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Optical light curves and spectra of the Type la supernova 1986G in NGC 5128 (Centaurus A) are presented. SN 1986G was discovered approximately one week before maximum light. The initial rate of decline of the B light curve was remarkably fast and characteristic of the infrequently observed Pskovskii photometric class ß = 12. Although the spectral evolution closely resembled that of the more common "slower" photometric classes of Type la supernovae, subtle differences in the maximum-light spectra were detected. The expansion velocity of the photosphere of SN 1986G decreased rapidly at early phases, suggesting that the outer-envelope density gradient was less steep than in supernovae with smaller values of ß. SN 1986G appears to have been heavily obscured (E(B -V) = 0.90 ± 0.10) by the dust lane of NGC 5128. This circumstance accounts for the strong interstellar-absorption lines of Ca n H and K and Na ID observed in the spectra as well as for several weaker absorption features that we identify with the diffuse interstellar bands.SN 1986G provides graphic confirmation of the existence of intrinsic differences in the optical light curves and spectroscopic properties of Type la supernovae. Consequently, these objects must be used with considerable caution as cosmological standard candles. On the basis of the very close resemblance of SN 1986G to SN 19711 in NGC 5055, we derive a relative distance of D NGC5128 / D NG c5055 = 0.39 ± 0.04. Further distance estimates are hampered due to the lack of other well-observed Type la supernovae with ß = 12.

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We present the results from our program to determine the evolution of the galaxy interaction/merger rate with redshift using the unique star-forming characteristics of collisional ring galaxies. We have identified 25 distant collisional ring galaxy candidates (CRGCs) in a total of 162 deep Hubble Space Telescope Wide Field/Planetary Camera-2 images obtained from the HST Archives. Based on measured and estimated redshifts, these 25 CRGCs all lie in the redshift interval of 0.1 ≤ z ≤ 1. Using the local collisional ring galaxy volume density and the new "standard" cosmology, we find that in order to account for the number of identified CRGCs in our surveyed fields, the galaxy interaction/merger rate, parameterized as (1 + z) m , must increase steeply with redshift. We determine a minimum value of m = 5.2 ± 0.7, though m could be as high as 7 or 8. We can rule out a non-evolving (m = 0) and weakly evolving (m = 1-2) galaxy interaction/merger rate at greater than the 4 sigma level of confidence.