Not so-dark: High resolution HI imaging of J0139+4328 and identification of an optical counterpart

TL;DR

This study uses high-resolution VLA imaging to identify an optical counterpart of a candidate dark galaxy, revealing it as a gas-rich, low-mass galaxy consistent with known relations, emphasizing the importance of high-resolution data.

Contribution

First high-resolution interferometric follow-up of a dark galaxy candidate, demonstrating the necessity of detailed imaging for accurate identification and characterization.

Findings

Identified an optical counterpart to the HI source J0139+4328.

Found the galaxy has a stellar mass of 3 x 10^6 M_Sun and high gas richness.

Confirmed the galaxy's properties align with known stellar-to-HI mass and baryonic Tully-Fisher relations.

Abstract

Dark galaxies - systems rich in neutral hydrogen (HI) gas but with no stars - are a common prediction of numerous theoretical models and cosmological simulations. However, the unequivocal identification of such sources in current HI surveys has proven challenging. In this work, we present interferometric follow-up observations with the VLA of a former dark galaxy candidate J0139+4328, originally detected with the single-dish FAST telescope. The improved spatial resolution of the VLA data allow us to identify a faint optical counterpart and characterize the galaxy. Located at a distance of about 31 Mpc, J0139+4328 has a stellar mass of 3 x 10^6 M_Sun and a relatively high gas richness of M_HI/M_star = 18. Despite its high ratio, the galaxy is consistent, within the scatter, with the stellar-to-HI mass relation of HI-selected samples in the literature and with the baryonic Tully-Fisher relation (BTFR), although its kinematic measurement is subject to large uncertainties. This case highlights the potential of modern high-sensitivity HI surveys for detecting low surface brightness, gas-rich galaxies, but underscores the need for careful interpretation of low-resolution HI data, with potentially large centroid errors, and for sufficiently deep optical imaging to ensure robust identification.