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A Hitchhiker’s Guide to Galactic Gastrophysics Part

II

John Everett & Snežana Stanimirović

(UW Madison)

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• How do we constrain models of the ISM? OR• “How do we keep theorists honest?”

• One important way: By measuring properties and volume filling fraction of ISM phases.

?McKee & Ostriker (1977) de Avillez & Breitscwerdt

(2005)

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Questions:- Do ISM phases really exist?- How does material transition from one phase to

another?- What is the filling factor of CNM and WNM?- How does the filling factor of CNM and WNM

vary with interstellar environments and across the Galaxy ?

Answer: The Arecibo Millennium++ survey

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One important step: The Millennium Survey @ Arecibo

• Heiles & Troland (2003): observed 79 continuum sources sources; t=15 min, S>a few Jy.

• Systematic, detailed, well-calibrated survey, good statistics.

• Statistics of CNM & WNM T, N(HI), and fractions.

• Showed that ~50% of WNM is thermally unstable.

Heiles & Troland (2003)

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• 60% of HI is WNM.• This is >10x higher than what MO predicts.• Weird, large number of LOSs with no CNM at

all!

One important step: The Millennium Survey @ Arecibo

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So, the Millennium Survey …

• Statistically emphasized some unexpected properties of the ISM.

• And triggered a whole lot of interesting numerical simulations! Importance of MHD turbulence, dynamically triggered phase conversion, an incredible range of scales etc.

• But talking about immaturity, have a look at the distribution of Millennium sources….

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Millennium survey sources

79 sources: while roughly uniform distribution for l=160-250,very under-sampled distribution of sources elsewhere.

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Millennium++ survey sources

1125 sources: uniform distribution over the whole AO sky!

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The obvious next step: The Arecibo Millennium ++

survey• There are 1125 continuum sources within the

AO sky with S > 1 Jy. • With ~1 hr (on average) per source we would

need 1125 hrs of telescope time.• Why only AO? Simply the best for this work

based on small beam + amazing sensitivity. Also, extremely well calibrated and understood system.

• Single-pixel receiver is fine for this project.

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Questions:- What is the structure of the Galaxy’s Halo?- What is the nature of the disk/halo interfaces?- How does matter transition btw phases?- Does matter accrete onto the Galaxy and in what phase?

Answer: GALFA-HI ++ or Studying gastrophysical processes in the Halo

Does the Milky Way need an Environmental Impact Statement?

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GALFA = Galactic Science with ALFA

(www.naic.edu/alfa/galfa)

An important contribution:

interfaces btw Halo clouds and Halo

(On-going) GALFA-HI survey: 12,734 deg2 @ 3.5’, v=0.2 km/s,

• Primarily observing commensally with e-gal & continuum surveys.

• Smooth, stream-lined observations, successful combination of data from many

GALFA projects.

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GALFA caught CHVC186+19-114 while breaking up

• De-acceleration by ram-pressure

• Evidence for low column density fluff.

• Part of a larger complex?

(Stanimirovic et al. 06)

(arcmin)

(1018)

“Companion cloud”: one of the smallest HVCs, 7’x9’, Ultra Compact HVC

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Peek et al. (2007)

Torn-off ‘condensations’de-accelerated by ram

pressure.

Again, lots of “fluff” at N(HI)<1019 cm-2

lurking in the Halo.

Such level of detail and disruption has not been

seen before.

Details of Cloud/Halo Interaction:

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“Fluff” with N(HI)<1018 cm-2

GALFA observations: Peek et al., in preparation

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

Ingredients: Halo properties, dark matter, magnetic field, turbulence --- all unknowns.

Contrasting observations with simulations can constrain models.

Cloud/Halo Interaction: Theoretical Perspective

Quilis & Moore(2001)

GAS ONLY WITH DARK MATTER

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A very special “HVC”: The Magellanic Stream

- D = 20 or 60 kpc.

- The only gaseous stream we know of.

- The closest tidal tail.

Putman et al. (2003)

GALFA-HI image: 1125 deg2 !(Stanimirovic et al. 07)

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The next steps:• GALFA-HI++: • Observe commensally with 3rd generation e-gal surveys at AO• With ALFALFA++ with t=40 sec/beam finish off the whole AO sky, requires ~3000 hr of AO time. • With AGES++ with t=300 sec/beam cover 3000 deg2 would require 10,000 hrs of AO time.

• GALFA-Stream survey: ~1000 deg2, t=300 sec/beam, 3000-4000 hrs required. • More beams would greatly help!

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Thank you !

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t=1-3 hours/source

- 20 additional CNM clouds with ~ a few <1018 cm-2.

-Continuation of the usual CNM population?

- Changing our ideas about cloud survival, phase transition, interactions.

- And curiously, at least one of these cold CNM clouds is <100 pc away!

(Stanimirovic & Heiles 05)

But take a deep look with Arecibo….

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The tip of the Stream:

HI integrated intensity

• Multiple streams• Lots of small (~6’) compact clouds•To survive must have low column density stuff around them. •OVI interfaces too.

Samantha Hoffman, SS, Putman

GALFA observations

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Note:• Josh’s cloud is a real beauty because it is big with high

column density, has such extreme velocity, and therefore makes these rich interface chunks.

• But there are so many other clouds with lower velocity and column density. They also interact with the Halo but their “shreds” are at lower column densities we can not detect. To detect them we need a deeper survey.

• Simulations predict lots of wispy stuff at a continuous range of col. Densities. Even in the case of Josh’s clouds we are still seeing the peaks of distribution. And those peaks offer a very complicated picture.

• Observations show so much irregular stuff, simulations show so much more ordered structure. Very far away!

• “Chaff” is at such low N(HI) and pops up all the time --- there is so much lower column density stuff that we are missing with current surveys.

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Note:Magellanic Stream is the only gaseous stream we know of around the Milky Way. It is also an incredible tidal structurerevealing a huge amount of tidal debris in our close proximity.Real opportunity to investigate tidal tails in detail, we know they are important in other places (e.g. Virgo and dark galaxies connection).

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Key questions:1. What are the processes that govern how galaxies from,

work, and evolve?2. How does the ISM affect galaxy formation?

3. ISM turbulence --- structure formation:

4. Small-scale end of the turbulent spectrum, where does the turbulent spectrum dissipates and how?

5. Do ISM phases exist? What are properties and lifetimes of phase interfaces? How do they evolve?

6. Where is the edge of the Galaxy?7. Halo interfaces?8. Small scale structure of the vertical Galactic HI distribution.5. GLIMPSE follow-up of dark clouds and bubbles (various

molecular lines).6. GLAST follow-up

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Potential surveys:• The need to go deeper: GALFA-HI++ = Full AO

sky, deep HI survey to study:- cloud/Halo interfaces, - the Magellanic Stream, also - disk/halo interface region.

• The need for molecular/magnetic field follow-ups:- GLIMPSE follow-up on dark clouds (magnetic field, and various molecular lines), targeted with many lines.- GLAST: high-latitude clouds

• Variability, large monitoring survey:– Small-scale structure– turbulence

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How does the ISM affect galaxy formation?

• In simulations radiative cooling serves to transfer matter from hot to cold phase, while heat conduction and feedback drive gas in the opposite direction. An ad hoc wind is implemented as the model does not produce winds. Some predictions are ok but the predicted pressure is much higher.

• Milky Way as a prototypical disk galaxy.

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Or how do you build a disk galaxy? Numerical simulations are reaching the bottle-neck:

• Current best galaxy simulations:• Stellar disk is made from the gas disk but quickly, by z=0.9 70% of gas

has been converted into stars.• Highly warped disk and unrealistic rotation curve (due to angular

momentum transport problem but other problems too).

• Essential ingredients for numerical simulations:• Need to resolve the multi-phase ISM in the presence of star formation

and feedback (input from massive stars and SNe)• Need to include radiative cooling near SF and Sne regions.

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So….

• Got to make partnerships! Sure we need to observe billion galaxies to understand galaxy formation, but we also need to understand key unknowns of the ISM processes.

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Why are surveys with Arecibo so special for Galactic science ?

A very unique combination:

1. Sensitivity

2. Resolution (3.5’)

3. Full spatial frequency coverage simultaneously

AC0 HVC -- LDSAC0 HVC -- GALFA

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Don’t think I’ll have time for this. But demonstration of how high velocity resolution

is opening a new window in Galactic HI studies is nice.

• Questions:

- What is the structure of the Galactic disk/halo interface region?

- What goes on in the extreme outer parts of the Galaxy?

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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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A spectacular example of small,compact low-velocity HI

clouds at b~18:• size: 4’-12’

• v: 2-4 km/s

• Tk < 400 K• N(HI)=2x1019 cm-2

• Vlsr: -20 km/s but “follow” disk HI

@ 3’ @ 36’

Too small to be

seen in low-res.

surveys…

Need for high velocity resolution !

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Almost continuous distribution of cloudy structure from the disk to the

intermediate-velocity gas

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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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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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h~1500 pc

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

interface

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Summary:

~60% of the GALFA survey has been completed. Arecibo’s resolution zooming in on the unexplored

interfaces btw the Galactic disk and the halo, as well as HVC/Halo interfaces.

“Cloudy” Galactic disk/halo interface region populated with cold HI clouds in the inner and outer Galaxy.

Evidence for active interaction btw HVCs and the Halo and HVC’s breakup into smaller UCHVCs opportunity to study Halo properties.

The tip of the Magellanic Stream contains many “mini-HVCs”, most likely, of tidal origin.