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Thin, Cold Strands of Hydrogen in the Riegel-Crutcher Cloud

Thin, Cold Strands of Hydrogen in the Riegel-Crutcher Cloud. Naomi McClure-Griffiths Australia Telescope National Facility, CSIRO SINS meeting 21 May 2006. TSAS and the structure of the CNM.

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Thin, Cold Strands of Hydrogen in the Riegel-Crutcher Cloud

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  1. Thin, Cold Strands of Hydrogen in the Riegel-Crutcher Cloud Naomi McClure-Griffiths Australia Telescope National Facility, CSIRO SINS meeting 21 May 2006

  2. TSAS and the structure of the CNM • Heiles (1997) suggested that TSAS might be explained by highly elongated structures aligned along the line of sight • The perpendicularly aligned counterparts have very small NH • Possibly detected by Braun & Kanekar (2005) & Stanimirovic & Heiles (20005) • The structure of the cold neutral medium (CNM) is difficult to study: • Generally study the CNM by measuring HI absorption towards continuum sources • Good for studying the temperature, density but provides little information about the structure of the CNM • High resolution HI self-absorption allows imaging of the CNM, but with confusion from a varying background (e.g. Gibson et al, 2000, 2005) • The Riegel-Crutcher cloud towards the Galactic Centre is one of the best known self-absorption features with a very bright, uniform HI emission background

  3. SGPS Galactic Centre Survey • Extension to the SGPS to cover the Galactic Centre • Covers -5º ≤ l ≤+5º and -5º ≤ l ≤+5º • Angular resolution of 100” • 967 pointings • Sensitivity: 1 - 2 K

  4. The Riegel-Crutcher Cloud • Discovered by Heeschen (1955) • Mapped by Riegel & Jennings (1969), Riegel & Crutcher (1972), Mongomery et al (1995) • CaII and NaI measurements give: • Distance 125 ± 25 pc • Thickness 1 - 5 pc • On the edge of the Local Bubble

  5. Interpolating the Background Widths <0.07 pc, length ~17 pc

  6. Temperature and Column Density • Some profiles are saturated, allowing us to derive optical depth and temperature (Ts ~ 40 K) • Comparable to all previous estimates • Filaments are unresolved with widths of <7x10-2 pc • Average column density for the base is NH ~ 6x 1020 cm-2 • Typical column densities for the filaments is NH ~ 1x 1020 cm-2 • Not exceptional properties for the CNM Column density map

  7. Thermal Pressure? • The thermal pressure is: nT = NH/s, where s is the thickness of the filaments • Two possible values for the thickness: • Filaments are cylindrical, thickness is <0.07 pc: • n ~ 450 cm-3, nT ~ 1.8 x 104 K cm-3 • They are in pressure equilibrium with the standard nT~4000 K cm-3, so the thickness is ~0.4 pc: • n ~ 100 cm-3

  8. Magnetic Field Structure • 56 stellar polarization measurements from 200 pc - 2 kpc (Heiles 2000) • Mean polarization angle: <p> = 53 º ± 11º, aligned very well with the filaments • Chandrasekhar-Fermi (1953) method gives a B ~ 40 G (errors of a factor of two expected) • B-field comparable to values obtained for similar densities in molecular clouds (Troland & Heiles 1986, Crutcher 1999) Vectors aligned with B-field

  9. Magnetic Dominated Structure • Excellent alignment of the filaments and the magnetic field • Filaments extremely straight • Suggests that the gas follows the magnetic field, rather than the magnetic field following the gas • For the magnetic field to dominate, the magnetic pressure must exceed the gas pressure • Gas (thermal + non-thermal) pressure, Pi = 2 ~ 6 x 104 K cm-3 • B2/8 > Pi: so B>14 G • Easily reached by the 40 G estimation from the stellar polarization

  10. Conclusions • Some cold HI appears to exist in thin filaments • The Riegel-Crutcher cloud seems to have thin threads that are <0.07 pc • Aligned with the local magnetic field • It seems that we are observing the sort of ‘skinny’ structure that Heiles (1997) suggested for TSAS (but bigger and denser) • Do these filaments scale to even smaller scales? • What role do magnetic fields play in general in producing the CNM structure?

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