Astronomy Picture of the Day 2018 Apr 06
- Slides: 23
Astronomy Picture of the Day: 2018 Apr 06 NGC 289 AS 2001 Galactic Astronomy
Structure of the Milky Way Halo Globular clusters Dust Gas Disc Sun AS 2001 Bulge Galactic Astronomy
Galactic Halo • Halo Structure and Kinematics of • Globular clusters ( ~ 1% of Halo mass ) • Individual halo stars ( e. g. subdwarfs, RR Lyrae ) • MW globular clusters N ~ 160 • Bright, easily recognisable Mv ~ -7. 5 mag • Most are old and metal poor • Well outside the disc => little or no dust obscuration • Distances / ages ( found by main sequence fitting ) • RR Lyra variables • Horizontal Branch ( HB ) variable stars. Period < 1 day • Tracer of Halo stars, also in Globular Clusters • Distances from Mv ~ +0. 6 mag AS 2001 Galactic Astronomy
MW Globular Cluster Distribution • Roughly spherical, centrally concentrated. • Show us the direction / distance to Galactic Centre. • Most at R ~1 - 25 kpc. (a few out to ~100 kpc). AS 2001 Galactic Astronomy
Two Types of Globular Cluster 1% • Halo Clusters 10% 100% solar • Metal Poor : [ Fe / H ] < -1 • Spherical Halo • No Net Rotation • Disk Clusters • Metal Rich : [ Fe / H ] > -1 • Thick Disk : H ~ 2 kpc • Rotating like disc stars 25 kpc 2 kpc 0 0 -2 kpc -25 kpc AS 2001 0 25 kpc -5 kpc 0 +5 kpc Galactic Astronomy
RR Lyrae in Baade’s Window • RR Lyrae : MV ~ + 0. 6 => standard candle for distances • Baade’s window : ( l ~ +1 o, b ~ -3. 9 o ) with low dust obscuration Galactic Centre • Dust-corrected counts of RR Lyra variables, looking through Baade’s window, vs apparent magnitude d. N / dm. V • Peak counts / mag at mv ~ +15. 3 • Distance modulus: m - M = 5 log( d / 10 pc ) = 15. 3 -0. 6 = 14. 7 mag • => Distance to Galactic Centre : R 0 = 8. 7 ± 0. 6 kpc AS 2001 Galactic Astronomy
Baade’s Window : ( l , b ) = (+1, -4)o AS 2001 Galactic Astronomy
Halo Star Number Density Profile • • • Discover RR Lyra stars by periodic brightness variations. Number counts vs magnitude => number densities vs distance. Well above / below Galactic Plane => small corrections for dust extinction slope ~ -3 log n. RR Rmin Rmax log r • Power-law slope ~ -3 from 1 to 25 kpc AS 2001 Galactic Astronomy
Total Mass of Halo Stars • Spherical power-law halo model : Practice integrating power laws! • Rmin ~ 1 kpc, Rmax ~ 25 kpc, R 0 ~ 8. 5 kpc • ~ 105 Msun / RR Lyra star => r 0 ~ 10 -4 Msun / pc 3 • => Mass of halo stars : Mhalo ~ 3 x 109 Msun • Compare to Dynamical mass : M = V 02 R 0 / G ~ 1011 Msun • Oort limit on local disc mass density : 0. 13 Msun / pc 3 AS 2001 Galactic Astronomy
Halo Kinematics • Big picture : halo stars disc stars • Solar neighbourhood : Halo stars near Sun appear as high velocity stars V > 100 km/s AS 2001 Galactic Astronomy
Orbits from LSR Kinematics • Orbits : y v = Vesc-V 0 ( e = 1 ) escape orbit v = 0 ( e = 0 ) circular orbit Galactic Centre LSR v = -V 0 ( e = 1 ) e = (A-B)/A B x v LSR • Velocities : A u V 0 Vesc AS 2001 Galactic Astronomy
Escape Velocity • Vesc = velocity needed to escape the Galaxy • measures of the depth of the gravitational potential well • To stay in Galaxy : • Here ( u, v, w ) are relative to LSR (small correction). • Highest velocity stars near the Sun => lower limit on Escape Velocity : AS 2001 Galactic Astronomy
How we find Orbit Eccentricity • Orbit eccentricity from ( u, v ) velocities (Botlinger diagram) +400 e = 0, ± V 0 circular orbit LSR v + V 0 ( km/s ) prograde e=1 retrograde e = 0. 5 - 400 AS 2001 - 400 u ( km/s ) 400 e = (A-B)/A B A Galactic Astronomy
… and the “Apo-galacticon” Radius • Rmax from ( u, v ) velocities (Botlinger diagram) +400 Rmax = 8. 5 kpc v + V 0 ( km/s ) 15 kpc 40 kpc - 400 AS 2001 Rmax = infinity V = Vesc u ( km/s ) +400 Galactic Astronomy
How we identify Halo stars • Subdwarfs (sd. G, K, M) are metal-poor (Pop II) halo stars • Metal absorption lines alter star colours : V fl U B V Main sequence disc stars subdwarfs l • More lines at shorter wavelengths • “Line Blanketing” => Blue light absorbed is re-radiated as red light • Subdwarfs are below and left of main sequence on HR diagram • Colour index change measures metallicity B-V U-B d ( U - B ) = 0. 2 mag for [ Fe / H ] = -2 AS 2001 Why no bluer sub-dwarfs here? B-V Galactic Astronomy
Galactic Orbits vs Metallicity • Halo stars : • Subdwarfs • RR Lyrae • Globular Clusters +400 +200 v +V 0 0 ( km/s ) Disc stars -400 1. 0 Halo stars • Mix of prograde and retrograde orbits • No net rotation • High eccentricity • Low metalicity e 0. 0 AS 2001 -3 -2 -1 0 [ Fe / H ] Galactic Astronomy
The Fossil Record in Stars • Population Orbits • Pop II Halo radial eccentric • Pop I Disc ~circular Ages Metals old low young + middle-age high • Clues to Formation of Galaxy • Halo structure: n* ~ R-3 (1 - 25 kpc) AS 2001 Galactic Astronomy
Current Research : “Spaghetti Halo” • • Halo may be built up by infall of many dwarf galaxies. Galactic tides spread these out into long star streams. Now being discovered (e. g. GAIA satellite). Stars also seen escaping from some Globular Clusters. simulation AS 2001 Galactic Astronomy
Summary of MW Halo Properties • Halo stars ( high-velocity subdwarfs, RR Lyrae ) + Globular Clusters. No dust in halo. • Metal poor -2 < [ Fe / H ] < -1 ( 1 -10% solar ) • Spherical distribution : n* ~ R-3 ( 1 - 25 kpc ) • Halo mass in stars : < 1010 Msun • Eccentric ( more nearly radial ) orbits. • Mix of prograde and retrograde (no net rotation) • Used to find LSR orbit radius : R 0 ~ 8. 5 kpc Circular velocity : V 0 ~ 220 km/s Escape velocity : Vesc > 400 km/s. • Also: “Thick Disc” of stars and globular clusters, intermediate age, metals, prograde rotation, flattened (~10 x 2 kpc). AS 2001 Galactic Astronomy
Thanks for Listening AS 2001 Galactic Astronomy
Botlinger Diagram (Eggen 1964) v ( km/s ) => u ( km/s ) => AS 2001 Galactic Astronomy
Original Botlinger Diagram AS 2001 Galactic Astronomy
Original Botlinger Diagram AS 2001 Galactic Astronomy
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