2. 1 yes, signal! physical properties of diffuse hi gas in the galaxy from the arecibo millennium...
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Yes, signal!
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Physical Properties of diffuse HI gas in the Galaxy from the Arecibo Millennium Survey
T. H. Troland
Physics & Astronomy Department
University of Kentucky, USA
Orsay, September 14, 2005
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Collaborator
C. Heiles (Berkeley, USA)
Carl Heiles explains magnetic field measurements to the next speaker.
Son, it’s like this…
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1. Diffuse HI gas in the Galaxy
“Diffuse” gas means non self-gravitating gas.
Diffuse HI gas appears to exist in two distinct phases in approximate pressure equilibrium:
I see!
CGPS
21cm HI
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Cold Neutral Medium (CNM)
Observed in 21cm HI absorption (including self absorption)
T 50 K, nHI 50 cm-3.
CGPS, 21cm HI (Perseus region)
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Warm Neutral Medium (WNM)
Observed in 21cm HI emission
T 5000 K, nHI 0.5 cm-3 (nHI higher in morphologically distinct shells & envelopes)
Dickey & Lockman
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Some questions about diffuse HI in Galaxy
1. What is the range of TK, NHI, Vturb in the CNM and in WNM?
2. Are the two phases physically distinct or only observationally distinct?
3. What are the mass fractions and volume filling factors of the CNM and WNM?
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Some questions about diffuse HI in Galaxy
4. How strong is the magnetic field (HI Zeeman effect)
5. What is the relative importance of thermal gas pressure, turbulent gas pressure and magnetic pressure in diffuse HI gas?
6. What is the mass-to-flux ratio in diffuse HI gas?
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Some questions about diffuse HI in Galaxy
7. How do these physical characteristics compare with predictions from theory, e.g. McKee & Ostriker 1977, 3-phase ISM in equilibrium (MO77)?
Good question!
?
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2. Arecibo Millennium Survey
Survey of Galactic HI absorption & emission toward 66 extra-galactic continuum sources (most with |b| > 10o).
Results sample CNM and WNM along random lines of sight in local Galaxy.
Arecibo telescope
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Millennium Survey
Publications to date by Heiles & Troland ApJS, 145, 329 (2003a) Paper I ApJ, 586, 1067 (2003b) Paper II ApJS, 151 271 (2004) Paper III ApJ, 624, 773 (2005) Paper IV
Arecibo telescope
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Millennium Survey
Toward each continuum source, we obtain in Stokes I:
1. HI opacity profile, e-
2. “Expected” HI emission profile, Texp(v)
1st & 2nd HI spatial derivatives removed from 2.
Analogous profiles also obtained for Stokes Q, U, V.
Heiles, ApJ, 551, L105 (2001)
3C18
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2a. Fitting opacity profile (Stokes I)
Opacity profile (v) fitted to Gaussians, each assumed to represent an isothermal CNM component.
Fit results - o, vo & Vtot for each CNM component
3 CNM components
3C18
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2b. Fitting emission profile (Stokes I)
Emission profile fitted simultaneously to (1) + (2) where:
(1) Emission of isothermal CNM components previously identified in (v).
(2) Emission of WNM Gaussians (1 or 2), each assumed to represent a component not detected in (v).
Radiative transfer effects included (CNM absorption)
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Fitting emission profile (Stokes I)
(2) WNM component
(1) CNM emission (sum of 3 components)
Heiles, ApJ, 551, L105 (2001)
3C18
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Fitting emission profile (Stokes I)
Fit results - NHI & Tkmax for each WNM component, and Ts and NHI for each CNM component
Assuming Ts = TK for CNM, we can also derive Vturb for each CNM component from Vtot .
Tkmax Vtot2 is maximum TK allowed by Vtot.
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2c. Fitting Stokes V opacity profile
V (v) fitted to sum of derivatives of CNM components in I (v) (Zeeman effect)
Fit results – Blos (and error ) for each CNM component
Instrumental errors carefully evaluated, they precluded reliable fits for Blos in WNM components.
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Fitting Stokes V opacity profile
CNM component (1 of 6)
I opacity profile
Blos = 11 3.1 G
V opacity profile dI/dv Blos = 5.6 1.0 G
Paper III
3C 138
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Fit Results - Summary
CNM components – Ts, NHI, Vturb, Blos
WNM components – Tkmax, NHI
Above Arecibo telescope
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3. Results of Arecibo Millennium Survey
Identified 143 CNM components toward 48 sources.
Identified 143 WNM components toward 66 sources.
Beneath Arecibo telescope
Statistics (sources with |b| > 10o)
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Results of Arecibo Millennium Survey
Statistics of HI Zeeman effect (all sources)
Obtained (Blos) < 10 G for 69 CNM components.
Detected Blos in 22 CNM components (at 2.5 level).
Arecibo telescope
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3a. Temperatures (CNM & WNM)
Number of CNM & WNM components vs. Tkmax Vtot2
CNM components form a distinct population with low T.
Paper II
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Temperatures (CNM)
Number of CNM components vs. Ts
median Ts = 48K
Very low Ts no grain heating
Solid line: |b| > 10o
Dotted: |b| < 10o
Paper II
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Temperatures (WNM)
Number of WNM components vs. Tkmax
At least half of WNM has Tkmax < 5000 K, cooler than thermally-stable equilibrium value of 8000 K. (Not consistent with MO77.)
Paper II
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3b. nHI (CNM & WNM)
CNM pressure estimated from CI & CII absorption lines in the uv (Jenkins & Tripp 2001).
P/k 3000 cm-3 K ( 3 ), so nHI 3000/T
TCNM 20-100 K nHI,CNM 150 – 30 cm-3
TWNM 1000-10,000K nHI,WNM 3 – 0.3 cm-3
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3c. Mass & volume statistics (WNM)
Statistics of N(HI) for WNM suggest:
WNM amounts to 60% of all HI by mass (much more than classical MO77 equilibrium theory predicts)
WNM has volume filling factor 50% in GP (very rough)
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3d. Turbulent velocity widths (CNM)
Number of CNM components vs. turbulent velocity dispersion (0.42 FWHM)
median Vturb = 2.8 km s-1 FWHMPaper IV
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3e. Blos in CNM
Blos vs. N(HI)los for CNM components
Crosses have |Blos| > 2.5
Blos
N(HI) 1020 cm-2
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Blos in CNM
Blos typically 5 G
Median value for total magnetic field 6.0 1.8 G (Paper IV)
B = 6 G!
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3f. Energetics in CNM
Data from Millennium Survey permit comparisons in CNM among relevant energies:
1. Thermal motions (gas pressure, Ptherm)
2. Turbulent motions (turbulent pressure, Pturb)
3. Magnetic field (magnetic pressure, Pmag = B2/8)
4. Gravitation (mass-to-flux ratio)
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Energetics in CNM
Turbulent Mach number
Vturb is FWHM in km s-1
therm
turb
K
turbturb P
P
T
VM
22
92
See Paper IV for details
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Energetics in CNM
Number of CNM components vs. Mturb
Most CNM components have highly supersonic turbulence (typically, Mturb 3).
supersonic
Paper II
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Energetics in CNM
Thermal plasma parameter
B in G
See Paper IV for details
mag
thermHIKtherm P
P
B
nT
23108.3
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Energetics in CNM
Turbulent plasma parameter
Vturb is FWHM in km s-1
B in G
mag
turbturbHIturb P
P
B
Vn
2
2
16.0
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Energetics in CNM
Mass-to-flux ratio (M/)
A measure of ratio of gravitational to magnetic energies in a self-gravitating cloud.
M/ conserved as long as flux freezing is maintained (so M/ in CNM may determine M/ in self-gravitating clouds).
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Energetics in CNM
Mass-to-flux ratio (M/)
M/ > 1 magnetically supercritical
M/ < 1 magnetically subcritical, self-gravitating cloud supported by B
N(H) in cm-2
B in G
B
N
M
MH
criticalM
21/ 108.3
/
/
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Energetics in CNM
Median parameters of the CNM (but wide dispersion)
Parameter Value
Vturb (FWHM) 2.8 km s-1
TK 50 K
B 6 G
nHI 55 cm-3
NHI 0.5 1020 cm-2
Arecibo telescope
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Energetics in CNM
Energy balance in the CNM
Parameter Ratio Median value
Significance
M2turb Pturb/Ptherm
14 CNM highly supersonic
therm Ptherm/Pmag0.29 Pmag slightly dominates Ptherm
turb Pturb/Pmag1.9 Pturb Pmag (near magnetic
equipartition)
M/Gravitational to magnetic energy
0.03 CNM magnetically subcritical (magnetically dominated)
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4. Some key conclusions
1. CNM and WNM appear to be physically distinct phases (T distributions very different)
2. About half of WNM has T < 5000 K, thermally unstable (c.f. de Avillez, Audit & Hennebelle)
3. WNM comprises more than half of the diffuse HI
4. CNM relatively cool, <T> 50 K, some components have T < 20K
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4. Some key conclusions
5. Median field strength in CNM is Btot = 6.0 1.8 G
6. CNM is highly turbulent, in near magnetic equipartion (Pturb Pmag)
7. CNM is magnetically subcritical (so self-gravitating clouds formed from CNM without loss of magnetic flux will be magnetically dominated)
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5. The B-n relationship in the diffuse ISM
Regime n(cm-3)
Btot
(G)
Data
WIM (DIM) 0.2 5 RMs &DMs
WNM 1-10(in shells & envelopes)
5-10* Zeeman effect in HI emission
CNM 30-150 6* Zeeman effect in HI absorption
Dark cloud envelopes
few 100 to 1000
10-20* Zeeman effect in OH emission
*Many sensitive upper limits
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END
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The B-n relationship in the diffuse ISM
Conclusion
Evidence now clear that B largely unrelated to n in low density ISM over 3+ orders of magnitude.
How does high density ISM form from low density ISM??
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