A Study of Two Diffuse Dwarf Galaxies in the Field

We present optical long-slit spectroscopy and far-ultraviolet to mid-infrared spectral energy distribution fitting of two diffuse dwarf galaxies, LSBG-285 and LSBG-750, which were recently discovered by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We measure redshifts using Hα line emission and find that these galaxies are at comoving distances of ≈25 and ≈41 Mpc, respectively, after correcting for the local velocity field. They have effective radii of r _eff = 1.2 and 1.8 kpc and stellar masses of M _∗ ≈ (2-3) ×10⁷ M _o. There are no massive galaxies () within a comoving separation of at least 1.5 Mpc from LSBG-285 and 2 Mpc from LSBG-750. These sources are similar in size and surface brightness to ultradiffuse galaxies, except they are isolated, star-forming objects that were optically selected in an environmentally blind survey. Both galaxies likely have low stellar metallicities [Z _∗/Z _o] < -1.0 and are consistent with the stellar mass-metallicity relation for dwarf galaxies. We set an upper limit on LSBG-750's rotational velocity of ∼50 km s^-1, which is comparable to dwarf galaxies of similar stellar mass with estimated halo masses <10¹¹ M _o. We find tentative evidence that the gas-phase metallicities in both of these diffuse systems are high for their stellar mass, though a statistically complete, optically selected galaxy sample at very low surface brightness will be necessary to place these results into context with the higher surface brightness galaxy population.