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A Hitchhiker’s Guide to
Galactic Gastrophysics Part II
John Everett & Snežana Stanimirović
(UW Madison)
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?
McKee & Ostriker (1977)
de Avillez & Breitscwerdt
(2005)
• 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.
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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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One important step: The Millennium
Survey @ Arecibo
• 60% of HI is WNM.
• This is >10x higher than what MO predicts.
• Weird, large number of LOSs with no CNM at all!
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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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Does the Milky Way need an
Environmental Impact Statement?
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
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(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.
An important contribution:
interfaces btw Halo clouds and Halo
GALFA = Galactic Science with
ALFA (www.naic.edu/alfa/galfa)
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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?
(1018)
(Stanimirovic et al. 06)
(arcmin)
“Companion cloud”:
one of the smallest
HVCs, 7’x9’, Ultra
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Compact HVC
Details of Cloud/Halo
Interaction:
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.
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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
Cloud/Halo Interaction:
Theoretical Perspective
Quilis & Moore
(2001)
GAS ONLY
WITH DARK MATTER
Ingredients: Halo properties, dark matter, magnetic field,
turbulence --- all unknowns.
Contrasting observations with simulations can constrain
models.
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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.
GALFA-HI image: 1125 deg2 !
(Stanimirovic et al. 07)
Putman et al. (2003)
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The next steps:
• GALFA-HI++:
• Observe commensally with 3rd generation egal 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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But take a deep look with Arecibo….
-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)19
GALFA observations
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
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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 structure
revealing 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 ?
AC0 HVC -- LDS
GALFA
A very unique
combination:
1. Sensitivity
2. Resolution (3.5’)
3. Full spatial frequency
coverage simultaneously
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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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High latitude HI at 3’:
‘Fingers’ @mild forbidden
velocities streaming out of
the Gal. Plane
b~20
b~12
“low-velocity clouds”
b~5
l~183
Galactic
Plane
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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’
Need for high velocity
@ 36’
Too small to be
seen !in low-res.
resolution
surveys… 31
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/s
Vdev=~15 km/s
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Possible mechanisms for maintaining
clumpy disk/Halo interface
Vdev increases
1. Galactic Fountain
(Shapiro & Field 1976,
Houck & Bregman 1990).
2. Shell fragmentation
(Norman & Ikeuchi 1989).
with Rg !
Cloud HI mass
spectrum can
test this
Clouds in
3. Final stage of the infalling IGM
simulations
100-600 pc
(Maller & Bullock 2004;
Kaufmann et al. 2006; Santillan et al. 2007)
Need dust
4. Photolevitation (Franco et al. 91)
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Halo clouds are most likely a general
property of the disk/halo interface
h~1500 pc
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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.
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