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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)

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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

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GALFA = Galactic Science with ALFAFind more @ www.naic.edu

ALFA = Arecibo L-band Feed Array

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GALFA = Galactic Science with ALFA

(www.naic.edu/alfa/galfa)

(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.

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A new way of running large surveys: commensal (parallel)

observing• Dedicated spectrometer• Scans: basket and drift +

special calibration• Combine images from various

projects

Extragalactic survey:

GalacticSurvey:

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GALFA’s “art”: filling in the jigsaw puzzle

GALFA-HI consists of many individual projects

TOGS

TOGS

Effective integration time per pointingTOGS = Turn On Galfa Spectrometer,

in || with e-gals survey

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Why is Arecibo + ALFA so special for Galactic science ?

A very unique combination:

1. Sensitivity2. Resolution (3.5’)3. Full spatial frequency

coverage simultaneously

AC0 HVC -- LDSAC0 HVC -- GALFA

…especially makes difference at high Galactic latitudes…

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Local HI (peak brightness image)

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Stanimirovic et al. (2005)

Galactic extra-planar gas in the inner and outer Galaxy: zooming in on the Galactic disk-

halo interface region

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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…

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Peek et al. (2007)

Torn-off ‘condensations’de-accelerated by ram

pressure.

Zooming in on Cloud/Halo Interfaces:

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A lot of “fluff” with N(HI)<1018 cm-2 lurking in the MW halo

GALFA observations: Peek et al., in preparation

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The many streams of the Magellanic Stream

Stanimirovic et al. [2008, (ApJ) astro-ph/0802.1349]

14Artist: Ron Miller

Artist:Jon Miller

Galactic Halo

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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.

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Parkes HI observations (15’ resolution)

StreamBridge

Leading Arm

SMC

LMC

Tip ofThe Stream

Putman et al.(2003)

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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.

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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.

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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.

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Recent excitement: Can we track down where do the Stream filaments come

from?

- 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?)

Nidever et al. 07

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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

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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?

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The Magellanic Stream: Velocity

Field:400 (Clouds) to -400

(tip) km/s

SMC

Putman et al. (2003)

GALFA-HI image: ~900 deg2 !N=3x1018 cm-2 (3-sigma, v=20 km/s)

LMC

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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!

GALFA observations

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Observed velocity gradients and stream morphologies

• 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?).

Steep velocity gradient

No velocity gradient

Moderate velocity gradient

Moderate velocity gradient

S1 S2

S3 S4

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SMC

Bridge

Leading arm

Stream

Main ‘stream’bifurcated

Very distant stream

2 younger streams

Connors et al. (2006):

detailed spatialstructure of the Stream

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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.

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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.

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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 HoffmanUW undergrad.

student

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Cloud properties

Angular size: peaks ~10 arcmins.90% of clouds have size 3-35’. characteristic size?!Even from images: large abundance of small, compact clouds.

Peak HI column density

N(HI) ~1x1019 cm-2

Size (arcmin)

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Cloud propertiesNumber of clouds/observed area Angular size (‘)

Central velocity(km/sec)

• 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.

Gal. Latitude (deg) Gal. Latitude (deg) Gal. Latitude (deg)

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Cloud properties

Two central velocity peaks (at -405 and -350 km/s); not a selection effect.

kinematic bifurcation as suggested by Connors et al.

Central velocity (km/sec)

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~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.”

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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.

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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.

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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.

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Independent constraint: Jin & Lynden-Bell (astro-ph/0711.3481)

GC

M. Clouds Tip

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

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Evidence for evaporating Stream

clouds?

STIS and FUSE observations

(Fox et al. 2005)

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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.

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Sembach et al. (2001)

Metallicity ~ Leading Arm of the MS.H2 clump (T~200 K) tidally stripped from the SMC.

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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.

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Thank you !

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Science highlights:

2. Spectacular HVC/Halo Interfaces

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CHVC186+19-114: caught while breaking up • Clear velocity

gradient• De-acceleration by

ram-pressure?• Rotation?• Part of a larger

complex?

(arcmin)

(1018)

“Companion cloud”: one of the smallest HVCs, 7’x9’, Ultra Compact HVC[N(HI)=5x1019 cm-2]

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Cloud “shreds” can be classified as Mini- and Ultra-Compact HVCs

3. Compact HVC ~30’ 2. Hybrid HVC 1. HVC

~deg

4. Mini HVC7-10’

5.Ultra-Compact HVC ~4’

• Is this a real sequence?• What defines

diff. HVCs?• How many

MHVCs, UCHVCs are there, what are they?

Putman et al. (1999)

Hoffman (2004) Bruns & Westermeier (2004)

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10-5 cm-3 10-4 cm-3

Ingredients: Halo properties, dark matter, magnetic field. Future: compare observations with models (Power, Putman) --> Halo density.

Cloud/Halo Interaction: Theoretical Perspective

Quilis & Moore(2001)

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Almost continuous distribution of cloudy structure from the disk to the

intermediate-velocity gas

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Stanimirovic, Dickey et al. (2002)

If in pressure confinement, then Halo density ~10-3 cm-3 at z~50

kpc.

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Science highlights:

1. Cold HI clouds in the Galactic disk/halo interface

region

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Galactic Plane

b~5

b~12

b~20

High latitude HI at 3’:

‘Fingers’ @mild forbidden velocities streaming out

of the Gal. Plane

“low-velocity clouds”

l~183

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• No distances, but NaI measurements in progress.

• If pressure confined & P/k = 3000 K cm-3 D 200-3000 pc

z = 60-600 pc L = 1-5 pc

M(HI) = 0.03-3 M

• If 10x lower pressure D = 5 kpc z = 1500 pc L = 12 pc

M(HI) = 8 M

What are the low-velocity clouds?

Numerous, small & cold, discrete HI clouds at z>100 pc ordinary CNM clouds but

“displaced” from the disk.

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Low-velocity clouds are common at different Galactic longitudes

l = 34

b = 15

V = -15 km/sVdev=~15 km/s

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Cousins of “Lockman’s clouds”? Lockman’s clouds:

Cloudy disk/halo interface in theInner galaxy.

GALFA Cloudy disk/halo interface is present in the outer Galaxy too.

Could be tracing the same population, but clouds in the anti-center appear smaller and colder with larger deviation velocities.

Lockman (2002)

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Several groups studying (low-velocity) Halo clouds

Location in the Galaxy

|Vdev|(km/s)

Size (pc) FWHM (km/s)

D(kpc)

|z|(pc)

Lockman Inner (l=28)

~15 25 12 7.5 950

Stil+ Inner(l=45)

~30 10 6 0.2-3 80

Dedes+ Outer(l=215)

~70 a few 3-7 5 1500

Stanimi+ Outer(l=183)

~30 1-8 4 0.2-3 60-900

• At R>R Halo clouds smaller and colder (selection effect?)• At R>R Halo clouds have smaller Vdev (probably not a selection effect).

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In progress: studying internal structure of (larger) Halo clouds

GBT @ 9’ GALFA @ 3’

Large clouds decompose into smaller clumps @ high resolution.

Bill Dirienzo, SS, Lockman, Muller

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1.Galactic Fountain (Shapiro & Field 1976, Houck & Bregman 1990).

2. Shell fragmentation (Norman & Ikeuchi 1989).

3. Final stage of the infalling IGM (Maller & Bullock 2004; Kaufmann et al. 2006; Santillan et al. 2007)

4. Photolevitation (Franco et al. 91)

Possible mechanisms for maintaining clumpy disk/Halo

interfaceVdev increases

with Rg !

Cloud HI massspectrum can

test this

Clouds in simulations100-600 pc

Need dust

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The key questions to distinguishing btw different scenarios:

1.Do clouds’ properties (deviation velocity, size, FWHM) change with Galactocentric radius?

2. What is the origin and importance of these clouds?

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h~1500 pc

Halo clouds are most likely a general property of the disk/halo

interface

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~40% of sky is covered by “clouds” that do not take part in Galactic rotation High Velocity Clouds (HVCs)

Wak

ker,

UW

Mad

ison

MagellanicClouds

Magellanic Stream

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IGM (Bryan & Norman)

How to get fromsmooth, warm

IGM to complex ISM on

star-forming

scales?

GLIMPSE: Milky Way, ISM

?

?

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What are the details of this transformation from smooth, hot gas at kpc-scales down to

cold gas at star-forming scales?

Recent theory advances: Galaxy formation is a multi-phase and

multi-scale process (Maller & Bullock 2004):

Hot gasScale ~kpc

Warm cloudsScale ~102 pc

??HVCs??

Cold gas in the disk

Scale ~pc

Future star-formation

fuel

ThermalInstability,

Fragmentation

?????Can we trace clouds to the disk?How do clouds fall? Disrupted?

Or smoothly infalling?Why lack of observational signatures?

Do clouds have sub-structure?Isolated clouds vs complexes vs filaments?

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Disk/Halo Interface

orTransition

h~ 200 pch~1500 pc

Galactic Halo, and especially the interface region btw the

disk and the halo holds the key records…

Hot Galactic Halo, or corona

Galactic disk

@200,000 ly

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These questions are important for the Galaxy but for far-away galaxies as well!

Need: large-area surveys with high angular resolution to zoom in on the disk-halo interactions!

• How do Galactic disk and halo exchange matter? • What’s the internal structure of the Galactic Halo?• What determines the size and morphology of HVCs?• Can we trace outflowing gas from the disk into the halo?• Can we trace infalling gas from the halo into the disk?

What we want to find out:

… and that’s what GALFA is about ! GALFA = Galactic Science with ALFA International collaboration (~80 members) @www.naic.edu/alfa/galfa/

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Braun & Thilker (2004)

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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Recent excitements• Lots of Hα detections along the Stream but the ionization

source is still a puzzle! Photoionization by escaping Milky Way photons not enough!

• Bland-Hawthorn et al. 07: cascade of shocks along the Stream! Up-stream clouds fragment due to KH instability, lagging fragments smash into the following clouds causing collisional ionization. Stream must be constantly replenished.

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Observing:Routine, smooth, consistent,little user interaction

Specialized Data ReductionGALSPECT

TO: Norberto Despiau

LSFScalibration

Basket-weave scans

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In simulations, most of the Stream gas follows SMC/LMC orbit, but there is a significant more distant

component at velocity<-420 km/sec.

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The tip of the Stream before ALFA: probing the density of

Galactic Halo

Stanimirovic, Dickey et al.

(2002)Arecibo

observations

Stream clouds are pressure confined by the Halo.nh(15 kpc) = 10-3 cm-3 / nh(45 kpc) = 3x10-4 cm-3