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
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
Elliptical Galaxies
•Smooth structure and symmetric, elliptical contours
•Subtype E0 - E7 defined by flattening
•En where n = 10(a-b)/a
where a and b are the projected major and minor axes
(doesn’t tell what the 3-D shape is)
Lenticulars or S0 Galaxies
•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
•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
Irr I
•No 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: asymmetrical with 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 optional bells and whistles
•Mixed types:
E/S0, Sab, Sbc
•Mixed barred/normal:
SA (unbarred), SB (barred), SAB (in between)
•Inner rings:
S(s) (arms out of ring), S(r) (arms in ring), S(rs)
•Outer rings:
(R) S
•Extended spiral, irr types: Sm (between spiral and Irr), Im (magellanic),
Sd (extreme Sc), Sdm (between Sd and Im)
•“t-types” scale
Added in later editions of the Reference Catalog
(de Vaucouleurs2, Corwin 1976)
E0  S0  Sa  Sb  Sc  Im
-5
-1
1
3
5
10
(t-type)
Schematic Diagram of Revised Hubble Classification
E
E+ S0- S0 S0+ Sa Sb Sc Sd Sm
Im
Cross section of diagram
No Bar
Limitations:
Ring
shaped
Spiral
shaped
•E  Im is not a linear sequence of one parameter
•Rings and bars are not independent
•Does not take into consideration mass or other
important parameters. All based on optical
surface brightness morphology.
Bar
Luminosity Classification or “DDO System”
van den Bergh (1960) -
working at David Dunlop
Observatory in Ontario,
Canada - hence the “DDO”
In spirals and irregular galaxies, some properties correlate with galaxy
mass rather than type. For spirals, the key parameter is arm
development (i.e. arm length, continuity and width relative to size)
Sc I - long, well-developed arms
Sc III - short, stubby arms
Sc IV - dwarf, spiral galaxy -faint hint of spiral structure
Revised DDO - van den Bergh (1976):
Placed disk galaxies into 3 parallel classes based on luminosity:
Gas-rich, anemics and lenticulars
Anemics have weak and diffuse spiral arms and low level of ongoing SF
Parameters which change systematically from Lenticular to Gas-rich
•Mean stellar age
•Gas fraction
•Recent SF
Yerkes System (Morgan 1958)
Strong correlation noted between the nuclear light concentration (how big the bulge
is) and its integrated spectrum. Type is based on this one parameter - integrated
spectral type.
•E, S0
•S
•Irr
K-type spectrum
F-K stars dominate
A stars dominate
Nomenclature:
gS2
Spectral type (dominant stars) Hubble type flattening
(i.e. bulge/disk)
E - elliptical
10(a-b)
a, af, f, fg, g, gk, k
D - S0
a
S - spiral
B - barred
I - Irregular
R - rotationally symmetric but no S or E structure
Kennicutt (1992)
Galaxies shown in
order of increasing
Hubble type from
top to bottom.
A couple of galaxy classes not addressed in these systems….
Dwarf Ellipticals – dE
•much less luminous
than the normal elliptical
galaxy.
•Typically a few kpc
across and contain 1
million stars.
NGC 205
Dwarf Spheroidals – dSph
•overall low star density
•appear as a cluster of faint
stars.
The Sculptor system (Shapley
1938) was the first to be
discovered.
dSph are the low-luminosity
counterparts of dEs.
Leo I dSph
Morphological Distributions
The morphological type of galaxy present depends to some extent on where you
look (more detailed discussion of this later…). Some key results:
•The Local Group includes a
significant number of very faint
galaxies. Of the ~35 galaxies,
only the 3 brightest (M31, MW
and M33) are spirals, the
remainder are equally divided
between irregular and dwarf
elliptical /spheroidal galaxies.
•Galaxies outside of clusters (in the “field”) are biased towards late-type (Sc)
spirals. A typical field sample might be 80% S galaxies, 10% S0 galaxies, and 10%
E galaxies. Within rich clusters, the distribution is dominated by early-type
systems (Dressler 1980). An intermediate density cluster will have 40% S
galaxies, 40% S0 galaxies, and 20% E galaxies. A high density cluster will have
10% S, 50% S0, and 40% E.
Automated Classification
Visual classification is inherently time consuming and different observers are
unlikely to agree in ambiguous cases. This motivates the development of
algorithms to automatically and impartially classify galaxy images - very
important for large surveys like 2MASS and SDSS.
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
Naim, Ratnatunga & Griffiths (1997) use 4 parameters: blobbiness, asymmetry,
filling factor and elongation.
Naim et al. (1995) used artificial neural nets to classify galaxies into the
numerical T types. Achieved uncertainty of +/- 1.8 in T which is comparable to
the dispersion between observers.
For distant galaxies (greater than z=0.5), classification is difficult because of
small angular size and apparent faintness of galaxies. HST galaxies (z~1)
classified by 2 experts (Ellis and van den Bergh) and also using A and C
parameters of Abraham.
For faint galaxies, C parameter alone is fairly good.
For brighter galaxies, C is degenerate between E and S0.
Abraham et al. (1996)
The Gini Coefficient and M20 parameter
Used in economics to measure distribution of wealth in population
G = relative distribution of flux in galaxy’s pixels (Abraham et al. 2003)
G=0 for completely egalitarian society (uniform surf brightness)
G=1 for absolute monarchy (all flux in single pixel)
Constant
2 x area= G
galaxy
M20 = 2nd order moment of the brightest 20% of the galaxy measures concentration
Mergers
more light
in fewer pix
E/S0/Sa
G
Sb/Sbc
Lotz et al. 2005
more uniform
surface
brightness
Sc/Sd/Irr
less
concentrated
M20
centrally
concentrated
...but we haven’t seen the end of visual classification!
No matter how good the automated classifications become, the human eye is still
better at determining patterns than neural networks (e.g. detecting spiral
structure, smoothness)
Galaxy Zoo is a project to employ volunteers to classify galaxies imaged in the
Sloan Digital Sky Survey
About 250,000 people have participated in this project to visually classify about
400,000 galaxies. Each galaxy receives over 20 classifications and the results are
used together to determine the true classification.
Some results? go to http://www.galaxyzoo.org/