The Optical Counterpart to the Extremely Luminous X-Ray Source near Holmberg IX: A Possible Supershell in a Tidal Tail

Using Hα imaging we have identified a nebular counterpart to the bright (f_x(⁰.²-^{4 }keV) ∼4 × 10^-12 ergs cm^-2 s^-1) X-ray source X-9 (Fabbiano 1988) near the dwarf galaxy Ho IX, a close companion of M81. Nebular spectroscopy reveals strong [S II] and [O I] lines, indicating that this is a shock-heated nebula. The oxygen abundance from shock models is 0/H ≈10^-4. However, at the distance of M81 (3.63 Mpc) the diameter of the nebula is about 250 pc, twice the size of the largest supernova remnants (SNRs) in M33 and about the size of "supershells" in nearby galaxies. This nebula is associated with a faint, blue, possibly nonstellar source. The most likely explanation is that the nebular and X-ray emission are from a supershell due to multiple supernovae from a recently formed cluster or OB association. The SNR lies at an angular distance of about 2' (2 kpc) from Ho IX, so they may not be directly related. However, the SNR appears to be associated with the tidal tail extracted from M81 by past interactions with M82 and NGC 3077. However, the large inferred size, the relatively hard, soft X-ray spectrum, and lack of strong radio continuum emission make the SNR interpretation tentative at this time, and others scenarios including that this is an accreting, isolated white dwarf or neutron star are discussed.