Galaxies Astro 530 Fall 2015 Prof. Jeff Kenney. CLASS 4 September 14, 2015 Structure of Stellar Disks & IntroducJon to KinemaJcs

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1 Galaxies Astro 530 Fall 2015 Prof. Jeff Kenney CLASS 4 September 14, 2015 Structure of Stellar Disks & IntroducJon to KinemaJcs 1

2 How does stellar disk form? Gas, which is collisional and dissipates energy through collisions, seples to a rotajng 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. 2

3 Why rotajng gas cloud forms thin rotajng disk Gas blob (nearly) all random mo(ons result in inelasjc collisions, which dissipate energy KE random à other forms of energy Gas blob Gas blob Gas blob ordered mo(ons (net rotajon) do not result in collisions (orbits are parallel & non- intersecjng) KE ordered à remains ~constant Only mojons supporjng cloud in direcjons other than rotajon direcjon are random so if parjcles are collisional the cloud collapses in all direcjons other than rotajon direcjon 3

4 VerJcal distribujon of stars near Sun in Milky Way sum thin disk thick disk halo Note that the A stars have a very small scale height (MS A stars have age<100 Myr) Why do different types of stars have different verjcal distribujons? G,K stars have large age range but many are old > 3 Gyr 4

5 What relajon do we expect between verjcal scale height z e and verjcal velocity dispersion <v 2 > 1/2? σ z2 =<v z2 > = A π G Σ z e a. A = 2 if ρ = ρ o sech 2 (z/z e ) b. A = 1.7 if ρ = ρ o sech (z/z e ) c. A = 1.5 if ρ = ρ o exp (z/z e ) sech 2 has ~constant density near disk midplane ( core ) exp is peaky ( cuspy ) near disk midplane sech 2 theorejcal expectajon from (too) simple model exp or sech closer to observajons A good way to es(mate the mass surface density Σ of disks: measure <v 2 > and z e 5

6 τ<100 Myr (O,B stars) ~100 pc

7 Structure & kinemajcs of Milky Way disk: key points from table 2.1 Increase in velocity dispersion corresponds to increase in scale height and decrease in mean rotajonal velocity Velocity dispersion of disk stars increases with age of stars There are different velocity dispersions in different direcjons (R, φ, z) why? Halo is not part of disk, and its origin is physically disjnct. But there are halo stars located in the solar neighborhood and within the disk. 7

8 VerJcal velocity vs. age Nearby main sequence F and G stars O = low metallicity stars (Z < 0.25 Z sun ) 8

9 Evolu(on of stellar disks GravitaJonal interacjons between stars and either GMCs or spiral arms transfer energy to the stars, heajng them up dynamically, thereby increasing their verjcal mojons and their average height above the disk midplane internal, conjnuous process origin of gradual trend of increasing velocity dispersion with age of stars 9

10 VerJcal distribujon of stars near Sun in Milky Way sum thin disk thick disk halo Note that the A stars have a very small scale height (MS A stars have age<100 Myr) Why do different types of stars have different verjcal distribujons? G,K stars have large age range but many are old > 3 Gyr 10

11 Thick disks in spiral galaxies 34 late- type, edge- on, undisturbed, disk galaxies spanning a wide range of mass Disk thickness appears to scale with circular velocity (~galaxy mass) for both thin and thick disk components not well understood thin thick Galaxy mass - > Galaxy mass - > Yoachim & Dalcanton

12 Thick disks in spiral galaxies 34 late- type, edge- on, undisturbed, disk galaxies spanning a wide range of mass Milky Way RaJo of verjcal scale heights for thick and thin components has range ~1.5-5 (factor of ~3) Yoachim & Dalcanton

13 Not all spiral galaxies have thick disks! Superthin Galaxy UGC7321 MaPhews+ 1999; MaPhews 2000 WIYN R- band LSB galaxy, i=88 deg, V max ~100 km/s VerJcal scale height in H- band Single component fit at r=0 (minor axis) exponenjal fit 2.9 =140 pc (~smallest value known) h r /h z = 14 (~largest value known) Possible 2- component fit to disk at r= +/- 60 Sech 2 fits, scale heights 3.8,8.7 It is possible that even this superthin disk has complex structure and internal dynamical heajng but probably no merger so no thick disk & not much of a stellar halo Simplest & least evolved stellar disks? 13

14 Thick disks in spiral galaxies 34 late- type, edge- on, undisturbed, disk galaxies spanning a wide range of mass Disk thickness appears to scale with circular velocity (~galaxy mass) for both thin and thick disk components not well understood thin thick superthin UGC7321 Galaxy mass - > Galaxy mass - > Yoachim & Dalcanton

15 Evolu(on of stellar disks GravitaJonal interacjons between stars and either GMCs or spiral arms transfer energy to the stars, heajng them up dynamically, thereby increasing their verjcal mojons and their average height above the disk midplane internal, conjnuous process origin of gradual trend of increasing velocity dispersion with age of stars Mergers of (small) galaxies with the Milky Way galaxy gravitajonally disturb the stars in the disk, heajng them up dynamically & maybe forming new stars in a disturbed & thicker gas disk external, discrete random events origin of thick disk 15

16 VerJcal disk structure - - caveats SJll debated whether Milky Way (& other galaxies) has disjnct thick disk component, or gradual increase in z e with age (e.g. Bovy etal 2012) 16

17 VerJcal disk structure - - caveats SJll debated whether Milky Way (& other galaxies) has disjnct thick disk component, or gradual increase in z e with age (e.g. Bovy etal 2012) Dust exjncjon near disk midplane makes it difficult to accurately measure verjcal distribujons of stars in most galaxies (NIR beper than opjcal) 17

18 VerJcal disk structure - - caveats SJll debated whether Milky Way (& other galaxies) has disjnct thick disk component, or gradual increase in z e with age (e.g. Bovy etal 2012) Dust exjncjon near disk midplane makes it difficult to accurately measure verjcal distribujons of stars in most galaxies (NIR beper than opjcal) Stellar populajon differences with z make it difficult to accurately interpret verjcal distribujons of stars would like to know distribujon of each stellar populajon separately AND distribujon of stellar mass (these are hard but worthwhile!) 18

19 Which galaxy is making a thick disk? Minor mergers can make thick stellar disks in 2 ways: 1. Pre- exisjng (old) stellar disk is dynamically heated by gravitajonal interacjon 2. Gas from either galaxy inijally has disturbed configurajon, stars form in it 19

20 20

21 galaxy kinemajcs types of mojons & their amplitudes Disordered or random mojons velocity dispersion Disk gas 5-30 km/s Disk stars 5-50 km/s Bulge & ellipjcal stars km/s Ordered or systemajc mojons - velocity 1. RotaJon Disk stars and gas ~5-400 km/s 2. Non- circular mojons Spiral arm density waves ~5-100 km/s Bar streaming mojons ~5-200 km/s Starburst winds km/s AGN ~ ,000 km/s 21

22 HI intensity = HI surface density OpJcal starlight HI gas vs. opjcal in NGC 3521 HI velocity HI velocity dispersion Note spider diagram papern to velocity field Walter etal 2008 THINGS VLA

23 RotaJon curve and observed velocity field of rotajng disk galaxy RotaJon curve Approaching blueshiwed Receding redshiwed Dark- halo potenjal ρ(r) ~ v H 2 / (r 2 +a H2 ) Isovelocity contours: lines of equal line- of- sight velocity v los Velocity field of inclined & rotajng disk with only circular mojons ( spider diagram )

24 Geometry of disk galaxies Mihalas & Binney 1981, p galaxy rotajon axis Ω Plane of sky coordinates Plane of galaxy coordinates Line of nodes: intersecjon of planes of sky & galaxy tan φ = cos i tan θ if π/2 < φ,θ < π/2 tan (π- φ) = cos i tan (π- θ) if π/2 < φ,θ < 3π/2 24

25 Line- of- sight velocijes in disk galaxies assume a galaxy disk (or annular ring in the galaxy) lies in a well- defined plane v los v los observed line- of- sight velocity v o = v sys = systemic velocity of galaxy Π =radial velocity in plane of disk Θ = azimuthal velocity in plane of disk Z = velocity perpendicular to plane of disk 25

26 Line- of- sight velocijes in disk galaxies assume a galaxy disk (or annular ring in the galaxy) lies in a well- defined plane v los vlos observed line- of- sight velocity v o = v sys = systemic velocity of galaxy Π =radial velocity in plane of disk Θ = azimuthal velocity in plane of disk Z = velocity perpendicular to plane of disk One cannot uniquely determine all these galaxy velocity components from the observajons! InformaJon is limited since we can directly measure only 1 velocity component, the one along the line of sight (no plane of sky mojons (proper mojons) for most galaxies) 26 Make assumpjons, check if velocity field consistent with assumpjons

27 Simplest case: only circular mo6ons in disk v los If only circular mojons in disk, then Π =Z=0, Θ(R,θ) = V rot (R) No systemajc radial mojons in plane of disk Π =0 No systemajc mojons perpendicular to plane of disk Z = 0 Then all azimuthal mojons are circular mojons Θ(R,θ) = V rot (R) v los V rot (R) Sky coordinates Galactocentric coordinates Angle between plane of galaxy & plane of sky 27

28 Simplest case: only circular mo6ons in disk v los If only circular mojons in disk, then Π =Z=0, Θ(R,θ) = V rot (R) No systemajc radial mojons in plane of disk Π =0 No systemajc mojons perpendicular to plane of disk Z = 0 Then all azimuthal mojons are circular mojons Θ(R,θ) = V rot (R) v los V rot (R) Sky coordinates Galactocentric coordinates Along major axis θ = 0 o, cos 0 o = 1, v los (ρ,φ) = v 0 + v rot (R) sin i Only azimuthal mojons along major axis Along minor axis θ = 90 o, cos 90 o = 0, v los (ρ,φ) = v 0 Only radial mojons along minor axis Angle between plane of galaxy & plane of sky 28

29 Observed velocity field in case of only circular mojons in disk plane isovelocity contours 29

30 Velocity cut along major axis (line of nodes) 30

31 Note papern of velocity field for solid body rotajon 31

32 Velocity cut along minor axis Why is this useful? 32

33 Velocity cut along minor axis Important since it tells us there are no radial (non- circular) mojons in disk! 33

34 RotaJon curve from velocity field Galactocentric radius (arcsec or kpc) RotaJon curve: VelociJes along line of nodes, averaged about center, and corrected for inclinajon 34