1 / 46

Physical Properties of Diffuse HI Gas in the Galaxy from the Arecibo Millennium Survey

This study presents the findings from the Arecibo Millennium Survey, focusing on the physical properties of diffuse HI gas in the Galaxy. We differentiate between two distinct phases of gas: the Cold Neutral Medium (CNM) and the Warm Neutral Medium (WNM). The CNM is characterized by a temperature of approximately 50 K and a density of around 50 cm⁻³, while the WNM has a temperature of about 5000 K and a density of around 0.5 cm⁻³. We explore various questions about thermal gas pressure, turbulent gas pressure, and magnetic fields to understand the dynamics and characteristics of diffuse HI gas.

licia
Télécharger la présentation

Physical Properties of Diffuse HI Gas in the Galaxy from the Arecibo Millennium Survey

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 2

  2. 1

  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??

More Related