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

TL;DR

This study uses high-resolution HI imaging from the VLA, together with Pan-STARRS1 optical data, to reassess the FAST-detected candidate J0139+4328 previously labeled as a dark galaxy. The VLA data reveal a faint optical counterpart coincident with the HI emission, yielding a low-stellar-mass, highly gas-rich dwarf with $M_{HI}/M_* \approx 18$ and $M_{HI} = 5.9 \times 10^{7}\,M_\odot$, while $V_{rot}$ remains uncertain due to resolution limits. J0139+4328 sits near the $M_{HI}$-$M_*$ and BTFR relations within scatter, suggesting it is not a true dark galaxy but rather an ultra-diffuse-like, gas-rich dwarf at $D \approx 31$ Mpc. The work underscores the value of interferometric HI follow-up for robust counterpart identification and cautions against interpreting low-resolution HI detections as dark systems without deep optical confirmation.

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.

Figures (9)

• Figure 1: Identification of the optical counterpart. Left: H i contours on optical image obtained by stacking PS1 $g$, $r$, and $i$-band images. The H i contours are [6.5, 10, 15, 20] $\times$$10^{19}$ atoms cm$^{-2}$, where the lowest contour is $\sim$2-$\sigma$. The thick red cross marks the VLA centroid, while the thin orange cross marks the FAST centroid from X23. Right: Color image of the galaxy from $g$ (blue), $r$ (green), and $i$-bands (red), smoothed by a Gaussian kernel of 2 pixels. Masked areas are in black.
• Figure 2: VLA global H i profile of J0139+4328 in the rest frame of the source. Dotted lines indicate the extent used for creating the moment zero map and the dashed line denotes the determined systemic velocity.
• Figure 3: Moment one map with total intensity H i contours at [3, 5, 10]-$\sigma$ overlaid in black. White contours are iso-velocity contours separated by 2 km s$^{-1}$. Full gray lines denote the area from which the position-velocity slice in Fig. \ref{['p3:fig:pvslice']} was extracted.
• Figure 4: Position-velocity slice at the $PA = 150$$^{\circ}$. Data contours are shown in white at [2, 4]-$\sigma$ with full (dashed) lines denoting positive (negative) emission. Orange horizontal lines denote the $W_{50}$ spread. In the lower left corner, we denote the beam size (horizontal line) and velocity resolution (vertical line).
• Figure 5: Isophotal fitting. Upper panels: The left panel displays the smoothed image with overlaid ellipses whose corresponding parameters are indicated by red stars in the lower panels. Masked regions appear in white. The central panel presents the model reconstructed from all fitted ellipses whose parameters are shown as blue circles in the bottom panels, while the right panel shows the residual obtained by subtracting the model from the data image. Lower panels: Ellipticity and position angle as functions of the semi-major axis length from the second iteration of the fitting procedure. Blue points represent all fitted ellipses from this run, and red stars correspond to those displayed in the upper left panel. The vertical black line indicates the radius equivalent to the PSF’s FWHM after smoothing, and the shaded gray area marks the region used to determine the overall geometry (by taking the median within this range).