Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593

0.22 MB·English

by  John Kormendy

DRAFTVERSIONFEBRUARY3,2008
PreprinttypesetusingLATEXstyleemulateapjv.6/22/04
PSEUDOBULGESINTHEDISKGALAXIESNGC7690ANDNGC45931,2
JOHNKORMENDY3,4,MARKE.CORNELL3,DAVIDL.BLOCK4,JOHANH.KNAPEN5,EMMAL.ALLARD5
DraftversionFebruary3,2008
ABSTRACT
We present K-band surface photometryof NGC 7690 (Hubble type Sab) and NGC 4593 (SBb). We find
s
that, in bothgalaxies, a majorpartofthe “bulge”is asflat asthe diskandhasapproximatelythesame color
as the innerdisk. In otherwords, the “bulges” ofthese galaxieshavedisk-like properties. We concludethat
these are examplesof “pseudobulges”– thatis, productsofsecular dynamicalevolution. Nonaxisymmetries
such as bars and oval disks transport disk gas toward the center. There, star formation builds dense stellar
6 componentsthatlooklike–andoftenaremistakenfor–merger-builtbulgesbutthatwereconstructedslowly
0 outofdiskmaterial. Thesepseudobulgescanmosteasilyberecognizedwhen,asinthepresentgalaxies,they
0 retaindisk-likeproperties.NGC7690andNGC4593thereforecontributetothegrowingevidencethatsecular
2 processeshelptoshapegalaxies.
NGC4593containsanuclearringofdustthatismorphologicallysimilartonuclearringsofstarformation
n
thatareseeninmanybarredandovalgalaxies.Thenucleardustringisconnectedtonearlyradialdustlanesin
a
J thegalaxy’sbar. Suchdustlanesareasignatureofgasinflow. Wesuggestthatgasiscurrentlyaccumulating
inthedustringandhypothesizethatthegasringwillstarburstinthefuture. TheobservationsofNGC4593
8
thereforesuggestthatmajorstarbursteventsthatcontributetopseudobulgegrowthcanbeepisodic.
1
Subjectheadings:galaxies:evolution—galaxies:individual(NGC4593,NGC7690)—galaxies:photometry

1 —galaxies:spiral—galaxies:structure
v
3
9 1. INTRODUCTION driven to small radii where it reaches high densities, feeds
3 starbursts, andbuilds“pseudobulges”. Becauseoftheirhigh
Internal secular evolution of galaxies is the dynamical
1 stellar densities and steep density gradients, pseudobulges
redistribution of energy and angular momentum that causes
0 superficiallyresemble–andoftenaremistakenfor–bulges.
galaxiesto evolveslowlybetweenrapid(collapse-timescale)
6 Following Sandage (1961) and Sandage & Bedke (1994),
transformation events that are caused by galaxy mergers.
0
Renzini(1999)adoptsthis definitionof a bulge: “It appears
/ Drivingagentsincludenonaxisymmetriesinthegravitational
h potential such as bars, oval disks, and global spiral legitimatetolookatbulgesasellipticalsthathappentohave
p aprominentdiskaroundthem[and]ellipticalsasbulgesthat
structure. Kormendy (1993) and Kormendy & Kennicutt
- for some reason have missed the opportunity to acquire or
o (2004,hereafterKK)reviewthegrowingevidencethatsecular
maintain a prominent disk.” We adopt the same definition.
r processeshaveshapedthestructureofmanygalaxies.
t The fundamental way that self-gravitating disks evolve – Our paradigm of galaxy formation then is that bulges and
s
ellipticals both formed via galaxy mergers (e.g., Toomre
a provided that there is an efficient driving agent – is by
: spreading (Lynden-Bell & Kalnajs 1972; Lynden-Bell & 1977; Steinmetz & Navarro 2002, 2003), a conclusion that
v is wellsupportedby observations(see Schweizer 1990fora
Pringle 1974; Tremaine 1989; see Kormendy & Fisher
i
X 2005 for a review in the present context). In general, review). Pseudobulgesare thereforefundamentallydifferent
frombulges–theywerebuiltslowlyoutofthedisk.Twowell
r it is energetically favorable to shrink the inner parts by
a expandingtheouterparts. Inbarredgalaxies,onewellknown developedexamples, one in the unbarred galaxy NGC 7690
and one in the barred galaxy NGC 4593, are the subjects of
consequence is the production of “inner rings” around the
thispaper.
end of the bar and “outer rings” at about 2.2 bar radii. The
Hierarchicalclusteringandgalaxymergingarewellknown.
most general consequence of secular evolution, and the one
Secular evolution is less studied and less well known. We
that is of interest in this paper, is that some disk gas is
are therefore still in the “proof of concept” phase in which
1BasedonobservationsmadewiththeAnglo-AustralianTelescope. it is useful to illustrate clearcut examples of the results of
2BasedinpartonobservationsmadewiththeNASA/ESAHubbleSpace secularevolution.Thispapercontinuesaseries(seetheabove
Telescope, obtainedfromtheDataArchiveattheSpaceTelescopeScience reviewsandKormendy&Cornell2004)inwhichweillustrate
Institute (STScI). STScI is operated by the Association of Universities thevarietyofdisk-likefeaturesthatdefinepseudobulges.
for Research in Astronomy, Inc., under NASA contract NAS5–26555.
The observations of NGC 7690 are associated with program IDs 7331 2. OBSERVATIONSANDDATAREDUCTIONS
(NICMOS–MassimoStiavelli)and6359(WFPC2–MassimoStiavelli).The
observationsofNGC4593areassociatedwithprogramIDs7330(NICMOS 2.1. AATInfraredImagingandDataReduction
–JohnMulchaey),and5479(WFPC2–MatthewMalkan).
NGC 7690 and NGC 4593 show dust absorption features
3 Department of Astronomy, University of Texas, 1 University Station,
Austin, Texas 78712-0259; [email protected], near their centers. Also, if star formation were in progress,
[email protected] there would be a danger that the brightness distributions at
4 Cosmic Dust Laboratory, School of Computational and Applied visiblewavelengthswouldbeaffectedbystrongvariationsin
Mathematics, UniversityoftheWitwatersrand, PrivateBag3,WITS2050,
mass-to-light ratios. We therefore base our results on near-
Johannesburg,SouthAfrica;[email protected]
5 Centre for Astrophysics Research, University of Hertfordshire, infrared photometry, and we check later that they are not
Hatfield, Herts AL10 9AB, United Kingdom; [email protected], greatly affected by stellar population gradients or by dust
[email protected] absorption.
2 Kormendyetal.
Near-infraredimages of both galaxies were obtained with Cosmic ray hits and bad pixels in the NICMOS data
the InfraRed Imaging Spectrograph 2 (IRIS2; Tinney et wereremovedusingthetvzapcommandinJonHoltzman’s
al.2004)atthe f/8CassegrainfocusoftheAnglo-Australian (http://astronomy.nmsu.edu/holtz/xvista)
Telescope (AAT). We used IRIS2 in its wide-field imaging implementationof VISTA (Lauer et al. 1983; Stover 1988).
mode; this provides a field of view of 7′.7 × 7.′7 sampled This replaces user-selected pixels with the median of the
at a scale of 0′.′4486 pixel- 1. The bandpass was K for surrounding 5 × 5 pixels. Cosmic ray hits in the WFPC2
s
both galaxies. We imaged NGC 7690 for a total on-source image of NGC 7690 were removed using the STSDAS task
exposure time of 56 m. These observations were taken on CRREJ. The central, bad 1 – 2 columns in the NICMOS
2004July1. ForNGC4593,weobtained52mofon-source datawerefixedbylinearlyinterpolatingtheneighboringpixel
exposure piecewise during the nights of 2004 June 30, July valueslinebyline. GapsinthemosaicedWideFieldCamera
1,July2,andJuly4. Individualimagesfromdifferentnights imagewere also filled via linear interpolation. TheHST PC
werecombinedduringthereductionprocess. V-band image of NGC 4593 that is used in Figure 3 was
Ourobservingtechniquesweresimilartothosedescribedin cleanedofcosmicraysusingtheIRAFscriptL.A.COSMIC
Knapenetal.(2003)andinBlocketal.(2004).Individual8s (vanDokkum2001)andVISTA tvzap.
exposures were co-added into raw images of 56 s exposure Finally,anyremainingskybackgroundlevelwascomputed
time. In contrast to our earlier work and taking advantage as the average of the modes of the pixel values in several
of the large IRIS2 field of view compared to the size of the sky boxes chosen to be free of galaxy light or interfering
galaxies, we did not alternate telescope pointings between objects. The galaxies fill the HST Planetary Camera (PC)
the target and a nearby, blank background field. Instead, and NICMOS fields of view. For the PC, the sky flux was
we used a grid of four pointings, each of which imaged the measured on the Wide Field Camera images and scaled to
galaxyinonequadrantofthedetectorarray. This“quadrant- the PC pixels. For the NICMOS image, the sky was taken
jitter”methodhastheadvantageofhighobservingefficiency, as zero for NGC 7690. For NGC 4593, the sky value for
becausenoseparatebackgroundexposuresareneeded,butthe the NICMOS image was chosen to optimize the agreement
disadvantagethatthegalaxyimagemustberemovedfromthe betweenthemajor-axiscutsasmeasuredontheNICMOSand
rawexposureswhenconstructingbackgroundskyimages. AATimagesintheradiusrange3′′≤r≤13′′.
The AAT K-band images were reduced in IRAF (Tody
s
1986) using our own procedures. The NGC 7690 data 2.3. SurfacePhotometry
reductions were relatively simple, because the images were Before fitting ellipses and calculating profile cuts,
obtained at low airmass on a single photometric night. The interferingforegroundandbackgroundobjectswereidentified
galaxy was observed in quadrant-jitter mode, so it was usingSourceExtractor(Bertin&Arnouts1996)andmasked.
centeredinadifferentquadrantofthedetectorineachframe Any remaining stars were identified visually and masked as
of a consecutive series. We constructed a flat-field frame well.
for each detector quadrant from the median of all images Positionangleandellipticityprofilesasafunctionofmajor-
with no galaxy in that quadrant. The images were adjusted axisradiusr werederivedfromellipsefits usingthe method
for any additive offset before computing the median. The of Bender & Möllenhoff (1987) and Bender et al. (1988) as
NGC 7690 field had few stars, and none remained in the implemented in MIDAS (Banse et al. 1988) by Bender and
final flat field. Once the quadrants were flattened, the bySaglia(2003,privatecommunication).Positionanglesare
galaxyquadrantswereregisteredandstackedusingamedian measured east of north. Some profiles were extended using
combine, again correcting for additive offsets. The final ellipsefitsmadebyGASP(Cawson1983;Davisetal.1985),
stacked frame was quite flat, so a single constant sky value which is slightly more robustat low S/N or when isophotes
was subtracted. In contrast, the NGC 4593 images were areincompleteattheedgeofthefieldofview.
taken over 4 nights, at high airmass, and in more variable Surfacebrightnesscutsalongthemajoraxis,theminoraxis,
conditions. Thebackgroundshapevariedenoughsothatthe and(forNGC4593)thebarwereproducedusinga program
simpleassumptionsusedforNGC7690didnotworkforNGC that averages pixel values in a 25◦-wide, pie-shaped wedge.
4593. We constructeda flat-fieldframefromalloftheNGC Therefore, more pixels are included at large radii where the
4593imagesasbefore.Afterflat-fielding,wesubtractedfrom S/N is low. Masked pixels are left out of the average. The
each galaxyframe a sky frame constructedfromthe median cutsonoppositesidesofthegalaxycenterwereaveraged.
ofthe4framestakennearestintimetothegalaxyframebut The various cut profiles were shifted in mag arcsec- 2 to
withthegalaxyinadifferentquadrant.Starsinthefieldwere matchupaswellaspossible.TheK-bandzeropointforNGC
s
maskedbeforeconstructingtheflat-fieldandskyframes. 7690wasderivedfrom2MASS,5′′-and7′′-radius,circular-
ThereducedNGC7690imagehasPSFFWHM=1.′′0and aperture photometry applied to the HST NICMOS F160W
theNGC4593imagehasFWHM=1.′′6. (H-band)image. The K-band zeropointfor NGC 4593 was
s
transferredfrom2MASSbymeasuringthegalaxymagnitude
2.2. AdditionalArchivalImages
in the Large Galaxy Atlas image (Jarrett et al. 2003) and in
The images used for surface photometry were archival ourAATimagewithinthelargest(radius=95′′)aperturethat
HubbleSpaceTelescope(HST)WFPC2F606Wimages,HST fits within our image. For NGC 7690, theV-bandzeropoint
NICMOSF160Wimages,archival2MASSimages(extended was derived from aperture photometry by Wegner (1979).
source catalog tile for NGC 7690 and a Large Galaxy Atlas For NGC 4593, the WFPC2 image is saturated, making
imageforNGC4593),andtheAATK-bandimagesobtained calibrationvia aperturephotometryproblematic. Instead we
s
withIRIS2.Toimprovesignal-to-noise,the2MASSJ,H,and usedthe transformationfromF606WtoJohnsonV givenby
K imageswereaddedtogether.Thebackgroundwasremoved Holtzman et al. (1995), assuming thatV- I =1.25 from the
s
fromtheNGC76902MASStilebymaskingthestarsandthen aperturephotometrybyMcAlaryetal.(1983)astabulatedby
fittingandsubtractingaquadraticsurface.Thebackgroundin Prugniel&Héraudeau(1998).
the2MASSimageofNGC4593wasalreadysufficientlyflat. Thecutprofilesweresmoothedbyaveragingvaluesinbins
PseudobulgesinNGC7690andNGC4593 3
ofwidth0.04inlogr.Profilesweretruncatedwheretheywere contrast,alens6isdefinedasashelfofnearlyconstantsurface
affectedbyseeingatsmallradiiandlowS/Natlargeradii. brightnessseenalongboththemajorandminoraxes. Lenses
The V - K cut profile was created from calibratedV and haveintrinsicaxialratios∼0.9±0.05intheequatorialplane
s
K composite profiles. TheV composite profile was created (see the above papers), whereas bars in early-type galaxies
s
froma cut fromthe Planetary Camera image inside a radius have axial ratios of ∼0.1 to 0.3 in the equatorialplane (see
of 20′′, and the cut from the Wide Field Camera beyond Sellwood&Wilkinson1993forareview).Inotherwords,the
that. Similarly,theK compositeprofilewascreatedfromthe shapedistributionsoflensesandbarsdonotoverlap. Lenses
s
NICMOSprofileinside10′′andtheAATprofileoutsidethat. and bars are physically different. In fact, barred galaxies
TheV- K images were created by taking the ratio of the often contain both components, with the bar filling the lens
s
sky-subtractedWFPC2 F606WV and AAT K frames, after inonedimension(seetheabovepapers). However,unbarred
s
convolving the WFPC2 frames with gaussians to match the galaxies can also have strong lenses (e.g., NGC 1553:
AATPSFs. Figures1and3showsthelogarithmsoftheratio Freeman1975;Kormendy1984).Notefurtherthatlensesand
images. “inner rings” (brightrings that generally encircle the end of
thebar)arealsodifferent(e.g.,Sellwood&Wilkinson1993;
Buta&Combes1996): innerringsarerelativelydarkinside.
3. NGC7690 Also, rings are narrow in the radial direction and therefore
NGC 7690 is an unbarred, Sab galaxy illustrated in the must have small radial velocity dispersions, whereas lenses
Carnegie Atlas (Sandage & Bedke 1994). In Tully (1988) are observed to have large radial velocity dispersions (e.g.,
group 61 - 0 +16; its group’s mean recession velocity of Kormendy1984). Themainbarsinearly-typegalacticdisks
1495±59 km s- 1 together with a Hubble constant of 71 have radii of ∼ 1.4 exponential scale lengths (Erwin 2005),
km s- 1 Mpc- 1 (Spergel et al. 2003) imply a distance of 21 and–asnotedabove–theradiioflensesandinnerringsare
Mpc. For a Galactic absorption of A ≃0.045 (Schlegel et closelysimilartothesizesoftheirassociatedbars. Distinctly
B
al.1998)andatotalB-bandapparentmagnitudeofB =13.0 smaller versions of all of these phenomena – nuclear bars,
T
(de Vaucouleurs et al. 1991: RC3), the absolute magnitude nuclear lenses (often but not always associated with nuclear
is M =- 18.7. NGC 7690is thereforea reasonablynormal, bars), and nuclearrings– also occur, usually but notalways
B
low-luminosity disk galaxy. Its relatively early Hubble type ingalaxiesthatalsocontainlarger,“main”barsorovals. We
implies that it should have a substantial bulge (e.g., Simien belabor these points because lenses are less well known in
& de Vaucouleurs1986). In particular, NGC 7690is earlier theastronomicalcommunitythanbarsorinnerrings. The6′′
in Hubble type than the relatively sharp transition observed × 3′′ shelf in the brightness distribution of NGC 7690 is a
to occur at Sb/Sbc between early-type galaxies that mostly nuclearlenswithoutanassociatednuclearormainbar.
contain classical bulges and late-type galaxies that mostly The nuclear lens is 1.5 to 2 K mag arcsec- 2 higher in
s
contain pseudobulges (see KK for a review). NGC 7690 surfacebrightnessthanthe inwardextrapolationofthe outer
neverthelessprovestocontainawelldevelopedpseudobulge. disk’s almost-exponential brightness profile. It therefore
ThepseudobulgeofNGC7690wasdiscoveredbyCarollo forms part of what would conventionally be identified as
et al. (1998); their V-band image is included in Figure 13 the bulge. We would like to classify a bulge as the E-like
of KK. An HST F160W image from Carollo et al. (2002)is part of a galaxy. In practice, this is not straightforward
shownintheupper-rightpanelofourFigure1. Qualitatively, for non-edge-on galaxies. Therefore, Carollo et al. (1999)
thecenterofNGC7690looksdisk-like,somuchsothatinthe adoptthe surrogatedefinition that a bulge is the central part
“Bulge?”columnofTable2inCarolloetal.(1998),theydo of the galaxy that is brighter than the inward extrapolation
notevenlistNGC7690ashavingan“IB”=“irregularbulge”, of the outer disk’s exponential brightness profile. By this
whichisessentiallyequivalenttowhatwecallapseudobulge. definition,thenuclearlensinNGC7690ispartofthegalaxy’s
Instead, the entry is “No?”. NGC 7690 was chosen for this bulge. But bulges and elliptical galaxies generally have
paper because we wanted to investigate the (pseudo)bulge simplebrightnessprofilesconsistingofasingleSérsic(1968)
quantitativelywithnear-infraredsurfacephotometry. logI∝r1/nprofile(possiblywithacuspycore)andnotashelf
The results are shown in Figure 1. The blue and red µ(r) inthebrightnessprofilesuchastheoneinNGC7690.
pointsaremajor-andminor-axisbrightnesscutscalculatedas Thesecondpseudobulgeindicatoristhatthenuclearlens–
discussedin§2.3.ExcellentagreementbetweentheAATand theshelfintheinnerbrightnessprofile–isclearlyadisk.This
2MASSprofilesprovidesanimportantcheckofourreduction wasalreadyevidentfromthe imagesin Carollo etal. (1998,
procedures. Theǫ(r)andPA(r)pointsareisophoteellipticity 2002), as shown here in Figure 1. The nuclear disk in the
and position angle profiles calculated by fitting elliptical top-rightpanelhasthesameapparentflatteningandposition
isophotes to the images identified in the key. Calculations angleastheouterdiskshowninthemiddleimagepanel. We
of the V - Ks color profile (top-left plot) and image (lower- seethisresultquantitativelyintheǫandPAplots. Ther=6′′
right)arediscussed in §2.3. Theprofilesareplottedagainst shelfinthebrightnessprofilehaspreciselythesameellipticity
logr to illustrate the behavior at small and large radii and andPA as the disk fartherout. It is notplausiblethat thisis
also againstlinear radius r so that an exponentialouter disk duetointernalabsorption,becausetheV-bandellipticitiesand
–anddeparturesofthe innerprofileaboveit– caneasily be position angles (crosses in Figure 1) agree with the K-band
s
recognized. ones at all radii that are relevant to our conclusions. Since
Fig.1showsthreequantitativesignaturesofapseudobulge: theK-bandextinctionisaboutone-tenthasbigastheV-band
s
First, the central component illustrated by Carollo and
collaboratorsformsaclearshelfinthebrightnessdistribution 6 Lens components should not be confused with lenticular galaxies,
at major- and minor-axis radii of 6′′ and 3′′, respectively. i.e., with the name for S0galaxies (Sandage 1961) that is adopted by de
Vaucouleursandcollaborators(e.g.,RC3).ManyS0galaxiesdonotcontain
That is, it has the morphology of a lens (Freeman 1975;
lenses,andmanySaandlater-typegalaxiesdocontainlenses.Thisconfusion
Kormendy 1979, 1981; Buta & Combes 1996), not a bar. isunfortunate,butitisthoroughlyembeddedintheliterature.
A bar would form a shelf in the major-axis profile only. In
4 Kormendyetal.
FIG.1.— NGC7690pseudobulge–topimage:18′′×18′′HSTNICMOSF160WimagefromCarolloetal.(2002);middle:91′′×91′′AATKs-bandimage
discussedin§2;theimagesareshownatdifferentlogarithmicintensitystretches. Bottom:V- KscolorimageconstructedfromtheaboveKsframeandfroma
V-band(F606W)imagetakenwithHSTbyCarolloetal.(1998;theyalsoshowanunsharp-maskedimage).Theintensitystretchislinearinmagarcsec- 2;white
correspondstoV- Ks=0.8,andblackcorrespondstoV- Ks=3.5.Themiddleandbottompanelsshowthesamefieldofview.Northisupandeastisatleftinall
images. Alarge-scale(9′′×9′′)V- HcolorimageobtainedwithHSTisillustratedinCarolloetal.(2002).Theleftpanelsaboveshowsurfacephotometryas
afunctionoflogradius(top)andlinearradius(bottom).BrightnessandV- Ksprofilesare25◦-widecutsalongthemajoraxis(PA=129◦)andminoraxis(PA
=39◦)color-codedasindicatedinthekeyandwithoppositesidesofthegalaxyaveraged. Isophoteellipticitiesǫandmajor-axispositionanglesPAarederived
fromisophotefitstotheimagesindicatedinthekey.
extinction(e.g., Cox2000),thisimpliesthatextinctiondoes not significantly affect the parameters at either wavelength,
PseudobulgesinNGC7690andNGC4593 5
unless unrealistically gray extinction is postulated. From 4. NGC4593
r ≃ 6′′ outward, the ellipticity and PA change very little,
NGC 4593 is a structurally normal SBb galaxy (Sandage
consistentwithanearlycircularmaindiskatnearlyconstant
1961; Sandage & Bedke 1994). We adopt a distance of
inclination. Weconcludethatthenuclearlens,i.e.,theouter 35 Mpc (Tully 1988 group 11 - 29), then B =11.67 (RC3)
partof what would conventionallybe identifiedas the bulge implies that M = - 21.2. NGC 4593 is thTerefore a high-
ofNGC7690,isasflatasthegalaxy’sdisk. Thisisthemost B
luminosity disk galaxy. Moreover, it has a well developed
clearcutsignatureofapseudobulgeinNGC7690(seeKKfor
engineforsecular evolutionin the formof a high-amplitude
pseudobulgeclassificationcriteria).
bar. Secular evolutionis likely to be rapid. Our photometry
Thesurfacebrightnessofthenucleardiskisnearlyconstant
of NGC 4593 proves to be consistent with this expectation
interiortoitssharpouteredge.Notsurprisingly,theellipticity
(Figure2).
drops rapidly inward as the profile of this highly-inclined
galaxy becomes dominated by a less flattened center. This
feature may be but is not necessarily a classical bulge. It 4.1. Near-InfraredPhotometry:EvidenceforaPseudobulge
would be no surprise if the pseudobulge part of NGC 7690 NGC 4593 is a well known Seyfert I galaxy (Lewis et
wereembeddedinaclassicalbulge,becausesecularevolution al. 1978; MacAlpine et al. 1979). The nuclear point source
canbuildapseudobulgeinsideapre-existingbulge(seeKK). is very bright (HST F606W mag ∼ 17.1 at r ≤ 0.′′09); it
Butseveralargumentssuggestthataclassicalbulgeisnotthe completelyswamps the centralbrightnessdistributionof the
main feature near the center. First, the PA profile shows a galaxy(MRK1330inFigure1ofMalkanetal.1998). Since
clear twist centered at r≃3′′. The ellipticity starts to drop thisis notstar light, it is importantthatwe subtractitin our
suddenlytowardthecenterattheradiusr≃6′′wherethetwist analysis of the brightness distribution. We did this for the
starts,anditcontinuestodropthroughouttheradiusrangeof HST NICMOS image by subtracting a PSF calculated with
the twist. Thiscombinationis a characteristicofa weakbar Tiny Tim(Krist&Hook2004)andscaledintotalintensity
or weak spiral arms. Bars and spiral arms are disk features. to remove the diffraction spikes and other small-scale PSF
Thus, eveninteriorto its sharpouteredge, the signsare that structure in the image as well as possible. The result is
thegalaxyisdominatedbyadisk. the image in the upper-right panel of Figure 2; it was used
Thethirdpseudobulgeindicatoristhelackofasteepcolor to calculate the NICMOS points in the profile plot panels.
gradient between a bluish disk and a red and much older Of course, it is not possible to recover the stellar brightness
bulge. The V - K color image (bottom-right in Figure 1) distribution very close to the center. We were conservative
s
and profile both show that, apart from a few irregular dust andtruncatedthe brightnessprofile at r < 1′′. Uncertainties
∼
featuresthatareeasilymaskedoutinthephotometry,thereis intheHSTPSFsubtractiondonotaffectourconclusions.Itis
no significant color difference between the inner part of the importanttonotethatthiswouldnotbetrueatground-based
outerdisk and the nucleardisk. Indeed, theV- Ks profile is resolution.TheAATKsprofiledoesnotshowtheprofilekink
completelycontinuousfromthedisk-dominatedoutergalaxy at logr=0.5 (see Figure 2 and discussion, below), because
tothebulge-dominatedinnergalaxy. Thisisanexampleofa thecentral,fainterpartoftheprofileisfilledinbytheseeing-
generalphenomenon:(pseudo)bulgeanddiskcolorscorrelate convolvedSeyfertnucleus.
(Peletier&Balcells1996;Gadotti&dosAnjos2001).Bulges A comparison of the major-axis, bar, and minor-axis
and disks both show large ranges in colors, but “bulges are brightness cuts then shows that the “bulge” dominates the
more like their disk[s] than they are like each other” (Wyse brightness distribution at major-axis radii r<20′′. Is this a
et al. 1997). Courteau et al. (1996) and Courteau (1996) classicalbulgeorisitapseudobulge?
interpret such correlations as products of secular dynamical The ellipticity profile shows that NGC 4593 contains a
evolution. As notedabove,thereis nosign in NGC 7690of pseudobulge, with no sign of a classical bulge component.
adiscontinuityinstellarpopulationbetweenanold,non-star- The apparentellipticity of the outer disk is ǫ≃0.25±0.05.
formingbulgeandayoungerdisk,asonewouldexpectfora Remarkably, the ellipticity of the bulge is also 0.25±0.05
classicalbulge(Sandage&Bedke1994). overthewholeradiusinwhichwecanmeasureit;i.e.,interior
Notethatthesmallcolorgradientfromouterdisktonuclear tothebarandexteriortotheregionclobberedbytheSeyfert
lensimplies,sinceKs extinctionsaresomuchsmallerthanV nucleus. Note again that HST resolution is important: the
extinctions, that the shelf in the brightness profile is a real extrasmoothingcaused by the ground-basedPSF makesthe
featureinthestellardensityandnotanartifactofabsorption. isophotes as measured with the AAT significantly rounder
Several classification criteria agree, so it seems safe to than those measured with HST NICMOS. But the NICMOS
concludethat NGC 7690containsa pseudobulge. This may datashowthatthepseudobulgeofNGC4593isasflatasits
ormaynotcoexistwithasmallclassicalbulge–wecannotbe disk.
certainfromthedataathand,althoughnoobservationpoints IntheabsenceofPAinformation,itmightbepossiblethat
compellinglytoaclassicalbulgecomponent. the “bulge” of NGC 4593 is really a nuclear bar. Nuclear
We conclude that NGC 7690 is an example of secular bars are common in barred galaxies, and since they have
evolution in action. This is particularly interesting because arbitrarypositionangleswithrespecttotheirassociatedmain
the Sab galaxy NGC 7690 is earlier in type than the bars,anelongated“bulge”couldbeaverticallythicknuclear
galaxies7 that most commonly contain pseudobulges. Also, bar rather than a vertically thin disk. However, the PA is
there is no obvious engine for secular evolution, such as essentially the same at small radii as in the outer disk. For
a bar, oval distortion, or global spiral structure. NGC this to be caused by a nuclear bar would require the added
7690thereforecontributestotheaccumulatingevidencethat accident that the nuclear bar is aligned with the major axis
secularevolutionoccursinawidevarietyofgalaxies. of the galaxy. This is not impossible, but the more likely
explanationisthatthebulgeisnearlycircularinitsequatorial
7Wenote,however,thatevenS0scancontainpseudobulges(KK). planeandasflatasthedisk.
Ineither case, the observationsimply a pseudobulge. The
6 Kormendyetal.
FIG. 2.— NGC4593pseudobulge–topimage: 18′′×18′′HSTNICMOSF160WimagefromCarolloetal.(2002);middle: 191′′×191′′AATKs-band
imagediscussedin§2; theimages areshownatdifferent logarithmic intensity stretches. Bottom: 284′′ ×284′′ B-bandimagefromtheCarnegie Atlasof
Galaxies(Sandage&Bedke1994). Northisupandeastisatleftinallimages. Theleftpanelsshowsurfacephotometryasafunctionoflogradius(top)and
linearradius(bottom).Brightnessprofilesare25◦-widecutsalongthemajoraxis(PA=101◦),bar(PA=57◦),andminoraxis(PA=11◦)withoppositesidesof
thegalaxyaveraged.Isophoteellipticitesǫandmajor-axispositionanglesPAarederivedfromisophotefitstotheimagesindicatedinthekey.
cleanestsignatureof a well developedpseudobulgeis that it is very flat. However, since bars are disk phenomena, the
PseudobulgesinNGC7690andNGC4593 7
observation of a nuclear bar would also be evidence for a so thatV-band isophote fits are possible and both ǫ and PA
pseudobulge.BothclassificationcriteriaarereviewedinKK. canbemeasured. AsinthecaseofNGC7690,weconclude
A second feature of the profiles in Figure 2 is that the near-infrared measurements, which are much less
consistent with a pseudobulge but does not by itself prove affected by internal absorption, are reliable. This confirms
that one is present. The (pseudo)bulge profile has a kink ourconclusionsabouttheflatteningofthepseudobulge.
at logr ≃ 0.5 (r ≃ 3′′). Such features are not seen in Weemphasizeagainthat,exceptintheveryredbutnarrow
classical bulges or ellipticals. The above radius is too large dustring and in the central arcsec, the pseudobulgein NGC
for the kink to be a cuspy core like those in ellipticals 4593is the same color,V- K ≃3.2±0.1, as the outerdisk
s
(see Lauer et al. 1995; Faber et al. 1997). Pseudobulges atr≃100′′. Bothareabout0.4magredderinV- K thanis
s
are a consequence of more complicated physics than the NGC 7690. These results again are consistentwith Wyse et
violent relaxation and dissipation that builds ellipticals. If al.(1997):Bulgesanddisksbothshowlargerangesincolors,
theygrowby star formationin gasthathasbeen transported but“bulgesaremoreliketheirdisk[s]thantheyarelikeeach
to the center, exactly how the star formation proceeds and other.”TheredcolorofNGC4593hasbeennotedpreviously
what kind of density profile it produces are controlled by a (Santos-Lleó et al. 1995; Shaw et al. 1995). The inner ring
complicated interplay between star formation (as described and the spiral arms between r ≃ 30′′ and 80′′ are slightly
byaSchmidt1959–Kennicutt1998a,blaw)andthefactors bluer than the rest of the disk. Inner ring formation is part
– e.g., resonances – that determine where the gas stalls. ofthecanonicalsecularevolutionpicture,andspiralstructure
Nuclear star formation rings seen in many barred and oval is a signature of outward angular momentum transport. But
galaxies are a hint that the stellar density that “rains out” thedust-freepartsofthepseudobulgeinteriortothedustring
of the gas distribution may have more complicated radial areasblueastheoutersprialstructure. Sothecolordataare
featuresthandoellipticalgalaxies,inwhichviolentrelaxation consistentwithslowpseudobulgegrowth.
smoothsthedensityinradius. So pseudobulgesarelikelyto That growth may be episodic. TheV - K color image in
s
havealargervarietyofprofileshapesthandoellipticals. We Figure 3 is instructive. It and the V-band image both show
seetwoexamplesofthisvarietyinthepresentpaper. a dust lane on the rotationally leading side of the bar that
becomescurvedandeventually–nearthecenter–tangentto
4.2. ComparisonofV andKs Images: TheInnerDustRing thenucleardustring. Simulationsofgasresponsetoabarred
asEvidenceforEpisodicPseudobulgeGrowth
potential (see especially Athanassoula 1992; KK provide a
Dust is more important in NGC 4593 than in NGC 7690. review)suggestthatsuchdustlanesoccuratshockswherethe
Therefore we verify in this subsection that dust absorption gasanddustarecompressed. Velocitydiscontinuitiesacross
doesnotcompromisetheaboveconclusions. Examinationof dustlanesobservedinHI(e.g.,Lindbladetal.1996;Regan
thedustfeaturesalsoleadstoanewresult,namelyahintthat et al. 1997)and HII (Zurita et al. 2004)providecompelling
thestarburststhatcontributemosttopseudobulgegrowthmay support. If shocks are present, it is inevitable that gas loses
beepisodic. energy and falls toward the center. All this is central to
Aninnerdustspiralisfaintlyvisibleinthetop-rightpanel the secular evolution picture. Intense starbursts, often in
ofFigure2.Itsrelationshipwiththerestofthegalaxyismade theformof nuclearrings, arecommonlyassociatedwiththe
clear in Figure 3, which shows V-band images that can be above phenomena (KK provide a detailed review) and are
compared with K images in Figure 2. Figure 3 also shows widely interpreted as the result of the gas inflow. This star
s
a V - K color image; major-axis cuts through this image formation is part of pseudobulge growth. Interestingly, in
s
are shown in the V - K color profile in the top-left panel NGC 4593, we see a nuclear dust ring, not a starburst ring.
s
of Figure 2. We show separately the color profiles east and Thissuggeststhatgasisaccumulatingasaresultofbar-driven
westofthecenter;thered“peaks”identifywherenarrowdust inflowbutthatit isnotcurrentlystarbursting(see also Oliva
featurescrossthemajoraxis. etal.1999).Sincehighgasdensitiesfavorhighstarformation
TheV-bandHST PCimage(Figure3)showsastrongdust rates(Schmidt1959;Kennicutt1998a,b),itisreasonablyto
ringwithmajor-axisradius≃5′′. Itcausestheredringthatis assume that the dust and associated gasring will turn into a
themostobviousfeatureoftheV- K colorimage. Itisalso star-formingringsometimeinthefuture.
s
seen as the red peak in both the east and west color profiles Finally, we return to the one-armed, spiral dust lane that
at logr≃0.7 in Figure 2. Interior to the dustring, the faint continuesinwardfromthedustringtothecentralregionthat
dust spiral seen in Figure 2 is, not surprisingly, much more is dominated by the Seyfert nucleus (Malkan et al. 1998;
obviousatV bandinFigure3. Itscrossingsofthemajoraxis Figure 3 here). Spiral dust lanes interior to star formation
are seen in the colorprofilein Figure2 asV- K maximaat ringsare alsoseenin manygalaxies(see KK).Elmegreenet
s
logr≃0.6on the westside (opencircles)and atlogr≃0.4 al.(1998)suggestthattheyareasignthatsomegascontinues
on the east side (filled circles). Fortunately, the dust spiral tosinktowardthecentereveninteriorto theradiusatwhich
proves to be very narrow. Over a substantial radius range mostgasstalls. Additionalpseudobulgegrowthand feeding
interior to the dust ring, the colors of the pseudobulge on oftheSeyfertnucleusareplausibleconsequences.
thewest side ofthe centerareclosely similar tothose in the In summary, theV-band HST image and theV - K color
s
outer disk. There is no sign of significant reddening or (by imageinFig.3furthersupportourconclusionthatbar-driven
inference) absorption at these radii. The minor axis on the gas inflow continues to grow a pseudobulge in NGC 4593.
southsideofthecenterissimilarlyfreeofabsorptionexcept They also emphasize a new aspect to the secular evolution
inthedustspiral. Weusedone-sidedbrightnesscutsinthese picture: the strong starbursts that are seen in many galaxies
twodirectionstomeasureV-bandellipticitiesat2.′′5≤r≤4′′. and that are interpreted as an importantpart of pseudobulge
These are shown as the crosses in the ǫ profile in Figure growthmay,atleastinsomegalaxies,beepisodic.
2. TheV-band ellipticities, carefully measured to avoid the
dust, agree with the K values. The same is true at radii 5. POSSIBLEEFFECTSOFGALAXYINTERACTIONSAND
s
outsidethe dustring, where dustabsorptionis small enough MERGERS
8 Kormendyetal.
FIG. 3.— NGC4593V-bandHST images(topleft,topright,andbottomright)fromMalkanetal.(1998)andV- Ks colorimage(bottomleft). ThePSF
oftheSeyfertnucleuswasmodeledusingTiny Timandhasbeensubtracted. TheV-bandimagesareshownatdifferentlogarithmicintensitystretches. The
intensitystretchoftheV- Kscolorimageislinearinmagarcsec- 2;whitecorrespondstoV- Ks=2.9,andblackcorrespondstoV- Ks=4.7.Thetop-leftpanel
is30′′×30′′andcanbecompareddirectlywiththe18′′×18′′top-rightpanelinFigure2.Bothshowadustfeaturestartingtangentialtothewelldefineddust
ringandspiralingintotheSeyfertnucleusatthecenter.Theotherthreeimagesare191′′×191′′andshowthesamefieldofviewasthemiddleimagepanelin
Figure2. Theyillustratetherelationshipbetweenthedustringwiththebar. Almostradialdustlanesinthebarturnintospiralsclosetothecenterandbecome
tangentialtothedustring.Allofthedustlanesareveryred,asshownbytheV- Ksimageandbythecolorcutsinthetop-leftpanelofFigure2.
Kannappan et al. (2004) suggest that accretions of small etal. 1994;Rix etal.1995;García-Burilloetal. 2003). The
galaxiesareanalternativewaytobuildcold,diskysubsystems observation that so many prominent pseudobulges occur in
in galaxies. They find a correlation between blue-centered, barredand ovalgalaxies– that is, ones that contain obvious
star-forming bulges and evidence of tidal encounters with engines for secular evolution – is one argument that galaxy
galaxy neighbors. Such processes must happen; embedded interactions do not produce most pseudobulges(KK discuss
counterrotating components provide the clearest examples others). These arguments are statistical; they do not much
(e.g., NGC 4826,Braunetal. 1994;Rubin1994; Walterbos constrain individual galaxies. It is reasonable to ask (as
PseudobulgesinNGC7690andNGC4593 9
the referee did): could accreted material account for our Hernquist 1991; Elmegreen et al. 1991, although see also
observations? Sellwood2000).Therefore,althoughitistemptingtowonder
The answer for NGC 7690 is “maybe, but there is no whetherthepresentinteractionhassomethingtodowiththe
evidence for this”. The answer for NGC 4593 is “probably barredstructureofNGC4593,thematuremorphologyofthe
no”. SB(r) structuresuggests thatthe galaxyhasbeen barredand
First, we need to understand what kind of accretion is evolvingsecularlyfor a long time. Thestructure ofthe dust
possible.Majormergersthathappenedrecentlyinthehistory features–anearlyradialdustlaneontheleadingsideofthe
of both galaxiesare, we believe, excluded. Tóth & Ostriker barbecomingtangenttoanucleardustring–areaclearand
(1992) emphasized that disks are fragile; they are easily cleansignofevolutioninaction(Athanassoula1992).
destroyed by dynamical stirring produced by even a low- Finally, the spiral structure of NGC 4593 is not unusual
massprojectile.Bothgalaxieshaveflatcomponentsneartheir or suggestive of the influence of the interaction. One arm
centers. Accretionofahigh-massgalaxyoronethatalready starts on the inner ring at the end of the bar and the other
hasabulgewoulddestroysuchasubsystemandleavebehind starts“about15degreesdownstream”fromtheendofthebar
a big classical bulge that we do not see. This is especially (Sandage & Bedke 1994). This behavior is similar to that
relevant in NGC 4593, which remains flat all the way in to of NGC 2523, another prototypical SB(r) galaxy (Sandage
theSeyfertnucleus. Whatcouldmostsafelybeaccretedisa 1961). NGC2523isnotasymmetric,anditismoreisolated
bulgeless,gas-richdwarfgalaxy;itsfluffystellardistribution than NGC 4594 (the nearest substantial companion, NGC
wouldgettornapartbytidaleffectsatlargeradii,anditsgas 2523B, is more than 7 NGC 2523 radii away in projection
coulddissipateitswayintothecentralregions. and has a recession velocity 365 km s- 1 larger than that of
NGC2523;RC3).
5.1. NGC7690 Thereforewebelievethatthepresentweakandtemporary
The galaxy is isolated on the Digital Sky Survey. The interaction with PGC 42399 is not responsible for the
closest galaxies listed by the NASA/IPAC Extragalactic main features of the structure of NGC 4593, including the
Database (NED) that have measured recession velocities pseudobulge.
within 1000 km s- 1 of that of NGC 7690 are ESO 240-
G12 (projected distance = 54.′5 = 49 radii of NGC 7690) 6. CONCLUSION
and ESO 240-G4 (projected distance = 59.′0 = 53 radii of NGC 7690 (Sab) and NGC 4593 (SBb) provide clean
examples of relatively early-type galaxies whose “bulges”
NGC 7690), where the radius of NGC 7690 at 25 B mag
arcsec- 2 is 1.′1 (RC3). In an HI survey to look for galaxy are more disk-like than any elliptical galaxy. In particular,
pairs (Chengalur et al. 1993), it was not listed as a pair. It elliptical galaxies are never flatter than axial ratio ≃ 0.4
(Sandage et al. 1970; Binney & de Vaucouleurs 1981;
has a normal, two-horned, single-dish HI velocity profile
Tremblay & Merritt 1995), whereas part (NGC 7690) or
with almost no asymmetry (Davies et al. 1989; Chengalur
essentially all (NGC 4593) of the bulges of the present
et al. 1993); many isolated, late-type galaxies have more
galaxies are as flat as their outer disks. We conclude that
asymmetric velocity profiles. No asymmetry is seen in The
both galaxies contain pseudobulges – that is, high-density,
CarnegieAtlasofGalaxies(Sandage&Bedke1994),andwe
centralcomponentsthatweremadeoutofdiskgasbysecular
see none in our deep AAT images. No interaction appears
evolution.InNGC7690,bluecolorsimplythatstarformation
to bein progress. We cannotexcludea pastminoraccretion
andhencepseudobulgegrowtharestillinprogress. InNGC
event,butnosmokinggunpointstoone.
4593, gas appears currently to be accumulating in a ring
5.2. NGC4593 that plausibly will form stars in the future. Our results are
examplesofthegeneralconclusion(seeKKforareview)that
There are clear signs that NGC 4593 is interacting with
seculardynamicalevolutionoccursnaturallyandoftenindisk
PGC 42399. Any such interaction is fast – the velocity
difference of 320 km s- 1 (NED) is large compared with galaxies,whether(NGC4593)ornot(NGC7690)anengine
fortheevolutionisreadilyrecognized.
plausible galaxy rotation velocities. This does not favor a
To further investigate secular evolution,
strong interaction. Also, NGC 4593 is brighter than PGC
a desirable next step would be to quantify bar strengths by
42399by a factorof 7.7 (NED),so the perturberis notvery
measuringbartorques,theratioofthebar-induced,tangential
massive. Ontheotherhand,itsprojecteddistancefromNGC
force to the mean radial force as a function of radius (Buta
4593 is only about two disk radii. The spiral structure of
&Block 2001;Laurikainen&Salo 2002; Blocketal. 2001,
NGC 4593 is slightly distorted toward PGC 42339. Some
2004; Laurikainenet al. 2004). This ratio can be as high as
tidal stretching and possibly some tidal tickling of the wave
0.6 in strong bars, emphasizing how efficient bars can be in
patterns(spiralarmsandbar)inthegalaxyareplausible.
redistributing angular momentum. It would particularly be
Still,thestructureofthegalaxyis–apartfromtheabove–
worthwhile to look for correlations between maximum bar
completelynormal(Sandage&Bedke1994).Thegalaxyhas
torques and quantifiable consequences of secular evolution
an inner ring at the end of the bar, as do many other barred
(e.g., ring-to-diskand pseudobulge-to-diskmass ratios) in a
galaxies. Suchringsarebynowreasonablywellunderstood
statisticallyrepresentativesampleofgalaxies.
asproductsoflong-term,bar-drivensecularevolution. Inner
rings are signs that evolution has been going on for a long
time. Simulations suggest that the SB(r) phase comes after JK is sincerely grateful to Mrs. M. Keeton and the
the SB(s) phase, after outward angular momentumtransport Board of Trustees of the Anglo American Chairman’s Fund
has slowed the pattern speed of the bar and after the bar for the financial support that made possible his visit to
has had time to rearrange disk gas into an inner ring (see South Africa during which this paper was written. He
KK for a review). This is relevant because simulations also thanks the Cosmic Dust Laboratory and the School of
also suggest that tidal interactions can trigger bar formation ComputationalandAppliedMathematicsoftheUniversityof
(e.g., Noguchi 1987, 1988; Gerin et al. 1990; Barnes & the Witwatersrand for their hospitality. JHK acknowledges
10 Kormendyetal.
support from the Leverhulme Trust in the form of a work. We also thank Roberto Saglia for his version of the
Leverhulme Research Fellowship. We thank the referee Bender photometry pipeline. This research has made use
for a careful reading that led to substantial improvements of the NASA/IPAC Extragalactic Database (NED), which
to this paper. JHK and EA wish to thank the staff of is operated by the Jet Propulsion Laboratory, California
the AAT, and in particular Dr. Stuart Ryder, for their Institute of Technology, under contract with NASA. This
excellent support during his observing run with IRIS2. The research also used the HyperLeda electronic database at
authors are grateful to Ron Buta, to Bruce and Debra http://www-obs.univ-lyon1.fr/hypercat and
Elmegreen, and to Ivanio Puerari for making data from the image display tool SAOImage DS9 developed by
their programswith DLB and JHK available for the present SmithsonianAstrophysicalObservatory.
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