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Yes, signal!

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

Collaborator

C. Heiles (Berkeley, USA)

Carl Heiles explains magnetic field measurements to the next speaker.

Son, it’s like this…

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

Cold Neutral Medium (CNM)

Observed in 21cm HI absorption (including self absorption)

T 50 K, nHI 50 cm-3.

CGPS, 21cm HI (Perseus region)

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

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?

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?

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

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

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

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

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)

Fitting emission profile (Stokes I)

(2) WNM component

(1) CNM emission (sum of 3 components)

Heiles, ApJ, 551, L105 (2001)

3C18

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.

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.

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

Fit Results - Summary

CNM components – Ts, NHI, Vturb, Blos

WNM components – Tkmax, NHI

Above Arecibo telescope

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)

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

3a. Temperatures (CNM & WNM)

Number of CNM & WNM components vs. Tkmax Vtot2

CNM components form a distinct population with low T.

Paper II

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

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

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

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)

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

3e. Blos in CNM

Blos vs. N(HI)los for CNM components

Crosses have |Blos| > 2.5

Blos

N(HI) 1020 cm-2

Blos in CNM

Blos typically 5 G

Median value for total magnetic field 6.0 1.8 G (Paper IV)

B = 6 G!

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)

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

Energetics in CNM

Number of CNM components vs. Mturb

Most CNM components have highly supersonic turbulence (typically, Mturb 3).

supersonic

Paper II

Energetics in CNM

Thermal plasma parameter

B in G

See Paper IV for details

mag

thermHIKtherm P

P

B

nT

23108.3

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

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).

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

/

/

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

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)

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

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)

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

END

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??