First Hubble Space Telescope observations of the brightest stars in the Virgo galaxy M100 = NGC 4321

Abstract

As part of both the Early Release Observations from the Hubble Space Telescope (HST) and the Key Project on the Extragalactic Distance Scale, we have obtained multiwavelength BVR Wide Field/Planetary Camera-2 (WFPC2) images for the face-on Virgo cluster spiral galaxy M100 = NGC 4321. We report here preliminary results from those observations, in the form of a color-magnitude diagram for approximately 11,500 stars down to V approximately 27 mag and a luminosity function for the brightest blue stars which is found to have a slope of 0.7, in excellent agreement with previous results obtained for significantly nearer galaxies. With the increased resolution now available using WFPC2, the number of galaxies in which we can directly measure Population I stars and thereby quantify the recent evolution, as well as test stellar evolution theory, has dramatically increased by at least a factor of 100. Finally, we find that the stars are present in M100 at the colors and luminosities expected for the brightest Cepheid variables in galaxies.

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I N ‘1’}110 V I I{.(JO C; AI,AXY M]()() :.- NG<; 4321a
W} ’;N1)Y 1,. I“ILI’:l’;I) MAN] , }IAILIW II’. h4 Al )OIO;2, ]’]I;’J’]I;l{ 11. S’J’II;’J’SON 3
SIIAUN M. G, llUGllltS4, JON A, llol/’J’Xh4AN5, .lI’:IU;MY. IL. MolJl,l)G
JOIIN ‘J’. ‘J’lLAlJGlH17, JOIIN S. GA I, I, AG}l I’;IL 11112, GI1,l)A l;. II AI,l,I:S”J’}IHL6
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813 Santa IIarbara. St.
l’asadmla, California 91101
a IIasd o]i c}bsc]vaiions wit]] t.lIe N A S A /lIXA IIvbbk Space ?kksco])c:,
‘J’dcsc.o])c Sc.iencc institute, wllid~ is o])c!ratd by AU ILA, IIIC.
5-?6555.
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ulldcr NASA Co]ltract No. NTAS
] ‘J’lIe observatories, (kncg;ic lllst.itut.ioli of Washinp;tclll, l’asadc)la, CA 91101
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ll~stitutc of ‘J’cc.lino]ogy, l’asadcna, (;A 91125
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IIlfrard l’rocmsing; and Analysis CCIItCI, ( C a l i f o r n i a
{ l)ominion Astrophysical Obscrvat.m-y, 5071 W. Saanidi l{d., VictoIia 11(; V8X 4M(i
4 1{.oyal (;remlwicll observatory, Madi]lglcy l{oacl, Calnbridgc, UK, C113 OI;Z
“ l,owd] Olxmvatory, Mars 11 ill Noa.d, l’lag;stafl’, AZ 86001

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~ 11<$$(), WcStc)ll ]). ()., A(:’1’ ’2611, Australia
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s li]liversityof h4iclliga~l, l)ept. of Astrmloll)y, l)c.]lllisol] IIldg., AI III A]])o], hll 48109-1090
“ sp:icc’’J’clcscopc scicllcc’ ll)stitlltc, 3700 s:lll Marti]l l)l’iYc, llalti II){) I”c, AJl) ?1218
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0 lkq)t o f l)hys; cs (SL Ast]cmo]ny, lh)]nbcr~ :)01, JC)IIIIS ]]o])k; IIs Univ., 3400 N. C’lIaI-lcs St.,
‘2 I’llivcrsity of WiscoIisi]I, M~disc)ll, l)c])t. of Astlollo]lly, 475 N. (;lla]te] St., Madison. \\] .5370G
IS I)cl)al. tIllcll~ of ‘J ICH-INtIial h4a~; nctism,Carllq;ic l~lstitutioll of ll~ashill~,toli, d ’241 IIroad lll-alICl I
1{(1. S.}V., Washington 1).(;. 20015
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]5 }]a, ,,a](l (:c)ll(’gy!,(kilter fc)r Astro])llysic.s, 6( I (;ardcn Stlcct Callll)rid~;e, h4A 0213S
16 $tcward obscnatory, U]livmsity of Arizolia., ‘J’UCSOII, AX 85721
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to v
Slc)pc
\Vith
w2?7 ma?; md a lu]ninosity fu]lc.tion for tltc Lriglltcst blue stars wllicll is fcJulld tc) have a
of O.7, ill cxcdlcnt a~;rcc]nwt with ]jrcvious results obtai]lcd for sir,] lific.ant]y ]Iealcr $;alaxics.
tllc illc.rcascxl resolution now availab]c using WIIII’C2, tllc ]Iu]nbcr of galaxies ill l~llicll we can
directly measure l’o])ulatio]l 1 stars slid tlIerclJy quantify the recent cvc)lutioll, as well as tcsl, stellar
evolution theory, has dralna.tically illc.rcascd I)y at lust a factor of 100. l“inal]y, wc fiIId that stars
arc })rescnt in h41 00 at tllc colors aIid lulnillosities cx])ectcd fc)r the bri~lltmt Cc])lleid varial)lcs ill
p,alaxics.
S’Ubjcct }Lcadings: g;alaxies: distances - ?;alayics: individual
Cephcids - stars: 011 stars
MIOO : NGC 4 3 2 1 ) s t a r s :

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:1
1. IN’J’1{.OI) IJ(:’J’ION”
Afl 100= NC; (; 4311 ) is alunlinous sl}iral ~;al:isy ill tllc \ri IgO cluster tliat is scelI lIe:i Ily f:iceoll,
iritl~ am a])]) aleIIt axial ratio of 0.87; ‘1’u]ly 1988). IIavinc very w’ell-ddi]lcd Sy)[llnct[ic s])ir:il aIII)s>
it is classified l)y SaIIdagc and ‘J’a]Ill Il;iIIII (1981) as Se(s) 1 and by de Vaucoulcurs ct 01. (1916) its
S,\ll(s)l)c. l’m]n tllc ground this p;alaxyis IIot }+’cll-]c~sc)ll’c(l, al]d little wits })lcviously k]low]l of its
stc,llar ]jo])ulatiolls.
As ]m]t of the
phase.s C)f WI’’I’(:7,
F;clrly ]klcasc Obscrw/t20tL }}rogram associatd w i t h tile i]litial collll[lissic)Iiill~;
h410(l was illlap)d in tllrcc filters a]ld at two epochs.
‘J’lle ~)u IJ)osc of these
ol)scrvations was to ])rovidc bot]l a test c~f tllc C.orrectivc’ o])tics and a fcasi})ility study of Vir~,o
cluster okmvations for tllc Extmgalactic l)istoncc S’cak Kcy ]’rojcct. ‘J’l Ie Key l’rc)jcct. was begu II
in l)CC.CIIIIMN 1992 using WII’/l)C (WI I’1’C1 ) to observe 1,1 Ie ILmIby galaxy h481; a clist:iltcc’ to that
o~]~xy l)as~d 011 t]l~ ]] S’J’ djSC.OVC!I.y of 3(I IICW (;clj]lcids ]Ias Icc.cIItly bccq I ])ublishcd l)J’ l’rccd~lnall Ct
<> .
(Il. 1994. ‘J’IIc c.alibratio]l })IC)CCXIUIC aIICl a. discussion of tlic stdlar ]Jo]julatiolts was :,i~’cll ill llugl Ies
ct al. 1994. ‘J’hc ])riloary ai~[l of tllc l{cy l’rojcct is to ]nca.sure Ce])llcid dista]lcm to about tlvo
dOZCII ncmrhy galaxies useful for c.alibratillg a nu]nbcw of scc.olldary distallc.c lnctllods. 111 addition,
the l{cy l’reject tarp;ct list includes 4 galaxiesi]l tllc Virgo c.lustm, ollc of wllic.h is h4100.
A ~vcll-clctcll[lil~ccl distance to tlic Virp;o cluster is iln])c)rtalit for many reaso]ls. I)UC lJOLIL
to its ])roximity and tllc fact that it c.ontai]ls a wiclc range of galaxy ty])cs, a large llu]l)bcl of
scc.olldary-distance tcc.llniqucs llavc hell a])])licxl to this cluster. ‘J’lledistallcc totl]c Virgo clustcr
haslmn afocal] )oilltf or{liscllssiollb ec.allsc]J llblis}lc[lva lucsllaves} )aIllLc(l awidc range: tllc’long>
and tllc ‘sliort’ estimates of the extmgalactic distance scale. It islLow gcllmally agrd that tllc
distances to galaxies within tllc general vicinity of tllc l,ocal Group (out to N 1 MI)c) llavc becN
obtailicxl to an ac.c.uracy and prm.isioll of :1 10
(% (1’ukup;ita ct ul. 1993). ]Iut curl-e]]t c5ti111atcx for
tllc distance to Virgo (15 - 21 M])c: c.{g. Jacohy d [/1. 19! X?;UP t o 2 7 MI)c: Sallda~,c IXEl)diflcr

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4
l)y tile uncertainty com]no]lly associated wit} I tllc far-field value of tile llUbblc CX)llstallt itself.
‘J’lIc core of tlIc Virq;o cluster is devoid of sl)iral ~,alaxies; lIc!ltce tlIe lIIost a]) I) I<)]) ri.atc tal~,et
ill \“jrSo is M](N), which is 4° NW of h487. l’indi]l~ Cc])llcids ill Ml 00 will not lead 10 a dccisi\’c
IIleasu Icvnmt of the Vil”go distallcc siIIcc the Vir~,o dustcv is tlioup,]it to mtcllcl fmln 10 to 18 JII)c
(’1’oIIry d 0[. 1990), aIid furthm in Vir~o Scmth.
NcvcrtlIcIless, a dista]lc.c to a Vir~,o ~alaxj is a
clitical test, of tl]e coln]mtin.g distant.c scales<
l~urtllcllito]e, all illc.leasillp; a]nou]lt of inforlllatic)ll
almut tlie structure of Virgo is l)cc.olni]lc available. lbr c!xa]il])lc, ]“c)uque ct ale (1990) have found
that tllc 20 ncmrest spirals to MIOO (within ‘20), arc ().37 ~1 0.17 mag closer than t]lc I:ilgo clust,cl
INea II. Virf;o is loc.atd a.])] )roxilnatdy 16 h4])c away according; to tllc snort distalice scale; lLCIICC
:? O-clay Cc])llcids in Virgo woulcl bc ])rcxlictd tc) llavc < V > =
2(j IIlat,. If Vir?;ois located at tl~c
long distancc c)f a})]jroximatdy
mcmcthan a.lllaf;llitll{lcfailltcr.
27 M])c (Sandag\c 1993), the ‘X)-clay Cc:]) llcids arc ])]cdic.tecl to bc
I+’or tliesc reasons, we IIavc ]~lac.cd high })riority oIt c)bsc] ving M 100.
in the long; term, direct Cc~)llcid distances to a sig;llific.ant saln])lc of Virgc) p;alaxies \vill settle
tile controvcwsy ovm tllc the shc)rt and long distallc.c scales for tllc cluster itself a vital step, since
it is currently t,hc uncertainty ill the distallc.c ratllcr tllall tllc velocity, that ])lays the critical role
ill the ])rcscnt c.ontrovcmy over tllc: value of tlic IIubblc c.clllstant..
h40reovm both l,lle Vir20 and
l’c)rnax clusters have an important role to play in calibrating secondary distallc.e indicators which
tic tllc local clistallc.e lncasurc:ncmt.s to the glc)hal Cxj)allsic)ll.
IIcfom tl[c rc])air of tllc: c)l)tics fcm II S’J’, rcscllvccl studies of tllc mc)st lu?liinous stars in sj)iral
galaxies were c.c)nfincxl ])rilnalily to t,llosc within tllc: l,ocal (;rc)u]) or within nearby gIoups suclI as
tlIcm associated with M81 aTId Ml 01. (11’or a rcwiew of studies of lulninous su]wrgiants in cxterllal
g a l a x i e s see, for cmmplc, ]Iumphreys 1987; Cc])llcid stuclics have km reviewed by Madorc &
l~rccdman 1991; and J acoby ct al.
l~ri~;htcxt objc:c.ts ill Virgo ~;alasicx
1 992). R.c!ccntly>
have been carrid
solnc ground -ba.scd Ineasurancnts of tllc
out under exc.dlcnt sewing conditions (W

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Page 6

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fralrlc, and a tiny s]]utter slladin~; corlc!c.tioli. Subscqllcvltly, tllc fI’alllcs ill Cacll filter \vcle Coll)billd
to reject c.os]nic r’ays. ]’ixc!]s ill cac]I fr’alnc: t]Iat dc!vjatc:d by Inolc! t]Ia II ~ Si~;Illa fl’o]n t]le II IWiII iII tlIC
ICI I~ai IIi IIg frames wcm rcjcctcxl; ]lcip,llbcm of these pixds wcuc rcjcctcd with a 3 sig,ma threshold.
l’i]ially, tllc averaged fra]ncs wcm Jlat,kllcxl usiIIF; flats fro]n the ‘J’V trots tc) rmnove lligl~ s])atial
frequency ICS])OIISC variatic)ns, al)d usill~; a model of tllc II S’J’ ])Ius \\~lI’1’C; illu]nillatiol] ])attmll to
1 cmovr lc)w s])atial frequency valiatio)ls.
l“roln suhscquellt o]l-orbit flats, tllc flat-f icldi]lg used fo]
the Mlo(l flii IIIC!S is cstimatd to he F;OOd lx) a f(!w ]K!rc.ellt 1111s, with peak cr]ors of w 10(70 ill tllc
corllcrs of cwcll chil).
An initial ])ass at, IIIC photometry of tile a.vcra~cd, cc)s)~]ic-lay-clcallc{l frames was lnade using
tile a])erturc p]lotometry routine availal~le in the softwarw })ac.kagy l) AOl’110”1’ 11 (e.g. Stetson ]992
and Icfcrcvlccs tllercin). l,ater, profile-fitting ])llotomctry was c)btaincd (as dcscribcd twlc)~v) OIL tllc
original, unavcragd frames. A comparison of tllc a.])crturc ])llotomct.ry between the avera~;ml al~d
ullavcrap;cd frames yiddcxl difl’ercnces amountinc to less than 1 ‘x).
]’rofilc-fit.ting p]lotolnct.ry of t]lc ori~;illal fralncs w a s ])erforlncd usi]lg; I) AO1’IIC)’J” 11 and
AI, I, II’ILAMII; (Stctscm 1994a).
‘J’]Ie l)oilit-s])rca(l functions (1’SII’S) were obtained wit]l tllc
stall(]-a]onc: routine hfilJ],’J’1]’Sl<’ (Stct,soll 10Wb). IIc!c.a,usc of a sliortagc of Lrigllt, WCII-CX])OSCII
Ycl llllsiltllratd stars in tllc Virgo images, I’SII’S were
field ill ti~ bltauri. A se])arate l’SII’ was p;mlcratcd fc)r
dctcrlni]ld froln images of tllc ca]ibratioll
cdl c.lli])/filter c,o)nhil)atic)ll; those for tile
tlirec Wide-1’icld Ca]tncra chips were allowed to vary quadratically as a fu]lctio]l of ])osition in tile
i]nage, wl]ilc that for the l’lanctary Calncra was trmtcd as bci]lg co]lstant,. l)uring tllc Al,l,l’1{.Ahll;
reductions thcmsdves, i]lner and outer radii for the sky an]iu]i were set at 0.7 and 6.0 ])ixds for tile
W}(’C c]lips, and 0.7 and 8.0 ])ixds for tllc I’C clli]).
‘J’llc! inner radius was set at this low value ill
order to I ) determine t,llc sky as C.1OSC to tllc stars as possible, 2) clctcrlnille tllc! sky brig; lltnms froln
pixels wllmc tllc star is as hrigllt as l)ossil)lc with rcx+I)cct to tllc sky, al}d 3) dctcrlninc tile sky flom

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-7-
tllosc ~~ixds fmlll wllicli it IIas IIot bcclI l]cc.cssaly to sul)tract a lalp;c alnoullt of star flux. ATotc
that sky CIetc!rlllillatic]lls were I)c:riodical]y rdilld aftcl all dctcc.tcd Olljccts IIad becll ]Jrovisiollally
subtracted fmln tllc! ima~cs; thus, tllc ]Jl)otcmctric. lnag]litucles should bc rclativdy unafl’cctd by
the ])a.tclli]lms of t.lle difl’usc flux in h410() that occurs on s])atial scales lal-gcr tlla.rl afe~v I)ixels.
"l"ltcfrallle-tc) -fralllc: Ic])catal)ility c) fstals Il~easllrccl ill W~1’1’(:2ilnages -
botll tllosc~ofh 4100
and c)f L) (;entauri, a s wc!ll as nulncrous si)nul ated ill]ages
was found to ljcI ~ 0.03 lnag ])(!)
cx~~osurc for stals fmln just below tile lna~nitude lcvd of incil)ic]it sal,uratiomto a Inagnitudc lCVC1
abc)ut foul ]nagllitudcs fainter. llclc)lv that, tllcelIcJlsill cleascclillJ casollablea~lcc 1Itelltlvitll what
u’ould be ex])ectcd froln plloto]l statistics. l+;xtcrna] c]rors will bc large], and cannot be estil[la.ted
frc)~l~{)~l]le])cat c)bscrva.tic)l~ssi]lce tllcfic]d al~vays fclli(lclltical lyc)ll tllc!cllil)s. A furtlicr sourccof
uncertainty comes from tile rcc.mlt]y disc,ovcnd ])roble]lt wit]] c.llalgc-trallsf ercfficicllcy (see below).
‘J’llc w Gmtauri i]nagcs suggest that ])ositic)ll-~lc] jcll(lf~llt systcnlat,ic errors may be as large as of
clrdcr ~- ‘i’(Yo (NO. ] lnag), root-)~lc>all-sq~lare, but this rcxu]t is bad only oll a few do?.en stars in
only six sc])aratc WII’J’C2 obscvvatic)lis.
‘1’lle transformation from the instrulnc!lita] (11’439\V, 11’bSSW,
11’702W) to the standard
Jollllson- liron-(;ousins l] Vi{. system ~va,sacc.ol~l])lisllecl in twos tcJm.
l“irst, usingd ata for starsin
the field of theglobu]ar clusterti Gm, atra)isforlllatic)ll was ]nadc from tllc W1’1’’~2illstrlllllclltal
magnitude systmn to tllc WII’J’(; l systcun , as ddiIId I)y IIarris ct 01. ( 1 9 9 3 ) . l)etails of t h i s
C:ilibl’atioli ]JroCcxlurc u~ldcrtakcll by tlic WI’’] ’(;2 tcalll ale giv~]l by II Oltz,ll-tall Ct (1/.
(1994).
l’iniilly, tllc transformations givcll by JIarris cl al. (1991) were used to transform totlle standard
l)VR Systc’111.
Subsequent to t,llcsc observations, it was discovered that tllcrc arc cllarge-trallsfcr ~)roblmls
w i t h tile W]I’]’c2 chips at t]jc o])crating tc!m])craturc used for tlicxw obsc:rva.tions (-77c). ‘J’his
cllar~;c-transfer problcm ]cads to variations ili d’cctive sensitivity depending 011 tlte row number

Page 8

-8-
i]i ~vllic.11 all objcc.t falls. ‘J’ltc cfl’cct gives a .gradiu~t of a])]lroxi]natcly 1 ()(% ill tile ~)]lc)[o]netry f o r
the calibration data. USCC1 here, but is ]Jossib]y rcxluccd ill tllc MIOO data. which has a significant
hackg;rou~ld (from the light ofunrc!scdvcd stars). IIccause oft,llis eflect, as well as scatter in tllc
flig}lta]ld ~;rolll~cl ol~scrvatic)llso ftllccalihratioli ficlcls, tllcl)liotc)lrlc:tric zcro])ctiIlts could bcofi’by
several (.5-10) perce)ltj and there is ])ossihly a. g;radimlt in the ])llotolnetry across tlic \flO(l fralllcx
lmcling to an cflcctivc acl(litiollal rmscrrorofw 3 - b ])crcclit wllc!n objcctsc)vcu. tlleelltircficld
are considered. l’or these reasons, the ])llotomctr.y ])rcxclltcxl i]) this pa])c)r slLould be cc)llsidcrcd as
])rclililillaryollly.
3. (:Ol,olL-MACJNIg’Ul)l(;l )IAGILAMSAN1) I,UMINC)SI”J’Y JI’UN(D’l’I[)N
.41,1,1+’1{Ahll~] )l)otoIl~ctry~vas obtained fora total of30,S51 stars and stal-lik(~objcctsil~ tllc
tllrccWl~~fiel(ls,al~d afurtller lS80c)bjcxtsill t,lIc:l’~flcld. Not all objects \vclcdetectable ina]l
filters, llolvelfcl,a llclvaliousgooc ll~c:ss-of fitc.litcliatvcrcosc(l to fllrtl]cr lilllit t]lcsal[l])]cof objects
discussed below. Stringent (;111 rcstrictic)ns were ]Jlaccxl C)JI t.llc data to lninimiz,e tllc ccmtalnillatic)ll
by clearly resolved, non-stc]lar objects.
1 ‘J’llcrmulting; V versus (II--V) color-~~lagilitu (le(liagra~]~
fcJrtl]c! M~l~Cclli]) 3issllc)\v]) i]~l’igllrc2. l’lotte{l areNll,500s tars~vit}iA l,l,l’ILANll; ~lI1 < 1 . 4 .
A b o u t 12%oftl~c total objects 1~lcasllrccl ~ve]celilllillatcclol~ thcba.sis ofthisstringcllt ~111 limit.
‘J’IIc brightest blue su])crgia]lts in i,lic colol-l[~agllitu[lc dia~;ra]n liavc V w 22..5, 11- V ~ 0.2
lnag. l’mscni. also arc rcd su])c:rp;iants wit]) V ~Q 2.2.7, 11 - V w 1.5 msg. ‘J’llc sc)lid lines rcprmc’l~t
] l“c)r r e a d e r s w h o arc not l) Aol’llorJ’” aflic.ic)llados, cl]] is the ratio of t]lc Observed IIIIS
])ixcl-to-])ixel residual of the actual ilnagc data from tllc best-fitting scaled mode!] 1’S1” to that rlns
residual prcdictcx] fro)n readout l~oisc, photoIi statistics, flat-ficldillg ullccrtaillty, and alltic.i})atcd
intcr])olation imprecision (i. e., the ratio of ohsc:rvccl to cx])cct.cxl rlns residual). If the noise ]nodel
is correct, tllc average value of (;111 sllou]d bc C.IOSC to unity; any object with ~111 >> 1 is probably
IIol)stcllar.

Page 9

:)
ttle IIlcall Ccplicid instability stri}) ((lmuibcd Mow). 1+’ai]ltc] tlIa]I \/ =22 IIlag C.c)llt all)illali[)ll by
forc:,roulld Galactjc fidd stars is ncp;ligiljlc ill tllc 1.8 squ are arc]ni)l area of a single l\ ”l’C2 clli])
(Ilal,call & %ncira 1 9 8 1 ; Ratnatullga & IIal,call 1985); t,lle*cforc, csse]ltjally all of Ille ]ncasurcd
sources colitributi]ig to l“igure 2 bclollg to 14100.
Stars with clll > 1.4 were flagp;wl as ])otcntial star cluster candid a,t~!s. A ])]ot of c]I] VCrSUS
]na.gllitudc Kwdcd t,llat over tllc raII~;c c}f tlic l)riglltcst 2 ]na~,]ijtudw, allnc)st all Ol)jccts IIad large!
(;111 values (> 1 .4). ‘J’hc brig] ltcst objc!cts in tllc initial caudidate list wmc tllcn ills})ccted by cyc to
clilnillatc! s])urious objects or illcglllar-slla])c(l astcrislns that wc!]c )nom likely duc to cllallce ilnagc
crc)ivdil~g. h40st of these clusters arc very blue (11 - V N ().2 map;) al~d are i,llus youn?,. llan-y of
tllcscobjcc.tshavc 1+ ’Wllh4 il~tcl~sity-cli alllctclsc)f < O.h arcscc and thus would a])])car to bcstcllar
ill lnost ilnag;es taken with existing grouncl-based tclcsco})cs.
lntcrcsting]y, tllcrc al)])cars to be a
ri])g-like concentration of compact star clusters around tllc ])cri])]lcry of tlic nuc]car disk. ‘J’llcsc
clusters will discussed in ]nc)rc detail ill a future ]ja])cr (Nladc)rc et al. 1994).
f\ luminosity funct. io]i for tile blue su])c!r:; iants is show]] ill l“ig;urc 3.
A line of S1O])C 0.7
(themcm slope dctcrmillcd by l“rccdman (1985 )forasam]lleof IOnca.rby spiral and irregular
galaxies) is su])cr])oscd 0)1 tlLc MIOO data; tlic agrccmcnt is excellent.
‘J’llis result extends and
co]lfir]ns earlier work i]](licati] lg;tl]at, to witl]in the unccrtaintics, tllc slope of tl]c ul)])er cnd of tllc
ILllllillosityfllllc.tioll appears tohc uliivcrsal forgalaxics with awidc rangcof morj)llologica] ty})cs
a]ld mctal]icitics.
1]] l’ig;urc2 t,hc ~cphcid instability strip (Madorc& l’rcccl]nan 1991) is ])lottcd
been slliftcxl by 31.3 magl[itudcs in apparent distant.c lnoclu]us (an illustrative mean
after ltavillg
value c)f the
Virg;onlodulus, basccl on ]ncasurcmcnts of 7indc})cmdcnt scco]ldary distance i]ldicators tabulated
by Jac.oby et al.1992). in ‘J’able 1, tl]c cxpcctcd (unreddcncd) V magnitudm for Ccphcids o f
20, 40, and 60 clays arc givc]l for a})])arc]it, distallcc moduli of 30.9 a.]ld 31.5 mag, Lasc(i OJ1 tllc

Page 10

lo-
at)ovecitcd ]JcricJC1-lll]lli]lcJsity calibratio]l.
‘J’hcsc distance lnoduli corresI)oJId t o lfi Mpc and 20
hl~jc rm])cc.tivcly. A 40-da.y (Ucl)lieid will lla.vc < V >N 25.1 ]IIag if Virgois at tlic clmer distallce
atld < V >W 2.5.7 mag if Vi]go is at tlIe fartltcr dista]lm. lfthc Virgo c,lustmis as distant as 27
hlpc orp N = 3 2 . 2 map;), th~ 60-day (kq)licids would still have < V >W 25.9 ]na?;. a ma:;nitudc
limil well within the mac.h of WIJ1’~2 (sew ]$igurc 2).
‘J’a.blc 1 - lJnrcxldcvIcd (;c])llcid V hfla~;]litudcs as a l’unc.ticm of IJistance
‘J’l]c cx])osure times for the IJarly Iklc(lsc 06scrwJtio?ls at cac]i illdividua] cI)oclt w’erc s]Lort
(ill total less than 1800 scc at V and 1200 scc at 1{). IIcvlce an ilnprovclncnt ill sig]lal-tc)-noise
ratio can be achicvcd witli lollp;cr Cx])osures.
lL40reovcr, the cflc:cts c)f crc)wdillg can Lc dccrc!ascd
by Ivorkingat alowmsurfaceb ri.ghtncsslcvcl at greaterr adial distancefroln the )iuc.lcus. ]+’inally,
tllc l’c can be used to full advantage (with its illcrcasccl sa]n])ling of the 1’S1’) by ])ositionill?; it
away from the nucleus in an active star-forlrlillgrc:gioll wl]cm &:])l\cids are cx])c:c.ted to be locat.cd.
IIascd on t}lcsc ])rcliminary data wc conclude that tl]c discovery c)f Ck])lleicl variables in h1100 is
feasible using WII’I’C2, and OU] schcdulcd l{c!y l’rojcxt observations (opti]nizcd as described above)
arc ]iow underway.
4. SUMMA1{.Y
Above all, tile results of this paper demonstrate tllc im])ressive ca})ability of IIS’J’ for studying
tile msolvd stdlar ]~o])ulations in galaxies well beyond tl)c I,ocal Groul).
IIVIL color-] nagitudc
.

Page 11

11-
diagralns and ii IIlmi]losity functio]l for tlIc l)rip;lltest I)lue supcrgi:i]lts lla~’c l~ccll ]ncasurml ii I a
galaxy at a distance rcpresc]ltativc of tllc V irgo dustm.
‘J’liesc results dc]nojistratc} lliat we Iiow
Itavc tl(c potcntia] to study tl}e rich variety of galaxies ~)rcswlt ill tllc ViI~,o clustel” with cc)m])arab]c
rcsc}lution to that c)f several galaxies in our OWI1 ],ocal Grou]);
]norcover for galaxies clcmr Illan
those in tl]c Virgo cluster, the rmoluticnl will bc even g;rc’atm.
III Mloo, we find the $Iopeofthc
lumillmity function to he in agrccmelit with that prcvious]y )[ncasurcd for su])c!rp,iallts ill IIcarby
galaxies. %veral ])ro)nincmt blue clusters and dc]lse lcgiolls ill associations IIavc Ijcwri idc’llifiecl.
Star-like objects with map;nitudm and colors ex])cxtml for ~cplieid variah]c!s at the distance of
Virgo ILavc been detected ill M1OO, thus clcmollstrati]lg tlIc fcasil~ility of dcterlninilt~ light cur~rcs
fclr actual ~c])llcid variables in the Inost distant of our tarp;ct galaxies. l’ollow-u]) c)bselvations c)f
a]) a[ljacent and ])artially ovmla])])illg fic!ld in MIOO llavc IIOW bcgu]l with tlic aim of discovering
and dctcrlnining periods for ~cpllcids in this galaxy, an i)n])orta.nt step in tile 11 S’1’ licy l’rojcct o]~
the l;xtragalactic l)istance Scale.
Acknowlcxlgmcmts
Su])])ort for this work was })rovidcd by NASA thrc)ugll grant llu~n}>cr 2227-87A fm]n tllc Spat.c
‘1’clcsco])c Scimlcc IIlstitute wl}ich is o]w.ratcxl by the Association of Universities for l{cscarch
in Astrolic)my IIIC.
u n d e r hlASA ~o~ltract NAS5-26555.
JIII’M was su])l)ortcd ill part I)y the
J e t l’ro])ulsion I,aboratory, ~alifornia IIlstitutc of ‘J’ecllnology, u]ldm tllc s])onsc)rsl}i]) o f tile
Astrop]lysic.s l)ivision of NASA’s Office of Sl)acc Scic]lce and A1)])lications. WI, }*”S ~vorli on tllc
extra.galactic distance scale is supported in part by NSII’ Gra.llt No. ASrJ’ 91-16496. ‘J’his rcscarcll
has Indc use of the NASA/11’A~ l’;xtragalactic l)atabasc (Nl;l)) which is o])crated by tllc Jet
l’ropulsioll l,ahoratory, ~altecl~, under contract with NASA.

Page 12

12-
l“igurc (;a]}tions
l“ig. 2 V versus (11 - V) color-l l~a.f;llit~l~lc: fliar;ra.111 for ~ll,500stalsJllcasl11ccl by }lI,l,lc’l{Ah41;
on \\T1’’C;2c.lli]j 3. ‘1’lic solicllillc:s ill{Iicatc tllc])ositioIlc)f tllc un]eddcmcx] lncan 02])lleid instability
stri]) for a dista]lce ]nodulus of 30.3 lnag (18 M])c.). ‘J’lIe c.c)]Ics])c)]l( li]]g]l ]ap, ]]itl](lcle\ ’cls al)l)mpriatc
for this distance Itndulus for lmriods of 10, 20, 40, a]id 60 days a]c laflmllcd.
l“ig. 3- V-band lulninmity function for tllc brightest blue stars in Ml 00.
ILefcrcnccx
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‘Jklty, 1{. 11. 1988, Ncalby Galaxies catalog, (~all~bridgc lJIIjv. l’rcss: ~ambridg;e, l;ngland)

Page 15

,,,
. .,.
.
. . .
.
l“?-c+clrll all et ~] .
l’i f;u I-C ]