Admin. 11/9/171. Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/2. Optional Discussion sections: Tue. ~11.30am (period 5), Bryant 3; Thur.

~12.30pm (end of period 5 and period 6), start in Pugh 170, then Bryant 3 [if just a small group we move to my office - 302 Bryant].

3. Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm, Bryant 302 (but email me if coming on Wed.).

4. Homework 9: is due Fri. Nov. 10th 11.59pm via Canvas e-learning under “Quizzes”

5. Reading this week: Ch. 0-3, 4.1-4.3, 5-14, 156. Midterm 2: results discussed in class.7. Observing project deadline was Thur. Nov. 2nd 20178. Final exam - Tue. 5th Dec., in class.9. Email me Astro-news, jokes, tunes, images: ast1002_tan-l@lists.ufl.edu10. Printed class notes? Name tags?

Key Concepts: Lecture 31: Galaxies

The Distance Ladder

Galaxy Types: Spirals, Ellipticals, Irregulars, Dwarfs

Spiral Density Waves

Mergers of Galaxies and Galaxy Evolution

Active Galaxies - more evidence for supermassive black holes

The Distance Ladder(see Ch. 14 in text book)

Different methods for measuring distance work only over certain ranges.

They also have different accuracies (radar & stellar parallax methods being more accurate).

Calibration of the larger distance methods requires overlap of application of the techniques to the same astronomical object.

Methods shown here allow us to measure distance to many nearby galaxies.

The Classification of Galaxies• Galaxies can be classified by how they appear on the sky

– How flattened the spheroid is– How prominent the disk and spiral arms are– If there is a bar

• Hubble devised what he thought may be an evolutionary sequence

Elliptical Galaxies• Only a smooth spheroidal

component• Hubble class subdivides them

•E0 - circular•E7 - most elongated

• No prominent disk• Composed of old reddish stars• Little dust, gas or ongoing star

formation

M87 - E0

E7

Spiral Galaxies• Have disk with two or more arms

– Bulge is old and red– Disk has gas and star formation

• Hubble sequence (Sa, Sb, Sc)– size of nuclear bulge vs. disk– tightness of spiral arms

• Sa - tightest pattern & large bulge• Sc - open pattern & smallest bulge

• S0 or lenticular– Have disk but no arms

Sb

Sc

M83 classed as SBbbar

NGC 4565 - Edge on Sb

Sombrero Galaxy Sa M84 S0

What are Spiral Arms?• Spiral arms are regions with a higher

density of gas, dust & stars• The rotation speed in these galaxies is

approximately constant with radius. • So, why do the arms not get more

tightly wound up?

Answer: the spiral arms are “density waves”

A familiar example of a Density Wave

Spiral Arms• Spiral arms are density waves

–As the gas and stars orbit the galaxy, they change their speed as they approach and leave the wave, so they spend more time in the arm, becoming bunched up.

–This is similar to what happens to cars in a traffic jam

• Because the gas densities are higher in spiral arms, they tend to be traced by star formation regions

The Spiral of the Milky Way• Hydrogen atoms emit radio

waves, with a wavelength of about 21cm–Due to change in

alignment of proton & electron

–Radio waves pass through dust unaffected

• Can be used to map the spiral arms of our galaxy–Use Doppler shift and

rotation of Galaxy to determine the distance

Irregular Galaxies

• No spiral structure or nuclear bulge

• Dominated by OB Stars & regions of ionized gas (created by the hot OB stars)

Large Magellanic Cloud

Dwarf Galaxies• The smallest galaxies are

dwarf ellipticals• No current star formation• About the same number of

stars in a globular cluster• Tend to be found near larger

galaxies• The most common type of

galaxy

Leo I

Mergers of GalaxiesGalaxies are relatively big compared to the space between them,

and so can sometimes undergo interactions…

Typical size ~100,000 ly, typical separation ~1,000,000 ly

Interacting Galaxies• Galaxies tend to form in groups • Over time dynamical friction causes them to merge• Interactions occur primarily though gravity

– In addition to mergers, Tidal Forces can also tear bits of the galaxies apart

– No stars actually collide• Major effects

– Causes most strange looking galaxies– Disks are destroyed - produce Elliptical type galaxies– “Starbursts” can be stimulated– Can produce tails and shells of stars

Galaxy collision

Milky Way and Andromeda Collisionwill occur in a few billion years

The Antennae Galaxies Antennae with HST

• Star clusters in formation

• Bands of dust and gas

The Cart Wheel Galaxy• A Splash

encounter• One galaxy

passes through the other

• Causes a wave to travel out

Galaxy Evolution

• Interactions are one major driver of the evolution of galaxies.

• The merger of two gas-rich spiral galaxies can result in an elliptical galaxy with relatively little gas. The gas was turned into stars during the merger in a “Starburst”.

When Galaxies Collide video

The Masses of Galaxies

• The stars & gas in galaxies are supported against gravity by their orbits

• Use Doppler shift to measure orbital velocities

• Use Newton’s adaptation of Kepler’s third law to measure the masses of galaxies

• Typical mass 1010-1012 Msun

for large galaxies

The Masses of Galaxies

• Spiral disks tend to have flat or rising rotation curves–Thus, as in the Milky Way, mass continues to increase as you

move outward –The total amount of mass is about 10x greater than that

expected from the stars & gas • More missing mass!: Further evidence for Dark Matter

Active Galaxies• Normal Galaxies

–Gas, dust & stars–Star formation

• Active Galaxies–Powerful compact energy source in

nucleus: AGN (Active Galactic Nucleus) can outshine entire galaxy

–Not due to normal stars• Manifestations

–Variable luminosity: changes over several years

–Strong & broad emission line spectra–Radio emission and “jets”–X-rays, gamma rays, UV emission

NGC 4151

Quasars were the first type found in the 1960s

Radio Galaxies (a certain kind of active galaxy)

• Radio telescopes found about 0.01% of galaxies had very bright radio emission

• Radio jets of charged particles originate in nucleus of galaxy• Radio lobes can be up to 1-10 Mpc across• The galaxy is usually an elliptical and often interacting or disturbed

Cen A RadioCen A Optical

Cygnus A - The first Radio Galaxy Identified

Radio Image

Optical Galaxy

Radio Image

The Galaxy

HST Image of Disk

and Jet

M87

The Black Hole paradigm to explain AGN

• Supermassive hole = 106 - 109 Msun

• Release gravitational energy as matter falls in

• Rotating matter organizes into a disk• Hot inner parts of disk emit brightly

in x-ray-optical• Rotating BH acts like particle

accelerator to produce radio jets

Active Galactic Nuclei

Evidence for Black Holes• Rapid variability requires small size• Very efficient release of energy

– 10% of mass energy (E=mc2) of material falling into black hole

• Dynamics (motions of stars and gas in the centers of these galaxies) indicate large nonstellar mass.