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AY16
March 20, 2008 Galaxies
Galaxies
A modern topic: 1920 Shapley-Curtis Debate
Evidence against galaxies as external
1. Proper motion of M31 (van Maanen)
2. Shapley’s GC Distances
3. “Nova” 1885a in M31
Killer evidence for:
1. Hubble’s discovery of Cepheids in 3
galaxies and their distance determinations.
What Are Galaxies?
1. Artifacts of the Formation Process
2. Tracers of Test Particles of Larger
Dynamics
3. Froth on an Ocean of Dark Matter
4. Objects Deserving Detailed Study in Their
Own Right
We hold these truths …
Galaxies have a broad range of properties
There exist connections between these
properties and other parameters (location,
location, location ---- formation +
evolution)
We must understand these connections to use
galaxies to understand the cosmological
model.
Morphology
Hubble’s Tuning Fork
Sa
Sb
Sc
S0
E0
Sd
Irr
E6
SB0
Ellipticity =
10(a-b)/a
< ~ 7 observationally
SBa
SBb
SBc
SBd
Irregular Galaxies
LMC = IBm
M82 = Irr II
= I0
Morphological classification is just taking the
grossest, simplest observational properties
and moving the bins around until they make
sense. Relate form to physics.
Regarding S0 galaxies, Hubble said “at present,
the suggestion of cataclysmic action at this
critical point in the evolutional development
of nebulae is rather pronounced.”
Hubble thought his diagram was an
evolutionary sequence!
Hubble Types are now
(1) Not considered to be “evolutionary”
(2) Considerably Embellished!
by Sandage, deVaucouleurs, van den Bergh, ++
(1) Irr  Im plus I0
(2) Sub classes added Sa, Sab, Sb, Sbc, Sc,
Scd, Sd, Sdm, Sm, Im and S0/a = slight
signs of structure in the disk
(3) S0 class well established + rings, mixed
types and peculiarities
e.g. SAbc(r) p = open Sbc galaxy with an
inner ring and some
peculiarities
SX(rs)0 = mixed S0 galaxy with mixed
ring morphology
SBdm
= barred very late type spiral
galaxy
deVaucouleurs Expansion
Other Embellishments
S. Van den Bergh introduced luminosity classes
in the 1960’s  for spirals, L is a function of
appearance.
I = giant ---- V = dwarf
this was used for a while to estimate H0. (ugh!)
in the 1970’s he introduced the Anemic
sequence: very low surface brightness disks
which is probably connected to the “stripping”
of spirals in the field
Discovery of Anemic spirals and other effects
(e.g. the morphology-density relation)
spawned the “Nature” vs “Nurture” debate:
Are S0’s born or made? Do field S0’s
exist?
Morgan in the 1950’s introduced spectral
types for galaxies a, af, f, fg, g, gk, k
which never caught on (but E+A galaxies
are now a hot topic – emission + A type)
Finally, in the 1960’s the search for active
galaxies and radio galaxies caused Morgan
to introduce another classification scheme
D galaxies --- E galaxies with apparently
extended envelopes.
cD galaxies --- Centrally located D’s
N galaxies --- Compact Nuclei
Plus other types like Seyferts + LINERS (both
specroscopic) and Zwicky’s compact and
“post-eruptive” galaxies…
M81 3.6μ
M81 Spitzer 3.6, 8.0 + 24 μ
M87
M87 Deep AAT USM
2μ
M101 W. Keel Optical
M101 UIT
R. Gendler
Ring galaxy
Crashing galaxies = The Antennae
•
Arp Introduced Peculiar Galaxies
(1966) Atlas of Peculiar Galaxies, mostly
interacting. Some 30% of al NGC objects
are in the Arp or Vorontsov-Velyaminov
catalogs. (Arp vs Sandage .)
Arp also introduced us to our limitations sue
to surface brightness considerations:
We can’t see galaxies that are too small or that
are too big (low Surface brightness)
 THE LAMPPOST SYNDROME
By the numbers:
In a blue selected, magnitude limited, z=0
sample,
1/3 are E + S0, 2/3 are S + I
20% 15%
60% < 10%
For Spirals
~ 1/2 A
~ 1/4 X
~ 1/4 B per unit volume is
something else again.
T Types
-6 = cE
-5 = E
-4 = E+
-3 = S0-2 = S0
-1 = S0+
0 = S0/a
1 = Sa
2 = Sab
3 = Sb
4 = Sbc
5 = Sc
6 = Scd
7 = Sd
8 = Sdm
9 = Sm
10 = Im
A = Unbarred
X = Mixed
B = Barred
etc.
Quantitative Morphology
Elliptical Galaxy Surface Brightness Profiles
What is the shape of the galaxy? What is
its integrated light?
(A) Hubble Law (one of 4)
I(r) = I0 (1 + r/r0)-2
I0 = Central Surface Brightness
r0 = Core radius
Problem(!) 4π ∫ I(r) r dr diverges.
(B) deVaucouleurs r1/4 Law
-7.67((r/re)1/4 - 1)
I(r) = Ie e
a.k.a. 10-3.333333
re = effective radius = ½ light radius
Ie = surface brightness at re
Roughly, I0 = e7.67 Ie ~ 103.3333 Ie ~ 2100 Ie
re ~ 11 r0
This function is integrable.
(C) King Profile
derived to fit isothermal spheres to globular
star clusters, includes a tidal cutoff term
with rc ~ r0, and rt = tidal radius
I(r) = IK [(1+r2/rc2)– 1/2 - (1 + rt2/rc2)-1/2]2
(D) Oemler Truncated Hubble Law
I(r) = I0 (1
2
-2
–(r/b)
+ r/r0) e
(pre computers)
Typical Numbers
I0 ~ 15 – 19 magnitudes /sq arcsec in B
<I0> ~ 17 m/sq”
for Giant Elliptical Galaxies,
r0 ~ 1 kpc
rc ~ 10 kpc
N4494
King Profiles
Spiral Galaxies
Profiles are on average (over the spiral arms)
Exponential Disks
I(r) = IS e -r/rs
Freeman (1970) found
IS ~ 21.65 mag/sq” B for 28 of 36
galaxies
rS ~ 1 – 5 kpc, function of Luminosity
Spirals are Composite
Spirals have both bulges (like E galaxies) and
disks.
From the deVaucouleurs Law
∞
LBulge = 2 ∫ I(r) πr dr = 7.22 π re2 Ie
0
LDisk = 2π
∫ IS e-r/rs r dr
 D/B = 0.28 (rs/re)2 IS/Ie
Disk to Bulge Ratio
Sombrero (M104) HST
Sombrero
Spitzer
Spiral Galaxy Structure
What gives Spiral Galaxies their appearance?
There are 2 main components (plus others less
visible)
Disk --- rotationally supported
--- thickness is a function of the local
vertical “pressure” vs gravity
Spiral Pattern --- Three models
Density Wave
Tidal Interactions
SPSF = self propagating star formation
Density Waves  Lin’s “Grand Design”
spirals (M81, M83)
Interaction Induced Spirals  Good Looking
spirals with Friends (M51)
Self Propagating Star Formation --detonation waves, SF driven
by SF,  “Flocculent”
Spirals
M81
Classic
Grand
Design
Spiral
Another GD Spiral
M51 Interacting System
Optical
Molecular Gas -CO
M33 A Flocculent System
NGC4414 another Flocculent S
Spiral Structure
Some Definitions:
Number of Arms = m, most spirals have
m=2, i.e. twofold symmetry
Arm Orientation:
Leading
rotation
Trailing
Density Wave Theory
Developed over many years by first Bertil
Lindblad, then C.C. Lin, then Frank Shu:
Quasi-stationary Spiral Structure Hypothesis
(spiral pattern changes only slowly w. time)
+
Density Wave Hypothesis
Pattern is a SF pattern driven by density change
Follow the Mass
Density Response of
Stellar Disk
Gravitational Field
due to Stars & Gas
+
Density Response of
Gaseous Disk
Total material
needed to maintain
the field
||
=
TOTAL RESPONSE
Density
Wave
Models
+
Bar
Potential
2
Toomre
model for
the Antennae
Galaxy Magnitudes!!!
Galaxy magnitudes are measured many
different ways!!!
Isophotal (to a limiting radius in mag/sq”
Metric (to a fixed size in kpc)
Integrated Total (very hard!)
Petrosian (to a fixed SB relative to the center)
Kron
(similar)
Properties vs Morphology
Type vs Color  driven by star formation
rates and histories
Color Gradients  most galaxies get bluer
with increasing radius
(combination of SFR + [Fe/H])
Color vs Magnitude  mostly for E’s
Morphology Density
Color vs Type (Optical)
Type
E
S0
Sa
Sb
Sc
Sd
Im
B-V
U-B
SB
0.93
0.91
0.86
0.75
0.60
0.57
0.46
0.46
0.44
0.29
0.16
-0.02
-0.10
-0.23
20.9
21.1
21.6
21.8
21.9
22.3
21.4
.
S/T =
LBulge/Ltot
which correlates
with type.
Dressler
MorphologyDensity
Gas Content (HI) versus type
Type
E
S0
Sa
Sb
Sc
Im
MH/M
10-6 to 10-3
0.005
0.03
0.05
0.1
0.2 to1.0
Luminosity versus Internal Motions
L versus σ for E’s = Faber –Jackson
L versus rotation for S’s = Tully-Fisher
L α σα , ΔVα; Α ~ 2.5 to 4
Diameter versus Luminosity
L α D2
Surface Brightness versus Luminosity
(and central SB vs Luminosity)

The Fundamental Plane
There exists a plane in several observable
dimensions on which most E galaxies and
similar objects lie.
Re = f (σ,L) or f (σ, L, [Fe/H])
Ditto for Spirals
TF relation implies that the mean global
M/L for spirals varies by at most x2 over
x100 in luminosity
For Spirals, Tully-Fisher Relation
if L ~ M
and rotation curves
flat and galaxies
similar in surf B = 
M ~ v 2R
R~ M/v2
L ~ 4 R2
R ~ (L/4)1/2
L ~ v4/4
Summary
1. Galaxies come in many forms (morphology)
2. Properties of galaxies correlate with type
3. Generally brightness falls with R in a
predictable way
4. Galaxy types correlate with density
5. Spiral structure can form several ways
6. Gravity rules! FP and TF relations show that
the properties of galaxies are governed by M