A New Identification Method and Sample of Counter-Rotating Disk Galaxies in SDSS-IV MaNGA DR17

Abstract

Counter-Rotating Disk (CRD) galaxies have two co-spatial stellar disks rotating in opposite directions, and provide a rare opportunity to study external gas accretion and dynamical assembly processes. In the 16th data release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, only 64 CRDs were visually identified. Using this as a training sample, we developed an automated pre-selection method that reduces the number of galaxies requiring visual inspection by removing systems unlikely to host counter-rotation. Applying this method to MaNGA Data Release 17, we identified 126 confirmed CRDs and an additional 143 candidate galaxies, more than doubling the MaNGA CRD sample. With this extended sample, we analyzed their Baldwin, Phillips, and Terlevich (BPT) emission-line diagrams and compared them with a matched control sample of early-type galaxies (ETG). We found no statistically significant difference in photoionization sources between CRDs and the ETG control sample, indicating emission-line diagnostics cannot solely be used to identify counter-rotating disks, nor do they correspond to a distinct present-day photoionization signature. Our method facilitates efficient discovery of CRDs in large spectroscopic surveys, enabling stronger statistical studies of their formation and evolution.

Figures (6)

• Figure 1: Stellar velocity ($V_\star$; top) and stellar velocity dispersion ($\sigma_\star$; bottom) maps for MaNGA ID 1-136248 from the MaNGA DAP. The $V_\star$ map exhibits two redshifted and two blueshifted regions characteristic of counter-rotation, while the $\sigma_\star$ map exhibits two spatially distinct peaks associated with the overlapping stellar disks.
• Figure 2: MaNGA ID 1-78381. Left Plots of the full $V_\star$ with PA$_{\text{kin}}$ superimposed and the recorded velocity of the spaxel pairs along PA$_{\text{kin}}$. Right Plots of the full $\sigma_\star$ with PA$_{\text{kin}}$ super imposed and the recorded velocity dispersion of the spaxel pairs along PA$_{\text{kin}}$. The positive velocity extrema were flagged in spaxel pairs 0-13 and 33-40, and the negative velocity extrema were flagged in spaxel pairs 23-30 and 55-58. The sigma extrema were flagged in spaxel pairs 14-23 and 41-48. The dashed horizontal line is the height used in find_peaks, and the dot-dashed vertical line is the central spaxel pair.
• Figure 3: Spatially resolved Baldwin–Phillips–Terlevich (BPT) classification maps for three representative CRDs (MaNGA IDs 1-54623, 1-584701, and 1-386322). Colors indicate the dominant ionization mechanism in each spaxel: star formation (cyan), composite (green), Seyfert (red), LINER (magenta), and ambiguous (gray).
• Figure 4: Stellar velocity ($V_\star$; left), H$\alpha$ velocity (center), and spatially resolved Baldwin–Phillips–Terlevich (BPT) classification map (right) for MaNGA ID 1-600853. In the BPT panel, cyan spaxels indicate star-forming (SF) regions, green indicate composite emission, and pink indicate AGN/LINER-like excitation.
• Figure 5: Physical and kinematic properties of the 126 confirmed CRDs identified with FindingCRDs. Columns list: (1) MaNGA ID; (2) TTYPE morphological parameter from the MDLM-VAC; (3) stellar mass; (4) effective radius; (5) logarithmic velocity dispersion within one effective radius; (6) kinematic detection criterion (CR, 2$\sigma$, or both); (7) stellar kinematic position angle; (8) H$\alpha$ kinematic position angle; and (9) qualitative alignment of the gaseous disk with respect to the inner or outer stellar disk.