@ARTICLE{2025A&A...704A.224L,
       author = {{Lumbreras-Calle}, A. and {Fern{\'a}ndez-Ontiveros}, J.~A. and {Infante-Sainz}, R. and {Akhlaghi}, M. and {Montoro-Molina}, B. and {P{\'e}rez-D{\'\i}az}, B. and {del Pino}, A. and {Vives-Arias}, H. and {Hern{\'a}n-Caballero}, A. and {L{\'o}pez-Sanjuan}, C. and {Guerrero}, M.~A. and {Eskandarlou}, S. and {Ederoclite}, A.},
        title = "{Andromeda's tenuous veil: A likely Milky Way nebula projected toward M31}",
      journal = {\aap},
     keywords = {ISM: clouds, ISM: general, ISM: kinematics and dynamics, ISM: lines and bands, galaxies: individual: M31, Astrophysics of Galaxies},
         year = 2025,
        month = dec,
       volume = {704},
          eid = {A224},
        pages = {A224},
     abstract = "{Context. A large, faint nebula was unexpectedly discovered near M31
        using narrowband [O III] images. Its apparent size and the lack
        of a clear counterpart at other wavelengths make it unique and
        challenging to explain. Aims. We aim to determine whether the
        nebula is extragalactic and vast or associated with the Milky
        Way filamentary structure. This will enable us to constrain its
        physical properties and assess its nature. Methods. We obtained
        deep narrowband [O II]3727 and H{\ensuremath{\alpha}}+[NII]
        observations with the JAST80 telescope at the Observatorio
        Astrof{\'\i}sico de Javalambre, as well as high spectral
        resolution spectroscopy (R \raisebox{-0.5ex}\textasciitilde
        5000) at four locations within the region of interest using the
        MEGARA integral field unit at the Gran Telescopio Canarias.
        Results. We found extended [O II] emission along two near-
        parallel strands to the [O III], offset by six arcmin. The
        nebular spectra reveal up to six emission lines from [O
        III]4959,5007, H{\ensuremath{\beta}}, [N II]6583, and [S
        II]6716,6731. Their receding velocities are above
        {\ensuremath{-}}40 km s$^{{\ensuremath{-}}1}$, far from the
        systemic velocity of M31 ({\ensuremath{-}}300 km
        s$^{{\ensuremath{-}}1}$ ). The fluxes and velocities are
        remarkably consistent for the same lines across different
        regions of the nebula. Conclusions. The nebular properties
        suggest a location within the Milky Way rather than being
        physically associated with M31. The most likely scenario
        suggests a resolved ionization structure in a Galactic nebula
        with a separation between [O II] and [O III] on the order of a
        few parsecs. The observed receding velocities would be
        unprecedented for an object physically linked to M31 but are
        common for nearby gas filaments. Their consistency across the
        nebula would also be highly unusual if it were larger than a
        kiloparsec. The analysis of the emission line ratios, line
        widths, and morphology suggests the possibility of it being an
        interstellar gas filament with an additional source of
        ionization to explain the [O III] emission. However, the complex
        properties of this object call for further observations to
        confirm its nature.}",
          doi = {10.1051/0004-6361/202453413},
archivePrefix = {arXiv},
       eprint = {2412.08327},
 primaryClass = {astro-ph.GA},
       adsurl = {https://ui.adsabs.harvard.edu/abs/2025A&A...704A.224L},
      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}