1. 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.
BM 4.1

2. Hubble Tuning Fork diagram (Hubble 1936)
Ellipticals
Lenticular (S0)
Spiral and Barred Spiral
Irregular

3. 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.

4. 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

5. 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)

6. 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)

7. 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, often interacting

8. 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
Only includes massive galaxies (doesn’t include dwarf spheroidals, dwarf irregulars, blue compact dwarfs)
Three different parameters for classifying spirals is unsatisfactory because the parameters are not perfectly correlated.
Bars are not all-or-nothing. There is a continuum of bar strengths.

9. 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
(t-type)

10. Schematic Diagram of Revised Hubble Classification
Cross section of diagram
E E+ S0- S0 S0+ Sa Sb Sc Sd Sm Im
No Bar
Ring
shaped
Spiral
shaped
Limitations:
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

11. 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
Van den Berg speculates that all disk galaxies are born as gas-rich spirals and gradually evolve to anemic and finally S0’s.

12. 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 K-type spectrum
S F-K stars dominate
Irr A stars dominate
Nomenclature:
g S 2
Spectral type (dominant stars) Hubble type flattening
(i.e. bulge/disk) E - elliptical (a-b)
a, af, f, fg, g, gk, k D - S a
S - spiral
B - barred
I - Irregular
R - rotationally symmetric but no S or E structure

13. Galaxies shown in order of increasing Hubble type from top to bottom.
Kennicutt (1992)
Galaxies shown in order of increasing Hubble type from top to bottom.

14. 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

15. 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:
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.
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.

16. 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.

17. Abraham et al. (1996)

18. 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)
2 x area= G
Constant
galaxy
M20 = 2nd order moment of the brightest 20% of the galaxy - measures concentration

19. Mergers E/S0/Sa G Sb/Sbc Sc/Sd/Irr M20 more light in fewer pix
Lotz et al. 2005
more uniform
surface brightness
Sc/Sd/Irr
M20
centrally
concentrated
less
concentrated

20. gim2d bulge fraction B/T versus gim2d smoothness s2.
gim2d bulge fraction B/T versus gim2d smoothness s2. Symbols indicate visual classifications as defined in Fig. 4. Shaded regions indicate the adopted automated classifications. Of the high-B/T galaxies, most visual bulges (red circles) are smooth (s2 ≤ 0.08), while visual discs and intermediates are more likely to show clumpiness or asymmetry (s2 > 0.08). We assign the high-s2 galaxies to the automated disc sample (blue shading). However, many visual discs and intermediates still remain in the low-s2 region containing candidates for the automated bulge sample (pink shading) which leads us to explore the additional cuts in Figs 8 and 9.
Judy Y. Cheng et al. MNRAS 2011;412:
Using surface brightness fitting to classify ~1000 galaxies from SDSS

21. ...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 “citizen science” project started in 2007, employing volunteers to classify galaxies imaged in the Sloan Digital Sky Survey, HST, and other galaxy survey projects.
Well over 250,000 people have participated in this project to visually classify about a million galaxies. Each galaxy receives over 20 classifications and the results are used together to determine the true classification.
Some results? go to