TiNy Titans HI: Discovering Satellites via HI Gas in an Isolated, Compact Group of Dwarf Galaxies

TL;DR

This study analyzes the HI content of the isolated, compact dwarf group dm1623+15 at $d\approx145$ Mpc to assess whether the system is gravitationally bound and to search for faint satellites. Using VLA HI maps and Gemini optical spectroscopy, the authors reveal two new HI-rich satellites, 1623e and 1623f, and several HI bridges that connect multiple group members, indicating ongoing interactions. The group’s dynamical and gas properties are broadly consistent with compact dwarf groups in the TNG50 simulation, though the satellite mass function is top-heavy and may reflect a multi-host or tidal-dwarf origin for some members. The results demonstrate that HI maps can uncover optically faint satellites and highlight the role of intermediate-density cosmic web environments (tendrils) in fostering gas-rich dwarf mergers that build up larger systems over 1–3 Gyr.

Abstract

We report on the HI content of an isolated, compact group of 6 dwarf galaxies at a distance of 145 Mpc. The distribution and kinematics of the HI, including multiple gaseous bridges, indicate the group is a gravitationally bound system. The HI maps further reveal two newly discovered dwarf satellites easily identified by their gas but only barely visible in optical images. The four dwarf group members previously identified in SDSS have 9.06 < log(Mstar/Msun) < 9.43 and 9.42 < log(MHI/Msun) < 9.73. The two newly discovered dwarf satellites have log(Mstar/Msun) = 6.10 with log(MHI/Msun) = 8.71 and log(Mstar/Msun) = 7.07 with log(MHI/Msun) = 9.18. New Gemini optical spectra link the HI detections and their optical counterparts. The group's 3D velocity dispersion (188 km/s), mass-to-light ratio (M_L/B ~44), dynamical-to-baryonic mass ratio (Mdyn/Mbary ~ 21), size (69 kpc), and gas fraction (0.56) are all consistent with the compact dwarf groups in the TNG50 simulation. The group has a top-heavy satellite mass function that is inconsistent with predictions for LMC-sized hosts and may instead be two or more groups coming together. A Voronoi tessellation reveals the group resides in a tendril outside the intersection of two filaments. These intermediate density environments within large scale structure provide the conditions needed for groups of star forming, gas-rich dwarf galaxies to form and eventually merge. Our results further show that it is possible to uncover fainter dwarf satellites around dwarf galaxy hosts via HI maps.

Figures (9)

• Figure 1: Ground-based view of TNT Dwarf Galaxy Group dm1623+15: broadband optical grz image from the Legacy Sky Survey. Newly identified group members discovered via their HI content, 1623e and 1623f, are circled.
• Figure 2: HI intensity (moment 0) and HI velocity field (moment 1) for TNT dwarf galaxy group dm1623. The synthesized beam of $\sim$7.5$^{\prime\prime}$$\times$ 6.5$^{\prime\prime}$ is shown in the lower left-hand corner.
• Figure 3: Discovered Low Mass Satellites. The HI morphology of the isolated dwarf group dm1623 overlaid on a g-band optical image of the system. The HI contours are at the levels of 7.8, 10.4, 13.0, 18.2, 23.4, 28.7, 33.9, 39.1 $\times$ 10$^{20}$ atoms cm $^{-2}$, and the size of the restoring beam is indicated with the filled ellipse in the lower right hand corner. The arrows indicate the six dwarf group members with the lettering scheme adopted in this work. The newly discovered low mass satellites (e and f) are easily identified via their gas content but are only barely visible in the optical image.
• Figure 4: Channel maps from the image cube resulting from the combined B, C, and D array data for dm1623. HI line emission is overlaid as contours on the g-band optical image. The contour levels are at 0.36 (2$\sigma$), 0.6, 0.8, 1.0, and 1.2 mJy beam$^{-1}$. The synthesized beam of $\sim$7.5$^{\prime\prime}$$\times$ 6.5$^{\prime\prime}$ is shown in the bottom-left corner. Each dwarf group member is labeled in the channel map closest to its peak velocity (v$_{50}$).
• Figure 5: Gemini spectra zoomed into the H$\alpha$ emission line (dashed blue line). The Gaussian fit used to determine the central wavelength is overlaid (dotted blue line). Two neighboring [NII] lines are noted by vertical red lines to show we do not expect blending to affect our redshift determinations. The sulfur doublet can be seen around 695 nm.