1. Gas in the Local Group James Binney & Filippo Fraternali Oxford University

2. Outline Missing baryons Infall and HVCs Extraplanar gas in external galaxies The hot halo Conclusions

3. Missing baryons Negative v los of M31 ) M LG =4.8£10 12 M ¯ (Kahn & Woltjer 59 ff) b / m =0.17 (Spergel et al 03) If M M 31 '1.5M MW (cf Wilkinson & Evans 99) But L V (MW) ' 1.5£10 10 L ¯, so M * ' 3-5£10 10 M ¯ Implies most baryons missing Klypin, Zhao & Somerville (02) have M MW =10 12 M ¯ and half baryons missing

4. Still infalling? Muller Oort & Raimond (63) found HI at highly anomalous velocities HVCs mapped at ever higher sensitivity ! Leiden- Dwingeloo (Hartman & Burton 1997) & HIPASS (Barnes et al 01) surveys Are HVCs distant & massive? (Oort 70; Blitz et al 99) Efforts to detect massive extragalactic clouds in other groups repeatedly failed (Pisano & Wilcots 03) Clouds usually have detectable H emission (Tufte et al 02; Putman et al 03)

5. Extraplanar gas Some HVCs associated with LG galaxies (Magellanic Stream; Andromeda clouds) Most are within MW and of low mass (Westmeier 03) Extend to N<10 19 cm -2 at which HI hard to detect (Hoffman et al 04; Richter et al 05) Significant covering factor Have complex shapes (Richter et al 05) Local clouds show net infall v ' 50 km/s (de Heij et al 02; Wakker 04)

6. Outside view Counterparts of HVCs now studied in external galaxies (M101: van der Hulst & Sancisi; NGC 5668: Schulman et al 94-6; NGC 891, NGC 2403: Swaters et al 97 ! Fraternali, Oosterloo & Sancisi 04)

7. Extra-planar gas in NGC 891 Sancisi & Allen 1979 N H 5 10 20 cm -2 Swaters et al. 1997 N H 7 10 19 cm -2 Oosterloo et al. 2005 N H 1.7 10 19 cm -2 Sancisi & Allen 1979 N H 5 10 20 cm -2 Swaters et al. 1997 N H 7 10 19 cm -2 Oosterloo et al. 2005 N H 1.7 10 19 cm -2 Sancisi & Allen 1979 N H 5 10 20 cm -2 Swaters et al. 1997 N H 7 10 19 cm -2 Oosterloo et al. 2005 N H 1.7 10 19 cm -2

8. NGC891: Low rotation of extra-planar gas Fraternali 2005 v rot ~15 km s -1 kpc -1

9. NGC 2403. Distance: 3 Mpc. Type: Sc. Inclination ~ 62. Non-interacting. Very similar to M33

10. NGC2403: Extra-planar gas Extra-planar gas 130 km/s Forbidden gas Fraternali, Oosterloo, Sancisi, van Moorsel 2001 Thin disc model

11. NGC2403: Non circular motions Thin discExtra-planar gas V Lagging halo Thin disc

12. Non-circular motions

13. NGC 6946: Extra-planar gas and SF Boomsma PhD 2005 WRST

14. Summary (observations) Extra-planar detected up to 15 kpc from plane Rotation lower than the disc High velocities (100-200 km s -1 ) Global inflow motion Link with star formation? Evidence for accretion?

15. How common is halo gas? Halo gas (HI) found and studied in 7 galaxies: NGC891, N2403, N6946, N253 (Boomsma et al. 2005), N4559 (Barbieri et al. 2005), UGC7321 (Matthews & Wood 2003), NGC2613 (Irwin & Chaves 2003).

16. Extra-planar gas and star formation Matthews & Wood 2003, ApJ UGC 7321 ROSAT HI Optical VLA NGC 253 - ATCA Boomsma et al. 2005, A&A SFR=0.01 M yr -1 SFR>10 M yr -1

17. Dynamical models A barotropic [p=p( )] fluid in a gravitational field corotates ( Poincaré, 1893 ) Hydrostatic models for non-barotropic fluid show gradient in rotation velocity but also high temperatures ( Barnabé, Ciotti, Fraternali, Sancisi, A&A, submitted ) Previous works: Galactic fountain: gas circulation (disc-halo-disc) ( Shapiro & Field, ApJ 1976; Bregman, ApJ 1980 ) Ballistic models: disagreement between predicted gradient in rotation velocity and H data ( Collins, Benjamin & Rand, A&A 2002 )

18. Fountain model (Shapiro & Field, ApJ 1976; Bregman, ApJ 1980) Clouds ejected from circular orbits with distributions in v, Axisymmetry exploited to build pseudo-data cube New work (Fraternali & B 05): Clouds move ballistically as in Collins, Benjamin & Rand, A&A 02, but may not be visible until z max or r max Clouds return to disk on first or second passage through z=0 <4% of SN energy needed

19. Dynamical model Continuous flow of particles from the disc to the halo Initial conditions: distribution of kick velocities Potential: exponential discs + bulge + DM halo Integration in the (R,z) plane, then distribution along At each dt projection along the line of sight Stop at the first or second passage through the disc Pseudo-cube to be compared with HI data cube

20. Model constraint: vertical distribution V kick ~ 75 km s -1 M halo ~2 10 9 M

21. N891: inflow/outflow Travel times Energy input <4 % of energy from SNe

22. NGC 891: Lack of low angular momentum Fast rotating gas NEED FOR LOW ANGULAR MOMENTUM MATERIAL

23. NGC2403: lagging gas Thin disc Thick disc 60 o V kick ~ 70 km s -1 M halo ~ 5 10 8 M

24. NGC2403: inflow/outflow Thin disc gas Extra-planar gas Radial outflow NEED FOR INFALLING MATERIAL V VRVR VzVz

25. Second-passage models V VRVR VzVz V VRVR VzVz

26. Phase-change models NGC 2403 NGC 891Fast rotating gas

27. Phase-change models Vertical motions

28. N2403: substructures

29. Inside view

30. High Velocity Clouds Complex A M 10 6 M O d 8-10 kpc v 100 km s -1 Wakker et al. 2003; Wakker & Van Woerden 1997 Complex C v 100 km s -1 If d 10 kpc M 10 7 M O Low metallicity Z=0.1-0.3 solar (Tripp et at. 2003) Forbidden gas v 100 km s -1 M 5 · 10 6 M O Filament v 80 km s -1 M 10 7 M O

31. Summary (models) Models reproduce the vertical extent with reasonable energy input (<4 % SN energy) Failure in NGC2403: lack of inflow Need for accretion Failure in NGC891: lack of low angular momentum Need for drag Seen from inside, a successful cloud model would look like HVC population But must reverse outflow and diminish rotation

32. The WHIM CDM simulations without feedback suffer from overcooling Natural solution: fast mass loss during GF Direct evidence from M outflow ' M SF (Pettini et al 01; Steidel et al 04) Also manifest connection of outflow to HVCs (NGC 6946 and …) So expect accumulation of gas @ NGC 253 Boomsma et al 05

33. The hot halo Munch (52) detected Ca II and Na I interstellar lines at |v-v LSR |>20 km/s even at high b Spitzer (56) argued that cold absorbing clouds must be confined by pressure p/k B '10 4 K cm -3 of gas with T' T vir At T vir, M gas = 0.52£10 9 (R max /R 0 ) M ¯ So CDM requires M>10 11 M ¯ halo to extend to R max '1Mpc

34. Copernicus, HST and FUSE detect absorption in C IV, O VI, etc O VI important because ionize E(O V)=114eV; O VI emission peaks @ T = 3£10 5 K

35. HI emission & O VI absorption Consistent with O VI at interface of HI and WHIM Possible evidence that O VI expanding relative to HI Sembach et al 02

36. Interaction of HVCs with WHIM Density contrast T vir /T HI ' 100-10 4 Analogous to a transonic sprinkler Ram-pressure drag (Benjamin & Danly 97) = 21 N 19 /(n -3 v 200 ) Myr T flight ' 100 Myr Drag important

37. Evidence for drag Structure of leading arm of Magellanic stream Head-tail structure of HVCs (Bruns et al 01) Z < Z ¯ for complex C HVC CHVCsPutman et al 03

38. Problems Fountain circulates large mass through extraplanar gas: M HI ' 5£10 8 M ¯ every 100 Myr If ejected gas loses 10% of its angular momentum, halo will become corotating if not extensive (M gas = 5£10 8 (R max /R 0 ) M ¯ ) Naively expect moving clouds to be ablated Net inflow and low Z (10% Z sun ) imply condensation prevails

39. Conclusions CDM predicts that most baryons are hidden Observations of external groups & galaxies show that HVCs lie at 10 – 100 kpc distances HVCs are generated by star formation The basic fountain model does not reproduce: lag in rotation & net infall Much evidence for interaction of HI with WHIM Likely that lag & infall result from interaction with WHIM LCDM predicts that WHIM contains bulk of LG baryons & extends to > 1Mpc