Galaxy Mass

• Star Number/Density– Counting stars in a given volume– Use volume of galaxy to estimate mass– Good idea?

• Rotation Curve– Best for spirals (s for ellipticals)– Based on Kepler’s Third LawMass(R) = R v2/GMass(R) = mass within radius RFor Sun

• V=220 km/s• R=7.5 kpc• M ≈ 1011 M

– Velocity – Mass– M/L values

• Sa/Sb > Sc/Sd

• Binary Galaxies– Greater uncertainty– M/L = 20-80

• Galaxy Clusters– Even greater uncertainty– M/L up to 1000!

Dark Matter

• Evidence– Spiral Rotation Curves

• Large mass halo

• 2-10 x disk mass

– Gravitational Lensing• Amount of mass found by degree of lensing

• Mass of clusters > Luminosity of clusters

• 90% total mass of Universe!

WIMPs vs MACHOs

• WIMPs– Weakly Interacting

Massive Particles

– Massive neutrinos

– Fast moving

– “Hot” Dark Matter (HDM)

– Detection: Very difficult

• MACHOs– Massive Compact Halo

Objects

– Stars, planets, black holes

– Slow moving

– “Cold” Dark Matter (CDM)

– Detection: Microlensing

Microlensing

• Massive object distorts light• Observe many stars, look for brightening• MACHO surveys

– MACHO

– EROS

– OGLE

– And many more….

• Results?

Many MACHOs detected – but not enoughOther benefits from surveys

Evolution of Galaxies

• Depends upon the evolution of its stars

• Low Mass stars– More common– Slower

• More complex than Pop I, Pop II– [Fe/H] variations between galaxies– [Fe/H] variations within galaxies (gradient)– Why?

[Fe/H]• Comes from supernovae• Which come from massive stars• Which come from large areas of star formation• Which come from large gas/dust clouds• Which are found mainly in disks (mid-plane)• So….• [Fe/H] gradient (in R, z) linked to all of the

above

• Halo Objects – Very low [Fe/H]

• Formation independent of disk

• Globulars formed before disk

• What triggered the formation of galaxies?

• Early Universe not smooth

• Fluctuations ~ 30 million pc

• Density Enhancements needed

• Gravity versus Gas Pressure

• Jean’s Length

m=hydrogen• Jean’s Mass

Minimum mass that forms ~ 100,000 M

metersT

R

T

Gm

kR

J

J

2/1

5

2/12/1

101.2

8

9

kgTM

RM

J

JJ

2/12/319

3

109.3

3

4

• Further evolution information– Star formation/evolution – colors of galaxies– Consistency of surface brightness laws– Contraction leads to galaxy type?

• Angular momentum

• Magnetic fields

• Turbulence

• Chemical composition

– Current observations of galaxy formation• Deep surveys (HUDF)

Fully formed large galaxies seen at great distance – implications?

• Or are they still forming at this distance?

• Ellipticals first, spirals later?

• Or galaxy building blocks first?

• Depends upon form of dark matter

Galaxy formation – star formation, when did it start? 1996 2002

Are we sure?

Other galaxy evolution problems

• Bars– Few seen at great distance– Later evolutionary effect? Tidal effect?– Influences later evolution

• Gas motion inward/outward

• Alters [Fe/H] gradient (big bar – shallow gradient)

• Enhances star formation

• Bulge structure

• Alters galaxy type?