WALLABY pilot survey: Blinded by the light -- discovery of a fourth member in the ESO 179-013 system

Abstract

We present new ASKAP/WALLABY HI observations of the nearby dwarf galaxy system ESO 179-013 (Kathryn's Wheel), the nearest known collisional ring galaxy, located 10 Mpc away in the Local Void. The system is composed of three previously known dwarf galaxies embedded in a large HI envelope, with a newly discovered fourth member identified through HI and radio continuum emission behind a bright foreground binary. Galaxy D exhibits the highest star formation rate in the group and deviates from the HI mass-diameter relation, suggesting it is a compact, gas-rich dwarf missed due to stellar foreground contamination. The HI data reveal for the first time an extended HI envelope around the whole system, the neutral gas counterpart of the star-forming ring and gas bridges among members, suggesting a more complex interaction history than the previously proposed collisional ring scenario. ESO 179-013 thus provides a rare opportunity to study hierarchical assembly and gas dynamics in underdense environments and demonstrates the power of blind HI surveys in identifying faint members of low-mass compact groups.

Figures (4)

• Figure 1: H i column density contours from ASKAP/WALLABY overlaid on a composite RGB image from CTIO/DECam on the left and on the 1.4 GHz radio continuum from ASKAP/WALLABY on the right. In the left image, red corresponds to H$\alpha$, green to [O iii], and blue to broadband $V$ (as in parkerKathrynsWheelSpectacular2015). The H i contours are at column densities of $2^n \times 8.1 \times 10^{19}$ cm$^{-2}$ for $n = 0, 1, 2, \dots$, tracing the large-scale neutral gas envelope. The ASKAP synthesized beam is shown as a yellow filled circle in both panels: 30$^{\prime\prime}$ for the H i data and 6.8$^{\prime\prime}$$\times$6.3$^{\prime\prime}$ for the radio continuum. The arrows indicate the positions of the galaxies (A, B, C, and D) and the ring.
• Figure 2: H i integrated intensity (moment 0 map) of the ESO 179-013 system shown over different velocity ranges. The middle ellipse highlights galaxy A, the largest ellipse marks the ring, the northeastern circle identifies the galaxy B, and the small ellipse to the west indicates galaxy C. (a) Full velocity range, 675--1050 $\mathrm{km\,s^{-1}}$, highlighting the extended gas envelope; (b) three-dimensional visualization of ESO 179-013 with iso-surface levels of different column density of neutral gas, an interactive version of this figure is available in the online journal; (c) 675--777 $\mathrm{km\,s^{-1}}$, gas component of galaxy B; (d) 777--900 $\mathrm{km\,s^{-1}}$, emission from the ring, galaxy C, and the interaction among group members; (e) 900--1050 $\mathrm{km\,s^{-1}}$, emission dominated by galaxy D. Black ellipses mark the optical radii of the three known galaxies and the best fitting ellipse to the ring. The black continuous line indicates the plume possibly associated with galaxy B. The black filled circle in panel (a) is the beam.
• Figure 3: Channel map of the H i emission for ESO 179-013. Each panel displays a 415$^{\prime\prime}$$\times$415$^{\prime\prime}$ region with contours of the H i emission at different velocities. Contours are drawn at the $[2.6, 6.1,...,33.9]\times10^{19}\,\,\mathrm{\mathrm{cm^{-2}}}$ levels. The red ellipses mark the optical positions of the ring and galaxies A, B, and C. The beam is shown as a yellow circle in the first panel. The background image is the continuum subtracted H$\alpha$ from CTIO parkerKathrynsWheelSpectacular2015 and the white strip marks a detector gap.
• Figure 4: H i kinematics of the ESO 179-013 system from ASKAP/WALLABY data; (a) is the flux-weighted velocity field, and (b) is the velocity dispersion map. PV diagrams along the kinematic axis of the system (c), and along the optical position of the ring (d). Galaxy D is detected as a vertical tail in both panels. The ring shows a sinusoidal pattern that suggests that rotation is the main component of the line-of-sight velocity. The largest ellipse marks the best-fitting optical ellipse of the H$\alpha$ ring, while the smaller ellipses indicate the optical extents of galaxies B, A, and C (from left to right). North is up and East is left. The black filled circle is the beam. The contour levels in the PV diagrams are $2^n \times 3\times1.8\,\mathrm{mJy\,beam^{-1}}$ ($n =$ 0, 1, 2, 3).