Galaxy Characteristics

• Surface BrightnessAlternative to LuminosityI(R) = Flux/area = erg/s/cm2/arcsec2

I(0) – center fluxI(R) = at radius R

• Define R where I=25, R25

• Often R25 defines edge• “I” depends on wavelength

Ellipticals

• Widest range of characteristics– Size, Brightness

• LE>25 billion L – LE<few billion L • Surface Brightness goes as

Re = effective radius (1/2 of light)

For n>1, b=1.999n-0.327

n=4, de Vaucouleurs Law, R1/4 law – large E

1/ /1

)()( n

eRRbe eRIRI

Core brightness - radius/overall luminosity

Large Luminosity Ellipticals

faint core

large core radius

Shape

Large Ellipticals – more spherical (E0)

Motion of stars – very random

Virial Theorem – relation of kinetic energy to potential (gravitational) energy of a system

Virial Theorem

)4(

02

12

02

2

2

2

2

rG

v

G

RvM

vv

mvr

GMm

KEGE

Mass distribution related to density, velocity

Can also relate surface brightness, givesa 1/R dependence (I = 1/R)

Systems that follow this are “relaxed”

Relaxed systems seen in some spiralgalaxy bulges, globular clusters

Non-relaxed systems found in other spiral galaxy bulges, ellipticals – indicatesthey have yet to “settle down”

Faber-Jackson Relation

Velocity dispersion = (km/s)

Related to Elliptical galaxy luminosity

Lv ≈2 x 1010 L (/200 km/s)4

Why?

Useful for getting distances to Ellipticals

Other Characteristics

• Stellar population – Cool star– Brightest = KM Supergiants

• Colors – Depends on luminosity/size/composition

Brighter/larger = redder & metal rich

Fainter/smaller = bluer & metal poor

• Gas - X-ray

• Mass – way big – Dark Matter!

X-rayVisible

Elliptical Characteristics

cD E dE dSph

MB-22 to -25 -15 to -23 -13 to -19 -8 to -15

Mass (M) 1013 - 1014

108 - 1013 107 - 109 107 - 108

D25, kpc 300-1000 1-200 1-10 0.1-0.5

<M/L > >100 10-100 ~10 5-100

Spirals

• Observed features depend on wavelength– Bright stars, spiral arms – visible, UV– Faint stars, dust – IR– Gas – IR, radio

• Which wavelength makes the galaxy brighter?

• How does the surface brightness vary?

• Surface Brightness Relation –

• hR = scale height (typically 1-10 kpc)

• Also have variation of brightness with hz

typically hz = 0.1 hR

• Spirals with small I(R) have lots of H I – why?

RhReIRI /)0()(

zhzeRIzRI /)(),(

Gas in Spirals

• H I dominant in disk

• Motion of disk – motion of H I

• Spider Diagrams – show velocity of disk motion– What should that look like?

• H I visible beyond optical edge

• Molecular gas – confined to inner galaxy

• Sc, Sd, Sm galaxies – lots of H I

• S0 – Little gas– Some exceptions – ring-like structures

• Motion of gas – mass of galaxy– Tilt of galaxy– Velocity varies along disk

• Goodie – another formula!

Vsys = velocity of galaxy through space

V(R) = velocity at radius R (rotation curve)

i = tilt from perpindicular (i=0 face on)

= angle from motion towards/away

This can be solve for the

rotation curve

cossin)(),( iRVViRV sysr

• What can velocities tell us?– Amount of velocity doesn’t correspond to

what we see!– More mass is needed to produce velocities!– Dark Matter (in halo)!

• Tully Fisher Relation

L vmax

vmax = maximum rotation velocity

≈ 4

depends on wavelength

Spiral Structure

• Use spiral arm tracers to map, measure– CO, H II, blue stars

• Why do spirals exist?– Density wave theory– Stochastic theory– ?

• Bars– Move at own rate, solid body rotation– Inward/outward motion?– S0, Sa bars – stars– Sb, Sc, Sm – stars, gas, dust

• Bulges– Elliptical (simple to triaxial)– Peanut shaped– Metallicity – metal poor– I(R) like Elliptical galaxies

• Black holes, Rings, other messes…

Irregulars

• No consistent characteristics

(what did you expect?)

• Often lots of gas and dust

• Inconsistent star formation histories

• Like spirals in brightness (sort of), but fainter

• Some with bars

Spiral Characteristics

Sa Sb Sc

MB -17 to -23 -17 to -23 -16 to -22

Mass (M) 109-1012 109-1012 109-1012

D25, kpc 5-100 5-100 5-100

<M /L> 6.2 4.5 2.6

<Mgas/Mtotal> 4% 8% 16%

Spiral/Irr Characteristics

Sd/Sm Im/Ir

MB -15 to -20 -13 to -18

Mass (M) 108-1010 108-1010

D25, kpc 0.5-50 0.5-50

<M /L> ~1 ~1

<Mgas/Mtotal> 25% 5-9%