The Milky Way and Nearby Galaxies

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The Square Kilometer Array
The Milky Way and Nearby Galaxies
The Dynamic Interstellar Medium
John M. Dickey University of Minnesota Geraldton
- August 2003
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• There are galaxies today whose ISM is similar
• to that of the Milky Way at z 4
• low metallicity (Z lt -2)
• relatively disordered velocity field
• HI rich, extended gaseous envelopes
• We can study these nearby dwarfs with the SKA
• at the same resolution we now get on the Milky
  Way.

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SN explosions
winds
nucleosynthesis
Star formation
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SN explosions
winds
nucleosynthesis
Star formation
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SN explosions
Hot gas
winds
shells
chimneys
Turbulence
nucleosynthesis
Diffuse clouds
Molecular clouds
Star formation
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• Brightness temperatures
• are determined by
• optical depth, and
• excitation temperature
• Typically the brightness temperature
• is a few K or less for these thermally
• excited lines.

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The aperture filling factor, f
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?s (arc sec)
Time to survey 500 s.d. (days)
?s (arc min)
?T 1K, ?v 1 km s , Tsys 20 K, ? 21 cm
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?s (arc sec)
Time to survey 500 s.d. (days)
?s (arc min)
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How will the SKA improve our mapping capabilities
? It depends on the filling factor, f !
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An example the US LNSD array
log B (m)
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A much improved model for 2003 !!!
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B
1
1 10 100 1000
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38.2
A
A
1
1
Ae
Tsys
25.5
12.7
Ae
B
15 log ( )

A
Ao
Bo
1
for Tsys 20 K, 2 lt B lt 200
1
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B 20 km
B 10 km
B 3 km
B 1 km
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?s (arc sec)
Time to survey 500 s.d. (days)
1 km
3 km
10 km
20 km
?s (arc min)
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20 km
10 km
3 km
1 km
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?s (arc sec)
Time to survey 500 s.d. (days)
1 km
3 km
10 km
20 km
?s (arc min)
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?s (arc sec)
Time to survey 500 s.d. (days)
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1 km
3 km
10 km
100 days
20 km
?s (arc min)
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We make this improvement at all wavelengths and
at all sensitivities, for large and small areas.
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The Southern Galactic Plane Survey

• SGPS
• ?21-cm line
• 210 sq. deg. area
• 2 ?syn
• Trms 1 K noise
• ?v 1 km s -1
• ATCA 375m config
• R 0.5 sq. deg. hr.-1

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Leafy Sea Dragon (Australian marine animal) photo
by Victoria Graham
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This area 18 sq. deg. Telescope time total 36
hours resolution 2
In 36 hours the SKA can map this area with
resolution 8 ...
or with brightness sensitivity better by a
factor of 4 ... or with velocity resolution
narrower by a factor of 15 ...
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Parkes-only map - resolution 15 (this is only
a factor of 7 lower than the SGPS).
The improvement in resolution of the SKA over the
SGPS is a factor of 15 !
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NH3
For studying regions of star formation ammonia
(NH3) is very useful because of its high ncrit.
Mosaic surveys have been done with the VLA, but
only over small areas.
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Orion NH3 Mosaic

• VLA D array mosaic
• 10 hours integration
• 32 square arc-minutes
• 0.3 km/s channels
• rms noise 0.5 K
• 9 ?syn FWHM
• R 9 x 10-4 sq. deg. / hr.

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• Conclusions
• The Milky Way and nearby galaxies
• have a variety of interstellar environments
• that can be studied with the SKA using
• several spectral line tracers.
• These are thermally excited transitions, so
• the brightness temperature is a few tens of
• degrees or less.
• To survey (mosaic) with these lines requires
• filling factor, f, greater than 10-3 !!!
• Mapping speed depends strongly on f.
• For this kind of mapping, the SKA will
• make as big an improvement over the VLA
  (EVLA)
• as the HST made over ground-based 2m
• optical telescopes !

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Orion A region. Grey scale is shocked H2 (2 ?)
emission. Contours are a VLA mosaic of NH3
emission (from Wiseman and Ho, 1999).