Transcript Galaxy Characteristics

Galaxy Characteristics
• Surface Brightness
Alternative to Luminosity
I(R) = Flux/area = erg/s/cm2/arcsec2
I(0) – center flux
I(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

b  R / R 
I ( R)  I ( Re )e
1/ n
e

1
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
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
GE  2 KE  0
Mass distribution related to density, velocity
 GMm  1 2 
 2 mv   0
r
2

v  v 
Can also relate surface brightness, gives
a 1/R dependence (I = 1/R)
 v2  R
M
G
 v2 

G (4r 2 )
Systems that follow this are “relaxed”
Relaxed systems seen in some spiral
galaxy bulges, globular clusters
Non-relaxed systems found in other
spiral galaxy bulges, ellipticals – indicates
they have yet to “settle down”
Faber-Jackson Relation
Velocity dispersion = s (km/s)
Related to Elliptical galaxy
luminosity
Lv ≈2 x 1010 L (s/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!
Visible
X-ray
Elliptical Characteristics
cD
MB
E
-22 to -25 -15 to -23
1013 108 - 1013
Mass
14
10
(M)
D25, kpc 300-1000 1-200
<M/L > >100
10-100
dE
dSph
-13 to -19
-8 to -15
107 - 109
107 - 108
1-10
0.1-0.5
~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 –
I ( R)  I (0)e
 R / hR
• hR = scale height (typically 1-10 kpc)
• Also have variation of brightness with hz
I ( R, z)  I ( R)e
 z / hz
typically hz = 0.1 hR
• Spirals with small I(R) have lots of H I – why?
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!
Vr ( R, i )  Vsys  V ( R) sin i cos f
Vsys = velocity of galaxy through space
V(R) = velocity at radius R (rotation curve)
i = tilt from perpindicular (i=0 face on)
f = angle from motion towards/away
This can be solve for
the
rotation curve
• 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 a
vmax = maximum rotation velocity
a≈4
a 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
MB
Mass (M)
D25, kpc
<M /L>
<Mgas/Mtotal
>
Sa
-17 to -23
109-1012
5-100
6.2
4%
Sb
-17 to -23
109-1012
5-100
4.5
8%
Sc
-16 to -22
109-1012
5-100
2.6
16%
Spiral/Irr Characteristics
MB
Mass (M)
D25, kpc
<M /L>
<Mgas/Mtotal>
Sd/Sm
-15 to -20
108-1010
0.5-50
~1
25%
Im/Ir
-13 to -18
108-1010
0.5-50
~1
5-9%