1. ‘GoldenEye’ in action: Mapping the Galaxy with GALFA Snežana Stanimirović (UW-Madison)

2. “GoldenEye” (1995): the 17th James Bond movie
At the end of “GoldenEye”
James Bond goes to Puerto Rico
searching for a gigantic
satellite dish…

3. …And he finds the 305-m Arecibo radio telescope, the largest radio
telescope in the world!
“The Arecibo Observatory is part of the
National Astronomy and Ionosphere Center (NAIC), a national research center operated by Cornell University under a cooperative agreement with the National Science Foundation (NSF).”
Built in 1963 but still lots of exciting
scientific capabilities….

4. In this talk: What do we know and don’t know about the Galactic Halo
How do we study Galactic Halo
GALFA survey in a nutshell: Why? How?
Science Highlights:
1. Cloudy transition region bw the Galactic
disk and the halo
2. The tip of the Magellanic Stream
3. Some really fast clouds out there

5. What do we know and don’t know about the Galactic Halo?

6. A more schematic view of dramatic “links” btw the Galactic disk and halo
Disk/Halo
Interface or
Transition
Galactic disk
h~650 ly
h~5000 ly
Hot Galactic Halo,
or corona
@200,000 ly

7. Most of the Galactic gas is in the form of
atomic hydrogen (or HI) and can be
mapped by radio telescopes
Leiden/Argentine/Bonn survey
36 arcmin resolution
Most of this gas belongs to the Galaxy. However…

8. High Velocity Clouds (HVCs)
~40% of sky is covered by “clouds” that
do not take part in Galactic rotation
High Velocity Clouds (HVCs)
Wakker, UW Madison
Magellanic
Stream
Magellanic
Clouds

9. We don’t know where HVCs come from, but we know that:
Supernovae blow large
holes in the Galactic disk
Some gas is being
grabbed from “outside”….
The Galaxy has a large
hot corona through
which HVCs move
-> Disk & Halo must be
talking to each other,
but HOW?
McClure-Griffiths et al. (2006)

10. What we want to find out:
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?
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!
… and that’s what GALFA is about !
GALFA = Galactic Science with ALFA
International collaboration (~80 members) @

11. Luckily James Bond saved the GoldenEye
Luckily James Bond saved the GoldenEye. In fact, the GoldenEye is more powerful than ever because of ALFA...

12. ALFA and what do we measure with a radio telescope?

13. ALFA = Arecibo L-band Feed Array
… To survey the sky much faster!

14. The 21-cm line of atomic hydrogen
every
107 years
Cosmos: The Swinburne
Astronomy
Online Encyclopedia
Hydrogen atom

15. Measuring Motions: Spectral Line Maps
Modified from Alyssa Goodman

16. Velocity from Spectroscopy
Observed Spectrum
Telescope 
Spectrometer
1.5
1.0
Intensity
0.5
All thanks to Doppler,
Radio astronomers work
with CUBES instead of images
0.0
Radial Velocity in km/sec
-0.5
100
150
200
250
300
350
400
Modified from Alyssa Goodman
"Velocity"

17. GALFA’s “art”: covering the whole sky visible from Arecibo
Effective integration time per pointing

18. Why is Arecibo + ALFA so special for Galactic science ?
A very unique combination:
Large bucket, or “Sensitivity”
Big dish, or “Good resolution (3’)”
A single big dish, or “Full spatial frequency coverage”
AC0 HVC -- LDS
AC0 HVC -- GALFA

19. What do we find?

20. The Galactic disk has a blanket of small clouds, seen for the first time
l= 34deg
b= 15deg
V=-15 km/s

21. Spectacular examples of small,compact low-velocity HI clouds above the Galactic disk
 Clouds “follow” the disk, but fall behind in velocity
Previous
surveys
Too small to be
seen in low-res.
surveys…
Stanimirovic et al. (2006)

22. What do we know about these clouds?
Numerous, small, discrete, cold (400 K = 260 F) HI clouds, found at various locations above the Galactic disk.
Typical cloud properties:
ly above the Galactic disk cloud size is ~10 ly
cloud mass is a few x M(Sun)
We study cloud properties and motion and compare those with theoretical predictions.

23. What mechanisms produce & maintain a very clumpy disk/Halo interface ?
Galactic Fountain
(Shapiro & Field 1976,
Houck & Bregman 1990).
2.Final stage of the infalling material
(Maller & Bullock 2004
Kaufmann et al. 2005)

24. 1. Galactic Fountain & ‘cannonball’ clouds
COLD
HOT
Idea: Clouds condense at higher altitudes and are now falling down, like little cannons, back onto the disk.
Cloud properties are telling us how effective fountain flows are in various places in the Galaxy.

25. 2. New ideas of how galaxies form: Anisotropic cloud infall and clumpy outer disks ?
Kaufmann et al. (2005)
density
temperature

26. The disk/halo transition region is very clumpy, not smooth as previously thought.
Galactic disk
h~650 ly
Hot Galactic Halo,
like Solar corona
What we still don’t know:
what role these clouds have in the transfer of
matter & energy btw the disk and the Halo.
@200,000 ly

27. The Magellanic Stream a huge starless tail of gas trailing behind the Magellanic Clouds
GALFA
LMC
SMC
Putman et al. (2003)
Observations with the Parkes
Telescope in Australia

28. What is the tip of the Stream telling us?
Still highly controversial: how was the Stream formed?
where did the Stream gas come from? how far away is
the Stream?
Main suspects: gravitational vs gas dynamic forces.
Models are increasingly complex..
The tip = the formation point of the Stream.
Multiple streams at the tip give support to gravitational
models as those are the only one that are able to produce
such structure.
Morphology and clumpiness of different streams tells us
they were not drawn from the same source or at the same
time.
Stanimirovic et al. (2007)

29. HVCs that move extremely fast
Details of Cloud/Halo Interaction (Peek et al. 2006:
HVCs that move extremely fast
Torn-off ‘condensations’
are being de-accelerated.
Differential drag:
n(halo)xD = F[observed params]
Such level of detail and disruption has not been seen before.

30. 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.
Already revising our Galactic knowledge:
- The transition region bw the disk and the halo is not smooth but made up of lots of small, cannon-like HI clouds.
- The tip of the Magellanic Stream consists of four distinct filaments, most likely produced by gravitational interactions bw the Magellanic Clouds and the Galaxy.

31. Thank you !