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PH607 – 3
1 Galaxy Classification
2 Spirals
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
http://www.galaxyzoo.org/
Four main types of galaxies –
Hubble proposed a scheme for classifying galaxies
in his 1936 book, The Realm of the Nebulae
 Ellipticals (27% locally)(E)
 Lenticulars (S0 or SB0)
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 Lens – bulge/disc, no arms, little dust
 S01, S02, S03 – strength of dust
absorption, S01 has none
 SB01, SB02, SB03 – prominence of bar
 Spirals (77%)– normal (S or SA) or barred (SB)
 Sa – Sc , ragged Sd
 depending on bulge/disk ratio,
tightness of spiral arms, gas content,
bright young stars
 Irr: Irregulars (3%) do not fit into above
categories
 d: Dwarves
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
otherwise.
A detailed description of Galaxy classifications can be found at:
http://nedwww.ipac.caltech.edu/level5/Haynes/Haynes_contents.html
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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
prototype):
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 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) ring 28″ diameter of 25 kpc
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The Arm tightness. Spiral galaxies have outstretched,
curving arms suggestive of a whirlpool or pinwheel.
Hubble 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
disc
▪ 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
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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, i,
of the spiral arms:
r  Ae B
or
dr
 Br
d
or
d log( r )
B
d
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The tan i = 1/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:

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smaller bulges
more "grand design" spiral structure
Leo trio:
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Messier 65 Sa, 11 Mpc :
M66, SAB(rs)a : Spitzer Infrared false-colour
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Triangulum, 900kpc, M33, SA(s)cd:
M51a and M51b – whirlpool SA(s)bc + SB0pec, 7 Mpc
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Hubble: Optical
The infrared emission from dust
M31, 780kpc Andromeda
NGC3982 21 Mpc,
SAb
SAB(r)b
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2.2 Irregulars
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."

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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
Two types of Irregular galaxies:
• 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
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Cloud, below. Such galaxies are sometimes referred to
as "Magellanic Irregular" galaxies.
NGC1427a:
•
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.
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.
2.3 Spiral Properties
As a fiducial, the Milky Way:

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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
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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) = I(0) exp(-R/Ro)
is given by 2 I(0) Ro2 ,
where I(0) is the (extrapolated) surface brightness at the
center of the disk, and R0 is the disk's exponential scale
length.
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
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comes from a rather thin component which is reasonably well-fit
by
I(z) =
where
I(z=0) sech2 (z/2zo)
.
sech(z)=2/[exp(z)+exp(-z)].
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.
From the Ursa Major Group at 15.5 Mpc:
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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 disc.
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:
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