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

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

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

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

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

Galaxy Classification Sa Sb Sc

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

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

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

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

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

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

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.

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.

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

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

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.

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!

Masses and Other Properties of Galaxies

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.

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

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

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”

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

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

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

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