Galaxy Formation and Evolution Chris Brook Modulo 15 Room 509 email: [email protected].
Dec 18, 2015
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Lecture 4: Merging and Environmet
How to make a galaxy
Create Hydrogen, Helium and dark matter in a Big BangQuantum fluctuations to cause some regions to be denser than others
Ensure a large amount of dark matter, so there is enough mass to ensure the dense regions collapse due to gravity
Fragmenting gas within the collapsed regions forms stars
Energy from Massive Stars, Supernovae, and Active Galactic Nuclei (AGNS) heats and expels the gas
Add Dark Energy so the Universe expands at correct rate
Heavier elements, Carbon, Oxygen Iron etc are formed in the stars and supernova explosions
Small collapsed regions merge to build larger structures, resulting in the Galaxies we see
Mergers and interactions inherent in CDM Cosmology
Galaxies live in Different Environments
Classical Hierarchical
Our view that Galaxies are “Island Universes” has changed in the last 50 years
Interactions: not just driven by theory!
Galaxy Surveys noted "peculiar" & closely paired galaxies showing distortions and tails
As interactions are short lived (108yr), their apparent rarity is misleading
Star formation and star bursts apparent in some interacting systems
The difference in cluster and field indicates that environment can affect morphology
Interactions: not just driven by theory!
HST images at low z
Not Interactions: not just driven by theory!
HST images at high z
Galaxies have an of average of 3 “major mergers” (ratio 1:3)during their lifetimes (counting only since they have acquired 1% of their mass)
Dynamical Friction
Dynamical Friction: simplified derivation
Dynamical Friction: simplified derivation
Tidal Stripping
Tidal stripping occurs when a larger galaxy pulls stars and gas from a smaller galaxy
Stars from the outer regions of the low mass galaxy are stripped first
Tidal Stripping: the Roche Limit
How far must a star be from the satellite before it is lost to the galaxy ?
NOT: where the r-2 force of the satellite and galaxy are balanced You need to include the fact that the satellite is also orbiting the galaxy
Centrifugal forces are also important.
In the rotating frame, the star's energy E = ½V2 + Φ(r) is not conserved.
Instead, the Jacobi Integral EJ = ½V2 + Φeff(r) is conserved, where
Φeff(r) = Φ(r) - ½ | Ω × r |2 is the effective potential in a rotating frame
where Ω refers to the satellite's orbit and r has origin at the Center of Gravity
Consider the simplest case:
A small satellite in circular orbit about a massive galaxy (m<<M).
Along a line connecting m and M (separation R), with origin at m:
Φeff(x) = -GM/ |R-x| - Gm/|x|-½Ω2(x-R)2
Then find the turning points: substitute for Ω2 = GM / R3; differentiate w.r.t. x; set to zero and solve for x = rJ:
rJ = R(m / 3M)1/3 Roche radius or tidal radius
Tidal Stripping: Observed Effects
“Galactic Cannabilism”Tidal forces and dynamical friction play important roles in the accretion of low mass galaxies onto high mass galaxiesMany Milky Way and M31 satellites are undergoing this process.e.g. as Sagittarius dwarf galaxy orbits the Milky Way, tidal forces pulled out long debris trails that completely encircle the Milky Way.
“Galactic Cannabilism”
An even more massive stream is found in M31, indicating a more massive accreted satellite.
“Galactic Cannabilism”
An even more massive stream is found in M31, indicating a more massive accreted satellite.
Major Mergers
Major Mergers
Major Mergers
Holmberg 1941
Toomre 1972
Major Mergers
Prograde merger
Toomre 1972
Major Mergers
Prograde merger
Animation: Frank Summers STScI
Ellipticals from MergersIf we merge two galaxies which are dominated by stars rather than gas, or if we simply consider the stellar populations, the cores of the two galaxies merge into a single elliptical galaxy.
Most stars in the tidal tails fall back into the new galaxy.
Although elliptical galaxies can be made by the merger of spiral galaxies, it is unclear what percentage of all elliptical galaxies form this way.
It is also unclear whether Ellipticals from from dry mergers i.e. from merging of two spirals which have little gas..
Or from wet mergers, i.e. between two gas rich galaxies. In this case, the merger would need to trigger a starburst, and/or AGN, that drives out the gas, resulting in the “death” of the galaxy, as any gas which is not expelled would reform a disc.
Ellipticals from Mergers
Dry mergers at z<0.7
Ellipticals from Mergers
This simulation includes massive black holes in the centre of each galaxy, which merge during the last stages of the interaction, creating an AGN.The exact processes that allow black holes to merge are the subject of ongoing theoretical work
Gas Rich Mergers
Low mass galaxies gas rich…. especially at high redhsiftSo mergers are dominated by gas! Very different resultsSee e.g. Brook et al. 2004, Springel 2005, Governato et al 2009
Geha et al. 2006
Gas Rich Mergers
Low mass galaxies gas rich…. especially at high redhsiftSo mergers are dominated by gas! Very different resultsSee e.g. Brook et al. 2004, Springel 2005, Governato et al 2009
trend for isolated galaxies
Geha et al. 2006
Stellar Mass-Halo Mass
Gas rich mergers
One affect of the early stages of the interaction is to form a barThe causes gas to lose angular momentum and be funeled to the central regions, fuelling star formation
Gas rich mergers
The final stages of Mergers also result in gas flows to central regions, resulting in starbursts and feeding AGN.
At high redshift, massive gas rich mergers are common, and are associated with quasars.
Arp 220
Environmental Effects
The Environment: Cluster vs the Field
log density
Fract
ion of
Pop
ula
tion
ES0Sp+Irr
Environmental Effects
The Environment: Cluster vs the Field
log density
Fract
ion of
Pop
ula
tion
ES0Sp+Irr
Environmental Effects: Galaxy Harassment
If galaxies move fast with respect to each other (cluster environment), then dynamical friction is weak.
Inside galaxy clusters, galaxies experience multiple weak encounters as they pass by other galaxies.
Each minor encounter may alter the shape of the spiral galaxy and to strip off some of its outer, weakly bound stars.
This process is called Galaxy Harassment. Galaxies slowly shrink as they lose their stars.
Galaxy Harassment may transform low luminosity cluster galaxies from the disturbed spirals that are seen at z=0.4 to the dwarf ellipticals that are seen today
Environmental Effects: Galaxy Harassment
Intra-cluster stars comprise ~20% of the stars of clusters such as Virgo.That is, stars that are not associated with galaxies, i.e. which have been stripped from their parent galaxy, presumably by harassment
Environmental Effects: Ram Pressure Stripping
Three galaxies in the Virgo cluster, with warps in the HI, indicative of gas being stripped as the galaxies fall through the inter-cluster medium
As a galaxy moves through intergalactic gas, ram pressure is capable of stripping the galaxy of much of fits inter-stellar gas.It is given by:
P = ρV2
Low mass galaxies gas rich…. especially at high redhsiftSo mergers are dominated by gas! Very different resultsSee e.g. Brook et al. 2004, Springel 2005, Governato et al 2009
trend for isolated galaxies
Ram Pressure Stripping ofSatellite galaxies
These gas rich dwarf galaxies are isolated
Geha et al. 2006
Environmental Effects: Ram Pressure Stripping
Environmental Effects: Strangulation
When ram pressure removes all gas, resulting in a halt in star formation, we call the process galaxy strangulation
The images show spiral galaxies with a range of colors. The red spirals live on the outskirts of clusters. Presumably they have ceased forming stars due to the loss of gas through strangulation.
Environmental Effects: Assembly BiasAccording to EPS formalism, halo bias depends only on halo mass. However, N-body simulations show that halo bias also depends on halo assembly time20% youngest halos 20% oldest halos all halos
Halos that assemble earlier are more strongly clustered than halos of the same mass that assemble later (are younger). This is known as assembly bias
Old halosAll halosYoung halos
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