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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 u 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 u “Diffuse” gas means non self-gravitating gas. u Diffuse HI gas appears to exist in two distinct phases in approximate pressure equilibrium: I see! CGPS 21cm HI

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

Warm Neutral Medium (WNM) u Observed in 21cm HI emission u T  5000 K, n HI  0.5 cm -3 (n HI higher in morphologically distinct shells & envelopes) Dickey & Lockman

Some questions about diffuse HI in Galaxy 1.What is the range of T K, N HI,  V turb 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! ?

2. Arecibo Millennium Survey u Survey of Galactic HI absorption & emission toward 66 extra-galactic continuum sources (most with |b| > 10 o ). u Results sample CNM and WNM along random lines of sight in local Galaxy. Arecibo telescope

Millennium Survey Publications to date by Heiles & Troland u ApJS, 145, 329 (2003a)Paper I u ApJ, 586, 1067 (2003b) Paper II u ApJS, (2004)Paper III u ApJ, 624, 773 (2005)Paper IV Arecibo telescope

Millennium Survey u Toward each continuum source, we obtain in Stokes I: 1.HI opacity profile, e -  2.“Expected” HI emission profile, T exp (v) u 1 st & 2 nd HI spatial derivatives removed from 2. u Analogous profiles also obtained for Stokes Q, U, V. Heiles, ApJ, 551, L105 (2001) 3C18

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

2b. Fitting emission profile (Stokes I) u Emission profile fitted simultaneously to (1) + (2) where: u (1) Emission of isothermal CNM components previously identified in  (v). u (2) Emission of WNM Gaussians (1 or 2), each assumed to represent a component not detected in  (v). u 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) u Fit results - N HI & T kmax for each WNM component, and T s and N HI for each CNM component u Assuming T s = T K for CNM, we can also derive  V turb for each CNM component from  V tot. T kmax   V tot 2 is maximum T K allowed by  V tot.

2c. Fitting Stokes V opacity profile u  V (v) fitted to sum of derivatives of CNM components in  I (v) (Zeeman effect) Fit results – B los (and error  ) for each CNM component u Instrumental errors carefully evaluated, they precluded reliable fits for B los in WNM components.

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

Fit Results - Summary u CNM components – T s, N HI,  V turb, B los u WNM components – T kmax, N HI Above Arecibo telescope

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

Results of Arecibo Millennium Survey Statistics of HI Zeeman effect (all sources) u Obtained  (B los ) < 10  G for 69 CNM components. u Detected B los in 22 CNM components (at 2.5  level). Arecibo telescope

3a. Temperatures (CNM & WNM) Number of CNM & WNM components vs. T kmax   V tot 2 u CNM components form a distinct population with low T. Paper II

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

Temperatures (WNM) Number of WNM components vs. T kmax u At least half of WNM has T kmax < 5000 K, cooler than thermally-stable equilibrium value of 8000 K. (Not consistent with MO77.) Paper II

3b. n HI (CNM & WNM) u CNM pressure estimated from CI & CII absorption lines in the uv (Jenkins & Tripp 2001). P/k  3000 cm -3 K (  3  ), so n HI  3000/T u T CNM  K  n HI,CNM  150 – 30 cm -3 u T WNM  ,000K  n HI,WNM  3 – 0.3 cm -3

3c. Mass & volume statistics (WNM) Statistics of N(HI) for WNM suggest: u WNM amounts to  60% of all HI by mass (much more than classical MO77 equilibrium theory predicts) u WNM has volume filling factor  50% in GP (very rough)

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

3e. B los in CNM u B los vs. N(HI) los for CNM components Crosses have |B los | > 2.5  B los N(HI)  cm -2

B los in CNM u B los typically  5  G u Median value for total magnetic field 6.0  1.8  G (Paper IV) B = 6  G!

3f. Energetics in CNM u Data from Millennium Survey permit comparisons in CNM among relevant energies: 1.Thermal motions (gas pressure, P therm ) 2.Turbulent motions (turbulent pressure, P turb ) 3.Magnetic field (magnetic pressure, P mag = B 2 /8  ) 4.Gravitation (mass-to-flux ratio)

Energetics in CNM Turbulent Mach number u  V turb is FWHM in km s -1 See Paper IV for details

Energetics in CNM Number of CNM components vs. M turb u Most CNM components have highly supersonic turbulence (typically, M turb  3). supersonic Paper II

Energetics in CNM Thermal plasma parameter uB in  G See Paper IV for details

Energetics in CNM Turbulent plasma parameter u  V turb is FWHM in km s -1 u B in  G

Energetics in CNM Mass-to-flux ratio (M/  ) u A measure of ratio of gravitational to magnetic energies in a self-gravitating cloud. u 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/  ) u M/  > 1 magnetically supercritical u M/  < 1 magnetically subcritical, self-gravitating cloud supported by B N(H) in cm -2 B in  G

Energetics in CNM u Median parameters of the CNM (but wide dispersion) ParameterValue  V turb (FWHM) 2.8 km s -1 T K 50 K B 6  G n HI 55 cm -3 N HI 0.5  cm -2 Arecibo telescope

Energetics in CNM u Energy balance in the CNM ParameterRatioMedian value Significance M 2 turb P turb /P therm 14CNM highly supersonic  therm P therm /P mag 0.29P mag slightly dominates P therm  turb P turb /P mag 1.9 P turb  P mag (near magnetic equipartition) M/  Gravitational to magnetic energy 0.03CNM 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,  50 K, some components have T < 20K

4. Some key conclusions 5.Median field strength in CNM is B tot = 6.0  1.8  G 6.CNM is highly turbulent, in near magnetic equipartion (P turb  P mag ) 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 Regimen (cm-3) B tot (  G) Data WIM (DIM)0.2  5 RMs &DMs WNM1-10 (in shells & envelopes) 5-10* Zeeman effect in HI emission CNM  6* Zeeman effect in HI absorption Dark cloud envelopes few 100 to * Zeeman effect in OH emission * Many sensitive upper limits

END

The B-n relationship in the diffuse ISM Conclusion u Evidence now clear that B largely unrelated to n in low density ISM over 3+ orders of magnitude. u How does high density ISM form from low density ISM??