6. Global integrated properties of SagDIG
A summary of the basic parameters of SagDIG is given in Table 2. Except where otherwise stated, the data listed are from this paper. The parameters listed in Table 2 are: (1,2) - equatorial coordinates of the galaxy center; (3) - the standard angular dimensions; (4) - the heliocentric radial velocity (data for the four first lines are from NED); (5) - the radial velocity with respect to the Local Group centroid (Karachentsev & Makarov 1996); (6,7) - interstellar reddening and extinction (Schlegel et al. 1998); (8,9) - integrated colour and apparent magnitude of the galaxy; (10) - the central surface brightness in V; (11,12) - the apparent reddening-corrected I-magnitude of the TRGB, and the median reddening corrected colour of the RGB measured at ; (13) - the mean metallicity estimated from using the calibration by Lee et al. (1993); (14-17) - the distance modulus and linear distance from the Milky Way, from the Local Group centroid, and from M 31; (18,19) - the total absolute magnitude and the standard linear diameter of the galaxy; (20,21) - the hydrogen mass-to-luminosity ratio and the total (virial) mass-to-luminosity ratio, for which the total luminosity in B obtained through and the hydrogen and total mass from Lo et al. (1993) have been used.
Table 2. Observed and derived properties of SagDIG
As the presented data show, SagDIG is one of the faintest, smallest irregular systems in the Local Group. According to its values of and , SagDIG seems to be an usual gas-rich dIrr. Its low central brightness and low luminosity follow the common relation between these parameters for dwarf galaxies (Caldwell et al. 1998). But the derived low mean metallicity of SagDIG displaces it from the main sequences [Fe/H] vs. and [Fe/H] vs. for dwarf galaxies (Lee 1995; Grebel & Guhathakurta 1999). The unusually low metallicity of SagDIG needs, of course, an independent confirmation. But if correct, studying the gas in SagDIG would become an interesting task because it could be the lowest metallicity dIrr known, with Z smaller than that of I Zw 18 even.
Being at more than 1 Mpc both from the Milky Way and from M31, SagDIG is apparently situated outside these two subsystems of the Local Group. Together with Sex A, Sex B, NGC 3109, Antlia and UGCA 438, it forms a scattered envelope of the Local Group populated by dIrr galaxies. In fact, SagDIG seems to be a rather isolated dwarf galaxy whose autonomous evolution proceeds without strong tidal influence from massive neighbours.
© European Southern Observatory (ESO) 1999
Online publication: December 2, 1999