Astronomía Extragaláctica y Cosmología ObservacionalDepto. de Astronomía (UGto)

AstronomAstronomíía Extragala Extragalááctica ctica y y

CosmologCosmologíía Observacionala Observacional

Prof. Dr. CProf. Dr. Céésar A. Carettasar A. Caretta

Lecture 1 Morphological Classification of Galaxies

Brief Historybefore the concept of galaxies – the nebulæ

• first observations• first theoretical-philosophical proposals• catalogs of nebulæ• attempts to discover the nature of the nebulæ• the “Great Debate”• “The Realm of Nebulæ”

movie

Classification systemsthe Hubble “tuning-fork” diagramextensions to the Hubble sequencenon-optical bandsnew proposals for classification systemsthe morphological types of galaxies

Astronomía Extragaláctica y Cosmología ObservacionalDepto. de Astronomía (UGto)

First observations

M31 (NOAO, EUA)

• 964 d.C. – Abd al-Rahman al-Safi [Persia]: notes the observation of Al Bakr (Andromeda) “Book of Fixed Stars”

• 1519 – Fernão de Magalhães [ocean]: notesobservation of Magellanic Clouds

Milky-Way (G. Garradd, Australia) Magellanic Clouds (MSO, Australia)

Milky-Way (C. Cook)

First observations

• 1576 – Thomas Digges [England]: “breaks” the fixed sphere of stars“A Perfit Description of the Caelestial Orbes”

• 1610 – Galileo Galilei [Italy]: resolves the MW (Milky Way) into stars with his telescope“Sidereus Nuncius” and “The Sideral Messenger”

• 1750 – Thomas Wright [England]: stars distributed in shells, MW is one of these shells“An Original Theory or New Hypothesis of the Universe”

• 1755 – Immanuel Kant [Germany]: diffuse (nebulous) celestial objects are “island universes”,similar to the MW“Allgemeine Naturgeschichte und Theorie Des Himmels”

First theoretical-philosophical proposals

Catalogs of nebulæ

• 1654 – G. B. Hodierna“De admirandis coeli characteribus” (41 nebulae)

• 1755 – A. N. L. de La Caille(42 nebulae in the Southern Celestial Hemisphere – South Africa)

• 1784 – Charles Messier“Catalogue de nébuleuses et des amas d’etoiles que l’on découvre parmi les étoiles fixes, sur l’horizon de Paris” (103 nebulae) – M1 to M110 nowhttp://www.seds.org/messier/http://www.ipac.caltech.edu/2mass/gallery/messiercat.html

• 1864 – John Herschel (results of Herschel family work)“The General Catalogue of Nebulæ” (5079 nebulae)

• 1888-1908 – John L.E. Dreyer“New General Catalogue of Nebulæ” (NGC) ≈15000“Index Catalogues” (IC)http://ngcic.org/dss/dss_ngc.htm

M1

M110

Attempts to discover the nature of nebulæ (the Herschel family)

F. Wilhelm Herschel

Karoline Lucretia HerschelJohn F. W. Herschel

• 1787 – first observational model for MW (William)• 1791 – two types of nebulæ: (i) luminous fluid

(ii) star system• 1864 – spiral nebulæ avoid the MW (John)

Largest telescopes of XVIII century (1,3m)Discovery of Netuno (1781)Planetary Nebulæ (1784)

Attempts to discover the nature of nebulæ (the Herschel family)

Distribution of 14650 galaxies of the catalogs UGC, ESO and MCG

http://www.eso.org/~mhilker/Gallery/gallery_lect.html

Attempts to discover the nature of nebulæ (new techniques and tools)

• 1797 – Pierre S. de Laplace [France]: nebular hypothesisfor the formation of solar system “Exposition du Système du Monde”

• 1826 – Photography

• 1832 – Thomas Henderson [Cape Obs., South Africa]: measures the parallax of αCent

• 1838 – Friedrich W. Bessel [Germany]: announces the first measurement of a stellar parallax (61 Cygni)

N. Nièpce (1826)J. M. Daguérre (1839)J. Herschel (1839) H. Draper (1880) – photographic plates (Orion)

Attempts to discover the nature of nebulæ (new techniques and tools)

M51

Leviathan:1.8 m (72”)

• 1845-1850 – William Parsons, the Lord Rosse [Ireland]: spiral nebulæ(M51, M99, M33, M74, M101)

• 1864 – William Huggins [England]: first spectroscopic observations ofdiffuse nebulæ (NGC6543: 3 lines: H, N and “nebulium” = OIII)

N6543~1868

70 nebulae:1/3 emission line spectra

(like M42)2/3 stelar spectra (“white”)

(like M31)

The “Great Debate”

• 1906-1922 – Jacobus C. Kapteyn [Netherlands]: countsof stars in 206 zones (Kapteyn Model)

“First Attempt at a theory of the arrangement and motion of the sideral system”

• 1912-1917 – Vesto M. Slipher [Lowell Obs., USA]:measured radial velocities of spirals

• 1918 – Harlow Shapley [1.5m (60”), Mt. WilsonObs. USA]: distr. of Globular Clusters (Shapley Model)

• 1916-1923 – Adriaan van Maanen [Mt. Wilson Obs, USA]:reported detection of “high speed” rotation (T ∼ 105 yrs) on M33 and M101(not confirmed later)

1913 – 4 ( 3 redshifts, M31 blueshift)1914 – 12 (11 redshifts, M31 blueshift)1925 – 43 (41 redshifts, 2 blueshifts)

∼ 100 kpc

∼ 15 kpc

N891

Harlow Shapley lecture and paper (1921)Spiral nebulae are members of the Great Galaxy

globular clusters are major structure elements of MWMW has lower surface brightness than spiral nebulæMW stars are redder, on mean, than spirals1885 (super)nova on M31, much brighter to be extragalacticrotation of spirals (van Maanen), would have supraluminal velocities if extragalactic

absorption

Herbert Curtis lecture and paper (1921)Spiral nebulae are “island universes”, like the MW

Kapteyn model – Shapley distances are overestimatedall other novæ on spirals are fainter than in MW (M31 ~100 kpc)spiral nebulæ avoid the MW plane (J. Herschel)spiral diameters have a large angular range (large dist. range)spirals have large radial velocities (Slipher), would escape from MWedge-on spirals present a dark band, like MW plane

absorption

The “Great Debate”

• 1920 – the Great Debate [Nac. Academy of Science, USA]Opposing views in

2 fundamental issues▪ the size of the MW▪ existence of external galaxies

• 1912 – Henrietta S. Leavitt [Harvard College Obs, USA]: discovered the period-luminosity relation for Cepheid variables

• 1925-1929 – Edwin Hubble [2.5m (100”), Mt. Wilson Obs., USA]: measured distances for N6822, M33 and M31 (300 kpc) using Cepheids

• 1929 – E. Hubble: Hubbles’s Law (expansion of the Universe): v = H0 D

• 1936 – E. Hubble: “The Realm of Nebulæ”(Hubble sequence...)

“The Realm of Nebulæ”

The Hubble “tuning-fork” diagram

Ellipticals

Normal Spirals

Barred Spirals

Irregulars (I, II)

EARLY → LATE UNCLASSIFIED[Hubble 1926, ApJ 64, 321; Hubble 1936 “The Realm of Nebulae”; Sandage 1975, “Galaxies and the Universe”]

E0 (M89) E5 (M59)

SBbc (NGC 1300)

Sa (M96)

IrrII (SMC)

E0 (M89)

E1 (M87)

E6 (M110)

E5 (M59)

E2 (M60)

E4 (M49)

No ellipticals more flattened than E7exist, probably because there is a stability limit for non rotating systems

* Do not confusewith “eccentricity”

Є = √ 1– b2/a2

ab

The Hubble “tuning-fork” diagram: criteria

Ellipticals – En

ε = (1 − b/a) ellipticity*n = 10ε

The Hubble “tuning-fork” diagram: criteria

Sa (M96)

Sb (M31)

Sc (M74) SBa (NGC 4643) SBb (M95) SBc (NGC 1365)

Normal (and Barred) Spirals – S(B)a, S(B)b, S(B)ca → spiral arms tightly wound and smooth

(not resolved into stars or HII regions), and dominant central bulge or bar

b → spiral arms more open and resolved, smaller bulge or bar

c → spiral arms very open, patchy and resolved into stars, star clusters and HII regions,bulge or bar not prominent

Order of importance: (i) openness of the winding of the spiral arms, (ii) size of the bulge or central bar relative to the disc (B/D), (iii) degree of resolution of the arms into stars and HII regions

IrrI (SMC)

IrrII (M82)

IrrI (LMC)

IrrII (N5195)

The Hubble “tuning-fork” diagram

Irregulars – IrrI, IrrIII → similar to the

Magellanic CloudsII → abnormal galaxies,

peculiar

Extensions to the Hubble sequence

S0 (NGC 3115)

• 1940 – Shapley & Paraskevopoulos [Proc. N. Ac. Sc. 26, 31]:added the S(B)d type {between S(B)c and Irr I}

• 1951 – Spitzer & Baade [ApJ 113, 413]: first reference to lenticular galaxies (already observed by Hubble) –with discs but not spiral arms

• 1954 – Gerárd de Vaucouleurs [AJ 60, 126]: discovereda weak spiral structure in LMC

• 1959 – G. de Vaucouleurs [Handbuch of Physik 53, 275]: introduced intermediate types and fine classifications:

SBb(r) (M95) transition between Sp types: a, ab, b, bc, cextension of the sequence beyond S(B)c: cd, d, dm, m, Im and I0transition between “Ordinary” and Barred Sp: SA, SAB and SBpresence or absence of rings: R, (r), (rs), (sr) and (s)

Extensions to the Hubble sequence: de Vaucouleurs criteria

Extensions:S(B)d → later spirals (weakly chaotic)S(B)m → transition Sp/Irr (LMC)Im → very irregular and loose (SMC)I0 → chaotic, very rich in gas

and SF regions

Rings:s → spiral arms begin directly

at the nucleus or a barrs, sr → intermediate ringsr → spiral arms begin at a ringR → external ring

Bars:A → unbarredAB → weakly barredB → barred

Obscuring matter:S0- → freeS0 → intermediate

amountS0+ → dark band

Extensions to the Hubble sequence: classification volume

Extensions to the Hubble sequence: T stage

• 1974 – G. De Vaucouleurs [In The Formation and Evolution of Galaxies, ed. J.R. Shakeshaft]:

E S0 Sa Sb Sc IrrI IrrII

E- E0 E+ S0- S00 S0+ S0a Sa Sab Sb Sbc Sc Scd Sd Sdm Sm Im I0

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11

Morphological Classification

Some comments on Morphological Classification:they were defined from the appearance of galaxies in photographic plates (optical)

they refer primarily to intrinsic luminous galaxies, but there exists a large population of dwarf galaxies (van den Bergh, 1960)(as luminosity decreases, first the rings become invisible, then the spiral arms, and finally the bar and disc – dwarfs are more frequently E and Irr)

the word “normal” for unbarred spirals and lenticulars do not mean that they are more common than barred: about 30% of the spirals and S0s are classified as barred, but this includes only the most extreme examples since this pattern may be unseen on near edge-on galaxies

although other classification systems were proposed, the Hubble sequence is still the most usedcurrently since it retains the main features (and thus accommodates the great majority of bright galaxies), and it correlates well with some astrophysical properties like bulge/disc ratio, gas content, star-formation strength, spectrum, integrate colors, chemical composition of ISM, etc

classifying galaxies is not an unambiguous task: since it is subjective in some aspects, distinctobservers may classify the same object differently

New proposals for classification systems...

• 1958 – Morgan (Yerkes) system [PASP 70, 364]: based strictly on theprominence of disc and bulge (E, S, B, I, D, N, L, db), on inclination (classes 1-7) and on a spectroscopic type corresponding to the neareststellar equivalent (a, f, g, k)

{only the N (light dominated by an unresolved nucleus) , D/cD [(supergiant)spherical galaxies with extended envelopes] and db (dumbbell) are still used from this system}

• 1960 – Sidney van den Bergh [ApJ 131, 215]: proposed a parallel sequenceof “anemic” spirals (A), putting the tuning-fork as a trident {anemic spirals are only common on rich clusters...}, and a lum.class (I-V) based on the degree of order of the spiralpattern {luminosity correlates with definition of structures!}

• 1982 – Elmegreen & Elmegreen [MNRAS 201, 1021]: proposed 12 arm classes: from chaotic/fragmented (1, called “flocculent”) to well-defined (12, called“grand design”) {classes correlate with luminosity}

• 1996 – Kormendy & Bender [ApJL 464, L119] : disky/boxy shaped ellipticals and bulges...

New proposals for classification systems...

• 1997 – S. van den Bergh [AJ 113, 2054] : proposed another “volume”classification system to include the dwarf galaxies (discovered by Shapley in 1939 [Proc. N. Ac. Sc. USA 25, 565]) and the cDs (discovered by Matthews et al. in 1964 [ApJ 140, 35])

Non-optical bands

classification in photographic plates (usually in blue light) loose certain components of galaxiesthat are not bright in this bandgalaxies at a substantial redshift have their UV light seen in the blue band!

in other bands, other physical properties dominate the galaxy emission

observing in the band B V R I J H K L Myou see the B band at z 0.0 0.24 0.48 0.81 1.74 2.66 3.92 6.75 9.67

X-rays UV Opt near IR far IR radioROSAT satellite UIT (Columbia shuttle) 1.3m tel. (KPNO) 2MASS IRAS (60µm) VLA

only high energy sources: young star clusters (obscured yellow bulge (old stars) old stars of bulge warm dust (normaly SF sites with no dustAGN and close binaries by dust) and weak AGN + blue SF regions (spiral dominate, arms less associated to SF absorption [ionized gas +

arms defined by dust) define (less absorption) regions – hot stars) nonthermal (magn. field)]

• Bill Keel’s homepage:http://www.astr.ua.edu/gifimages/m81series2.html

E1 (M87, VirgoA)

The morphological types

Ellipticals:have an elliptical appearance (look as spheroids or ellipsoids)have no particular structural featureshave no or little gas, but a population of relatively old stars and globular clusterstheir luminosity decreases very regularly from center to edgetheir absolute magnitudes range from among the most luminous galaxies known (MB ∼ -24), to dwarf ellipticals (dE)they include the most massive galaxies known

Lenticulars (or S0):normally they have a ‘lens-like’ shapehave two main components: bulge and disc, but without spiral armshave large bulges, with smooth light distribution likethe elliptical galaxies, and axial ratios b/a < 0.3the disc is very thin and sometimes may contain some dust (obscuring material)

S0 (M102, N5866)

The morphological types

Spirals:have a disc-like appearance, with more or less conspicuous spiral arms of enhanced luminosity, emanating from their central regionscentral bulge, or spheroidal component, resembles ellipticalsbut is smallsometimes present a bar crossing the bulge, from the end of which the spiral arms originate (normally less tightly wound)their discs are very flattened and with less steeply declining brightnesscontain much gas, young stars and active regions of star formation (HII regions)double spiral pattern is common, also a remarkable degree of symmetry with respect to the centre, but many more complicated configuration of spiral structure are known

SBbc (NGC 1300)

Irregulars:have amorphous shapes (with no nucleus, no disc,no spiral arms, and so on)are very rich in interestellar matter and young stars,and are often the location of major outbursts of star formation

I0 (M82)

Cartwheel

The morphological types

Peculiar (and interacting galaxies):have a strange appearance (with tails, jets, ring-like structures, distortions, etc), normally due to gravitational interactions or collision between galaxiesthey are amongst the most luminous extragalactic sources in the far IR due to enhanced SFthere is only a small % of peculiar systems currently, but thisincreases dramatically as we look further and further back in time! they are usually classified as having some Hubble type plus a“p”, indicating the peculiarity

Antennae

NGC 6872 + IC 4970

M87

BCD (N1705)

The morphological types

Dwarfs:are much less luminous (and massive) than “normal” galaxiesare hardly seen at distances far beyond the LGare the most abundant galaxies in the Universe, and “building blocks”frequently they orbit around larger galaxies as satellitesmost have low surface brightnessmay be of several types:

▫ dE – less luminous E (no current SF)▫ dSph – very low surface brightness spheroids (more massive, but

less centrally concentrated than GC), similar to dIrr in struct.▫ dS0 – less luminous lenticulars▫ dS – the last to be discovered [Schombert et al. 1995]▫ dIrr or dIm – less luminous irregulars (show active SF)▫ BCD – blue compact dwarves (with centrally concentrated SF, or HII

galaxies, if spread), the only ones with high surf. brightness

dE (M32)

dSph (Cass. Dwarf)

dS (N5474) dS0 (N4431) dIrr (IC 10)

LSB (UGC 285)

The morphological types

Low Surface Brightness Galaxies (LSB):most are dwarves, but there are many “normal” LSBtheir abundance (frequency) and properties are very badly knownsince they are very hard to detect

LSB (UGC 7332)

LSB (IC 342)

LSB (UGC 7698)

Further readings:

Papers:V. Trimble, 1995. PASP 107, 1133 – 1920 Shapley-Curtis DebateA. Sandage, 1975. In “Galaxies and the Universe”, eds. A. Sandage, M. Sandage &

J. Kristian, Univ. Chicago Press – Morphological classification of galaxies from before Hubble to 1975

A. Naim et al. 1995. MNRAS 274, 1107 – Expert astronomers × machines in classifying galaxies

Books:F. Combes, P. Boisse, A. Mazure & A. Blanchard 1995. “Galaxies and Cosmology”

(A&A Library), Springer-Verlag – chapter 1J. Binney & M. Merrifield 1998. “Galactic Astronomy” (Princeton Series in

Astrophysics), Princeton Univ. Press – chapter 4M.S. Longair 1998. “Galaxy Formation” (A&A Library), Springer-Verlag

– chapter 3M.H. Jones & R.J.A. Lambourne 2003. “An Introduction to Galaxies and

Cosmology”, Cambridge Univ. Press – chapter 2

Internet:http://www.astr.ua.edu/keel/galaxies/ – Bill Keel’s homepagehttp://nedwww.ipac.caltech.edu/level5/basic_data.html