Morphological Classification
Hubble proposed a scheme for classifying galaxies in his 1936
book, The Realm of the Nebulae
Four types of galaxies –
 Ellipticals (E): En, there n=10[1 –(b/a)], so E0 is round and
E7 is most flattened
 Lenticulars (SO or SB0)
 S01, S02, S03 – strength of dust absorption, S01 has none
 SB01, SB02, SB03 – prominence of bar
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Spirals – normal (S) or barred (SB)
 Sa – Sc depending on bulge/disk ratio, tightness of spiral arms,
and gas content
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Irregulars (does not fit into above category)
Hubble’s original tuning fork
Hubble thought this was an evolutionary sequence, so
ellipticals are “early-type” and spirals are “late-type”
Hubble classification scheme
Ellipticals
M89 – E0
Ellipticals
M87—E1
Ellipticals
M32—E2, dwarf
Ellipticals
M49-E4
Ellipticals
M59-E5
Lenticulars
M84—S0
Lenticulars
NGC5866 – S03
Spirals
M65--Sa
Spirals
M104—Sa
Sombrero
Spirals
M31 -- Sb
Andromeda
Spirals
M33--Sc
Spirals -- barred
M58--SBb
Extensions to the Hubble Sequence
De Vaucouleurs (1959) added three new classes: Sd,
Sm (e.g., Large Magellanic Cloud is an SBm), Im (e.g.,
Small Magellanic Cloud)
Extensions to the Hubble Sequence
De Vaucouleurs also introduces:
 Notation SA for unbarred galaxy (to match SB for
barred)
 Notation SAB for intermediate, weakly barred
systems
 Symbols (r) and (s) to indicate systems with and
without rings
 In Second (and Third) Reference Catalog, also
uses T-type ranging from –5 to 10:
 E = -5, E/S0 = -3, S0 = -2, S0/a = 0
 Sa = 1, Sab=2, Sb=3, Sbc = 4, Scd =6
 Sdm=8, Im=10
Spirals
M83—SAB(s)c
Spirals
NGC303—SAB(r)c
Extensions to the Hubble Sequence
Van den Bergh (1960) added luminosity
classes based on arm quality/length (DDO
System)
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I – strong, well-defined arms, most luminous
galaxies, MB < -21
V – chaotic, small arms, least luminous spirals,
usually Sd-Im, MB = > -17
Luminosity class is listed in Revised Shapley Ames
Catalog
What is missing from the Hubble Sequence?
Dwarf galaxies
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Faint, M > -18,
Dwarf Ellipticals, dwarf spheroidals, dwarf irregulars
There are probably lots of these, in the Local Group there are
>30!
Low Surface brightness galaxies
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Very difficult to detect!
Need dedicated surveys
Probably lots of these too!
Peculiar Galaxies
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In particular, interacting galaxies
Many cataloged by Arp in 1966
Dwarf Elliptical
Optical
Near-infrared
Dwarf Spheroidal
Leo I
Dwarf Irregular
IC 10
Low Surface Brightness Galaxy
Malin 1
Interacting Galaxies
Arp 295
Interacting Galaxies
The Mice
NGC 4676
Interacting Galaxies
The Tadpole
Arp 188
UGC 10214
Overview of Galaxy Properties
E
S0
Sa
Sb
Sc
Color
Red
Stellar
Pop.
Old
Old +
Intermediate
Old +
Intermediate +
Young
SFR
zero
low
higher
HI
(gas)
Zero/
low
low
modest
dust
Zero/
low
Dyn.
Bulge/halo
dom.
Sd Irr
Blue
Higher
highest
Disk dominated, so rotation
Intermediate +
Young
high
high highest
Lower (less
metals)
Overview of Galaxy Properties
As a fiducial, the Milky Way
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Radial Scale Length of 3-4 kpc
Blue Luminosity of ~ 1.5 x 10 L
Absolute blue magnitude, -20.7
Total Mass of ~1011 – 1012 M 
 Depending on how much dark matter there is
Overview of Elliptical Galaxies
About 20% of field galaxies are ellipticals
Most ellipticals are found in clusters!
There are a number of different types of
ellipticals
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E’s (normal ellipticals)
cD’s (massive bright ellipticals at the centers of
galaxy clusters)
dE’s (dwarf ellipticals)
dSph’s (dwarf spheroidals)
Note that these do not form a continuous
sequence, they are structurally, kinematically, and
physically different objects.
Overview of Elliptical Galaxies
Measure the size of ellipticals by its effective
radius – radius which encloses half the light
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For comparison the effective radius in an
exponential disk is 1.7 x the scale length
Sizes range from few tenths of a kpc (dE’s) to tens
of kpc (cD’s)
Absolute magnitudes range from –10 (dSph’s) to –
25 (cD’s), a factor of 106 in luminosity
Masses range from 107 M to 1013 - 1014 M
The Coma Cluster
Center of the Coma Cluster
cD – NGC 4881
Overview of Spiral Galaxies
About ¾ 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
Hubble Deep Field
Hubble Deep Field – zoomed in
Released March 9, 2004
11.3 days of observing!
Spiral Galaxies at different wavelengths
Spiral Galaxies at different wavelengths
Overview of Irregular Galaxies
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 at different wavelengths
Optical
Near-infrared
Catalogs and Atlases of Galaxies
In late 1700’s, Messier made a catalog of 109
nebulae so that comet hunters wouldn’t
mistake them for comets!
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~40 of these were galaxies, e.g., M31, M51, M101.
Most are gaseous nebulae within the Milky Way,
e.g., M42, the Orion Nebula
Some are stellar clusters, e.g., M45, the Pleiades
Catalogs and Atlases of Galaxies
New General Catalogue (Dreyer 1888)
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Based on lists of Herschel (5079 objects)
Plus some more for total of 7840
~50% are galaxies, catalog includes any non-stellar object
Index Catalogue (IC) – (Dreyer 1895, 1898)
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Additions to the NGC, 6900 more objects
See www.ngcic.org for online info
Shapley-Ames Catalog (Harvard 1932)
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Bright galaxies, mpg < 13.2
Whole-sky coverage, fairly homogenous
1246 galaxies, all in NGC/IC
Revised by Sandage & Tamman in 1981
Catalogs and Atlases of Galaxies
Uppsala General Catalog (UGC --Nilson 1973)
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Based on Palomar Observatory Sky Survey (POSS)
Size limited, a > 1 arcmin
13000 objects
ESO (European Southern Observatory) Catalog
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Similar to UGC in southern sky,  < 30
18000 objects
Morphological Catalog of Galaxies (MCG, Vorontsov-Vel’yaminov
et al)
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Based on POSS plates
32000 objects, -2 <  <-18
Catalogs and Atlases of Galaxies
Hubble Atlas (Sandage 1961)
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Present plates used by Hubble in developing classification system
plus explanation of system
Atlas of Galaxies Useful for measuring the Cosmological Distance
Scale (Sandage & Bedke 1988)
Nearby Galaxies Atlas & Catalog (Tully 1988)
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V < 3000 km/s
Reference Catalog of Bright Galaxies (RC3, deVaucoleurs et al
1991)
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B < 15.5, 23022 galaxies
Also, RC1 (1964, 2599 galaxies) and RC2 (1976, 4364 galaxies)
Carnegie Atlas (Sandage & Bedke 1994) – Images of galaxies in
the Revised Shapley Ames Catalog
Catalogs and Atlases of Galaxies
Catalogs of sources in x-ray, radio, infrared, etc.
More recent galaxy surveys – APM survey, CfA
Redshift Survey, 2dF redshift survey, Sloan Digital Sky
Survey (SDSS)
The list is fairly endless!
The NASA/IPAC Extragalactic Database (NED) is a
good source of information on galaxies, plus has
many galaxy catalogs on-line:
http://nedwww.ipac.caltech.edu/