The Role of High-Velocity Clouds in Galactic and Local Group Evolution

1. The Role of High-Velocity Clouds in Galactic and Local Group Evolution Joe Collins (University of Colorado) with: Mike Shull (University of Colorado) Mark Giroux (East Tennessee State University)

2. Outline I. Introduction to HVCs Complex C II. Highly Ionized HVCs

3. What are HVCs? • Most often detected in H I • Velocities incompatible with Galactic rotation: |VLSR|> 100 km/s • 1st detection: Muller, Oort, & Raimond (1963)

4. Typical H I Emission Spectra Simple cases: Galactic Galactic HVC HVC Or: Galactic IVC HVC

5. HVC Morphology • HVC Complexes angular size: 10 - 100 o o • Compact HVCs (CHVCs) (Putman et al. 2002)

6. HVC Morphology • Magellanic Stream (Putman et al. 2003)

7. The Sky in High-Velocity H I • Sky-covering fraction, 40 % down to N(H I)  10 cm (B. Wakker) 18.5 -2

8. Where are the HVCs? • No stellar component --> distance is difficult to obtain • Diameter d • Mass d • Are HVCs, • in Galactic halo? -->d < 50 kpc • Local Group objects? -->d1 Mpc 2

9. Significance of HVCs Galactic or Local Group “building blocks” • Halo: G-Dwarf Problem (Pagel 1994) --> infall of material onto Milky Way • Local Group: “missing dwarves” from cosmological simulations (Blitz et al. 1999)

10. Cosmological Simulations • Local Group location can be ruled out for the HVC complexes (B. Moore)

11. HVC Complexes Are halo objects, but what is their origin? I. Infalling objects from the IGM --> explain G-dwarf problem II. Condensations from Galactic fountain (Bregman 1980)

12. Origin of HVCs • How do you distinguish these scenarios? Metallicity: so, I. Infalling clouds: Z << Z (0.1 Z ) II. Fountain material: Z Z (X/H) Z(Z )  [X/H] = HVC solar (X/H) solar solar solar solar

13. Gas-Phase Abundances • Must be done in absorption --> need bright background targets • Must go to UV for useful absorption lines --> space-based observations but, DV  100 km/s which is, 0.4Å at 1200Å HVC

14. UV Astronomy • High-res UV spectroscopy in late-1990’s I. Hubble Space Telescope (HST) covers 1150Å-3000Å II. Far UV Spectroscopic Explorer (FUSE) covers 912Å-1200Å --> FUSE is better suited for metallicity studies of HVC Complexes

15. FUSE At KSC; June 14, 1999 Launch; June 24, 1999

16. The HVC Complexes (B. Wakker)

17. Complex C • Covers  2000 deg • Pierced by several quasar sight lines • unique laboratory for HVC studies (Wakker 2001) 2

18. Complex C • Distance constraint • lower limit: halo star absorption d > 6 kpc (Wakker 2001) • Previous metallicity studies (HST- GHRS) • Wakker et al. (1999): Mrk 290 sight line Z = 0.1 Z based on [S II/H I] • Gibson et al. (2001): 5 sight lines Z = 0.1-0.4 Z from [S II/H I] solar solar

19. The FUSE/HST Survey of Complex C (Collins et al. 2003) • FUSE data for 8 sight lines • HST STIS and GHRS data for 7 of the sight lines From Leiden-Dwingleoo H I Survey (Hartmann & Burton 1997)

20. Complex C Survey • Use N(H I) from 21-cm Effelsberg data (Wakker et al. 2001) • Measure column densities of: O I, N I, Si II, S II, Fe II --> [O I/H I] for metallicity!!!! • Can analyze high ions: O VI, N V, C IV, Si IV, Si III, C III

21. Complex C Survey • PG 1259+593 • Best FUSE data (610 ks) • STIS E140M data covering • 1150-1700Å IV Arch C H I G 19 -2 • N(H I) = 8.4 X 10 cm

22. PG 1259+593 H I 21cm N I 1199.55 Å O I 1039.23 Å Si II 1020.70 Å S II 1259.52 Å Fe II 1144.94 Å Equivalent Width: W =S(1-F )Dl l n

23. PG 1259+593 Equivalent Widths Curve of Growth

24. PG 1259+593 Results • Metallicity: [O I/H I] = 0.10 Z +0.05 -0.04 solar

25. Complex C Results • Metallicity: [O I/H I] = 0.10-0.25 Z --> infalling cloud is mixing with Galactic gas as it plunges towards disk • Fox et al. (2004) study of high ion ratios: [C IV/O VI], [Si IV/O VI], [N V/O VI] --> consistent with collisional ionization at the cloud interface. solar

26. II. Highly Ionized High-Velocity Clouds

27. Highly Ionized HVCs • FUSE survey of O VI • O VI doublet at 1031.93, 1037.63 • I.P. = 138 eV --> traces gas at T 10 K • High-velocity O VI is ubiquitous 60-85 % of extragalactic sight lines (Sembach et al. 2003) 5.5

28. The Sky in High-Velocity O VI • Do the O VI HVCs correlate with the H I HVCs?

29. O VI: H I: (Sembach et al. 2003)

30. The Highly Ionized HVCs M.W. motion L.G.B. • Nicastro et al. (2003) --> these trace a filament of the warm-hot intergalactic medium (WHIM) in the Local Group

31. The WHIM -1 100h Mpc box (Cen & Ostriker 1999) 5-7 -6 -3 WHIM: T ~ 10 K and n ~10 cm

32. Are these HVCs WHIM? Nicastro et al. (2002, 2003) propose that these objects trace Local Group WHIM

33. Mass in Local Group WHIM 14 NOVI = 10 cm T = 10 K Z = 0.1 Zsolar RWHIM = 1 Mpc –2 5.5 12 Mhot ~ 10 Msolar (Cen & Ostriker 1999)

34. Are these HVCs WHIM? Nicastro et al. (2002, 2003) propose that these objects trace Local Group WHIM The Evidence: 1) “Dipole” sky distribution of HVC velocities

35. Kinematic sky distribution can be explained in several ways  Local Group WHIM  infall to L.G.B M.W. motion L.G.B. • Galactic Infall • Rshell = 15 kpc • Vinfall = 50 km/s • standard Galactic rotation

36. Are these HVCs WHIM? Nicastro et al. (2002, 2003) propose that these objects trace Local Group WHIM The Evidence: 1) “Dipole” sky distribution of HVC velocities 2) Possible correlation with poorly-resolved z=0 OVII/VIII absorption (PKS 2155-304, 3C 273, Mrk 421, H 1821)

37. PKS 2155-304 HVC • Nicastro et al. (2002) detect O VII/VIII absorption in Chandra X-ray spectra. -500 0 500 LSR Velocity (km/s)

38. PKS 2155-304 HVC • Nicastro model the O VI/VII/VIII and find: n ~ 5 x 10 cm --> 1st detection of WHIM!? • This work ignores known detections of Si II and C IV (Sembach et al. 1999) • We have data for PKS 2155-304: • HST-STIS data: 1150-1700 Å • FUSE data -6 -3

39. PKS 2155-304 HVC O VI 1031.93 Å C II 1334.53 Å C III 977.02 Å C IV 1548.20 Å Si II 1193.29 Å Si III 1206.50 Å Si IV 1393.76 Å (Collins et al. 2004) --> detections in O VI, C II/III/IV, and Si II/III/IV

40. Ionization Modeling CLOUDY photoionization models (assume AGN background) • C II, Si II detections imply nH > 10 cm but, nWHIM ~ 10 cm !!! • O VI, C IV, Si IV are underpredicted  collisional ionization (shocks, conductive interfaces, etc) –3.5 –3 -6 -3

41. PKS 2155-304 HVC • Compare to Complex C where, n = 0.01 - 0.1 cm and, C IV, Si IV, O VI are collisonally ionized --> similar ionization pattern • PKS 2155-304 highly ionized HVC • ionization pattern is characteristic of halo gas • same population as H I HVCs at lower N(H) -3 [PG 1259+593 sight line; Fox et al. (2003)]

42. Survey of Highly Ionized HVCs Highly Ionized HVCs from Sembach et al. (2003) • FUSE data with S/N > 5 • HST-STIS (public April 2004) • E140M • G140M

43. The Sight Lines PG 1116+215 PHL 1811 PKS 1302-102 Ton S180 UGC 12163 3C 273 HE 0226-4110 MRC 2251-178 Mrk 509 Mrk 1513 PG 0953+414 And, PKS 2155-304 (Sembach et al. 1999; Collins, Shull, & Giroux 2004)  12 sight lines  Can consider whether these objects are Galactic halo HVCs or WHIM

44. The Sight Lines PG 1116+215 PG 0953+414 3C 273 PKS 1302-102 M.W. motion UGC 12163 Mrk 1513 L.G.B. Mrk 509 PHL 1811 PKS 2155-304 MRC 2251-178 HE 0226-4110 Ton S180 (Collins et al. 2005)

45. PG 1116+215 O VI 1031.93 Å N V 1238.82 Å C IV 1548.20 Å Si IV 1393.76 Å Si III 1206.50 Å C III 977.02 Å Si II 1526.71 Å O I 1302.17 Å

46. Ton S180 O VI 1031.93 Å N V 1238.82 Å C III 977.02 Å Si III 1206.50 Å C II 1036.34 Å Si II 1260.42 Å

47. PKS 1302-102 O VI 1031.93 Å N V 1238.82 Å C IV 1548.20 Å Si III 1206.50 Å C II 1334.53 Å Si II 1260.42 Å

48. Survey Results • Low ion (CII, SiII) detections in 9 of 12cases: • imply photoionization with log nH > –3.5 • are inconsistent with WHIM log nH = –5 to –6 • High ions (CIV, SiIV, OVI) • share similar kinematics with the low ions • arise via collisional ionization at cloud interface • Only 2 HVCs are detected solely in O VI • cannot rule out WHIM origin • multiphase objects similar to Galactic halo HI HVCs, albeit at low-Ntotal(H)

49. Conclusions • HVCs play a role in Galaxy evolution • Complex C • Metallicity, Z = 0.10.25 Zsolar • infalling cloud mixing with Galactic gas • FUSE Cycle 5: trace signatures of gas mixing • Role of hot gas: C IV, Si IV, O VI • Highly Ionized HVCs • trace low column density halo HVCs • C II, Si II > densities too high to trace WHIM • Investigate Galactic anti-center HVCs (FUSE Cycle 6 proposal pending)