Chapter 15

A Universe of Galaxies

The Hubble Deep Field10 day exposure –field located in the Big Dipper

Edwin Hubble

Edwin Hubbl

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Hubble’s Galaxy Classification

Spiral Galaxies

• galaxies like the Milky Way with arcing structures lying in a plane and emanating from the nuclear bulge

M81

M51

Lenticular Galaxies

• Galaxies that have disks but no spiral arms.

Barred Spiral Galaxies

• galaxies with a bar of stars running through the nuclear bulge

NGC 1097

NGC 4477 M91

NGC 4123

Elliptical Galaxies

• galaxies with an elliptical shape, no spiral arms, and little interstellar matter

M32 - E2

M87 - E1

NGC 4125

Irregular Galaxies

• galaxies that are asymmetrical and are sometimes just two or more galaxies colliding

Large Magellanic Cloud – a small irregular galaxy that orbits the MMilky Way

M87Centaurus A

Measuring Cosmic Distances

• Distances to other galaxies are measured using Main Sequence Fitting.

• This entails the use of a light source of known, standard luminosity called a standard candle.

• The distance can be found using the luminosity-distance formula

– Apparent brightness = luminosity/4d2

Comparison of the apparent brightness of stars in the Hyades Cluster with those of the Pleiades Cluster

The Pleiades Cluster is 2.75 times farther away because

(2.75)2 = 7.5 times dimmer

The same luminosities are assumed for all main sequence stars of the same color.

RR Lyrae and Cepheid Variable Stars

• These are both pulsating variable stars.

• Their pulsation periods are on the order of a few days.

• Using the period-luminosity relationship, distances to other galaxies can be estimated

Cepheid Period-luminosity Relation.

Cepheids of a particular period have very nearly the same luminosity.

Edwin Hubble

Edwin Hubbl

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http://www.seds.org

Right Ascension00 : 42.7 (h:m)

Declination+41 : 16 (deg:m)

Distance2900 (kly)

Visual Brightness3.4 (mag)

Apparent Dimension178x63 (arc min)

Andromeda Nebula M3, is actually another spiral galaxyAndromeda Nebula M31, is actually another spiral galaxy

• Astronomers discovered that the faster a spiral galaxy rotates, the more luminous it is.

• This relationship is called the Tully-Fisher relation, after its discoverers

Tully-Fisher Relation – using galaxies as standard candles

The Tully-Fisher relation.

Galaxy ObservationsGalaxy Observations

• During the 1920's Edwin Hubble and Milton Humason photographed the spectra of many galaxies with the 100 inch telescope at Mount Wilson.

• They found that most of the spectra contained absorption lines with a large redshift.

Red Shift and Distance

24 Mpc 1200 km/s

300 Mpc 15,000 km/s

780 Mpc 39,000 km/s

1220 Mpc 61,000 km/s

Galaxy Observations

• Using the Doppler effect, Hubble calculated the velocity at which each galaxy is receding from us.

• Using the period and brightness of Cepheid variables in distant galaxies, Hubble estimated the distances to each of the galaxies.

The Tully-Fisher Relation

Hubble’s Law

• Hubble noticed that there was a linear relationship between the recessional velocity and the distance to the galaxies.

• This relationship is know as Hubble’s Law:

V = H Drecessional velocity = Hubble’s Constant Distance

Hubble’s Law

• H is known as the Hubble constant.

• It’s true value appears to be somewhere between 55 to 75 km/s/Mpc.

• This means that a galaxy that is 1 megaparsec from Earth will be moving away from us at a speed somewhere between 55 to 75 km/s.

The Distance Chain or Ladder

Measuring Cosmic Age

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Raisin Cake ModelRaisin Cake Model

Like raisins in rising raisin cake, galaxies Like raisins in rising raisin cake, galaxies move away from each other in our move away from each other in our

expanding universe.expanding universe.

Measuring Cosmic Age

Typical Cube of GalaxiesTypical Cube of Galaxies

Homogeneous, isotropic universe?Homogeneous, isotropic universe?

NO!

The Birth of The Universe-“The Big Bang”

A very rough estimate for the age of the universe

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All of space and time were created in the Big Bang, which then expands. Analogous to the surface of a balloon.

Cosmological Red ShiftCosmological Red Shift

As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift.

Elliptical, Spiral and irregular galaxies at different ages.

Modeling Galaxy Birth

• The most successful models are based on the following assumptions:

• Hydrogen and helium gas filled all of space fairly uniformly early in the universe.

• The near uniformity had small perturbations which allowed for dense regions to exist.

Galactic CollisionsNGC 4038/4039 are a pair of colliding spiral galaxies

Hubble Space Telescope Photos Of Distorted Young Galaxies.

The larger number of distorted galaxies in the past suggests that collisions between galaxies were common during the first few billion years.

Star- Burst galaxies

While the Milky Way forms a new star about once per year, starburst galaxies can form over 100 new stars per year

Quasars and Active Galactic Nuclei

• Active galaxies are galaxies which are much more luminous than normal galaxies and have spectra that are nonstellar in nature.

•This indicates that the energy they emit is not simply the accumulated light of many stars.

•Most of the energy from active galaxies is in the radio and infrared portions of the spectrum.

Planck curves for Active and Normal Planck curves for Active and Normal GalaxiesGalaxies

Seyfert GalaxiesSeyfert Galaxies• Look like normal spiral galaxies except

with extremely bright central galactic nucleus.

•The luminosity of the nucleus can exceed that of the rest of the galaxy.

•Spectral lines are very broad, indicating rapid rotation.

•Luminosities can vary by large amounts in fractions of a year.

Active galactic nucleus in the elliptical galaxy M 87.

Jet of particles shooting outward from the nucleus at nearly the speed of light

Radio GalaxiesRadio Galaxies

• Active galaxies that emit most of their energy in the radio part of the spectrum.

•Comparable to Seyferts in total energy output.

•Usually associated with elliptical galaxies.

Two Types Of Radio GalaxyTwo Types Of Radio Galaxy

• Core- Halo Radio Galaxy: Energy is emitted from a small central nucleus, as with a Sayfert Galaxy.

•Lobe Radio Galaxy: Energy is emitted from enormous radio lobes. These lobes usually lie far beyond the galactic nucleus and are usually much larger than the visible part of the galaxy.

Radio image of the radio galaxy Cygnus A taken with the VLA.

~ 400,000 light-years

Active Galaxies show some or all of Active Galaxies show some or all of the following properties.the following properties.

• High Luminosities.

• Energy emission is nonstellar.

• Energy output can be highly variable.

• Often exhibit “jets” and other signs of explosive activity.

• Spectra show broad emission lines - indicate rapid internal motions.

Central Engine of Active GalaxyCentral Engine of Active Galaxy

NGC 1461 in Virgo ClusterNGC 1461 in Virgo Cluster

Energy EmissionEnergy Emission

• Although the rotating supermassive black hole model is now widely accepted, the actual mechanism for the energy production is uncertain.

• One popular model which explains some One popular model which explains some features is the synchrotron radiation model.features is the synchrotron radiation model.

Synchrotron RadiationSynchrotron Radiation

• A type of nonthermal radiation produced by high-speed charged particles, such as electrons, as they are accelerated in a strong magnetic field.

Synchrotron RadiationSynchrotron Radiation

Quasi-stellar Objects Quasi-stellar Objects (QUASARS)(QUASARS)

• Circa 1960, astronomers observe what appear to be faint blue stars identified with radio sources.

• These objects had odd spectral lines which appeared broadened and extremely redshifted.

• Quasars are :

• believed to be some of the oldest objects in the universe.

• some of the most distant objects from us.

• the most luminous objects known.

Radio Jet in the Quasar 3C 345, shows a blob of plasma moving away from the core at nearly the speed of light

Active Galaxy FormationActive Galaxy Formation

• Possible evolutionary track for galaxies may be as follows:–Quasars ---> –Radio/Sayfert Galaxies ---> –Normal spiral and elliptical galaxies.

• Black holes are always present, but reduce over time as they run out of fuel.

Artist’s conception of an accretion disk surrounding a super-massive black hole.

Doppler Shift of the emission lines in the nucleus of the elliptical galaxy M 87 indicates a 2-3 billion solar mass black hole

End of Section