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
A Universe of Stars?   A Universe of Hydrogen gas
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
• Details of ISM evolution best studied in our Galaxy
The Evolution of Matter

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

• 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
Milky Way in Optical Light (0.0005 mm)

 Stars obscured by dust
Milky Way in Far-infrared Light (0.0035 mm)

Old red stars with little obscuration
Milky Way at Sub-millimetre (l=0.240 mm )

Dust now seen as an emitter
Milky Way at Radio (21 cm)

 Atomic hydrogen gas (the basic stuff of the Universe)
The Milky Way at Radio (74 cm)

Ionized gas and magnetic fields
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?
   The CGPS Data Base

All images at 1 arcminute
Where is the CGPS?

                     Survey covers Galactic longitudes l
                     = 74.20 to 147.30 and latitude b =
                     - 3.50 to +5.50
The Dominion Radio Astrophysical Observatory
Milky Way at Radio (21 cm)

 Butler & Hartmann (1994), Leiden-Dwingeloo Survey, 35’ resolution
Atomic Hydrogen Image from a Single Antenna Radio Telescope

25-m Radio Telescope, Dwingeloo
Netherlands Foundation for Radio Astronomy
Atomic Hydrogen Image from a Radio Interferometer

7-element Interferometer, Penticton
Dominion Radio Astrophysical Observatory

equivalent diameter equals 600m
Slicing up the Milky Way Galaxy

                                      Velocity changes
                                      with distance
                      Sun             along the line of

                    Galactic Centre
Atomic hydrogen data “cube”

                              256 channels,
                              velocity resolution
                              1.2 km/s.
A top-down view of the hydrogen cube

     Outer spiral arm

                              The Perseus spiral arm

                                 The Local spiral arm
A Close-up view of the Perseus Arm
 Optical Image
   Stars and Ionized gas
   (Thanks to Alan Dyer)

Radio 21cm image
 Neutral Hydrogen gas
 (Perseus Spiral Arm)
Optical Image
  Stars and Ionized gas

Far-Infrared Image
 Dust Particles
  Optical Image
   Stars and Ionized gas

Radio 74 cm image
 Ionized Gas
Optical Image
 Stars and Ionized gas

Composite Image
 Hydrogen Gas
 Ionized Gas
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
W4 Superbubble

                                   Ha map
HI velocity channel map
                                   Dennison, Topasna, & Simonetti
Normandeau, Taylor, & Dewdney
                                   (1997); model overlay by Basu,
(1996, 1997); CGPS Pilot Project
                                   Johnstone, & Martin (1999)
Blowout from Galactic disk: Theory
                 MacLow & McCray (1988);MacLow, McCray,
                 & Norman (1989)
                 Compare to expansion in a uniform medium:
                                 1/ 5
                    125 
                 r                   L1/5 01/5t 3/5 .
                    154               0

                 Bubble can stall at radius
                                        1/ 2
                            27 
                 Rstall                     L1/ 2  0 / 4 Pe3/ 4 .
                            154              0

                 Therefore, bubble “blows out” if stalling
                 b         1.
 Blowout from Galactic disk: Theory
                                     Kompaneets (1960) analytic solution for
                                     ambient atmosphere   0 e - z/H. Pext=0.
                                     Solid lines: shock front =>
                                     r ( z, y)  2 H arccos    e 1 
                                                                  z/2 H   y2
                                                                          4H   2            
                                                                                    e z / H .
                                      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):
 z1      ln1 - y 2 H                                        More generally, r(z=0)
                       3.33  y 2 H  198.
        arcsin y 2 H 
                                          .                   ~2H in late stages. We
                                                              observe r(z=0) ~ 50 pc
Since rmax  2 H arcsin y 2 H   2.89 H for y 2 H  198,
                                                                H  25 pc.
and we observe rmax  74 pc for d  2.35 kpc ,
 H  25 pc.
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

                             H  17.3 pc.

                        Note: relatively small H =>
                        superbubbles may have limited
                        influence near Galactic plane.
Classical picture of Galactic gas scale height
e.g., Spitzer (1978)

        1  a   , where ceff  6 -10 km s-1 ,
g  3  109 cm s-2 , a  1,   1,

 a = ratio of magnetic energy density to kinetic energy density of clouds,
  = ratio of cosmic ray pressure to kinetic energy density.

  H  100  200 pc

  Consistent with large scale surveys of H I. But individual star-forming
  regions appear to be distinct.
Ionization front in a stratified medium (W4)

Initial ionization front around an   Ionization front around a wind-swept
H II region for RSt/H = 0.1, 0.3,    shell in the same atmosphere for n = 1,
0.5, 0.7, 0.9, 1, 2, 3, and 4.       5, 10, 15, and 20 cm-3. Require n > 10
Atmosphere  = 0 e - z/H.           cm-3 to fit observations of W4.
Breakout when RSt/H > 1.
                      Basu, Johnstone, & Martin (1999)
Evolution of Ionization Front
                        Basu, Johnstone, & Martin (1999) -
                        emission measure through ionized

                         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)?
Age of W4 Superbubble
t  c1n0/ 3 H 5/ 3 L1/ 3 .

c1  6.3 at current epoch.
H  25 pc, n0  10 cm3 , L0  3  1037 erg s1 ,
      t  2.5 Myr.
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 => not seen!
• Likely need to run MHD models for a more complete picture.
Atomic Hydrogen Mushroom Cloud
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
                    A jet, buoyant bubble, or something
The Mushroom Cloud: GW 123.4-1.5
English et al. (2000) => a buoyant supernova remnant.
Illustrate the effect with Zeus-2D numerical simulations.
Look at case in which Rstall < H.
  What’s Next? A Global Galactic Plane   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
A Global Survey: CGPS, VGPS and SGPS
CGPS 1+2 :

650  l  1800 .

2530  l  3570 .
180  l  670 planned
-50  l  180 proposed
 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 =>
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