The Diffuse Interstellar Medium ISM

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• The Diffuse Interstellar Medium (ISM)
• Lecture Topics
• the 21-cm line in emission and absorption
• heating-cooling balance and the phases of the
  ISM
• turbulence and structure in the ISM
• magnetic fields, polarization and Faraday
  tomography

John M. Dickey University of Tasmania February
2008
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• Observation of structure on a wide range of
  scales
• is not the same as observation of turbulence
• Can we learn from the observed structure in
• density
• velocity
• magnetic field
• temperature, pressure, ionized fraction
• how to determine the physical laws that govern
  the propagation of energy between scales ?

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• Characterizing Small Scale Structure in the ISM
• Spatial Power Spectrum (P1 or P2 or P3 for
  dimensions)
• Structure Function (D)
• (Cordes and Rickett 1998 Ap. J. 507, 846
• Lazarian and Pogosyan 2000 Ap. J. 537, 720
• Goldman 2000, Ap. J. 541, 701,
• Miville-Deschenes et al. 2003 Ap. J. 593, 831)
• Spectral Correlation Function
• (Padoan et al. 2003, Ap. J. 588, 881,
  Rosolowsky et al. 1999, Ap. J. 524, 887
• Ballesteros-Paredes et al. 2002, Ap. J. 571,
  334)
• Fractal dimensions
• (Westpfahl et al. 1999, Ap. J. 117, 868
• Stanimirovic et al. 1999, MNRAS 302, 417
• Elmegreen et al. 2001, Ap. J. 548, 749)

• Principal Component Analysis
• (Heyer and Schloerb, 1997, Ap. J. 475, 173)

• mhd wave spectrum
• (Cho et al. 2002, Heitsch et al. 2004)

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The Spatial Power Spectrum
1pc 1AU 1Rsun
The Cascade
In the ionized gas (the WIM) we see a continuous
spectrum over many orders of magnitude in scale
size.
Figure from Armstrong, Rickett, and Spangler
1995, Ap. J. 443, 209.
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Ramachandran et al. 2006 Ap. J. (Astroph -
0601242)
Dispersion measure variations toward PSR B193721
The structure function implied by these DM
variations
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(1 - the Structure Function) (the spatial
autocorrelation function) is the Fourier
Transform of the Spatial Power Spectrum
P2
1-D
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l21-cm measurement of the HI spatial power
spectrum (Green 1993MNRAS 262, 397)The power
law index is in the range -2.2 to -3.Typically
the range of scale sizes covered is 20 pc to
0.5 pc.
The Neutral Medium
power (Jy)2
(1 deg)-1 (2 arc-min)-1
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In most environments, including the
Magellanic Bridge and the local MW disk, the
slope of the 21-cm emission spatial power
spectrum steepens as we take broader and broader
velocity channels. The slope changes from (-2.25
to -3) to (-3 to -4). This fits the expected
transition from thin to thick (2d to 3d)
turbulence (-8/3 to -11/3) (Dickey et al. 2001
Ap. J. 561,264).
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An aside on the Large Magellanic Cloud and the
Magellanic Bridge

• In the HI Bridge between the LMC and the SMC
  the 21-cm emission shows the same power law
  structure function as in the MW disk.
• In the LMC there is an ordered (spiral)
  magnetic field, plus a disordered component
  similar to that in the MW.

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HI in the Magellanic Bridge Muller et al. 2004
Ap. J. 616, 845.
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Spatial Power Spectrum of HI in the Magellanic
Bridge (Muller et al. 2004)
the slope varies from -2.25 to -3 and steepens
with velocity averaging as in the MW.
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Is the turbulence generated on large scales,
either by Galactic processes or by collective
action of stellar winds and supernova
remnants? One possibility the breakdown of
shells, bubbles, and chimneys through gas dynamic
instabilities in the swept up matter.
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600 pc at 7 kpc
McClure-Griffiths, et al. 2000 A.J. 119, 2828.
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Some structures in the shell walls are easy
to understand, like these Rayleigh-Taylor drips.
Others are more problematic, like these narrow
ridges that seem to join at the bottom.
McClure-Griffiths et al. 2003, Ap. J. 594, 833.
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A CO cloud buried inside one of the HI drips of
GSH 277 (McClure-Griffiths 2006)
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A new chimney complex, similar to GSH277 but MUCH
larger on the sky. McClure-Griffiths and the
GASS team 2006.
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In HI, 21-cm absorption is better for tracing
structures smaller than 1 arc minute,
21-cm emission is better for structures larger
than 1. Absorption traces cool gas (10 lt T lt
150 K) which is a small fraction of the neutral
atomic gas by mass and volume. In some regions
the background 21-cm emission makes a cool
foreground HI cloud look dark (HISA HI
self-absorption)
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HI SGPS-II emission movie showing the
Riegel-Crutcher cloud in absorption
distance 100-150 pc
filament dimensions gt15 pc long x lt 0.1 pc wide
McClure-Griffiths et al. 2007
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One channel map of the R-C cloud with superposed
stellar polarization vectors from Heiles (2000,
A.J. 119, 923)
The magnetic field is probably involved in
shaping the long thin filaments of gas.
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Spatial Power Spectrum of the 21-cm Absorption
toward Cas A (Deshpande 2000)(simulation
fittedto the data)
0.02 pc
4 pc
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Brogan et al. 2006 Ap. J. 130, 698.
3C138 Continuum
21-cm absorption
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Optical Absorption Lines Studies of small scale
structure in the Na I absorption
M15 - Meyer and Lauroesch 1999 Ap.J. 520, L103.
0.3 pc
Time variation toward HD32039/40 -
Lauroesch, Meyer, and Blades 2000
0.2 pc
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Tiny Scales (103 AU to 103 km)

• HI absorption (VLBI observations)
• Interstellar scintillation and scattering
• Optical absorption lines toward multiple stars
• Time variations in pulsar absorption lines
• Time variations in pulsar dispersion measures
• Time variations in stellar absorption lines

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Scintillation Its not just for pulsars anymore!
Compact extragalactic continuum sources show
refractive interstellar scintillation (Low
Frequency Variables, Intra-Day Variables, Extreme
Scattering Events, )
Scintillation Regimes refractive scintillation
(riss) diffractive scintillation (diss) weak
scattering (weak) from Rickett (2002, PASA 19,
100)
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We can use refractive scintillation to study
source structure on 10 micro-arc-second
scales. We put the small scale structure of the
ISM to work as a giant refracting
telescope! Earth Orbit Synthesis of Intra-Day
Variables Dennett-Thorpe and de Bruyn 2003,
Astron. Astroph. 404, 113, Bignall et al. 2003,
Ap. J. 585, 653, Jauncy et al. 2003, Ast. Sp.
Sci 288, 63.)
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• The most dramatic example of refractive
  scintillation acting like an interstellar
  telescope is in pulsar parabolic arcs, where the
  ISM makes an interferometer (Stinebring et al.
  2001, Ap. J. 549, L97).
• The parabolic arc appears when pulsar dynamic
  spectra are Fourier transformed with respect to
  frequency and time delay.
• These are interpreted very convincingly as
  multipath interference, with a varying Doppler
  shift as well as varying path difference. The
  parabola results from a square law relationship
  between delay and Doppler shift.

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Parabolic Arcs in Pulsar Dynamic SpectraCordes
et al. 2006 Ap. J. 637, 346.
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Conclusion

• Soon we will fill in the gaps by measuring on all
  scales from a few thousand km to a few kpc
• density
• velocity (magnitude and direction)
• magnetic field (magnitude and direction)
• ionization fraction
• The Square Kilometer Array telescope will do it!

(Dickey et al. 2005 New Astr. Rev.)
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Ground state excitation of CI shows the
interstellar pressure distribution function is
bimodal 102 lt p lt 107
p107
p106
p105
p104
Jenkins and Tripp 2001 Ap. J. Supp. 137, 297.
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The Pressure Fluctuations associated with tiny
scale structure are not a problem.

• But why is the pressure
• distribution function bimodal?

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• What about the magnetic field?
• The magnetic field shows a spectrum of
  irregularities superposed on a large scale
  pattern. Does this reflect the same underlying
  turbulence as seen in the density and velocity
  fields? If not, can we understand why not? If
  so, which forces dominate the dynamics on what
  scales?

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The Penticton Lens - Gray et al. 1999, Uyaniker
et al. 2003
Structures in the linear polarization of the
Galactic synchrotron emission
due to Faraday rotation in the intervening
medium.
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Linear polarized emission from radio synchrotron
emission modulated by the intervening Faraday
rotation.
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• Polarized Intensity vs. Faraday Depth
• (Faraday Depth is Fourier conjugate to l2)

Brentjens and de Bruyn, 2005 Astron. Astrop. 441,
1217.
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de Bruyn et al. 2006 Astronomische Nachrichten
327, 487.
This is one plane from a cube of data.
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Structure Functions for the Emission Measure and
for the Rotation Measure in the Diffuse Ionized
Medium (Minter and Spangler 1996)
RM
EM
log (structure function)
0.1o 1o 10o
0.1o 1o 10o
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Structure function of the RM (and DM
)between spiral arms (top row) and through
spiral arms (bottom row) Haverkorn et al. 2006

Question On what scales does the magnetic
field dominate the gas pressure to drive the
dynamics of the cascade?