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Yes, signal!

2. 1. 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). Son, it’s like this….

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Yes, signal!

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  3. Yes, signal!

  4. 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

  5. Collaborator • C. Heiles (Berkeley, USA) Son, it’s like this… Carl Heiles explains magnetic field measurements to the next speaker.

  6. 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

  7. Cold Neutral Medium (CNM) • Observed in 21cm HI absorption (including self absorption) • T 50 K, nHI  50 cm-3. CGPS, 21cm HI (Perseus region)

  8. 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

  9. Some questions about diffuse HI in Galaxy • What is the range of TK, NHI, Vturb in the CNM and in WNM? • Are the two phases physicallydistinct or only observationally distinct? • What are the mass fractions and volume filling factors of the CNM and WNM?

  10. Some questions about diffuse HI in Galaxy • How strong is the magnetic field (HI Zeeman effect) • What is the relative importance of thermal gas pressure, turbulent gas pressure and magnetic pressure in diffuse HI gas? • What is the mass-to-flux ratio in diffuse HI gas?

  11. Some questions about diffuse HI in Galaxy • How do these physical characteristics compare with predictions from theory, e.g. McKee & Ostriker 1977, 3-phase ISM in equilibrium (MO77)? ? Good question!

  12. 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

  13. 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

  14. Millennium Survey 3C18 • Toward each continuum source, we obtain in Stokes I: • HI opacity profile, e- • “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)

  15. 2a. Fitting opacity profile (Stokes I) • Opacity profile  (v) fitted to Gaussians, each assumed to represent an isothermal CNM component. 3C18 Fit results - o, vo & Vtot for each CNM component 3 CNM components

  16. 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)

  17. Fittingemission profile(Stokes I) (2) WNM component 3C18 (1) CNM emission (sum of 3 components) Heiles, ApJ, 551, L105 (2001)

  18. Fittingemission profile(Stokes I) • Fit results - NHI& Tkmaxfor each WNM component, and Ts and NHI for each CNM component • Assuming Ts = TK for CNM, we can also derive Vturbfor each CNM component from Vtot. Tkmax Vtot2 is maximum TK allowed by Vtot.

  19. 2c. Fitting Stokes Vopacity 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.

  20. Fitting Stokes V opacity profile I opacity profile CNM component (1 of 6) 3C 138 V opacity profile  dI/dv Blos = 5.6  1.0 G Blos = 11  3.1 G Paper III

  21. Fit Results - Summary • CNM components – Ts, NHI, Vturb, Blos • WNM components –Tkmax, NHI Above Arecibo telescope

  22. 3. Results of Arecibo Millennium Survey • Identified 143 CNM components toward 48 sources. • Identified 143 WNM components toward 66 sources. Statistics (sources with |b| > 10o) Beneath Arecibo telescope

  23. Results of Arecibo Millennium Survey Statisticsof HI Zeeman effect (all sources) • Obtained  (Blos) < 10 G for 69 CNM components. • Detected Blosin 22 CNM components (at 2.5  level). Arecibo telescope

  24. 3a. Temperatures (CNM & WNM) Number of CNM & WNM components vs. Tkmax Vtot2 • CNM components form a distinct population with low T. Paper II

  25. Temperatures (CNM) Number of CNM components vs. Ts Very low Ts no grain heating Solid line: |b| > 10o Dotted: |b| < 10o median Ts = 48K Paper II

  26. 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

  27. 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

  28. 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)

  29. 3d. Turbulent velocity widths (CNM) • Number of CNM components vs. turbulent velocity dispersion (0.42  FWHM) median Vturb = 2.8 km s-1 FWHM Paper IV

  30. 3e. Blos in CNM • Blosvs. N(HI)los for CNM components Blos N(HI)  1020 cm-2 Crosses have |Blos| > 2.5 

  31. Blos in CNM • Blostypically  5 G • Median value for total magnetic field 6.0 1.8 G (Paper IV) B = 6 G!

  32. 3f. Energetics in CNM • Data from Millennium Survey permit comparisons in CNM among relevant energies: • Thermal motions (gas pressure, Ptherm) • Turbulent motions (turbulent pressure, Pturb) • Magnetic field (magnetic pressure, Pmag = B2/8) • Gravitation (mass-to-flux ratio)

  33. Energetics in CNM Turbulent Mach number • Vturb is FWHM in km s-1 See Paper IV for details

  34. Energetics in CNM Number of CNM components vs. Mturb • Most CNM components have highly supersonic turbulence (typically, Mturb  3). supersonic Paper II

  35. Energetics in CNM Thermal plasma parameter • B in G See Paper IV for details

  36. Energetics in CNM Turbulent plasma parameter • Vturb is FWHM in km s-1 • B inG

  37. 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).

  38. 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

  39. Energetics in CNM • Median parameters of the CNM (but wide dispersion) Arecibo telescope

  40. Energetics in CNM • Energy balance in the CNM

  41. 4. Some key conclusions • CNM and WNM appear to be physically distinct phases (T distributions very different) • About half of WNM has T < 5000 K, thermally unstable (c.f. de Avillez, Audit & Hennebelle) • WNM comprises more than half of the diffuse HI • CNM relatively cool, <T> 50 K, some components have T < 20K

  42. 4. Some key conclusions • Median field strength in CNM is Btot = 6.0 1.8 G • CNM is highly turbulent, in near magnetic equipartion (Pturb Pmag) • CNM is magnetically subcritical (so self-gravitating clouds formed from CNM without loss of magnetic flux will be magnetically dominated)

  43. 5. The B-n relationship in the diffuse ISM *Many sensitive upper limits

  44. END

  45. 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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