Photometry and kinematics of dwarf galaxies from the Apertif H I survey

Context. Understanding the dwarf galaxy population in low density environments (in the field) is crucial for testing the current Λ Cold Dark Matter cosmological model. The increase in diversity toward low-mass galaxies is seen as an increase in the scatter of scaling relations, such as the stellar mass–size and the baryonic Tully-Fisher relation (BTFR), and is also demonstrated by recent in-depth studies of an extreme sub-class of dwarf galaxies with low surface brightnesses but large physical sizes called ultra-diffuse galaxies (UDGs). Aims. We aim to select dwarf galaxies independent of their stellar content and to make a detailed study of their gas and stellar properties. We selected galaxies from the APERture Tile In Focus (Apertif) H I survey and applied a constraint on their i-band absolute magnitude in order to exclude high-mass systems. The sample consists of 24 galaxies, 22 of which are resolved in H I by at least three beams, and they span H I mass ranges of 8.6 ≲ log(M_{H I}/M_⊙) ≲ 9.7 and a stellar mass range of 8.0 ≲ log(M_⋆/M_⊙)≲9.7 (with only three galaxies having log (M_⋆/M_⊙) > 9). Methods. We determined the geometrical parameters of the H I and stellar disks, built kinematic models from the H I data using ^3DBarolo, and extracted surface brightness profiles in the g-, r-, and i-bands from the Pan-STARRS 1 photometric survey. We used these measurements to place our galaxies on the stellar mass–size relation and the BTFR, and we compared them with other samples from the literature. Results. We find that at a fixed stellar mass, our H I-selected dwarfs have larger optical effective radii than isolated optically selected dwarfs from the literature, and we found misalignments between the optical and H I morphologies for some of our sample. For most of our galaxies, we used the H I morphology to determine their kinematics, and we stress that deep optical observations are needed to trace the underlying stellar disks. Standard dwarfs in our sample follow the same BTFR of high-mass galaxies, whereas UDGs are slightly offset toward lower rotational velocities, in qualitative agreement with results from previous studies. Finally, our sample features a fraction (25%) of dwarf galaxies in pairs that is significantly larger with respect to previous estimates based on optical spectroscopic data.