Velocity measurement in the extensive [OIII] emission region 1.2° south-east of M 31

Context. The discovery of a broad, ∼1.5° long filamentary [OIII] 5007 emission ∼1.2° south-east of the M 31 nucleus has recently been reported. More than 100 hours of exposures of a wide field (3.48° × 2.32°) have allowed this pioneering detection based on 30 Å narrow-band filters and several small refractors equipped with large cameras. Aims. We report a first velocity measurement in this extensive [OIII] emission line region. Methods. We used the low-resolution spectrograph MISTRAL (R ∼ 750), a facility of the Haute-Provence Observatory 193 cm telescope. The velocity measurement is based on the Hα, [NII], [SII] and [OIII] lines. Results. The best solution to fit the spectrum indicates that the Hα and [OIII] emissions are at the same heliocentric line-of-sight velocity of −96±4 km s−1. This was measured within an area of ∼250 arcsec2 selected on a bright knot along the long filament of ∼1.5°, together with a [OIII]5007 surface brightness of 4.2±2.1×10−17 erg s−1 cm−2 arcsec−2. This agrees moderately well with the previous measurement. We also estimated the Hα/[NII] line ratio as ∼1.1. Conclusions. The radial velocities at which the Hα and [OIII] lines were detected seem to show that these hydrogen and oxygen atoms belong to the same layer, but we cannot exclude that another weaker [OIII] line, belonging to another structure, that is, at another velocity, is below our detection threshold. Different scenarios have been considered to explain this filamentary structure. The extra-galactic origin was excluded in favour of Galactic origins. We tentatively assume that this filament is a piece of a supernova remnant located at a distance of ∼0.7 kpc from the Sun, of which we only see a small fraction of the shells with a radius of ∼35 pc. The progenitor may be along the line of sight of the galaxy M 31, but this observation might also just be part of a large-scale filamentary structure that should be investigated further.