Kuchinski etal AJ 122 729

21 Pages·2001·0.55 MB·English

THEASTRONOMICALJOURNAL,122:729¨749,2001August
(2001.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A.
QUANTITATIVE MORPHOLOGY OF GALAXIES OBSERVED IN THE ULTRAVIOLET
L. E. KUCHINSKI,1 BARRY F. MADORE,2,3 W. L. FREEDMAN,2 AND M. TREWHELLA1
Received1999December23;accepted2001May8
ABSTRACT
We present a quantitative study of the far-ultraviolet (FUV) and optical morphology in 32 nearby
galaxies and estimate the (cid:147)(cid:147)morphological k-correctionˇˇ expected if these objects were observed
unevolved at high redshift. Using the common indices of central concentration (C) and rotational asym-
metry (A) to quantify morphology, we consider independently two phenomena that give rise to this k-
correction. Bandshifting, the decrease in the rest-frame wavelength of light observed through optical
—lters, is explored by measuring these indices in several passbands for each galaxy, and it is found to be
the primary driver of changes in C and A. In general, the optical trend found for decreasing C and
increasing A when going to shorter wavelengths extends to the FUV. However, the patchy nature of
recent star formation in late-type galaxies, which is accentuated in the FUV, results in poor quantitative
correspondence between morphologies determined in the optical and FUV. We then arti—cially redshift
our FUV images into the Hubble Deep Field (HDF) —lters to simulate various cosmological distance
e(cid:134)ects, such as surface brightness dimming and loss of spatial resolution. Hubble types of many galaxies
in our sample are not readily identi—able at redshifts beyond zD1, and the galaxies themselves are diffi-
cult to detect beyond zD3. Because only features of the highest surface brightness remain visible at
cosmological distances, the change in C and A between simulated high-z galaxies and their unredshifted
counterparts depends on whether their irregular features are primarily bright or faint. Our simulations
suggest that k-corrections alone are indeed capable of producing the peculiar morphologies observed at
high redshift; for example, several spiral galaxies have C and A indices typical of irregular or peculiar
HDF objects viewed at z”2. We brie(cid:209)y discuss some elements of a scheme to classify rest-frame UV
images, mergers, protogalaxies, and other objects for which classical Hubble types do not adequately
encompass the existing morphology.
Key words: galaxies: fundamental parameters ¨ galaxies: individual (M51, M63, NGC 1097, NGC 1313,
NGC 1512) ¨ galaxies: photometry
1. INTRODUCTION observed locally. Theoretical scenarios of structure forma-
tionintheuniversepredictasigni—cantfractionofmergers
The morphology of galaxies in local and distant popu-
and interacting systems at high redshift compared with the
lations provides clues about the physical processes that
localpopulation(e.g.,Baugh,Cole,&Frenk1996).Bycare-
shaped these systems, either at the time of their formation
fully comparing samples of nearby and distant objects, it
orduringtheirevolutionovertheageoftheuniverse.Inthe
may be possible to identify the characteristics of protoga-
framework of the traditional Hubble scheme, morphology
laxies and young star-forming systems and understand the
has been correlated with a number of fundamental under-
roles of monolithic collapse and mergers in shaping the
lying physical properties (for a comprehensive review see
galaxiesofthecurrentepoch.
Roberts & Haynes 1994 and references therein). Elliptical
There are two major drawbacks to using the Hubble
galaxies are dynamically hot systems supported by their
schemeinstudiesofgalaxyevolutionfromdeepsurveys:its
velocitydispersions,whilespiral(disk)galaxiesaredynami-
subjective nature and its lack of descriptive and discrimi-
cally cold and rotationally supported (e.g., Kormendy
nativepowerfortheirregular,starbursting,andinteracting
1982).Bulge-to-diskratiosre(cid:209)ecttherelativeimportanceof
systemsthatmaybemoreprevalentathighredshift.Classi-
dynamicallyhotandcoldpopulations(e.g.,Kent1986).The
—cationintoHubbletypesisbasedonqualitativeanalysisof
presence of a bar may imply secular evolution that builds
observable features in each galaxy, and it can thus di(cid:134)er
spiral bulges or produces starbursts (Pfenniger & Norman
from one observer to another. Multiple features are con-
1990; Courteau, de Jong, & Broeils 1996). Galaxies with
sidered and weighted together, making it difficult to
earlytypesintheHubblesequencetendtobemoremassive,
automate classi—cation on the Hubble sequence and
more luminous, and have less gas and a lower present-day
occasionally yielding internally contradictory suggestions
star formation rate than late-type systems (Roberts &
ofthetype(e.g.,bulge-to-diskratiovs.windingofthespiral
Haynes). However, it is expected that the morphology of
arms). In a comparison of classi—cations of D800 galaxies
high-redshift galaxies could be quite di(cid:134)erent from that
bysixexpertmorphologists,Naimetal.(1995)—ndadisper-
sion in revised Hubble T index of pD1.8, where a change
of 1.0 corresponds to the di(cid:134)erence between, e.g., Sa and
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
1InfraredProcessingandAnalysisCenter/JetPropulsionLaboratory, Sab. For peculiar galaxies, the level of agreement between
CaliforniaInstituteofTechnology,MS100-22,770SouthWilsonAvenue, di(cid:134)erentexpertsˇclassi—cationsissubstantiallylower(Naim
Pasadena,CA91125. &Lahav1997),andtheHubbleschemedoesnotdividethe
2Observatories of the Carnegie Institute of Washington, 813 Santa
peculiarsystemsintoanyfurthercategoriesbeyonddivision
BarbaraStreet,Pasadena,CA91101.
of the faint irregulars with no bulge or arms and the pecu-
3NASA/IPAC Extragalactic Database, California Institute of Tech-
nology,MS100-22,770SouthWilsonAvenue,Pasadena,CA91125. liargalaxiesthatmayhavetidalfeatures,mergers,orother
729
730 KUCHINSKI ET AL. Vol. 122
obvious disruptions. This ambiguity at the late end of the galaxies provides an essential basis for interpreting images
Hubble sequence is particularly problematic for surveys ofhigh-redshiftsystems.However,thiswavelengthregimeis
thatprobeevolutionofthegalaxypopulationbecausethere inaccessible from the ground, and the availability of UV
isevidenceofanapparentincreaseinthefractionofirregu- galaxy images obtained from rocket and orbital missions
larorpeculiargalaxiesathighredshift(e.g.,Brinchmannet has been limited until quite recently (e.g., OˇConnell 1997
al.1998;Driveretal.1998;Abrahametal.1996a). and references therein). The existing data suggest that UV
In recent years, renewed attention has focused on the and optical morphology are not often well coupled (e.g.,
quantitative classi—cation of galaxies as an objective, OˇConnell1997;Marcumetal.1997,2001;Kuchinskietal.
automated measure of their properties and evolution. The 2000,hereafterK00),andtheinterestedreaderisreferredto
emerging methods are, by design, easily applicable to the anearlydemonstrationofthesefactsbyBohlinetal.(1991).
currentgenerationoflargeCCDimagingsurveys.Abraham Thus it is difficult to interpret the in(cid:209)uence of bandshifting
et al. (1994) developed a numerical index for the central on observed di(cid:134)erences between the optical characteristics
concentration (C) of galaxies, following the Yerkes classi- ofnearbygalaxiesandtherecordedrest-frameUVappear-
—cation system of Morgan (1958) and the correlations ance of distant ones. Compared with their optical classi-
between concentration and Hubble type presented by —cations, spiral galaxies generally appear to have later
Okamura,Kodaira,&Watanabe(1984)andDoi,Fukugita, HubbletypesintheUV(OˇConnell1997;K00).Spiralarms
& Okamura (1993). These authors later (Abraham et al. andstar-formingringsaremoreprominentintheUVthan
1996b) added a quantitative measure of galaxy asymmetry at optical wavelengths, while bulges and bars are much
(A) to form a two-part classi—cation system that dis- fainter,nearlyinvisible,intheUV(Walleretal.1997;K00).
tinguishes three bins: E/S0 galaxies, spiral galaxies, and The UV radial pro—les of disk galaxies are (cid:209)atter than
irregular or peculiar systems. If data from di(cid:134)erent wave- opticalpro—les,andthecentralconcentrationappearstobe
lengths are available, it is possible to divide the latest-type lowerintheUV(OˇConnell1997;Fanellietal.1997a;K00).
bin further, between the Hubble Irr types and the mergers FUV images of late-type galaxies often appear much more
byusingacorrelationbetweencolorandasymmetrythatis fragmented than the optical view (OˇConnell 1997), which
validonlyfornoninteractinggalaxies(Conselice,Bershady, raises the possibility of mistaking an ordinary irregular
&Jangren2000;Conselice1997).Animportantstepinthe galaxyforamergingprotogalaxy(seealsoK00).Thee(cid:134)ects
development of this scheme was the classi—cation of local of bandshifting on elliptical and S0 galaxies are less pro-
galaxies based on their optical images, which provides the nounced than on spiral galaxies: their FUV emission is
(cid:147)(cid:147)calibrationˇˇ of C and A to Hubble types (Abraham et al. smoothly distributed but is more centrally concentrated
1996b; Brinchmann et al. 1998; Conselice et al. 2000). This than optical light (OˇConnell 1999). Although imaging
A-C classi—cation system has been used to study the dis- distant galaxies in the infrared would lessen bandshifting
tribution of morphologies in the Hubble Space Telescope e(cid:134)ectsbysamplingtherest-frameopticalinsteadoftheUV
(HST) Medium Deep Survey (Abraham et al. 1996b), the out to zD4, the rate of NIR data acquisition is currently
Hubble Deep Field (HDF; Abraham et al. 1996a), the slowerthanintheoptical.DeepNIRimagingofsmallareas
ground-based Canada-France Redshift Survey and using the NICMOS camera on HST suggests that the
Auto—b/Low-Dispersion Survey Spectrograph Redshift increase in peculiarity may not necessarily be due to band-
Survey (Brinchmann et al. 1998), and HST Near Infrared shifting (Teplitz et al. 1998; Bunker 1999; Corbin et al.
Camera and Multi-Object Spectrometer (NICMOS) deep 2000), which could be con—rmed more rapidly with large
images (Teplitz et al. 1998). Division of the morphological optical samples and a better understanding of the relation-
data into redshift bins demonstrates the increase in appar- shipbetweenUVandopticalmorphology.Ourmainpoint
ently irregular or peculiar galaxies at high redshift is that the vast majority of publications to date on high-
(Brinchmannetal.1998). redshiftmorphologyissubjecttobandshiftinge(cid:134)ects,hence
Tostudygalaxyevolution,itisimportanttounderstand the present work is important in light of reinterpreting
thebehaviorofCandAasafunctionofwavelengthinthe thoseresults.
local population before applying these indices to a sample In the absence of data, several attempts have been made
of objects observed at a range of redshifts (i.e., di(cid:134)erent to simulate the appearance of high-redshift galaxies by
look-back times). At long rest wavelengths, where stellar (cid:147)(cid:147)extrapolatingˇˇ to the UV morphology from optical
lightisagoodtracerofmass,lowdegreesofcentralconcen- images or by applying the cosmological e(cid:134)ects of surface
tration and symmetry re(cid:209)ect a lack of dynamical organiz- brightnessdimmingandlossofspatialresolution.Arti—cial
ation that may characterize either interacting systems or UV images of galaxies have been produced using template
those in the process of formation. At shorter wavelengths, spectral energy distributions (SEDs) for di(cid:134)erent Hubble
asymmetryandpatchinessaremorelikelytohighlightcases types to estimate the UV light in each pixel based on its
inwhichdustand/orrecentlocalizedstarformationstrong- opticalcolors(Abrahametal.1996a;Abraham,Freedman,
ly in(cid:209)uence the observed morphology. Thus it is not sur- & Madore 1997; Brinchmann et al. 1998). The apparent
prising that B-band images already show lower C and changeinmorphologyduetobandshiftingintheA-Cclas-
higherAthantheirR-bandcounterparts(Brinchmannetal. si—cation system has been quanti—ed by Brinchmann et al.
1998; Conselice 1997). In light of the physically di(cid:134)erent (1998) using this approach. They —nd that 13% of spiral
regimes probed by optical and UV light, it is necessary to galaxies are mislabeled as irregular or peculiar at zD 0.7,
understand how this morphological trend extends to the withthefractionrisingto24%atzD 0.9.Thisfractionmay
UVtodeterminewhethersimplequantitativek-corrections be expected to increase at higher redshifts as rest wave-
arefeasiblefortheconcentrationandasymmetryindices. lengthsmovefurtherintotheUV.However,itisimportant
Because the rest-frame far-ultraviolet (FUV) light of gal- tonotethatthesesimulationsdonotutilizeactualUVdata.
axiesisredshiftedintooptical—ltersatzD3andintonear- The SEDs used to infer UV (cid:209)ux from B[R color may not
infrared(NIR)onesatzD10,theUVmorphologyoflocal accurately re(cid:209)ect local conditions, especially in very dusty
No. 2, 2001 GALAXIES OBSERVED IN THE ULTRAVIOLET 731
regionsorinlocalizedstarbursts(Donas,Milliard,&Laget concentrationandasymmetry.Theprocedureforarti—cially
1995). Conselice et al. (2000) degrade the resolution and redshifting images to take into account dimming and
signal-to-noise ratio (S/N) of optical images of nearby gal- reduced spatial resolution is presented in (cid:176) 4, along with a
axiestosimulateothere(cid:134)ectsofdistanceandredshift.They qualitative discussion of these e(cid:134)ects on apparent mor-
—nd their technique for measuring asymmetry is robust to phology. Section 5 contains the results of the quantitative
S/Nvariations(forS/N[100)butshowsanapparentdrop morphologyanalysisandadiscussionofthetotalmorpho-
in the asymmetry index as spatial resolution decreases. logicalk-correctionforCandAbetweenlocalopticaldata
WorkingwithUVdatadirectlytoavoiduncertaintiesinthe and high-redshift galaxy images. In (cid:176) 6, we provide a brief
morphologicalk-correctionduetobandshifting,Giavalisco summary, compare our arti—cially redshifted galaxies with
etal.(1996a)andHibbard&Vacca(1997)rebinnedimages deepHST images,anddiscussimplicationsforthestudyof
and scaled the surface brightness to simulate redshifting. high-zgalaxies.Wealsoconsidertheshortcomingsofboth
Their qualitative analysis of the resulting morphology sug- theHubbleschemeandtheCandAindicesforUVgalaxy
geststhatevenintheabsenceofbandshiftingcosmological data,andweproposesomenewparametersthatmaymore
e(cid:134)ects cause the (cid:147)(cid:147)redshiftedˇˇ galaxies to have a later type adequatelydescribetheobservedmorphology.
and more irregular appearance (Giavalisco et al. 1996a, As emphasized by the referee there are certainly other
1996b;Hibbard&Vacca1997).However,withoutobjective waysofcreatingasymmetryandconcentrationindicesthat
criteria to describe morphology, it is difficult to determine donotdependonisophotesandmayindeedbemorerobust
whetherthemagnitudeofthise(cid:134)ectissufficienttoaccount (to centering errors in particular). In this paper we are
for the observed increase in irregular galaxies at high red- testing the methodology of Abraham (1999) since this is
shift. (We note in passing that there is a contradiction representativeoftheextantliteratureonthemorphologyof
regarding the trend of (cid:147)(cid:147)typeˇˇ based on symmetry with high-redshift (HDF) galaxies. Our data are publicly avail-
decreasing S/N when comparing the results of Conselice et ableforalternativetests,suchastheiterativeminimization
al. with those of Giavalisco et al. and Hibbard & Vacca. scheme as described in Conselice, et al. (2000) and in Ber-
Reconciling these di(cid:134)erences is beyond the scope of this shady,Jangren,&Conselice(2000).
paperbuttherefereesuggestedthatwealertreaderstothis
fact). Gardner et al. (1997) have measured the central con- 2. DATA
centration and asymmetry of —ve nearby galaxies on UV
2.1. SampleSelection
imagesresampledtosimulateHST imagesatredshiftsnear
Themorphologicalanalysisinthispaperwascarriedout
zD2,butthee(cid:134)ectsofsurfacebrightnessdimmingwerenot
ongalaxiesdrawnfromthesampleofUV¨opticaldatapre-
considered.Theyalso—ndthatspiralgalaxiestendtomove
sentedinK00,withtheadditionofafewgalaxiesforwhich
toalatertypebinintheA-Cclassi—cationsystem.Aquan-
there are UIT images but no optical data in our database.
titativestudyofthechangeinapparentmorphologyoflocal
The sample selection for the UIT Astro-2 mission, our
galaxies when viewed at high redshift is still necessary to
source of UV galaxy images, is discussed in detail in K00.
determine how the distant galaxy population di(cid:134)ers from
AlthoughtheUITsamplewasdesignedtocoverarangeof
thatofthecurrentepoch.
morphologiesfromellipticaltoirregularorpeculiar,itdoes
To compare the morphologies of galaxies at UV and
not statistically represent the local distribution of Hubble
opticalwavelengths,weobservedasetof32nearbygalaxies
types.Becausewewishtoinvestigateinparticularthemor-
in the FUV using the Ultraviolet Imaging Telescope (UIT)
phologyoftheirregularfaintgalaxiesseenathighredshift,
in low-earth orbit aboard the space shuttle Endeavour. In
oursampleisweightedtowardthespiralandirregularUIT
K00,wepresentedFUVandground-basedopticaldatafor
galaxies.Di(cid:134)erentsubsetsofthesampleareusedforvarious
these galaxies and discussed the morphology qualitatively
parts of our study: only galaxies that —t entirely on the
based on images and surface brightness pro—les. We —nd
optical data frames and do not su(cid:134)er central saturation in
that localized star formation features dominate the FUV
theoptical—ltersareusedtoprobethebehaviorofapparent
light and FUV¨optical color pro—les, in contrast to the
morphology with wavelength, and only galaxies that are
smoothunderlyingdistributionsseeninopticalimagesand
still detectable in the FUV after arti—cial redshifting are
pro—les. The strong deviations from a smooth disk or
used for the study of redshift-dependent e(cid:134)ects. We also
disk]bulge pro—le that are evident in the FUV highlight
arti—cially redshifted the U-band images of some galaxies
the difficulty of determining traditional structural parame-
into longer wavelength optical —lters to study the e(cid:134)ects of
tersfromrest-frameUVimagesandsuggestaneedtomove
moderate redshift; again only galaxies that —t entirely on
beyond the Hubble sequence to describe UV galaxy mor-
the optical frame were used. Table 1 contains some basic
phology.
data for each galaxy in the sample and denotes for which
In this paper, we present a quantitative analysis of the
parts of our investigation each was used. More detailed
morphologyoftheK00galaxysample.Weusecentralcon-
information about most of these galaxies and our obser-
centration and asymmetry parameters to quantify band-
vationaldataisgiveninK00.
shifting and cosmological e(cid:134)ects that are inherent in the
studyofhigh-redshiftgalaxies.Thisworkextendsthequali-
tative investigations of cosmological e(cid:134)ects on UV images 2.2. ObservationsandDataReduction
carried out by Giavalisco et al. (1996a) and Hibbard & FUV images were obtained using the UIT and under-
Vacca (1997). Unlike the Brinchmann et al. (1998) study of went the standard pipeline processing (Stecher et al. 1997).
redshift e(cid:134)ects on morphology, we do not rely on the U,B,V,R,andIimageswereobtainedoverthepastseveral
(cid:147)(cid:147)extrapolationˇˇ to UV morphology from optical images. years at Palomar Observatory, Las Campanas Observa-
Section2reviewsthesampleselectionanddataacquisition tory, and Cerro Tololo Inter-American Observatory. The
and reduction, most of which is discussed in detail in K00. CCD images were bias-subtracted, (cid:209)at-—elded, and com-
In (cid:176) 3, we present our method for measuring the central bined(wherenecessary)usingstandardproceduresinIRAF
732 KUCHINSKI ET AL. Vol. 122
TABLE 1
BASICDATAFORSAMPLEGALAXIESWITHFUVIMAGES
Redshifted Redshifted
Name HubbleTypea OpticalDatab FUVb Ub
NGC925................ SAB(s)d UBVR Y Y
NGC1068(M77)....... RSA(rs)b UB Y Y
NGC1097............... SB(s)b UBVI Y Y
NGC1313............... SB(s)d UBVI Y Y
NGC1365............... SB(s)b ... Y N
NGC1510............... S0pec UBVI N N
NGC1512............... SB(r)a UBVI Y Y
NGC1566............... SAB(s)bc UBVI Y Y
NGC1672............... SB(s)b UBVI Y Y
NGC2403............... SAB(s)cd UBVRI Y Y
NGC2841............... SA(r)b UVR N Y
NGC2903............... SAB(rs)bc UBVR Y Y
NGC3226............... E2pec BR Y N
NGC3227............... SAB(s)apec BR Y N
NGC3310............... SAB(r)bcpec VR Y N
NGC3351(M95)....... SB(r)b BVRI N N
NGC3389............... SA(s)c R Y N
NGC4038............... SB(s)mpec UVI Y Y
NGC4039............... SA(s)mpec UVI Y Y
NGC4214............... IAB(s)m BVRI Y N
NGC4258(M106)...... SAB(s)bc ... Y N
NGC4449............... IBm BVRI Y N
NGC4470............... Sapec ... Y N
NGC4486(M87)...... E0pec(cD) ... Y N
NGC4631............... SB(s)d ... Y N
NGC4647............... SAB(rs)c R N N
NGC4736(M94)....... (R)SA(r)ab UV Y Y
NGC5055(M63)....... SA(rs)bc UBVR N Y
NGC5194(M51)....... SA(s)bcpec UBVRI Y Y
NGC5236(M83)....... SAB(s)c UBRI Y Y
NGC5253............... Pec BVI N N
NGC5457(M101)...... SAB(rs)cd ... Y N
aDatafromtheRC3(deVaucouleursetal.1991).
bAvailabilityofcentralconcentrationandasymmetrydata,denotedby—lterfortheFUV¨
opticalcolumnandbyY(yes)orN(no)fortheredshiftsimulations.
(Tody1986),VISTA(Stover1988),andCCDPACKinStar- tures or unusual features that do not correspond to any
link.Artifactssuchasthe(cid:147)(cid:147)UITstripeˇˇandcosmicrayson otherHubbletypes.)Thecombinationofthesetwoparam-
the optical images were removed, and background sky eters, plotted as log A versus log C, has been shown by
levelsweredeterminedforeachimage.Wetransformedthe Abrahametal.(1996b)todividenearby(opticallyclassi—ed)
coordinatesoftheopticalimagestothesystemoftheFUV galaxiesintothreebroadmorphologicalbinscorresponding
imageforeachgalaxy,yieldinga—nalscaleof1A.14pixel~1. to the Hubble sequence: elliptical/S0, spiral, and irregular-
The optical images were then smoothed to the UIT peculiar-merger. This provides a basis for understanding
resolution of D3A. Finally, we masked out all foreground distant galaxies in terms of the well-studied (at optical
starsintheopticaldataandinterpolatedoverthoseregions wavelengths)localpopulation.Asnotedinseveralinstances
of the image. Data reduction procedures are described in below,theintrinsicpatchinessandlowsignal-to-noiseratio
moredetailinK00. of the FUV images have posed special difficulties for the
techniques typically used to measure C and A. We have
3. CONCENTRATION AND ASYMMETRY INDICES
thereforemodi—edthede—nitionsproposedbyAbrahamet
We have chosen to use the central concentration and al.(1994,1996a,1996b),whileretainingthegeneralideaofa
asymmetry indices as quantitative morphology parameters correlationbetweenhighconcentrationplussymmetryand
becausetheyareconceptuallysimpleandhavebeenusedin an ordered appearance. The e(cid:134)ects of these modi—cations
asigni—cantbodyofrecentwork,especiallyinthestudyof willbeexploredlaterinthissection.
distant galaxies (see references in the Introduction). In Itisimportanttode—necarefullyagalaxyˇspositionand
general,galaxieswithhighdegreesofcentralconcentration apertureinwhichtheconcentrationandasymmetryindices
and symmetry have an ordered, regular appearance and will be measured because these indices are known to be
those with low central concentration and large asymmetry sensitive to the centering and aperture size (Teplitz et al.
haveanirregularorpeculiarmorphology.(Hereweexplic- 1998;Conseliceetal.2000).Manyauthorsde—netheaper-
itly distinguish between the Hubble irregular type that is ture based on a threshold set at some small multiple of the
simplyanextensionofthelate-typespiralsequenceandthe measured sky noise and then use intensity-weighted image
generic peculiar type that has merger or interaction signa- moments to de—ne an ellipse (e.g., Abraham et al. 1996b;
No. 2, 2001 GALAXIES OBSERVED IN THE ULTRAVIOLET 733
Teplitzetal.1998).Alternatively,wenotethatthereisalso sidered below in our determination of uncertainties in C
the method of Bershady et al. (2000), which is not isophote and A. As a control, we use the same aperture for each
threshold dependent. Aperture centers are typically —lterˇs image of a galaxy to ensure that we compare the
obtainedbycentroidingonthebrightestpixel(Teplitzetal. samephysicalregionofthatgalaxyatallwavelengths.For
1998). However, the patchy appearance and prominence of the small number of galaxies for which we have only FUV
star-forming regions in the FUV images pose some signi—- data, we estimate the center and determine the ellipse pa-
cantdifficultiesforusingthesetechniquesinanunmodi—ed rametersfromisophotal—tstotheFUVimage.Inallthese
manner. Sigma-clipping often produces multiple small cases,theellipticityagreeswellwiththeRC3value(deVau-
regionswithinwhatvisuallyappearstobethegalaxyaper- couleurs et al. 1991), and these particular galaxies are
ture.Spiralarmsorlargeareasofstarformationcaninfact regular enough that the center is fairly unambiguous. The
be brighter than the geometric center of the galaxy (as ellipseparametersforeachgalaxyaregiveninTable2.Asin
de—nedbytheopticalcenterorbytheapparentcenterofan K00,themergerNGC4038/9isconsideredasinglesystem
ellipse enclosing the galaxy). Here we have used the avail- with a center determined from a segmented image, and the
ableopticaldatatoouradvantageinde—ningtheapertures. apertureforNGC3227alsoincludeslightfromitscompan-
(It should be noted that this would not usually be possible ionNGC3226.
in analyzing high-redshift galaxies). Isophotal —ts to outer The concentration index C is the fraction of total galaxy
parts of the longest-wavelength galaxy image (usually R or lightthatisemittedfromthecentralregioncomparedwith
I; images of the same galaxy in both —lters yield nearly the whole. In practice, di(cid:134)erent authors have measured C
identicalresults)determinetheellipticityandpositionangle using a variety of de—nitions (Doi et al. 1993; Abraham et
oftheaperture.Thecentroidofthislong-wavelengthimage al. 1994; Naim, Ratnatunga, & Griffiths 1997). We select a
isselectedasthe—xedaperturecenter.Theseellipseparam- verysimpleexpression:
eters are identical to those used in K00 for azimuthal
averaging to obtain surface brightness pro—les. The & I(i, j)
C\ (i,j>R:0.3Rmax) , (1)
maximum aperture radius is the largest radius at which & I(i, j)
there is still detectable light above the noise in the FUV (i,j>R:Rmax)
image, which was determined by visual inspection of the whereR denotes the radius of the elliptical galaxy aper-
max
light pro—les of K00. This procedure for aperture radius ture selected as described above. The calculation of total
selection yields a slightly di(cid:134)erent limiting surface bright- (cid:209)ux in the inner and outer elliptical apertures is performed
nessforeachgalaxy(dependingonS/N),whichwillbecon- on background-subtracted galaxy images. The choice of
TABLE 2
CONCENTRATIONANDASYMMETRYPARAMETERSFORFUVANDOPTICALIMAGES
Name va P.A.b C C C C C C A A A A A A
FUV U B V R I FUV U B V R I
NGC925 ......... 0.40 115 0.20 0.34 0.37 0.37 0.35 ... 0.72 0.22 0.17 0.13 0.15 ...
NGC1068........ 0.20 84 0.54 0.49 0.50 ... ... ... 0.28 0.18 0.13 ... ... ...
NGC1097........ 0.32 140 0.17 0.44 0.47 0.50 ... 0.53 0.64 0.27 0.21 0.19 ... 0.12
NGC1313........ 0.20 40 0.28 0.39 0.41 0.41 ... 0.37 0.64 0.34 0.22 0.21 ... 0.17
NGC1365........ 0.45 32 0.13 ... ... ... ... ... 0.72 ... ... ... ... ...
NGC1510........ 0.12 145 0.51 0.73 0.68 0.64 ... 0.50 0.25 0.21 0.16 0.15 ... 0.10
NGC1512........ 0.36 46 0.15 0.34 0.37 0.40 ... 0.41 0.69 0.12 0.08 0.06 ... 0.04
NGC1566........ 0.21 40 0.30 0.54 0.57 0.60 ... 0.60 0.47 0.25 0.18 0.15 ... 0.12
NGC1672........ 0.13 161 0.19 0.35 0.36 0.39 ... 0.42 0.61 0.27 0.21 0.17 ... 0.11
NGC2403........ 0.44 130 0.28 0.41 0.41 0.42 0.45 0.39 0.74 0.26 0.17 0.13 0.14 0.15
NGC2841........ 0.56 147 0.14 0.32 ... 0.39 0.39 ... 0.78 0.12 ... 0.10 0.08 ...
NGC2903........ 0.53 24 0.23 0.32 0.33 0.34 0.35 0.34 0.62 0.16 0.13 0.11 0.10 0.06
NGC3226/7...... 0.55 157 0.19 ... 0.30 ... 0.34 ... 0.60 ... 0.34 ... 0.33 ...
NGC3310........ 0.22 170 0.76 ... ... 0.75 0.73 ... 0.17 ... ... 0.16 0.12 ...
NGC3351........ 0.32 17 0.21 ... 0.35 0.39 0.40 0.43 0.42 ... 0.06 0.05 0.05 0.06
NGC3389........ 0.55 108 0.21 ... ... ... 0.31 ... 0.39 ... ... ... 0.19 ...
NGC4038/9...... 0.00 ... 0.14 0.18 ... 0.18 ... 0.20 0.89 0.63 ... 0.48 ... 0.42
NGC4214........ 0.18 132 0.54 ... 0.53 0.54 0.51 0.50 0.35 ... 0.14 0.14 0.12 0.13
NGC4258........ 0.65 150 0.72 ... ... ... ... ... 0.58 ... ... ... ... ...
NGC4449........ 0.36 60 0.51 ... 0.55 0.54 0.51 0.51 0.38 ... 0.19 0.17 0.15 0.13
NGC4470........ 0.30 0 0.33 ... ... ... ... ... 0.20 ... ... ... ... ...
NGC4486........ 0.10 0 0.39 ... ... ... ... ... 0.30 ... ... ... ... ...
NGC4631........ 0.80 86 0.37 ... ... ... ... ... 0.46 ... ... ... ... ...
NGC4647........ 0.20 135 0.22 ... ... ... 0.23 ... 0.55 ... ... ... 0.11 ...
NGC4736........ 0.22 95 0.54 0.58 ... 0.60 ... ... 0.20 0.10 ... 0.06 ... ...
NGC5055........ 0.47 102 0.25 0.38 0.41 0.42 0.43 ... 0.59 0.19 0.19 0.17 0.15 ...
NGC5194........ 0.30 30 0.22 0.30 0.28 0.30 0.30 0.31 0.54 0.30 0.24 0.25 0.24 0.25
NGC5236........ 0.10 80 0.23 0.25 0.27 ... 0.31 0.36 0.55 0.18 0.21 ... 0.13 0.11
NGC5253........ 0.57 43 0.66 ... 0.50 0.48 ... 0.37 0.22 ... 0.10 0.12 ... 0.08
NGC5457........ 0.00 ... 0.41 ... ... ... ... ... 0.69 ... ... ... ... ...
aEllipticityofthegalaxyaperture,measuredfromlongest-wavelengthopticalimage.
bPositionangleoftheadoptedellipticalapertureindegrees;0¡isnorth.Angleincreasestotheeast.
734 KUCHINSKI ET AL.
whatfractionofR constitutesthecentralregionissome- Freietal.(1996).(Asixthgalaxy,M63,wassaturatedinthe
max
what arbitrary; for convenience of comparison, we select Freietal.imagesandwasnotusedforthecomparison.)We
thevalueof0.3adoptedbyAbrahametal.(1994)andused calculateCandAfrombothsetsofimages(oursandtheirs)
in most subsequent analyses of galaxy concentration by and —nd p \0.016 and p \0.011. The limiting surface
C A
these and other workers. Noise in the images should not brightness on the FUV images, which was used to deter-
present a serious problem for our concentration index mine the aperture radius for measuring C and A, ranges
because it should be evenly divided between (cid:147)(cid:147)positiveˇˇ fromD24to25magarcsec~2.Forninegalaxieswithlimit-
(above the sky) and (cid:147)(cid:147)negativeˇˇ (below the sky) and thus ingsurfacebrightness”24.5magarcsec~2,wemeasuredC
should cancel out within each aperture. Concentration and A in an aperture extending only to 24 mag arcsec~2
indices for each galaxy at every available wavelength are and compared the values with those measured in the
giveninTable2. maximumaperturesize.Inthiscase,we—ndp \0.025and
C
TheasymmetryindexAisameasureofthe180¡rotation- p \0.012. The direction of the changes in C and A as
A
al symmetry of the galaxy. The basic mathematical de—ni- aperture radius increases depends on the detailed structure
tionisasfollows: ofthegalaxy,butitistypicalto—ndalargerconcentration
andalargerasymmetryinthelargeraperture.Addingthese
& oI(i, j)[I (i, j)o
A\ (i,j>R:Rmax) rot , (2) (independent) errors in quadrature yields our —nal uncer-
& I(i, j) taintyestimates:0.03forCand0.02forA.
(i,j>R:Rmax)
WehavealsousedtheFreisampletoexplorethee(cid:134)ectof
where the original image is rotated 180¡ to get I (i,j). By
rot using di(cid:134)erent mathematical de—nitions of the C and A
takingtheabsolutevalueofthedi(cid:134)erencebetweentheorig-
indices.ThissamplewasusedbyAbrahametal.(1996b)to
inal and rotated images, one introduces a systematic error
calibrate the A-C classi—cation system, and their C and A
because the sky noise always contributes positive values.
indicesforindividualgalaxiesaretabulated.We—rsttested
For galaxies with regular shapes, in which all pixels in the
our ability to recover the Abraham et al. values by using
aperturecontaingalaxylight,thiserrorisasmallfractionof
their de—nitions of C and A to measure indices on the Frei
A. On FUV images, which have low signal-to-noise ratios
sample.Theresultsareencouraging:ameandi(cid:134)erenceinC
andareintrinsicallypatchyorfragmented,thenoisemaybe
of0.001^0.015(1p)andinAof0.006^0.047,bothinthe
signi—cant. Abraham et al. (1996b) correct for the noise by
senseof(ourmeasurement[Abrahametal.).Wethenused
subtracting from A the measured asymmetry of a patch of
our de—nitions of C and A on the Frei sample to quantify
skywithasizeidenticaltotheaperture;butwetypicallydo
the systematic di(cid:134)erence between the two methods. The
not have enough sky on our galaxy frames to apply this
measurement of C di(cid:134)ers only in the aperture de—nition,
corrective technique. Instead, we consider the sky noise in
andwe—ndano(cid:134)setof*C(ours[Abraham)\0.03^0.02.
pixels that do contain galaxy light, and we also explicitly
Inspiteoftheverydi(cid:134)erentmethodsusedtomeasureA,the
takeintoaccountthefactthatmanypixelswithintheaper-
o(cid:134)set is still small: *A(ours[Abraham)\0.04^0.03.
turedonotcontainanygalaxylight.Weapproachthelatter
Theseo(cid:134)setsaresmallcomparedwiththeuncertaintiesthat
problem by summing over only those pixels with values
we have estimated above and small with respect to the
above a threshold of 1.5 p above the background, rather
sky errors of D0.07 quoted by Abraham et al.4 Our results
than all pixels in the aperture, when using equation (2) for
suggestthatitwillbepossibletoutilizetheAbrahametal.
A.Foropticalimages,mostorallthepixelsareusedinthe
A-Cclassi—cationschemeusingourvaluesofCandA,with
sum,whileintheFUV,thefractionmaybeaslowas10%.
the morphological bins adjusted by the calculated o(cid:134)sets
Tocorrectforskynoiseinthepixelsthatdocontaingalaxy
whereappropriate.Forananalysisofthee(cid:134)ectsofdi(cid:134)erent
light (and thus are used in the sum), we measure the asym-
aperturesizesseeConseliceetal.(2000),whohaveexplored
metryinapatchofskyaslargeaswecan—ndontheimage,
thissourceofuncertaintyinsomedetail.AlsoseeBershady,
thenrenormalizeittothenumberofpixelsactuallyusedin
Lowenthal, & Koo (1998) for a description of measuring
the sum. This correction factor is then subtracted from the
sizesforfaintsources.
asymmetrydeterminedforpixelsabovethethreshold.Thus
the noise-corrected expression for the asymmetry is given 4. ARTIFICIAL REDSHIFTING
by
To study the in(cid:209)uence on morphology of cosmological
& oI(i, j)[I (i, j)o e(cid:134)ectsassociatedwithlargelook-backtimes,wehavearti—-
A (i,j>p(i,j);np) rot [k , (3) cially redshifted galaxy images to values of z at which the
corr & I(i, j) A
*i,j>p(i,j);np+ rest-frame FUV —lter bandpass would coincide with the
where p(i,j) is the pixel value at position (i,j), np is the fourHST WFPC2—ltersusedtoimagetheHDF(Williams
threshold,k \N A ,andN isthenumberofpixels etal.1996).WesimulatetheHDFbecauseitcomprisesthe
A pix sky,pix pix
intheaperturewithvaluesabovethethreshold,andA deepestobservationofdistantgalaxiestodateandhasbeen
sky,pix
is the asymmetry per pixel due to sky noise, calculated by thesubjectofnumerousstudiesofmorphologyathighred-
dividingAofequation(2)forapatchofskybythearea(in shift.TheFUVrestwavelength(D1500(cid:211))isredshiftedinto
pixels) of the patch. Values of the correction termk range the broadband F300W —lter at zD1, the F450W —lter at
A
from „0.03 for optical data to 0.1¨0.2 for FUV images (A zD2, the F606W —lter at zD3, and the F814W —lter at
itself ranges from 0 to 1). The galaxy asymmetry values
calculatedwiththismethodaregiveninTable2. ¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
We estimate the uncertainty in C and A by considering 4Wenotethatthescatterinourcalculatedo(cid:134)setsissomewhatsmall
two factors: measurement error and errors that arise from comparedwiththeuncertainties:Thisislikelybecausewehaveusedthe
same images as Abraham et al. did. The quoted uncertainties include a
using apertures extending to di(cid:134)erent limiting surface
termthattakesintoaccountdi(cid:134)erencesinlimitingsurfacebrightnessfor
brightnesses.Toquantifytheerrorinmeasurement,weuse
di(cid:134)erentgalaxies,whichwillnotbeafactorincomparisonsusingthesame
—vegalaxiescommontooursampleandthedigitalatlasof imageofthesamegalaxy.
1" F300W F450W
M 101
F606W F814W
120"
0.5"
NGC 4214
F300W F450W
60" F606W F814W
FIG. 1.¨FUVandarti—ciallyredshiftedimagesofM101andNGC4214,orientedwithnorthupandeasttotheleft.TheFUVM101imageisa1311s
UITexposure,andtheredshiftedM101imagesarescaledastheywouldappearintheHDF.TheFUVNGC4214imageisa1061sUITexposure,andthe
redshiftedNGC4214imagesagainsimulatetheHDF.ThescalebarshownintheF300Wpanelforeachgalaxyisapplicabletoallredshiftedimagesofthat
galaxy.
60" 1" 1"
M 51 U z~0.6 F606W z~1.2 F814W
FIG. 2.¨U-bandandarti—ciallyredshiftedimagesofM51,orientedwithnorthupandeasttotheleft,showingthesimulatedappearanceofM51inthe
HDFattheredshiftsnotedinthelabels(centerandright).TheU-bandimagehashadforegroundstarsremoved.
736 KUCHINSKI ET AL. Vol. 122
TABLE 3
CONCENTRATIONANDASYMMETRYPARAMETERSFORARTIFICIALLYREDSHIFTEDIMAGESa
Name C(FUV,z1)b A C(FUV,z2) A C(FUV,z3) A C(FUV,z4) A C(U,z0.6) A C(U,z1.2) A
NGC925 ......... 0.23 0.56 0.22 0.56 0.22 0.52 ... ... 0.47 0.24 0.40 0.36
NGC1068........ 0.53 0.14 0.51 0.21 0.52 0.20 0.45 0.34 0.38 0.52 0.40 0.54
NGC1097........ 0.18 0.55 0.18 0.58 0.18 0.56 0.12 0.66 0.44 0.24 0.42 0.36
NGC1313........ 0.35 0.36 0.33 0.36 0.32 0.40 0.17 0.65 0.44 0.36 0.40 0.43
NGC1365........ 0.14 0.58 0.14 0.60 0.14 0.57 0.10 0.68 ... ... ... ...
NGC1512........ 0.18 0.49 0.19 0.49 0.17 0.53 ... ... 0.40 0.12 0.35 0.25
NGC1566........ 0.31 0.42 0.28 0.47 0.29 0.43 0.15 0.62 0.55 0.25 0.46 0.28
NGC1672........ 0.22 0.42 0.22 0.46 0.23 0.45 0.15 0.60 0.43 0.28 0.36 0.30
NGC2403........ 0.24 0.58 0.20 0.59 0.20 0.54 ... ... 0.44 0.16 0.42 0.20
NGC2841........ ... ... ... ... ... ... ... ... 0.33 0.10 0.30 0.09
NGC2903........ 0.21 0.50 0.22 0.56 0.21 0.62 ... ... 0.31 0.07 0.32 0.06
NGC3226/7...... 0.17 0.66 0.15 0.67 0.16 0.66 ... ... ... ... ... ...
NGC3310........ 0.72 0.26 0.71 0.15 0.72 0.23 0.47 0.18 ... ... ... ...
NGC3389........ 0.22 0.38 0.21 0.40 0.21 0.38 ... ... ... ... ... ...
NGC4038/9...... 0.14 0.82 0.13 0.81 0.14 0.80 0.11 0.72 0.23 0.62 0.19 0.62
NGC4214........ 0.54 0.18 0.50 0.27 0.52 0.32 0.39 0.48 ... ... ... ...
NGC4258........ 0.28 0.62 0.25 0.55 0.26 0.561 ... ... ... ... ... ...
NGC4449........ 0.56 0.15 0.54 0.19 0.51 0.42 0.40 0.31 ... ... ... ...
NGC4470........ 0.32 0.25 0.33 0.27 0.29 0.27 ... ... ... ... ... ...
NGC4486........ 0.28 0.42 0.29 0.40 0.33 0.36 ... ... ... ... ... ...
NGC4631........ 0.33 0.46 0.32 0.42 0.31 0.41 0.21 0.47 ... ... ... ...
NGC4736........ 0.40 0.19 0.37 0.13 0.37 0.12 0.22 0.38 0.59 0.06 0.54 0.16
NGC5055........ ... ... ... ... ... ... ... ... 0.36 0.16 0.39 0.16
NGC5194........ 0.15 0.53 0.15 0.54 0.14 0.54 ... ... 0.32 0.25 0.31 0.26
NGC5236........ 0.18 0.47 0.19 0.52 0.19 0.48 0.16 0.59 0.29 0.19 0.28 0.17
NGC5457........ 0.16 0.72 0.14 0.72 0.14 0.68 ... ... ... ... ... ...
aC,Aforaperturesadjustedtomatchthelimitingsurfacebrightnessforeachimage.
bC,AforFUVdataarti—ciallyredshiftedtozD1.OthercolumnheadshavesimilarformatforFUVorU-banddataarti—ciallyredshiftedtovariouszto
matchtheHDF—lters.
zD4. (The precise value of z depends on which FUV —lter values of both 0.05 and 0.5 and —nd that galaxies are more
was used for the UIT imaging and may vary from the easilydetectedwiththehigherq .Interestedreadersshould
0
approximateredshiftgivenbyupto^0.2;detailsofthetwo consultthispaperfordetails.Filteranddetectorcharacter-
FUV—ltersaregiveninK00.)Becausethereisatpresenta istics that are input to these formulae are taken from
paucity of U-band images of nearby galaxies, we also Stecher et al. (1997) for the UIT and Williams et al. (1996),
explore the e(cid:134)ects of moderate redshifts that move the TheWFPC2InstrumentHandbook(Biretta1996),andThe
U-band rest wavelength into the F606W —lter at zD0.6 HST Data Handbook (Voit 1997) for the HDF. Galaxy
and into the F814W —lter at zD1.2. We emphasize that distances are from Tully (1988) and are given in Table 1 of
there is no need to estimate the e(cid:134)ects of bandshifting in K00. Instead of convolving the resulting images with the
these simulations; we simply selected a redshift such that HST point-spread function (PSF) as Giavalisco et al. have
therestwavelengthisplaceddirectlyintothedesired—lter. done,wechoosethemoresimplemethodofsmoothingwith
Forexample,ahigh-redshiftgalaxyatzD3observedwith a circular Gaussian to match the PSF width of D3 pixels
WFPC2 in the F606W —lter is really being observed in its reported by Williams et al. This technique avoids the diffi-
rest-frame FUV, and a moderate redshift galaxy at zD0.6 culty of simulating in detail the complex PSF that results
inF606Wisreallybeingobservedinitsrest-frameUband. from HST optics and the (cid:147)(cid:147)drizzleˇˇ procedure used to
By comparing the arti—cially redshifted images with their combine HDF images. Our redshifted images were then
unredshifted counterparts, we can isolate the e(cid:134)ects of scaledtotheappropriateHDFexposuretimeforeach—lter,
surfacebrightnessdimmingandlossofspatialresolutionon and a sky background and sky noise were added based on
apparent morphology. Although this method restricts our the HDF values reported in Williams et al. We accounted
investigation to speci—c redshifts, we span the range of for foreground Galactic extinction (very small in most
zD0.6¨4, over which recent work has suggested intrinsic cases) by correcting the input image to zero extinction
evolutioninthegalaxymorphologies. before redshifting, then adding the appropriate extinction
We follow the procedures outlined by Giavalisco et al. for each —lter after the arti—cial redshifting was completed.
(1996a)toredshiftthegalaxyimagesarti—cially,usingtheir The B-band foreground extinctions for each galaxy are
equations (2), (5), and (7) to determine the resampling from Burstein & Heiles (1984), and the Galactic extinction
factor and surface brightness scaling. A cosmology with lawofCardelli,Clayton,&Mathis(1989)wasusedtocalcu-
)\1 and q \0.5 is assumed throughout, and we take lateextinctionsatotherwavelengths.
0
H \75 km s~1 Mpc~1 (e.g., Freedman et al. 2001). As galaxies are arti—cially redshifted to high z, their
0
Because this value of H was also used to estimate the appearance can change dramatically simply because of the
0
distancestosamplegalaxies,boththerebinningandbright- fading of low surface brightness features and simultaneous
nessscalingfactorsareindependentofH .Thedependence loss of spatial resolution. In some systems, only the bright
0
on q is much more complex, but Giavalisco et al. test regions may survive the e(cid:134)ects of dimming. Observed at
0
No. 2, 2001 GALAXIES OBSERVED IN THE ULTRAVIOLET 737
simulated zD4 in the F814W —lter of the HDF, the Im resolution at high redshift, which we shall refer to collec-
galaxyNGC4214appearsinFigure1asacompact,regular tively as the (cid:147)(cid:147)cosmological e(cid:134)ects,ˇˇ are then discussed in
object. For other galaxies, such as the Scd M101 (also terms of a —xed physical aperture size for each galaxy, i.e.,
shown in Fig. 1), only bright star-forming regions in the one that samples the same physical region on rest-frame
spiral arms are visible at high redshift; the nucleus is andsimulatedredshiftimages((cid:176)5.2).Wethencomparethe
quickly lost below the detection threshold. The spatial dis- —xed aperture results with those obtained by adjusting the
tribution of these features gives the arti—cially redshifted aperture based on each imageˇs limiting surface brightness,
images a patchy, fragmented appearance suggestive of a as an observer would do with real data ((cid:176) 5.3). Finally, we
later type galaxy or even multiple systems (see also Giava- combine the bandshifting and cosmological e(cid:134)ects to
lisco et al. 1996a). Most of the E/S0 systems in our sample discussthetotal(cid:147)(cid:147)morphologicalk-correctionˇˇbetweenthe
are either undetectable at high redshift or have shrunk to optical appearance of nearby galaxies and images of high-
theappearanceofpointsources.Thisstronglysuggeststhat redshiftsystems((cid:176)5.4).Inseveraloftheplotsofconcentra-
source counts at high redshift will be a(cid:134)ected by a lack of tion and asymmetry indices that are presented in this
early-type galaxies unless these systems have evolved sig- section,galaxiesaredividedintofourbinsbyHubbletype.
ni—cantly. At lower redshifts (z„1), cosmological e(cid:134)ects Symbols for the di(cid:134)erent bins are explained in the —gure
aremild,andthesimulatedgalaxiessimplylooklikefainter, captions. Note that not all galaxies have optical data in
moresmoothedversionsoftheirlocalcounterparts.Figure every—lter;thatis,aplotcomparingtheB-bandandFUV
2 shows the results for the Sbc galaxy M51, whose rest- morphologies will not necessarily include all the same gal-
frame U-band data have been redshifted into the WFPC2 axiesasonecomparingR-bandandFUVimages.
F606W and F814W —lters. The simulated F606W image
looks a great deal like the original U-band image, but by 5.1. Bandshifting
zD1.2,intheF814W—lter,thegalaxyisbeginningtolook
FUV values of the central concentration index (C) are
lessregularasthefaintinterarmlightfallsbelowthedetec-
predominantly lower than those measured on optical
tion threshold. It is clear from these images that surface
images of the same galaxy. Figure 3 (left) illustrates the
brightness e(cid:134)ects play a signi—cant role in determining the
behaviorofCasafunctionofwavelengthfor10representa-
apparent galaxy morphology and thus that cosmological
tive galaxies in our sample; galaxy names are shown to
dimmingcannotbeneglectedinananalysisofhigh-redshift
facilitate the discussion below. The change *C\C
objects. OPT
[C is shown as a function of C for all galaxies in
Inthenextsectionwewillattempttoquantifythee(cid:134)ects FUV FUV
Figure 4. Galaxies with large increases in C from FUV to
of redshift on morphology by using the concentration and
optical wavelengths, (i.e., large positive values of *C), such
asymmetryindices.Weconsideronlythosegalaxiesthatare
as NGC 1097 and NGC 1566, tend to be early- to
detected and resolved on the simulated high-redshift
intermediate-type spiral galaxies whose prominent optical
images. First, C and A were measured on the arti—cially
bulgesarefainttoinvisibleintheFUV(seealsoK00).This
redshifted images by using the procedures described in (cid:176) 3
e(cid:134)ect is enhanced in barred galaxies because of the domin-
anda—xed(cid:147)(cid:147)physicalˇˇaperturesize(i.e.,sizeinkiloparsecs
ance of red stars in the bar, as shown for the SBa galaxy
on the galaxy). For these measurements we select an aper-
NGC1512inFigure5.ThechangeinCislessdramaticfor
turesizeonthearti—ciallyredshiftedimage,thenscaleitby
late-typespiralgalaxiessuchasNGC1313,M51,andM63.
the appropriate bin factor to determine its size on the
Galaxies with ongoing, widespread star formation often
unredshifted image. This procedure isolates the e(cid:134)ects of
havesimilarC andC becausethelightinbothspec-
surface brightness dimming and reduced spatial resolution OPT FUV
tralregimesisdominatedbytheyoungstars.Thesesystems
by comparing the same physical region of the galaxy at
include the starburst NGC 3310 (Smith et al. 1996; also
di(cid:134)erentsimulatedredshifts.WethenrecalculatedCandA
shown in Fig. 5) and the irregulars NGC 42145 and NGC
using a method that mimics the procedure an observer
4449, as well as the starburst]active galactic nucleus
mightfollow:adjustingtheaperturesizeforeachimageby
(AGN) NGC 1068 (Ne(cid:134) et al. 1994). These galaxies have a
visualinspectiontoencloseonlythedetectablelight.Inthis
higherC thanthatofothersinoursample(seeFig.3for
case,thephysicalaperturesizevariesbasedonthelimiting FUV
NGC3310andNGC4214),suggestingthattheconcentra-
surface brightness of each simulated image. The apertures
tionindexmaybeusefulasanindicatorofstarburstactivity
selected for high redshifts are often smaller than the size
in the analysis of rest-frame FUV galaxy images. Con-
expected from scaling a galaxyˇs rest-frame aperture by the
versely,themergersystemNGC4038/9sharesthestarburst
bin factor for that redshift. This comparison is less direct
traitofsimilarC andC buthasuniformlylowvalues
than the one in which aperture size is —xed, but it is more FUV OPT
ofC,correspondingtoitspeculiarmorphology(Fig.3).The
indicative of analysis techniques for deep surveys. Table 3
starburst galaxy NGC 5253 is a conspicuous counter-
givestheCandAvaluesforthearti—ciallyredshiftedFUV
example to the trend toward lower C in the FUV. Its UV-
andU-bandimagesinwhichapertureswereadjustedbased
bright, centrally concentrated starburst produces a very
on limiting surface brightness. Di(cid:134)erences between these
highC ,whileolderstarssurroundingtheburstcontrib-
resultsandthevaluesfora—xedaperturesizearediscussed FUV
ute signi—cantly to the optical light and thus reduce the
in(cid:176)5.3.
valueofC .Overall,thechangeinCduetobandshifting
OPT
5. QUANTITATIVE MORPHOLOGY RESULTS ranges from nearly zero to a maximum of D0.4, or up to
40%asCtheoreticallyrangesfrom0to1.
In this section we discuss the results of our quantitative
morphology investigation. We —rst consider the e(cid:134)ects of
¨¨¨¨¨¨¨¨¨¨¨¨¨¨¨
bandshiftingalonebycomparingtheFUVandopticalmor-
5Fanelli et al. (1997b) —nd a starbursting core superposed on a faint
phologies of the sample galaxies within a —xed aperture
diskinNGC4214;however,ourapertureforC enclosesonlytheregion
((cid:176)5.1).Thee(cid:134)ectsofsurfacebrightnessdimmingandlossof containingFUVlightandthusdoesnotsampleOtPhTedisk.
738 KUCHINSKI ET AL. Vol. 122
FIG. 3.¨Valuesofthecentralconcentration(C)andasymmetry(A)indicesasafunctionofwavelengthfor10galaxiesinoursample
TheasymmetryvaluesA areconsistentlyhigherthan (70%) in some cases. We stress that this relation, while
FUV
A , with a marked trend toward larger *A\A usefulovertherangeofHubbletypesinoursampleoflocal
OPT OPT
[A in galaxies that are very asymmetric in the FUV galaxies, needs further testing of its validity, especially for
FUV
(seeFigs.3and4,right).UV-brightcircumnuclearstarfor- themergersandprotogalaxiesencounteredathighredshift.
mationinabrokenringdominatestheFUVlightofseveral We—ndoneparticulargalaxy,NGC4736,thathighlights
galaxies, including NGC 1097 (Fig. 3), NGC 1512 (Fig. 5), the limitations of using C and A indices for morphological
and NGC 3351. This produces large A values even comparisons. FUV and V-band images of this galaxy are
FUV
though the galaxies appear symmetric in optical light. In showninFigure5(bottom).AscanbeseeninFigure3,the
others, such as NGC 925 and NGC 2403, the underlying values of C and A for NGC 4736 change very little with
di(cid:134)use disk light is invisible in the FUV, and patchy light wavelength in spite of dramatic morphological di(cid:134)erences
fromyoungstarsdominatesA .ThemergersystemNGC betweentheFUVandopticalimages.Abrightstar-forming
FUV
4038/9standsoutinFigure3becauseofitsextremelyhigh ringproduceshighconcentrationintheFUV,whilealarge
A at all wavelengths compared with the other sample gal- bulgeincreasestheopticalvalueofC.Symmetryinthering
axies.Aswasthecasefortheconcentrationindex,theasym- thatdominatestheFUVimagesyieldsalowvalueofA ,
FUV
metryindicesoftheglobalstarburstNGC3310(seeFig.3) while an equally regular disk and bulge shape are
and the starburst]AGN NGC 1068 are not very depen- responsible for a lowA . The prevalence of star-forming
OPT
dent on wavelength. With the exception of NGC 4038/9 inner and circumnuclear rings in the FUV (K00 and refer-
(which is plotted to the far right as an asterisk but is ences therein) suggests that cases such as NGC 4736 may
excluded from the —tting procedure described below), not be uncommon and thus that the use of C and A to
Figure 4 (right) shows a tight correlation between *A and compare rest-frame UV and optical images may not ade-
A . The diagonal lines in Figure 4 show linear least- quatelydescribethedetailede(cid:134)ectsofbandshifting.
FUV
square—tstothedata;solidlinesarethe—tstodataforeach We now come to the most interesting general point of
—lter individually, and the dotted line is the —t to all data thisstudy.Inmarkedcontrasttotheresultsforopticaldata,
together.Thetwolinesineachpanelarenearlyidentical,so theFUVvaluesofCandAfailtosegregategalaxiesinour
A canbepredictedfromA usingthedotted-linerela- sampleintothebroadmorphologicalbinsoftheA-Cclassi-
OPT FUV
tion —cation system of Abraham et al. (1996b). Figure 6 shows
both FUV and optical C and A for the sample galaxies on
*A\A [A [(0.92^0.05) thelog Aversuslog Cdiagram.ThedashedlinesinFigure
OPT FUV
6aretheAbrahametal.(1996b)divisionsintomorphologi-
](A )](0.12^0.02) , (4)
FUV calbins,(shiftedbytheo(cid:134)setsbetweenourvaluesandtheirs
wherethestandarddeviationoftheresidualsis^0.07.The that were calculated in (cid:176) 3). Errors in log C and log A
maximum value for *A can be quite large, reaching D0.7 dependonthevaluesoftheseindices(wesimplypropagate