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 evolution

• 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”.

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

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 (=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

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?

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.

The CGPS Data Base

All images at 1 arcminute resolution

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, DwingelooNetherlands Foundation for Radio Astronomy

Atomic Hydrogen Image from a Radio Interferometer

7-element Interferometer, PentictonDominion Radio Astrophysical Observatory

equivalent diameter equals 600m

Slicing up the Milky Way Galaxy

Sun

Galactic Centre

Velocity changes systematically with distance along the line of sight.

Atomic hydrogen data “cube”

256 channels,

velocity resolution

1.2 km/s.

A top-down view of the hydrogen cube

The Perseus spiral arm

The Local spiral arm

Outer 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 Dust 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

HI velocity channel map

Normandeau, Taylor, & Dewdney (1996, 1997); CGPS Pilot Project

H map

Dennison, Topasna, & Simonetti (1997); model overlay by Basu, Johnstone, & Martin (1999)

Blowout from Galactic disk: Theory

MacLow & McCray (1988);MacLow, McCray, & Norman (1989)

Compare to expansion in a uniform medium:

r L t

1 2 5

1 5 4

1 5

01 5

01 5 3 5

/

/ / / .

Bubble can stall at radius

R L Psta ll e

2 7

1 5 4

1 2

01 2

01 4 3 4

// / / .

Therefore, bubble “blows out” if stalling parameter

bR

H s ta ll 1.

Blowout from Galactic disk: TheoryKompaneets (1960) analytic solution for ambient atmosphere e - z/H. Pext=0.

Solid lines: shock front =>

r z y H e ez H y

Hz H( , ) ./ / 2 12

4

2

2arcco s 12

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):

z

r

y H

y Hy H

r H y H H y H

r d

H

11 2

23 3 3 2 1 9 8

2 2 2 8 9 2 1 9 8

7 4 2 3 5

2 5

m ax

m ax

m ax

. . .

. . ,

.

ln -

a rcsin

S in ce a rcsin fo r

an d w e o b serv e p c fo r k p c ,

p c .

More generally, r(z=0) ~2H in late stages. We observe r(z=0) ~ 50 pc

H 2 5 p c .

Another H I shell: G132.6-0.7-25.3Normandeau, Taylor, Dewdney, & Basu (2000).

Apply aspect ratio argument using Kompaneets model and estimated distance (~2.2 kpc) to obtain

H 1 7 3. p c .

Note: relatively small H =>

superbubbles may have limited influence near Galactic plane.

Classical picture of Galactic gas scale height

e.g., Spitzer (1978)

Hc

gc

g

eff2

eff-1

-2

w h ere 6 - 1 0 k m s

cm s

1

3 1 0 1 19

, ,

, , ,

= ratio of magnetic energy density to kinetic energy density of clouds,

= ratio of cosmic ray pressure to kinetic energy density.

H 1 0 0 2 0 0 p c

Consistent with large scale surveys of H I. But individual star-forming regions appear to be distinct.

Ionization front in a stratified medium (W4)

Basu, Johnstone, & Martin (1999)

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 = e - z/H. Breakout when RSt/H > 1.

Ionization front around a wind-swept shell in the same atmosphere for n = 1, 5, 10, 15, and 20 cm-3. Require n > 10 cm-3 to fit observations of W4.

Evolution of Ionization FrontBasu, 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)?

Age of W4 Superbubble

t c n H L

c

H n L

t

1 01 3 5 3

01 3

1

03

03 7 1

6 3

2 5 1 0 3 1 0

2 5

/ / / .

.

, ,

.

a t cu rren t ep o ch .

p c , cm erg s

M y r.

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

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?

English et al. (2000)

The Mushroom Cloud: GW 123.4-1.5English 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 Survey

Dominion Radio Astrophysical ObservatoryNational Research Council of Canada

Australia Telescope Compact Array Commonwealth Science and Industrial Research Organisation

Very Large ArrayU.S. National Radio Astronomy Observatory

A Global Survey: CGPS, VGPS and SGPS

CGPS 1+2 :

6 5 1 8 00 0 l .

SGPS:

2 5 3 3 5 70 0 l .

VGPS:

1 8 6 7

1 8

0 0

0

l

l

p lan n ed

-5 p ro p o sed0

ConclusionsOnly 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