Download Elliptical Galaxies

Admin. 11/8/16
1. Class website http://www.astro.ufl.edu/~jt/teaching/
ast1002/
2. Optional Discussion sections: Tue. ~11.30am in
Bryant 3; Thur. ~12.35pm, start in Pugh 170, then
Bryant 3
3. Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm,
Bryant 302 (but email me if coming on Wed.).
4. Homework 10 due Mon. 14th Nov. by 11.59pm.
5. Reading this week: Ch. 0-3, 4.1-4.3, 5-14, 15, 16
6. Email me Astro-news, jokes, tunes, images:
[email protected]
7. Printed class notes? Name tags?
Key Concepts: Lecture 31: Galaxies
Galaxy Types: Spirals, Ellipticals, Irregulars, Dwarfs
Spiral Density Waves
Mergers of Galaxies and Galaxy Evolution
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
Active Galaxies - more evidence for supermassive black holes
E7
What are Spiral Arms?
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
• 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?
Sc
Answer: the spiral arms are “density waves”
NGC 4565 - Edge on Sb
M83 classed as SBb
A familiar example of a Density Wave
bar
Sombrero Galaxy Sa
M84 S0
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 Galaxies
Galaxies are relatively big compared to the space between them,
and so can sometimes undergo interactions…
Galaxy collision
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
Milky Way and Andromeda Collision
will occur in a few billion years
The Antennae Galaxies
Antennae with HST
• Star clusters in formation
• Bands of dust and gas
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”.
The Cart Wheel Galaxy
• A Splash
encounter
• One galaxy
passes
through the
other
• Causes a
wave to
travel out
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
Active Galaxies
Quasars were the first type found in the 1960s
• 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
NGC 4151
– Variable luminosity: changes over several years
– Strong & broad emission line spectra
– Radio emission and “jets”
– X-rays, gamma rays, UV emission
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
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
• More missing mass!:
Further evidence for Dark Matter
Cen A Optical
Cen A Radio
Cygnus A - The first Radio Galaxy Identified
The Black Hole paradigm to explain AGN
Radio Image
Optical Galaxy
Active Galactic Nuclei
Radio Image
M87
HST Image of Disk
and Jet
The Galaxy
• 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
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.