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Mechanisms of Galaxy Evolution

Explore the basics of galaxy merging and its impact on star formation. Learn about the physics of gravitational slingshot, dynamical friction, and the role of dark matter. Discover how galaxy mergers create spheroids, enhance star formation, and drive feedback. Gain insights into the correlation between structure and star formation history, the demographics of galaxy mergers, and the growth of the red sequence.

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Mechanisms of Galaxy Evolution

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  1. Mechanisms of Galaxy Evolution Things that happen to galaxies… Galaxy merging

  2. Galaxy merging : basics • Same physics as gravitational slingshot, just backwards…. • M = secondary mass, going at speed vM •  = local density • C depends on how vM compares with velocity dispersion of the matter around it… • Dark matter is important contributor to dynamical friction for galaxy mergers…

  3. Galaxy merging : dynamical friction Piontek, AIP

  4. Dark Matter merger

  5. Galaxy merger… i) Mergers create spheroids ii) Can lead to enhanced star formation iii) Merger initiates feedback which quenches SF (recall spheroids empirically associated with quenched SF) Springel, MPA

  6. Mergers create spheroids…

  7. Except when they don’t… • Minor mergers • Puff up disks (how much - active debate) • Toth/Ostriker, Velasquez/White, Hopkins et al! • Major mergers with lots of gas • May end up producing a disk-dominated remnant (Robertson et al. 2006).

  8. Mergers are responsible for the largest starbursts • The most intensely-SF galaxies are merging… Borne et al. 1999

  9. Average effects of major mergers… • Average enhancement in SFR of ~1.6x in major mergers between SF galaxies M>1010Msun (averaged over tmrg ~ 2Gyr) • Intense bursts short-lived • Not all mergers produce a burst • <10% of SF directly triggered by a major merger

  10. Mergers can drive feedback

  11. Correlation between structure and star formation history Blanton et al. 2003; ApJ, 594, 186 • A bimodal galaxy population - transition mass of 3e10 • Red sequence • Mostly non-star-forming • Bulk of galaxies bulge-dominated • Most massive galaxies • Blue cloud • Star-forming • Bulk of galaxies disk-dominated • Lower mass galaxies Blue, forms stars Red, non SF Low mass High mass -18 -20 -22 Absolute magnitude in i-band Cessation (quenching) of star formation is empirically correlated with the existence of a prominent spheroid

  12. Merger effects: • Spheroid creation in many major mergers • Minor mergers likely to leave a disk • Major mergers with high >50% gas fraction may give disk • Enhancement of SFR • Modest, some very intense short-lived events • Feedback • Observed winds • Correlation spheroids with quenching

  13. Merger rates/demographics • Morphologies • Close pairs / 2pt correlation functions

  14. I. Merger rates • Messed up morphologies?

  15. Merger demographics • Some mergers between early-types (hard to recognize)

  16. Merger demographics • Many mergers between late-type galaxies • Way to think of it as mergers between central galaxies in ~1012-1013.5 halos

  17. Close pairs… • Projected close pairs • Galaxies with separations < xxkpc on sky • Projected close pairs with spectra • Spectra of both galaxies, + separation<xxkpc • 2pt correlation function • Formalizing projected close pairs, can infer 3d close pair fraction…

  18. 2pt correlation function dP (r) = n (1+(r)) dV (r) = (r/r0)- w(rp) = DD/RR - 1

  19. I. Merger rates • Merger rates • 2 point correlation function --> fraction of galaxies in close pairs in 3D space (through deprojection) MB < -20 z~0.6 COMBO-17 z~0.1 2dFGRS M > 2.5x1010 M z~0.6 COMBO-17 z~0.1 SDSS/2MASS Bell et al. 2006

  20. Introduction The growth of the red sequence Can mergers drive growth of the red sequence? introduction merger rates assumptions results Summary II. Assumptions • Assume • Mergers between galaxies 2.5x1010 M galaxies  red galaxies with > 5x1010 M • All r<30kpc pairs merge (limit) • Timescale ~ 2πr / v • rav ~ 15kpc, v ~ 150km/s  timescale ~ 0.4Gyr • Very uncertain • Only way to make z<1 5x1010 Mgalaxy is through merging • Predict rate of growth of number of red galaxies with > 5x1010 M

  21. Introduction The growth of the red sequence Can mergers drive growth of the red sequence? introduction merger rates assumptions results Summary III. Results • IF all mergers between gals with M > 2.5x1010 M  red sequence galaxy M>5x1010 M • There are enough mergers to plausibly feed the growth of red sequence Observed number density of red galaxies with M>5x1010 M Predicted rate of growth

  22. Galaxy Mergers • Galaxy Merging • From dynamical friction (wake of particles behind secondary) •  Msecondary2/v2 • Makes spheroidal structures • If gas, enhances star formation (x2 on average) • Can drive intense stellar and AGN feedback • Early-type galaxies merge, produce very massive elliptical galaxies… • ~ 0.5-1 merger per massive galaxy z<1…

  23. The influence of halo mass(or galaxy clusters)

  24. Historical background • Dressler 1980 • Increased E/S0 fraction in denser environments

  25. More background • Blanton+ 05 • Environment a strong function of color • @ given color, environment not a strong function of sersic index (structure) • SFH depends on environment • Structure (morphology)-density relation is a secondary effect

  26. Galaxies in clusters • Redder • Early-type (more spheroid-dominated) • More massive ones + more very low-mass ones • Question was : do galaxies form different in clusters or do environmental processes make them different? • E.g., ram pressure stripping or tidal interactions?

  27. Ram-pressure stripping • Ram pressure stripping • P ~ hotv2 • Restoring force 2Gg* • Stripping if ram pressure > restoring force… • When of outer gas envelope called strangulation • Gunn & Gott 1972

  28. Tidal processes… • Galaxy tidal interactions / harassment • Interactions with dark matter halo / individual galaxies in the cluster • Tidal interactions • Drive gas to middle • Thicken disk (increase vel. disp) • Ben Moore, Kenji Bekki… Lake et al.

  29. Moore et al. 1999 • DM halo of a Milky Way galaxy • DM halo of a large galaxy cluster

  30. What is different between a galaxy cluster and a galaxy (with little satellites)? • The behaviour of the baryons is the main thing that is different… • Efficiency of galaxy formation low for low-mass halos/subhalos • Efficiency of galaxy formation maximal at 1012 solar masses • Some minor differences in assembly history (rather more recent assembly for a cluster) • Clusters filled with >107K gas, galaxy-sized halos are likely full of ~105K gas

  31. Similarities… • Ram pres. stripping • Tidal disruption

  32. What is the effect of cluster mass? • Why are red sequence galaxies red? • Merging + AGN feedback • This happens at 1012-1013 solar masses • Environment (ram pressure stripping, harassment) • This happens just in massive halos >1014

  33. Another view… • g-r vs. stellar mass • Weakly dependent on halo mass (bottom panel) • Weakly dependent on radius (centre panel) • Stellar mass is a much more important driver of properties than halo mass • Weak residual trend towards redness for more massive clusters (small radii) Van den Bosch et al. 2008

  34. What scales matter? Small scales (< 1 Mpc) matter Large scales (~ 6 Mpc) do not blue fraction as a function of density Blanton et al. 2005

  35. Summary • Dark matter scale-free • Behaviour baryons very scale-dependent • Galaxy clusters • Lots of ~L* galaxies --> tides / harassment • Lots of hot gas --> ram pressure • Appears that properties of L* galaxies determined before fall into a cluster • Clusters are a second-order effect for L* galaxies • Decisive for low-mass galaxies • Only <~1Mpc scales matter for galaxy formation • Support for the halo model

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