The Classification of Galaxies

Transcription

1 Admin. 11/9/17 1. Class website 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 Bryant]. 3. Office hr: Tuesday pm; Wed pm, Bryant 302 (but 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, , 5-14, Midterm 2: results discussed in class. 7. Observing project deadline was Thur. Nov. 2nd Final exam - Tue. 5th Dec., in class. 9. me Astro-news, jokes, tunes, images: ast1002_tan-l@lists.ufl.edu 10. Printed class notes? Name tags? 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 Distance Ladder (see Ch. 14 in text book) Key Concepts: Lecture 31: Galaxies The Distance Ladder Galaxy Types: Spirals, Ellipticals, Irregulars, Dwarfs 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 Spiral Density Waves Mergers of Galaxies and Galaxy Evolution Active Galaxies - more evidence for supermassive black holes

2 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 NGC Edge on Sb M83 classed as SBb Elliptical Galaxies bar M87 - E0 Sombrero Galaxy Sa M84 S0 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

3 A familiar example of a Density Wave 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 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 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

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

5 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 Collision will occur in a few billion years The Antennae Galaxies Antennae with HST Star clusters in formation Bands of dust and gas

6 The Cart Wheel Galaxy A Splash encounter One galaxy passes through the other Causes a wave to travel out When Galaxies Collide video 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 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 M sun for large galaxies

7 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 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 Optical Cen A Radio 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 Variable luminosity: changes over several years Strong & broad emission line spectra Radio emission and jets X-rays, gamma rays, UV emission NGC 4151 Cygnus A - The first Radio Galaxy Identified Radio Image Optical Galaxy

8 Active Galactic Nuclei Radio Image M87 HST Image of Disk and Jet The Galaxy The Black Hole paradigm to explain AGN Supermassive hole = 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.