arXiv:astro-ph/9601165v2 30 Apr 1996 aoy aSla Chile Silla, La vatory, LG the of to ⋆ requests properties any offprint The Send from LG. far the An- is of , the the member to one, in companions luminous only are are and three , 1994) Way, dromeda Milky al. dis- the recently et of the Ibata halo (including dwarf; nine Sagittarius (LG), in covered Group (dSph) Local spheroidal dwarf the known 12 the Among Introduction 1. galaxy dwarf Tucana Saviane Ivo the of photometry CCD 1 2 Abstract. ihtegatbace fglci lblrcutr we clusters globular [Fe/H]= galactic of a comparison branches direct estimate by the giant and From the tip Group. branch with Local giant the red the of of border color lo- the spheroidal at dwarf almost isolated cated an is Tucana that confirming ryugselrpplto.W eiethe intermediate derive We an for population. stellar evidence young no cluster or is globular for- Galactic There the formation. of single star epoch a the had at burst has mation Tucana color–magnitude that The indicates spread. diagram metallicity a for indication e words: el- Key dwarf and spheroidal dwarf M31 companions. and liptical Galaxy defined the relations metallicity– by magnitude general the brightness–absolute to con- surface we and participates which Tucana , from that resolved Tucana, firm of of parameters profile structural density the surface the profile, fterdgatbac eetmt itnemodulus distance a estimate we branch giant ( red the of a otn aais structure Galaxies: – content lar Tucana individual: ies: eprHTpooer.Fo the the pho- From photometry. to accurate HST complementary the deeper data our and make investigated calibration field tometric large The cana. ∼ 10.;1.91Tcn;1.21 11.;11.19.6 11.19.5; 11.12.1; Tucana; 11.09.1 11.06.2; are: codes thesaurus later) Your hand by inserted be (will no. manuscript A&A m nvri` iPdv,Dpriet iAtooi,vicolo Astronomia, di Dipartimento Padova, Universit`a di sevtroAtooiod oon,vaZmoi3,I–401 33, Zamboni via Bologna, di Astronomico Osservatorio 6 tr ntercnl icvrddafglx Tu- galaxy dwarf discovered recently the in stars 360 ae ndt olce tteErpa otenObser- Southern European the at collected data on Based − M ) 0 24 = epresent We 1 aais udmna aaees– parameters fundamental Galaxies: nioV Held V. Enrico , . 69 ± 0 . 6 orsodn o870 to corresponding 16, .V Held V. E. : V oa ru aais stel- Galaxies: – Group Local – and − I 1 2 . n imal Piotto Giampaolo and , 8 I C htmtyfor photometry CCD ± antd ftetip the of magnitude 0 . ,wt oclear no with 2, V luminosity ± 0Kpc, 60 Galax- elOsraoi ,I312Pdv,Italy Padova, I–35122 5, dell’Osservatorio 6Blga Italy Bologna, 26 yfitn h in rnhso 9,M,ad4 u to Tuc 47 ( and obtained M3, M92, he M31. of and data, branches Galaxy the giant our the to fitting companions By dSph might indistinguish- the be intermediate-age from that to able conspicuous seems stars Tucana a which bright from of red, population, presence very the nor indicate stars blue young eouiniaigreaching imaging resolution yD ot’ 19)i i eetrve.H obtained He review. recent his in (1994) a Costa’s Da by distance the given. for was mag determination 24.75 metallicity in of direct spheroidal limit No dwarf upper modulus. dE5 an a and as LG, the galaxy this of classification ( 19) aey&Mgel(92 rsne shallow a presented (1992) Lavery Mighell by & for re-discovery Lavery overlooked serendipitous (1990). was its 1985), before time al. some et (Corwin Catalog Galaxy and galaxies formation. spiral their giant for of models constrain halo environ- to the an from in opportu- galaxies different dwarf unique of ment a evolution the offers study Tucana to nity (1993). of Zinn location and isolated (1994), Da The Wyse (1995), & Caldwell Gallagher e.g., (1994), reviews, Costa recent of subject the are tutr ntesa itiuin nepnnillwfit length law scale a exponential yields An data distribution. the to star the sub- for in indication structure no showing Tucana of profile brightness ais r10p tadsac f90kc nextrapolated an kpc, 900 of brightness distance surface a central at pc 130 or radius) naslt antd M magnitude absolute an bnac.D ot 19)as rsne a presented also the (1994) in uncertainty Costa the Da to contributor abundance. largest the being ror ealct F/]= [Fe/H] metallicity a 93.Cseln .e l 19)peetdapors re- Irwin progress & a S. presented (1995) Demers al. II: et Leo M. e.g., Castellani (cf. 1993). present parameter second be mild bluer may a even that effect an suggest authors expect = the might [Fe/H] and one HB, as study), low this as by Tu- metallicity confirmed Since a (HB). has branch a horizontal cana are blue 1995) moderately stars, a Caldwell young and for (see evidence coll. no with and diagram color-magnitude Seitzer by Tucana of V 1 V − rlmnr htmti eut aebe reported been have results photometric Preliminary uaa lhuharaypeeti h Southern the in present already although Tucana, h rtrslso nogigde S mgn study imaging HST deep ongoing an of results first The ( – I oo-antd iga CD ae nlow- on based (CMD) diagram color-magnitude ) B − V oo-antd iga hwn neither showing diagram color-magnitude ) − m 1 V µ . − 8 0 = ,B V ± M − ∼ 0 25 = ) α 9 ⋆ 0 . 5dx h itneer- distance the dex, 25 . 3 hysgetdthe suggested They 23. 3 30 = 24 = ± . a arcsec mag 5 0 ASTROPHYSICS ′′ . 4. . ASTRONOMY 8 goercmean (geometric ± 12.1.2018 − 0 AND 1 . a and mag 2 . avalue (a 8 B − surface 2 and , V – 2 Ivo Saviane et al.: CCD photometry of the Tucana port of a study using the same data (obtained from the Observations were obtained in two runs, on September HST archive), and deep ground-based CCD observations. 15-16, 1993 and September 2–3, 1994, using EFOSC2 at They obtain a distance modulus consistent with that of Da the ESO 2.2m telescope. EFOSC2 was equipped with a Costa (1994), and marginally higher metallicity ([Fe/H] 1024 1024 pixels Thomson coated CCD (ESO #19); = 1.56 0.20), and claim that the spread in color on the pixel× size was 19 µm yielding a scale 0′′. 336 pixel−1 the− RGB± is larger than the one expected on the basis of and a field of view 5.′7 5.′7. The CCD conversion factor photometric errors. was 2.1 e−/ADU, and the× read-out noise, measured from In order to provide clues as to whether the star for- the overscan region, was 4 e− (cf. EFOSC2 User Manual). mation history and structure of dwarf spheroidals are af- Weather conditions and seeing were quite variable dur- fected by environment, we present a comprehensive CCD ing both runs, so the quality of the observations is not study of the stellar populations and structure of Tucana, uniform. A journal of the observations selected for this based on V, I CCD photometry over a relatively wide field study is given in Table 1. The useful images have been of 5.′7 5.′7. This area allows a complete coverage of the grouped into two sets: the first set includes the best images × ′′ galaxy. Photometry of the stars in our field provides a well (FWHM 1.2 ), most of which were taken on the single populated upper RGB, essential for obtaining reliable dis- best night≤ of Sept. 3, 1994 (under photometric conditions). tance and metallicity estimates. The plan of the present A second set includes all the images taken in medium, yet paper is as follows. Observations and stellar photometry acceptable, seeing conditions ( 1′′. 5 FWHM). All images are described in Sec. 2. Sect. 3 presents the I – (V I) with seeing worse than 1.5′′ FWHM∼ have been discarded. color-magnitude diagrams of Tucana. A new accurate− dis- tance determination is derived in Sec. 4 by locating the RGB tip on the I luminosity function. New estimates for Table 1. The journal of observations the mean metal abundance and abundance spread of stars on the red giant branch (RGB) are presented in Sec. 5. Surface photometry is presented in Sec. 6 where struc- tural and photometric parameters are derived. In Sec. 7 N filter texp Date UT X FWHM ′′ we discuss the properties of Tucana in the framework of 1 V 1800 1993 Sept. 15 01:40 1.32 1.4 the general relations for dwarf elliptical and spheroidal 2 I 2700 1993 Sept. 15 02:15 1.27 1.5′′ ′′ galaxies. 3 V 1800 1993 Sept. 15 03:09 1.23 1.3 ′′ 4 I 2700 1993 Sept. 15 03:44 1.22 1.5 5 V 1800 1994 Sept. 3 00:58 1.52 1.2′′ ′′ 2. Observations and data reduction 6 I 1800 1994 Sept. 3 01:33 1.42 1.2 ′′ 7 I 1800 1994 Sept. 3 02:06 1.35 1.5 8 V 1800 1994 Sept. 3 02:46 1.29 1.5′′ ′′ 9 V 1800 1994 Sept. 3 03:19 1.26 1.5 ′′ 10 I 1800 1994 Sept. 3 03:55 1.23 1.2 11 I 1800 1994 Sept. 3 04:29 1.22 1.1′′ ′′ 12 I 1800 1994 Sept. 3 05:02 1.23 1.2 ′′ 13 V 1800 1994 Sept. 3 05:42 1.25 1.2

Processing of the CCD frames was accomplished as follows. First, a constant offset was determined form the overscan region and subtracted from each frame (sam- ple bias frames are constant across the chip and stable in time). Both sky and dome flat-fields (the latter taken on a white screen illuminated by day light) were taken in the 1994 run. A comparison of dome and sky flats shows them to be consistent at the 1% level. For the 1994 ob- servations, master twilight-sky flats have been created for each filter, with residual stellar images removed by taking the median of individual exposures. For the 1993 run only dome flats were available. After preliminary processing, the frames were sorted Fig. 1. The combined V image of Tucana from our data with by filter and by seeing quality. Images from the “good see- good seeing ing” set and the “medium seeing” set were carefully regis- Ivo Saviane et al.: CCD photometry of the Tucana dwarf galaxy 3 tered to a common reference frame. Finally, the registered Table 2. Photometric errors and incompleteness frames were co-added, yielding four master images, and the 1.2′′ V image is shown in Fig. 1. The size of the Point

Spread Function (PSF) on the combined images shows no V (V − I) σV σI σ(V −I) Ni Nf /Ni σc degradation with respect to the individual frames. 21.24 1.60 0.014 0.010 0.021 300 0.99 0.03 Stellar photometry was performed using DAOPHOT 21.74 1.51 0.021 0.017 0.030 420 0.99 0.02 and ALLSTAR (Stetson 1987, 1994) in the MIDAS envi- 22.23 1.42 0.028 0.028 0.045 800 0.99 0.03 ronment. After a few experiments, we found that an ana- 22.73 1.33 0.041 0.044 0.071 1240 0.98 0.03 lytic Moffat function (with β = 2.5), along with a variable 23.22 1.21 0.071 0.068 0.093 2060 0.95 0.03 PSF model with quadratic dependence on distance from 23.71 1.09 0.108 0.109 0.153 2500 0.84 0.04 the frame center, provided the best representation of the 23.96 1.02 0.110 0.159 0.193 220 0.64 0.09 PSF. A PSF was iteratively constructed for each master frame starting with a set of 50 bright, non-saturated stars and progressively reducing∼ this sample to 20 stars by discarding poorly fit stars. In particular, visual∼ inspec- jects with sharp < 1 are isolated hot pixels while sharp tion of a PSF image created by adding an artificial star to > 1 corresponds to− PSF spikes and diffuse objects. an empty frame was employed to identify and reject any Absolute calibration is based on observations of Lan- PSF stars contaminated by faint components. ALLSTAR dolt’s (1992) standard stars on the photometric night of was run twice on the combined frames, using our best PSF Sept. 3, 1994. We have obtained the following calibrations and the aperture photometry catalogs. For each of the four (standard deviations of the residuals are 0.006 mag in V combined images, all stars detected and analyzed on the and 0.014 mag in I, respectively): first run of ALLSTAR were subtracted and the residual images searched again for faint undetected objects. Then, ALLSTAR was run again on the complete list of stars. V = v′ +0.053(V I)+23.69 (1) In order to analyze the degree of completeness and − the photometric errors as a function of magnitude and lo- I = i′ 0.055(V I)+22.47 (2) cation on the frame, artificial star experiments were per- − − formed using the DAOPHOT task ADDSTAR, along with a code written by one of us (I.S.) to perform automatically ′ ′ the individual experiments as well as the general program where v , i are instrumental magnitudes defined as fol- of simulations. Each experiment follows a scheme designed lows: to yield approximately constant number of stars per mag- ′ m = m +2.5 log(t + ∆ t) kλX. (3) nitude bin. The results of these simulations are summa- ap exp − rized in Table 2: columns 1 and 2 specify the central mag- map are the “total” magnitudes resulting from a growth- nitude of the V bins and mean colors, columns 3 to 5 are curve analysis of the standard stars in circular apertures the standard deviations in V , I, and (V I), column 6 to ′′ − up to a radius R = 6. 4 (we have verified that magni- 8 give the number of stars added (Ni) for each magnitude tudes measured within larger apertures, say R 10′′, are bin, the completeness ratio c = Nf /Ni (where Nf is the only 0.01 mag brighter on the average). ∆ t is≤ the aver- number of artificial stars eventually found and measured age shutter delay and kλ represents the extinction coef- by DAOPHOT), and its standard error. Magnitude (color) ficients in the V, I bands. For the shutter delay we have errors were derived from the rms scatter of the difference assumed 0.4 s (Veronesi 1995); assuming ∆ t = 0 would between input and measured magnitudes (colors) of the not affect the (V I) color measurements, while V , I artificial stars. Finally, σc was estimated from the scatter magnitudes would− be fainter by 0.02 mag. We used mean of c obtained from 10 individual experiments at a fixed extinction coefficients for La Silla kV = 0.14 and kI = 0.06, magnitude. Completeness is 1 for V 20.7 and falls obtained from photoelectric photometry on the standard below 20% for V 24.2. ∼ ≤ ≥ Johnson/Bessell system at La Silla (G. Clementini, priv. Star-like objects were selected out of the raw photo- comm.). These values are confirmed by the La Silla Ex- metric catalogs by using a set of selection criteria based tinction database of the Geneva Observatory. A plausible on the values of the DAOPHOT parameters related to the 10% fluctuation of extinction during the night may con- quality of the fit and image shape (χ and sharp). The dis- tribute± about 0.02 mag to the zero point uncertainty on tribution of the same parameters resulting from artificial the V and I magnitudes,± and less than 0.005 mag to the star experiments served as a training set, complemented (V I) color uncertainty. by careful scrutiny of the Tucana co-added images. Ob- −The aperture corrections needed to transform the PSF jects with χ > 1.5 turned out to be hot pixels (mostly magnitudes of the Tucana stars into “total” aperture mag- cosmic rays) or spikes around bright sources; similarly, ob- nitudes were then calculated as mean differences (CV and 4 Ivo Saviane et al.: CCD photometry of the Tucana dwarf galaxy

CI ) between raw PSF-fit magnitudes and the instrumental agram for the stars in the outer region. The red giant magnitudes, for the set of bright, isolated stars on refer- branch of Tucana is similar to the giant branch of old, ence images of Tucana taken on the photometric night metal-poor Galactic globular clusters. In the same figures, of Sept. 3, 1994. We paid particular attention to avoid the Tucana’s CMD is compared with the MI – (V I)0 systematic errors on the colors, and found that the instru- diagrams of 6 globular clusters from Table 10 of Da Costa− mental colors of the reference stars are almost independent & Armandroff (1990; hereafter DA90), suitably scaled to of aperture size (with an rms error of 0.004 mag) for radii the distance of Tucana. The average metal abundance of 3′′. 0 22 (by a factor of 3. Color-magnitude diagrams 2–3). In order to reduce the effects of field contamination on the However, in the magnitude interval shown in Fig. 2 color-magnitude diagrams and luminosity functions, we some contamination of background galaxies is expected. defined an “inner region” (or “galaxy region”) including According to Tyson’s (1988) galaxy counts, we expect all stars within an ellipse (area 5.8 arcmin2) with semi- 90 objects for 20

Fig. 3. As for Fig. 2, for the outer field

is some contamination due to foreground/background ob- jects, but that this is less than 5% (cf. previous discussion).

Fig. 2. The Tucana’s CMD, compared with gi- ant branches from Da Costa & Armandroff (1990). Open circles represent starlike objects; obvious faint galaxies are indicated Table 4. Fiducial sequence along the RGB by filled symbols, while crosses identify photometric blends; the dashed line represents the 50 % completeness level. Globular ′′ ′′ I (V − I) σ − (1. 2) σ − (1. 5) σ cluster fiducial loci are shifted to the same distance modulus as (V I) (V I) meas − Tucana, assuming (m M) = 24.69 and AI = E(V −I) = 0. Left 21.0–22.0 1.22 0.10 0.13 0.10 to right: M15 ([Fe/H]= −2.17), NGC 6397 ([Fe/H]=−1.91), 22.0–22.5 1.16 0.14 0.11 0.13 M2 ([Fe/H]=−1.58), NGC 6752 ([Fe/H]=−1.54), NGC 1851 22.5–23.0 1.06 0.18 0.18 0.11 ([Fe/H]=−1.29) and 47 Tuc ([Fe/H]=−0.71)

A fiducial line through the RGB was then derived by analyzing the color distributions in different magnitude We note a few starlike objects in the CMD at (V I) > bins. The magnitude range of each bin, the modal (V I) − − 2, just above the V completeness limit. Visual inspection color, and the color dispersion σ(V −I), are given in Ta- ′′ ′′ of these stars allows us to rule out that they are artifacts ble 4 for both the 1. 2 images and the 1. 5 images. Column near the limit of our photometry. Although there are a few 5 will be described in detail in Sec. 5. The modal color rep- obvious faint galaxies (filled symbols in Fig. 2), or photo- resents the mode of the (V I) distribution, after all the − metric blends (crosses), most of the red objects appear to outlier stars have been removed by an interactive proce- be star-like. It is of interest to understand whether these dure equivalent to a 3σ clipping. Trend of the color spread objects are red stars belonging to Tucana, since in that as a function of magnitude was smoothed by fitting a lin- case they might indicate the presence of a metal-rich com- ear relation to σ − vs. I: σ − =0.058 I 1.087. (V I) (V I) − ponent. There are 15 red (V I > 2.0) objects in the inner Stars within 2σ from the ridge line of the Tucana’s region and 35 in the outer region.− Assuming for Tucana an RGB define a clean sample of stars most likely belonging exponential profile implies an excess of 5.3 4.2 objects in to Tucana (hereafter referred to as “2σ” sample). For the the inner region, which is significant only± at the 1σ level. “2σ” sample, there are 339 and 87 objects in the inner and We conclude that there is no significant population of red outer region, or Σout/Σin = 0.07, indicating that there objects in Tucana. 6 Ivo Saviane et al.: CCD photometry of the Tucana dwarf galaxy

As already noted by Da Costa (1994), another feature ground/ foreground objects. The field object counts (prop- in the CMD of Fig. 2 is the absence of any significant num- erly scaled to the inner area) were obtained by counting ber of stars bluer than the RGB. Lavery & Mighell (1992) stars in the outer field, applying the same color selection noted a concentration of stars at (V I) 1, V 22 in as for the inner field, and allowing for the 16% of the their color-magnitude diagram. These− may∼ be part∼ of an stars which represent the estimated contribution of Tu- asymptotic giant branch or, as the authors point out, just cana stars in the outer region (cf. Sect. 3). The corrected a manifestation of their large photometric color errors. LF’s are shown if Fig. 4, and the corresponding tables are Our CM diagrams confirm the latter explanation: there is available from the authors. no excess of stars bluer than (V I) = 1 at the level of the RGB tip. More quantitatively, in− the inner region there are 8 stars with 22 1.4) the num- bers are 4 and 7 stars, respectively, yielding− an excess of Fig. 4. V and I luminosity functions before (dotted lines) 2.0 2.1 stars. Visual inspection of the objects brighter ± and after (solid lines) completeness corrections and field object than the RGB tip in the galaxy region shows that at least subtraction one has an elongated shape suggesting it is a blend or a background galaxy. Thus we conclude that the surface density of stars brighter than the RGB tip is entirely con- Figure 5 shows the RGB luminosity function of Tu- sistent with the star counts in the outer region over the cana along with the LF of Fornax (Sagar et al. 1990). In same magnitude range, and rule out any significant pop- the same figure, the LF of Tucana is also compared with ulation of AGB stars younger than 10 Gyr in Tucana. the theoretical LF from Bergbush & VandenBerg (1992) for a metal abundance [Fe/H] = 1.68 (Z = 0.0004). In − 4. Luminosity function and distance Fig. 6 we compare the observed V absolute magnitude of TRGB the RGB tip, MV , with the V luminosity of the RGB Assuming that Tucana does not contain very red stars, tip from the Bergbush & VandenBerg’s (1992) models, we have derived the V and I RGB luminosity functions for different ages and , assuming a distance TRGB (LF) using stars in the 2σ sample, with a correction for of 870 pc (see below). MV is a sensitive function of incompleteness in both V and I according to the results metallicity, becoming fainter as the metal content grows, of artificial star experiments and a correction for back- while it is little sensitive to age for stellar populations Ivo Saviane et al.: CCD photometry of the Tucana dwarf galaxy 7 older than 8 Gyr (in contrast, the LF can be a useful age indicator for younger stellar populations). If Tucana’s dominant population is similar to old globular clusters (a plausible assumption given the absence of any significant intermediate-age population), the location of the observed tip (shaded region in Fig. 6) is consistent with models with Z = 0.0004 ([Fe/H] = 1.68), in very good agreement with the metallicity estimates− given in Sect. 5.

Fig. 6. V absolute luminosity of the RGB tip in the Bergbush & VandenBerg (1992) model isochrones, for different ages and metallicities. The shaded region indicates the absolute mag- nitude and uncertainty of the Tucana’s RGB tip, after cor- rection for distance modulus. Data points connected by solid lines represent models with fixed metallicity (top to bottom: Z = 0.0001, 0.0004, 0.001, 0.002, 0.003, 0.004, 0.006)

yielding the bolometric luminosity of the RGB tip as a function of metallicity, and the bolometric correction as a Fig. 5. Tucana’s LF (dots with error bars) compared to that function of color: of Fornax (dotted histogram; Sagar et al. 1990), and to a the- oretical LF (solid line; models from Bergbush & VandenBerg 1992, with Z = 0.0004 and Age = 15 Gyr) BCI = 0.881 0.243(V I) (4) − − tip Mbol = 0.19 [Fe/H] 3.81. (5) The I magnitude of the tip of the first-ascent red gi- − − ant branch can be used to determine the distance to Tu- Assuming [Fe/H] = 1.82 for the metallicity (cf. Sec. 5), − cana, following the methods described in Mould & Kris- and (V I) =1.45 0.05 (cf. Fig. 2), we obtain BCI = − tip ± tian (1986) and Lee et al. (1993a). Distance estimates ob- 0.531 0.01, M TRGB = 3.46 0.04, M TRGB = 3.99 ± bol − ± I − ± tained using the RGB tip have been shown to be of com- 0.04, and M TRGB = 2.54 0.04. The distance modulus V − ± parable accuracy to using Cepheid or RR Lyrae variables to Tucana is then (m M)=24.69 0.16, corresponding − ± (Madore & Freedman 1995). This method has the consid- to 870 60 kpc. ± erable advantage that for old, metal-poor systems ( 2.2 < − [Fe/H] < 0.7, 2

(Lee et al. 1993a). quadratically the systematic uncertainty 0.03 mag on the (V I)−3.5 was obtained from a 3σ-clipped Gaussian absolute color calibration, we obtain a total uncertainty fit to the− color distribution of the giant stars in the range σ[δ(V I)]=0.03, and σ[Fe/H]=0.11 dex for the metal- 21.13 1.15, and found [Fe/H]= 1.83. We conclude that− the average metallicity of Tucana− is [Fe/H]=–1.82 0.20 and adopt this value in the present paper. ±

Fig. 7. The mean difference between Tucana’s fiducial RGB 5.2. RGB width points and fiducial RGB lines from Da Costa & Armandroff The RGB color spread of several dwarf spheroidals has (1990) is plotted against [Fe/H]. The error bars are the stan- been found to be larger than measurement errors, a result dard deviations of the residuals in (V − I). The solid line rep- resents a linear fit to the points (see text) generally interpreted as due to an abundance spread of the red giant population. This is not a general rule, how- ever. For instance, Smecker-Hane et al. (1994) have been Second, the metallicity of Tucana was determined by able to separate the contributions of the RGB and the direct comparison of the Tucana’s RGB with the globular intermediate-age AGB found in Carina, suggesting that cluster giant branches of DA90. The mean color differences the width of the giant branch is consistent with the RGB δ(V I)0 between the RGB sequences of the 6 GC’s in of Galactic globular clusters. The presence of a metal- Fig.− 2 and our Tucana’s giant branch have been calculated abundance spread has received in some cases spectroscopic using the brightest 1.8 mag of the sequence (I 22.5). confirmation, e.g., from spectrophotometry of individual The color differences are plotted in Fig. 7 against≤ globular giants in Draco (Lehnert et al. 1992) and Sextans (Suntzeff cluster metallicities. et al. 1993). For example, from a model atmosphere anal- In order to reduce the statistical noise, we used only ysis of 14 giants in Draco, Lehnert et al. (1992) conclude stars in the 2-σ sample. A linear fit to the mean color that the color spread of Draco’s giants is due to a 1 differences as a function of GC metallicities on the DA90 dex range in metal abundance. Such a range of chemical∼ system is abundance is traced back to chemical enrichment of the interstellar medium by supernova ejecta and stellar winds. δ(V I) = 0.274 [Fe/H] 0.498. (7) Here we have measured the RGB width from our pho- − 0 − − tometry to see if it is compatible with internal errors. The interpolated metal abundance of Tucana is [Fe/H] Clearly, tighter upper limits on an intrinsic abundance dis- = 1.81, with a dispersion about the regression line persion are expected from HST observations. Tab. 4 lists − ′′ σ(V −I) = 0.01 (standard deviation of the mean). Adding the width of the RGB as measured from the 1. 2 and the Ivo Saviane et al.: CCD photometry of the Tucana dwarf galaxy 9

1′′. 5 image sets. The two sets give similar results. By com- sition angle (PA) for a> 65′′. These are given in Table 5 paring the colors from the two sets, we have obtained an along with their standard errors. estimate of the internal errors which is independent of an A V surface brightness profile including galaxy + sky AGB contribution to the color spread. In a given magni- was then obtained as the median flux along ellipses with tude bin, we have calculated σ∆(V −I), the standard de- constant ellipticity and orientation, and plotted against viation of the difference between the color measurements geometric radius (rg = √ab). The azimuthally averaged ′′ of the same stars in the two image sets. This reflects the profile is essentially flat for rg > 140 , so that the mean ′′ combined measurements errors. Assuming that the color intensity for rg > 140 was chosen to represent the sky measurements are of comparable uncertainty in the two level. In this range, the rms scatter of the profile, binned image sets, and uncorrelated, we can estimate the color in 10′′ intervals, is 0.05%. Large-scale (> 20′′) peak-to- ∼ measurement error σmeas = σ∆(V −I)/√2. The values of valley fluctuations of the sky background in the master σmeas are listed in Table 4, whereas the 1σ color errors V frame, in the absence of azimuthal averaging, are of ′′ ′′ derived from artificial star experiments on the 1. 2 data the order 0.3%. By extrapolating the inner (rg < 100 ) are listed in Table 2. While, in principle, crowding exper- exponential profile out to this geometric radius, we esti- ′′ iments (which use a constant PSF) could underestimate mate that the galaxy light at rg = 140 is of the order the random measurement errors in the case of large PSF 0.1% of (or about 7.5 mag arcsec−2 below) the sky back- variations across the frame, in the present case random ground. This is comparable to the uncertainties on the errors estimated from simulations and from comparison sky level, and the profile can be reliably measured down −2 of different frames are perfectly consistent. Note that the to µV 29 mag arcsec , where the galaxy light contri- ∼ low value of σmeas for the faintest bin is an artifact of bution becomes comparable to 3σ sky fluctuations. The incompleteness in the 1′′. 5 frames. sky-subtracted V luminosity profile of Tucana is shown in The results in Tab. 4 and Tab. 2 show that the width Fig. 8a against geometric radius, along with an exponen- of the Tucana’s giant branch is not significantly different tial fit in the range 10′′ 100′′. As it is common for dwarf − from that expected on the basis of photometric errors. elliptical galaxies (cf. Caldwell et al. 1992), the surface Once measurement errors are critically and carefully as- brightness profile of Tucana is well fit by an exponential sessed, no convincing indication of an abundance spread law except for the center. is found in our data.

Table 5. Structural parameters for Tucana 6. Surface photometry and structural parameters

6.1. Surface photometry Parameter Value

In this section we derive the structural parameters of Tu- VT 15.15 ± 0.18 cana needed to compare this galaxy with the other LG MV −9.55 ± 0.27 ± dwarf spheroidals. Our procedure follows quite closely the µ0,V (obs) 25.05 0.06 methods of Caldwell et al. (1992). In order to measure α (pc) 123 ± 19 ± the luminosity profile of Tucana, all stars brighter than S0,V 24.30 0.19 ± I = 20 or redder than V I =1.84 were subtracted from Re (pc) 205 32 SB 25.43 ± 0.19 the coadded images using− DAOPHOT. Further, all bright e r (pc) 176 ± 26 stars, obvious background galaxies, and diffraction spikes c rs (pc) 166 ± 5 were interactively masked out. In this way we were left µ0,V (King) 24.76 ± 0.18 with stars mostly belonging to the RGB, superimposed c 0.72 ± 0.12 onto a diffuse, unresolved light component. Surface pho- 1 − b/a 0.48 ± 0.03 tometry was performed only on the V image, since flat- P.A. 97◦ ± 2◦ fielding of the outermost area of the I image was not of sufficient accuracy to derive an extended surface bright- ness profile. The cleaned master V image was processed as follows. After 2 2 rebinning, the frame was median filtered with a 5 5× box. The smoothed image thus pro- The scale length (α) and central extrapolated surface × duced is essentially elliptical, with intrinsic noise due to brightness of the fit (S0,V ) are specified in Table 5, assum- the presence of resolved stars with a distribution which ing a distance of 870 kpc. Note that all magnitude errors appears somewhat clumpy. Ellipses were fit to isophotes given in Table 5 reflect internal error estimates; a 0.02 in the radial range 37′′

The total magnitude of Tucana (VT = 15.15 0.18) was calculated by integrating the observed surface± bright- ′′ ′′ ness profile in the innermost region (0 < rg < 20 ), and adding a correction based on the exponential fit. Alterna- tively, VT was obtained from the observed profile alone, ′′ integrated out to rg = 150 , in which case the asymptotic value is VT = 15.05. The two methods give consistent re- sults within the quoted errors. The absolute magnitude MV = 9.6 mag is consistent, within the large uncer- tainties− inherent in this kind of measurements for dwarf spheroidals, with the values derived by previous authors. The effective geometric radius Re, as well as SBe, the mean surface brightness inside Re, were derived from the parameters of the exponential fit. The errors on all derived quantities were obtained by assigning their extreme values −2 Fig. 8. a Surface brightness in V (mag arcsec ) against the to S0,V and α. In the case of MV we have also taken into geometric radius for Tucana. The line represents an exponen- ′′ ′′ account a 0.2 mag error on the distance modulus. tial fit to the luminosity profile between 10 and 100 , with ′′ − A King law is also well fit to the Tucana surface bright- scale length 29 . b Surface density (arcmin 2) of resolved stars in the Tucana field. Abscissa is geometric radius. Also shown ness profile (we used in this case the same radial range as ′′ for the exponential fit). A fit was obtained allowing all pa- is an exponential fit with scale length 31 (solid line) rameters to vary. This yielded the central surface bright- −2 ness of the model profile, µ0,V = 24.76 mag arcsec (not to be confused with the intensity scale parameter of the King law), the “core radius” rs, and the concentration pa- rameter c, all listed in Table 5. For comparison, the core radius at which the surface brightness drops at half the ′′ ′′ central value is rc = 42 6 (176 26 pc). Uncertainties on these parameters were± estimated± by changing the sky level by 3σ, and the range over which the fit was made. ± Also in the case of the King fit, the central surface brightness of the fit profile, µ0,V (King), is higher than the −2 observed value, µ0,V (obs) = 25.05 0.06 mag arcsec . The latter was calculated as the median± surface brightness in the inner 3′′, together with its associated rms scatter. The dip is certainly real, since it is also seen in the I pro- file, and the B profile of Da Costa (1994). Inspection of the images seems to suggest a clumpy (“ring-like”) distri- bution of the bright giants in the central region, rather than evidence for a dust lane. Finally, Fig. 9 shows the integrated color of Tucana ′′ inside rg = 40 . The central (V I)=0.89 0.01 was obtained as the mean of the inner− 20′′ (V I =0±.87 inside 40′′). Adopting the mean relation (B V−)=0.8 (V I) − − derived for the giant branches of globular clusters (see Fig. 9. Median (V − I) color averaged on ellipses plotted Smecker-Hane et al. 1994), this value corresponds to (B against distance from the galaxy center. Dots with error bars − ′′ V )=0.70, in full agreement with the result of Da Costa represent mean values in 10 bins (1994). Ivo Saviane et al.: CCD photometry of the Tucana dwarf galaxy 11

6.2. Surface density of resolved stars Mould 1994; and references therein). The presence of car- bon stars in many dSph companions to the Galaxy, and We have also investigated the structure of Tucana us- in And II, (see Armandroff 1994 for a recent review) in- ing the surface density of stars as a function of distance dicates that took place over an extended from the galaxy center. Figure 8b shows the surface den- time period, probably in episodes alternated to quiescent sity profile obtained by counting stars in elliptical annuli phases. with a 5′′ step in semi-major axis, and fixed ellipticity A possible correlation between the star formation his- and position angle. Only stars in the magnitude range tory of dSph and their distance from the parent galaxy was 20.7