Astronomy 404/CSI 769 Extragalactic Astronomy

S. SatyapalASTR 404/CSI 769 1/27/05 Course Details ASTR 404, Galactic Astrophysics (3) Lecture Day/Time: Thursday 4:30-7:10 PM Lecture Place: Room 310, S&T I Prerequisites ASTR 328, Math 214, PHYS 308

S. SatyapalASTR 404/CSI 769 1/27/05 What are Galaxies? Just 80 years ago, we did not even know there were galaxies in our Universe …. Most distant objects we see in the Universe (~10,000 galaxies). 16 billion times fainter than what the human eye can see.

S. SatyapalASTR 404/CSI 769 1/27/05 The Menagerie of galaxies

S. SatyapalASTR 404/CSI 769 1/27/05 Tentative Lecture Schedule DateLecture Topics 27-Jan-05 Introduction - the Universe of galaxies and their discovery Astronomy terms, units, and Intro to Milky Way 3-Feb-05The Milky Way Galaxy/Gravitational Lensing and Dark Matter 10-Feb-05Galaxy Classifications - Hubble Sequence 17-Feb-05Properties along the Hubble Sequence 24-Feb-05Intro to The Interstellar Medium - Gas and Dust in Galaxies 3-Mar-05Review of Atomic Structure/Gaseous Nebulae continued 10-Mar-05Intro to AGN 24-Mar-05AGN Physics 31-Mar-05Galaxy Clusters 7-Apr-05Galaxy Evolution and Formation 14-Apr-05Observational Cosmology 21-Apr-05Observational Cosmology 28-Apr-05Project 5-May-05Presentations 12-May-05Final Exam

S. SatyapalASTR 404/CSI 769 1/27/05 Today’s Lecture Review of a few astronomical units of measurement Brief review of stars History of the Milky Way Star counts Morphology and major components of our Galaxy

S. SatyapalASTR 404/CSI 769 1/27/05 Brief History Late 1700s - Herschels counted stars in 683 regions of sky, assumed all are equally luminous. Concluded that Sun at center of a flattened system. What is the Shape of the Milky Way?

S. SatyapalASTR 404/CSI 769 1/27/ Kapteyn used a greater number of star counts and came to roughly the same conclusion Star Counts: If stars are distributed uniformly in space, then in any patch of sky, the total number of stars with flux less than a limiting flux, f is: Log(N(m’<m)) = C + 0.6m; where C = constant (Note: These formulae were derived on the board in class)

S. SatyapalASTR 404/CSI 769 1/27/05 logN m Actual star counts are less than would be predicted by this relationship at fainter flux levels (or larger magnitudes)

S. SatyapalASTR 404/CSI 769 1/27/05 Conclusion: stellar density not uniform but decreases with distance from Sun; faster in direction perpendicular to Milky Way and slower in the direction of the Milky Way Milky Way is a highly flattened disk

S. SatyapalASTR 404/CSI 769 1/27/ Shapley studied globular clusters; used distance derived from pulsating stars to determine that Sun is not at center of Milky Way. These were found at great distances above and below the plane of the Galaxy, where extinction effects are much less than that found along the Milky Way

S. SatyapalASTR 404/CSI 769 1/27/05 Definitely bound by gravity Contain large numbers of stars in a very small volume: 20,000-1,000,000 stars in a volume 20 pc in diameter very round and symmetrical in shape - very old -- among the first stellar complexes formed in the galaxy Globular clusters

S. SatyapalASTR 404/CSI 769 1/27/05 Distances from Variable Stars

S. SatyapalASTR 404/CSI 769 1/27/05 Morphology of Galaxy

S. SatyapalASTR 404/CSI 769 1/27/05

S. SatyapalASTR 404/CSI 769 1/27/05 Disk Young thin disk Old thin disk Thick disk

S. SatyapalASTR 404/CSI 769 1/27/05 Thin disk Diameter ~ 50 kpc Young thin disk scale height = 50 pc Old thin disk scale height = 325 pc Contains youngest stars, dust, and gas Contains Sun, which is 30 pc above midplane M * = 6  M sun M dust+gas = 0.5  10 6 M sun (scale height 0.16) Average stellar mass ~ 0.7 M sun L B ~ 1.8  L sun Population I stars in the Galactic plane Contains ~ 95% of the disk stars [Fe/H] ~ Age ~ < 12 Gyr Spiral structure seen in neutral H, HII regions, young O and B stars

S. SatyapalASTR 404/CSI 769 1/27/05 Thick disk Diameter ~ 50 kpc Scale height = 1.4 kpc M * = 2-4  10 9 M sun L B ~ 2  10 8 L sun [Fe/H] ~ (less metal rich than thin disk) Age ~14-17 Gyrs

S. SatyapalASTR 404/CSI 769 1/27/05 Gas and Dust

S. SatyapalASTR 404/CSI 769 1/27/05 Spheroidal Components Central bulge Stellar Halo Dark Matter Halo

S. SatyapalASTR 404/CSI 769 1/27/05 Central Bulge Diameter ~ 2 kpc Scale height = 0.4 kpc M * = 1  M sun L B ~ 0.3  L sun [Fe/H] ~ (less metal rich than thick and thin disk) Age ~10-17 Gyrs

S. SatyapalASTR 404/CSI 769 1/27/05 Stellar Halo Diameter ~ 100 kpc Scale height = 3 kpc number density distribution ~  r -3.5 M * = 0.1  M sun L B ~ 0.1  L sun [Fe/H] ~ (metal poor) Age ~14-17 Gyrs

S. SatyapalASTR 404/CSI 769 1/27/05 Dark Matter Halo? Rotation of Galaxy implies that there is a lot of mass in our Galaxy that we don’t see (ie, if we count up the mass from the stars that emit visible light, it’s much less than that implied by observing the motion of stars as a function of radius from the center of the Galaxy. How do we know that the stars in the disk rotate around the center of the Galaxy? How do we know the rotational velocity of the Sun? How do we know the rotation curve? (rotational velocity as a function of radius from the Galactic center?)