Imprints of tidal interactions on the stellar distribution of satellite galaxies: implications for dark matter deficient galaxies

Abstract

Interactions with the host galaxy strip stars and dark matter from the outer regions of satellite galaxies. Meanwhile, some stars from the central regions can migrate outward due to dynamical heating, producing an excess in the outer surface brightness relative to the extrapolation of the inner Sérsic profile. Recently discovered dark matter deficient galaxies (DMDGs) appear to be representative examples of such tidally disturbed systems. In this work, we investigate how the break radius, defined as the radius beyond which this surface brightness excess emerges, forms and evolves, by performing $N$-body simulations of a satellite galaxy interacting with a host, where the satellite serves as a plausible progenitor of a DMDG. Our simulations naturally reproduce a break radius consistent with that observed in DMDGs. We find that the break radius grows over time and exhibits a characteristic evolutionary behaviour: during each pericentric passage it briefly contracts due to tidal compression, and then rapidly and strongly expands as the satellite undergoes dynamical relaxation. After the satellite reaches a quasi-equilibrium configuration, the break radius shows only mild variations until the next pericentre. Across our suite of simulations, the ratio of the break radius to the effective radius remains approximately constant, even when we change the orbital parameters and internal structure of the satellite. Based on these findings, we develop a prescription for predicting the time evolution of the break radius, which can be used to constrain the tidal interaction history of satellite galaxies, including DMDGs and splashback galaxies.

Figures (14)

• Figure 1: Stellar surface brightness map obtained from a mock observation. The snapshot at $t=-2.3\,\mathrm{Gyr}$ from the fiducial simulation is analysed. Pixels fainter than 32 $\text{mag arcsec}^{-2}$ are masked to mimic the detection threshold of observational instruments, such as the Dragonfly Telephoto Array. Cyan lines represent the fitted isophotal contours, while the blue line marks the isophote corresponding to the break radius.
• Figure 2: Surface brightness profile of the simulated satellite galaxy. Snapshots are provided for the first two and last apocentres (upper panels) and pericentres (lower panels) from the fiducial simulation run. The look-back time is indicated at the top of each panel. For each snapshot, the mock observation analysis produces 100 surface brightness profiles along with 100 associated break radii. The blue points represent the surface brightness for the particular orientation that yields the median break radius. Grey solid line shows the Sérsic fit to the blue points within 1.5 times the effective radius. The identified break radius, marked with a red cross, is defined as the point where the surface brightness exceeds the Sérsic fit by at least 0.2 $\text{mag arcsec}^{-2}$. The error bar illustrates the 15–85th percentile range obtained from 100 random projections.
• Figure 3: Global properties of the satellite galaxy in the fiducial run. Blue line represents the median value obtained from mock observations using 100 random orientations, and the error bars show the 15th to 85th percentile range. Upper left: distance between the satellite and the centre of the host halo. The satellite orbit gradually shrinks due to the mass growth of the host. Lower left: DM-to-stellar mass ratio at a distance of 2.7 kpc from the centre of the satellite galaxy. The ratio decreases over time as tidal stripping transforms the system from a typical dwarf galaxy to a DMDG. The observational inference is shown by the pink shaded area danieli_still_2019. Upper right: evolution of the effective radius over time. The upper and lower boundaries of the pink shaded region indicate the effective radius of DF2, assuming a distance of 22.1 Mpc shen_tip_2021 and 18.9 Mpc cohen_dragonfly_2018, respectively. Lower right: evolution of the line-of-sight stellar velocity dispersion within the effective radius. The black dashed line and the pink shaded area represent the observed stellar velocity dispersion of DF2, along with its uncertainty danieli_still_2019.
• Figure 4: Evolution of the stellar component in the satellite galaxy model in simulations varying the initial effective radius of the satellite galaxy model, $R_{\mathrm{e,i}}$. Results from the fiducial run ($R_{\mathrm{e,i}} = 1.25$ kpc) are shown in black, while simulations with $R_{\mathrm{e,i}} = 0.75$ and 1.75 kpc are depicted in blue and red, respectively. Vertical dashed lines mark the times of pericentric passages. Upper: stellar bound mass fraction, $f_{\mathrm{bound}}^*$. Lower: break radius, $R_{\mathrm{b}}$. The lines represent the median from 100 random mock projections, and the shaded areas in matching colours indicate the 15th–85th percentile range. The dashed horizontal line represents the break radius of DF2 keim_tidal_2022. In the fiducial run, a distinct break radius emerges after the second pericentric passage, yet the galaxy retains its gravitational hold on all its stars until after the third passage, suggesting that both stripped and loosely bound stars contribute to the stellar distribution beyond the break radius. Simulations with larger $R_{\mathrm{e,i}}$ consistently show larger $R_{\mathrm{b}}$ throughout. In the simulations with larger $R_{\mathrm{e,i}}$, stars begin to be stripped and the break radius appears earlier than in the others. The stellar component is almost disrupted by the end of simulation with $R_{\mathrm{e,i}}=1.75$ kpc.
• Figure 5: A detailed examination of the evolution of the break radius between the fourth and fifth pericentric passages in the fiducial run. Upper left: Break radius. The solid line shows the median obtained from 100 random mock projections, and the black shaded region marks the 15th–85th percentile range. Lower left: Satellite orbit. Six illustrative snapshots, three near pericentre and three near apocentre, are chosen (crosses) to highlight significant changes in the break radius. Right: Surface brightness profiles for these selected snapshots (circles), along with their Sérsic fits (solid lines) and their corresponding break radii (vertical dashed lines). The colours correspond to those used in the left-hand panels. Profiles with the median break radius values are shown. The rapid and substantial growth of the break radius following the pericentric passage is linked to rapid, strong dynamical relaxation, whereas the smaller variations near apocentre stem from slower and gentler relaxation in the outer regions of the satellite galaxy.