Exploring the formation mechanisms of tidal structures in globular clusters of extragalactic origin

Abstract

Tidal structures around globular clusters provide valuable insights into cluster evolution and the hierarchical assembly of the Milky Way. Using wide-field imaging data from the DESI Legacy Survey combined with a color-magnitude matched-filter technique, we perform a systematic analysis of extra-tidal features in 28 Galactic globular clusters of likely extragalactic origin, representing the largest homogeneous sample examined in this context to date. The clusters display diverse morphologies: 12 exhibit tidal tails, 9 show diffuse envelopes, and 7 reveal no clear extra-tidal features. Notably, we report the first detection of an extended tidal structure around the Sagittarius-associated cluster Terzan 7. To explore the underlying drivers, we compare intrinsic properties, orbital dynamics, and possible accretion associations across morphological groups. From the parameter distributions, complemented by Kolmogorov-Smirnov tests, we find that total mass, escape velocity, concentration, tidal filling factor, pericentric radius, eccentricity and radial angle in action-angle coordinates are all likely group-sensitive parameters. These results suggest that both internal cluster properties and orbital configurations play important roles in shaping extra-tidal morphologies. In addition, the cluster's accretion history shows no clear correlation with the presence of tidal features, indicating that it is not a direct driver of outer structure formation. Overall, the diversity of tidal structures is unlikely to be governed by a single factor, but instead reflects the interplay between internal dynamical evolution and the external Galactic environment. This study provides the most comprehensive constraints so far on the physical processes driving extra-tidal structures in accreted globular clusters.

Figures (10)

• Figure 1: Color-magnitude diagrams for part of the cluster sample. Each panel displays the template Hess diagram (top) and the global background Hess diagram (bottom). CMDs for the remaining clusters are presented in Fig. \ref{['CMDappendix']}.
• Figure 2: Signal-to-noise ratio distributions for part of the cluster sample, derived from the matched-filter output. The red circle indicates the King tidal radius (${r_{\rm t}}^{\rm King}$) from the Harris catalog, while the dashed blue circle represents the dynamical tidal radius (${r_{\rm t}}^{\rm dyn}$) from the globular cluster database. The solid orange arrow points toward the Galactic center, and the dashed light-blue arrow indicates the direction of the cluster's orbital motion over a short timescale. The black contours correspond to significance levels from 1$\sigma$ to 3$\sigma$, with the outermost 1$\sigma$ contour shown in bold. S/N distributions for the remaining clusters are shown in Fig. \ref{['figure2appendix']}.
• Figure 3: Corner plot of intrinsic parameters. The selected parameters include total mass ($\log M$), escape velocity ($v_{\rm esc}$), central concentration ($c$), ratio of half-mass radius to dynamical tidal radius ($r_{\rm h,m}/r_{\rm t}^{\rm dyn}$), mass-loss fraction ($M_{\rm dis}/M_{\rm init}$), metallicity ($\rm {[Fe/H]}$) and relaxation-age ratio ($\log(\mathrm{age}/T_{\rm rh})$). The diagonal panels show the one-dimensional histograms of each parameter for the three morphological groups, while the off-diagonal panels display the corresponding two-dimensional scatter plots for all parameter pairs. Clusters are color-coded by morphological classification: red for G1, blue for G2, and black for G3. Pal 12 is highlighted with open marker.
• Figure 4: Corner plot of orbital parameters. The parameters included are pericenter distance ($R_{\mathrm{Peri}}$), apocenter distance ($R_{\mathrm{Apo}}$), orbital semi-major axis ($a$), inclination angle($i$), orbital eccentricity($e$), and radial angle in action-angle coordinates ($\theta_{\rm R}$). The mass-loss fraction ($M_{\rm dis}/M_{\rm init}$) is also included to test its potential coupling with orbital properties. The color-coding of morphological classifications, the use of symbols, and the presentation of diagonal/off-diagonal panels are the same as in Fig. \ref{['para1']}.
• Figure 5: Orbital parameter relationships for clusters in different morphological groups. (a) The distribution of pericentric distance ($R_{\rm Peri}$) versus radial orbital period ($P_{\rm R}$). (b) The time since the last pericentric passage ($T_{\rm Peri}$) versus the time since the most recent disk crossing ($T_{\rm Disk}$). Clusters are color-coded by morphological group: G1 in red, G2 in blue, and G3 in black. Pal 12 is highlighted with an open marker.