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Planetary nebulae in local group galaxies. VI - an observational determination that M32 is in front of M31
Ford, H. C.; Jacoby, G. H.; Jenner, D. C.
AA(California, University, Los Angeles, Calif.), AB(California, University, Los Angeles, Calif.), AC(California, University, Los Angeles, Calif.)
Astrophysical Journal, Part 1, vol. 223, July 1, 1978, p. 94-97. Research supported by the University of California (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Andromeda Galaxy, Elliptical Galaxies, Local Group (Astronomy), Planetary Nebulae, Radio Astronomy, Astronomical Maps, Color, Gravitational Effects, Interstellar Extinction, Spectral Line Width
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Twenty-one-centimeter integrated brightness maps of M31 are used to predict a color excess E(B - V) of approximately 0.5 if M32 is behind M31. A galactic cosecant law applied to M31 predicts a color excess between 0.5 and 0.6. It is concluded that M32 will be heavily reddened if it is behind M31. The predicted value of H-alpha/H-beta intensity ratio is 5.1 for a nebula in M32 if M32 is behind M31, whereas the predicted ratio is 3.2 if M32 is in front of M31. The observed ratio (2.96) in the planetary nebula M32-1 agrees with the latter value within the observational uncertainty. This is taken to be strong evidence that M32 is in front of M31. Additional arguments in favor of this conclusion are (1) the absence of dust clouds silhouetted against M32; (2) the absence of known visual absorption and color gradients across M32; and (3) the inconsistency of the intrinsic color of M32 and its integrated line strengths if it is behind M31. All considerations lead to the conclusion that M32 must be in front of M31. Byrd (1976) showed that gravitational perturbation by M32 can account for the velocity and morphological anomalies observed in M31. The encounter orbit which best reproduces the anomalies places M32 at the present time near to and in front of M31. It is concluded that the basic picture of gravitational perturbation of M31 by M32 is correct.

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