Galaxies Please press “1” to test your transmitter.

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

The Hubble Deep Field: 10-day exposure on an apparently empty field in the sky 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)

Shapes of Galaxies: Spirals NGC 5236 NGC 1530 NGC 1201NGC 2841 NGC 2811 M 81 NGC 488M 74 “Classical” Spirals Barred Spirals Type SBb Type SBc Type Sa Type Sb Type SabType Sc Type S0Type Sb

Shapes of Galaxies: Ellipticals/Irregulars Small Magellanic CloudM 87 NGC 205NGC 6822 Type E1 Type E6 Elliptical GalaxiesIrregular Galaxies

What type of galaxy is our Milky Way? 1.Elliptical. 2.Spiral. 3.Barred Spiral. 4.Irregular. 5.None of the above.

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

In which type of galaxy do you expect that stars are being formed at a higher rate? 1.Elliptical. 2.Spiral. 3.There should be no difference.

Remember: Spiral arms are density waves that trigger self- sustaining star formation!

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

Irregular Galaxies 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) Large Magellanic Cloud NGC 4038/4039 The Cocoon Galaxy

What is the type of this galaxy? 1.E0 2.S0 3.Sa 4.Sc 5.Irregular IC 342

What is the type of this galaxy? 1.E0 2.S0 3.Sa 4.Sc 5.Irregular M 82

What is the type of this galaxy? 1.E0 2.E4 3.E7 4.SBc 5.Irregular M 89

How could astronomers, for the first time, measure the distance to other galaxies? 1.Using the trigonometric parallax. 2.Using light travel time arguments. 3.Using Cepheid Variables. 4.Using cosmological redshift. 5.Measuring the time required for extragalactic space travel.

Distances to Other Galaxies a)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.

The Doppler effect provides a relation between the … and the … of a light source. 1.brightness; distance 2.temperature; wavelength of maximum energy output 3.radial velocity; distance 4.radial velocity; frequency shift 5.distance; frequency shift

Expansion Velocity: The Doppler Effect Blueshift (shorter wavelength) Redshift (shorter wavelength)

Distance Measurements to Other Galaxies: The Hubble Law E. Hubble (1913): Distant galaxies are moving away from our Milky way, with a recession velocity, v r, proportional to their distance d: v r = H 0 *d H 0 ≈ 70 km/s/Mpc is the Hubble Constant. => Measure v r through the Doppler effect → Infer the distance.

The Universe is 14 billion years old. How far away is, theoretically, the most distant galaxy that you could possibly observe? 1.14 billion parsec billion light years. 3.7 billion parsec. 4.7 billion light years. 5.There is no theoretical limit; it just depends on the size of your telescope.

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 Observe frequency of spectral lines across a galaxy. 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

What can we infer from the rotation curve of a galaxy? 1.Its distance. 2.Its mass. 3.Its luminosity. 4.Its morphhological type. 5.Its radius.

Rotation Curves of Galaxies Observe frequency of spectral lines across a galaxy. 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 → Infer the mass of the galaxy!

Masses and Other Properties of Galaxies

What makes up most of the mass in the Milky Way? 1.The central supermassive black hole. 2.Globular clusters. 3.Population I stars in the disk and spiral arms. 4.Neutral hydrogen gas, invisible in the optical, but visible in the 21 cm radio line. 5.Dark matter.

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. 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

Is the Local Group a rich or a poor cluster? 1.A poor cluster. 2.An intermediate rich/poor cluster. 3.A rich cluster. 4.Impossible to say without a bank statement.

Large Scale Structure 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 Especially in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies and tidal tails. Often triggering active star formation: Starburst galaxies NGC 4038/4039

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

What type of object / phenomenon should be associated with starburst galaxies (if the currently favored model is correct)? 1.White dwarfs. 2.Supermassive black holes. 3.Nova explosions. 4.Gamma-ray bursts. 5.Type Ia supernovae.

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