The stellar mass distribution in early-type disk galaxies: surface photometry and bulge-disk decompositions

0.52 MB·English

by  E. Noordermeer

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed5February2008 (MNLATEXstylefilev2.2)
The stellar mass distribution in early-type disk galaxies:
surface photometry and bulge-disk decompositions
E. Noordermeer,1,2⋆ and J. M. van der Hulst1
1KapteynAstronomical Institute, Universityof Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
7 2Universityof Nottingham, School of Physics and Astronomy, UniversityPark, NG7 2RD Nottingham, UK
0
0
2
accepted forpublicationinMNRAS,19-12-2006

n
a
J
ABSTRACT

5 We present deep B- and R-band surface photometry for a sample of 21 early-type
2 disk galaxies with morphological types between S0 and Sab and absolute B-band

magnitudes between -17 and -22. Six galaxies were also observed in I. We present
1
radialprofilesofsurfacebrightness,colour,ellipticity,positionangleanddeviationsof
v
axisymmetry for all galaxies, as well as isophotal and effective radii and total magni-
0
tudes.Wehavedecomposedtheimagesintocontributionsfromaspheroidalbulgewith
3
7 a general S´ersic profile and a flat disk with an arbitrary intensity distribution, using
1 an interactive, 2D decomposition technique. We caution against the use of simple 1D
0 decompositionmethodsandshowthattheycanleadtosystematicbiases,particularly
7 in the derived bulge parameters.
0 We study in detail the relations between various bulge and disk parameters. In
/ particular, we find that the bulges of our galaxies have surface brightness profiles
h
p rangingfromexponentialtoDeVaucouleurs,withtheaveragevalueoftheS´ersicshape
- parameter n being 2.5. In agreement with previous studies, we find that the shape of
o thebulgeintensitydistributiondependsonluminosity,withthemoreluminousbulges
r havingmorecentrallypeakedlightprofiles(i.e.highern).Bycomparingtheellipticity
t
s of the isophotes in the bulges to those in the outer, disk dominated regions, we are
a
able to derive the intrinsic axis ratio q of the bulges. The average axis ratio is 0.55,
: b
v with an rms spread of 0.12. None of the bulges in our sample is spherical, whereas in
Xi some cases, the bulges can be as flat as qb =0.3−0.4. The bulge flattening seems to
beweaklycoupledtoluminosity,moreluminousbulgesbeingonaverageslightlymore
r
a flattened than their lower-luminosity counterparts. Our finding that most bulges are
significantly flattened and have an intensity profile shallower than R1/4 suggests that
‘pseudobulges’, formed from disk material by secular processes, do not only occur in
late-type spiralgalaxies,but areacommonfeature inearly-typedisk galaxiesas well.
Mostgalaxiesin oursamplehaveradialcolourgradients,becomingbluer towards
largerradii. Although this can partly be explained by the radially declining contribu-
tionoftheredbulgestotheobservedlight,weshowthatdisksmustalsohaveintrinsic
colour gradients.
Key words: galaxies: photometry – galaxies: spiral – galaxies: structure – galaxies:
stellarcontent– galaxies:fundamental parameters– galaxies:kinematics anddynam-
ics.
1 INTRODUCTION knowledge of the contribution of the stars to the gravita-
tional potential. The contribution of the stellar component
Oneoftheoutstandingproblemsinourunderstandingofthe
is expected to be particularly important in early-type disk
structureandevolutionofgalaxiesistherˆoleplayedbydark
galaxies. Early-type disk galaxies are among the brightest
matter. Paradoxically, however, one of the main obstacles
galaxies in the universe, both in terms of total luminosity
towards an exactmeasurement oftheamountand distribu-
and of surface brightness (Roberts & Haynes1994). It is to
tionofdarkmatteris,inmanycases,formedbyourlimited
beexpectedthatinthesegalaxies,thestellarmassisdynam-
ically muchmoreimportantthaninlowluminosity andlow
surface brightness galaxies, which are generally believed to
⋆ email:[email protected]
2 E. Noordermeer & J. M. van der Hulst
be dark matter dominated (e.g. Carignan & Freeman 1988; tioncurvesforasubsetofthegalaxies from Paper I,andin
deBlok & McGaugh 1997; Swaters 1999; Cˆot´e et al. 2000). a future publication, we will compare them to the rotation
Historically, spiral galaxies were considered as a com- velocities expected from the visible matter in order to in-
bination of a more or less spherical bulge and a flattened fer the dark matter distribution. Here, we present another
disk. Bulges were normally modelled as miniature elliptical ingredient for our study, a study of the dynamical impact
galaxies, with surface brightnessfollowing an R1/4-law (fol- of the bulges and disks in our galaxies. We present B-, R-
lowingde Vaucouleurs1948,1958),whereastheazimuthally andI-bandsurfacephotometryforasubsampleof21galax-
averaged disk brightness was usually described with an ex- ies from Paper I, with morphological type ranging from S0
ponential profile (deVaucouleurs 1959; Freeman 1970). toSaband absoluteB-bandmagnitudes 17>MB > 22.
− −
It was readily noted that not all galactic disks are Wehavedecomposedtheimagesintocontributionsofaflat-
well described by simple exponential profiles at all radii teneddisk(includingrings,bars,etc.)andaspheroidalbulge
(Freeman 1970; van der Kruit 1979), and recent develop- withagenuine3Dstructure,usinganinteractive2Ddecom-
ments have shown that many bulges are not the sim- position technique. The radial distribution of stars in each
ple, structureless spherical bodies as previously thought ei- component is then carefully measured and luminosity pro-
ther. HST observations by Carollo et al. (1997, 1998) of files are constructed and fitted with a general S´ersic profile
the cores of 75 spiral galaxies revealed a wealth of nu- (bulge) and exponentialdisk.
clear structure, including nuclear spirals, rings and dust In addition to their use in the mass models described
lanes. Many of their galaxies showed signs of nuclear star above, our data also provide interesting information about
formation. Erwin & Sparke (1999, 2002) and Erwin et al. the internal structure of bulges and their relation with the
(2003) showed that many galaxies have nuclear bars and surroundingdisks.Wepresentananalysisonthevariouscor-
other disk-like structures in their centres. They noted relations between different bulge- and disk parameters and
that the central light concentrations in many spiral galax- brieflydiscusstheimplicationsfordifferentbulgeformation
ies seem to originate from highly flattened structures, mechanisms.
rather than from spherical bodies. The photometric pro- The structure of this paper is as follows. We describe
files of many spiral galaxy bulges are described better by thesampleselectioninsection2.Theobservationsanddata
exponential or R1/2-profiles than by the classical R1/4- reductionstepsaredescribedinsection3.Insection4wedis-
law (Andredakis& Sanders 1994; Andredakiset al. 1995; cuss the photometric analysis of the galaxies, including the
deJong 1996a; Courteau et al. 1996; Carollo 1999). Fi- derivation of radial surface brightness profiles, total magni-
nally, spectroscopic observations show that many bulges tudes, diameters, etc. In section 5, the consistency of our
are muchmore rotationally supported than elliptical galax- photometric results is checked by comparing different ob-
ies (Illingworth & Schechter1982;Kormendy & Illingworth servations of the same galaxies. In section 6 we present the
1982; Kormendy 1993; see also the recent results from the procedurethatwasdevelopedtodecomposeourimagesinto
SAURONprojectforanoverviewofthecomplexdynamics, bulgeanddiskcontributions.Wediscusstheimplicationsof
including disk-like rotation, in the centres of many spiral our results in section 7. Finally, the individual galaxies in
galaxies: Emsellem et al. 2004; Fathi2004). our sample are discussed in section 8, and a brief summary
All these observations led Kormendy & Kennicutt of ourstudyand themain conclusions arepresented in sec-
(2004) to postulate that many central light concentrations tion 9. For clarity, all long tables have been placed at the
arenotclassical, spheroidalbulgeswith asimilar formation end of the paper, in appendix A. In appendix B, we show
historyasellipticals, butratherdisk-likestructures,formed theimages andphotometric profilesfor all galaxies and the
by secular processes; they call these disk-like bulges ‘pseu- results from the bulge-disk decompositions.
dobulges’.
For a study of the global dynamics of spiral galaxies,
aproperunderstandingof thestructureof bulgesanddisks
2 SAMPLE SELECTION
is crucial. Irrespective of the exact formation mechanism
of (pseudo)bulges, they will have experienced a different The galaxies studied in this paper form a subset of the 68
evolutionary history than their surrounding disks. As a re- galaxies with Hi observations from Paper I, which them-
sult, their stellar populations, and corresponding mass-to- selves were selected from the WHISP survey (Westerbork
light ratios, are expected to be different. Accurate bulge- SurveyofHiinspiralandirregulargalaxies;Kamphuis et al.
disk decompositions are therefore needed to determine the 1996; van der Hulst et al. 2001). The WHISP sample con-
contribution of each component to the gravitational poten- sistsofgalaxiesonthenorthernhemisphere(δ>20◦),with
tial. Furthermore, the vertical structure of the bulge has a optical diameter and Hi line flux limits of D25 > 1′ and
strong influence on the peak rotation velocity in a galaxy: f >20mJyrespectively.ThesampleofPaper Iconsistedof
a flat, disk-like structurewill generally have a higher circu- all galaxies in the WHISP survey with morphological type
larrotationspeedintheplanethanasphericalsystemwith betweenS0andSab.Amoredetaileddescriptionofthepar-
the same projected surface density (see e.g. figure 2-12 in entsampleandthepropertiesoftheselectedgalaxiesisgiven
Binney & Tremaine 1987). in Paper I.
The current paper is part of a larger study of the re- Given the context of the present paper as part of our
lation between dark and luminous matter in nearby, early- larger study of rotation curves and dark matter in early-
type disk galaxies. In a previous paper (Noordermeer et al. type disk galaxies, the selection of galaxies from the par-
2005, hereafter paper I), we presented Hi observations of entsampleinPaper Iwasmainlybasedontherequirement
a sample of 68 such galaxies. In an accompanying paper to that good rotation curvescould bederived.In practice, the
thecurrentone(Noordermeer et al. 2006),wepresentrota- following, somewhat subjective criteria were used: 1) the
The stellar mass distribution in early-type disk galaxies 3
3 OBSERVATIONS AND DATA REDUCTION
Table1.Samplegalaxies:basicdata.(1)UGCnumber;(2)alter-
nativename;(3)morphologicaltype(takenfromNED1);(4)ab-
We have obtained images of all our galaxies in Harris R-
soluteB-bandmagnitude; (5)distance(fromPaperI).
and B-bands, with a few additional observations in Harris
UGC alternative Type MB D I-band;for UGC 6786, only R-banddata were available.
name mag Mpc Thebulkoftheobservationsdescribedherewereperformed
(1) (2) (3) (4) (5) during4observingrunsin2000and2001withthe1.0m Ja-
89 NGC23 SB(s)a -21.45 62.1 cobusKapteynTelescope(JKT)attheObservatorioRoque
94 NGC26 SA(rs)ab -20.32 62.6 de los Muchachos on La Palma3. A few observations were
624 NGC338 Sab -20.83 65.1
performed at the JKT in service mode in 2002. In all cases
1541 NGC797 SAB(s)a -21.12 77.0
we used the JAG-CCD camera which was equipped with a
2487 NGC1167 SA0- -21.88 67.4
2048×2048 SITe chip with a pixel size of 0.33′′ and a total
2916 – Sab -21.05 63.5
unvignettedfield-of-view of approximately 10′×10′.
2953 IC356 SA(s)abpec -21.22 15.1
3205 – Sab -20.89 48.7 A small number of observations were done using the
3546 NGC2273 SB(r)a -20.02 27.3 Wide Field Camera on the 2.5m Isaac Newton Telescope
3580 – SA(s)apec: -18.31 19.2 (INT) on La Palma3. It was equipped with four 4096×2048
3993 – S0? -20.19 61.9 EEV CCD’s with 0.33′′ pixels, offering a total field of view
4458 NGC2599 SAa -21.38 64.2 of 34×34′.
5253 NGC2985 (R’)SA(rs)ab -20.93 21.1 Finally, a few images were taken with the 8K Imag-
6786 NGC3900 SA(r)0+ -19.93† 25.9
ing Camera at the 2.4m MDM Hiltner telescope at Kitt
6787 NGC3898 SA(s)ab -20.00 18.9
Peak,Arizona.Itcontainsamosaicofeight4096×2048SITe
8699 NGC5289 (R)SABab: -19.48 36.7
CCD’swith 0.18′′ pixelsandatotalfieldofviewof24×24′.
9133 NGC5533 SA(rs)ab -21.22 54.3
TypicalexposuretimesforallobservationsontheJKT
11670 NGC7013 SA(r)0/a -19.20 12.7
11852 – SBa? -20.44 80.0 were 30 – 50 minutes per band, usually broken up into at
11914 NGC7217 (R)SA(r)ab -20.27 14.9 least 2 shorter exposures to enable cosmic ray rejection.
12043 NGC7286 S0/a -17.53 15.4 Sometimes, more but shorter exposures were taken to pre-
† NoB-banddataavailableinthisstudy;MB taken vent the bright cores of the images from saturating on the
fromLEDA2. chip.FortheobservationsatthelargerINTandMDMtele-
scopes,weusedcorrespondinglyshorterexposurestoachieve
roughly the same sensitivity as with our exposures on the
Hi velocity field must be well resolved (> 5 – 10 beams
JKT.
across) and defined over significant parts of the disks (i.e.
During some nights the observing conditions were
not confinedtosmall ‘patches’);2) thegas must bemoving
non-photometric due to thin cirrus-clouds, and in some
in regular circular orbits around the centre of the galaxy.
other nights the seeing was very poor. Useful images could
Strongly interacting galaxies, strongly barred galaxies, or
still be taken on those nights, but in the former case the
galaxies with otherwise distorted kinematics are excluded;
absolute photometric calibration was not reliable, whereas
3)theinclinationanglemustbewellconstrainedandprefer-
in the latter case the central light concentrations were
ably lie between 40◦and 80◦.
poorly resolved. Those problems were fixed by re-observing
Very few galaxies from Paper I satisfy all these condi-
thegalaxies with shortexposuretimeslater.Afewgalaxies
tionsandastrict application of thesecriteria would lead to
were observed in good conditions several times. Those
a very small sample. For the present paper, we have there-
observations are used in section 5 to check the consistency
fore relaxed the selection criteria and included a number
of our observations. In table A1 we list the observations of
of galaxies with e.g. bars, mild kinematical distortions or a
all galaxies.
more face-on orientation.
Although the sample selection criteria described above
Data reduction was performed within the IRAF envi-
may,atfirstsight,appearsomewhatirrelevantforastudyof ronment4. Standard procedures were followed to subtract
bulgeanddiskproperties,webelievethatourgalaxiesforma
readout bias and apply flatfield corrections. Separate expo-
representativesampleofearly-typediskgalaxies.InPaper I,
we showed that the galaxies in the parent sample resemble
verymuch‘average’early-typediskgalaxies.Theadditional
1 NED: The NASA/IPAC Extragalactic Database
selection for the current paper, based on the kinematical
(http://nedwww.ipac.caltech.edu), is operated by the Jet
properties described above, is unlikely to introduce a bias propulsion Laboratory, California Insitute of Technology, under
towards any particular bulge or disk properties. contract with the National Aeronautics and Space Administra-
In total, the current sample consists of 21 galaxies; a tion.
few basic characteristics of the members are given in ta- 2 LEDA: The Lyon Extragalactic Database,
ble 1. The distances in the table were taken from Paper I http://leda.univ-lyon1.fr/
andwerebasedonthesystemicvelocitiesderivedthere,cor- 3 The Jacobus Kapteyn Telescope and Isaac Newton Telescope
are operated on the island of La Palma by the Isaac Newton
rected for Virgo-centric inflow, and a Hubble constant of
Groupinthe Spanish Observatoriodel Roque de los Muchachos
75kms−1Mpc−1. The majority of the galaxies in our sam-
of the Instituto de Astrofisica de Canarias. Part of the data de-
ple are luminous (16 systems have M < 20). Of the 21
B − scribed in this study was collected via the service observation
galaxies, 16 areclassed as spirals (typeSaor Sab) and 3 as
schemeoftheIsaacNewtonGroup.
lenticulars (S0- – S0+); the remaining two fall in the tran- 4 IRAFisdistributedbytheNationalOpticalAstronomyObser-
sitional S0/a class. vatories,whichareoperatedbytheAssociationofUniversitiesfor
4 E. Noordermeer & J. M. van der Hulst
suresforeach galaxy werealigned usinganumberof bright backgroundgalaxies andanypossible artifacts notremoved
stars. They were then combined, using a simple rejection in the previous steps. For the foreground stars, we fitted
criterion to remove cosmic ray events and cosmetic defects a 2D analytical PSF to 20 bright, unsaturated stars on
≈
on thechip. theimage.Subsequently,allpointsourcesintheimagewere
Special care was taken in the subtraction of the back- detected, and the flux in each was determined. The PSF
groundlight.InthefirstthreerunsontheJKT,thecamera wasthenscaled tothemeasured fluxandmodelstellar‘im-
suffered from light-leaks which caused faint residual gradi- ages’ were subtracted from the image. This proved to work
ents in the background after flatfielding, especially in the in a satisfactory manner only for relatively faint stars. For
R-band images. In almost all cases, the residuals could be bright stars, the analytical PSF often did not represent the
removed by fitting a first- or second-order 2D polynomial stellar images sufficiently well, and significant residuals re-
to the background and subtracting it from the image. In a mained present in the image after subtraction. Saturated
few cases we used polynomials up to order 5. In all cases, stars,backgroundgalaxiesandotherartifacts intheimages
extreme care was taken to exclude the image of the galaxy could, naturally, not be removed with this method either.
from thefittingregion, soas not tosubtract any light from All these were removed by hand, masking out the affected
thegalaxy itself. regions from the image. Special care was taken to remove
In all other observations, including the B-band obser- faint haloes of bright stars. The same masks were used for
vations from the JKT runs with light-leaks, the flatfielding all images of a galaxy in different bands,toensure that the
workedverywellandresidualsweresmall.Fortheseimages, measured magnitudes and surface brightnesses refer to the
thebackgroundwasremovedbyfittingafirst-orderpolyno- same regions.
mial tothe emission-free regions.
Asanextracheckthatthelight-leaksdidnotaffectour
results, we re-observed UGC 2953 and 3546 in the fourth
observing run on the JKT, when the light-leaks were elimi- 4 PHOTOMETRIC PROPERTIES
nated.Comparisonoftheresultingphotometricprofilesfrom
Photometry was performed at concentric elliptical annuli,
theimageswithandwithoutlight-leaksshowsthatthegra-
usingtheELLIPStaskfromtheSTSDASISOPHOTEpack-
dientswere adequately removedand that theresulting pro-
age (Jedrzejewski 1987). The ellipses were defined at loga-
files are consistent within the errors (see section 5). rithmic intervals in semi major axis, starting at 0.3′′ and
Throughout the nights, standard star fields from ≈
increasing the radius of each successive ellipse by a factor
Landolt (1992) were observed at intervals of 1 – 2 hours.
1.1.
Using the instrumental fluxes of the stars and the magni-
tudesgivenbyLandolt,wefittedthemagnitudezero-points
and extinction coefficients, such that
4.1 Isophote orientation parameters
mLandolt=mobs+m0+e , (1)
·A The orientation parameters of the ellipses were determined
where m is the stellar magnitude given by Landolt, from theR-band images in two steps.
Landolt
mobs is our instrumental magnitude, m0 is the magnitude First, ellipses werefittotheimage with theposition of
zero-point, e is the extinction coefficient and is the air- the centre, position angle and ellipticity of each ellipse left
A
mass. The coefficients were determined for each night and as free fitting parameters. From these fits, the central posi-
each colour separately. The errors ∆m0 and ∆e on the fit- tions of the galaxies were determined. The bulges of most
tedcoefficientswereusedtoderivethephotometricerrorfor galaxies create a clear central peak in the image and the
each observation: centres of the inner ellipses converge usually to a narrowly
definedvalue; often thevariation between theinner ellipses
σphot = ∆m20+(∆e·A)2. (2) issmallerthan0.1′′.Inthesecases,theerrorintheabsolute
p
The resulting errors for all observations are given in ta- central position is dominated by the uncertainty in the co-
ble A1. The average photometric errors for our B-, R- and ordinate system (see section 3). In a small number of cases
I-bandobservationsarerespectively0.13,0.11and0.11mag, dustobscurestheverycentreoftheimage,andthelightdis-
and respectively 90, 76 and 83% have σ <0.2mag. tributionismoreirregular.Sinceinthosecasesthebrightest
phot
Coordinate systems were added to the images by reg- pixelslieatthepositionofleastobscuration,ratherthanat
istering a number of bright stars with known coordinates the true centre of the underlying light distribution, the in-
from the Digitized Sky Survey (DSS). Typical rms errors ner isophotes can not be used to determine the centre and
were of the order of 0.15′′ for the JKT images and 0.2′′for we rather used the ones at intermediate radii where the ef-
the larger INT and MDM images. Note, however, that the fectofdustiseithersmalleroraveragesoutovertheellipse.
uncertainty in the coordinate system of the DSS is about The central positions of those galaxies are, however, neces-
0.6′′(Russell et al.1990;V´eron-Cetty & V´eron1996).Thus, sarily less accurately determined than for the ones where
thecoordinate systems of our images are effectively limited we can see into the centre directly. The fitted centres are
bytheuncertaintiesin theDSSandhaveanaccuracy ofno listed in columns (2) and (3) in table A2; the uncertainty
betterthan 0.6′′. and its dominant source are given in columns (4) and (5)
Finally,theimageswerecleanedfromforegroundstars, respectively.
Inthesecondstep,wedidfitswiththecentreofeachel-
lipsefixedatthepositionjustdetermined,butwithelliptic-
Research in Astronomy, Inc., under cooperative agreement with ityandpositionanglestillfree.Thefittedorientationparam-
theNational ScienceFoundation. etersareshownbythesymbolsinthetoprighthandpanels
The stellar mass distribution in early-type disk galaxies 5
inthefiguresinappendixB.Intheouterpart ofthegalax-
ies,outsidetheregionwherebulgesandbarscomplicatethe
picture, they usually converge to more or less constant val-
ues(seeUGC3205,3580and6787forniceexamples).These
valueswerethenassumedtorepresentthetrueposition an-
gleandinclinationofthegalaxy.Insomecases,however,the
position angle and/or ellipticity never converge, and either
keep varyingtill thelast point (e.g. UGC 89, 1541 or 4458)
orshowseveralplateaus(e.g.UGC9133).Inthesecases,we
visually compared the fitted isophotes to the shape of the
galactic disksandthenestimated theposition angleandel-
lipticitybyeye.Notethatlargevariationsintheorientation
parameters in the very outer points (e.g. UGC 3993, 4458,
8699) can usually beattributed to asymmetries in thelight
distribution of the galaxy or to imperfect flatfielding; these
variations were generally not considered real. The adopted
values for the position angle and ellipticity are indicated
withthedashedlinesinthefiguresinappendixBandlisted
in columns (6) and (7) in table A2.
Fromtheellipticitywedeterminedtheinclinationofthe
galaxies using the standard formula Figure1.Exampleofthedeterminationoftheexactbackground
level for one of our images. The squares give the average inten-
cos2i= (1−ǫ)2−q02, (3) sities on each ellipse; the errorbars give the measurement error.
1 q2 The long-dashed line gives the average intensity of the outer el-
− 0
lipses,whichdefinesthezero-pointIbg oftheimage.Inthiscase,
with q0 theintrinsic flattening of thedisk, for which we as- Ibg=−0.46,indicatingthatwehaveslightlyover-subtractedthe
sumeda valueof 0.2 (cf.de Grijs 1998).Thederivedvalues background.Thedottedlinesgivetheassumederrorinthezero-
are given in column (8) in table A2. Note that they are es- point. The arrow indicates the outer point with significant flux
timates only; if the disks have different intrinsic flattening, fromthegalaxy.Seetextfordetails.
oriftheyareintrinsicallyelongatedratherthanaxisymmet-
ric,thanthetrueinclination maydeviatesignificantly from
these values. 24.26magarcsec−2 in theB-,R-and I-bandimages respec-
tively.
Theerrorsonthemeasuredintensitiesinthegalaxyare
now given by the quadratic sum of the measurement error
4.2 Surface photometry
∆I (r) and the zero-point error σ :
m bg
Once theorientation parameters were determined, we fixed
them for all ellipses and simply measured the average in- σI(r)= ∆Im(r)2+σb2g (4)
q
tensitiesateach radius.Thesameellipses wereusedforthe
Finally, the intensities and corresponding errors were
images in different colour bands.
calibrated and converted to surface magnitudes using the
Even though the background subtraction described in
transformations determined in section 3. Only points with
section 3 usually worked quite well, some small residuals
intensities larger than 2σ were considered. The resulting
always remained. To determinetheexact zero-points in the bg
photometric profiles are shown in the top middle panels on
images, aswellastheiruncertainties,welet theradiiofthe
the first row of the figures in appendix B. In the bottom
ellipses expand until they covered the entire images. Thus,
middlepanelsonthefirstrowofthefigures,thecolourpro-
intensitiesweremeasured notonlyon thegalaxies, butalso
files are shown. The errors on the points are given by the
inannulicenteredon,butmuchlargerthan,thegalaxies.A
quadraticsum oftheerrors on theindividualbands.Points
typical result is shown in figure 1.
areonlyshownwhenthetotalerrorissmallerthan0.5mag.
In this case, it is clear that no light from the galaxy is
detectedbeyondaradiusofabout90′′,sotheellipsesoutside
thisradiusprobetheskybackground.Theaverageintensity
4.3 Deviations from axisymmetry
on theouter ellipses definesthezero-point I of theimage
bg
and is subsequently subtracted from the intensities in the In addition to the average intensity on each ellipse, we also
galaxy. measured the higher-order harmonic components to study
The variation in the intensities on the outer ellipses deviationsfromperfectaxisymmetry.Theintensitydistribu-
measures the uncertainty in the zero-point. Note, however, tionalongeachellipsewasdecomposed intoaFourierseries
thattheseouterellipsescoverlargepartsoftheimage,such of thefollowing form:
that small scale irregularities in the background are aver-
I(α)=I0+I1cos(α+φ1)+I2cos(2(α+φ2))+..., (5)
aged out. To account for irregularities at smaller scales as
well, we adopted for the zero-point uncertainty σbg twice where α is the azimuthal angle along the ellipse, I0 is the
the rms variation in the intensities of the outer ellipses. In average intensity on the ellipse and I1 and I2 measure the
table A1 we give the magnitude level µ3σ corresponding to strength of the m=1 and m=2 Fourier components respec-
3σbg abovethesky;theaverage valuesare 26.95, 26.07 and tively.Thus,ahighvaluefor I1/I0 indicatesastrongly lop-
6 E. Noordermeer & J. M. van der Hulst
sided light distribution, whereas a high value for I2/I0 re- 5 INTERNAL COMPARISON OF
veals thepresence of a baror oval distortion. PHOTOMETRIC PROFILES
A statistically significant measurement of these higher
Several galaxies were observed during different nights and
ordertermsinthelightdistributionrequiresahighersignal-
with different telescopes (see table A1). For each of these
to-noise ratio than is necessary for the zeroth order term.
cases,onlythebestdataareshowninappendixBandlisted
The m=1 and m=2 terms were therefore only measured on
in tables A2 and A3. The data from the other observations
ellipsesforwhichtheaverageintensitywasrespectively2√2
have been used to assess the quality of our data and the
and 4 times σ .
bg reliability of thederivederrors. Theseobservationswere re-
The relative strengths and the phases of the m=1 and
duced independently, following the same steps as for the
m=2 components are shown in the bottom 4 panels on the
mainobservations.Thesamemaskswereused,however,for
right side of thefirst row of thefigures in appendix B.
all images (section 3), to make sure that measured surface
brightnesses correspond to the same regions in all images.
Similarly,thesamepositionanglesandellipticitieswereused
to derivethephotometric profiles.
In figure 2, we compare thephotometric profiles for all
the galaxies with multiple observations. In most cases, the
independentlyderivedprofilesagreeverywell,butthereare
4.4 Isophotal diameters, effective radii and total also some cases with significant differences.
magnitudes Errors in thephotometric calibrations lead to constant
offsets in a profile but leave its shape unchanged. Indeed,
For each galaxy and each band, several diameters were de-
systematicoffsetsbetween2profilesareseenforsomegalax-
rived. D25 and D26.5 are isophotal diameters, determined iesinfigure2(thedottedlinesgivetheweightedaverageof
at µ=25 and 26.5magarcsec−2 respectively. R20, R50 and thedatapoints), but in most cases, theseoffsets fall within
R80 are effective radii, containing respectively 20, 50 and the combined 1σ photometric errors (solid lines). In a few
80%ofthelight.Thediametersandeffectiveradiiaregiven
cases the offsets are larger than to be expected (notably
in table A2.
UGC 2953, 6786 and 79895). In the case of UGC 2953, the
Note that these diameters and effective radii are de-
differencesbetweentheindependentobservationscanbeex-
rivedfrom thephotometricprofilesasmeasured onthesky.
plained as the result of non-photometric conditions during
However,thesurfacebrightnessesofthedisksofthegalaxies
thenightsofthecomparison observations. Intables A1,A3
aredimmedbyGalacticforegroundextinction.Ontheother
andA4,wehavemarkedallobservationsthatweredoneun-
hand,theobservedsurface brightnessesare higherthanthe
dernon-photometricconditions.Notethatnon-photometric
truevaluesdueto thefact that we observethegalaxies un-
conditions do not affect the shapes of the photometric pro-
der a non-zero inclination angle. The raw isophotal diame-
files. Scale lengths, effective radii, bulge-disk ratios, etc.
ters derived above do therefore not correspond to the same
can still be accurately measured on such nights. Further-
physical surface brightness in the disks of these galaxies.
more,inmanycases,galaxiesthatwereobservedundernon-
In table A2 we also give the isophotal diameters Dc and
25 photometricconditionswerelaterre-observedwithshortex-
Dc thatcorrespondtoface-on,extinctioncorrectedlevels
26.5 posure times; these short exposures were then used to cali-
of µ = 25 and 26.5magarcsec−2. The correction for incli-
bratethedeepimagestakenbeforeandaccuratephotometry
nation was performed assuming that the galactic disks are
could still be achieved.
optically thin;nocorrections weremadeforinternalextinc-
TheoffsetsforUGC6786and7989areprobablycaused
tioncausedbydustinthegalacticdisksthemselves.Thecor-
bydifferencesbetweenthefiltertransmissioncurvesusedfor
rections for Galactic foreground extinction were performed
theobservationsontheJKTandtheMDMtelescope. Note
using thevalues from Schlegel et al. (1998).
that the offsets between the JKT and MDM profiles are
Total magnitudes cannot be derived from the images equally large (0.20magarcsec−2) for both UGC 6786 and
directly,as large parts of the images are sometimes masked
7989; this strengthens the interpretation of the offsets as a
to remove foreground stars, background galaxies, etc. (sec-
result of filter differences.
tion3),suchthatthemaskedimagescontainfluxfromparts
Apart from the systematic offsets between theprofiles,
of the galaxies only. To correct for this effect, model im-
randompoint-to-pointvariationsaregenerallysmall,except
ageswerecreated,basedonthephotometricprofilesderived
for the inner few arcseconds where differences in seeing be-
in section 4.2, interpolating over the masked regions. From
tween the observations lead to artificially large discrepan-
thesemodelimages,twoapparentmagnitudeswerederived.
cies.Insomecases,errorsinthesky-leveldeterminationlead
m is the total magnitude within the last measured point
lim to differences in the outer points of the compared profiles,
in the photometric profile; m25 is the apparent magnitude buttheyaregenerallywithintheerrorsderivedasdescribed
within the 25thmagarcsec−2 isophotal diameter D25. The insection4.2.Thelargestsky-subtractionerrorsareseenin
errors in the total magnitudes are usually dominated by
UGC4458,wheretheouterpointsofthetwocomparedpro-
theuncertaintyσ inthephotometriccalibrations(equa-
phot
tion 2, column (7) in table A1), with small contributions
from the zero-point uncertaintyσ and Poisson errors.
bg 5 The analysis in this section includes two galaxies, UGC 7989
The corresponding absolute magnitudes Mlim and M25 and 12713, which were observed during the same runs as the
weredeterminedusingthedistancesasgivenintable1and other galaxies in our sample, but which are not included in the
were corrected for Galactic foreground extinction using the remainder ofthis study. They areonlyshown hereas additional
values from Schlegel et al. (1998). comparisonmaterial.
The stellar mass distribution in early-type disk galaxies 7
Figure 2.Internal comparisonofthephotometricprofiles.Datapointsshow thedifferencebetween photometricprofilesfrommultiple
observationsofthesamegalaxy.Theobservationswhicharecomparedareindicatedaboveeachpanel;seetableA1fordetails.Errorbars
givethecombinederrorsoneachpoint.Dashedlinesgivetheweightedaverageofthepointswhicharenotaffectedbyseeingdifferences.
Solidlinesgivethecombined1σ photometricerrors.
files disagree at a level of 2 times the combined 1σ errors. count well for the uncertainties in the determination of the
Given thenumberofprofiles compared here,onecase of2σ skylevel. Photometric profileswhich were derivedfrom im-
disagreement is to be expected. The only cases where we ages observed with the INT or MDM telescope show small
observesignificantdifferencesbetweentheprofilesthatcan- deviations ( 0.1 0.2magarcsec−2) compared tothe pro-
∼ −
not be attributed to seeing effects or uncertainties in the filesderivedfrom theJKTimages. The deviationsmanifest
sky-level are the profiles which were observed with differ- both as systematic offsets between the profiles, as well as
enttelescopes(R-bandobservationsofUGC2953,6786and point-to-point variations within the profiles. These differ-
7989);asabove,thesedifferencescanprobablybeattributed ences are probably caused by differences in the filters used
to the different filters which were used for the observations for theobservations.
undercomparison here.
In conclusion, for galaxies that were observed under
photometric conditions, the photometric errors listed in ta-
6 BULGE-DISK DECOMPOSITIONS
bleA1seemtobereliable;fornon-photometricobservations
the errors are lower limits. The errors on individual data Many methodsexist to decompose thelight of spiral galax-
points in our photometric profiles are also realistic and ac- ies into contributions from bulges and disks. Traditionally,
8 E. Noordermeer & J. M. van der Hulst
riveddirectly from ourphotometricprofileswill beseverely
affected by systematic errors.
Anotherproblemwithbulge-diskdecompositionsisthe
degeneracy that exists between the different parameters
(MacArthuret al. 2003; de Jong et al. 2004). The data can
often be fitted by different combinations of parameters, so
that it can be difficult to reach unambiguous conclusions
about the truevalues.
To overcome the difficulties mentioned above, we have
developed a hands-on, interactive procedure that uses the
full 2D information from the images to separate them into
bulge and disk components. Our method bears some sim-
ilarities with the one used by Palunas & Williams (2000)
and can be summarised as follows. We model the bulges as
flattened axisymmetric spheroids, with intrinsic axis ratios
q and general S´ersic photometric profiles characterised by
b
effectiveradiusandmagnituder andµ andshapeparam-
e e
eter n (S´ersic 1968). The bulge parameters are determined
fromimageswhereaninitialestimateforthediskshasbeen
subtracted. Based on the fitted parameters, model images
Figure 3. The influence of projection effects on the central R-
band photometric profile of UGC 6786. The profile shown with of the bulges are created and subtracted from the original
squaresisderivedusingthestandardmethod, i.e.theintensities images.Alllightintheseimageisthenassumedtooriginate
aremeasuredalongellipseswithellipticityfixedatthevalueofthe from the disks, and their photometric profiles are used to
outerregions.Thecirclesdenotetheprofilewhenmeasuredalong derivethedisk light distributions.
ellipses with an ellipticity which better represents the isophotes A more detailed description of the separate steps is
ofthebulge. given in thefollowing subsections.
thedecomposition is performed on the photometric profiles
6.1 Bulge parameters
directly, fitting them with the sum of an exponential disk
and a certain profile for the bulge (usually either an r1/4, The first step in our procedure consists of deriving initial
exponential or general S´ersic profile). As this is a 1D pro- estimates for the bulge and disk parameters from an ana-
cedure, it is quick and can therefore be used to study large lytic fit to the photometric profile. As discussed above, the
samples of galaxies in short timespans (e.g. Baggett et al. central, bulge-dominated parts of the photometric profiles
1998;Graham2001;MacArthur et al.2003).However,pho- suffer from projection effects. In the outer parts, however,
tometricprofilessufferfromprojectioneffects.Theobserved theprofileisusuallydominatedbytheflatdisk,andprojec-
intensity at each point in a galaxy is a superposition of tion effects are not an issue. Thus, the initial estimates for
light from the bulge and disk. Because they have different thedisk are generally sufficiently reliable for thenext step.
intrinsic shapes, the contributions from both components Based on the initial estimates, an initial model image
come from different radii when the galaxy is observed un- ofthediskcomponentiscreatedbyextrapolatingthefitted
der a non-zero inclination angle, and the average intensity disk profile inwards. This model image is then subtracted
alongagivenisophoteisgenerallynotdirectlyrelatedtothe from the original image to obtain an image with, to first
true mean brightness at that radius. Thus, deriving bulge order, bulge-light only.
anddiskparametersfromazimuthallyaveragedphotometric From theresultingimage, thebulge-parameters can be
profiles will lead to systematic errors. A proper treatment obtained. We first determine the flattening of the bulge by
of the projection effects requires a full 2D decomposition fitting ellipses to the R-band bulge image with ellipticity
technique (e.g. Byun & Freeman 1995; deJong et al. 2004; and position angle as free parameters. In most cases, the
Laurikainen et al. 2005). position angle of the bulge isophotes is close to the value
The projection effects described above are particularly derived for the outer disk, indicating that the bulge is a
severe in the type of galaxies studied here, which have of- flattened spheroid with the plane of symmetry coinciding
ten large and luminous bulges. As an illustration, we show with that of the disk. In a few cases, the position angle
in figure 3 two R-band photometric profiles of the central in the centre differs from the outer values, indicating the
part of the bulge-dominated galaxy UGC 6786. The profile presence of a nuclear bar or other triaxial structure. The
shownwithsquaresisderivedfollowing ourstandardproce- presence of these non-axisymmetric structures is ignored in
duresofsection 4.2,that is, it showstheaverage intensities the following; we model all bulges as oblate spheroids that
measuredalongellipseswiththeellipticityfixedatthevalue are aligned with the outer disk and we simply average out
of the outer regions. The circles show the intensities when anynon-axisymmetric structuresalong theisophotes in the
measured alongellipses with amuchlowerellipticity,which next step. Because the bulges are more vertically extended
ismorerepresentativeoftheisophotesinthecentral,bulge- than the surrounding disks, their ellipticities are generally
dominated region. Clearly, the bulge of UGC 6786 is more lowerthanthosefoundfortheouterdisks.Fromtheaverage
centrally concentrated than is shown by our standard pho- ellipticity ǫ of the bulge isophotes, the intrinsic flattening
b
tometricprofile;anystructuralparametersofthebulgesde- of thebulge can be determined by rewriting equation 3 as
The stellar mass distribution in early-type disk galaxies 9
q2 = (1−ǫb)2−cos2i. (6) above, are shown in thebottom left panels in thefigures in
b 1 cos2i appendix B.
−
Here q is the intrinsic axis ratio of the bulge and i is the
b
inclination of thesystem, as derived in section 4.1.
Wenextderivephotometricprofilesforthebulgeinall
available colour bands by measuring the average intensities
onconcentricellipseswithfixedellipticityǫb.Intensitiesare 6.2 Disk parameters
only measured inside the radius where the bulge and disk
intensities from the original image are equal. The resulting To measure the light distribution in the disk, the model
profiles are shown with the data points in the figures in images of the bulge are subtracted from the original im-
appendix B. ages in each available colour. The resulting R-band images
To extrapolate the contribution of the bulge to larger are shown in the bottom right panels in the figures in ap-
radii,wefittedtheprofileswithageneralS´ersiclightprofile pendixB.
(S´ersic 1968): All remaining light in these images is then assumed to
originatefromthediskcomponent.Thisalsoincludesresid-
r 1/n ualsin thebulge-dominatedparts,whereourfittedanalyti-
I (r)=I exp . (7)
b 0,b (cid:20)−(cid:16)r0(cid:17) (cid:21) calprofile(equation8)doesnotexactlymatchtheobserved
light distribution.Note, however,that thedeviations of the
Here, I0,b is the central surface brightness and r0 is the observedbulgeprofilefromtheanalyticfitarealmostalways
characteristicradius.nisashapeparameter,thatdescribes
verysmall compared tothetotal over-densityofbulge-light
the curvatureof the profile in a radius-magnitude plot. For
over the inner disk (typically 0.05 – 0.1 vs. 2 – 4 magni-
n=4,equation7reducestothewell-knownDeVaucouleurs
tudes).The errorcaused byattributingtheresiduals to the
profile (deVaucouleurs 1948), whereas n = 1 describes a
disk, instead of to the bulge, will be negligible when calcu-
simple exponential profile. In the literature, equation 7 is
latingthecombineddynamicalimpactofbothcomponents.
usually written as
The radial distribution of the disk light is then deter-
mined by measuring the average intensities on the ellipses
r 1/n
Ib(r)=Ieexp(cid:20)−bn(cid:26)(cid:16)re(cid:17) −1(cid:27)(cid:21), (8) dtheefindeisdki,nousetcstidioenth4.e1.buTlhgee-rdeissuklttirnagnspihtiootnomraedtiruics,parroefislehsowfonr
with I = e−bnI the intensity at the effective radius in the bottom right panels in the figures in appendix B. In
e 0,b
re =bnnr0, the radius which encompasses 50% of the light. twocases, UGC6786 and6787, it wasfound,after thesub-
traction of the bulge, that the ellipticity derived from the
b is a scaling constant that is defined such that it satisfies
n
γ(b ,2n)= 1Γ(2n), with γ and Γ theincomplete and com- original image did not accurately represent the inclination
n 2 of the disk. In these two cases, the bulges are so dominant
pletegammafunctionsrespectively.Itcanbeapproximated
that,in theoriginal images, theyinfluencetheshapeof the
byb 1.9992n 0.3271for1<n<10(Graham2001),but
n
≈ − isophotes even in the outer parts. The correct inclination
in our fitting procedure, we determined b more accurately
n
angle of the disks could therefore only be determined after
by numerically solving theequation above.
the subtraction of the bulge, and for the final disk profile,
In magnitudes, equation 8 is written as
weusedvaluesdifferentthantheoneslistedintableA2(see
r 1/n individualnotesin section 8).Thecorrected inclination an-
µ (r)=µ +1.0857b 1 . (9)
b e n(cid:26)(cid:16)re(cid:17) − (cid:27) gle was also used to recalculate the flattening of the bulges
for these galaxies (equation 6).
Beforefittingtheprofileofequation9totheobserveddata, Finally,exponentialprofileswerefittedtotheobserved
it is corrected for seeing effectsusing theconvolutions from disk profiles:
Graham (2001).
The seeing-convolved profile is then fitted to the data r
I (r)=I exp , (10)
usingastandardnon-linearleast-squaresalgorithm.Forthe d 0,d −h
(cid:16) (cid:17)
fits to the B- and I-band profiles, we fixed the parameters
or in magnitudes:
nand r tothevaluesfoundfor theR-bandprofile, leaving
e
onlytheeffectivesurfacemagnitudeµ free.Thus,wemodel
e r
thebulges as fully mixed systems without colour gradients. µd(r)=µ0,d+1.0857 h , (11)
(cid:16) (cid:17)
Anyremaining colour gradients afterthesubtraction of the
bulgeinfluenceareattributedtothedisk.Thefittedprofiles with I and µ the central surface brightness, and h the
0,d 0,d
are shown as the solid lines in the figuresin appendix B. radialscalelength.Onlypointsoutsidethebulge-disktran-
The fitted bulge parameters ǫ , q , µ , r and n are sitionradiuswereusedforthefit.Theresultingparameters
b b e e
given in table A4. The effective surface brightness was also are listed in table A4; the corresponding profiles are shown
corrected for Galactic foreground extinction, using the val- as the solid lines in the figures in appendix B. In table A4,
uesfromSchlegel et al.(1998);thecorrectedsurfacemagni- wealsolistµc ,thecentralsurfacebrightnesscorrectedfor
0,d
tudeµc is listed in table A4 as well. Finally, a total magni- inclination and Galactic foreground extinction. The correc-
e
tude was derived by integrating equation 7 to infinity; the tionforinclinationwasperformedassumingthatthegalactic
resulting values m and M for the apparent and absolute disks are optically thin; the corrections for Galactic fore-
b b
magnitudesarealsolistedintableA4.Modelimagesforthe ground extinction were performed using the values from
bulge in the R-band, created using the parameters found Schlegel et al. (1998).
10 E. Noordermeer & J. M. van der Hulst
Figure4.ComparisonoftheR-bandbulgeanddiskparameters.Left:absolutemagnitudesofbulgevs.disk;middle:bulgeeffectiveradius
vs.diskscalelength;right:bulgeeffectivesurfacebrightnessvs.diskcentralsurfacebrightness.TheopensquaresindicateUGC624,the
circlesindicateUGC6786.
7 DISCUSSION lefthandpaneloffigure5,whereweshowthedistributionof
the BD luminosity ratio for all galaxies in our sample. The
The main purpose of the analysis described in this paper
average value of log(L /L ) is 0.23 0.47 in the R-band,
istoderivethecontributionofthestellar componentofour b d − ±
where theerror gives thestandard deviation of thesample.
galaxiestotheirgravitationalpotential.Inaforthcomingpa-
Apart from two galaxies, UGC 624 and 6786, which
per, we will calculate the contributions of the stellar disks
have unusually large bulges (as measured from their effec-
and bulges to the rotation curves, and use the results to
tiveradiir ),thereisaweak correlation between diskscale
constrainthecontentanddistributionofdarkmatterinour e
length and bulge effectiveradius(middle panelin figure 4).
galaxies.Meanwhile,someinterestingresultsconcerningthe
In the following, we will show that UGC 6786 differs from
structureandmutualdependenceofthebulgesanddisksof
the rest of the sample in many aspects, and we will inter-
ourgalaxiescanbederivedfromourstudyaswell.Thesere-
pretthisinsection8asanindicationthatthisgalaxyisnot
sultsareinsomewayscomplementarytothelargerstudiesof
really a disk galaxy, but rather an elliptical galaxy with an
e.g.Graham(2001),MacArthuret al.(2003),deJong et al.
additionaldiskofgasandstars.TheoffsetofUGC624may
(2004),Hunt et al.(2004)andLaurikainen et al.(2005).Be-
be explained as a result of its high inclination with respect
low, we discuss the most important points.
to the line of sight. There are indications that the image of
this galaxy is significantly affected by dust (note for exam-
plethestrong asymmetry in thecentre, visible in the I1/I0
7.1 A comparison of bulge and disk parameters
lopsidedness parameter, and by eyein thebulge-subtracted
Infigure4,wecomparethetotalluminosities,sizesandsur- image). Ifthereareindeedlarge amountsofdustpresentin
face brightnesses of the bulges and disks of the galaxies in thisgalaxy,itisconceivablethattheyintroducelargeerrors
our sample with accurate bulge-disk decompositions. It is intheshapeofthephotometricprofilesandintheresulting
clear that there is only a weak coupling between the bulge bulge-disk decomposition.
and disk parameters. Theothergalaxies seem tofollow theglobal trendthat
Thereis aweak trendofmoreluminousdisksharbour- large bulges reside in large disks, although the scatter is,
ing more luminous bulges, but the scatter around the rela- again, large. In the right hand panel of figure 5, we show
tion is large. Moreover, several biases in our sample selec- the distribution of re/h for all galaxies in the sample. The
tioncouldintroduceanartificialcorrelation:galaxieswitha averagevalueofthisratio is0.51 0.72 (theerrorindicates
±
highly luminous bulge and a faint disk might not be classi- thestandard deviation),butthisvalueisheavilyinfluenced
fied as disk galaxies, but rather be mis-identified as ellipti- by UGC 6786. The average value for the sample without
cals.Similarly,luminousdiskswithfaintbulgesmightnotbe UGC 6786 is 0.35 0.19, whereas it is further reduced to
±
classifiedasearly-typedisks,butratheraslate-typespirals. 0.32 0.15 if UGC 624 is excluded as well. Even this last
±
This latter effect is, however, not expected to be as strong value is significantly higher than the average re/h for the
as the former, since the morphological type classification is late-type spiral galaxies studied by Courteau et al. (1996).
basedonotherparametersthanbulge-diskluminosityratios ItisalsohigherthanthevaluesfoundbyGraham(2001)and
aswell.Moreover,oursamplecontainsanumberofgalaxies MacArthuret al.(2003),whobothnotedamildtrendofthe
with high-luminosity disks and faint bulges (e.g. UGC 94, ratiore/hincreasingtowardsearliertypespiralgalaxies,but
3205,3546),whicharestillclearlyrecognisableasearly-type onlyreachinganaverageofrespectively0.21and0.24forthe
disk galaxies. UGC 12043 even has no obvious bulge com- early types.
ponent at all, but is still classified as an S0/a galaxy. Itisnotimmediatelyclearwhatcausesthehigherr /h
e
In anycase, ourresultsindicatealarge rangein bulge- ratio for our sample, compared to the results of Graham
to-disk (BD) luminosity ratio and show that the common (2001) and MacArthuret al. (2003), but it could well be
belief thatearly-typediskgalaxies havelarge andluminous related to the different decomposition techniques used. We
bulges does not hold in all cases. This is also visible in the already argued in section 6.1 that 1D decompositions suf-