Annu. Rev. Astron. Astrophys. 1992. 30: 359-89 Copyright © 1992 by Annual Reviews. All rights reserved

3.2 SNe Ia in Nearby Galaxies

No supernova known to be of type Ia has been observed in a galaxy in which cepheids have been reached, but four SNe Ia have appeared in galaxies within just a few megaparsecs. SN 1937C in IC 4182 and SN 1895B and SN 1972E in NGC 5253 were observationally normal and only slightly extinguished, and are suitable calibrators provided their distances can be established. SN 1986G in NGC 5128 was both observationally peculiar and highly reddened (Section 2.3), and cannot be trusted as a calibrator (Section 2.3).

In Table 4 we adopt a distance modulus µ₀ = 28.2 ± 0.3(4.4 ± 0.6 Mpc) for IC 4182, as determined by Sandage & Tammann (1982) on the assumption that its three brightest red stars have a mean value of M_V⁰⁰ = -7.72. The spiral features and associations of IC 4182 made the identification of its member stars quite straightforward and photometry of the brightest stars was particularly easy because the surface brightness of the back-ground disk of IC 4182 is low. The main uncertainty regarding the distance determination is whether the red-star luminosity function in IC 4182 is filled. Sandage & Tammann also determined the distance to NGC 4214, the parent galaxy of SN 1954A, but this supernova is now regarded to have been of type Ib (Branch 1990). A shorter distance to IC 4182 has been derived by Pierce et al (1992) from I- and K-band photometry of its brightest red stars. Observations of cepheids in IC 4182 by Sandage and others with the Hubble Space Telescope will clarify the discrepancy.

van den Bergh (1989) reviews the distance to the NGC 5128 / 5236 group, which he takes to include NGC 5253. Based on the planetary nebula distance to NGC 5128 relative to M31 (Jacoby et al 1988), van den Bergh recommends µ₀ = 27.95 ± 0.13 (3.9 ± 0.2 Mpc) for NGC 5128 itself, and he concludes that most of the group members have distances in the range 3.2-4.6 Mpc. We adopt this recommendation and use 3.9 ± 0.7 Mpc for NGC 5253 in Table 4.

Supernova apparent magnitudes, corrected for Galactic extinction, are taken from LTCC91. For SN 1937C and SN 1895B we have converted from m_pg to B magnitudes using B = m_pg + 0.26 (Leibundgut & Tammann 1990).

Our interpretation of the absolute magnitudes in Table 4 is that all three supernovae were normal SN Ia. The excessively high brightness of SN 1895B is probably due to the ancient photometry (cf Cadonau & Leibundgut 1990) and should be given low weight. We therefore adopt M_B⁰ = - 19.6 ± 0.3. This value may still be affected by some extinction in the parent galaxies. From the precepts set out in Section 2.5 and the colors given by LTCC91 we have estimated the parent-galaxy extinction in Table 4. No colors are available for SN 1895B. The fully corrected absolute magnitudes M_B⁰⁰ are given in Table 4. From them we adopt as the best value M_B⁰⁰ = -19.8 ± 0.3.