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The tip of the Magellanic Stream: GALFA’s view Snežana Stanimirović (UW Madison)

The tip of the Magellanic Stream: GALFA’s view Snežana Stanimirović (UW Madison). Collaborators: Carl Heiles (UCB), Mary Putman (Michigan), Josh G. Peek (UCB), Steven Gibson, Kevin Douglas, Eric Korpela (part of GALFA collaboration). Outline: GALFA in a nutshell

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The tip of the Magellanic Stream: GALFA’s view Snežana Stanimirović (UW Madison)

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  1. The tip of the Magellanic Stream: GALFA’s viewSnežana Stanimirović(UW Madison) Collaborators: Carl Heiles (UCB), Mary Putman (Michigan), Josh G. Peek (UCB), Steven Gibson, Kevin Douglas, Eric Korpela (part of GALFA collaboration)

  2. Outline: GALFA in a nutshell A few flavors of GALFA’s Science: 1. Cold HI clouds in the Galactic disk/halo interface region 2. Spectacular HVC/Halo Interfaces, & new HVCs Zooming in on what goes on in the Galactic Halo? The “many streams” of the Magellanic Stream Summary

  3. ALFA = Arecibo L-band Feed Array GALFA = Galactic Science with ALFA Find more @ www.naic.edu

  4. (On-going since 2005)GALFA-HI survey: • 12,734 deg2 @ 3.5’, v=0.2 km/s, S~0.1K • Primarily observing commensally with e-gal & continuum surveys. • Smooth, stream-lined observations, • successful combination of data • from many GALFA projects. GALFA = Galactic Science with ALFA (www.naic.edu/alfa/galfa)

  5. A new way of running large surveys: commensal (parallel) observing • Dedicated spectrometer • Scans: basket and drift + special calibration • Combine images from various projects Galactic Survey: Extragalactic survey:

  6. GALFA’s “art”:filling in the jigsaw puzzle GALFA-HI consists of many individual projects Effective integration time per pointing TOGS = Turn On Galfa Spectrometer, in || with e-gals survey TOGS TOGS

  7. Why is Arecibo + ALFA so special for Galactic science ? A very unique combination: Sensitivity Resolution (3.5’) Full spatial frequency coverage simultaneously AC0 HVC -- LDS AC0 HVC -- GALFA …especially makes difference at high Galactic latitudes…

  8. Local HI (peak brightness image)

  9. Galactic extra-planar gas in the inner and outer Galaxy: zooming in on the Galactic disk-halo interface region Stanimirovic et al. (2005)

  10. High angular and velocity resolution: opening a new parameter space for Galactic science • size: 4’-12’ • v: 2-4 km/s • Vlsr: -20 km/s but “follow” disk HI @ 3’ @ 36’ Too small to be seen in low-res. surveys…

  11. Zooming in on Cloud/Halo Interfaces: Peek et al. (2007) Torn-off ‘condensations’ de-accelerated by ram pressure.

  12. A lot of “fluff” with N(HI)<1018 cm-2 lurking in the MW halo GALFA observations: Peek et al., in preparation

  13. The many streams of the Magellanic Stream Stanimirovic et al. [2008, (ApJ) astro-ph/0802.1349]

  14. Galactic Halo Artist: Jon Miller Artist: Ron Miller

  15. Main Stream Specs: > 40 yr of research and still lot of unknowns • First HI detection of the Stream in 1965, Dieter (1965). • Long, thin filament (100º x 10º) with a bead-like sequence of clouds (Mathewson et al. 1974). • No stars found so far in the Stream • The largest diffuse cloud known (9 x 107 Solar masses)! • The only clear example of a gaseous halo stream in the MW’s proximity.

  16. Parkes HI observations (15’ resolution) Tip of The Stream Stream LMC Bridge SMC Leading Arm Putman et al. (2003)

  17. Observational Perspective • From 6 discrete concentrations, MSI (close to the Clouds) to MSVI (at the tip) [Mathewson et al. 1974] •  • Network of filaments and clumps with two double-helix-like features: fully-sampled Parkes Multibeam surveys [Putman et al. 03, Bruns et al. 05] • Chaotic appearance around Dec ~0 deg, dying off. • Only high-resolution, Arecibo, view: Stanimirovic et al. (2002). • Braun & Thilker (2004): Westerbork observations suggested that the MS has a significant northern extension.

  18. Theoretical Perspective: two competing ideas • Tidal origin theory: gravitational stripping of the SMC gas by the Galaxy. Stream is 1.5 Gyr old and is trailing on an almost polar orbit at 65 kpc. [Gardiner & Noguchi 96; Connors et al. 06] • Ram Pressure origin theory: ram-pressure stripped gas from MCs by an extended diffuse halo around the Galaxy. Stream is 0.5-1 Gyr old, and is falling into the Galaxy [Moore & Davis 94; Mastropierto et al. 05] • Models focus on reproducing observed features: distribution of HI column density and velocity field.

  19. How far away is the tip of the Magellanic Stream? • Tidal models: • early models: 60-70 kpc; • latest model: + a more distant component at 170-200 kpc. • Ram pressure models: distance declining along the Stream to 25 kpc. • In all models, tip is the oldest portion of the MS, and represents the gas originally pulled out of the Clouds. • Age of the MS (in most models) ~1 Gyr.

  20. Recent excitement: Can we track down where do the Stream filaments come from? Nidever et al. 07 • The SMC ? • The SMC and the Magellanic Bridge ? • Nidever et al. 07: one filament originates from an over-density • in the LMC (blown out by star formation?)

  21. Latest orbits: Clouds are not bound ! New (HST) LMC & SMC’s proper motions, Kallivayalil et al. 06. New Clouds’ orbits, Besla et al. 07  LMC is only on its 1st passage around the MW! Neither tidal or ram pressure stripping would have had enough time to produce the MS --> new ideas needed! LMC

  22. Studying “gastrophysical” processes in the MW halo • To what extent interactions btw the Stream and the MW halo determine the structure of the Stream gas? • What is the origin of Stream filaments? • What does the Stream tell us about the properties of the MW halo? • Can shocked and ionized Stream gas represent new fuel for Galactic accretion? • How does the Stream influence gaseous structure in the MW disk? • Is the Stream being continuously replenished? At what rate? Which effect this has on the Magellanic Clouds?

  23. The Magellanic Stream: Velocity Field:400 (Clouds) to -400 (tip) km/s SMC LMC GALFA-HI image: ~900 deg2 ! N=3x1018 cm-2 (3-sigma, v=20 km/s) Putman et al. (2003)

  24. GALFA observations The tip of the Stream:HI integrated intensity • Several streams! • Coherent, large, continuous “streams” (S1 to S4) up to Dec~25deg. • Confirm significant extension of the MS • (Braun & Thilker ‘04) • But also lots of small discrete HI clouds!

  25. Observed velocity gradients and stream morphologies Moderate velocity gradient Steep velocity gradient S1 S2 S3 S4 Moderate velocity gradient No velocity gradient • S2, S3 & S4 show gradual decrease in velocity gradient. • S1 diffuse; S2, S3 & S4 have clumpy morphology and similar spatial origin. • S1 could be more recently formed from the Bridge. Less clumpy, so significantly younger (distance?).

  26. Connors et al. (2006): detailed spatial structure of the Stream Leading arm 2 younger streams Main ‘stream’ bifurcated Bridge SMC Stream Very distant stream

  27. Connors et al. 06: summary • General gas distribution in the Magellanic System, plus the spatial and kinematic bifurcation of the MS, can be reproduced by purely gravitational interactions. • Main MS filament formed 1.5 Gyrs ago in the main encounter btw the SMC, LMC and the MW. Further ‘tidal kicks’ from encounters with the LMC 1.05 and 0.55 Gyrs ago resulted in spatial, then kinematic bifurcation. • A very distant part of the MS, formed 2.2 Gyrs ago in an encounter btw the SMC and the LMC, is at a distance of 170-220 kpc. • Two tidal tails drawn <200 Myr ago from the Bridge follow the main MS filament along most of its length.

  28. Where do the new streams come from: observations vs simulations • Organized, large-scale structure at the tip suggests: • - S2, S3 & S4: 3-way splitting of the main MS filament • - Gas had enough time to cool and fragment. • - S1: formed more recently from the Bridge. • - Less clumpy, so significantly younger (distance?). • Observational picture is far more complicated, but comparison with the tidal model is encouraging. Spatial splitting: A big + for tidal models.

  29. But there is all this clumpy structure…. Samantha’s catalog of ~180 clouds: N(HI), angular size, velocity profiles. Cross-correlated with catalogs of HVCs and mini-HVCs  “purely” MS sample Samantha Hoffman UW undergrad. student

  30. Cloud properties Size (arcmin) Peak HI column density N(HI) ~1x1019 cm-2 Angular size: peaks ~10 arcmins. 90% of clouds have size 3-35’.  characteristic size?! Even from images: large abundance of small, compact clouds.

  31. Number of clouds/observed area Central velocity (km/sec) Cloud properties Angular size (‘) Gal. Latitude (deg) Gal. Latitude (deg) Gal. Latitude (deg) • Number of clouds decreases steeply towards the MS tip. • Clouds with angular size <20’ mainly close to the Clouds. •  Possible increase in distance along the Stream.

  32. Cloud properties Central velocity (km/sec) Two central velocity peaks (at -405 and -350 km/s); not a selection effect.  kinematic bifurcation as suggested by Connors et al.

  33. ~15% of clouds have multi-phase (warm & cold gas) structure • “Cold cores” with FWHM ~13 km/s • Tk < 1000-1500 K. • “Warm envelopes” with • FWHM ~25 km/s • Kalberla & Haud 06: 27% of sight lines have multi-phase structure at positive Stream velocities. • Wolfire et al. (1995): • “We predict that no cold cores are • expected at z>20 kpc in a • T = 106 K halo.”

  34. What physical processes are responsible for clumpy morphology? 1. Thermal Instability: warm gas cools, becomes thermally unstable and fragments --> characteristic fragment size. For typical warm gas (T~8000 K) with a typical SMC volume density, then expected size is (cool) ~100-200 pc, timescale ~20-30 Myr ().  Thermal instability will have a significant effect on MS structure. 2. Kelvin-Helmholtz (KH) instability: warm stream moving through a hot ambient medium will develop instability at the interface region and fragment. Timescale is ~ 1 Gyr, most likely not important for the Stream. 3. Ram pressure: surprisingly no cometary or head-tail structures indicative of ram pressure. Most likely a secondary effect, gravity dominates.

  35. Let’s say we make small fragments through TI, but can they survive in the hot MW halo? YES! Classical evaporation theory (McKee & Cowie 1977): “Critical radius” for stable clouds is ~200 pc. Clouds are evaporating, but this process takes about ~1 Gyr. We expect 106 K halo gas. Sembach et al. 03 found OVI: evidence for ionized gas surrounding the MS with T<106 K. Bottom line: a warm tail of gas tidally pulled from the Clouds will quickly become thermally unstable and start to fragment into smaller condensations. These condensations will be evaporating but can stick around for a long time though. This simple picture could explain the very clumpy morphology we observe.

  36. Can clumpy structure constrain the distance of the MS tip? • TI could be the dominant structure-shaping agent. • Thermal fragments of ~200 pc in size require a distance of ~70 kpc to explain the observed angular size of clouds (~10’). • 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. •  The MS tip can not be too distant, <~150 kpc. •  Need to reconsider conditions for multi-phase medium and pressure of the MW halo.

  37. Independent constraint: Jin & Lynden-Bell (astro-ph/0711.3481) GC 75 kpc • Geometrodynamical model: • the stream is in the plane containing the G. center • energy & momentum are conserved along the stream. •  The tip of the Stream is 70-75 kpc from the G. center M. Clouds Tip

  38. Evidence for evaporating Stream clouds? STIS and FUSE observations (Fox et al. 2005)

  39. STIS and FUSE observations Metallicity ~ what is found in the Stream. Cloud tidally stripped from the main body of the Stream and ionized by the pervading radiation field of the the Milky Way.

  40. Sembach et al. (2001) Metallicity ~ Leading Arm of the MS. H2 clump (T~200 K) tidally stripped from the SMC.

  41. Summary: • GALFA is surveying the Galaxy with high angular and velocity resolution. Completion date = mid 2011. • Diverse and rich science case + legacy products for the astronomy community at large. • The tip of the Magellanic Stream consists of several “streams”. • Evidence for spatial and velocity “bifurcation” gives support to the tidal model by Connors et al. 06. • The clumpy HI structure of the Stream can be interpreted (at least partially) as being due to thermal instability. If this is the major shaping process, then the tip is at a distance of ~70 kpc.

  42. Thank you !

  43. Science highlights: 2. Spectacular HVC/Halo Interfaces

  44. CHVC186+19-114: caught while breaking up • Clear velocity gradient • De-acceleration by ram-pressure? • Rotation? • Part of a larger complex? (1018) “Companion cloud”: one of the smallest HVCs, 7’x9’, Ultra Compact HVC [N(HI)=5x1019 cm-2] (arcmin)

  45. Cloud “shreds” can be classified as Mini- and Ultra-Compact HVCs 3. Compact HVC ~30’ HVC ~deg 2. Hybrid HVC Putman et al. (1999) 4. Mini HVC 7-10’ 5.Ultra-Compact HVC ~4’ • Is this a real sequence? • What defines diff. HVCs? • How many MHVCs, UCHVCs are there, what are they? Hoffman (2004) Bruns & Westermeier (2004)

  46. Cloud/Halo Interaction: Theoretical Perspective 10-4 cm-3 10-5 cm-3 Quilis & Moore (2001) Ingredients: Halo properties, dark matter, magnetic field. Future: compare observations with models (Power, Putman) --> Halo density.

  47. Almost continuous distribution of cloudy structure from the disk to the intermediate-velocity gas

  48. If in pressure confinement, then Halo density ~10-3 cm-3 at z~50 kpc.  Stanimirovic, Dickey et al. (2002)

  49. Science highlights: 1. Cold HI clouds in the Galactic disk/halo interface region

  50. High latitude HI at 3’: ‘Fingers’ @mild forbidden velocities streaming out of the Gal. Plane b~20 b~12 “low-velocity clouds” b~5 Galactic Plane l~183

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