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Galaxies

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Galaxies

Relativity and Astrophysics

Lecture 16

Terry Herter

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Outline

Discovering Galaxies

Classifying Galaxies Hubble (morphological) classification

Different types of galaxies

Note: Most galaxy photos from Astronomy Picture of the

Day (APOD) on the web – you can find them (and credits for images) by searching the name of the object.

Galaxies

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Galaxies

A galaxy is a collection of stars, gas and dust along w/ associated starlight, magnetic fields and cosmic rays.

Four broad categories based on morphology (appearance): E elliptical S spiral (normal & barred) S0 lenticular I irregular

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Ellipticals Galaxies

Range from spherical to highly flattened with designations E0 to E7

Contain old stars (Pop II) Very little gas and dust 1-200 kpc in diameter Mostly found in clusters of galaxies Average spectral type: K 106 to 1013 Msun

NGC 4636 (E0/S01)

NGC 3377 (E5-6) NGC 3115 (E7/S01)NGC 4406 (E3)NGC 4278 (E1)

Galaxies

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M87

Giant Elliptical Galaxy

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Spiral Galaxies

Flattened systems which have a thin disk Display spiral structure Divided into barred (SB) and

unbarred (S) spirals Further subdivided into classes

a, b, and c; e.g. SBb, Sc, ... where a large nuclear bulge & tightly

wound spiral arms c small nuclear bulge & loosely

wound spiral arms Young (Pop I) and old (Pop II) stars Copious amounts of gas and dust 5-50 kpc in diameter Found mostly in the “field” (outside clusters of galaxies) Average spectral type: A, F, G, K 109 to 1011 Msun

M33 - Sc

Galaxies

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Sc: Whirlpool (M51) SABc: Southern Pinwheel (M83)

Spiral Galaxy Images

SBc: M109Sa: M65

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Lenticulars (S0 Galaxies)

Like spiral galaxies in shape and color but no spiral arms

Flattened systems which are morphologically between ellipticals and spirals.

M85

NGC 5866

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Irregulars

By definition, irregular in shape Mostly young stars (Pop I)

Lots of gas and dust

1-10 kpc in diameter

Found in the field (outside clusters)

Average spectral type: A, F 108 to 1010 Msun

M82

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Hubble Tuning Fork – For classifying galaxies

Ellipticals

E0 E7 S0

Normal Spirals

Barred Spirals

Sa

SBa

Sb

SBb

Sc

SBc

The classification scheme is strictly morphological and does not necessarily imply an evolutionary sequence.

Galaxies

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Thumbnail Version of Hubble Tuning Fork

Source:M65, M77, M84, M101: IPAC Multiwavelength gallery; M49, M89, M91, M109: NOAO Image Gallery;M95: Nial R. Tanvir (through SEDS)

M101, ScM77, SbM65, Sa

M49, E4 M84, S0 (E1?)M89, E0

M109, SBcM91, SBbM95, SBa/b


Other Types of Galaxies

Dwarfs 106 to 108 stars

Peculiar Exploding, Rings, Disrupted

Seyfert Very Bright Nucleus

Interacting Tidal Effects, Tails (pairs)

QSO Collapsed Nuclei?

Galaxies

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Dwarf Galaxies

Leo I

NGC 1313

Interacting Galaxies

Cartwheel Galaxy

The Antennae (NGC 4038 and NGC 4039)

Galaxies

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Interacting Galaxies

Arp 104 – NGC 5216 (top right) and NGC 5218

Arp 273


The Local Group

100 kpcLMC

SMC Milky Way

M33M31

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SMC(Dwarf)

50kpc

Nearby galaxies – SMC

47 TucGlobularCluster

LMC(Dwarf)

50kpc

Nearby galaxies – LMC

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M31(Sb)

700kpc

Nearby galaxies – M31

Great galaxy in Andromeda

NGC205 (dE5) = M110

M32 (cE2)=NGC 221


Notes on Classifying Galaxies

Classifying of galaxies by their morphology is a bit problematic Appearance can depend on exposure time, wavelength, and

angular resolution. Thus can change with redshift – even if no intrinsic evolution

Classification Requirements Homogeneous data set Unique classification Flexible and unambiguous notation

Unfortunately these requirements are often violated Can get ambiguities and inconsistencies

Some examples of how galaxy morphology can change with exposure time and wavelength are given in the next slide.

Galaxies

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Cen A: Two Different Scales

The image of the peculiar galaxy Cen A (NGC 5128) but with different image display scales. How would YOU classify this galaxy? The same effect could be achieved with different exposure times. [Image is IIIaJ, 4680A from the Digital Sky Survey (DSS) via NASA Extragalactic Database (NED).]

Cen A: Two Different Wavelengths

The image of Cen A (NGC 5128) but with a two different wavelengths. Both images are from APOD (Astronomy Picture of the Day)

Left image: antwrp.gsfc.nasa.gov/apod/ap030806.html Right image: antwrp.gsfc.nasa.gov/apod/ap040624.html

Galaxies

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Centaurus A (Peculiar Galaxy)

Nearest Radio Galaxy

Centaurus A (Peculiar Galaxy)

In the Infrared

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HI + cont.

8.0 um3.6 um

B Color visual

M81 at different wavelengths

0.23 um (yellow), 0.16 um (blue)

24 um

70 um 160 um

M81: H-alpha emission

V image from NED H-alpha in blue, V in red + green

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M81: Blue light

Color enhanced image using B & V bands. Color mix is V=red, B=green, B/V=blue.

Ratio of B to V images


Galaxy Classification Systems

Hubble System (of Classification) Introduced in by Edwin Hubble in 1936 book, The Realm of

the Nebulae

Definitive expose by Alan Sandage in 1961, The Hubble Atlas of Galaxies

A number of other classification systems exist De Vaucouleurs’ Extension of the Hubble System, 1959

Yerkes System (W. W. Morgon 1958, 1959, 1970)

David Dunlap Observatory (DDO) System (S. van den Bergh 1960, 1976)

Note – these system are based on visual appearance

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