Shells, Bubbles, Worms, and Chimneys: Highlights from the Canadian Galactic Plane Survey

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Shells, Bubbles, Worms, and Chimneys Highlights
from the Canadian Galactic Plane Survey

• Shantanu Basu
• U. Western Ontario

Michigan State University October 19, 2000
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A Universe of Stars?
A Universe of Hydrogen gas
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The Interstellar Medium

• The matter between the stars, mostly hydrogen
  gas
• A complex balance between the conversion of gas
  to stars and the feedback from stars, e.g.,
  massive stars at the end of their life
• The ISM holds the key to understanding star
  formation
• The ISM plays a key role in understanding galaxy
  formation and evolution
• Details of ISM evolution best studied in our
  Galaxy

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The Evolution of Matter
The ecosystem of galaxies
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The Milky Way Galaxy is the only galaxy close
enough to see the details of the Galactic
Ecosystem.

• Challenges
• The Galactic plane encircles the Earth
• A large area of sky must be observed
• The Galaxy is a 3-dimensional object
• Must untangle the third dimension
• High Angular resolution is need to see the
  details in the context of the larger picture
• A very large data base
• A large range of wavelengths must be covered to
  see all major components of the ISM
• Several telescopes will be required

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Milky Way in Optical Light (0.0005 mm)
Stars obscured by dust
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Milky Way in Far-infrared Light (0.0035 mm)
Old red stars with little obscuration
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Milky Way at Sub-millimetre (l0.240 mm )
Dust now seen as an emitter
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Milky Way at Radio (21 cm)
Atomic hydrogen gas (the basic stuff of the
Universe)
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The Milky Way at Radio (74 cm)
Ionized gas and magnetic fields
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Objectives of the CGPS

• Observing Goals
• Create a high-resolution (1arcminute),
  3-dimensional map of the interstellar medium of
  the Milky Way. The first large-scale, spectral
  line, aperture synthesis survey ever made.
• Construct a Galactic Plane Survey Data base of
  the distribution of major constituents of the
  interstellar medium.

• Science Goals
• How does the interstellar medium evolve?
• Explore the evolutionary relationship between the
  phases and states of the interstellar medium.
  How do galaxies convert diffuse primordial
  hydrogen to stars and the building blocks of
  life?
• What energizes and shapes the medium?
• Characterize the energy sources and modes of
  energy transport
• Is the Milky Way a closed system?
• Explore the vertical structure out of the disk.
  Is there mass and energy exchange between the
  disk and extragalactic space?

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The CGPS Data Base
All images at 1 arcminute resolution
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Where is the CGPS?
Survey covers Galactic longitudes l 74.20 to
147.30 and latitude b - 3.50 to 5.50
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The Dominion Radio Astrophysical Observatory
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Milky Way at Radio (21 cm)
Butler Hartmann (1994), Leiden-Dwingeloo
Survey, 35 resolution
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Atomic Hydrogen Image from a Single Antenna Radio
Telescope
25-m Radio Telescope, Dwingeloo Netherlands
Foundation for Radio Astronomy
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Atomic Hydrogen Image from a Radio Interferometer
7-element Interferometer, Penticton Dominion
Radio Astrophysical Observatory equivalent
diameter equals 600m
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Slicing up the Milky Way Galaxy
Velocity changes systematically with distance
along the line of sight.
Sun
Galactic Centre
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Atomic hydrogen data cube
256 channels, velocity resolution 1.2 km/s.
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A top-down view of the hydrogen cube
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A Close-up view of the Perseus Arm
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Optical Image Stars and Ionized gas (Thanks
to Alan Dyer)
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Optical Image Stars and Ionized gas
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Optical Image Stars and Ionized gas
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Optical Image Stars and Ionized gas
Composite Image Hydrogen Gas Dust
Ionized Gas
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W4 A Chimney to the Galactic halo?
A chimney may be blown out by a cluster of
massive hot stars at the bottom
Intense ultra-violet radiation leaks out of the
galaxy
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W4 Superbubble
Ha map Dennison, Topasna, Simonetti (1997)
model overlay by Basu, Johnstone, Martin (1999)
HI velocity channel map Normandeau, Taylor,
Dewdney (1996, 1997) CGPS Pilot Project
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Blowout from Galactic disk Theory
MacLow McCray (1988)MacLow, McCray, Norman
(1989) Compare to expansion in a uniform medium
Bubble can stall at radius
Therefore, bubble blows out if stalling
parameter
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Blowout from Galactic disk Theory
Kompaneets (1960) analytic solution for ambient
atmosphere r r0 e - z/H. Pext0. Solid lines
shock front gt
where y, between 0 and 2H, parameterizes the
evolution of the bubble. Dashed lines
streamlines
Can fit the observed aspect ratio of W4 (Basu,
Johnstone, Martin (1999)
More generally, r(z0) 2H in late stages. We
observe r(z0) 50 pc
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Another H I shell G132.6-0.7-25.3
Normandeau, Taylor, Dewdney, Basu (2000). Apply
aspect ratio argument using Kompaneets model and
estimated distance (2.2 kpc) to obtain
Note relatively small H gt superbubbles may have
limited influence near Galactic plane.
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Classical picture of Galactic gas scale height
e.g., Spitzer (1978)
a ratio of magnetic energy density to kinetic
energy density of clouds, b ratio of cosmic ray
pressure to kinetic energy density.
Consistent with large scale surveys of H I. But
individual star-forming regions appear to be
distinct.
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Ionization front in a stratified medium (W4)
Initial ionization front around an H II region
for RSt/H 0.1, 0.3, 0.5, 0.7, 0.9, 1, 2, 3,
and 4. Atmosphere r r0 e - z/H. Breakout when
RSt/H gt 1.
Ionization front around a wind-swept shell in the
same atmosphere for n 1, 5, 10, 15, and 20
cm-3. Require n gt 10 cm-3 to fit observations of
W4.
Basu, Johnstone, Martin (1999)
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Evolution of Ionization Front
Basu, Johnstone, Martin (1999) - emission
measure through ionized region.
Ionizing photons initially escape atmosphere,
then trapped by wind-swept shell, then break out
of the top part of shell. Competition of n2
dependence of recombination rate vs. diverging
streamlines.
Eventually, some 15 of ionizing photons escape
through the top of shell. If this is typical of
superbubbles, can it explain the Reynolds layer
(scale height of free electrons 1 kpc)?
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Age of W4 Superbubble
Age agrees with estimates for age of cluster OCl
352 at the base of the superbubble consistent
with bubble powered by stellar winds.
Dynamics of W4 Superbubble

• Numerical hydrodynamic simulations predict lack
  of collimation at large height and
  Rayleigh-Taylor instability gt not seen!
• Likely need to run MHD models for a more
  complete picture.

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Atomic Hydrogen Mushroom Cloud
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The Mushroom Cloud GW123.4-1.5
English et al. (2000)
Challenges to conventional superbubble models 1)
narrow stem width and large cap to stem width
ratio 2) bulk of mass in cap 3) excess of H I
emission, not a deficit A jet, buoyant bubble, or
something else?
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The Mushroom Cloud GW 123.4-1.5
English et al. (2000) gt a buoyant supernova
remnant. Illustrate the effect with Zeus-2D
numerical simulations. Look at case in which
Rstall lt H.
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Whats Next? A Global Galactic Plane Survey
Dominion Radio Astrophysical Observatory National
Research Council of Canada
Australia Telescope Compact Array Commonwealth
Science and Industrial Research Organisation
Very Large Array U.S. National Radio Astronomy
Observatory
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A Global Survey CGPS, VGPS and SGPS
CGPS 12
SGPS
VGPS
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Conclusions
Only a small fraction of the Galaxy has so far
been mapped in 1 arcminute resolution. Some of
what has been learned

• First close-up views of exotic phenomena
  (chimney, mushroom) related to the disk-halo
  interaction (matter and radiation transport) in
  our Galaxy
• First comparison of observed superbubble(s) with
  theoretical models. Evidence for highly
  stratified ISM near star-forming regions gt
  superbubbles have limited influence near Galactic
  Plane significant fraction of ionizing photons
  can escape to high latitudes
• Widespread complex polarization patterns - a
  tracer of magnetic field and ionized medium

In the future, expanded CGPS VGPS SGPS will

• Observe nearly full Galactic longitude range at
  1 arcminute resolution
• Focus on individual disk-halo interaction
  candidates to higher latitude
• Explore star formation by focusing on atomic gas
  around molecular clouds