Magellanic Stream as a template for galaxy evolution Snežana Stanimirović (UW Madison)

1. Magellanic Stream as a template for galaxy evolutionSnežana 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. Galaxies grow mainly via accretion Dekel et al. 09 • 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. Keres et al. 08

3. The Magellanic Stream: Velocity Field:400 (Clouds) to -400 (tip) km/s b=-25 b=-50 SMC GALFA-HI image: 3’ resolution, N=3x1018 cm-2 GALFA = Galactic science with the Arecibo L-band Feed Array (ALFA) LMC Putman et al. (2003)

4. From 100 to ~200-deg long Stream 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 SMC LMC Putman et al. (2003)

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

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

8. Cloud properties Size (arcmin) Peak HI column density N(HI) ~1x1019 cm-2 Stanimirovic et al. 08 • 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.

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. 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. Multi-phase clouds: Column density and Mach number • Single-phase • Multi-phase • Multi-phase clouds prefer higher HI column densities, 1.5-4x1019 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)

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. Mystery of cloud survival? Heitsch & Putman 09 HVCs dropped in an isothermal halo with n~10-4 cm-3 at 10 kpc disintegrate quickly. • Fragments have: M(HI) = 100 – 104 M; <Mach number> ~ 0-2 • HVCs dropped at z=10 kpc can travel for ~100 Myr. • Replenishment rate of ~2-0.4 M/yr -- large! ~2x109 Mover 1 Gyr required • Something must slow down this process! • Substantial stabilizing Stream-halo interface?

14. Constraining Stream-halo interfaces • Lou Nigra’s PhD at UW-Madison (+ Gallagher, Lockman, Nidever, Majewski) • GBT observations, most sensitive to date, σ=1x1017 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 velocity field HI column density N=3-5x1018 cm-2 Not possible to see in previous observations Nigra et al. 09

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. How effective are dynamical instabilities? 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.

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

18. Kinematics of the Stream-halo interface: “Cylindrical cow” or stacking analysis -385 Reaching down to ~1017 cm-2 “Interface” slowed down behind the center of the cylinder. Detailed profile comparison with models in progress. Nigra et al. -305 RA

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. Thank you !