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Mechanisms of Galaxy Evolution. Things that happen to galaxies… Galaxy merging. Galaxy merging : basics. Same physics as gravitational slingshot, just backwards…. M = secondary mass, going at speed v M  = local density

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mechanisms of galaxy evolution

Mechanisms of Galaxy Evolution

Things that happen to galaxies…

Galaxy merging

galaxy merging basics
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…
galaxy merger
Galaxy merger…

i) Mergers create


ii) Can lead to enhanced

star formation

iii) Merger initiates

feedback which

quenches SF

(recall spheroids

empirically associated

with quenched SF)

Springel, MPA

except when they don t
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).
mergers are responsible for the largest starbursts
Mergers are responsible for the largest starbursts
  • The most intensely-SF galaxies are merging…

Borne et al. 1999

average effects of major mergers
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
correlation between structure and star formation history
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

merger effects
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
merger rates demographics
Merger rates/demographics
  • Morphologies
  • Close pairs / 2pt correlation functions
i merger rates
I. Merger rates
  • Messed up morphologies?
merger demographics
Merger demographics
  • Some mergers between early-types (hard to recognize)
merger demographics1
Merger demographics
  • Many mergers between late-type galaxies
  • Way to think of it as mergers between central galaxies in ~1012-1013.5 halos
close pairs
Close pairs…
  • Projected close pairs
    • Galaxies with separations < xxkpc on sky
  • Projected close pairs with spectra
    • Spectra of both galaxies, + separation
  • 2pt correlation function
    • Formalizing projected close pairs, can infer 3d close pair fraction…
2pt correlation function
2pt correlation function

dP (r) = n (1+(r)) dV

(r) = (r/r0)-

w(rp) = DD/RR - 1

i merger rates1
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

ii assumptions

The growth of the

red sequence

Can mergers drive

growth of the red



merger rates




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
iii results

The growth of the

red sequence

Can mergers drive

growth of the red



merger rates




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


number density

of red galaxies

with M>5x1010 M


rate of growth

galaxy mergers
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…
historical background
Historical background
  • Dressler 1980
    • Increased E/S0 fraction in denser environments
more background
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
galaxies in clusters
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?
ram pressure stripping
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
tidal processes
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.

Moore et al. 1999
    • DM halo of a Milky Way galaxy
    • DM halo of a large galaxy cluster
what is different between a galaxy cluster and a galaxy with little satellites
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
  • Ram pres. stripping
  • Tidal disruption
what is the effect of cluster mass
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
another view
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

what scales matter
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

  • 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