A Study of Two Diffuse Dwarf Galaxies in the Field

TL;DR

This study characterizes two isolated diffuse dwarf galaxies through spectroscopy and spectral energy distribution fitting, revealing their distances, sizes, stellar masses, metallicities, and kinematic properties, highlighting their unique environment and star-forming nature.

Contribution

It provides detailed observational data on two newly discovered isolated diffuse dwarf galaxies, including their distances, metallicities, and kinematic limits, expanding understanding of such galaxies in low-density environments.

Findings

Both galaxies are at distances of approximately 25 and 41 Mpc.

They have stellar masses around 2-3×10^7 solar masses.

Gas-phase metallicities may be higher than expected for their stellar mass.

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 $M_\star\approx2$-$3\times10^{7}~M_\odot$. 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.