ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney. Class 15 October 24, 2018 Tidal Interac/ons for Galaxies & Star Clusters

Transcription

1 ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 15 October 24, 2018 Tidal Interac/ons for Galaxies & Star Clusters

2 /mescales of Local Group a small loose group of galaxies calculate /mescales..but now par/cles are galaxies not stars! N = 3 (# of large galaxies) R = 0.5 Mpc n = 10 gal Mpc - 3 σ = 200 km/s t cross = 2x10 9 yr < t H so group should be virialized t relax = 10 9 yr < t H so 2- body encounters have changed orbits t coll = yr ~ t H so collisions & mergers important in groups all $mescales short so galaxy- galaxy gravita$onal encounters (both direct collisions & near misses) important in galaxy groups

3 Tidal forces Gravita/onal force ac/ng on extended body If we subtract the force at the center of mass, we get the differen$al gravita$onal force = $dal force To observer at center, the near & far sides are experiencing accelera/ons which differ from its own

4 Tidal disrup/on of dwarf galaxy companion of M31 Tidal arms NGC 205

5 Tidal arms Interac/ng pair of large spiral galaxies

6 Tidal tails of halo globular cluster Palomar 5 Palomar 5 Op/cal image The outer parts of Pal 5 are being /dally disrupted by gravita/onal force of MW as Pal 5 orbits MW Stellar /dal tails extend from Pal 5 6

7 effects of /dal interac/ons removal of outer par/cles by $dal stripping add KE to unstripped par/cles by $dal hea$ng trigger forma/on of bars and 2- arm spirals (both m=2 modes) in disks of galaxies

8 Tidal forces Gravita/onal force ac/ng on extended body If we subtract the force at the center of mass, we get the differen$al gravita$onal force = $dal force To observer at center, the near & far sides are experiencing accelera/ons which differ from its own

9 examine gravita/onal force exerted by companion galaxy M (~point source) on 3 stars within extended galaxy m R M a N a C a F companion galaxy M gravita)onal accelera)on on 3 stars: NEAR a N = GM/(R- r) 2 CENTER a C = GM/R 2 FAR a F = GM/(R+r) 2 r r extended galaxy m

10 view of differen/al accelera/on (/dal accelera/on) across extended galaxy R M Δa N Δa F companion galaxy M )dal accelera)on on 2 stars: NEAR Δa N = a N - a C + a C (2r/R) = +2GMr/R 3 FAR Δa F = a F - a C - a C (2r/R) = - 2GMr/R 3 r r extended galaxy m toward companion away from companion

11 Q: Why does the earth orbit the Sun, but the /dal force on the earth is greater from the Moon?

12 Tidal force falls off as R - 3 Gravita/onal force on earth from Sun greater than from Moon (since Gravita/onal force falls off as R - 2 ) but Tidal force on earth from Moon greater than from Sun (since Tidal force falls off as R - 3 )

13 QUESTION The water /des on the 2 sides of the earth are the same size, but the /dal arms in interac/ng galaxies are generally different sizes WHY?

14 Lunar /des on earth r R actual photo of earth & moon taken by OSIRIS- REX spacecran r/r<<1 for earth- moon system (r/r = 1/30) so the 2 /dal bulges on the earth are symmetric

15 Tidal arms in interac/ng galaxy pair Arp84 R r r/r~1 for these interac/ng galaxies so the 2 /dal arms of the northern galaxy are NOT symmetric

16 QUESTION & ANSWER The water /des on the 2 sides of the earth are the same size, but the /dal arms in interac/ng galaxies are generally different sizes WHY? In many galaxy interac/ons r/r~1 NOT r/r<<1 (i.e., distance at closest approach is comparable to galaxy size) so 2 /dal tails are produced but they are not symmetric

17 Tidal stripping

18 Radial distribu/on of starlight in globular cluster M92 Sharp trunca/on at /dal radius due to /dal interac/on with Milky Way Galaxy Tidal radius

19 when is material /dally stripped from object m? R r M m

20 Tidal radius can a mass hold it itself together by self- gravity against 5dal force across it exerted by the gravity from another body? Is not where the gravita/onal force from m and M are equal Is where gravita/onal force from m ( F m ) is equal to the difference in gravita$onal force from M at center and edge of m ( ΔF Mce ) m F m ΔF Mce M r /dal

21 when is material /dally stripped from object m? R r M m masses must be orbi/ng but first we will ignore that fact

22 when is material /dally stripped from object m? R r M m masses must be orbi/ng but first we will ignore that fact

23 When is material /dally stripped from an object? 1. Rough approxima/on: ignore the fact that the masses M and m must be orbi/ng each other. i.e., ignore centrifugal forces r /dal = (m/2m) 1/3 R stuff at distance r from center of m will be stripped if r>r /dal

24 When is material /dally stripped from an object? 1. Rough approxima/on: ignore the fact that the masses M and m must be orbi/ng each other. i.e., ignore centrifugal forces r /dal = (m/2m) 1/3 R Q: When are /dal impacts on m large?

25 When is material /dally stripped from an object? 1. Rough approxima/on: ignore the fact that the masses M and m must be orbi/ng each other. i.e., ignore centrifugal forces r /dal = (m/2m) 1/3 R Q: When are /dal impacts on m large? A: when r /dal is small - > when R small, m small, M large - > close interac$on & big difference in mass

26 When is material /dally stripped from an object? 1. Rough approxima/on: ignore the fact that the masses M and m must be orbi/ng each other. i.e., ignore centrifugal forces r /dal = (m/2m) 1/3 R 2. Beter approxima/on: take into account centrifugal force associated with circular orbit of m around M r /dal = (m/3m+m) 1/3 R [deriva/on given in S&G, with r /dal - > r J, R - > D ]

27 When is material /dally stripped from an object? 1. Rough approxima/on: ignore the fact that the masses M and m must be orbi/ng each other. i.e., ignore centrifugal forces r /dal = (m/2m) 1/3 R 2. Beter approxima/on: take into account centrifugal force associated with circular orbit of m around M r /dal = (m/3m+m) 1/3 R [deriva/on given in S&G, with r /dal - > r J, R - > D ] 3. No approxima/on: the orbits of most galaxies and star clusters are not circular. Solu/on is complex. Effects largely determined by distance of closest approach. Get rough solu/on by using #2, with R= distance of closest approach

28 Interes/ng behaviors of /dal radius 5dal stripping as runaway process mass m of stripped objects drops as it gets stripped, so /dal radius r /dal shrinks could be runaway process r /dal = (m/2m) 1/3 R is it stripped or not?? for elongated orbit /dal radius shrinks as object gets closer to center (R gets smaller), so more stuff stripped but /dal radius expands as object gets further from center (R gets larger), so stuff can get re- captured!!

29 changes in /dal radius on elongated orbits r t large /dal radius r t when far away (large R) R orbit direc/on R r t smallest /dal radius r t at closest approach (smallest R) R r t /dal radius becomes large again aner closest approach when R gets large again may recapture stars lost at closest approach!

30 Structure of /dal tails To central body (Milky Way) Direc/on of 2 /dal arms rela/ve to orbital mo/on of main body: arms roughly fall along orbit of main body (WHY?) Inner tail (closer to central body) is the leading tail par/cles orbit faster Outer tail is trailing tail par/cles orbit slower (this is as expected - - the orbital /me is faster closer to the central body)

31 Some halo globular clusters originate from the /dally disrupted Sagitarius dwarf galaxy, currently merging with the Milky Way 31

32 Simula/on of /dal disrup/on of Sagitarius dwarf galaxy

33 Simula/on of /dal disrup/on of Sagitarius dwarf galaxy

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35 interes/ng behaviors seen in this simula/on: 1. /dal interac/on triggers bar and spiral arms (both m=2 structures, like /des) in large galaxy 2. small galaxy originally in circular orbit outer parts /dally stripped, inner core sinks to center. this sinking process is like mass segrega/on massive thing sinks to center, less massive par/cles (stars) move outwards

36 bars in galaxies something interes/ng that happens in a disk can drive some stars out of disk plane to make par/cular type of bulge (pseudobulge) cause large gravita/onal torques which drive material radially inward or outward drive secular evolu$on of disk galaxies

37 Classic barred spiral NGC 1300 NGC 1300 HST Dust lanes along leading edges of stellar bar (generally true) Not much star forma/on along bar (not always true) Spiral arms emerge from bar ends (not always true) 37

38 Bars exist in spirals, S0s, dwarf irregulars NGC 5020, barred spiral >50% of spirals & S0s have bars NGC 4608, barred S0 LMC, barred irregular (Sm) don t need gas to have bar (unlike spiral arms) - - stellar orbits can support a bar in some dwarfs the bar is offset from galaxy center 38

39 What are bars? Descrip5on of morphology: Elongated linear feature of extra stellar mass density In disk plane: bars elongated ~2.5:1-5:1 Perpendicular to disk plane: most stars near disk plane but at some radii in some bars stars get far from disk to make peanut- shaped bulge 39

40 Stellar orbits in bars Athanassoula (1992) models Many stellar orbits highly elongated in direc/on of bar. Such stars make up the bar. R CR Corota/on radius Unlike stars in density wave spiral arm, stars in a bar STAY IN THE BAR Figure shows some of the closed orbits in a simple model of a barred galaxy. These closed orbits are only a subset of all the stellar orbits, but there are enough stars on similar orbits in many real galaxies to make a bar. 40

41 stellar bars have large m=2 component θ light intensity bar θ Meaning of m=2: Azimuthal distribu/on of something (e.g. light, mass) described by I(θ) = I 0 cos(mθ), where m=2 as you go around once, you encounter 2 peaks & 2 troughs 41

42 Tidal forces External gravita/onal force ac/ng on extended body If we subtract the force at the center of mass, we get the differen$al gravita$onal force = $dal force Tidal forces have dominant m=2 component To observer at center, the near & far sides are experiencing accelera/ons which differ from its own

43 Bars can be triggered by /dal interac/on or minor merger /dal interac/ons have strong m=2 component, so ideal for triggering m=2 bars or spiral arms NOTE: the bars in these galaxies are not necessarily caused by this /dal encounter! 43

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45 Does this galaxy have a bar? NGC 253 in op/cal 45

46 Bars easier to see in NIR NGC 253 in NIR (2MASS) 46

47 Bars easier to see in NIR dust ex/nc/on at op/cal λs can hide bars NGC 253 in op/cal NGC 253 in NIR (2MASS) Bar frequency higher in NIR studies than op/cal studies, since the underlying stellar mass distribu/on is traced beter in NIR. Stellar bar does not appear to extend above the dust in disk, indica/ng that the stellar bar is NGC 253 is a THIN DISK component 47

48 ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Basics on the Forma)on of the Elements these slides won t be covered in class. please review before Lecture 16 (Mon Oct 29)!

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50 Lunar /des on earth r R r/r<<1 for earth- moon system (r/r = 1/30) so the 2 /dal bulges on the earth are symmetric