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.

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

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.

Hubble’s Tuning Fork
Sa Sb Sc Sd
E0 E6
S0
SB0
Ellipticity =
10(a-b)/a
< ~ 7 observationally
SBa SBb SBc SBd

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

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 H
0
. (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 Spitzer 3.6, 8.0 + 24 μ

M87

M87 Deep AAT USM

M101 W. Keel Optical

M101 UIT

R. Gendler

Crashing galaxies = The Antennae

•

(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.

-6 = cE 2 = Sab A = Unbarred
-5 = E 3 = Sb X = Mixed
-4 = E+ 4 = Sbc B = Barred
-3 = S05 = Sc
-2 = S0 6 = Scd
-1 = S0+ 7 = Sd
0 = S0/a 8 = Sdm etc.
1 = Sa 9 = Sm
10 = Im

Elliptical Galaxy Surface Brightness Profiles
What is the shape of the galaxy? What is its integrated light?
(A) Hubble Law (one of 4)
I
0
I(r) = I
0
(1 + r/r
0
)
-2
= Central Surface Brightness r
0
= Core radius
Problem(!) 4π
I(r) r dr diverges.

(B) deVaucouleurs r
1/4
Law
e
-7.67((r/r e )1/4 - 1) a.k.a. 10
-3.333333
r e
= effective radius = ½ light radius
I e
= surface brightness at r e
Roughly, I
0
= e
7.67
I e
~ 10 r e
~ 11 r
0
This function is integrable.
3.3333
I e
~ 2100 I e

(C) King Profile derived to fit isothermal spheres to globular star clusters, includes a tidal cutoff term with r c
I(r) = I
K
~ r
0
, and r
[(1+r
2
/r c
2
) t
= tidal radius
– 1/2
- (1 + r t
2
/r c
2
)
-1/2
]
2
(D) Oemler Truncated Hubble Law
I(r) = I
0
(1 + r/r
0
)
-2 e
–(r/b)2
(pre computers)

Typical Numbers
I
0
~ 15 – 19 magnitudes /sq arcsec in B
<I
0
> ~ 17 m/sq” for Giant Elliptical Galaxies, r
0
~ 1 kpc r c
~ 10 kpc

King Profiles

Profiles are on average (over the spiral arms)
Exponential Disks
I(r) = I
S e
-r/r s
Freeman (1970) found
I
S
~ 21.65 mag/sq” B for 28 of 36 galaxies r
S
~ 1 – 5 kpc, function of Luminosity

Spirals have both bulges (like E galaxies) and disks.
From the deVaucouleurs Law
L
Bulge
= 2
I(r) πr dr = 7.22 π r
0
∞ e
2
I e
L
Disk
= 2π
I
S e
-r/rs r dr

D/B = 0.28 (r s
/r e
)
2
I
S
/I e
Disk to Bulge Ratio

Sombrero (M104) HST

Sombrero Spitzer

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

M33 A Flocculent System

NGC4414 another Flocculent S

Some Definitions:
Number of Arms = m, most spirals have m=2, i.e. twofold symmetry
Arm Orientation:
Leading rotation
Trailing

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

Gravitational Field due to Stars & Gas
Total material needed to maintain the field
Density Response of
Stellar Disk
+
Density Response of
Gaseous Disk
||
=
TOTAL RESPONSE

2

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)

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

Type B-V U-B SB .
E 0.93 0.46 20.9
S0 0.91 0.44 21.1
Sa 0.86 0.29 21.6
Sb 0.75 0.16 21.8
Sc 0.60 -0.02 21.9
Sd 0.57 -0.10 22.3
Im 0.46 -0.23 21.4

S/T =
L
Bulge
/L tot which correlates with type.

Gas Content (HI) versus type
Type M
H
/M
E 10
-6 to 10
-3
S0 0.005
Sa 0.03
Sb 0.05
Sc 0.1
Im 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 σ
α
, ΔV
α
; Α ~ 2.5 to 4
Diameter versus Luminosity
L α D
2
Surface Brightness versus Luminosity
(and central SB vs Luminosity)


There exists a plane in several observable dimensions on which most E galaxies and similar objects lie.
R e
= 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

if L ~ M and rotation curves flat and galaxies similar in surf B =

M ~ v
2
R R~ M/v
2
L ~ 4

R
2
R ~ (L/4

)
1/2
L ~ v
4
/4


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