Galaxy Formation and Evolution Chris Brook Modulo 15 Room

2 Lecture 4: Merging and Environmet

How to make a galaxy Create Hydrogen, Helium and dark matter in a Big Bang Quantum 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 (10 8 yr), 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

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 = ½V 2 + Φ(r) is not conserved. Instead, the Jacobi Integral E J = ½V 2 + Φ eff (r) is conserved, where Φ eff (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: Φ eff (x) = -GM/ |R-x| - Gm/|x|-½Ω 2 (x-R) 2 Then find the turning points: substitute for Ω 2 = GM / R 3 ; differentiate w.r.t. x; set to zero and solve for x = r J : r J = 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 galaxies Many 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

Holmberg 1941

Toomre 1972 Major Mergers Prograde merger

Toomre 1972 Major Mergers Prograde merger

Animation: Frank Summers STScI

Ellipticals from Mergers If 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 redhsift So mergers are dominated by gas! Very different results See 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 redhsift So mergers are dominated by gas! Very different results See 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 bar The 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 Fraction of Population E S0 Sp+Irr

Environmental Effects

The Environment: Cluster vs the Field log density Fraction of Population E S0 Sp+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 = ρV 2

Low mass galaxies gas rich…. especially at high redhsift So mergers are dominated by gas! Very different results See e.g. Brook et al. 2004, Springel 2005, Governato et al 2009 trend for isolated galaxies Ram Pressure Stripping of Satellite galaxies These gas rich dwarf galaxies are isolated Geha et al 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 Bias According to EPS formalism, halo bias depends only on halo mass. However, N-body simulations show that halo bias also depends on halo assembly time 20% youngest halos20% oldest halosall 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 halos All halos Young halos