1. 1 Magellanic Stream as a template for galaxy evolution Snežana Stanimirović (UW Madison) Outline:  Latest observational results: extension and small-scale structure of the Stream  Small-scale HI structure of the MS: “Gastrophysical” processes in the Galactic halo  Implications for accreting flows in general

2. 2 Even at z=0 accretion is very important “Hot” accretion ~ cold accretion at z~0. Large galaxies esp accrete from satellites. What are physical properties of accretion flows? How much do galaxy halos flavor accretion flows? How much would actually reach the disk? Magellanic Stream is the closest gaseous halo stream. Galaxies grow mainly via accretion Keres et al. 08 Dekel et al. 09

3. 3 The Magellanic Stream: Velocity Field: 400 (Clouds) to -400 (tip) km/s SMC Putman et al. (2003) GALFA-HI image: 3’ resolution,  N=3x10 18 cm -2 GALFA = Galactic science with the Arecibo L-band Feed Array (ALFA) LMC b=-50 b=-25

4. 4 SMC Putman et al. (2003) LMC Data: LAB survey Bruns et al. 05 Braun & Thilker 04 Stanimi. et al. 08 Westm. & Koribal. 08 Nidever et al. 09 Nidever et al. 09, submitted From 100 to ~200-deg long Stream

5. 5 Latest observational (HI) results:  The Stream is significantly more extended than previously thought: WSRT+ GALFA-HI + HIPASS + GBT [Stanimirovic et al. 08, Westmeier & Koribalski 08, Nidever et al. 09]  The northern Stream has a significant abundance of small- scale HI structure. Several filaments + clouds.  Why is this important? (i) How much of the hidden low-density “fluff” in the Galactic halo has yet to be discovered? Missing baryons problem. (ii) What shapes the large-scale structure of the MS? What is this telling us about the orbits of the Clouds? E.g. “interaction time”. (iii) What shapes the small-scale structure of the MS? How does the Stream, and accretion flows in general, age?

6. 6 ram pressure old tidal new tidal  LMC X Putman03 At the MS tip it’s much easier to distinguish btw diff. models: models significantly different + the MS has smaller spread Predicted velocity gradient along the Stream

7. 7 Velocity gradient along the Stream ram pressure old tidal new tidal  LMC X Putman03 ∆ GALFA Gravity is important for large-scale structuring and kinematics of gaseous flows.

8. 8 Cloud properties Angular size: peaks ~10’. 90% of clouds have size 3-35’. In agreement with expectations for thermal fragments @ 60-70 kpc. Thermal (dynamical) instabilities are important for structuring gaseous flows. Peak HI column density N(HI) ~1x10 19 cm -2 Size (arcmin) Stanimirovic et al. 08

9. 9 ~15% of clouds have multi-phase (warm & cold gas) structure “Cold cores”: FWHM ~13 km/s (range 3 to ~20 km/s) “Warm envelopes”: FWHM ~25 km/s Matthews et al. 2009: cold HI, T~70 K  Stream has multi- phase medium Kalberla & Haud 06: 27% of sight lines have multi- phase structure at positive Stream velocities. Gaseous flows at significant distances can have multi-phase medium

10. 10 Conditions for the existence of the multi-phase medium? Wolfire et al. (1995): multi-phase clouds pressure confined by the hot halo can exist at distances <20 kpc. Sternberg et al. (2002): multi-phase clouds confined by dark matter can exist at distances <150 kpc. Expected: P = 30-300 K cm-3 Measured: P = 500 - 2000 K cm-3 Model underestimates Halo pressure. Reconsider conditions (Halo properties & phase conversion) for multi-phase medium in the Halo?

11. 11 Multi-phase clouds: Column density and Mach number Multi-phase clouds prefer higher HI column densities, 1.5-4x10 19 cm -2. Turbulent Mach number = motion of cold cores inside warmer envelopes = 0 to 2 subsonic/transonic  not very turbulent, no strong internal dynamics (e.g. CNM in the MW has Mach>3) -Single-phase -Multi-phase

12. 12 Gravitational confinement? At dist = 60 kpc, M(grav) ~ 100-1000 x M(HI)  Gravitational confinement would require unreasonable amount of dark matter. If in free expansion, mean expansion time < 10 Myr, very short.  If clouds are stable & long-lived, pressure confinement the easiest explanation.

13. 13 Mystery of cloud survival? Fragments have: M(HI) = 10 0 – 10 4 M  ; ~ 0-2 HVCs dropped at z=10 kpc can travel for ~100 Myr.  Replenishment rate of ~2-0.4 M  /yr -- large! ~2x10 9 M  over 1 Gyr required Something must slow down this process! Substantial stabilizing Stream-halo interface? Heitsch & Putman 09 HVCs dropped in an isothermal halo with n~10 -4 cm -3 at 10 kpc disintegrate quickly.

14. 14 Constraining Stream-halo interfaces Lou Nigra’s PhD at UW-Madison (+ Gallagher, Lockman, Nidever, Majewski) GBT observations, most sensitive to date, σ=1x10 17 cm -2 Small-scale HI: head-tail clumps and narrow filaments transverse to the main filament and lagging in velocity. Gas streamers and coherent structures expected for dynamical instabilities HI column density HI velocity field N=3-5x10 18 cm -2 Not possible to see in previous observations Nigra et al. 09

15. 15 How effective are dynamical instabilities? Bland-Hawthorn et al. 07 Shocks destroy low-N gas and eat into the high-N gas. At the tip of the Stream ablation should be the strongest.

16. 16 Bland-Hawthorn et al. 07 Excess of both low and high-N material relative to the model. PDF almost Gaussian, not highly-peaked. Suggests that ablation rate is slower than what predicted. How effective are dynamical instabilities?

17. 17 Kinematics of the Stream-halo interface: “Cylindrical cow” or stacking analysis

18. 18 - 385 - 305 Reaching down to ~10 17 cm -2 “Interface” slowed down behind the center of the cylinder. Detailed profile comparison with models in progress. Nigra et al. RA Kinematics of the Stream-halo interface: “Cylindrical cow” or stacking analysis

19. 19 Summary :  The Magellanic Stream is a laboratory for understanding the aging process of gaseous accretion flows.  Stream is more extended than previously thought. Abundance of filaments and small HI clouds.  Gravity dominant for large-scale HI structure.  Small-scale HI structure: evidence for thermal & dynamical instabilities, yet “calmer”, multi-phase and longer-lived environment.  Extended low column density Stream-halo interface may be a stabilizing agent. Deep radio observations show broad Gaussian N(HI) PDF with lagging velocities.  Detailed profile analysis under investigation by Lou Nigra.

20. 20 Thank you !