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Galaxies AS7007, 2012 Lecture 2: the Milky Way and Local Group

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2012-03-23 Galaxies AS7007, 2012 Outline I Lecture 2: The Milky Way and Local Group • The Milky Way Galaxy – Disk, Bulge, Stellar Halo, Dark Halo – The Galactic Centre – Galactic Rotation • The Local Galaxy Group – Local Group “Geography” & Inventory – The Large and Small Magellanic Clouds – The Magellanic Stream – Satellites of the Milky Way – The Andromeda Galaxy & M33 • Outside the Local Group The Milky Way Galaxy The Milky Way Galaxy • Disk (Thin & Thick) • Bulge • Stellar Halo • Dark Halo • Nucleus (Supermassive Black Hole) The Milky Way Galaxy The Milky Way Galaxy • Spiral galaxy of type Sbc or SABbc • The concept of populations: • Contains about 2 — 4 ×10 11 stars – Three types with increasing age: population I, II & III. Pop III stars are the first to form in the universe. z – No strict dividing line between the types φ R – Less used today, except pop III which is a hot topic Galactic in the high-redshift Universe Sun centre • Correlation between age and metallicity (amount of Galactic coordinates heavy elements) → can obtain information both = − − about when and where the stars formed n(,,) R z S n(0,0,S ) exp[ R/ hR ( s )] exp[ z/ hz ( s )] hR: Scale length, h z: Scale height S: Stellar type 1 2012-03-23 The Milky Way Stellar Disk I The Milky Way Stellar Disk II • Radius of the disk: > 15 kpc • Stars form in clusters and associations Open clusters: • Scalelength h r of the disk: 2—4 kpc • × 9 – Few hundred stars at most • Disk luminosity: 15—20 10 Lsolar × 10 – Luminosity 100--30000 Lsolar • Stellar Disk mass: 6 10 Msolar • Thin disk: – Core radius ~ few pc – Young (Only ~5% more than 1 Gyr old) – Scaleheight h z : 300—400 pc – Contains 95% of all disk stars & all the young ones – Mass segregation – High metallicity • Stars form in clusters and associations • Thick disk: • Associations: – Scaleheight: 1000—1500 pc – Not gravitationally bound Lower metallicity – – Forms temporary systems The Milky Way Bulge Does the Milky Way have a classical bulge or pseudobulge? The jury is out… • Flattened (a/b ≈ 0.6), radius ~ 1 kpc • Possibly contains bar (2—3 kpc long) • Rotates in same direction as disk stars, but slower (≈ 100 km/s) • Contributes 20% of the MW luminosity • Stars several Gyr old, but younger than in halo Classical bulge ≈ • Average stellar metallicity 0.5 Z solar Classical bulge: Resembling a small elliptical galaxy, formed through mergers Pseudobulge: Disk-like properties, formed internally (so-called ”secular evolution”). No mergers required. The Milky Way Halo I The Milky Way Halo II • Somewhat flattened, but rounder than bulge • Globular clusters • Radius ≈ 50 kpc – Up to 1 million stars – Total mass ∼ 10 5 Msolar ∝ -3.5 • Stellar density r – No dark matter (at least not • Total mass in halo stars: ~ 10 9 Solar masses anymore) 1/1000 of all local stars belong to halo – Core radius < 1 pc • – Tidal / truncation radius 20-30 pc • Eccentric orbits, sometimes retrograde – About 150 objects known, ages 10—14 Gyr (oldest objects in the Galaxy) Latest stuff (not in textbook): May have two components – Typically very metal-poor (inner and outer halo) with different properties 2 2012-03-23 Intermission: Which of these The Milky Way Dark Halo is not a globular cluster? • Radius > 100 kpc • Contributes ~ 90% of the mass inside 100 kpc • Content unknown… The Milky Way Gaseous Disk The Milky Way Gaseous Disk Neutral hydrogen • 4—8 × 10 9 solar masses HI × 9 • 2—4 10 solar masses H 2 (but uncertain) • Dust ∼ 1 % of HI mass The Milky Way Gaseous Disk The Milky Way Gaseous Disk Ionized hydrogen HI H2 HII 3 2012-03-23 The Milky Way Centre The Milky Way Centre • Infrared light shows a dense star cluster which peaks at the center, near Sagittarius A*. • The high velocities of the stars require a mass of ∼ 6 ∼∼2 x 10 Msolar within 1 pc • Stars are only 1000 AU apart • Collisions every ≈≈≈ 10 6 years! • The centre of the star cluster may host a Supermassive Black Hole (although somewhat lightweight) 1 m Radio observations Galactic Rotation Galactic Rotation • Differential rotation • Neutral hydrogen: 21 cm line • Distance Sun-centre: 8 Kpc • Sun’s Velocity around the centre 220 km/s • One revolution in 250 Myr Galactic Rotation The Local Group • The Local Galaxy Group – Local Group “Geography” & Inventory – The Large and Small Magellanic Clouds – The Magellanic Stream – Satellites of the Milky Way – The Andromeda Galaxy & M33 4 2012-03-23 Local Group “Geography” The Local Group Inventory • Radius ∼1.2 Mpc • Held together by gravity – decoupled from Hubble flow • 3 spirals: Milky Way, M31, and M33 • Two more massive galaxies: – Irregular Large Magellanic Cloud – Small (dwarf) elliptical galaxy M32 • The rest are dwarf galaxies (dI, dE, dSph) with MV > -18 The Local Group Inventory The Large Magellanic Cloud (LMC) • The Local Group does not contain: –Blue compact dwarf galaxies –Dwarf spirals –Massive ellipticals –Active galaxies The Small Magellanic Cloud (SMC) The Magellanic Clouds LLMMCC SSMMCC Diameter 24 deg. 7 deg. Distance 50 kpc 63 kpc Total mass 6 ××11009 s.s.mm.. 22×× 11009 s.s.mm.. Luminosity ~10% of MW ~1% of MW HI mass 7 ×× 11008 s.s.mm.. 66..55×× 11008 s.s.mm.. ZZ 0.70 solar 0.25 solar M(HI)/M(total) 00..0099 00..3322 5 2012-03-23 The 11 “Classical” Satellites The Magellanic Stream & Bridge of the Milky Way • Magellanic Bridge: • LMC – HI bridge between LMC and SMC • SMC ≈ Fornax – Size 20 kpc • But dark matter theory predicts Mass: 2 ×10 8 solar masses HI • Sagittarius – factor of ~ 10 more! – Contains stars formed 10—25 Myr ago • Leo I (DDO 74) – Could have formed 200 Myr ago when LMC and • Sculptor Are there dark galaxies…? SMC where the closest • Leo II (DDO 93) • Magellanic Stream: • Sextans Carina – Gas trailing behind LMC and SMC • • Ursa Minor – Wraps 1/3 around the sky • Draco (DDO 216) Ultrafaint dwarf galaxies Stellar Populations • Some of the most dark matter-dominated systems known Resolved Unresolved • About 10 discovered in the Sloan Digital Sky Survey after 2005 Individual stars can be analyzed Integrated spectroscopy / Applicable for Milky Way star clusters photometry only and the most nearby galaxies The most common case in extragalactic astronomy Finding ultrafaint dwarf galaxies using Finding ultrafaint dwarf galaxies using resolved stars resolved stars Actual picture of the sky Resolved stars in same area Just old, metal-poor stars Same, filtered and smoothed (foreground stars dominate) (galaxy size assumed) 6 2012-03-23 Andromeda, Milky Way, M33 The Andromeda Galaxy & M33 - The Big Spirals of the Local Group- • Hubble type: Sb, Sbc/Sc, Sc • Luminosity: 1.5 × MW, 1 × MW, 0.35 × MW • Andromeda & The Milky Way have warped disks, probably caused by interaction with M32 and Magellanic Clouds • Milky Way & Andromeda may collide in ~ 5 Gyrs M31 (Andromeda) M33 (NGC 598) Outside the Local Group 7.
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