpreted as being a signature of infalling
Following Dressler (1980) and Binggeli et al. (1987), we
populations(Tully & Shaya1984; Conselice et al.2001).
definethelatterforeachgalaxyasthenumberofobjects
However,the differencesbetweenthese velocitydistribu-
per square degree within a circle that includes the ten
tions are not or only marginallysignificant— the “most
nearest neighbours, independent of galaxy type. Only
different” pair of distributions according to the K-S test
certainclustermembersareconsidered. Forcomparison,
are the bright dE(nN)s and the dE(bc)s, which have a
we also show the same distributions for different Hubble
probabilityof6.6%for the nullhypothesis. The mainis-
types (Fig. 6, inset), i.e., for the rather strongly cen-
sueherearethesmallsamplesizes: onlyafractionofthe
trally clustered giant early-type galaxies, as well as for
galaxiesofeachsubsamplehasmeasuredvelocities(num-
the weakly clustered and probably infalling spiral and
bersaregiveninparenthesesintheleftcolumnofFig.5),
irregular galaxies (e.g., Binggeli et al. 1987).
whichareavailablefromtheNEDfor193ofour413dEs,
As a confirmation of the impression from the spa-
and,e.g.,foronly19ofour39brightdE(nN)s. Similarly,
tial distribution, the bright dE(nN)s are preferentially
measurementsoftheskeworkurtosisofthedistributions
found in regions of moderate to lower density, similar
donotyieldvaluesthatdiffersignificantlyfromzero. We
to (and at even slightly lower densities than) the dis-
can thus only state that the rather asymmetric, multi-
tribution of irregular galaxies, in accordance with the
peaked distributions of the faint dE(nN)s, the dE(di)s,
findings of Ferguson & Sandage (1989). This implies
andthedE(bc)swouldbeconsistentwithourabovecon-
that they, as a population, are far from being virial-
clusion that they are mostly unrelaxed populations, but
ized. The densities then increase slightly going from the
that more velocity data is needed to perform a reliable
bright dE(nN)s to the dE(bc)s, dE(di)s, and the faint
quantitative comparison of velocity distributions.
dE(nN)s, in this order. Still, all of these are distributed
similarly to the irregular and spiral galaxies in the clus- 5.4. Remarks on possible selection biases
ter, again implying that they are unrelaxed or at least
The different spatial distribution of dE(N)s and
largelyunrelaxedgalaxypopulations,andconfirmingthe
dE(nN)s was long considered a fundamental and well-
impression from their projected spatial distribution. In
founded observation, but has recently been questioned
contrast, both bright and faint dE(N)s are located at
by Cˆot´e et al. (2006). These authors argued that galax-
larger densities, and display a distribution comparable
ies with high central surface brightness (HSB, with
to the E and S0 galaxies, in agreement with the re-
µ . 20 mag arcsec−2 or B . 14.55) would
sults of Ferguson & Sandage (1989). This would sug- g,central
have been preferentially classified as non-nucleated in
gest that they are a largely relaxed or at least partially
the VCC, which may have lead to a selection bias in
relaxed population. Note, however, that the Es alone
the VCC that artificially relates spatial distribution to
(without the S0s) are located at still higher densities.
nucleus presence. We test this conjecture by considering
Conselice et al. (2001) pointed out that only the Es ap-
the following points:
pear to be a relaxed galaxy population, while all others,
(1) If the dE(nN)s were objects in which nuclei have
including the S0s, are not — thus, the dE(N)s presum-
preferentially gone undetected due to a too large cen-
ably are not fully relaxed either.
tralsurfacebrightness,thedE(nN)s’surfacebrightnesses
We performed statistical pairwise comparisons of the
should, on average, be significantly higher than those of
distributions of our dE subsamples with respect to den-
the dE(N)s. However, the mean surface brightness in r
sity, similar as for the axial ratios in Sect. 5.2. The K-S
within the half-light aperture has very similar median
test probabilities for the null hypothesis that two ob-
valuesforthebrightdE(nN)s(µ =22.65magarcsec−2)
served distributions stem from the same underlying dis- r
andthebrightdE(N)s(µ =22.63magarcsec−2),which
tributionaregivenaspercentagesinFig.7. Eventhough r
makes such a bias unlikely. Furthermore, the distribu-
Virgo early-type dwarfs. III. Subpopulations 7
tions of surface brightnesses of the two subsamples are count measurement errors and the different S/N levels
similar — a K-S test yields a probability of 84% for the for objects of different magnitudes and surface bright-
null hypothesis that they stem from the same underly- nesses. Such a study is beyond the scope of the present
ing distribution. Certainly, measurements of the very Paper and will be presented in a forthcoming Paper of
central surface brightness, which are possible only with this series.
high-resolution observations, would provide a more di- Nevertheless, in order to tackle the question about
rect argument here. However, since both nucleated and whether the dE subsamples differ in their color proper-
non-nucleated dEs within a given magnitude range have ties,wepresentinFig.9theinneru−g(“age-sensitive”)
similar surface brightness profiles (Binggeli & Cameron versusi−z (“metallicity-sensitive”)colorsforthe bright
1991), their effective surface brightness and central sur- (m ≤ 15.67) dEs, measured within an aperture of
r
face brightness are closely correlated. a = 0.5a . This approach guarantees relatively small
hl,r
(2) Only one single galaxy among our 39 bright errors (typical values are shown in the lower left corner
dE(nN)s (2.5%) is bright enough to fall among of the Figure) that need not be taken into account indi-
Cˆot´e et al.’s definition of a HSB dE. In contrast, 14 of vidually. For eachdE subsample, we indicate its median
our 121 bright dE(N)s (12%) would qualify as HSB dE. color values with the black symbols drawn with thick
Therefore, it appears highly unlikely that a significant lines.
number of dE(nN)s possess nuclei with similar relative However,adirectcomparisonofthese valueswouldbe
brightnesses as those of the dE(N)s that were not de- biased by the existence of a color-magnitude relation: if
tected by Binggeli et al. (1985). differentsubsampleshad,onaverage,significantlydiffer-
(3) None of the dEs in Cˆot´e et al.’s own sample that ent magnitudes, they would be offset in our color-color
werepreviouslyclassifiedasnon-nucleated,buthavenow diagram even if they followed exactly the same color-
beenfoundtohostaweaknucleus,actuallyareHSBdEs. magnitude relation. We therefore compute an approxi-
(4) Since we are interested in the distributions of our mate correctionfor this effect: first, we perform a linear
subsamples with respect to density in the cluster, we least squares fit to the color-magnitude relations (r ver-
translate Cˆot´e et al.’s conjecture about the spatial dis- sus u−g and r versus i−z) of our full dE sample, clip-
tributionofthe dEsintooneaboutthedistributionwith ping one time at 3σ and excluding the dE(bc)s because
respect to density: if the different density distributions of their blue inner colors. We then derive the median
ofbrightdE(N)sanddE(nN)s(seeabove)wouldprimar- r magnitude of each subsample, and use the linear fit
ily be caused by a surface brightness selection effect, a to compute its expected color offset from the sample of
significantlylargerfractionofthehighsurfacebrightness dE(nN)s, whichwe choose as reference. The so obtained
objectsshouldbe locatedatlowerdensities ascompared correctedmediancolorsareshowninFig.9asblacksym-
to the lower surface brightness objects. To test for this bols drawn with thin lines, and are connected with lines
possible bias, we plot the mean r band surface bright- to their uncorrected values.
nesswithinthehalf-lightapertureagainstlocalprojected The dE(bc)s exhibit, as expected, the bluest colors of
density for the combined sample of bright dE(N)s and all subsamples, basically by definition, since we focus
dE(nN)s (Fig. 8). No correlationis seen, ruling out that hereonthe inner galaxycolors. While the correctedcol-
such a bias is present in our data. orsofthedE(di)saresimilartothoseofthedE(nN)s,the
We point out that it might of course still be the case dE(N)s are, on average,redder in i−z and significantly
that most of the dE(nN)s host weak nuclei that are be- redderinu−z. Giventheverysmallcolorcorrectionand
low the VCC detection limit, as discussed in Sect. 2.2. largesamplesize ofthe dE(N)s, this canbe considereda
However,whatis atstakehere is the questionwhether a robust result. In the inset shown in Fig. 9, we compare
significant number of dE(nN)s should already have been the median values of the dE(nN)s and dE(N)s to two
classifiedas dE(N)s by the VCC, and whether this could model tracks from stellar population synthesis calcula-
account for the population differences that we find. The tions (Bruzual & Charlot 2003). Both tracks represent
above arguments clearly rule out such a bias. We can stellar populations formed through a single burst of star
thus conclude that the bright dE(N)s and dE(nN)s are formation that exponentially decays with time (τ = 0.5
indeed distinct dE subpopulations that differ in their Gyr),usingPadova2000isochronesandaChabrierIMF.
clustering properties (Figs. 6 and 7), as well as in their The tracks are curves of constant metallicity (grey solid
shapes (Figs. 3 and 4). line: Z = 0.008, grey dotted line Z = 0.004); ages
increase from bottom to top and are marked at 3, 6,
6. COLORANALYSIS 10, and 14 Gyr (the latter mark is outside of the plot
Since our morphological subdivision of the dEs into area for the Z = 0.008 track). Our measured values lie
several subpopulations is now established, the next step along of the Z = 0.008 track, illustrating that, within
would obviously be to compare their stellar population the framework of our simplified stellar population mod-
properties. Given that the SDSS imaged every galaxy els, the color difference between dE(nN)s and dE(N)s
in five bands, it should be able to provide some insight could be interpreted as a difference in age. According
into their stellar content, even though it is basically im- to this simple approach, the dE(N)s would be, on aver-
possibletodisentangleagesandmetallicitieswithoptical age, a few Gyr older than the dE(nN)s. However, the
broadbandphotometryalone. However,theissueiscom- measurements also fall roughly along a virtual line con-
plicatedbythefactthattheuandz bandimages,which necting the 6 Gyr points of each model, showing that
would be very important for an analysis of the stellar they might also be interpreted as a difference in metal-
content, have a very low S/N (see Sect. 3). It is there- licity. While this color offset between the dE(N)s and
fore important to perform a thorough study of the dE dE(nN)swouldbequalitativelyconsistentwiththestudy
colors and color gradients that properly takes into ac- by Rakos & Schombert (2004), who find the dE(N)s in
8 Lisker, Grebel, Binggeli, & Glatt
the Coma and Fornax clusters to have older stellar pop- galaxies had been forming stars until their infall into
ulations than the dE(nN)s, reliable conclusions need to the cluster, resulting in a younger stellar population on
await a more comprehensive color study of our dEs. average in the case of a later infall (neglecting possible
metallicity differences). However, as stressed in Sect. 6,
7. DISCUSSION robust conclusions need to await a more detailed multi-
7.1. Interrelations between subclasses color study of our dEs.
The galaxy harassment scenario (Moore et al. 1996)
The bright dE(nN)s and dE(di)s are both unrelaxed
describesthestructuraltransformationofalate-typespi-
populations of relatively bright dEs shaped like thick
ralintoaspheroidalsystemthroughstrongtidalinterac-
disks. ThisalsoappliestothedE(bc)s, whichcouldthus
tions with massive cluster galaxies. A thick stellar disk
be candidates for being the directprogenitorsof the for-
may survive and form a bar and spiral features that can
mer: thepresentlybluecentersofthedE(bc)swillevolve
be retained for some time, depending on the tidal heat-
to typical dE colors within 1 Gyr or less after the cessa-
ingofthegalaxy(Mastropietro et al.2005). Harassment
tion of star formation (Paper II). Therefore, the bright
could thus form disk-shaped dEs, and dE(di)s in partic-
dE(nN)s and dE(di)s could constitute those disk-shaped
ular. Moreover,itpredictsgastobefunneledtothecen-
dEs where centralstar formationhas alreadyceased. To
ter and form a density excess there (Moore et al. 1998),
test this hypothesis, we make the following considera-
which would be well suited to explain the central star
tions. Thereare39brightdE(nN)s,aswellas30dE(di)s
formation in the dE(bc)s. Therefore, it appears possible
withm ≤15.67mag,7ofwhicharenon-nucleated. This
r that harassment could form disk-shaped dEs that first
adds up to 69 “non-star-forming, disk-shaped dEs”, 23
appearasdE(bc)sandthenpassivelyevolveintodE(di)s
(33%) of which are nucleated. Among the dE(bc)s there
and bright dE(nN)s as their star formation ceases (see
are 15 galaxies with m ≤ 15.67 mag, 6 (40%) of which
r Sect.7.1). Itmightalsoprovideawaytoformthefainter
are nucleated. Thus, the fraction of nucleated galax-
non-nucleateddEs,assumingthatthe tidalforceshavea
ies would be compatible with our hypothesis within the
stronger effect on the shape of less massive galaxies, re-
errors, with the caveat that nuclei might still form in
sulting inrounder objects onaverage. However,inorder
the centersofsomedE(bc)s (see Oh & Lin2000andPa-
to explain all these subclasses by a single process, one
per II), which would raise the nucleated fraction of the
wouldneed to invoke a correlationbetween the presence
dE(bc)s.
ofanucleusandofdiskfeatures,asdiscussedinSect.7.1.
Now,43%ofthenon-star-forming,disk-shapeddEsare
Ram-pressure stripping (Gunn & Gott 1972) of dwarf
dE(di)s, i.e., showdiskfeatures (notonly anoveralldisk
irregulars (dIrrs) could be responsible for the fact that
shape). IfthedE(bc)swouldcontainthesamefractionof
the disk-shaped bright dE(nN)s do not show disk fea-
galaxies that display disk features, we would expect 6.5
tures like the dE(di)s: dIrrs typically have no nucleus,
suchobjectsamongthe15dE(bc)s,withastandarddevi-
and ram-pressure stripping exerts much less perturb-
ationof1.9. Theobservednumberof4lieswithin1.3σof
ing forces than a violent process like harassment, thus
theexpectedvalueandcouldthusstillbereconciledwith
probably not triggering the formation of bars or spiral
the above picture. However, since not only the dE(di)s,
arms. Commonly discussed problems with this scenario
butalsothe brightdE(nN)s aredisk-shaped,whydothe
are the metallicity offset between dEs and dIrrs (Thuan
latter not display disk features like the dE(di)s? This
1985; Richer et al. 1998; Grebel et al. 2003) and the too
could either indicate a correlation between the presence
strong fading of dIrrs after cessation of star formation
of a significantly bright nucleus and the presence of disk
(Bothun et al. 1986). Also, the flattening distribution of
substructure, or it could imply that there is more than
Virgo cluster dIrrs – with intrinsic (primary) axial ra-
one formation path towards disk-shaped dEs.
tios≥0.5formostgalaxies(Binggeli & Popescu1995)–
is not quite like that of our bright dE(nN)s. On the
7.2. Formation mechanisms
other hand, significant mass loss due to stripped gas
If dEs originated from galaxies that fell into the clus- might affect the stellar configurationof the galaxies and
ter, how long ago could this infall have taken place? couldthuspossiblyaccountforthedifference. Moreover,
Conselice et al. (2001) derived a two-body relaxation the flattening distribution of the dIrrs is similar to that
timefortheVirgodEsofmuchmorethanaHubbletime. of the faint dE(nN)s (cf. Fig. 9 of Binggeli & Popescu
Even violent relaxation, which could apply for the case 1995),suggestingthatthese–andpossiblynotthebright
of infalling or merging groups, would take at least a few dE(nN)s – might be stripped dIrrs.
crossing times t , with t ≈1.7 Gyr for the Virgo clus- TidallyinducedstarformationofdIrrsmightbeableto
cr cr
ter (Boselli & Gavazzi 2006). Therefore, the majority overcometheproblemsoftheram-pressurestrippingsce-
of dE(N)s or their progenitors should have experienced nario: the initially lower metallicity and surface bright-
infall in the earliest phases of the Virgo cluster (which ness of a dIrr are increased by several bursts of star
is a rather young structure, see Binggeli et al. 1987 and formation (Davies & Phillipps 1988), during which the
Arnaboldi et al.2004),ortheycouldhaveformedindark galaxyappears as blue compact dwarf (BCD). After the
matter halos along with the cluster itself. All other dE lastBCDphaseitfadestobecomeadE,therebyprovid-
subclasses are largely unrelaxed populations, implying ing an explanation how BCDs could be dE progenitors,
that they have formed later than the dE(N)s, probably which has frequently been discussed (e.g., Bothun et al.
from(continuous)infallofprogenitorgalaxies. Ourcolor 1986;Papaderos et al.1996;Grebel1997;PaperII).The
analysisinSect.6wouldsupportthis view,since itfinds last star formation burst might occur in the central re-
that the inner colors of the dE(N)s can be interpreted gion, consistent with the appearance of the dE(bc)s.
with an older stellar population than the dE(nN)s. This In addition to the number of possible formation sce-
would be expected if one assumes that the progenitor narios, the role of the nuclei provides another unknown
Virgo early-type dwarfs. III. Subpopulations 9
element. If dE(N)s and dE(nN)s would actually form a results for both Virgo and Fornax cluster galaxies, it is
continuumofdEswithrespecttorelativenucleusbright- also clear that this zoo of different dE subclasses is not
ness as suggested by Grant et al. (2005), their signif- only specific to the Virgo cluster. Similarly,a significant
icantly different population properties could be inter- numberofComaclusterdEsshowatwo-componentpro-
pretedwithacorrelationbetweenrelativenucleusbright- file and are flatter than the normal dEs (Aguerri et al.
nessandhostgalaxyevolution. Suchacorrelationcould, 2005). Moreover, Rakos & Schombert (2004) found the
for example, be provided by nucleus formation through dE(N)s in Coma and Fornax to have older stellar popu-
coalescence of globular clusters (GCs): the infall and lationsthanthedE(nN)s,consistentwithourcoloranal-
merging of several GCs – resulting in a rather bright ysisoftheVirgoclusterdEs. Thus,althoughtherelative
nucleus like in a dE(N) – takes many Gyr (Oh & Lin proportionsofthe dEsubclassesmightvarybetweenthe
2000), consistent with the dE(N)s being in place since dynamically different Virgo,Coma,and Fornaxclusters,
long. The dE(nN)s, on the other hand, were probably the dE variety itself is probably similar in any galaxy
formed more recently, leaving time for only one or two cluster in the present epoch. We thus consider it im-
GCs, or none at all, to sink to the center. portantthatfuturestudiesofdEsdonotinterminglethe
differentsubclasses,butinsteadcomparetheirproperties
7.3. Remarks on previous work with each other, e.g., their stellar content or kinemati-
cal structure. This will eventually lead to pinning down
Results similar to ours were derived by
the actual significance of the various suggested forma-
Ferguson & Sandage (1989), who also subdivided
tionpaths,therebyunveilinganimportantpartofgalaxy
VirgoandFornaxclusterdEswithrespecttomagnitude
cluster formation and evolution.
and the presence or absence of a nucleus. In accordance
with our results, they found that the dE(N)s are
centrally clustered like E and S0 galaxies, while the
bright dE(nN)s are distributed like spiral and irregular We thank the referees for useful suggestions that
galaxies. They also found the axial ratios of the bright helped us improve the paper. We gratefully acknowl-
dE(nN)s to be flatter than those of the dE(N)s. edge support by the Swiss National Science Founda-
However, despite these similar results, their magni- tion through grants number 200020-105260and 200020-
tude selection of “bright” and “faint” subsamples is ac- 105535. We thank J. Gallagher for stimulating discus-
tually quite different from ours. We initially selected sions, and W. Lo¨ffler for his untiring computer support.
only dEs with mB ≤ 18.0 mag (the completeness limit This study would not have been possible without the
of the VCC), yielding a sample range of about 4.5 mag wealth of publicly available data from the SDSS. Fund-
in B, and then subdivided our full sample at its median ing for the SDSS has been provided by the Alfred P.
r magnitude. In contrast to that, Ferguson & Sandage SloanFoundation,theParticipatingInstitutions,theNa-
(1989) included VCC galaxies with mB < 17.5 in their tional Science Foundation, the U.S. Department of En-
bright subsample,whichthereforestillspansarangeof4 ergy, the National Aeronautics and Space Administra-
mag. Their faint subsample contains VCC galaxieswith tion, the Japanese Monbukagakusho, the Max Planck
mB > 18.4, which are not included in our study and al- Society, and the Higher Education Funding Council for
ready lie within the luminosity regime of Local Group England. The SDSS Web Site is http://www.sdss.org/.
dwarf spheroidals (e.g., Grebel et al. 2003). Therefore, The SDSS is managed by the Astrophysical Research
theirandourstudy canbeconsideredcomplementaryto Consortium for the Participating Institutions. The Par-
some extent, in the sense that we probe different lumi- ticipatingInstitutionsaretheAmericanMuseumofNat-
nosityregimeswithourrespectivesubsampledefinitions. ural History, Astrophysical Institute Potsdam, Univer-
sity of Basel, Cambridge University, Case Western Re-
8. CONCLUSIONS serve University, University of Chicago, Drexel Univer-
We have presented a quantitative analysis of the in- sity, Fermilab, the Institute for Advanced Study, the
trinsic shapes and spatial distributions of various sub- Japan Participation Group, Johns Hopkins University,
samples of Virgo cluster early-type dwarfs (dEs): bright the Joint Institute for Nuclear Astrophysics, the Kavli
and faint (non-)nucleated dEs (dE(N)s and dE(nN)s), Institute for Particle Astrophysics and Cosmology, the
dEswithdiskfeatures(dE(di)s), anddEswithbluecen- Korean Scientist Group, the Chinese Academy of Sci-
tersdE(bc)s). ThedE(bc)s,dE(di)s,andbrightdE(nN)s ences(LAMOST), LosAlamos NationalLaboratory,the
are shaped like thick disks and show basically no cen- Max-Planck-Institute for Astronomy (MPIA), the Max-
tral clustering, indicating that they are unrelaxed pop- Planck-Institute for Astrophysics (MPA), New Mexico
ulations that probably formed from infalling progenitor State University, Ohio State University, University of
galaxies. As opposed to that, the dE(N)s (both bright Pittsburgh, University of Portsmouth, Princeton Uni-
and faint) are a fairly relaxed population of spheroidal versity, the United States Naval Observatory, and the
galaxies, though an oblate intrinsic shape is favored for University of Washington.
themaswell. ThefaintdE(nN)s appeartobe somewhat This research has made use of NASA’s Astro-
intermediate: theirshapesaresimilartothedE(N)s,but physics Data System Bibliographic Services, and of the
theyformalargelyunrelaxedpopulationasderivedfrom NASA/IPACExtragalacticDatabase(NED)whichisop-
their clustering properties. Takentogether,these results erated by the Jet Propulsion Laboratory, California In-
defineamorphology-densityrelationwithin thedEclass. stitute of Technology, under contract with the National
GiventhatFerguson & Sandage(1989)derivedsimilar Aeronautics and Space Administration.
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