M82: the exploding galaxy?

Abstract

IT is nearly 20 yr since M82 was accepted as the archetypal exploding galaxy. Its most spectacular feature is the system of luminous Hα filaments radiating from the core of the galaxy in the direction of the minor axis (ref. 1 and K.T. and D.J.A. in preparation), and the discovery of a velocity gradient within these filaments led to the conclusion that ionised material had been ejected from the nucleus at approximately 10³ km s⁻¹ by a cataclysmic explosive event some 10⁶ yr ago². A later and more extensive spectroscopic study refined and generally confirmed the explosion hypothesis³. It had been assumed that the emission was intrinsic to the filaments, but additional radio and optical data cast doubt on this assumption and hence the whole concept of an explosive event. The well ordered symmetrical pattern exhibited by the broad band polarisation measurements⁴ was interpreted as a consequence of reflection of the light from the main body of the galaxy by an extensive halo of dust particles⁵. Furthermore, the emission lines were also found to be polarised identically to the continuum^6,7; this strongly implied that they were also seen in reflection and, therefore, the observed velocity differences had to be reinterpreted in terms of motions of the scattering particles. An explosion model then becomes most unlikely, because then the dust would scatter exclusively redshifted light, whereas both blue and redshifts are observed⁸. M82 is enveloped in a giant HI cloud which engulfs the whole of the M81 group^9,10 and Solinger et al.¹¹ therefore suggested that M82 is merely an interloper in the group, moving through this dusty HI cloud in such a way that its drift velocity accounts for the observed Doppler field seen in reflection. We draw attention here to some spectacular new spectroscopic data, which we believe to be irreconcilable with these more recent scattering models and which naturally lead us to resurrect an expulsion hypothesis.