Galaxies II – Dr Martin Hendry

1. Galaxies II – Dr Martin Hendry 10 lectures to A3/A4, beginning January 2008

2. 3. Galaxy Formation and Evolution • How did galaxies form?… • A detailed explanation is one of the main unsolved problems in astronomy. • We understand the general picture: gravity assembles galaxies and clusters by causing growth of primordial density perturbations

3. CMBR fluctuations are the seeds of today’s galaxies Galaxy formation is sensitive to the pattern, of CMBR temperature fluctuations

4. 3. Galaxy Formation and Evolution • How did galaxies form?… • A detailed explanation is one of the main unsolved problems in astronomy. • We understand the general picture: gravity assembles galaxies and clusters by causing growth of primordial density perturbations • A recipe for galaxy formation has ingredients: gravity (dark matter) gas physics (stars) background cosmological model

5. Studying the statistics of the present-day galaxy distribution is a powerful tool for constraining cosmological parameters. Can compare real observations with the results of computer simulations – e.g. 2dF galaxy redshift survey

7. Studying the statistics of the present-day galaxy distribution is a powerful tool for constraining cosmological parameters. • Can compare real observations with the results of computer simulations – e.g. 2dF galaxy redshift survey. • Simulations follow the evolution of galaxies as structure grows • Clear that mergers and interactions are common.

11. Galaxy mergers and interactions • What happens when galaxies merge? • Their stars don’t collide, but some of the galaxies’ K.E. is • transferred to the random motion of the stars. • Galaxies experience a ‘drag’ force, known as • Dynamical Friction Drag force on each galaxy depends on:- ,mass of galaxy , velocity of galaxy , mass density of neighbour

12. Mass of galaxy enters via Newton’s law of gravitation, so the drag force will be a function of Can show via dimensional analysisthat (3.1)

13. Mass of galaxy enters via Newton’s law of gravitation, so the drag force will be a function of Can show via dimensional analysisthat (3.1)

14. Mass of galaxy enters via Newton’s law of gravitation, so the drag force will be a function of Can show via dimensional analysisthat (Can also use this formula to consider the lifetime of e.g. globular clusters orbiting a parent galaxy – see example sheets) (3.1)

15. High speed ‘collision’ of 2 disk galaxies • Galaxies are not slowed down enough to become a bound pair. • Galaxies separate, but their disks are ‘dishevelled’: stars acquire random motions, causing disks to ‘puff up’. • Can form spiral arms or bars

16. e.g. spiral arms of M81

17. e.g. bar of M95

18. High speed ‘collision’ or ‘fly-by’ of 2 disk galaxies • Galaxies are not slowed down enough to become a bound pair. • Galaxies separate, but their disks are ‘dishevelled’: stars acquire random motions, causing disks to ‘puff up’. • Can form spiral arms or bars • Multiple ‘close encounters’ may destroy disks all together; explains lack of disk galaxies in the cores of rich clusters

20. Slower ‘collision’ or ‘fly-by’ • Much greater disturbance – particularly if co-planar and direction of fly-by aligned with direction of motion • Relative velocity of stars in galaxy A and B is significantly smaller; stars spend a long time in close proximity • Interaction can draw out a long tidal tail which may persist for several Gyr

22. Slower ‘collision’ or ‘fly-through’ • Head-on collision can produce a polar ring galaxy

24. Slower ‘collision’ or ‘fly-through’ • Head-on collision can produce a polar ring galaxy Computer model, by J. Toomre

25. Why should a polar ring form? Suppose the galaxy is in virial equilibrium before the interaction (3.2)

26. Why should a polar ring form? Suppose the galaxy is in virial equilibrium before the interaction If the interaction happens quickly (impulse approximation) then the potential energy of the galaxy doesn’t have time to change appreciably. Stars in the galaxy gain K.E. from the disturber  Galaxy K.E. increases to  Galaxy thrown out of equilibrium (3.2) (3.3)

27. Why should a polar ring form? After some time virial equilibrium will be restored. When virialised, the galaxy’s total energy satisfies (3.4)

28. Why should a polar ring form? After some time virial equilibrium will be restored. When virialised, the galaxy’s total energy satisfies Just after the interaction, (3.4) (3.5)

29. Why should a polar ring form? Once virial equilibrium has been restored Comparing with eq. (3.3) we see that, just after the interaction, the galaxy has gained of K.E., but by the time it has virialised again it has lost of K.E. One way this can happen is to convert excess K.E. into P.E. – e.g. a shell of galactic material expands outwards (3.6)

32. Why should a polar ring form? Once virial equilibrium has been restored Comparing with eq. (3.3) we see that, just after the interaction, the galaxy has gained of K.E., but by the time it has virialised again it has lost of K.E. Tidal tails and streams can achieve a similar result. (3.6)

33. Formation of dust lanes • Sometimes, instead of the ‘disturber’ drawing out a tidal tail from the first galaxy, the first galaxy can ‘tidally strip’ gas and dust from the disturber (the disturber becomes the disturbed) • Can leave behind a dust lane – fresh supply of gas and dust which can kick start new star formation (even in an elliptical) e.g. Centaurus A (also strong radio source)

36. Even slower ‘collision’, leading to a merger • Interactions with lower approach velocities may lead to a merger – possibly after an elaborate ‘courtship’ • Final appearance of galaxies depends on mass and speed of the perturber, and orientation during interaction.

40. Even slower ‘collision’, leading to a merger • Interactions with lower approach velocities may lead to a merger – possibly after an elaborate ‘courtship’ • Final appearance of galaxies depends on mass and speed of the perturber, and orientation during interaction. • Close passage of two gas-rich spirals can produce a Starburst galaxy: * disk gas is pulled away from near circular orbits * gas clouds collide at high speed, causing shocks * compresses gas to very high density, triggering large amounts of star formation

44. Star Formation Models • Since galaxies contain stars, to understand galaxy formation we need to understand star formation. • This is a much more complicated problem than following the evolution of the dark matter – which only needs gravity.

45. Star Formation Models • Since galaxies contain stars, to understand galaxy formation we need to understand star formation. • This is a much more complicated problem than following the evolution of the dark matter – which only needs gravity. • We need to understand: • how and where do stars form in galaxies? • what determines stellar masses? • what determines stellar luminosities? • what determines stellar chemical compositions? • how do all of these depend on galaxy type, age, redshift ?

46. Star Formation Models • A good way to compare star formation models with observations is by spectral synthesis: We can compute a synthetic spectrum for our model galaxy, accounting for: We can then compare with observed spectrum to ‘best fit’ galaxy properties • age of the galaxy • chemical composition (metallicity) • initial mass of stars and gas • rate at which new stars form • redshift of observation

47. Spectral energy distribution, from the models of Bruzual & Charlot (2003). Assuming solar metallicity (i.e. same chemical abundances as the ISM in the vicinity of the Solar System). Single burst of star formation at time Ages range from 1 million years (0.001 Gyr) to 13 Gyr Note that the spectrum shape evolves very little between about 4 Gyr and 13 Gyr (See also examples sheet 5)

48. Although spectrum shape varies little with age, the strength of spectral absorption lines depends on metallicity: Lines associated with metals stronger in metal-rich galaxies (But some degeneracy here, as older galaxies are usually also more metal-rich, as metals build up after many generations of star formation) Ca H, K Fe, Mg TiO O2

49. Ca H, K Fe, Mg TiO O2 • Note the sharp drop in flux • for , known as the • Lyman Break • This occurs because: • There aren’t many stars hot enough to produce UV photons in great numbers • Any UV photons that are produced can ionise HI clouds, so a large fraction of them are absorbed before they reach us • We saw the Lyman break in • The spectra of S0 and Sb • Galaxies, in Section II