1. Galaxies Star systems like our Milky Way
Star systems like our Milky Way
Contain a few thousand to tens of billions of stars.
Large variety of shapes and sizes

2. Large variety of galaxy morphologies:
Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies
Large variety of galaxy morphologies:
Spirals
Ellipticals
Irregular
(some interacting)
The Hubble Deep Field:
10-day exposure on an apparently empty field in the sky

3. Shapes of Galaxies: Spirals
NGC 1201
NGC 2841
NGC 5236
Type S0
Type Sb
NGC 2811
M 81
Type SBb
Barred Spirals
“Classical” Spirals
Type Sa
Type Sb
NGC 1530
NGC 488
M 74
Type Sab
Type Sc
Type SBc

4. Shapes of Galaxies: Ellipticals/Irregulars
M 87
Small Magellanic Cloud
Type E1
Elliptical Galaxies
Irregular Galaxies
NGC 205
NGC 6822
Type E6

5. What type of galaxy is our Milky Way?
Elliptical.
Spiral.
Barred Spiral.
Irregular.
None of the above.
:45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

6. Galaxy ClassificationSa Sb Sc

7. Gas and Dust in Galaxies
Spirals are rich in gas and dust
Ellipticals are almost devoid of gas and dust
Galaxies with disk and bulge, but no dust are termed S0

8. The Hubble Sequence of Galaxies
“Early Types”
“Late Types”
Almost devoid of gas and dust; little or no star formation
Rich in gas and dust; active star formation

9. Irregular Galaxies Often: result of galaxy collisions / mergers
Often: result of galaxy collisions / mergers
Often: Very active star formation (“Starburst galaxies”)
Some: Small (“Dwarf galaxies”) satellites of larger galaxies
(e.g., Magellanic Clouds)
The Cocoon Galaxy
Large Magellanic Cloud
NGC 4038/4039

10. What is the type of this galaxy?
M 74 E0 S0 Sa Sc
Irregular
:45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

11. What is the type of this galaxy?
M 82 E0 S0 Sa Sc
Irregular
:45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

12. What is the type of this galaxy?
M 87 E0 E4 E7
SBc
Irregular
:45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

13. Distance Measurements to Other Galaxies (I)
Cepheid Method:
Using Period – Luminosity relation for Cepheid variables
b) Type Ia Supernovae
(collapse of an accreting white dwarf in a binary system):
Type Ia Supernova have well known standard luminosity → Compare to apparent magnitude → Find its distance
Both are “Standard-candle” methods:
Know absolute magnitude (luminosity) → compare to apparent magnitude → find distance.

14. Distance Measurements to Other Galaxies (II): The Hubble Law
E. Hubble (1913):
Distant galaxies are moving away from our Milky way, with a recession velocity, vr, proportional to their distance d:
vr = H0*d
H0 ≈ 70 km/s/Mpc is the Hubble Constant.
=> Measure vr through the Doppler effect → Infer the distance.

15. The Extragalactic Distance Scale
Many galaxies are typically millions or billions of parsecs from our Galaxy.
Typical distance units:
Mpc = Megaparsec = 1 million parsec
Gpc = Gigaparsec = 1 billion parsec
Distances of Mpc or even Gpc  The light we see has left the Galaxy millions or billions of years ago!!
 “Look-back times” of millions or billions of years

16. Galaxy Sizes and Luminosities
Vastly different sizes and luminosities:
From small, low-luminosity irregular galaxies to giant Ellipticals and large spirals, a few times the Milky Way’s size and luminosity

17. Rotation Curves of Galaxies
From blue / red shift of spectral lines across the galaxy
→ infer rotational velocity
Plot of rotational velocity vs. distance from the center of the galaxy:
Rotation Curve
Observe frequency of spectral lines across a galaxy.

18. Rotation Curves of Galaxies
From blue / red shift of spectral lines across the galaxy
→ infer rotational velocity
Plot of rotational velocity vs. distance from the center of the galaxy:
Rotation Curve
Observe frequency of spectral lines across a galaxy.
→ Infer the mass of the galaxy!

19. Masses and Other Properties of Galaxies

20. Dark Matter
Adding “visible” mass in
stars,
interstellar gas,
dust,
etc., we find that most of the mass is “invisible”!
The nature of this “dark matter” is not understood at this time.
Some ideas:
Brown dwarfs, small black holes, exotic elementary particles.

21. Clusters of Galaxies
Galaxies do generally not exist isolated, but form larger clusters of galaxies.
Rich clusters:
1,000 or more galaxies, diameter of ~ 3 Mpc, condensed around a large, central galaxy
Poor clusters:
Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center

22. Our Galaxy Cluster: The Local Group
Milky Way
Andromeda galaxy
Small Magellanic Cloud
Large Magellanic Cloud

23. Large Scale Structure Superclusters = clusters of clusters of galaxies
Superclusters = clusters of clusters of galaxies
Superclusters appear aligned along walls and filaments.
Vast regions of space are completely empty:
“Voids”

24. The Furthest Galaxies
The most distant galaxies visible by HST are seen at a time when the Universe was only ~ 1 billion years old.

25. Interacting Galaxies
Cartwheel Galaxy
Particularly in rich clusters, galaxies can collide and interact.
Galaxy collisions can produce
ring galaxies and
tidal tails.
NGC 4038/4039
Often triggering active star formation: Starburst galaxies

26. Ultraluminous Infrared Galaxies
Starburst Galaxies
Starburst galaxies are often very rich in gas and dust; bright in infrared:
Ultraluminous Infrared Galaxies
M 82
Cocoon Galaxy

27. If the hypernova model is correct, then GRBs should be associated with galaxies with very active star formation.