Upcoming class schedule
Thursday March 15 2pm AGN evolution
(Amy Barger)
th
Monday March 19 Project Presentation
(Brad)
nd
Thursday March 22 postponed to
make up after spring break..
Spring break March 26­30
then back to normal as of Tuesday 3rd April
th
GALAXIES 626
Lecture 16:
The effects of environment
The colour­magnitude diagram
Red sequence
Note the
bimodality of
galaxy colours
blue sequence
Sloan DSS data
The colour­magnitude diagram
Dependence on
environment
Fraction of red galaxies depends strongly on density. . Density­dependence present at all luminosities. Bright and faint galaxies show trend with density
Moving galaxies from the blue
to the red peak
 What are the implications for the galaxy transformation mechanism?
 Blue & red peaks are correlated with density,
But Density evolves. Galaxies must move from one to the other
 How long do they take? If they take too long, then the gap between the peaks will be filled in.....
Timescales for Galaxy Transformation
How rapid must the blue→red transition be?
Two gaussian model Red Peak
always fits the data well – there is no room for an intermediate population.
colour evolves rapidly if timescale for star formation to stop is short
Blue Peak
if transformations occur uniformly in time:
 need τ<0.5 Gyr

Summary
• There are distinct populations of red (“early”) and blue (“late”) galaxies
• The mean colors and spread of the two populations depends only weakly on environment
• But the relative abundance of the populations is a strong function of environment.
• Blue galaxies move quickly into the red population as their environment changes – Why?
Evolution with redshift
How does the dependence on environment
change with redshift?
Galaxy clusters: z=0
z=0.39
z=0.83
• Local galaxy clusters are dominated by passively evolving galaxies with high formation redshifts
• How does the evolution compare with the general field?
• Nature or nurture: clusters are built from groups. How do groups evolve?
Butcher­Oemler Effect
• Concentrated clusters at high redshift may have Blue fraction
more blue galaxies than concentrated clusters at low redshift
• But blue fraction depends strongly on luminosity and radius so care needs to be taken to evaluate blue fraction at same luminosity limit, and within same (appropriate) radius. Cluster SFR evolution
• “Butcher­Oemler effect” also seen in the general field
• Is the effect stronger in clusters?
Field
2dF
Clusters
0 0.3 1
Redshift
Nakata et al. (2005)•
Based on sparsely­
sampled [OII] Postman, Lubin & Oke 2001
spectroscopy
van Dokkum et al. 2000
• Suggests fraction of star­
Fisher et al. 1998
forming galaxies evolves Czoske et al. 2001
relatively weakly in clusters
•
Red galaxy fraction
Red galaxy fraction
Evolution of the red peak
High density
All galaxies
MV < ­20 Low density
Redshift
Tying star formation to structure growth
Groups
Clusters
Low redshift groups
• Relation between star formation rate and local density Field
Field
Groups
• There are fewer spiral galaxies in groups than in the field, at the same redshift.
• No evidence for more disturbance/irregularities in group galaxies
Spiral fraction
Field
Spiral fraction
E/S0 fraction
Morphologies: Field
Groups
Groups
Vel. Dispersion (km/s)
The connection between star Field
formation rate, morphology Groups
and environment
Distributions are corrected for differences in luminosity function between group and field
S0
Elliptical
Early spiral
Late spiral
Like clusters, groups contain passive spirals: disk morphology but low star formation rates
Evolution in groups
Fraction of non­SF galaxies
• Use [OII] equivalent width to find fraction of galaxies without significant star formation
• most galaxies in groups at z~0.4 have significant star formation – in contrast with local groups
Fraction of non­SF galaxies
Groups
Fraction of non­SF galaxies
Field
Group SFR evolution
• Fraction of non­SF galaxies increases with decreasing redshift
• for both groups and field
• Insensitive to aperture effects
Summary
• More star formation in groups at z=0.5 than at z=0
• On average, groups at 0<z<0.5 have less star formation and fewer spiral galaxies than the field.
• Passive spiral galaxies are a key component of groups at z=0.0­0.5
Environmental Mechanisms
Why should galaxy properties depend on the environment?
 Collisions / harassment
 "Strangulation"
 Ram­pressure (actual effects)

 Different history...
Ram Pressure Stripping
Mostly important in clusters where the surrounding gas
density is high....
Ram Pressure Stripping
ICM
Clusters are not empty – in fact, most baryons are in a diffuse form, the “intra cluster medium”
As a galaxy travels through the ICM, it feels a force, the “ram pressure”
aaaaaa
2
ρh v 2 πGΣ R Σ g  R 
The gas disk will be to radius R if:
Gravitational Force due to ICM
restoring force
Ram pressure Stripping -
ICM
simulations
Galaxy NGC 4388 Expels Huge Gas Cloud NGC 4388 is a member of the the Virgo Cluster. It is classified as an active galaxy. One hypothesis holds that the gas was stripped away as NGC 4388 made its way through the intergalactic medium of the Virgo Cluster. Galaxy interactions
Gravitational interaction can
produce major shifts in the gas and stars of the galaxy
Major mergers produce massive star forming
events but more minor repeated events
can gradually remove the gas from a galaxy
Galaxy Collisions, Tides and Harassment
•Tidal truncation
•Slow encounter
•Depends on gradient of potential
•Big impact on the dark halo, but not significant for stellar component
•Impulsive heating •Fast encounter
•Importance increases as relative velocity decreases
•Harassment
•The cumulative effect of repeated encounters
Galactic mergers
Most dramatic examples: major mergers between galaxies
of comparable mass. Large morphological changes as a
consequence of the interaction.
Observationally and theoretically, find that major mergers
are uncommon - perhaps ~1 such merger in the lifetime
of the Universe for a large galaxy in the field.
Examples of galaxy collisions
in the real universe and in a
simulation (Moore et al 1995)
The Antennae Galaxy
Best place for major mergers - small groups of galaxies
Galaxies in rich clusters are generally less vulnerable to
mergers, despite the very high density of galaxies
because the velocities are higher.
Galaxy Collisions, Tides and Harassment
Galaxy 1
size x force Perturbation to gradient Time of velocity of star encounter
in galaxy 1
m
Δv ¿=2
r
b
V
M
Perturber, galaxy 2
ΔE =
 
GM
b3
2
r
2
b
V
4G M m
4
3b V
Change of internal energy of galaxy 1
2
r2
Minor mergers between galaxies of very different masses
are much more common.
Example: the Magellanic clouds
Bound satellites orbiting within the
extended halo of the Milky Way
(~50 kpc distance)
Eventually will spiral in and merge
Sagittarius dwarf galaxy is another
satellite which is now in process of
merging…
• What effect does the merger have on the disk galaxy?
• How fast does the spiral-in and merger occur?
Dynamical Friction
Why does the orbit of a satellite galaxy moving within the
halo of another galaxy decay?
Stars in one galaxy are scattered by gravitational perturbation
of passing galaxy.
Stellar distribution around the intruder galaxy becomes
asymmetric - higher stellar density downstream than upstream.
Gravitational force from stars produces a `frictional’ force
which slows the orbital motion.
How quickly will the LMC merge with the Milky Way?
Simple estimate - dynamical friction time:
t friction »
V
»
V
3
200 km/s
∣dV / dt∣ 4 pG 2 Mnm ln L
~3
1010 Solar
masses
Galactic density at LMC for flat rotation curve estimate
3 x 10-4 Solar masses / pc3
With these numbers, estimate orbit will decay in ~3 Gyr
Close satellite galaxies will merge!
Strangulation - removal of the gas halo
Removes surrounding
gas and so stops infall
and star formation
Strangulation - removal of the gas halo
Summary of Mechanisms
Ram­pressure Needs dense ICM and high velocities ­ clusters
Collisions / harassment
Mostly in groups where slow galaxy interactions are common
"Strangulation"
Removal of the gas halo: no more fuel supply
Similar to ram­pressure stripping but much easier!
Mechanisms
Ram­pressure Density too low
Needs dense ICM and high velocities ­ clusters
Collisions / harassment
Groups are preferred place!
"Strangulation"
Removal of the gas halo: no more fuel supply
Similar to ram­pressure stripping but much easier!
Transformation too rapid
Mechanisms
Collisions / harassment
Groups are preferred place!
What next? Look at individual galaxies to see if they provide evidence to support the “big picture”
The best mechanism to explain the observational data – but most effective in galaxy groups rather than clusters
Summary
 Clear bimodality of galaxy properties
 More dense environments... there is less star formation
 A higher fraction of passive/red galaxies
 But...
 Star Forming/blue galaxies have similar properties in all environments (only the fraction changes)
 “Passive” galaxies exist in all environments
 Galaxy transformation is rapid!
 Its most likely to happen in galaxy groups rather than in clusters End