Is the Pegasus Dwarf Galaxy a Member of the Local Group?

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Galaxies: Individual (Pegasus Dwarf Irregular Galaxy: Ddo 216), Galaxies: Stellar Content, Galaxies: Photometry, Local Group, Cepheid, Distance Scale

Scientific paper

Deep VI CCD photometry of the Pegasus dwarf irregular galaxy shows that tip of the red giant branch (RGB) is located at I = 21.15+-0.10 mag and (V-I) = 1.58+-0.03. Using the I magnitude of the tip of the RGB(TRGB), the distance modulus of the Pegasus galaxy is estimated to be (m-M)o = 25.13+-0.11 mag(corresponding to a distance of d = 1060+-50kpc). This result is in a good agreement with the recent distance estimate based on the TRGB method by Aparicio[1994, ApJ, 437, L27], (m-M)o = 24.9 (d = 950kpc). However, our distance estimate is much smaller than that based on the Cepheid variable candidates by Hoessel et al.[1990, AJ, 100, 1151], (m-M)o = 26.22+-0.20(d = 1750+-160 kpc) mag. The color-magnitude diagram illustrates that the Cepheid candidates used by Hoessel et al. are not located in the Cepheid instability strip, but in the upper part of the giant branch. This result shows that the Cepheid candidates studied by Hoessel et al. are probably not Cepheids, but other types of variable stars. Taking the average of our distance estimate and Aparicio's, the distance to the Pegasus galaxy is d = 1000+-80 kpc. Considering the distance and velocity of the Pegasus galaxy with respect to the center of the Local Group, we conclude that the Pegasus galaxy is probably a member of the Local Group.

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