The Brightest Stars in Nearby Galaxies IX: Comparison of Ground-Based and HST Photometry of the Brightest Stars in IC 4182

Abstract

The distance to the Sdm dwarf galaxy IC 4182 is known from the measurements of its Cepheids made with the prerepair HST by Saha et al. [ApJ, 425,14(1994)]. IC 4182 is the parent galaxy of the type Ia supernova 1937C. Because of this it is important for calibrating the SNe Ia prime distance indicator as well as other secondary indicators. We compare HST photometry for the brightest stars in IC 4182 with two independent sets of Palomar ground-based data, one using the Argelander step-scale method tied to a photoelectric sequence; the other done with a CCD camera. Finding charts are given for all the stars that were measured. The three purposes of the study are (1) to test the photometric zero points used for the HST Cepheid measurements, (2) to obtain the magnitudes of the brightest blue and red stars in IC 4182 and to compare them with similar data in other galaxies that have Cepheid distances, and (3) to assess the effect of crowding on the brightest star problem by comparing the HST and the ground-based data where the angular resolutions differ by a factor of 10. For stars brighter than V = 22, the V band photometric zero points for the HST and the Palomar CCD data agree to within 0.05+/-0.03 mag, with an rms scatter of 0.16 mag. The I band zero points agree to within 0.02+/-0.04 mag, with an rms scatter of 0.14 mag. These confirm the magnitude zero points used in the HST Cepheid photometry in IC 4182. The mean apparent magnitudes of the three brightest blue stars are <B3> = 20.34, <V3> = 20.11, <R3> = 20.18, and <I3> = 20.12. The respective absolute magnitudes, using the Cepheid modulus of m-M = 28.36, are -8.02, -8.25, -8.18, and -8.24. The corresponding data for the brightest red stars are <B3> = 22.36, <V3> = 20.76, <R3> = 19.92, and <I3> = 18.94. The absolute magnitudes are -6.00, -7.60, -8.44, and -9.42. Problems of membership of the candidate stars are discussed. The absolute magnitudes in B and V fit well with similar data for the brightest stars in other galaxies, increasing confidence in such stars as secondary distance indicators.