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Galaxies
Chapter 20
Galaxy Classification
In 1924, Edwin Hubble
divided galaxies into different
“classes” based on their
appearance.
Why begin here?
•Hubble classification serves as the
basic language of the field.
•The morphological sequence reflects
a fundamental physical and, in some
ways, evolutionary sequence, which
offers important clues to galactic
structure, formation and evolution.
Hubble Tuning Fork diagram
(Hubble 1936)
Ellipticals
Lenticular (S0)
Spiral and Barred Spiral
Irregular
Spiral Galaxies
•Disk + spiral arms + bulge (usually)
•Subtype a b c defined by 3 criteria:
•Bulge/disk luminosity ratio
•Sa: B/D>1 Sc: B/D<0.2
•Spiral pitch angle
•Sa: tightly wound arms Sc: loosely wound arms
•Degree of resolution into knots, HII regions, etc.
Barred Spiral Galaxies
•Contain a linear feature of nearly uniform
brightness centered on nucleus
•Subclasses follow those of spirals with
subtypes a b and c
MW may be
SBb, depending
on prominence
of the bar.
Spiral Galaxies
Comprise about 2/3rds of bright galaxies
Grand Design Spiral - well defined spiral structure
Flocculent - less organized spiral design
Spirals clearly contain much gas and dust
Most starlight is from young, blue stars - ongoing
star formation
Sizes - radius = 10 to 30 kpc
Masses - M = 107 to 1011 Msun
Milky Way and Andromeda are
both bright, spirals
MV ~ -21 or LV ~ 2 x 1010 LV,sun
Elliptical Galaxies
•Smooth structure and symmetric, elliptical contours
•Subtype E0 - E7 defined by flattening
•En where n = 10(a-b)/a
a and b are the projected major and minor axes
(not necessarily a good indicator of the true 3-D shape)
Types of Elliptical Galaxies
Giant Ellipticals – few 100kpc
across, 1 trillion stars
MV ~ -23 or LV ~ 1011 LV,sun
Dwarf Ellipticals (dE) - as small as
1 kpc across, 1 million stars
MV ~ -18 or LV ~ 109 LV,sun
Ellipticals:
• contain little or no gas (only ~1% of mass)
• no current star formation
• mostly older, redder stars - like those in the
halo of our galaxy
• Unlike halo stars, metal abundances of these
stars are high (about twice solar metallicity)
NGC 205
Dwarf Spheroidal - Classification dSph
Very low stellar density and surface brightness
MV ~ -14 or LV ~ 3 x 107 LV,sun
Leo I dSph
S0 Galaxies (Lenticulars)
•Smooth, central brightness concentration (bulge similar to E) surrounded by
a large region of less steeply declining brightness (similar to a disk)
•No spiral arm structure but some contain dust and gas
•Originally thought to be transition objects between Sa and E but typical S0 is
1-2 mags fainter than typical Sa, E (van den Bergh 1998)
Irregular Galaxies
NGC 4485-Irr II
M82-Irr II
LMC - Irr I
•Little morphological symmetry
•Lots of young, blue stars and interstellar material
•Smaller than most spirals and elliptical galaxies
•Two major subtypes:
•Irr I: spiral-like but without defined arms, show bright knots with O,B stars
•Irr II: contain many dust lanes and gas filaments (e.g. M82) - explosive
General trends within
Hubble sequence E  Sc:
 Decreasing Bulge/Disk
 Decreasing stellar age
 Increasing fractional gas
content
 Increasing ongoing star
formation
Limitations of the Hubble Classification Scheme
1.
Only includes massive galaxies (doesn’t include dwarf
spheroidals, dwarf irregulars, blue compact dwarfs)
2.
Three different parameters for classifying spirals is
unsatisfactory because the parameters are not perfectly
correlated.
3.
Bars are not all-or-nothing. There is a continuum of bar
strengths.
de Vaucouleurs’ Revised Hubble Classification System
(de Vaucouleurs 1958, Handbuch der Phys. 53, 275)
(de Vaucouleurs2 1964, Reference Catalog of Bright Galaxies)
Basic idea: retain Hubble system, but add lots of additional options:
Rings (inner and outer), range of bar-like structures….
Cross section of diagram
No Bar
E
E+ S0- S0 S0+
Sa Sb Sc Sd Sm Im
Ring
shaped
Spiral
shaped
Limitations:
Rings and bars are not independent
Does not take into consideration mass or other
important parameters.
Bar
Morphological Distributions
The range and frequency of different morphological types is sensitive to
the sample of galaxies studied. Some key results:
•The Local Group is the only sample that includes a significant number
of very faint galaxies. Of the ~35 galaxies in the Local Group, only the 3
brightest (M31, MW and M33) are spirals, the remainder are equally
divided between irregular and dwarf elliptical /spheroidal galaxies.
•Within rich clusters of galaxies,
the population of bright galaxies
is dominated by Ellipticals.
•Samples of galaxies outside of
clusters (in the “field”) are mainly
Spirals.
Coma cluster – red=Ellipticals;
blue=Spirals; green contours= X-rays
Automated Classification
Visual classification  time consuming and different observers may not always
agree. This motivates the development of algorithms to automatically and
impartially classify galaxy images - very important for large surveys like the
Sloan Digital Sky Survey and deep extragalactic fields.
Examples: Abraham et al. (1994, 1996):
Concentration parameter C - fraction of light within ellipsoidal radius
0.3 x outer isophotal radius (1.5 above sky level).
Asymmetry parameter A - fraction of light in features not symmetric
wrt a 180 degree rotation
Photometric Properties of Galaxies
Astronomers measure brightness
in units of magnitude.
faintest
During the 2nd century BC,
Hipparchus ranked all visible stars –
brightest were magnitude 1, faintest
were magnitude 6.
To our eyes, a change of one
magnitude = a factor of 2.512 in flux
(i.e. logarithmic scale).
To convert from flux units (in W/m2 or
erg/s/cm2) to magnitudes:
mag2 - mag1 = 2.5 log (F1/F2)
brightest
Photometric Properties of Galaxies
To measure the brightness distribution of galaxies, we must
determine the surface brightness of the galaxy.
Surface brightness is flux (magnitude)
within 1 unit of angular area on the sky
(square arc seconds).
 (mag/arcsec2)
I is Intensity (W/m2/str) where 1 str =
4.25 x 1010 arcsec2
R
15
B
20
Surface Brightness is
independent of distance since flux
decreases d2, but the area
subtended by 1 square arc
second increases as d2.
25
30
Radius R
Surface Brightness profiles for
Elliptical Galaxies (and bulges of Spirals)
B
log I ~ - R 1/4
R1/4
I(R) = Ie exp{-7.67[(R/Re)1/4-1]}
“deVaucouleurs law” (1948)
or “r1/4 law”
Re = effective radius containing 50% of luminosity
(factor of 7.67 chosen to make this so)
Ie = flux at Re
Re = (aebe)1/2
-for major,minor axis
Io = Ie103.33 = 2138Ie (central flux)
Spiral Galaxies
Bulges
•Luminosity profiles fit r1/4 law
•Structure appears similar to
Elliptical galaxies, except bulges
are more flattened (oblate
spheroids).
NGC 7331
Sb galaxy
R-band
isophotes
Disks
log I ~ -R
•Most are well-represented by an exponential profile
I(R) = Ioe-R/Rd (Freeman 1970)
Central surface brightness
Disk scale length
NGC 7331
(Rd)
I
•Bulge dominates in center and again at very large radii
(if bulge obeyed r1/4 to large R)
•Disk dominates at intermediate radii
•Rd ~ 1 - 10 kpc
•Disk in many spirals appear to end at some Rmax around
10 to 30 kpc or (3-5Rd)
Multi-wavelength view of Galaxies
Photons of different energies are created by different physical phenomena.
Spiral galaxy M31 (Andromeda) 2 million ly away (700 kpc). 1” on the sky = 3pc
HI (neutral H
gas) – 21 cm
•
•
•
•
•
•
Star formation - Infrared
CO (molecular
gas) – 3.6
micron
Radio observations allow us to study HI
(neutral Hydrogen) and molecular gas
CO displays sharp drop with radius
Traces spiral arms
CO more associated with arms than HI which
permeates galaxy (except in center)
Spiral galaxies vary in the amount of
molecular to neutral Hydrogen (50% to 10%)
CO velocity map shows rotation
Stars & hot gas
(optical)
X-ray
Visible
M82
Irregular galaxy
Infrared light produced
by recent star
formation inside dusty
clouds – dust absorbs
light and reradiates at
longer wavelengths.
Infrared
Radio
X-ray emitting gas
heated to high temps
by supernovae and is
expanding in galactic
wind.
Radio – 21-cm neutral
Hydrogen gas
Spiral Structure
The winding dilemma
• If spiral arms are coherent physical structures displaying
differential rotation like the stars and gas, they would have wound
up so tightly by now that they would not be visible.
 The stars and gas have made dozens of revolutions around the
center of their galaxy over the age of the Universe so spiral
structure should be “smeared out” if caused due to winding…
Spiral Structure
A leading theory for galactic spiral arms is spiral density waves.
Density Waves in Traffic
Density Waves in Spiral Galaxies
Spiral arms are caused by the compression of gas as it orbits the Galactic
center and encounters density waves (moving more slowly). The
compression of gas causes stars to form which we see as spiral arms.
•Cloud approaches arm at a relative
speed of ~100km/s.
•Arm acts as gravitational well,
slowing down the cloud.
•Arm will alter orbits of gas/stars,
causing them to move along arm
briefly.
•Compresses HI gas and gathers
small MCs to form GMCs.
•GMCs produce O&B stars.
•Stellar radiation disrupts the clouds.
Spiral Structure
How do density waves form?
If stars in the disk of a spiral galaxy are on slightly eccentric orbits, and the
position angle of these ellipses vary with radius, a spiral-shaped density wave
can be formed from a set of nested ovals.
Density wave theory is really based on the premise that mutual gravitational
attraction of stars and gas clouds at different radii can offset the spiral’s
tendency to wind-up.
This produces a pattern which rotates rigidly within the galaxy disk.
Galaxy Spectra
Useful information about
galaxies can be learned by
looking at the spectrum
Spectra are dominated by
the brightest stars in the
galaxy
Elliptical galaxy spectra
dominated by old, K-type
stars. They have strong
absorption lines seen in
stellar spectra
Spiral galaxies have
absorption but also bright
emission lines from star
forming gas regions
heated by newly formed
stars
Kennicutt (1992)
Spiral galaxies – Rotation Curves and Mass
Spiral disks seen at inclination i
a
b
q = cos i where q=b/a
(b=semiminor and a=semimajor axis)
Where vr is radial velocity
and vr,0 is radial velocity at
galaxy center
For M31, vr along the
major axis is -30 km/s
and central velocity is 270 km/s. Velocity
curve is fairly constant
out to 36 kpc from
center and yields vc ~
230 km/s.
Evidence of Dark Matter in Spiral Galaxies
Rotation curves
determined from
Doppler shifts in
spectral emission lines
Vr stays roughly
constant with R as far
as luminous matter
can be detected
Masses from 1011 to
2x1012 Msun
50% to 80% of mass is
Dark matter.
DM does not have to
be confined to the
disk – distributed
spherically
Elliptical galaxies – Velocity Dispersion and Mass
Added width of the absorption lines in
elliptical galaxies reveals large random
motions of stars – not much rotational
motion as observed in spirals.
K Giant star
The velocity dispersion σ can be used
to estimate the galaxy’s mass.
Use Virial Theorem - relation between
kinetic and gravitational potential
energy for a system in equilibrium
NGC 2549 S0 galaxy
2K = -U
K = ½ M <v2> and U ~ -GM2/r or
U = -0.4 GM2/rh where rh is half mass radius
Let <v2> = 3σ2 then M = 7.5 σ2rh/G
For the dSph Leo I, rh = 290 pc and σ = 8.8
km/s giving
M/L ratio of 8 indicates significant DM
Determining Distances to Other Galaxies
Cepheid variable stars, when found
in other galaxies, tell us the
distance to those galaxies since
their luminosities can be determined
using the period-luminosity relation
Cepheids
With their high luminosities (~10,000 Lsun),
Cepheid variables extend the distance
scale to nearby galaxies, out ~25 Mpc (80
million light years).
Determining Distances to Other Galaxies
Tully-Fisher Relation
(a broadened line)
• Galaxy rotation measured via the Doppler shifting of the 21cm atomic
hydrogen line.
• Rotation speed (width of H line) is proportional to the galaxy’s mass.
• Galaxy luminosity is also proportional to galaxy mass (number of stars).
• The correlation between luminosity and rotation speed is referred to as the
Tully-Fisher relation.
Determining Distances to Other Galaxies
Type 1a SN
Type-I supernova result from
the detonation of white dwarf
stars when their mass (slightly)
exceeds 1.4 Msun.
The brightness of the explosion
should be (roughly) the same
for every Type-1 supernova.
Type-I Supernovae are standard candles. Knowing their luminosity,
and comparing to their measured flux, yields the distance via the
inverse-square law.
Useful for determining distances out to (3 billion light
years - 1 Gpc).
Determining Distances to Other Galaxies
Hubble’s Law
In 1912, Vesto Slipher
discovered that with few
exceptions, every galaxy is
receding from us,
i.e. has redshifted spectral
lines.
Redshift is defined by:
z = Dl/l
• In the 1920’s, Edwin Hubble
discovered that more distant
galaxies (using distances
determined from Cepheids) are
receding faster (have larger
redshifts).
• The relationship, well fit by a
straight line, is called Hubble’s
Law.
Hubble’s Law is written:
Recessional Velocity (in km/sec) = Ho  distance (in Mpc)
V = Ho D
where Ho is Hubble’s constant (slope of the line)
Universal Expansion
· You can think of space as the surface of
a balloon. As the balloon expands, the
space between galaxies stretches.
· This means that the wavelength of light
emitted by galaxies is also stretched as
space expands. The wavelength of light
is redshifted – we call this a
Cosmological Redshift.
If the rate of expansions stays constant over time, and all
objects are together at t=0, current distance between two
objects is
d = v to
Then,
where to is current age of Universe
v = (1/to)d
Same as Hubble’s law where Ho = 1/to
The value 1/Ho is called the Hubble Time  age of the
Universe if expansion is constant.
Ho has units of km/s/Mpc to express velocity and distance
in convenient units.
Each distance technique has uncertainties which then add
to the error in determining the Hubble Constant
Current values hover around Ho = 70 km/s/Mpc with an
error of +/- 8 km/s/Mpc
The Cosmic
Distance Ladder
Hubble’s law allows us
to measure distances to
the “ends of the visible
universe,”
(~13 billion light years).
It is less accurate for distances
< 100 Mpc because of the
“peculiar” velocities of galaxies
(i.e. motions affected by local
gravitational fields).