Stellar CoRGI in MaNGA: Stellar Counterrotating Galaxies Identified in the MaNGA Survey

TL;DR

We address how disk galaxies assemble their stellar components by studying stellar counter-rotation (CR), using MaNGA integral-field data to identify and analyze CR disks. The authors combine visual kinematic misalignment screening with non-parametric LOSVD recovery to compile 120 CR galaxies (65 reliable, 55 probable), 74 of which are new, and perform two-component spectral decomposition for about a third to derive ages and metallicities of both disks. They uncover a robust inner/outer CR dichotomy linked to disk scale, age, and metallicity, and show that the CR disk is formed from externally accreted material through multiple channels (mergers with gas-rich satellites, filamentary accretion, or gas exchange with neighbors). Gas-phase metallicities generally exceed the stellar CR metallicities, and CR galaxies inhabit a range of environments, indicating CR formation is a heterogeneous process shaped by external gas accretion and dynamical evolution, with implications for disk assembly and angular-momentum growth across cosmic time.

Abstract

Stellar counterrotating (CR) galaxies are systems hosting two large-scale stellar components rotating in opposite directions -- a main, preexisting galaxy body with an older stellar population and a younger CR stellar disk likely formed from externally accreted gas. Such systems offer a unique opportunity to study disk assembly by analyzing the stellar populations of each component. Using integral field spectroscopic data from the SDSS-IV Mapping Nearby Galaxies at Apache Point Observatory survey, we identified a sample of 120 CR disk galaxies (65 reliable and 55 probable systems) by inspecting their kinematic maps and analyzing the shape of the stellar line-of-sight velocity distribution, which was recovered nonparametrically. Of these, 74 CR galaxies have not been reported in previous studies. For one-third of our sample, we further derived the ages and metallicities of stars for both disks via a spectral decomposition technique. We show that the observed spatial bimodality -- where the CR disk either is concentrated in the central region (inner counterrotation) or dominates the outer part of the galaxy (outer counterrotation) -- is driven by differences in the stellar mass and angular momentum of the CR disk. The wide range of stellar metallicities observed in CR disks suggests that no single source of external material is solely responsible for the formation of counterrotation in all galaxies; instead, proposed mechanisms such as merger with gas-rich satellites, accretion from cosmic filaments, and exchange of gas between neighboring galaxies can dominate in individual cases.

Figures (13)

• Figure 1: Eight examples of different types of kinematic misalignment. Each individual panel shows a composite RGB image of the galaxy taken from the Legacy Imaging Surveys (Dey2019_legacysurveys, http://legacysurvey.org/), kinematic maps are taken from MaNGA-DAP (stellar velocity $V_\star$, dispersion of stellar velocity $\sigma_\star$, and H$\alpha$ gas velocity $V_\mathrm{gas}$), hexagon shows MaNGA FoV, dashed line lies along the major axis (by the position angle from the NSA catalog). From left to right, from top to bottom: 1-189376 --- 2$\sigma$, Srot, CR-GS; 1-210611 --- CR-GS, $\sigma$-elong.; 1-245176 --- 2$\sigma$, NRR; 1-384400 --- gas-polar; 1-247630 --- 2$\sigma$, star-polar, gas-polar; 1-339111 --- KDC; 1-200602 --- Gas-mis; 1-318518 --- other.
• Figure 2: Top: Color-magnitude diagram for CR sample overlaid on a background computed from the RCSED catalog Chilingarian2017ApJS..228...14C. All MaNGA galaxies were cross-matched with the RCSED and are plotted as transparent purple dots. Bottom: M$_\star$--SFR diagram with the CR galaxies. GSWLC catalog Salim2016ApJS..227....2S was used to construct the grey background. The top and bottom green dashed lines correspond to specific star formation rates (sSFR) of $\log \textrm{sSFR} = -10.8$ yr$^{-1}$ and $\log \textrm{sSFR} = -11.8$ yr$^{-1}$, respectively. For both panels, the bright red stars denote the reliable CR sample (Tab. \ref{['Tab:Rel_samp']}), while small red stars indicate the probable sample (Tab. \ref{['Tab:Prob_samp']}).
• Figure 3: Sample of galaxies with reliable CR. The panel description is similar to Fig. \ref{['fig:im-prob']}. Numbers in the upper right corner of the maps indicate the minimum (blue) and maximum (red) values of the corresponding parameter.
• Figure 4: Same as Fig. \ref{['fig:im_kin_maps_rel']}, but for the probable sample of CR galaxies.
• Figure 5: Subset of CR galaxies with distinct X-shaped structures in the non-parametrically recovered stellar LOSVD. The left panel displays the stellar velocity field derived from MaNGA spectra using a single-component NBursts analysis, with the pseudo-slit oriented along the major axis. The central panel presents the LOSVD extracted along the pseudo-slit. The solid red and blue lines represent the modeled rotation velocities $v_i$ (Eq. \ref{['eq:v_prof']}), of the main and CR disks, respectively. The dashed lines above and below each $v_i$ profiles indicate $v_i \pm \sigma_i$, where $\sigma_i$ is the velocity dispersion of the corresponding disk. A negative radius corresponds to the left side of the map (the region with negative X-coordinates). The right panel displays a 2D representation of the LOSVD model. Cyan lines indicate $v_i$ and $v_i \pm \sigma_i$ from the single-component NBursts analysis of the MaNGA spectrum, while the yellow lines are derived from the LOSVD model.