A Study of Two Diffuse Dwarf Galaxies in the Field

Abstract

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 $M_{\star }\approx 2$-$3\times 10^7{M}_{\odot }$. 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.

16 pages, 6 figures; accepted for publication in ApJ

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