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Galaxy fly-bys sustain bar-halo friction and bar slowdown in disk galaxies

Rumi Kodama, Rimpei Chiba, Tetsuro Asano, Junichi Baba, Michiko Fujii

Abstract

Bars in disk galaxies slow down as they transfer their angular momentum to their dark matter halo via dynamical friction from near-resonant orbits. This bar-halo dynamical friction can become ineffective once phase mixing erases the phase-space gradient around the main resonances. We present fully self-consistent $N$-body simulations of a Milky Way-like disk galaxy with a single dwarf-galaxy fly-by in prograde and retrograde orbits before, during, and after bar formation. In our models, the fly-bys do not trigger a long-lived tidal bar; the bar forms on essentially the same time as in the isolated model. After the encounter, however, all perturbed models develop bars that are stronger and slower than in the isolated one. The final pattern speed depends little on the encounter time, but it does depend on the encounter direction relative to the disk rotation: prograde encounters slow the bar more than retrograde ones. The angular-momentum evolution shows that the disk loses its angular momentum and the halo gains it, consistent with bar-halo friction. By probing the particle distribution of the halo in angle-action space, we demonstrate that the isolated bar enters a metastable, saturated state with a flattened distribution in the phase space around the bar's corotation resonance, whereas a dwarf passage excites long-lived fluctuations in the halo that restore the phase-space gradients near the corotation and thereby sustain the bar-halo friction. This mechanism explains the continued slowdown and growth of bars after fly-bys. It may be relevant to the Milky Way, whose bar formed near the epoch of a major ancient accretion event, suggesting that an early encounter could have influenced the subsequent secular evolution of the bar.

Galaxy fly-bys sustain bar-halo friction and bar slowdown in disk galaxies

Abstract

Bars in disk galaxies slow down as they transfer their angular momentum to their dark matter halo via dynamical friction from near-resonant orbits. This bar-halo dynamical friction can become ineffective once phase mixing erases the phase-space gradient around the main resonances. We present fully self-consistent -body simulations of a Milky Way-like disk galaxy with a single dwarf-galaxy fly-by in prograde and retrograde orbits before, during, and after bar formation. In our models, the fly-bys do not trigger a long-lived tidal bar; the bar forms on essentially the same time as in the isolated model. After the encounter, however, all perturbed models develop bars that are stronger and slower than in the isolated one. The final pattern speed depends little on the encounter time, but it does depend on the encounter direction relative to the disk rotation: prograde encounters slow the bar more than retrograde ones. The angular-momentum evolution shows that the disk loses its angular momentum and the halo gains it, consistent with bar-halo friction. By probing the particle distribution of the halo in angle-action space, we demonstrate that the isolated bar enters a metastable, saturated state with a flattened distribution in the phase space around the bar's corotation resonance, whereas a dwarf passage excites long-lived fluctuations in the halo that restore the phase-space gradients near the corotation and thereby sustain the bar-halo friction. This mechanism explains the continued slowdown and growth of bars after fly-bys. It may be relevant to the Milky Way, whose bar formed near the epoch of a major ancient accretion event, suggesting that an early encounter could have influenced the subsequent secular evolution of the bar.
Paper Structure (11 sections, 3 equations, 10 figures, 2 tables)

This paper contains 11 sections, 3 equations, 10 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Initial conditions
  4. $N$-body simulations
  5. Results
  6. Time evolution of the bar under perturbations
  7. Time evolution of the disk's angular momentum
  8. Saturation and recovery of dynamical friction
  9. Fluctuations in the dark matter halo induced by the dwarf fly-by
  10. Summary
  11. Pendulum Hamiltonian

Figures (10)

  • Figure 1: The trajectory of the dwarf galaxy in prograde (left) and retrograde (right) orbits. Gray dots indicate the disk stars, and the arrow in the disk shows the rotation of the disk. The arrows above the curve indicate the direction of the motion. Alt text: scatter plots showing the distribution of disk stars, lines showing the orbit of the dwarf galaxy, and arrows showing the disk rotation directions.
  • Figure 2: Snapshots of the discs for models isolated, p00, p10, p25, r00, r10, and r25 from top to bottom. The time indicates the time from the beginning of the isolated model simulation. The second left panel shows the snapshot just after the pericentre passage (0.32, 1.3, and 2.8 Gyr for models 00, 10, and 20) of the dwarf. Alt text: surface density plots for 0.15, 0.49, 1.13, 2.59, 2.94, 4.89, and 8.81 Gyr.
  • Figure 3: Top: The distance between the dwarf and disk center as a function of time. Bottom: Time evolution of the bar pattern speed. The colors indicate each model; black, red, orange, yellow, blue, green, and cyan correspond to models isolated, p00, p10, p25, r00, r10, r25, respectively. Vertical dotted lines mark the pericenter-passage times (0.32, 1.3, and 2.8 Gyr, respectively) for the $t=0$, 1.0, and 2.5 Gyr starting-time models. Alt text: The top panel is a six-line graph. The bottom panel is a line graph with seven lines showing the pattern speeds of the bars of the seven models as a function of time, 0 to 10 Gyr. The pattern speeds slow down with time, but the slowdown stalls in the isolated model.
  • Figure 4: Time evolution of the $m=2$ mode maximum amplitude, $A_{2,\rm max}$. Colors and vertical lines are the same as Figure \ref{['fig:pattern_speed']}. Alt text: Graph with seven lines showing the maximum Fourier amplitude for $m=2$ as a function of time between 0 and 10 Gyr. Perturbed models show a Fourier amplitude higher than that of isolated model.
  • Figure 5: Time evolution of the $z$-component of the disk angular momentum. Colors and vertical lines are the same as Figure \ref{['fig:pattern_speed']}. Alt text: Line graph with seven lines showing the z-component of the disk angular momentum for the isolated and perturbed models as a function of time for 0 and 10 Gyr. The angular momentum transfer stalled in the isolated model but continued in the perturbed models.
  • ...and 5 more figures