First Light with the 120-lens Dragonfly Spectral Line Mapper

Abstract The Dragonfly Spectral Line Mapper (DSLM) is a 120-lens distributed aperture narrowband imaging telescope that is operating as a pathfinder for an upcoming 1140-lens instrument being built to directly image the circumgalactic medium of nearby galaxies. DSLM has been on sky since 2023 October, collecting representative data for individual galaxies in H α , [O iii ], and [N ii ]. In this paper, observations obtained for three targets (NGC 6946, NGC 891, and NGC 7479) are presented. These galaxies span a range of recessional velocities and foreground contamination levels, enabling a comprehensive evaluation of the experimental design. Diffuse H α structures down to 10 −19 erg s −1 cm −2 arcsec −2 on 10′ scales are mapped by our data. We report the discovery of new extended structures in diffuse H α and [N ii ] emission around NGC 6946 and NGC 891. We also map multiple new structures surrounding NGC 6946; however, due to the low recessional velocity of this object and the significant presence of galactic H α emission at low galactic latitudes, we cannot confirm whether these gas clouds are associated with NGC 6946 or are of Galactic origin. We present a framework using the “ sbcontrast ” tool which can be broadly applied to evaluate the limiting depth of wide-field narrowband imaging observations. The surface brightness limits obtained with DSLM are consistent with our theoretical predictions and, in fields with low foreground contamination, the imaging depth is not limited by instrument systematics. Future dedicated surveys on galaxies selected by recessional velocity to avoid sky emission lines are projected to reach surface brightness limits of 10 −20 erg s −1 cm −2 arcsec −2 on scales of a few arcmin, placing the circumgalactic medium of nearby galaxies within reach.