Andromeda's tenuous veil: A likely Milky Way nebula projected toward M31

Abstract

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. We aim to determine whether the nebula is extragalactic or located within the Milky Way. This will enable us to constrain its physical properties and assess its nature. To do so, we obtained deep narrowband [O II]3727 and H$α$+[NII] observations with the JAST80 telescope at the Observatorio Astrofísico de Javalambre, as well as high spectral resolution spectroscopy (R~5000) at four locations within the region of interest using MEGARA at the Gran Telescopio Canarias. We found extended [O II] emission along two near-parallel strands to the [O III], offset by six arcmin. The nebular spectra reveals up to 6 emission lines from [O III]4959,5007, H$β$, [N II]6583, and [S II]6716,6731. Their receding velocities are above $-$40 km s$^{-1}$, far from the systemic velocity of M31 ($-$300 km s$^{-1}$). The fluxes and velocities are consistent for the same lines across different regions of the nebula. The nebular properties suggest a location within the Milky Way rather than being physically associated with M31. The most likely scenario is a resolved ionization structure in a Galactic nebula with a separation between [O II] and [O III] 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 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.

Figures (6)

• Figure 1: Image of the [O III] emitting nebula (known as SDSO) and its surrounding area, including the Andromeda galaxy (top right). The [O III] flux is shown in cyan, while H$\alpha$+[N II] is shown in red. The green squares (surrounded by green circles) mark the location of the MEGARA pointings in and around the nebula presented in this work. The dashed square shows the area covered by our full-depth observations with the JAST80 telescope targeting [O II] and H$\alpha$ with narrowbands $J0378$ and $J0660$, respectively. The RGB imageof the nebula and M31 was presented in Drechsler23, and was obtained by Marcel Dreschler, Xavier Strottner, and Yann Sainty.
• Figure 2: Narrowband images obtained using JAST80 on the [O III] nebula. Top.$J0378$ image, tracing the [O II]3727 emission line. Bottom.$J0660$ image, tracing H$\alpha$ and [N II] emission. In both images, in blue, we show the contours of areas where we detect emission in each image. In dotted, red lines, we encircle the area where the full depth in the images is achieved.
• Figure 3: MEGARA spectra of the pointings in the first night (four in the blue range, three in the red), in the observed wavelength frame. Top. The LR-B spectra, centered in the wavelength range from H$\beta$ to [O III] 5007. Bottom. The LR-R spectra, from [N II] 6583 to [S II]6731. The emission lines of interest are marked in both plots. The lines that are not marked are sky emission lines, with H$\alpha$ being contaminated by one.
• Figure 4: Line fluxes measured with MEGARA at GTC in the different pointings in the region of the nebula on the first night of observations. They correspond to the integrated flux of the best-fitting Gaussian function to the line profile. The lines that are missing have not been detected by our code in the spectra, except for the red lines ($\lambda$>6000 Å) in P3, where those observations were not made.
• Figure 5: Receding barycentric velocities derived from different emission lines and pointings. For each pointing, the first column corresponds to results from the first night of observations, and the second one to the second night. Color code and lines missing as in Fig. \ref{['fig:fluxes']}.