Galaxies - Cornell Astronomy

Galaxies
Galaxies
Relativity and Astrophysics
Lecture 16
Terry Herter
Outline
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Discovering Galaxies
Classifying Galaxies
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Note:
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Hubble (morphological) classification
Different types of galaxies
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
Galaxies
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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):
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E
S
S0
I
elliptical
spiral (normal & barred)
lenticular
irregular
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Galaxies
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Ellipticals Galaxies
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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 4278 (E1)
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NGC 4406 (E3)
NGC 3377 (E5-6)
Galaxies
NGC 3115 (E7/S01)
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Galaxies
M87
Giant
Elliptical
Galaxy
Spiral Galaxies
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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
M33 - Sc
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
Galaxies
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Galaxies
Spiral Galaxy Images
Sc: Whirlpool (M51)
SABc: Southern Pinwheel (M83)
Sa: M65
SBc: M109
Lenticulars (S0 Galaxies)
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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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Galaxies
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Galaxies
Irregulars
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By definition, irregular in shape
Mostly young stars (Pop I)
Lots of gas and dust
1-10 kpc in diameter
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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
Normal Spirals
Ellipticals
Sa
E0
E7
S0
SBa
Sb
Sc
SBb
SBc
Barred Spirals
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The classification scheme is strictly morphological and does not
necessarily imply an evolutionary sequence.
Galaxies
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Galaxies
Thumbnail Version of Hubble Tuning Fork
M65, Sa
M89, E0
M49, E4
M77, Sb
M101, Sc
M84, S0 (E1?)
Source:
M65, M77, M84, M101: IPAC Multiwavelength gallery;
M49, M89, M91, M109: NOAO Image Gallery;
M95: Nial R. Tanvir (through SEDS)
M95, SBa/b
M91, SBb
M109, SBc
Other Types of Galaxies
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Dwarfs
Peculiar
Seyfert
Interacting
QSO
106 to 108 stars
Exploding, Rings, Disrupted
Very Bright Nucleus
Tidal Effects, Tails (pairs)
Collapsed Nuclei?
Galaxies
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Galaxies
Dwarf Galaxies
Leo I
NGC 1313
Interacting
Galaxies
Cartwheel Galaxy
The Antennae (NGC
4038 and NGC 4039)
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Galaxies
Interacting
Galaxies
Arp 104 – NGC 5216 (top
right) and NGC 5218
Arp 273
The Local Group
M33
M31
LMC
SMC
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Milky Way
Galaxies
100 kpc
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Galaxies
Nearby galaxies – SMC
47 Tuc
Globular
Cluster
SMC
(Dwarf)
50kpc
Nearby galaxies – LMC
LMC
(Dwarf)
50kpc
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Galaxies
Nearby galaxies – M31
M32 (cE2)
=NGC 221
M31
(Sb)
700kpc
NGC205 (dE5)
= M110
Great galaxy in Andromeda
Notes on Classifying Galaxies
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Classifying of galaxies by their morphology is a bit
problematic
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Classification Requirements
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Homogeneous data set
Unique classification
Flexible and unambiguous notation
Unfortunately these requirements are often violated
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Appearance can depend on exposure time, wavelength, and
angular resolution.
Thus can change with redshift – even if no intrinsic evolution
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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Galaxies
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).]
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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
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Galaxies
Centaurus A (Peculiar Galaxy)
Nearest Radio Galaxy
Centaurus A (Peculiar Galaxy)
In the Infrared
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Galaxies
M81 at different wavelengths
0.23 um (yellow), 0.16 um (blue)
3.6 um
70 um
B
Color visual
8.0 um
24 um
160 um
HI + cont.
M81: H-alpha emission
V image from NED
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H-alpha in blue, V in red + green
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Galaxies
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
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Hubble System (of Classification)
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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
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A number of other classification systems exist
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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
Galaxies
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