ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 4 Sept 10, 2018 The Milky Way Galaxy: Star Clusters
finish disk of Milky Way 2
good view of edge- on stellar disk in S0 galaxy NGC 4452 HST 3
we d like to examine ver/cal structure of Milky Way disk Study stars in directions ~perpendicular to disk plane 4
Density of stars (of par/cular type S) in Milky Way disk Use cylindrical coordinates (R,φ,z) and integrate over azimuthal angle φ h R = (radial) scale length of disk h z = (vertical) scale height of disk n(0,0,s) = density of stars of type S at galaxy center Radial distribution: Exponential is excellent fit to light profiles of many disk galaxies. Some galaxies have deviations from this. Scale length differences for different types of stars in some but not all disks (generally not as important as type differences in vertical distributions) Hard to study in Milky Way Vertical distribution: Nothing special about exponential, gives OK rough fit for some types of stars Very different scale heights for stars of different type or age (age is main parameter, but age is harder to determine than type) 5 Possible to study in detail in Milky Way
τ<100 Myr (O,B stars) ~100 pc
Key points of table 2.1 scale height of disk stars increases with age of stars increase in scale height corresponds to increase in velocity dispersion and decrease in mean rota?onal velocity There are different velocity dispersions in different direc/ons (R, φ, z) why? Halo is not part of disk, and its origin is physically dis/nct. But there are halo stars located in the solar neighborhood and within the disk. 7
Ver/cal velocity vs. age Nearby main sequence F and G stars O = low metallicity stars (Z < 0.25 Z sun ) 8
cartoon of spiral galaxy disk showing spa/al extent of stars of different ages 9
Forma?on of Stellar Disks Gas, which is collisional and dissipates energy through collisions, sebles to a rota/ng thin gas disk. Stars form in giant molecular clouds (GMCs) of dense gas, which are embedded within a thin disk of gas. The youngest stars are therefore in a disk with the same thickness as the layer of star- forming dense gas. 10
Evolu?on of stellar disks Gravita/onal interac/ons between stars and either GMCs or spiral arms transfer energy to the stars, hea/ng them up dynamically, thereby increasing their ver/cal mo/ons and their average height above the disk midplane internal, con/nuous process origin of gradual trend of increasing velocity dispersion with age of stars Mergers of (small) galaxies with the Milky Way galaxy gravita/onally disturb the stars in the disk, hea/ng them up dynamically & maybe forming new stars in a disturbed & thicker gas disk external, discrete random events origin of thick disk 11
video of thick disk forma/on from merger. hbps://www.youtube.com/watch?v=uualarnj0os 12
disks exist in many astrophysical systems planetary rings 10 10 cm planets around stars 10 15 cm AGN accre/on disk 10 16 cm galaxy stellar disk 10 22 cm why doesn t the universe have any larger disks? 13
Why don t clusters of galaxies have disks? A. they don t have enough gas B. their gas can t cool C. disk forms but gets disrupted by /dal encounters D. dark maber doesn t form disks E. not enough /me to form them 14
Why don t clusters of galaxies have disks? A. they don t have enough gas B. their gas can t cool C. disk forms but gets disrupted by /dal encounters D. dark maber doesn t form disks E. not enough /me to form them 15
ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 4 Sept 10, 2018 The Milky Way Galaxy: Star Clusters
Why are star clusters interes/ng? They are sub- units of a galaxy that allow us to learn an incredible amount about the forma'on and evolu'on of our galaxy We can easily measure the age and elemental abundances and distances of star clusters (easier than for individual stars) by using CMDs Use together with their loca?ons in galaxy and their mo?ons to /ghtly constrain models of the forma/on & evolu/on of the Milky Way Galaxy 17
forma/on scenario for spiral galaxies including the Milky Way 18
Distribu/on of globular clusters in Milky Way all are old, with ages 10-13 Gyr metal- rich [Fe/H]>- 0.8 metal poor [Fe/H]<- 0.8 Nearly all metal- rich globulars are in the disk or bulge Blue HB: very metal- poor Red HB: moderately metal- poor Nearly all metal- poor globulars 19 are In the halo
forma/on scenario for spiral galaxies including the Milky Way stars & star clusters that formed before they joined galaxy are now in halo 20
Star clusters most (all?) stars form in clusters, clusters disperse over /me Globular cluster Large: 10 5-10 6 stars Open cluster Small: 10 2-10 4 stars
Rosebe nebula a star forming region which has just made a young Open star cluster (<10 Myr old)
Westerlund 2 young star cluster & star forming region (< 10 Myr) HST image 23
Pleiades star cluster Age: 125 Myr Mass: 800 M sun Lum: 4500 L sun M/L = 0.2 M sun/ L sun Distance: 132 pc 24
How much does the mass- to- luminosity ra/o vary among star clusters? A. it is nearly constant, to within a factor of ~2 B. it varies by a factor of ~10 C. it varies by a factor of ~100 D. it varies by a factor of >1000 25
How much does the mass- to- luminosity ra/o vary among star clusters? A. it is nearly constant, to within a factor of ~2 B. it varies by a factor of ~10 C. it varies by a factor of ~100 D. it varies by a factor of >1000 26
M67 star cluster Age: 4000 Myr Mass: 2000 Msun Lum: 2100 Lsun M/L = 1.0 Msun/Lsun Distance: 860 pc The M67 star cluster has a similar mass to the Pleiades but is much older. 27 It no longer has luminous massive stars, so the M/L is much higher than the Pleiades.
NGC 3603 star cluster Age: < 5 Myr Mass: 7000 M sun Lum: 20,000,000 L sun M/L = 0.00035 M sun/ L sun Distance: 6500 pc NGC 3603 is a very young star cluster. It is 10x as massive as the Pleiades but has an enormous luminosity 4000x higher because it s/ll has nearly all of its massive, high luminosity stars. The ra/o of light- to- mass is very high or the inverse, M/L, is very low.
47 Tuc globular star cluster Age: 12 Gyr Mass: 700,000 M sun Lum: 500,000 L sun M/L = 1.4 M sun/ L sun Distance: 4500 pc This globular cluster has 100x more mass than the young cluster NGC 3603, yet is 4x 29 less luminous since it is so old
Pleiades star cluster Age: 125 Myr Mass: 800 M sun Lum: 4500 L sun M/L = 0.2 M sun/ L sun Distance: 132 pc 30
Pleiades cluster color- magnitude diagram CMDs of star clusters are incredibly powerful tools, which /ghtly constrain the age & chemical composi/on & distance of their stars, and therefore of a piece of a galaxy. It s important to understand how we interpret them, to get preby 31 accurate values of age, metallicity and distance.
cluster life/me main sequence star life/me O B A F G K M the main sequence life/me of a star at the turn- off point of the main sequence is equal to the age of the star cluster 32
Pleiades cluster color- magnitude diagram 16 Myr 100 Myr observed colors & magnitudes of Pleiades stars Isochrone (line connec/ng stars of different masses but same age & same elemental composi/on, based on models of stellar structure & evolu/on) 33
Pleiades cluster color- magnitude diagram 16 Myr 100 Myr Unresolved Binaries are displaced to higher L s & different T s compared to Single stars single stars isochrone Isochrone (line connec/ng stars of different masses but same age & same elemental composi/on, based on models of stellar structure & evolu/on) 34
Pleiades cluster color- magnitude diagram 16 Myr dust no dust 100 Myr Unresolved Binaries are displaced to higher L s & different T s compared to single stars isochrone Single stars Isochrone (line connec/ng stars of different masses but same age & same elemental composi/on, based on models of stellar structure & evolu/on) 35
what is dust? 36
Interstellar dust grains These examples (from solar system) are much larger than typical interstellar dust grains Small specks of solid maber, (~10 6-10 9 molecules) Range of sizes, but typical sizes: d dust ~ 0.1-1 µm or d dust ~ 0.4 µm Chemical composi/on: silicates (like sand) or carbon compounds (like soot or graphite) Origin: evolved stars
Forma/on of dust in outlowing envelope of red giant star (before planetary nebula phase) Origin of dust: ejected outer envelopes of evolved stars Ejected gas cools and clumps into par/cles as gas cools, it changes from ions to atoms to molecules to clumps of molecules (dust)
Labelled components in color- magnitude diagram (CMD) of globular cluster 39
Dust: ex/nc/on & reddening Photons are absorbed or sca9ered in interac/ons with dust grains (collec/vely ex/nc/on ) Ex/nc/on highly wavelength- dependent, most effec/ve if d dust > λ Short wavelength photons more affected than long wavelength photons leads to reddening of light 40
dust absorp/on & scabering blue photons λ B d dust θ B scattering absorption dust heated red photons λ R θ R scattering θ R < θ B absorption dust heated blue photons go shorter distance before being absorbed or scattered, and are scattered by a larger angle
dust absorp/on & scabering blue red λ B λ R d dust θ B scattering θr scattering θ R < θ B both scattering & absorption very sensitive to wavelength of photon -- most effective if d dust >= λ typical dust d dust ~ 0.4 µm = λ Blue so blue light easily scattered & absorbed but red light with λ Red = 0.7 µm has wavelength larger than typical dust particle so is less easily easily scattered & absorbed
Scabering of sunlight by molecules & dust in earth s atmosphere Blue light is scabered more than red light, since it has shorter wavelength (wavelength selec/ve scabering à reddening ) Why sky is blue. Why sun looks red at sunset & sunrise.
dust ex/nc/on & reddening a bigger effect at op/cal than NIR wavelengths Milky way galaxy in op/cal (0.4-0.7µm)(B V R) Milky way galaxy in infrared (1.2, 1.6, 2.2µm)(J H K) 2MASS 44
op/cal depth 0 x+δx x 0 Δx x x F λ (x+δx) = F λ (x) [1 κ λ Δx] κ λ = opacity = rate at which light is being absorbed & scabered at wavelength λ (property of material) df λ dx = - κ λ F λ F λ (x) = F λ (x 0 ) e τ λ = op/cal depth F λ (x 0 ) - κ λ (x- x 0 ) = F λ (x 0 ) e - τ λ = apparent brightness that would be measured without dust ex/nc/on in magnitudes = A λ = 1.086 τ λ 45
op/cal depth 0 x+δx x 0 τ λ Δx x x F λ (x+δx) = F λ (x) [1 κ λ Δx] κ λ = opacity = rate at which light is being absorbed & scabered at wavelength λ (property of material) df λ dx = - κ λ F λ F λ (x) = F λ (x 0 ) e τ λ = op/cal depth F λ (x 0 ) - κ λ (x- x 0 ) = F λ (x 0 ) e = apparent brightness that would be measured without dust - τ λ ex/nc/on in magnitudes = A λ = 1.086 τ λ 46
Ex/nc/on toward 3 different stars in Milky Way Compared to visible wavelengths, ex/nc/on much greater in the UV and much less in the IR. Different ex/nc/on laws along different lines- of- sight due to differences in composi?on and size distribu?on of dust grains IR visible UV à 47
Pleiades cluster color- magnitude diagram 16 Myr dust no dust 100 Myr Unresolved Binaries are displaced to higher L s & different T s compared to single stars isochrone Single stars Isochrone (line connec/ng stars of different masses but same age & same elemental composi/on, based on models of stellar structure & evolu/on) 48
Pleiades cluster color- magnitude diagram 16 Myr no dust 100 Myr dust dust both decreases the flux (increases the magnitude) and reddens the light 49
Pleiades cluster color- magnitude diagram 16 Myr no dust 100 Myr dust dust both decreases the flux (increases the magnitude) and reddens the light dust mimics older age 50
Distribu/on of globular clusters in Milky Way all are old, with ages 10-13 Gyr metal- rich [Fe/H]>- 0.8 metal poor [Fe/H]<- 0.8 Nearly all metal- rich globulars are in the disk or bulge Blue HB: very metal- poor Red HB: moderately metal- poor Nearly all metal- poor globulars 51 are In the halo
how does the metallicity difference between disk and halo fit into the origin story?
Gas is the raw material for star forma/on, but where does the gas in galaxies come from? 1. primordial (from Big Bang) 2. reprocessed & recycled, through stars Figuring out how much the raw material in stars has been recycled through previous genera/ons of stars offers powerful evidence on galaxy evolu/on!
Solar system elemental abundances Solar abundances: Hydrogen: M H /M gas = 0.74 Helium: M He /M gas = 0.24 heavies (Everything else): M h /M gas = 0.02 = Z sun ( metals )
Elemental abundances in astronomy Where elements made: A=1-5 (H, He, Li, Be, B) mostly in Big Bang A=6-100+ (C,N,O.) mostly in stars + SN Astronomy defini/ons of abundance ra/os: [A/B] = log (# A atoms/#b atoms) star (# A atoms/#b atoms) sun [Fe/H] is logarithmic ra'o of Fe/H in star rela/ve to sun Fe is preby good indicator of overall heavy element abundance. SomeEmes [Fe/H] represents average heavy- element abundance & not just Iron. 55
Range of heavy element abundances in Milky Way stars: Z = 10-5.5 - > 3 Z sun Z sun = M h /M gas = 0.02 [Fe/H] = - 5.5 - > 0.5 There are no stars in Milky Way with primordial = Big Bang abundances, i.e., no Popula/on III stars. Popula/on I Z ~ Z sun, [Fe/H]~0 (roughly like Sun) Popula/on II Z ~ 0.1-10 - 5 Z sun, [Fe/H]~- 1- >- 5 (metal poor) Popula/on III Z 0,, [Fe/H] <- 6 (no metals) All stars in Milky Way formed from gas that was polluted at least a bit by previous genera/on(s) of stars! 56
only got this far in class 4. 57