A Normal Stellar Disk in the Galaxy Malin 1

0.57 MB·English

by  A. J. Barth

PreprinttypesetusingLATEXstyleemulateapjv.6/22/04
ANORMALSTELLARDISKINTHEGALAXYMALIN11
AARONJ.BARTH
DepartmentofPhysicsandAstronomy,4129FrederickReinesHall,UniversityofCalifornia,Irvine,CA92697-4575;[email protected]
ABSTRACT
Sinceitsdiscovery,Malin1hasbeenconsideredtheprototypeandmostextremeexampleoftheclassofgiant
lowsurfacebrightnessdiskgalaxies.ExaminationofanarchivalHubbleSpaceTelescopeI-bandimagereveals
thatMalin1containsanormalstellardiskthatwasnotpreviouslyrecognized,havingacentralI-bandsurface
brightnessofµ =20.1magarcsec- 2 andascalelengthof4.8kpc. Outtoaradiusof∼10kpc,thestructure
0
of Malin 1 is that of a typical SB0/a galaxy. The remarkably extended, faint outer structure detected out to
7 r≈100kpcappearstobeaphotometricallydistinctcomponentandnotasimpleextensionoftheinnerdisk.In
0 termsofitsdiskscalelengthandcentralsurfacebrightness,Malin1wasoriginallyfoundtobeaveryremote
0 outlierrelativetoallotherknowndiskgalaxies. Thepresenceofadiskofnormalsizeandsurfacebrightness
2 in Malin 1 suggests that such extreme outliers in disk properties probably do not exist, but underscoresthe
importanceoftheextendedouterdiskregionsforafullunderstandingofthestructureandformationofspiral
n
galaxies.
a
J Subjectheadings:galaxies:individual(Malin1)—galaxies:spiral—galaxies:structure

3

2 1. INTRODUCTION density peaks in voids (Hoffman,Silk,&Wyse 1992) or by
secularevolutionandradialmixinginordinarydiskgalaxies
v ThegalaxyMalin1hasremainedasingularlyunusualand
8 puzzling objectfor nearly two decades. As described in the (Noguchi2001). Asthemostdiffuseandgas-richgiantLSB
1 discoverypaperbyBothunetal.(1987)andinlaterworkby galaxyknown,Malin1isalsoaparticularlyinterestinglabo-
0 Impey&Bothun (1989), it has the largest radial extent of ratoryforunderstandingissuesofdiskstabilityandthresholds
1 fortheonsetofstarformation(Impey&Bothun1989).
any known spiral galaxy, with low-surface brightness emis-
0 This paper presents a reexamination of an archival Hub-
sion extending out to ∼ 100 kpc, and its disk was found
7 ble SpaceTelescope (HST) image of Malin 1, which reveals
to have an extrapolated central surface brightness of only
0 µ (V)≈25.5 mag arcsec- 2, far fainter than any galaxy pre- thatthe galaxy’sopticalmorphologyis rather differentfrom
h/ vi0ously known. The exponential scale length of this disk whathadpreviouslybeendeterminedbasedonground-based
p was determined to be ∼ 70 kpc. Despite the faint surface imaging. It is shown that Malin 1 actually contains a fairly
- brightness of its disk, Malin 1 is a massive galaxy with a normalstellardisktypicalofanearly-typebarredspiral,and
ro totalopticalluminosityof MV ≈- 22.9mag (Pickeringetal. tihsfayttbhaesestdanodnatrhdecprirteesreianfcoerobfetihnigscdlaissksiMfieadlians1andLoeSsBngoatlsaaxty-
t 1997). Based on these properties, it is consideredthe proto-
s despite its having the most extended low surface brightness
a typical giant low surface brightness (LSB) galaxy. Even in
outer regions of any known disk galaxy. This is not merely
: comparison with other giant LSB galaxies discovered sub-
v a semantic distinction, as this new observation significantly
sequently (Bothunetal. 1990; Sprayberryetal. 1993, 1995;
i truncates the range of parameter space (in terms of central
X Beijersbergenetal. 1999), the properties of Malin 1 are un-
surfacebrightnessandscalelength)populatedbyrealgalaxy
usual; no other galaxy disk has been found with a surface
r
disksbyremovinganextremelyaberrantdatapointfromthe
a brightnessand scale length that approachthese extremeval-
observed galaxy population. A new optical spectrum of the
ues. Furthermore, it is one of the most gas-rich galaxies
known, with an HI mass of ∼5×1010 M⊙ (Pickeringetal. galaxyisalsousedtoderiveitsbulgevelocitydispersionand
toreexaminetheclassificationofitsactivenucleus.
1997;Matthews,vanDriel,&Monnier-Ragaigne2001). The
Malin1hasaredshiftof0.0825(Pickeringetal.1997),cor-
HIrotationcurvemeasuredbyPickeringetal.(1997)reaches
anasymptoticvalueofv =190kms- 1andextendstoradiias respondingto a luminosity distance of DL =370 Mpc and a
r projectedangularscaleof1.53kpcarcsec- 1 foracosmology
large as 110 kpc fromthe galaxycenter, implyinga dynam-
ical mass of order 1012 M although the disk inclination is withH0=71kms- 1 Mpc- 1, ΩM =0.27,andΩΛ=0.73. For
⊙
consistency,observedquantitiestakenfromtheliteratureand
uncertainandthevelocitystructureshowssomewarpingand
listedinthispaperhavebeenrescaledtothiscosmology.
noncircularmotions.
Theimplicationsofthisobjectforunderstandingtheover-
allpopulationofdiskgalaxiesaresignificant.Asemphasized 2. ARCHIVALDATAANDOBSERVATIONS
by Bothunetal. (1997), galaxies like Malin 1 would likely ImagesofMalin1wereobtainedwiththeWFPC2camera
beunderrepresentedinmagnitude-limitedgalaxysurveysbe- aspartofHSTprogramGO-5946(PI:Impey)on1996March
cause of surface brightness selection effects. Even if such 10.ThegalaxywasobservedforoneorbiteachintheF300W
extremegiantLSBgalaxiesarerare,whichappearstobethe (U-band)andF814W(I-band)filters,withthegalaxyplaced
case(e.g.,Minchinetal.2004),thepropertiesofMalin1pose onthe WF3 CCD nearthecenterofthefullWFPC2 field of
interestingchallengestotheoriesofgalaxyformation. Mod- view. Eachorbitwassplitintofourindividualexposuresfor
elsofdiskformationincolddarkmatterhalosarenoteasily
abletoreproducethepropertiesofsuchenormouslyextended 1 Basedonobservations madewiththeNASA/ESAHubbleSpaceTele-
disks, although scenarios have been proposed which might scope, obtained fromtheDataArchive attheSpaceTelescope ScienceIn-
stitute,whichisoperatedbytheAssociationofUniversitiesforResearchin
account for the formation of Malin 1-type galaxies as rare
Astronomy,Inc.,underNASAcontractNAS5-26555.
2 BARTH
removalofcosmic-rayhits. TheF300Wexposureswereread pointsourcecomponentwasaddedtoallowforthepossibility
out with 2×2 pixel binning on the CCD, while the F814W of unresolved emission from the active nucleus. The point-
images were read out in the standard unbinnedmode. Total spreadfunctionfortheF814Wfilterwasgeneratedusingthe
exposure times were 2100 s in each filter. This dataset was TinyTimsoftwarepackage(Krist1993).
previouslyanalyzedbyO’Neiletal.(2000),whostudiedthe Figure2 showsthe results of the GALFIT modeling. The
propertiesoffaintgalaxiesinthesameWFPC2fieldasMalin structure of the galaxy is reproduced well overall, although
1,buttheydidnotdiscussthestructureofMalin1itself. therearesystematicresidualsaroundthebulgeandinnerbar
Theimageswereretrievedfromthe HST data archiveand region. These residuals could be reduced somewhat if the
individualexposureswere combined using standard tasks in constraintontheSérsic indexofthe barwererelaxed,atthe
IRAF/STSDAS2. Figure 1 shows a portion of the F814W expenseof havingan unrealisticbar profilewith a verycen-
WFPC2 mosaic centered on Malin 1. An image of the en- trally peaked structure. However, whether the bar profile is
tire WFPC2 mosaic is shown by O’Neiletal. (2000). If the constrainedor notin the fit hasalmost no effecton the cen-
fullHST imageisviewedwithextremelyhighcontrast,some tralsurfacebrightnessorscalelengthofthediskcomponent,
of the very low surface brightness outer spiral structure can sincethediskparametersaremainlydeterminedfromthere-
justbarelybeseenatradiiofupto∼1′fromthegalaxycen- gionaroundr=4′′- 6′′whereitdominatesthegalaxyprofile.
ter, and the faint spiral features match the structures visible Figure3showsthegalaxy’sradialprofileasmeasuredfrom
in Fig. 2 of Bothunetal. (1987). However, the S/N in the the WFPC2 image and from the GALFIT model. The ra-
WFPC2imageattheselargeradiiistoolowtoperformuse- dial profiles were measured using the IRAF task ELLIPSE,
fulmeasurementsofthesurfacebrightnessoftheouterdisk. whichfitsellipticalisophotestotheimageataseriesoffixed
TheF300WimagealsohasverylowS/Nandwillnotbedis- semimajor axis lengths, following the methods described
cussedfurtherinthispaper. by Jedrzejewski (1987). Conversion from HST F814W fil-
AnewopticalspectrumofMalin1wasobtainedattheKeck ter magnitudesto standardCousinsI-bandVega-basedmag-
II telescope on 2005 May 16 UT with the ESI spectrograph nitudes was done using the SYNPHOT package in IRAF,
(Sheinisetal. 2002). The exposure time was 1800 s with a assuming an S0-type spectrum from the spectral atlas of
0.′′75-wide slit, yielding a spectrum with R≈6000 over the Kinneyetal. (1996). The brightness profiles include a cor-
range 3800–10000Å and S/N ≈20 per pixel in the contin- rectionforGalacticextinction(AI=0.067mag;Schlegeletal.
uumat5100Å.Theslitwasorientedattheparallacticangle 1998),aK-correctionof0.08magfortheI band,determined
for the midpointof the observation. Spectralextraction was using the Kinneyetal. (1996) S0 template spectrum, and a
doneusinga1′′ width,andtheextractedechelleorderswere correction for cosmological surface brightness dimming of
wavelength-calibrated using observations of HgNe, Xe, and 0.34 mag. Brightness profiles derived from 0.′′2-wide slices
CuAr lamps and flux-calibrated using an observation of the alongthemajorandminoraxesofthebarareshowninFig-
standardstarBD+28◦4211.Thetenindividualechelleorders ure4. Asidefromsmalldeviationsinthediskduetothespi-
were combinedinto a single spectrum, weightedby the S/N ralarms,themajorandminoraxisbrightnessprofilesarevery
in the overlapregionsbetween orders. SeveralK-giantstars symmetricaboutthenucleus.
werealsoobservedonthesamenightduringtwilightforuse Anotherissuetoconsideristheeffectofinclinationonthe
asvelocitytemplates. disksurfacebrightness.Onesimplemethodthatiscommonly
usedtocorrectforinclinationistoapplytherelationµ
corrected
3. MEASUREMENTSANDRESULTS = µobserved- 2.5Clog(a/b), where a and b are the major and
minor axis lengths of the disk, andC is a parameter whose
3.1. Imaging
valuerangesfrom0foranopticallythickdiskto1fortheop-
TheWFPC2imageclearlyrevealsthemorphologyofMalin ticallythincase(deJong1996;Seigar&James1998). From
1atsmallangularscaleswherepreviousground-basedimages theGALFITdecomposition,thediskaxisratiob/ais0.858,
wereunabletodiscernthedetailsofitsstructure. Thegalaxy consistentwiththemeanellipticityofǫ=1- (b/a)=0.14mea-
is seen to have a compactbulge dominatingthe lightprofile suredbytheELLIPSEroutineoverthedisk-dominatedregion
outtor≈1′′,abaroflength∼6′′,andanearlyface-ondisk fromr=4 to6arcsec. Thiscorrespondstoa surfacebright-
with a hintofspiralstructurethatcanbe tracedoutto∼6- nesscorrectionof- 0.17magintheopticallythincase. Given
7′′. The disk morphology appears smooth and there are no the approximatenature of this methodand the small magni-
obviousdust lanes, knots, or star-formingcomplexes. Apart tudeofthecorrection,wechoosenottoapplythiscorrection
fromtheremarkablebutnearlyinvisibleouterLSBstructure, tothemeasuredquantities,butweconsideritasaminoreffect
Malin1hasthemorphologyofanSB0/agalaxy. thatwouldtendtoincreasethecentralsurfacebrightnessbya
To decompose the galaxy’s structure into its subcompo- smallamount.
nents,modelfitstotheWFPC2imagewereperformedusing ThestructuralparametersderivedfromtheGALFITmodel-
the2-dimensionalfittingpackageGALFIT(Pengetal.2002). ingarelistedinTable1. Whiletheformalfittinguncertainties
Thecomponentsusedinthemodelfitincludedanexponential fromGALFITarenegligiblysmall,theactualuncertaintieson
disk, a Sérsic-lawbulge, anda bar. Since GALFITdoesnot the derived model parameters are predominantly systematic
includebarmodelssuchasaFreeman(1966)baroraflatbar duetorealdeviationsofthegalaxycomponentsfromthesim-
(Prietoetal. 1997), the bar componentwas modeledusinga ple fitting functions used by GALFIT and are therefore dif-
Sérsicprofilewiththeindexnconstrainedtobe≤0.5toap- ficult to estimate. Trial GALFIT runs with slightly different
proximatea bar with a flat core, and with this constraintthe fitting models (i.e., without constraints on the bar Sérsic in-
barhasn=0.5inthebest-fittingmodel.Inaddition,acentral dexnorwithoutincludingapointsourcecomponent)yielded
magnitudedifferencesof order ∼0.2 mag for the bulge and
2 IRAFisdistributed bytheNational OpticalAstronomyObservatories, bar componentsrelative to the best-fit model listed in Table
whichareoperated bytheAssociation ofUniversities forResearch inAs-
1,whichgivessomeindicationofthelikelysystematicuncer-
tronomy,Inc.,undercooperativeagreementwiththeNationalScienceFoun-
dation. taintiesinthedecomposition. Differentchoicesofbulgeand
NORMALSTELLARDISKINMALIN1 3
barmodelsoromissionofthenuclearpointsourceledtorel- thebulge, bar, andnormaldiskof thegalaxy. Since thesur-
atively small changes in the exponential disk parameters, at facebrightnessofthediskwithinseveralarcsecondsofthenu-
thelevelof<0.1magintotalmagnitudeand<5%inscale cleusismuchfainterthanthesurfacebrightnessofthebulge
length. orbar,itisnotsurprisingthatitwouldbedifficulttorecognize
thediskinground-basedimagingofaverageseeing. Interest-
3.2. Spectroscopy
ingly, Bothunetal. (1987) note that the nucleus “appears to
AspreviouslyshownbyImpey&Bothun(1989),thebulge besurroundedbyextendednebulosityofsomewhathighsur-
spectrumisconsistentwithapredominantlyoldstellarpopu- face brightness”. In retrospect, this “nebulosity” must have
lation. The stellar velocitydispersionofthe bulgewasmea- beenstarlightfromthediskitself.
suredfromthe Keck spectrumby directfitting ofbroadened RecentworkbyAguerrietal.(2005)ontheI-bandphoto-
anddilutedstellarspectrainthewavelengthdomain,follow- metricpropertiesofSB0galaxiesprovidesanidealcompari-
ing the methods described by Barthetal. (2002). The fit sonsampletoexaminethestructureofMalin1inthecontext
wasperformedovertherestwavelengthrange5210–5480Å, ofgalaxiesofsimilarHubbletype. Basedonasampleof14
whichcontainsFeλ5270andseveralotherreasonablystrong nearbygalaxies,theyfoundthatSB0galaxiesgenerallyhave
features. TheMgblineswereexcludedfromthefitsincethe nearlyexponentialbulgeprofileswithn=1.48±0.16andre
galaxyspectrumhasalowerFe/αabundanceratiothantypical intherange0.3to1.0kpcwith ameanof0.6kpc,so Malin
nearbyKgiantstars,asisgenerallyfoundforhigh-dispersion 1’s bulge properties are typical for its Hubble type. With a
galaxies. Weak [NI] λ5200 emission appears to be present bulgeabsolutemagnitudeofMI=- 20.9magandσ=196km
as well. Fits were performedusing nine template stars with s- 1,Malin1alsofallswithinthenormalrangeofSB0galax-
spectral types between G8 III and K3 III were. The best fit iesintheFaber-Jacksonrelation(seeFig. 5ofAguerrietal.
wasfoundwithaK0star,givingσ=196±15kms- 1,where 2005).
the final uncertainty is the sum in quadrature of the fitting ThediskofMalin1alsohaspropertiessimilartotheSB0
uncertaintyfromthebest-fittingtemplate(10kms- 1)andthe sample. Figure 7 plots the central I-band surface brightness
standarddeviationoftheresultsfromfittingallninetemplates against disk scale length. While Malin 1 falls toward the
(11kms- 1). Figure6showsthefitofthebroadenedKOstar lower end of the sample in terms of its surface brightness,
tothespectrumofMalin1overthisregion. it is still consistent with the normal range for this Hubble
Thevelocitydispersionfittingcodewasalsousedtogener- type. The only aspect of Malin 1’s structure that appears
atestarlight-subtractedspectraintheregionssurroundingthe different from the SB0 sample is the ratio of bulge to disk
HβandHαemissionlines. Usingthebest-fittingK-typestel- radius. Aguerrietal. (2005) find a surprisingly tight cou-
lartemplateandallowingforafeaturelesscontinuumdilution pling with re/h = 0.20±0.01 for their sample, while Ma-
yieldedanadequatecontinuumsubtractionovertheseregions, lin 1 has re/h=0.13. A portion of this difference could be
givingapureemission-linespectrum(Figure5). the result of the different bar model adopted for the GAL-
Reddening-corrected emission line flux ratios measured FITanalysis,however,sinceAguerrietal.(2005)usedFree-
from the nuclear spectrum are [OII] λ3727 / [OIII] λ5007 manbarorflatbarmodelsintheir1-dimensionalradialpro-
=3.1,[OIII]λ5007/Hβ =1.9,[OI]λ6300/Hα=0.3,and file fits, anddifferentassumptionsforthe barprofilewill di-
[NII] λ6584 / Hα = 0.85. These measurements are consis- rectly affect the flux inferred to arise from the outer part of
tent with a LINER classification (Heckman 1980; Hoetal. the bulge. In any case, a bulge-to-disk scale length ratio of
1997a). Impey&Bothun(1989)hadpreviouslyfound[OIII] 0.13is well within the normalrange forlow or highsurface
/Hβ =4.8andclassifiedMalin1asahigh-excitationSeyfert brightnessspiralgalaxiesofarangeofHubbletypes(deJong
galaxy; presumablythis resulted fromnothavingperformed 1996;Seigar&James1998;Beijersbergenetal.1999).Over-
a starlight subtractionand thereforemissing most of the Hβ all, these results indicate that out to r≈10 kpc, the optical
flux. Approximately30%ofS0–SagalaxiescontainLINER structureofMalin1doesnotdeviateverysignificantlyfrom
nuclei(Hoetal.1997b)soMalin1isnotunusualinexhibit- thegeneralpopulationofearly-typebarredgalaxies.
ing this type of activity. There is at best weak evidence for The low-surfacebrightnessstructureat largeradiiappears
a broad component to the Hα emission line. Model fits in- to be a photometrically distinct component of the galaxy’s
cluding3Gaussiancomponentsforthethreenarrowemission structurerather than a smoothextensionof the normalinner
linesaresomewhatimprovedbytheadditionofabroadcom- disk. Foradiskscalelengthof4.8kpc,theextrapolatedsur-
ponent (having a best-fitting width of FWHM = 2010±80 facebrightnessoftheinnerdiskatr=50′′(equivalentto∼75
kms- 1),buttherelativelylowS/Nofthespectrumprecludes kpc)wouldbeundetectablyfaint: 36.1magarcsec- 2 intheB
anydefinitiveconclusionsregardingthepossiblepresenceof band,whichisfarfainterthantheactualouterdiskbrightness
abroad-linecomponent. of µV ≈27 mag arcsec- 2 at this radius(Bothunetal. 1987).
There may be a break in the exponentiallight profile of the
4. DISCUSSION disk at radii greater than ∼7′′, but deeper images would be
The measurementsdescribed above indicate a very differ- neededtotracethesurfacebrightnessprofileinthetransition
entstructureforMalin1thanthatwhichhaspreviouslybeen regionbetweentheinnerdiskandtheextendedouterdisk.
found.Bothunetal.(1987)fitthegalaxyprofilewithamodel GiventhepresenceofthisnewlyidentifieddiskinMalin1,
consistingof an r1/4-law bulge and an exponentialdisk, and shoulditstillbeconsideredanLSBgalaxy?LSBgalaxiesare
determined that the bulge had r = 2.′′9±0.′′5, which cor- generallyclassifiedassuchbasedontheextrapolatedcentral
e
responds to 4.4 kpc for the cosmology assumed in this pa- surfacebrightnessoftheir disk components. A crucialpoint
per. Pickeringetal. (1997) foundsimilar resultsfromnewer is that the LSB classification applies only to the disk com-
ground-basedimaging data. The HST image shows that the ponent. Giant LSB disk galaxies have high surface bright-
bulge radius is smaller than this value by nearly an order of nessbulgesthatdominatethelightofthecentralregions(e.g.,
magnitude,anditseemslikelythatthebulgeregiondescribed Sprayberryetal.1995). Thus,thetotallightprofilesofgiant
byBothunetal.(1987)wasactuallythecombinedlightfrom LSBgalaxiesapparentlyalwayshavehighsurfacebrightness
4 BARTH
centralregions,andaradialprofiledecompositionisrequired Bothunetal.(1987)arefaroutsidetherangesfoundfornor-
in order to determine whether a galaxy should be classified malspiralsorevenforothergiantLSBgalaxies. Inthegiant
as having a giantLSB disk or not. Clearly, to performsuch LSBsampleofSprayberryetal.(1995),forexample,thedisk
classifications in a meaningfulway, observations with suffi- scalelengthsrangefrom5to13kpcandthefaintestdiskhas
cientspatialresolutiontodecomposethebulgeandinnerdisk µ (0)=24.2mag arcsec- 2. Clearly, however,whenthe nor-
B
componentsaccuratelyarerequired. mal disk of Malin 1 is comparedwith the disk propertiesof
While there is perhapsno universalagreementon the sur- other galaxies in these diagrams, it is no longer an outlier.
facebrightnessthresholdforagalaxydisktobeconsideredan It might be argued that the extended outer structure in Ma-
LSBdisk,acommoncriterionisacentralsurfacebrightness lin1stillhastheformofadiskwithaverylowextrapolated
fainterthanµ =23.0magarcsec- 2 (Impey&Bothun1997). centralsurfacebrightnessandlargescalelengthandthatthis
B
AsnotedbyBothunetal.(1997),agalaxywithacentralsur- extended disk remains an outlier relative to all other known
facebrightnessfainterthanthislevelwouldrepresenta>4σ galaxies. However, it seems morereasonable and consistent
deviationfromthedistributionofsurfacebrightnessfoundby tocomparethenormal,centraldiskofMalin1withthoseof
Freeman (1970) and would therefore be a very unusualout- othergalaxies, andto considerits extendedouter diskin the
lieriftheactualdistributionofdisksurfacebrightnesseswere contextoftheextremeouterdisksofothernearbyspiralgalax-
describedbyFreeman’slaw. To comparethediskproperties ies,whichsometimesdisplaystructuressimilarto(albeitless
ofMalin1withthiscriterion,theBmagnitudeoftheMalin1 extendedthan)thatofMalin1.
diskwasestimatedbyperformingsyntheticphotometryonthe Recent work has revealed a great deal of detail and a
S0 templatespectrumfromKinneyetal. (1996). The(B- I) diversity of properties among the extreme outer disk re-
colorindexforthisspectrumis2.2mag.Assumingthis(B- I) gionsofsomenearbyspiralgalaxies. Deepnarrow-bandob-
colorforthediskofMalin1,ithasanestimatedµ0(B)=22.3 servations of some late-type spirals have detected H II re-
magarcsec- 2,whichwouldnotqualifyitasanLSBdisk3. In gions far beyond the galaxies’ optical radii (Fergusonetal.
theclassificationsystemofMcGaugh(1996),Malin1would 1998b; Lelièvre&Roy 2000), and GALEX observations of
havean“intermediatesurfacebrightness”disk. the nearby spiral M83 have detected UV-bright stars in the
Another related criterion that has been applied to distin- outerdiskatdistancesofgreaterthan20kpcfromthegalaxy
guish low and high surface brightness galaxies is a “dif- center(Thilkeretal.2005). Intheseexamples,theregionsof
fusenessindex”forthedisk,whichcombinescentralsurface recent star formation have filamentary, spiral-like structures
brightnessandscale lengthintoa single parameter. Thedif- withanappearancesimilartotheouterdiskregionsofMalin
fuseness index criterion given by Sprayberryetal. (1995) is 1. Fergusonetal.(1998a)notedthatthedistantouterregions
µ (0)+5log(h)>27.0 for a galaxy to be classified as a gi- ofspiraldiskshave physicalproperties(suchas low column
B
ant LSB disk. Based on the GALFIT results and the as- densitiesofgasandhighgasmasstostellarmassratios)that
sumed(B- I)colorindexof2.2mag,thediskinMalin1has are very similar to the inferred properties of Malin 1. It is
µ (0)+5log(h)=25.0,whichlieswellinsidethehighsurface nowapparentthatthissimilarityisnotmerelyacoincidence–
B
brightnessrangeforthisparameter.Thus,basedoneithercen- the parameters previously found for Malin 1 were precisely
tralsurfacebrightnessordiffuseness,thediskofMalin1out those of its extreme outer disk region. Extended outer disk
tor≈10kpcshouldnotbeconsideredanLSBdiskaccording structurehasrecentlybeenfoundouttor≈40kpcinM31as
toconventionalcriteria. well (Fergusonetal. 2002; Ibataetal. 2005), but in the case
Malin1stillmightbeconsideredanLSBgalaxybasedon of M31 the extended outer structure appears to be a smooth
theaveragesurfacebrightnessoveritsentirediskarea, how- continuation of the exponential profile of the main disk, in
ever. Accordingto Pickeringetal. (1997), the galaxy’stotal contrast to the photometrically decoupled outer structure of
luminosityisM =- 22.9±0.4mag. TheHST measurements Malin 1. The stellar disk of the late-type spiral NGC 300
V
yield M =- 22.4mag for the combinedbulge, bar, and nor- also follows a single exponential profile from its inner re-
I
maldiskcomponents. Assuming(V- I)=1.3magforanS0 gions to the outer disk at distances up to 10 scale lengths
galaxy as determined from SYNPHOT, this corresponds to (Bland-Hawthornetal.2005).
M =- 21.1mag.Thus,thecomparisonbetweentheHSTand New observations of early-type barred galaxies
V
ground-basedmeasurementsindicatesthatthemajorityofthe have revealed surprising outer disk structures as well.
galaxy’stotalopticalluminosityliesintheextendeddiskre- Erwin,Beckman,&Pohlen (2005) have found that at least
gion beyond the normaldisk. It would be useful to confirm 25%of SB0-SBb galaxieshave photometricprofilescharac-
this result with new ground-based imaging data and a sin- terizedbyanouterexponentialdiskcomponentwithalarger
gle profile decompositionover the entire radial range of the scalelengththantheinner,maindiskcomponent.Thetransi-
galaxy;theHST imagecouldbeusedtoexcludebackground tion from the inner to outer disk occurs at 3–6 scale lengths
galaxiesandimprovethemeasurementaccuracyfortheouter of the inner disk and at surface brightnesses of 22.6–25.6
diskbrightnessprofile. mag arcsec- 2, and spiral structure is often seen in the outer
Incomparisonwithallotherknowndiskgalaxies,Malin1 disk region. Erwin etal. callsuch structures“antitruncated”
has always appeared as a unique and distant outlier in plots disks. Malin 1 appears to fit within this category, although
of central surface brightness against disk scale length (see, deeperimageswouldbeneededtodeterminethestructureof
e.g.,Bothunetal.1987;Sprayberryetal.1995;Bothunetal. thetransitionregionbetweentheinnerandouterdisks.
1997; Dalcantonetal. 1997; vandenBosch 1998). The Simulationsby Peñarrubiaetal. (2006) have exploredone
previously measured disk scale length and central surface possible mechanism that could form such highly extended
brightness (h ≈ 70 kpc; µ (V) = 25.5 mag arcsec- 2) from outer disks, by the tidal shredding of dwarf galaxies in the
0
outskirtsofanM31-sizedgalaxy.Withaninitialapocenterof
3Forcomparison,Fukugitaetal.(1995)findanaverage(B- I)colorof2.0 r=75kpc,thedwarfsatellitesweretidallydistortedintoex-
magforS0galaxies;usingthisvalueinsteadoftheSYNPHOTcolorwould tendeddiskshavingspiral-likefilamentarystructure,extend-
yieldµ0(B)=22.1magarcsec- 2. ingtoover100kpcfromthecenterofthelargergalaxy. The
NORMALSTELLARDISKINMALIN1 5
tidaldebristendsto evolve into a nearly exponentialprofile, anormalearly-typebarredspiralgalaxy,andthecentralsur-
withscalelengthsofupto50kpc,dependingonthecompact- face brightness of its disk is ∼ 4 magnitudes brighter than
nessoftheoriginalstellardistributioninthedwarfgalaxy. the value originally found by Bothunetal. (1987). Thus, it
CouldothergiantLSBgalaxiespossessinner,highsurface seemsreasonabletoconsiderMalin1notasanLSB galaxy,
brightness disks that have gone undetected? Several of the butratherasagalaxywiththenormalstructuralcomponents
otherknowngiantLSBgalaxieslieatdistancescomparableto ofanearly-typebarredspiral,whichisembeddedinaremark-
thatofMalin1(e.g.,Sprayberryetal.1995),andinadequate ablyextended,opticallyfaint,andgas-richouterstructurebe-
spatialresolutioncouldcauseanormaldisktobemissedina yonditsnormaldisk.
bulge-diskdecomposition. Thiswouldbeanaturalpitfallof Bothunetal.(1997)remarkthatgiantLSBgalaxiessuchas
fittingasimple2-componentmodeltoagalaxywithabright Malin1 are “trulyenigmaticin that’normal’formationpro-
centralregion(includingabulgeanddisk)andaverydiffuse cesseswereatworktocreatethebulgecomponentbutnocon-
and extended outer disk that was brighter than a simple ex- spicuousstellardiskeverformedaroundthisbulge.” Malin1
tensionoftheinnerdiskprofileatlargeradii. Theouterdisk isperhapsasomewhatlessenigmaticgalaxyonceitsnormal
woulddominatethefitatlargeradiiandthereforedetermine diskhasbeenrecognized.
the scale length and central surface brightness of the expo-
nentialdiskcomponentinthe fit, andthe inner,highsurface
brightness disk, if present, would be subsumed in the outer I am grateful to Joe Shields and Karen O’Neil for discus-
partofthebulgeprofile. Deep, high-resolutionimagingofa sions thatmotivated this work and to Luis Ho, Marc Seigar,
representativesampleofgiantLSBgalaxieswouldbeuseful Peter Erwin, Chien Peng, and the anonymous referee for
toclarifythenatureoftheseobjects. manyhelpfulsuggestions. SupportfortheKeckobservations
wasgenerouslyprovidedbytheUCIrvinePhysicalSciences
5. CONCLUSIONS Innovation Fund. The author wishes to recognize and ac-
ThestellardiskofMalin1haslongbeenthoughttobean knowledgetheverysignificantculturalroleandreverencethat
extreme and distant outlier compared to the disk properties thesummitofMaunaKea hasalwayshadwithintheindige-
of all other known spiral galaxies. The archival HST data nousHawaiiancommunity.Wearemostfortunatetohavethe
demonstrates that the central structure of Malin 1 is that of opportunitytoconductobservationsfromthismountain.
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6 BARTH
TABLE1
STRUCTURALPROPERTIESOFMALIN1.
Parameter Value
Bulgere 0.6kpc
BulgeSérsicindexn 1.24
BulgeImagnitude 17.0mag
BarImagnitude 17.6mag
PointsourceImagnitude 20.6mag
Diskµ0(I) 20.1magarcsec- 2
Diskscalelengthh 4.8kpc
Bulgevelocitydispersion 196±15kms- 1
NOTE.—PropertieslistedherearedeterminedfromtheGALFIT
decompositionoftheWFPC2imageandfromtheKeckspectrum.
Themagnitudes listed aboveinclude acorrection forGalactic ex-
tinctionof0.067mag.Additionally,aK-correctionof0.08magand
acorrectionforcosmologicalsurfacebrightnessdimminghavebeen
appliedtothevalueofthediskcentralsurfacebrightnessµ0(I).
NORMALSTELLARDISKINMALIN1 7
FIG. 1.—InnerportionoftheHSTWFPC2F814WmosaicofMalin1,displayedusingalogarithmicstretch,showingthebulge,bar,anddiskcomponents.
ThefaintnoisyverticalandhorizontallinesaretheboundariesbetweentheWFPC2CCDs.
8 BARTH
FIG.2.—GALFITdecompositionofMalin1.Leftpanel:A20′′×20′′portionoftheWFPC2image.Middlepanel:GALFITmodelincludingbulge,bar,and
diskcomponents.Rightpanel:Residualsaftersubtractionofgalaxymodel.
NORMALSTELLARDISKINMALIN1 9
FIG.3.—I-bandsurfacebrightnessprofilefortheinnerregionofMalin1(crosses),measuredwiththeIRAFELLIPSEtask.Modelcomponentsarethecentral
pointsource(dot-dashedcurve),bulge(dotted),bar(shortdashed),anddisk(long-dashed),withthetotalgalaxymodelplottedasasolidline. Thelowerpanel
displaystheellipticityprofileforthegalaxyandfortheGALFITmodel.
10 BARTH
FIG. 4.—Surfacebrightness cutsalongthemajorandminoraxes ofthebar(upperandlowerpanels, repsectively). Crossesrepresent measuredsurface
brightnessaveragedovera2pixel-wideextractionthroughtheimage. Modelcomponentsarethecentralpointsource(dot-dashedcurve),bulge(dotted),bar
(shortdashed),anddisk(long-dashed),withthetotalgalaxymodelplottedasasolidline.