A Study of Two Diffuse Dwarf Galaxies in the Field

Abstract

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 SuprimeCam Subaru Strategic Program (HSC-SSP). We measure redshifts using Ha line emission and find that these galaxies are at comoving distances of approximate to 25 and approximate to 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-star approximate to (2-3) x 10(7)M(circle dot). There are no massive galaxies (M-star > 10(10) M-circle dot) 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(star)/Z(circle dot)] < -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 similar to 50 km s(-1), which is comparable to dwarf galaxies of similar stellar mass with estimated halo masses <10(11)M(circle dot). 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.