COLLECTIONS OF STARS, GAS (and DARK MATTER): HUGE VARIETY OF TYPES NORMAL GALAXIES COLLECTIONS OF STARS, GAS (and DARK MATTER): HUGE VARIETY OF TYPES

What are the three major types of galaxies?

Hubble Ultra Deep Field

Hubble Ultra Deep Field

Hubble Ultra Deep Field Spiral Galaxy

Hubble Ultra Deep Field Spiral Galaxy

Hubble Ultra Deep Field Elliptical Galaxy Elliptical Galaxy Spiral Galaxy

Hubble Ultra Deep Field Elliptical Galaxy Elliptical Galaxy Spiral Galaxy

Hubble Ultra Deep Field Elliptical Galaxy Elliptical Galaxy Irregular Galaxies Spiral Galaxy

Basic Galactic Facts SIZES 1 kpc -- 100 kpc across 109 M -- 1013 M dwarf galaxies down to 107 M BASIC STRUCTURES SPIRAL -- Milky Way & most overall ELLIPTICAL -- most dwarfs and giants IRREGULARS -- often ``satellites'’ LOCATIONS Isolated (Field) Groups (Local group includes ~50) Clusters (100s to 1000s of galaxies) Superclusters (clusters of clusters!) Voids (galaxies not there)

ELLIPTICAL GALAXIES Ellipticity: E0 -- E7 (round to flatest) Projected on the sky: # = (1 - b/a) x 10 Can be more elliptical than they are seen to be (Projection Effect) Often really tri-axial Some (e.g. Centaurus A) include dust disk -- big elliptical swallowed a spiral!

Elliptical Galaxy Shapes M49 is close to a circle: E2 M84 is an “average” E3 M110 is a dwarf E (Andromeda satellite): E5

SPIRAL GALAXIES Classify by Hubble Type: S0; Sa--Sc; SBa--SBc (tuning fork diagram) S0: disk seen, but no spiral arms Sa: prominent nucleus, tightly wound arms Sb: significant nucleus, moderate arms Sc: small nucleus, patchy loose arms SBa, SBb, SBc: central bar from which arms emerge Milky Way is a SBb (weak bar though, and between SBb and SBc)

“Regular” Spiral Types

M101-pinwheel galaxy 51 HST images

Barred Spiral Types

S0 (lenticular) Galaxies S0 and SB0 have disk and bulge but no visible spiral arms

Irregular Galaxies: Magellanic Clouds

Irregular Galaxies: Interactions and Starbursts

Key Info on Galaxy Types Colors: E's red, S's various, Irr's, blue Populations: E: Pop II Irr: Pop I S: disk, Pops I+II; halo, Pop II Sizes: E's 1-100 kpc, S's: 3-50 kpc; Irr's 1-15 kpc # of Stars: Ellipicals: 107 (dwarfs) to 1013 (cD central dominant) Spirals: 1010 - 1012 Irregulars: 107 - 1011

Gas Content and Star Formation Gas Content: E: up to 30% of ordinary matter (baryonic) mass, but very hot (T > 107 K) S: typically 5-15% of baryonic mass, in many ISM phases (10 K < T < 106 K) Irr: typically 20-50% of baryonic mass, in many ISM phases Star Formation: E: very little, if any, currently (so RED) S: moderate amount in disk (some BLUE with many YELLOW and RED) Irr: often lots currently (so BLUE)

Thought Question Why does ongoing star formation lead to a blue-white appearance? A. There aren’t any red or yellow stars B. Short-lived blue stars outshine others C. Gas in the disk scatters blue light

Thought Question Why does ongoing star formation lead to a blue-white appearance? A. There aren’t any red or yellow stars B. Short-lived blue stars outshine others C. Gas in the disk scatters blue light

What Determines Galactic Shapes? The quasi-spherical shapes of Ellipticals as well as halo and bulge stars in spirals arise from their stars' original RANDOM VELOCITIES. The orbits of stars in spiral galaxies come from stars mainly forming in a flattened disk, supported by ROTATION. The odd shapes of Irregulars are not well understood but many probably arise from tidal distortion by bigger galaxies.

Formation of a Spiral Galaxy

Summary: The Hubble Sequence Like stars on the Main Sequence galaxies are born into a type in the Hubble Sequence and they DO NOT usually move along the Hubble Sequence.

Mergers and Cannabalism The biggest E and S galaxies have almost certainly MERGED WITH comparable sized galaxies, or CANNABALIZED several smaller galaxies over billions of years. Typically S+S  E, S+E  E, E+E  E So as the universe ages the fractions of: S's goes down, E's up. BUT sometimes mergers induce more star formation and spiral disks Cen A: new dust disk from S swallowed by big E

Where are they found? Most Es (except dwarfs) near CENTERS OF CLUSTERS. Most S's: in the FIELD or toward EDGES OF CLUSTERS. Irr's locations are less well known; probably like Spirals. Coma Cluster about 100 Mpc away

DISTRIBUTION OF GALAXIES Most galaxies are in clusters; Most clusters are part of superclusters. Our Local Group has about 50 members. MW + LMC, SMC, Draco, Fornax, Sculptor, Leo etc is one sub-group; Andromeda (M31) + M32, M33, NGC 147 and more is another sub-group Total extent about 1 Mpc (M31 is 700 kpc from MW)

The Local Group

Some Properties of CLUSTERS The nearest CLUSTER is the Virgo Cluster, about 15 Mpc away; Clusters vary in size and richness, from 100 up to over 5000 galaxies. Within clusters, E's and S0's dominate the central parts (90% or so) but S's and SB's dominate the outskirts of clusters. Start here on 4/9

Cluster Merger Movie Many clusters grow through mergers of smaller clusters Some clusters are still growing today Collisions can heat gas in clusters (intracluster medium) to ~108K, giving off X-rays START HERE ON 11/28

THE COSMIC DISTANCE LADDER Review of VARIABLE STARS Giants and supergiants will PULSATE in the INSTABILITY STRIP; above the MS for A and F stars, where variations in He opacity drive increases and decreases in R and T. Some variable stars calibrate distances All RR LYRAE stars are nearly the same luminosity, some 70 times the Sun's. Periods between 2 and 24 hours. CEPHEID VARIABLES have luminosites proportional to their periods from ~200 L (for 1 day) to ~10000 L (for 50 days) Both are "STANDARD CANDLES" that allow DISTANCE DETERMINATIONS to NEARBY CLUSTERS and many, relatively nearby GALAXIES. Start here on 11/27

Next Step: Tully-Fisher Relation There is a very strong correlation between rotational speeds and luminosities for Sc galaxies. Why? Roughly: rotation speed ~ mass ~ luminosity Measure brightness and estimate luminosity, then distance. The 21 cm H I line is broader in the faster rotating galaxies; IR magnitudes give better estimates of total brightness This Tully-Fisher relation is good out to 200 Mpc!

Cosmic Distance Ladder, Illustrated Type 1a SNe take us out beyond 1 Gpc Start here on 11/10/09

White-dwarf supernovae can also be used as standard candles Type Ia SNe as Standard Candle

Apparent brightness of white-dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years)

HUBBLE's LAW Back in 1920's Edwin Hubble found that nearly all galaxies showed REDSHIFTS! Even more interesting, the fainter the galaxy, therefore, probably the more distant the galaxy, the greater the redshift. When distances (r) were calibrated using Cepheid variables, Hubble found: v = H0 r where v = c (/) is the “expansion velocity” and z = / is the redshift. (So v = cz)

Galaxy Spectra and Hubble’s Law Discover Hubble's Law

Using Hubble’s Law If one knows enough galaxy distances from independent measurements and has z's for all of those galaxies then one gets a value for H0 = average of all (v/r) measurements. This yields: 50 < H0 < 100 km/s/Mpc and most likely, H0 = 72 km/s/Mpc (with an error of 3 km/s/Mpc) An example: say a line of 5000 Å is seen at 5500 Å z = / = (5500 Å -5000 Å)/5000 Å = 0.10 So v = cz = 0.10 x 3.00 x 105 km/s = 3.00 x 104 km/s If H0 = 75 km/s/Mpc, then r = v/ H0 = (30,000 km/s)/(75 km/s/Mpc) = 400 Mpc

Distances between faraway galaxies change while light travels Cause of Hubble's Law Distances between faraway galaxies change while light travels Astronomers think in terms of lookback time rather than distance distance?

Copernican Principle, Expanded VERY IMPORTANT POINT: The expansion of the Universe shown by the Hubble Law should be independent of location in the Universe. EVERYONE WOULD SEE AN EQUIVALENT EXPANSION AWAY FROM THEM. In other words, we do not believe we are at a “special” place in the universe.