\B aade’s red sheet" resolved into stars w ith H ST 1 in the B lue
C om pact D w arf G alaxy V II Zw 403
arXiv:astro-ph/9708240v1 26 Aug 1997
R .E.Schulte-Ladbeck,M ary M .C rone2
U niversity ofPittsburgh,Pittsburgh,PA 15260
U .H opp
U niversitatssternwarte M unchen
R eceived
1
;
accepted
Support for this work was provided by N A SA through grant num ber A R -06404.01-95A
from the Space Telescope Science Institute,
w hich is operated by the A ssociation of U niversities for R esearch in A stronom y,
Incorporated,under N A SA contract N A S5-26555.
2
C urrent address: Skidm ore C ollege,Saratoga Springs,N Y 12866
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A B ST R A C T
H ST W FPC 2 observations of the nearby Blue C om pact D warf G alaxy
V IIZw 403 (= U G C 6456) resolve single stars dow n to M I -2.5,deep enough
to identify red giants. T his population has a m ore uniform spatialdistribution
than the young m ain-sequence stars and supergiants,form ing the structure
know n as \Baade’s red sheet". W e conclude that V IIZw 403 is not a prim eval
galaxy.
Subjectheadings: G alaxies | Irregular,Blue C om pact D warf
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1. Introduction
In 1972,Searle & Sargent announced that two prototypicalBlue C om pact D warf
G alaxies (BC D s) were \the rst m etal-poor system s ofPopulation I to be discovered".
T hey posed the question ofw hether BC D s are young galaxies in the sense that m ost of
their star form ation happened recently,or old galaxies in w hich star form ation occurs in
intense bursts separated by long quiescent intervals.
A n old galaxy m ust show evidence for the presence ofan old stellar population. Since
BC D s are generally too sm alland too distant to resolve into stars from the ground,studies
oftheirstellar populations have been based on analyses ofglobalgalaxy colors and spectra.
In his 1991 review ,T huan discussed severalobservations w hich,taken together,m ade him
favor the old-galaxy hypothesis. Based on spectralsynthesis m odeling ofIU E spectra,
Fanelliet al. (1988) found that the star-form ation history ofBC D s is best characterized
by m ultiple,discrete star-form ing episodes rather than by a single burst or a continuous
star-form ation rate. T huan (1983) obtained near-infrared photom etry ofBC D s and argued
he had found an old population ofred giants. U nfortunately,as T huan recognized,it is
di cult to discrim inate between a population ofyoung red supergiants and one ofold
red giants using only the totalinfrared colors ofa galaxy. C am pbell& Terlevich (1984)
obtained C O indices and asserted that the population detected in the infrared is prim arily
com posed ofsupergiants from the current starburst. Loose & T huan (1986),K unth et al.
(1986),K unth et al. (1988) and Papaderos et al. (1996) studied the opticalm orphology
ofBC D s. T hey found that m any BC D s show m ore extended and regular m orphologies in
red im ages than in blue im ages,w ith colors becom ing redder w ith increasing distance from
the starburst centers. T hese observations have been interpreted to indicate the presence of
\Baade’s red sheet",the signature ofan old stellar population,in som e BC D s. H owever,
there are BC D s w hich do not appear to exhibit these extended red halos.
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T hree BC D s ofbeen observed w ith H ST /W FPC 2,two ofw hich are extrem ely m etal
poor and thus good candidates for prim evalgalaxies. But because these BC D s are at
distances greaterthan 10 M pc,even the extrem ely high spatialresolution ofH ST could not
resolve any faint old stars in these galaxies. M rk 966,studied by T huan et al. (1996),has
an oxygen abundance of1/10 ofsolar. T his BC D is som ew hat resolved: T huan et al. nd
40 point-like sources around and projected onto an unresolved disk,w hich they identify
w ith an old globular cluster system . SBS 0335-052 has an oxygen abundance of1/40 of
solar. T huan et al. (1997) nd young super-star clusters ofthis galaxy are resolved,but
its disk is not. T he irregular m orphology and blue color ofthe disk lead T huan et al. to
suggest that this is probably a young galaxy. In I Zw 18,the m ost m etal-poor galaxy on
record w ith an oxygen abundance estim ated to be between 1/40 to 1/60 ofsolar,H unter
& T hronson (1995) successfully resolve individualm assive stars w ith W FPC 2. H owever,
any evolved descendants oflower-m ass stars are below the detection lim it;once again,
inform ation about the galaxy’s youth is lim ited to the integrated colors ofthe disk.
V II Zw 403 has recently attracted attention w ith the RO SAT /PSPC discovery ofan
extended hot gas out ow powered by the present starburst (Papaderos et al. 1994). T he
oxygen abundance ofV II Zw 403 is about 1/15 ofsolar;its H I m ass is 2x108 M
(Tully
et al. 1981). Prior to the observations presented in this paper,severalpieces ofevidence
suggested an old population m ightbepresentin V IIZw 403.Itbelongsto the m orphological
type iE in the classi cation ofLoose & T huan,w hich designates irregular star-form ing
centers surrounded by an ellipticallow -surface-brightness envelope. T he red color ofthe
unresolved disk is interpreted to indicate an old stellar population (Schulte-Ladbeck &
H opp 1997). C arozziet al. (1974) presented an opticalspectrum ofV II Zw 403 w hich
displays a continuum break in the area ofthe C a H and K and the G -band absorption
features. V II Zw 403 resolves into single stars and/or star clusters from the ground (e.g.,
Fisher & Tully 1979,Tully et al. 1981,K arachentsev et al. 1994,H opp & Schulte-Ladbeck
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1995),but only a few ofthe m ost lum inous objects can be discerned.
W e here present W FPC 2 observations ofthe m ost nearby ofthe well-studied BC D s
(K unth & Sevre 1986),V IIZw 403 = U G C 6456.W e dem onstrate thatby choosing a BC D
that is nearby,we can use H ST to resolve \Baade’s sheet" into single stars. W e thereby
show conclusively that V IIZw 403 is not a young galaxy.
2. O bservations & D ata R eduction
T he H ST W FPC 2 observations ofV II Zw 403 are sum m arized in Table 1. W e re-ran
the pipeline calibration w ith im proved calibration les,rem oved cosm ic rays w ith C R R EJ,
corrected forC T E using a 4% ram p,and corrected forgeom etric distortion using the im age
available from ST ScI.
Plate 1 displays the three-color (F814W ,F555W ,and F336W ) im age ofthe PC chip,
illustrating the m orphology ofthe centralregion ofV IIZw 403.T he blue starsare clum ped
together,and are often associated w ith nebulosity,w hereas the faintred starsform a nearly
uniform sheet across the im age. T here are severalthousand point sources in the PC alone
(Table 1)! In the interest ofprocessing allstars identically and lim iting contam ination by
foreground and background sources,we include only stars in the PC in our analysis.
W e perform ed photom etry on H -subtracted im ages. Subtracting H had three e ects.
First,a few stars are line-em ission sources;these decreased in brightness by as m uch as a
m agnitude. Second,m any ofthe fainter stars (m F 555W < 26) that were detected at the 3
levelin the im ages w ithout H
subtraction are no longer detected at this level,prim arily
due to increased noise. T his changes the appearance ofthe \red tangle" (see below ). T hird,
the fainter stars w hich were stilldetected in F555W becam e slightly brighter (by less than
0.1 m ag for m F 555W < 26),probably thanks to better sky subtraction.
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W e used D A O PH O T /A LLSTA R to do psf tting for the crow ded eld. W e chose
relatively isolated starsin the im age to determ ine the psf,after nding thatthisproduced a
better t than using T inyT im or independent observations. W e calibrated the photom etry
w ith the M ay 1997 updated SY N PH O T tables,and used the C ool& K ing (1995)m ethod to
m axim ize the perform ance ofD A O PH O T .A LLSTA R residuals for the F555W and F814W
ltersrem ain below 0.1 m ag form agnitudes less than 25.5.ForF336W the residuals exceed
0.1 m ag at about 24.0 m ag.
W e checked the com pleteness ofour photom etry for each H -corrected im age by
adding a distribution offalse stars consistent w ith the m agnitude distribution ofthe real
stars. W e ran 100 sim ulations for each im age,adding 10% ofthe num ber ofrealstars each
tim e,in order to m aintain the approxim ate levelofcrow ding in the originalim age. T he
percentage ofrecovered stars indicates that there is no signi cant incom pleteness brighter
than m F 555W = 26,m F 814W = 25,and m F 336W = 25.
Foreground galactic reddening for V II Zw 403 is E(B-V ) = 0.025 (Burstein & H eiles);
we correct for the corresponding extinction using the tables provided in H oltzm an et al.
(1995).Tully etal. (1981) nd large,anom alousinternalextinction near the large southern
H II-region,w hich is included in the PC im age (see Plate 1). H owever,in our color-color
diagram (cd. Fig. 1),we see no signi cant reddening for m ost stars. T herefore,we applied
no correction beyond the one forforeground extinction. Itisquite possible that,as in other
m etal-poor dwarfgalaxies (C alzettiet al. 1997),reddening is m uch larger for the ionized
gasthan forthe di use stellarpopulation. W e transform ed to U ,V ,and Im agnitudesusing
the color term s in H oltzm an et al. W e note that there are signi cant uncertainties in the
transform ation to U ,w hich lim itthe accuracy ofourextinction estim ate,butdo notchange
the shape ofthe [(V -I),I]color-m agnitude diagram (C M D ).
Figure 1 show s color-m agnitude diagram s for stars in the PC .W e include allobjects
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detected to at least 3 w hich were successfully t to the psf. G iven the very sm allsize
ofthe PC and the latitude l= 37:3 ofV II Zw 403,the m odels ofR atnatunga & Bahcall
(1985)predict negligible foreground contam ination.
3. D iscussion
3.1. T he T ip-of-the-R ed-G iant-B ranch D istance to V II Zw 403
Lee et al. (1993) showed that the tip ofthe red giant branch (T RG B) consistently
occursatan absolute IM agnitude M I -4,w ith only a slightdependence on m etallicity. W e
use this m ethod to determ ine the best distance estim ate to date for V IIZw 403.
Figure 2 show s our I-band lum inosity function for the red stars only;to bring out the
red giant branch,we include only stars redward of(V -I)o= 0.7. T he T RG B is so obvious
as a sudden rise at Io= 24.3 that we did not need to use an edge- nding algorithm . (O ther
sources oferror are m uch larger!) Follow ing the m ethod ofLee et al.,we need a value of
[Fe/H ]to determ ine M bol;T R G B . It is di cult to determ ine [Fe/H ]from the colorofthe \red
tangle",however,any reasonable value for [Fe/H ]does not change M bol;T R G B by a large
am ount. U sing (V -I)o 1.5 and a m etallicity [Fe/H ]of-1.2,we nd an absolute m agnitude
for the T RG B in V II Zw 403 ofM Io= 4.10. O ur error budget for the distance m odulus is
sum m arized in Table 2. T he largest source oferror is that from the R R Lyrae distance
calibration ofthe T RG B,w hich we take to be 0.15 m ag,follow ing Sakaiet al. (1997). T he
0:4
distance m odulus is 28.4 0.09 0.16,yielding a distance of4:8+ 0:
5 M pc.
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3.2. T he Stellar C ontent of V II Zw 403
Fig.1 contains a wealth ofinform ation on the stellar content and star-form ation
history ofV IIZw 403,but it needs to be decoded. Because the spatialdistribution ofgas
and stars,the num ber ofresolved point sources,and the m orphology ofthe [(V -I)o,M Io]
C M D ofV II Zw 403 are very sim ilar to those ofLocalG roup D warfIrregular G alaxies
(dIrr),we letourqualitative interpretation ofthe stellarcontentofV IIZw 403 be guided by
recent results ofground-based observationaland m odeling work on the Pegasus D warfand
N G C 6822 (A paricio & G allart 1994,A paricio & G allart 1995,G allart et al. 1996 a,b,c),
including its suggested application to H ST data by A paricio et al. (1996). W e now trace
through the evidence for both recent star form ation and an older underlying population.
1)T hatV IIZw 403 isactively form ing starsatthe presenttim e isdem onstrated by the
strong line em ission seen as nebulosity in the H - im age,and also through its contribution
to the V lter in Plate 1. T he H II-regions are tracers ofstars w ith ages < 10 M yr.
2) M ain-sequence (M S) stars and blue supergiants younger than 50 M yr populate
Fig.1b in a strip extending from [(V -I)o,M Io] [-1,-2]to [0.5,-9]. T he young m assive
stars also form a distinct band in the [(U -V )o,V o]C M D .C ontrary to w hat is seen in the
[(V -I)o,M Io]C M D ofthe Pegasus D warfand N G C 6822,the \red tangle",the clum p at
(V -I)o 1 in Fig.1b (w hich includes RG B stars,old and interm ediate-age asym ptotic-giant
branch (A G B) stars,and interm ediate-age blue-loop (BL) stars) is N O T the single m ost
prom inent feature ofthe C M D .Instead the \blue plum e" at (V -I)o 0,w hich contains M S
and BL stars,is equally wellpopulated. So,w hile V IIZw 403 clearly exhibits a population
ofold stars,it has a m uch m ore abundant population ofyoung stars than these LG -dIrrs.
W e note that in our C M D ,especially in the ST lters,the low -lum inosity portion ofthe
\blue plum e" is visibly split into two distinct color bands. T he bluer band is the M S,
and the redder band m ight m ark the location ofthe blue turn-around ofthe blue loops.
{9{
T he BL contains evolved interm ediate-m ass and high-m ass stars w ith ages from 50 M yr
to severalhundred M yr. A few high-lum inosity (M Io< -4) stars are at (V -I)o 0.3. Such
a population is not apparent in the observed C M D s ofthe dIrrs,possibly rem oved by
foreground-subtraction procedures; however, very few foreground stars are expected in
V IIZw 403 ow ing to itshigh galactic latitude and the sm allsize ofthe PC .T hispopulation
does appearin the sim ulated C M D s (e.g. Fig. 5c ofG allartet al. 1996b). W e interpret the
objects as high-m ass BL stars in evolutionary stages between the M S and the R SG phase.
T he [(U -V )o,V o)]C M D also displays quite a few blue and yellow supergiants. T he single
star located at [(V -I)o,M Io] [-0.7,-5.5]is a very bright H
point source w ithin the large
southern H IIregion,w hich rem ainsanom alously blue even afterH subtraction. Som e blue
stars are w ithin this H IIregion,but there are also blue stars w ith surrounding H II-regions
distributed across the im age.
3) T he red-supergiant (R SG ) strip extends from [(V -I)o,M Io] [1,-4]to [2,-9],and
contains stars w ith ages sim ilar to those ofthe stars located in the \blue plum e" ofthe
C M D .T he R SG stars stand out on Plate 1,scattered across the PC im age.
4) T he A G B stars are located along a band at M Io -4.3 to -5.3 and extending from
(V -I)o 1.5 to 3.0. T he age ofm ost ofthe A G B stars is severalG yr. A single star located
at (V -I)o 3.5,M Io -6.3 m ight be a younger (a few 100 M yr old) A G B star. T he m ere
presence ofthe \red tail" im plies that the stellar m etallicities for the A G B ofV II Zw 403
are larger than Z 0.001,or,for Z = 0.02,larger than 1/20 ofsolar. T he red-tailextends
alm ost as far to the red as that ofN G C 6822,w hich has an oxygen abundance of1/5 of
solar,and for w hich the red extension ofthe A G B ts to the next available grid point
(Z= 0.004)in the stellar m odels ofBertelliet al. (1994).W e suggest thatthe upper lim it to
the stellarm etallicitiesofinterm ediate-age starsliesbelow 1/5 ofsolar.Itisinteresting that
the stellar m etallicities ofthe interm ediate-age stars bracket wellthe oxygen abundance
{ 10 {
deduced from the gas.
5) T hat feature w hich harbors the truly prim evalstellar population ofV II Zw 403 is
the \red tangle". It extends from [(V -I)o,M Io] [0.5,-2]to [1.5,-4]. T he \red tangle" is
potentially populated by stars w ith ages spread over the entire history ofthe galaxy,from
about 0.2 to 15 G yr. A s discussed by A paricio & G allart (1994),the \red tangle" can be
split into three alm ost verticalbands. T he reddest band is populated by RG B and A G B
starsw ith agesranging from 1 to 10 G yr,the centralportion ispopulated by the oldestand
m ostm etalpoorRG B and A G B starsw ith agesexceeding 10 G yrand representing the rst
generation ofstars form ed in this galaxy,w hile the blue part is populated by BL stars w ith
ages ofa few hundred M yr. Plate 1 illustrates \Baade’s red sheet" as a background sheet
ofvery num erous,very faint red stars w ith a fairly uniform distribution across the face of
the galaxy.
4. C onclusions
T he m orphology ofthe [(V -I)o,M Io]C M D ofV IIZw 403 resem blesthatofthe well-studied
dIrrs N G C 6822 and the Pegasus D warf,but the ratio ofblue-plum e to red-tangle stars is
highest for V II Zw 403,suggesting that its present-day star form ation rate is the highest
ofthe three galaxies. T he C M D ofV IIZw 403 is populated by stars w ith ages covering a
m ajor fraction ofthe age ofthe U niverse. T his result em phasizes a point m ade previously
by Tully etal.(1981),nam ely thatBC D sshould notbe considered a separate classofdwarf
galaxy,but rather the high-star-form ation extrem e in the continuum ofdIrr galaxies.
0:4
T he T RG B yields a distance 4:8+ 0:
5 M pc,placing V IIZw 403 beyond the M 81 group.
Forthe rsttim e,\Baade’s red sheet" is resolved in a BC D .It is m ade ofA G B and RG B
stars,w ith ages in excess ofseveralG yr. V II Zw 403 is not a \young" galaxy form ing its
{ 11 {
rst generation ofstars at the present epoch,as is som etim es proposed for BC D s.
T hiswork wasfunded through H ST archivalgrantA R -06404.01-95A .U H acknow ledges
support from the SFB 375.
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R E FE R E N C E S
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C om parative and G lobalProperties",eds. A .Layden,R .C .Sm ith & J.Storm ,115
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C .Leitherer,p. 290
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{ 13 {
K arachentsev,I.D .,T ikhonov,N .A .,Sazonova,L.N .1994,A & A S,106,555
K unth,D .,M artin,J.M .,M aurogordato,S.,V igroux,L.,1986,in \Star-Form ing D warf
G alaxies and R elated O bjects",eds. D .K unth,T .X .T huan,and J.T .T .Van,89
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T his m anuscript was prepared w ith the A A S LATEX m acros v4.0.
{ 14 {
Fig. 1.| C olor-m agnitude and color-color diagram s for resolved stars in the PC .(a) C M D
for the \raw " photom etry for ST lters in the Vega m agnitude system . (b) T he sam e C M D
but w ith H
subtraction,dereddening, and transform ation into Johnson-C ousins V and I.
T he prim ary di erence is caused by H
subtraction { see text for discussion. T he absolute
M Io scale on the right is based on our T RG B distance determ ination. (c { d) [(U -V )o,V o]
C M D and color-color diagram . T he clum p and linear feature in the color-color diagram are
consistent w ith an isochrone ofa few M yr. W e show a reddening vector for E(B-V )= 0.1.
Fig.2.| Lum inosity function in Iforstarsredward of(V -I)o
0.7.T he tip ofthe red giant
branch isclearly visible atIo= 24.3.T he sm allerstep in the distribution atIo= 23.7 isdue to
the A G B stars.
E1
Fig. 3.| T hree-color H ST PC 2 im age of the Blue C om pact D warf G alaxy V II Zw 403,
show ing the active,centralstar-form ing region ofthe galaxy.Im agesfrom the ltersF814W ,
F555W ,and F336W were com bined using the IR A F C O LO R package. T he blue starsclum p
together, w hereas the older, faint red stars form a uniform background sheet across the
im age. N orth is to the lower right.
E1
N O T E T O ED IT O R :T he follow ing is the caption for Plate 1. W e couldn’t gure out
w hether there is a platecaption com m and,so here it is as Fig. 3.
{ 15 {
Table 1. W FPC 2 Exposures ofV IIZw 403.
Filter
Exp. tim e (s)
D ata sets
Point sources
F555W
4200
u2pq0504t,u2pq0505t,u2pq0506t
5171
F814W
4200
u2pq0507t,u2pq0508t,u2pq0509t
3478
F336W
5600
u2pq050at,u2pq050bt,u2pq050ct,u2pq050dt
788
F656N
2400
u2pq0501t,u2pq0502t,u2pq0503t
139
{ 16 {
Table 2. Errors for T RG B D istance.
Source
Error (m ag)
R esiduals from psf tting ....
0.05
T ip m easurem ent ............
0.05
G alactic extinction ...........
0.02
Internalextinction ...........
0.05
Totalrandom errors ......
0.09
R R Lyrae distance scale ......
0.15
A bsolute m agnitude ofT R BG
0.10
Transform ation to Iand V ...
0.03
Photom etric zero points ......
0.02
Totalsystem atic errors ...
0.18