WALLABY: an untargeted search for H I-bearing ultra-diffuse galaxies uncovers the first known ultra-diffuse galaxy pair

TL;DR

The study demonstrates an untargeted HI-based search for H I-bearing ultra-diffuse galaxies (UDGs) using WALLABY, yielding a core sample of 10 UDGs plus 12 additional diffuse systems and revealing the first known UDG pair (UDG-1 and UDG-2) with a 22 kpc separation and a 34 km s^{-1} velocity difference. Photometric modeling from deep Legacy Survey data provides structural parameters (μ_{0,g}, R_e) and stellar masses, while WALLABY HI data supply gas masses, kinematics, and evidence for rotation-like velocity gradients. The pair exhibits higher HI-to-stellar mass ratios and ongoing kinematic features, with dynamical masses highly sensitive to the chosen method (inclination-corrected w_{20} vs baryonic Tully–Fisher), underscoring the need for higher-resolution HI data. Environmentally, the pair is locally isolated but lies in the Hydra cluster infall region, suggesting a potential future merger and ram-pressure effects as it migrates toward the cluster; collectively, the results illuminate possible evolutionary links between gas-rich field HUDs and quiescent cluster UDGs.

Abstract

Using the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) we performed an untargeted search for H I-bearing ultra-diffuse galaxies (UDGs). We identified a core sample of 10 UDGs defined by $μ_{g,0}\ge24$ mag arcsec$^{-2}$ and $R_{e}\ge1.5$ kpc, and a broader sample including 12 additional faint diffuse galaxies ($μ_{g,0}\ge23.7$ mag arcsec$^{-2}$ and $R_{e}\ge1.3$ kpc). Within the core sample, we highlight the first discovery of a UDG pair. Their projected separation is just 75 arcsec (22 kpc at 61.9 Mpc), with a central H I velocity difference of 34 km s$^{-1}$. The North-Western UDG (WALLABY J104513-262755-UDG-1) has a larger H I reservoir, $\log_{10}(M_{HI}/\rm M_{\odot}) = 8.95\pm0.03$, compared to the South-Eastern UDG (WALLABY J104513-262755-UDG-2), $\log_{10}(M_{HI}/\rm M_{\odot}) = 8.60\pm0.04$. UDG-1's stellar mass and star formation rate are also approximately an order of magnitude larger at $\log_{10}(M_*/\rm M_{\odot}) = 8.07\pm0.12$ and $\log_{10}(SFR/\rm M_{\odot}~yr^{-1}) = -1.26\pm0.12$ respectively. The pair has an isolated local environment, with no other galaxies or H I sources within 30 arcmin (525 kpc) and $\pm1000$ km s$^{-1}$. However, in the context of the larger-scale structure, the pair is located outside the virial radius of the Hydra cluster, with its position on the phase-space diagram indicating that it is infalling into the cluster. The identification of this H I-bearing UDG pair raises important questions around the formation of such a unique system and the evolution of UDGs in a transitional phase before ram pressure stripping and cluster infall.

Figures (31)

• Figure 1: The three colour ($grz$-band) Legacy Survey image of WALLABY J104513-262755. The H i contours levels equal to the local column density rms ($2\times10^{19}$ cm$^{-2}$) and $1\times10^{20}$ cm$^{-2}$ are overlaid in white. The dashed contour represents the edge of the SoFiA mask. The north-western galaxy is labelled UDG-1 and the south-eastern galaxy is labelled UDG-2. The WALLABY beam (30 arcsec) is shown in the lower left corner.
• Figure 2: Sérsic models of UDG-1 and UDG-2 in the $g$-band. (a) Model image of UDG-1, UDG-2 and three foreground stars. (b) Residual image. (c) Radial $g$-band surface brightness profiles of UDG-1 (left) and UDG-2 (right). The data profile, shown by the points, is the median of pixel values at a given radius. The model profile is shown by the solid line. The Sérsic indices of UDG-1 and UDG-2 are 0.7 and 0.4 respectively. (d) $g$-band image with H i contours (purple and pink) and optical aperture (cyan) overlaid. The dashed line corresponds to the edge of the SoFiA mask, and the lowest contour level shown represents the local rms. The optical apertures have the central coordinates, position angle and axis ratio from the Sérsic models and a radius equal to $R_e$. The WALLABY beam is shown in the lower left corner.
• Figure 3: Smoothed, co-added image created with the $g$, $r$, $i$ and $z$-band images and by convolving with a 2D boxcar kernel with a size of 2.6 arcsec by 2.6 arcsec. The dashed contour represents the edge of the SoFiA mask.
• Figure 4: H i properties of the UDG pair. (a) The moment 0 (intensity) map. The H i contours levels equal to the local column density rms ($2\times10^{19}$ cm$^{-2}$), $1\times10^{20}$ cm$^{-2}$ and $2\times10^{20}$ cm$^{-2}$ are overlaid. The dashed contour represents the edge of the SoFiA mask. Apertures that separate the H i emission of each UDG are shown in cyan. (b) The moment 1 (velocity) map. The same H i column density contours from (a) are shown. The axes used for the position velocity diagrams are shown in red. (c) The WALLABY masked spectrum (black) and unmasked spectrum (grey) measured in a 23$\times$23 pixel box. The central velocity are shown by vertical dashed lines and $w_{20}$ is shown by the shaded regions. (d) The spectra for UDG-1 (purple) and UDG-2 (blue) measured from the masked cube in the H i apertures from (a). The central velocities are shown by vertical dashed lines and $w_{20}$ are shown by the shaded regions. (e) Position-velocity diagram of UDG-1 along the axes shown in (b). Blue contours show [0.0019, 0.0037] Jy/beam ([1, 2]$\times$ the local rms). (f) Position-velocity diagram of UDG-2 along the axes shown in (b). Blue contours show 0.0019 Jy/beam (the local rms).
• Figure 5: Location of the UDG pair. (a) Legacy Survey g-band image centred on the UDG pair (H i mask outline shown) with sources from NED that lie within a 15 arcmin radius highlighted. (b) The location of the UDG pair (purple square marker) with respect to the larger scale structure. The Hydra cluster and virial radius is shown in orange. The other WALLABY sources and sources from 6dFGS, 2MRS and MILLIQUAS with redshifts $<0.015$ are shown in blue and black respectively. The UDG pair is 2.17 deg in projection from the cluster centre with a line of sight velocity difference of $\sim600$ km s$^{-1}$.