A Study of Two Diffuse Dwarf Galaxies in the Field
  We present optical long-slit spectroscopy and far-ultraviolet to
near-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
H$\alpha$ line emission, and find that these galaxies are at comoving distances
of ${\approx}25$ and ${\approx}41$ Mpc, respectively, after correcting for the
local velocity field. They have effective radii of $r_\mathrm{eff}=1.2$ and 1.8
kpc and stellar masses of $M_\star\approx2$-$3\times10^{7}~M_\odot$. There are
no massive galaxies ($M_\star>10^{10} M_\odot$) 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 ultra-diffuse galaxies, except they
are isolated, star-forming objects that were optically selected in an
environmentally blind survey. Both galaxies likely have low stellar
metallicities $[Z_\star/Z_\odot] < -1.0$ and are consistent with the stellar
mass-metallicity relation for dwarf galaxies. We set an upper limit on
LSBG-750's rotational velocity of ${\sim}50$ km s$^{-1}$, which is comparable
to dwarf galaxies of similar stellar mass with estimated halo masses
$<10^{11}~M_\odot$. 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.
