3. Distance and metallicity from the colour-magnitude diagram
The I vs. CMD of the resolved stars in the SagDIG frame is shown in Fig. 4. SagDIG lying close to the direction of the Galactic center, has a CMD heavily contaminated by foreground stars. Fig. 5 shows the CMD of the aforementioned nearby field. In both figures, only stars with SHARP and CHI ALLFRAME parameters in the intervals and in both filters have been plotted, being 1513 in Fig. 4 and 648 in Fig. 5. Comparison of both CMDs shows that for , the former is almost free from foreground stars, while most of the stars redder than that value and brighter than must be Galactic members. Two features can be recognized to originate in SagDIG: the blue sequence with , extending up to and the sequence of red stars at to 1.5, extending up to . The former is identified as being produced by young main sequence and He-burning blue-loop stars while the latter can be recognized to be the red giant branch (RGB), and should also contain asymptotic giant branch (AGB) stars.
The distance to SagDIG can be obtained from the tip of the RGB (TRGB), which for metal poor systems like SagDIG (see below) can be assumed to be at (Da Costa & Armandroff 1990).
The magnitude of the TRGB has been obtained applying a Sobel filter (kernel [-1,0,+1]; see Myler & Weekes 1993) to the luminosity function of stars with . To minimize the effects of foreground contamination, only the stars at less than 1´ from the center of SagDIG have been used (see below for the dimensions of the galaxy). The resulting luminosity function and Sobel filtered luminosity function are shown in Fig. 6. The TRGB corresponds to the peak at (bottom panel), the one at at being produced by a density fluctuation inside the RGB. The error can be estimated as 1/2 of the peak width at 62% of its maximum and turns out to be .
We have adopted a Galactic reddening for SagDIG of or from the IRAS/DIRBE map (Schlegel et al. 1998). The above values yield a distance modulus of or Mpc, where the standard error includes uncertainties in the exact location of the TRGB, extinction, and photometric calibration. SagDIG is at Mpc from the barycenter of the Local Group and at 1.34 from M31. The closest galaxies to SagDIG are DDO 210, at 0.35 Mpc, and NGC 6822, at 0.56 Mpc. Hence SagDIG seems to be a rather isolated galaxy in the periphery of the Local Group.
The average magnitude of the three brightest stars in a galaxy is a simple, frequently used method to estimate the distance. However, for low surface brightness galaxies without very young stars this method usually leads essentially to overestimating the distance. For illustrative purposes, we have obtained the distance to SagDIG by this method also. The mean apparent magnitude of the three brightest blue stars can be estimated from our VI photometry using the relation obtained from blue  standard stars of Landolt (1992). Applying this to the stars with in Fig. 4 the mean apparent magnitude of the three brightest ones is , or if the brightest blue star [, ] is assumed to be a foreground contaminator and neglected. Moreover, the integrated B magnitude of the galaxy can be estimated using the values and derived in Sect. 4 and the relation by Makarova & Karachentsev (1998). It results . Using the standard relation from (Karachentsev & Tikhonov 1994) with it is obtained (26.36) if the brightest star is (is not) considered, which is about 1 mag more than the distance modulus via TRGB or more than 50% larger in the distance. It must be noted that Cesarsky et al. (1977) obtained , which would result in and that these authors claimed a distance modulus of , close to the TRGB estimate. However Cesarsky et al. estimate was based on rough eye photometry on photographic plates and could well be affected of severe blending.
The metallicity of SagDIG was measured by Skillman et al. (1989) from a low surface brightness HII region in the galaxy. They obtained . Alternatively, we have estimated the mean metallicity of the stars in SagDIG from the mean colour of the RGB at , i.e. 0.5 mag fainter than the TRGB. At this level we have a mean assuming a reddening of . Lee et al. (1993) provided a calibration for the metallicity of the RGB based on the colours at for Galactic globular clusters (Da Costa & Armandroff 1990): [Fe/H] . Using this calibration we obtain a value for the mean metallicity of [Fe/H] . This value is quite smaller than the Skillman et al.'s one. However, it must be noted that it corresponds to intermediate and old stars in the galaxy and, more important, that Lee et al.'s relation is used in extrapolation, since the least metallic globular cluster (M 15) used by Da Costa & Armandroff (1990) has [Fe/H]. In any case, SagDIG lies at the extreme metal-poor end of dwarf galaxies. Fig. 7 shows the RGB fiducials of Da Costa & Armandroff (1990) overplotted to the distance and reddening corrected CMD of SagDIG.
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999