초록 ▼

조석 상호작용을 통한 나선 팔 형성과 특성 연구
${\circ}$ 조석 섭동에 의해 항성 원반에 두 개의 나선 팔 형성
- 최 접근점 통과 후 0.6 Gyr가 흐른 뒤 나선 팔 출현
- 나선 팥은 내부 Lindblad 총명 반경 바깥쪽에만 존재
${\circ}$ 생성된 밀도파는 촘촘히 감긴, 대수형(logarithmic) 나선 팔
- 방위각 방향에 대한 기울기(pitch angle)가 $i\sim3^{\circ}-4^{\circ}$

조석 상호작용을 통한 나선 팔 형성과 특성 연구
${\circ}$ 조석 섭동에 의해 항성 원반에 두 개의 나선 팔 형성
- 최 접근점 통과 후 0.6 Gyr가 흐른 뒤 나선 팔 출현
- 나선 팥은 내부 Lindblad 총명 반경 바깥쪽에만 존재
${\circ}$ 생성된 밀도파는 촘촘히 감긴, 대수형(logarithmic) 나선 팔
- 방위각 방향에 대한 기울기(pitch angle)가 $i\sim3^{\circ}-4^{\circ}$
${\circ}$ 밀도파의 패턴 속도는 $\Omega_p\sim6$ - 10km/s/kpc
- 섭동체의 최 접근점에서의 궤도 운동 각속도와 유사
- 밀도파의 은하 회전은 완벽한 강체 운동이 아님
- 바깥쪽에서 느린 회전 속도
${\circ}$ 생성된 나선 팔의 세기는 최 접근점에서 섭동체가 미치는 조석력에 비례
- 섭동체의 질량이 은하 질량의 0.1-0.2배 이고 최 접근점 거리가 35-45 kpc일 때, $F\sim2-4%$
${\circ}$ 조석 섭동에 의해 생성된 나선 팔의 수명은 약 1 Gyr
- 나선 팔과 항성의 상호작용에 의해 가열된 항성 원반이 나선 팔을 약화시킴

Abstract ▼

A large fraction of disk galaxies that possess relatively strong spiral arms, including grand-design spiral galaxies such as M51 and M81, have satellite galaxies or close companions. This strongly suggests that spiral arms in these galaxies might be triggered by tidal forces from orbiting nearby gal

A large fraction of disk galaxies that possess relatively strong spiral arms, including grand-design spiral galaxies such as M51 and M81, have satellite galaxies or close companions. This strongly suggests that spiral arms in these galaxies might be triggered by tidal forces from orbiting nearby galaxies. Although the literature is abound with theoretical studies on tidal encounters of galaxies, most previous work focused on the morphological changes or formation of tidal tails and bridges resulting from very strong tidal interactions. While galaxy disks consist of both stellar and gaseous components, only a few studies included the gaseous component in their simulations for tidal interactions. Consequently, the detailed mechanisms that lead to the formation of spiral arms through not-so-strong tidal encounters and the physical properties of resulting spiral arms are not well understood, notwithstanding the effects of the gaseous medium. We quantitatively measure their physical properties including arm type, pitch angle, arm strength, and longevity.
In this work, we investigate the tidal evolution of a disk galaxy with a point mass perturber. Our main objective is to quantify the physical properties of spiral arms, such as arm type, pitch angle, arm strength, longevity, etc., that form from tidal perturbations. We use the GADGET numerical code that simultaneously solve equations for stellar particles and interstellar gas dynamics. The halo and central bulge are treated using fixed gravitational potentials, while the disk with an exponential surface density distribution is realized by particles. We assume that the disk is infinitesimally thin and neglect the effect of finite disk thickness. The point mass perturber that has a mass of 10 or 20 per cent of the disk galaxy is set to move on a parabolic orbit with the pericenter distance of 35 or 45 kpc from the disk galaxy. The back reaction of the galaxy on the orbit of the perturber is not taken into account. The main results can be summarized as follows.
(1) The tidal interaction provides a stellar disk with density perturbations at the location of the inner Lindblad resonance, which are subsequently swing amplified and emerge as a two-armed global spiral pattern. The arms become prominent about 0.6 Gyr after the pericenter passage of the perturber and last for about 1 Gyr. The heating of disk stars through the gravitational scattering off the arms causes the arm strength to be reduced.
(2) The arms that form in our simulations are all well described by tightly wound, logarithmic spiral arms. The arm pitch angle is $i\sim3^{\circ}-4^{\circ}$, with a larger value corresponding to a stronger spiral arm.
(3) The spiral pattern does not obey an exact rigid-body rotation ground the galactic center. The arm located outer parts of the disk tends to rotate slower, resulting in a further winding of the pattern as it revolves. The averaged pattern speed of the arm is found to be $\Omega_p\sim6$ - 10km/s/kpc, which is very similar to the angular velocity of the perturber near the pericenter. With these small values of the pattern speeds, the regions containing the arms are in between the inner Lindblad resonance and corotation resonance radii.
(4) The amplitude of density variation corresponding to the m = 2 mode relative to the background mean density is a linearly increasing function of the radius and is less than 50% for R < 15 kpc. But, the corresponding F parameter that measures the perturbed gravitational force relative to the background axisymmetric force remain more or less constant over a range of radii. We find that the F value of the spiral arms that form is proportional to the strength of tidal force at the pericenter, giving $F\sim2-4%$ for a perturber with mass of 0.1 to 0.2 times the galaxy mass and with the pericentric distance of 35 to 45 kpc.
The simulation results reported in this work do not take into account the effects of the gaseous component on the formation of stellar density waves. In future work, we will redo the calculation by including gas dynamics as well as by using a larger number of stellar particles (say, up to $10^6$) in order to enhance the numerical resolution. We also plan to relax some of the assumptions made in the current models so that the new models handle the galactic halo and bulge as live, account for the effects of finite to disk thickness, and follow the orbital evolution of the perturber self-consistently.

목차 Contents

• 제 1 장 서론...10
• 제 2 장 은하 및 섭동체 모형...13
• 1 절 은하 모형...13
• 1. 헤일로 모형...13
• 2. 중앙 팽대부 모형...13
• 3. 은하 원반 모형...14
• 4. 은하 원반의 회전 속도...15
• 2 절 섭동체 모형...16
• 제 3 장 수치코드: GADGET...19
• 제 4 장 수치 실험 결과...22
• 1 절 고립 은하의 진화...22
• 2 절 조석 상호작용...26
• 1. 나선 팔 형성 과정...26
• 2. 나선 팔의 물리적 특성...29
• 제 5 장 결론...38
• 제 6 장 참고문헌...41