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Home , Spiral galaxy

210 Outline

• The Milkyway – Globular clusters, Galactic nucleus, Nuclear bulge, This Class (Lecture 37): Spiral Arms and the Disk The • We rotate around the center of the Galaxy. Stardial 2 due Friday • There is something fishy about the MW rotation curve. Next Class: • Other , other rooms Galaxies, Structure of the • Recycling centers of the Universe • Is everyone’s rotation curves full of halibut?

Music: Amy Hit the Atmosphere – Counting Crows Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

The Milky Way Astronomy: The Big Picture Moving from the birth/death of to a better understanding of the Galaxy!

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 http://home.arcor-online.de/axel.mellinger/mwpan_aitoff.html What is it? Our Place • Shapley showed that we are not the center of the Galaxy in the 1920s. – 2nd Copernican revolution! • All of the globular clusters are orbiting around a point in Sagittarius– 26000 lyrs or 8000 away. • That must be the center of our Galaxy. Globular clusters

Sun 8 kpc

In the Galaxy

Astronomy 210 Spring 2005http://antwrp.gsfc.nasa.gov/apod/image/0001/milkyway_cobe_big.jpg Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

The Structure of Our Galaxy The Disk

• Disk – All kinds of stars, many (30 kpc) • The disk of our younger Galaxy contains – Open clusters – Gas and dust (8 kpc) most of its • Halo visible mass – Old, red dwarfs and – 90% of the The disk in infrared light giants Galaxy’s stars – Little gas and dust • Its where “the action” occurs – Globular clusters – formation, nebulae, etc.. • Bulge • Relatively thin – Mixture of halo and disk Side View – 700 parsecs thick vs. 30,000 parsecs across

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Spiral Arms? Hints of Spiral Arms

• Other disk galaxies show spiral arms • We plot the – Made of O- & B-type stars, locations of nearby diffuse nebulae, and giant O- and B-type molecular clouds stars in our Galaxy • How do we know our • Find the stars are arranged in arms Galaxy has them? • Our is • It’s the problem of not in-between seeing the forest for the spiral arms trees • But we can’t see beyond a few thousand light years • What about the rest of the Galaxy?

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Seeing the Galaxy Visible and Radio in Hydrogen Emission

• Look for 21-cm wavelength photons – Emitted by interstellar hydrogen – most abundant stuff! – Easily pass through gas & dust along the way – Map the Galaxy!

M83 Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 The Galaxy’s Spiral Arms The

• The 21-cm radio emission shows • Our Galaxy’s disk is surrounded by a spherical halo of the spiral arms (below) stars & globular clusters • We find five main arms in the – Red dwarfs and red giants – old stars Galaxy (right) – Only about 2% the number of stars in the disk • There is some structure to the halo – Denser towards the center – Two sets of globular clusters • Outer clusters – older, spherical distribution The disk in infrared light 100000 ly • Inner clusters – slightly younger (but still old), flattened spherical distribution Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

The Bulge

• The region where the disk and the halo merge – About 2,000 pc across Our Galaxy – Contains about 10% of the Galaxy’s stars • Mix of primarily old stars, but also contains some young stars and gas & dust 26000 ly • Like an extension of both the disk and halo

100000 ly

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 The Center of Our Galaxy The Galactic Nucleus

• Buried in the center of the bulge • 8,000 pc away • Incredibly dense region of stars and gas

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Do Galaxies Spin? Rotation of the Galaxy

• Similar to the planets orbiting the Sun, the stars and gas of the Galaxy the nucleus • How does the Galaxy rotate? • Like a CD? DVD? • How about disk vs. halo? • Measure Doppler shifts to Spiral galaxies really suggest it. BTW, our Galaxy find out. probably looks more like the right galaxy. Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Solid vs. Differential Rotation Rotation of the Galaxy

• Stars in the disk all orbit the Galaxy in the same direction – Stay in the disk (they may drift up and down) – roughly circular • Stars in the halo and bulge orbit the Galactic nucleus randomly – No organization to the orbits – Many very elliptical orbits Same angular speed Same linear speed (degrees per year) (parsecs per year)

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Is the Solar System Moving Rotate on.. Too? • Different than the solar system. • In Solar System, the mass is centralized. Yes… the whole – V = 2 πa/ P and P 2 = a 3 Galaxy has – So, V ∝ 1/a ½ differential rotation– us • For a galaxy, there is substantial mass at all radii, so the included orbital velocities of stars and gas change differently with orbital radius in a galaxy than in the solar system. • Using the Sun’s orbital speed and Kepler’s 3rd Law, we can calculate the mass of the Galaxy inside our orbit! • Measure our movement with respect to the globular The Sun orbits at 220 km/s clusters. or about 500,000 mph– 230 million years per orbit! Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Wow! That’s fast! The Rotation of the Galaxy

Stop and think about it. • That’s traveling to Chicago • Since we know our speed, we can in 1 second! measure the orbital speed of the • But MW is big! other stars. • Only orbited 50 times! • v is constant from 2kpc out. • Last time the Sun was here, the dinosaurs were just starting out. • ¼ way around, they were extinct!

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Velocities Are Odd. Velocities Are Odd.

• v ~ constant after 2 kpc • Can be used to get mass interior

• Recall that 2 v R 10 GM M (r) = circ M 8( kpc) = 8×10 solar masses v = G circ R ∝ • And if vcirc is a constant, then M R • This means that there is more mass farther out • Can be used to get mass interior M (16kpc) = 2M 8( kpc) = 6.1 ×1011 solar masses v2 R M (r) = circ G • But once outside all visible matter, stars, gas, dust, the mass should be constant, then MG 1 v = ∝ circ R R

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Rotation Curve Shows Whaa? Hidden Mass • Velocity does not • The farther a star is drop off with stars, from the center, the gas, or dust slower it should orbit • It is still constant, • Observations show that speed actually or even increasing increases or is • There must be a lot constant with distance of mass at farther from the center radii that is not • There must be a lot of mass in the outer glowing at any parts of the Galaxy wavelength. • But only 20% of the • ! Galaxy’s light is outside the orbit of the Sun • The mass in the outer part of the Galaxy is dark

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Dark Matter Dark Matter • What is this dark matter? • The dark matter in the Galaxy • Must have mass and must not glow. To be precise, must be is in greatly extended halo very dim. – Up to 90% of the Galaxy’s mass is • Dark matter is of unknown origins, although several dark matter! hypotheses exist: – Galaxy may have over a trillion solar masses total!

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 “Spiral Nebulae”

• Dim, diffuse “nebulae” with Astronomy: spiral patterns • Spiral structures catalogued The Big Picture mid-1800s by Lord Rosse Moving from our Galaxy outward! (Ireland) ModernRosse’s M51 M51 picture sketch

“Leviathan” 1.8 m telescope Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Those weird Spiral Nebulae?

• Dim, diffuse, “interstellar” nebulae with • In 1923, Hubble resolved M31, the spiral structure were seen in the 17 th century. Andromeda “”, into stars • Some disagreement on what they were. • If these stars were like the stars in our Galaxy, then M31 must be far away! – Kant: Our galaxy is a spiral “island universe” and the other spiral nebulae are the same and far away • Estimated the distance to M31 to be 300,000 parsecs (modern estimate is – Herschel and others: Milky Way is all there is in Hubble at Mt. Wilson the Universe, and the spiral nebulae are nearby. 700,000) Observatory More prevalent idea. • Andromeda is an “island universe” like our own Galaxy.

Sun Galaxy

The Andromeda “Nebula”

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Galaxies – Fundamental Galaxies – Fundamental “Ecosystems” of the Universe “Ecosystems” of the Universe

• Galaxies “fill” universe. • The cosmic engines that • Typical separation ~ 10 6 turn gas and recycles the pc or 1 Mpc gas the stars eject back • In between no star • Most distance is 1000’s of formation occurs – Mpc away “nothing happens” in • Galaxies are huge masses intergalactic space. of stars • Can classify them • Range in size from large (MW-like) to small “Dwarf” – 1 billion to 100’s of billions of stars

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Classes of Galaxies Galaxy Types: Overview

• Spirals (S) Spirals Ellipticals Irregulars – Basic structure: disk and bulge – Medium to large galaxies 9 12 5 13 8 10 – The disk has the young blue Mass ( MÍÍÍ) 10 – 10 10 – 10 10 – 10 stars, while the bulge has older 8 10 5 11 7 9 red stars Luminosity ( LÍÍÍ) 10 – 10 10 – 10 10 – 10 • Ellipticals (E) – Pure bulge, no disk component Diameter (kpc) 5 - 200 1 - 200 1 - 10 – Large range in sizes – All older red stars Color Disk: bluish-white Reddish-yellow Bluish-white • Irregulars (Ir) Bulge: reddish - yellow – Well… odd, irregular structure – Smaller galaxies – Mostly young blue stars

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Spiral Galaxies Effect of Viewing Angle Face-on • Spirals are classified on the amount of bulge component (and how tightly the arms are wound) • These are designated as Sa, More bulge Sb, Sc, in order of and tightly decreasing bulge Easier to see wound the spiral arms Sb Edge-on 90° More disk means more ongoing ! Easier to see the More disk and loosely wound Sc dust in the disk Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Barred Spirals Why do we see Spiral Arms?

• About 20% of spiral are barred • They are easily spirals seen as the • The spiral arms branch off from a straight bar of stars that passes arms contain through the central bulge numerous bright • They are designated with an O and B stars “SB” rather than the usual “S” that illuminate for spiral galaxies dust in the arms • The classes of barred spirals are SBa, SBb, and SBc • However, stars overall are evenly distributed throughout the disk

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 The Winding Problem Density Waves

• Spiral arms • If the arms are stationary, are caused they should by waves wind up and in the gas disappear and dust • This is not – Make the observed gas clump • Spiral arms up are not a permanent – Like an collection of stars interstellar traffic jam • Star, gas, and dust • Increased density of gas and dust sparks formation of new pass through the spiral arms O- and B-type stars that light up the spiral arm

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Elliptical Galaxies Varieties of Elliptical Galaxies Giant elliptical • Like a ’s bulge • Ellipticals come in a great range of – Mostly old, redder stars, little gas and dust masses – No disk organization, stars on random orbits • The largest are giant ellipticals – Up to 100+ times more massive • Classified by how elliptical they appear than the Milky Way – E0 (spherical) to E7 (elongated) • The smallest are the dwarf ellipticals – 10,000 to a million times less massive than the Milky Way Dwarf – Some only a few times larger than elliptical a ! • Of course, there are also “garden variety” ellipticals Type E0 Type E3 Type E6 Spherical Elongated – About 100 times smaller than to equal in size to the Milky Way Garden variety

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 Hubble’s “Tuning Fork” Irregular Galaxies Classification Scheme • Chaotic systems of stars • Prominent examples: The – Two of the Milky Way’s satellite galaxies • Generally smaller galaxies – Thousands to tens of times smaller than the Milky Way • Chaotic systems of stars – No disk, no elliptical structure • Dominated by young, blue stars

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

What Type of Galaxy is Masses of Galaxies the Milky Way? • The Milky Way is a • As with the Milky spiral galaxy Way, we measure the speed of a – Probably type Sb galaxy’s rotation • It might be a barred – Use Kepler’s 3rd Law to spiral! calculate mass – Type SBb? • Like the Milky Way, other galaxies have a flat rotation curve • Indicates a halo of dark matter

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 X-Ray Halos Dark Matter

• Some galaxies show a halo of X-ray emitting gas • Dark matter fills the Universe!!!! – Hot! - Millions of • Normal matter makes up less degrees in temperature than 10% of the Universe • Not much mass in • Dark matter is spherically the gas itself distributed all around galaxies • Its existence indicates in a huge halo. a powerful gravitational field – Over ten times the mass found in the galaxy’s stars – Again, dark matter !

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005

Nucleus of the Galactic Nuclei • Rapid orbits of radio • Orbital velocities around Optical Image the nucleus indicate a 10 sources around the million M black hole! Milky Way’s center Sun indicate a 2.5 million solar mass black hole X-Ray Image at its nucleus! • Do other galaxies show evidence for such supermassive black holes as well?

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005 1.2 billion solar M87’s Central Monster masses within region the size of the Solar System • Jet of M87 • 5000 light- year blow- torch! • Probably from the disk of the black hole at the center • 3 billion solar masses!

~ 800 ly

Astronomy 210 Spring 2005 Astronomy 210 Spring 2005 Apr 25, 2005 Apr 25, 2005