1 Magellanic Stream as a template for galaxy evolution Snežana Stanimirović (UW Madison) Outline: Latest observational results: extension and small-scale.

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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

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• 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

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The Magellanic Stream: Velocity Field:

400 (Clouds) to -400 (tip) km/s

SMC

Putman et al. (2003)

GALFA-HI image: 3’ resolution, N=3x1018 cm-2

GALFA = Galactic science with the Arecibo L-band Feed Array (ALFA)

LMC

b=-50

b=-25

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SMC

Putman et al. (2003)

LMC

Data:LAB surveyBruns et al. 05Braun & Thilker 04Stanimi. et al. 08Westm. & Koribal. 08Nidever et al. 09

Nidever et al. 09,submitted

From 100 to ~200-deg long

Stream

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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?

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ram pressure

old

tidal

new

tida

l

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

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Velocity gradient along the Stream

ram pressure

old

tidal

new

tida

l

LMC

X Putman03∆ GALFA

Gravity is important for large-scale structuring and kinematics of gaseous flows.

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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) ~1x1019 cm-2

Size (arcmin)

Stanimirovic et al. 08

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~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

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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?

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Multi-phase clouds: Column density and Mach number

• 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)

-Single-phase-Multi-phase

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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.

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Mystery of cloud survival?

• 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 Mover 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.

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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 column density HI velocity field

N=3-5x1018 cm-2

Not possible to seein previous observations

Nigra et al. 09

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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.

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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?

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Kinematics of the Stream-halo interface: “Cylindrical cow” or

stacking analysis

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-385

-305

Reaching down to ~1017 cm-2

“Interface” slowed downbehind the center of the cylinder.

Detailed profile comparisonwith models in progress.Nigra et al.

RA

Kinematics of the Stream-halo interface: “Cylindrical cow” or

stacking analysis

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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.

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Thank you !