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1 Galaxy Classification
2 Spirals
3 Ellipticals
4 Comparison
5 Multiwavelength
1. Galaxy Classification
External galaxies occur in a wide variety of shapes and sizes. In
the first systematic attempt to quantify their morphology, Hubble
produced his "tuning fork" diagram in the 1920s:
The Zoo: Sloan Digital Sky Survey
Four main types of galaxies –
Hubble proposed a scheme for classifying galaxies
in his 1936 book, The Realm of the Nebulae
 Ellipticals (20%)(E)
 Lenticulars (SO or SB0)
 S01, S02, S03 – strength of dust
absorption, S01 has none
 SB01, SB02, SB03 – prominence of bar
 Spirals (77%)– normal (S) (SA) or barred (SB)
 Sa – Sc depending on bulge/disk ratio,
tightness of spiral arms, and gas
 Irregulars (3%)(do not fit into above category)
Galaxies on the left are designated early type galaxies,
and those toward the right are called "late types." These
labels arise because Hubble believed that this diagram
represents an evolutionary sequence. We now believe
A detailed description of Galaxy classifications can be found at:
2 Spiral Galaxies
2.1 Classification
de Vaucouleurs introduced several important features:
-barred spirals (through
mixed types SA-SAB-SB)
-Scd-Sd-SdmSm-Im (m means Magellanic, the LMC being the
 Rings. Galaxies are divided into those possessing ringlike structures (denoted ‘(r)’) and those without rings
(denoted ‘(s)’). So-called ‘transition’ galaxies are given the
symbol (rs).
Hoag’s object: (183Mpc)
Spiral galaxies have outstretched, curving arms
suggestive of a whirlpool or pinwheel.
distinguished different sub-classes according to the
tightness of the arms and the size of the nucleus. He
called these Sa, Sb, and Sc.
▪ Sa - tightly-wound, smooth arms, and a bright central
▪ Sb - better defined spiral arms than Sa
▪ Sc - much more loosely wound spiral arms than Sb
▪ Sd - very loose arms, most of the luminosity is in the
arms and not the disc
Normal spiral galaxies are designated S* or SA*.
Barred spiral galaxies are designated SB*
Definite spiral structures are seen in some 61% of
galaxies. These structures often extend throughout
most of the galaxy’s visible disk, which have scale
lengths to 15 kpc or more.
Although individual galaxies often show irregularities
in the light distribution within the spiral patterns the
underlying spiral geometry is highly regular.
Logarithmic Spirals: The "*" is chosen from a, b or c, and
was originally classified on the basis of the pitch angle of
the spiral arms:
The derivative r'(θ) is proportional to the parameter b. In
other words, it controls how "tightly" and in which direction
the spiral spirals.
In the extreme case that b = 0, the spiral becomes a circle
of radius a.
Conversely, in the limit that b approaches infinity the
spiral tends toward a straight line.
Note that late-type spiral galaxies (Sc's) also tend to have:
smaller bulges
more "grand design" spiral structure
M65 Sa :
Triangulum, 900kpc, M33, SA(s)cd:
M51a and M51b – whirlpool SA(s)bc + SB0pec, 7 Mpc
2.2 Irregular
Certain galaxies lack either an obvious spiral structure or
nuclear bulge, appearing instead as a random collection of
stars with no obvious order.
These are designated "Irr" for "irregular."
Make up a few % of the field galaxy population
Generally smaller, sizes of a few kpc
Absolute magnitudes of –13 to –20
Masses of 108 to 1010 M
Irregular galaxies come in two types:
• Irr I which are in some sense a logical extension of the
Hubble tuning fork, having characteristics "beyond"
those of class Sc - high gas content, dominant presence
of a young population.
Irr I galaxies may show bar-like structures and
incipient spiral structure like the Large Magellanic
Cloud, below. Such galaxies are sometimes referred to
as "Magellanic Irregular" galaxies.
The LMC Large Magellanic Cloud
Irr II which are galaxies which defy classification
because of some form of disturbance. M82, shown below,
is undergoing an intense period of star-formation.
2.3 Spiral Properties
 As a fiducial, the Milky Way
 Radial Scale Length of 3-4 kpc
 Blue Luminosity of ~ 1.5 x 1010 L
 Absolute blue magnitude, -20.7
 Total Mass of ~1011 – 1012 M
(depending on how much dark matter there is).
About 90% of galaxies in the field are spirals
Most spirals are found in the field (in groups)
Spiral galaxy scale lengths run from ~1 kpc (dwarfs)
to ~50 kpc
Absolute magnitudes ranging from –16 to –23, that’s
a factor of ~1000 in luminosity!
Masses ranging from 109 to 1012 M
Surface Brightness. At large radii, face-on disk galaxies
typically have exponential luminosity profiles; the log of
the surface brightness falls as a linear function of radius,
The total luminosity of an exponential disc profile
I(R) = Io exp(-R/Ro)
is given by 2I o Ro2 ,
where I0 is the (extrapolated) surface brightness at the
center of the disk, and R0 is the disk's exponential scale
At smaller radii, the luminosity profile may deviate either
above or below the exponential line; the former are known
as `Type I' profiles, the latter as `Type II' ( Barred Spirals).
Observations of edge-on disks show that most of the
luminosity comes from a rather thin component which is
reasonably well-fit by
I(z) =
I(z=0) sech2 (z/2zo)
Colours. The integrated colours of disk galaxies reveal trends
with morphological type; S0 and Sa galaxies are red, while Sc
and Sd galaxies are blue. These trends reflect different rates of
star formation; broadband colors are sensitive to the average
star formation rate over the last 108 years.
2.4 Barred spirals
A large fraction of disk galaxies have bars: narrow linear
structures crossing the face of the galaxy.
In barred S0 galaxies the bar is often the only structure visible in
the disk.
In types SBa and later the bar often connects to a spiral pattern
extending to larger radii (e.g. NGC 1300). Viewed face-on, bars
typically appear to have axial ratios of 2.
The surface brightness within the bar is often fairly constant. Some
bars appear to be ‘squared off’ at the ends. The true 3-D shapes of
bars are difficult to determine, but many appear to be no thicker
than the disks they occur in; if so then bars are the most flattened
triaxial systems known
Bars in edge-on galaxies are hard to detect photometrically;
however, kinematic signatures of barred potentials have been
used to infer their presence in some edge-on systems. What is
noteworthy is that such edge-on bars appear to be associated with
boxy or peanut-shaped bulges.
M58 SBb:
3 Elliptical Galaxies
When you have seen one elliptical galaxy, you have pretty much
seen them all. Here is a picture of one such system, the nearby
elliptical M32:
An elliptical galaxy shows no spiral structure and can vary
from almost round (what Hubble called E0) to almost cigar
shaped (called E7).
This classification is based on our perspective from Earth
and not on the actual shape. So, elliptical galaxies are
designated "E#," where # refers to their apparent flattening:
o # = 10(1 - b/a)
M89: E0
M59 E5
Apparently round ellipticals are E0s
The flattest ellipticals observed are E7s.
Do not have perfect elliptical isophotes – typical deviations of
a few %
Deviations from ellipses can be classified as disky or
Boxy galaxies tend to be more luminous, slower rotators
Disky: normal and low luminosity ellipticals, which have
nearly isotropic random velocities but are flattened due to
3-D shapes – ellipticals are predominantly triaxial
Oblate: A = B > C (a flying saucer)
Prolate: A > B = C (a cigar)
Triaxial A > B > C
(A,B,C are intrinsic axis radii)
Find that galaxies are mildly triaxial:
A:B:C ~ 1 : 0.95 : 0.65 (with some dispersion ~0.2)
Triaxiality is also supported by observations of
isophotal twists in some galaxies (would not see
these if oblate or prolate)
It was once thought that the shape of ellipticals varied
from spherical to highly elongated. The Hubble
classification of elliptical galaxies ranges from E0 for those
that are most spherical, to E7, which are long and thin.
It is now recognized that the vast majority of ellipticals
are of middling thinness, and that the Hubble
classifications are a result of the angle with which the
galaxy is observed.
In between the ellipticals and the spirals are the S0s
which have
o very large bulges
o weak disks
o no spiral structure
Surface Brightness. Many normal bright (Mv < -17)
elliptical galaxies and the bulges of spirals have a
projected luminosity distribution that follows a de
Vaucouleurs. (or R1/4) law. The surface brightness, I, of
the bulge of the galaxy (measured in units of L pc-2)
shows a radial dependence according to:
where Re is the radius of the isophote containing half the
total luminosity, and Ie is the surface brightness at Re.
This is often referred to as a r1/4 law – and the distribution is
sometimes called a de Vaucouleurs profile.
Note that this law is a purely empirical fit with no physical
basis. However, any theory of elliptical galaxy formation
must reproduce it.
Cores of Ellipticals
• Seeing corrections are important; moreover, it's
generally not possible to make corrections without
some assumptions about the underlying luminosity
• Few E galaxies actually have flat luminosity profiles at
small radii; instead, the profiles rise inward to the last
measured point .
• Cores may exhibit unusual kinematics; for example,
about a quarter of all elliptical galaxies have cores
which appear to counter-rotate with respect to the rest
of the galaxy .
• Although such `kinematically decoupled' cores are
generally not photometrically distinct, several E
galaxies with decoupled cores have features in their
line-strength profiles coincident with the kinematically
decoupled regions .
• A few nearby E galaxies have nuclear star clusters with
densities much higher than the cores they reside in;
some of these nuclei may be rotating, disk-like systems.
Shapes of Ellipticals
• Projected axial ratios range from b/a = 1 to ~0.3, but
not flatter (Schechter 1987).
• Apparent ellipticity is generally a function of projected
radius, with a wide range of profiles.
• Isophotal twists are common. Because it is highly
unlikely that intrinsically twisted galaxies could be
dynamically stable, such twists are generally interpreted
as evidence for triaxiality .
• Elliptical galaxies are not elliptical; isophotes may
depart significantly from perfect ellipses. A Fourier
analysis of isophotal radius in polar coordinates implies
that most E galaxies are either `boxy' or `disky' .
Kinematics of Ellipticals
• The rotation velocities of bright E galaxies are much
too low to account for the flattenings we observe;
fainter E galaxies, however, rotate at about the rates
implied by their shapes (Davies 1987).
• E galaxies may exhibit minor-axis rotation; more
generally, the apparent rotation axis and the apparent
minor axis may be misaligned. While in most galaxies
these misalignments are modest, a few galaxies appear
to rotate primarily about their minor axes.
Larger galaxies have fainter effective surface brightnesses. Mathematically
effective radius, and
(Djorgovski & Davis 1987) where Re is the
is the mean surface brightness interior to Re.
, we can substitute the previous correlation and see that
and therefore:
meaning that more
luminous ellipticals have lower surface brightnesses.
More luminous elliptical galaxies have larger central velocity
dispersions. This is called the Faber-Jackson relation (Faber &
Jackson 1976). Analytically this is:
Shells & Other `Fine Structures'
• The surface brightnesses of E galaxies do not always
decline smoothly with radius. When a smooth
luminosity profile is subtracted from the actual surface
brightness, `shells' or `ripples', centered on the
galaxy, are seen.
• The fraction of field E galaxies with shell-like features is
at least 17% and possibly more than 44%.
• The colours of shells indicate that they are composed of
stars. In many cases the shells are somewhat more
blue than the galaxies they occupy.
• Shell systems have a variety of morphologies; some
galaxies have shells transverse to the major axis and
interleaved on opposite sides of the center of the
galaxy, while other galaxies have shells distributed at
all position angles .
• Profile subtraction sometimes reveals other kinds of
structures in E galaxies, including embedded disks,
linear features or `jets' (not the jets seen in AGNs!), `Xstructures', etc..
Gas & Dust in Ellipticals
• When examined with sufficient resolution, 25% to more
than 40% of E galaxies show features due to dust
• The dust lanes seen in E galaxies imply that the
absorbing material is distributed in rings or disks. Dust
lanes may be aligned with either the major or minor
axes, or they may be warped.
• E galaxies contain modest amounts of cool and
warm gas, although not as much as is found in S
galaxies. A few E galaxies have extended disks of
neutral hydrogen.
• X-ray observations indicate that many ellipticals contain
10^9 to 10^10 solar masses of gas at temperatures of
K; this hot gas typically forms a pressuresupported `atmosphere' around the galaxy.
Tidal Features
• Elliptical galaxies in rich galaxy clusters often exhibit
luminosity profiles which fall below a de Vaucouleurs
law at large radii. Such downturns are often attributed
to tidal truncation in the mean field of the cluster
• In contrast, E galaxies with close companions often
have luminosity profiles which rise above a de
Vaucouleurs law at large radii. These features may be
plausibly blamed on tidal interactions.
• E galaxies in closely interacting systems sometimes
exhibit outer isophotes which are visibly eggshaped and/or offset with respect to the centers of their
galaxies. Again, tidal effects are strongly implicated
(K82, Borne et al. 1988).
• On very deep exposures, some E galaxies are seen to
have `plumes' or `tails', while others (e.g. NGC 5128)
show rather irregular luminosity distributions. Tail-like
features may be signatures of major mergers
involving one or more dynamically cold disk galaxies
(Schweizer 1987).
4 Galaxy Constituents:
Spiral galaxies contain:
stars (population I and II) , gas, dust
Elliptical galaxies contain:
o stars (population II only – (i.e. old) stars)
Irregular galaxies are harder to classify. They usually
o stars (population I (young stars) – in other words there
are significant amounts of gas in the galaxy which is
being transformed into young stars – with ages as
short as a few million years) and some population II) ,
star-forming regions , gas (a higher proportion than in
The Large Magellanic Cloud at optical wavelengths
Mass M
Absolute mag
Luminosity L
M/L (M / L
Diameter (kpc)
105 - 1013
-9 -> -23
3 x 105 - 1011
10 – 4 x 1011
-15 -> -21
108 – 2 x 1010
2 – 20
108 – 3 x 1010
-13 -> -18
107 - 109
1 – 200
II and old I
5 - 50
I in arms, II
and old I
1 – 10
I, some II
of Almost none
Total fraction
Sd Irr
Old Old +
Old +
Intermediate Intermediate +
+ Young
Star Form zero low
HI (gas) Zero/ low
high highest
Dynamics Bulge/halo Disk dominated, so
The Colour-Magnitude Diagram:
Colour: Large automated imaging surveys are better at
defining a galaxy's colour rather than morphology.
it is more natural to describe a galaxy as being on the
‘red sequence’ or ‘blue sequence’ rather than being an
‘early type’ or ‘late type’.
This interpretation also has the advantage that galaxy
colours are directly related to the star formation, dust and
metal-enrichment history of the galaxy and can thus be
more readily interpreted in theoretical models
The bimodal distribution of red and blue galaxies as seen
in analysis of Sloan Digital Sky Survey data[2] and even in
de Vaucouleurs' 1961 analyses of galaxy morphology.
Three features:
the red sequence,
the green valley
the blue cloud.
The red sequence includes most red galaxies which are
generally elliptical galaxies. The blue cloud includes most
blue galaxies which are generally spirals. In between the
two distributions is an underpopulated space known as the
green valley which includes a number of red spirals.
Unlike the comparable HR diagram for stars, galaxy
properties are not necessarily completely determined by
their location on the color-magnitude diagram. The
diagram also shows considerable evolution through time.
Colour (U-R) versus stellar mass relations for different
Panel (a): void-like environments while
Panel (f) cluster-like environments.
[Conclude: Hubble sequence applies to other properties.]
5 Multiwavelength Views of Galaxies
Our view of is greatly affected by the observing wavelength –
the infrared penetrates deeper than optical radiation.
M101, a nearby Sc galaxy:
The Whirlpool: M51…below Chandra (X-ray) and ISO (mid-IR):
The x-ray image (left) highlights the energetic central regions of
the two interacting galaxies. Much of the diffuse glow is from multimillion degree gas. Many of the point-like sources in the x-ray
image are due to black holes and neutron stars in binary star
Mid-IR light (right) is well-suited to studying star formation and
tracing dust in spiral galaxies. This image not only shows the galaxy
cores and spiral arms, but nicely illustrates the knots of star
formation occurring in the arms of M51.
M104: (SAa, 9 Mpc) Spitzer's infrared view of the starlight, piercing
through the obscuring dust, is easily seen, along with the bulge of
stars and an otherwise hidden disk of stars within the dust ring.
NGC253 at infrared,optical and X-ray wavelengths
M81 at optical wavelengths and using the 21cm wavelength HI tracer of
atomic hydrogen gas. The spiral structure is clearly shown in this image,
which shows the relative intensity of emission from neutral atomic hydrogen
gas. In this pseudocolor image, red indicates strong radio emission and blue
weaker emission.