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Magnetically Dominated Strands of Hydrogen in the Riegel-Crutcher Cloud

Magnetically Dominated Strands of Hydrogen in the Riegel-Crutcher Cloud. Naomi McClure-Griffiths CSIRO Australia Telescope National Facility ATNF Science Symposium 30 April 2007. HI Morphological Structure. What is the structure of the cold neutral gas?

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Magnetically Dominated Strands of Hydrogen in the Riegel-Crutcher Cloud

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  1. Magnetically Dominated Strands of Hydrogen in the Riegel-Crutcher Cloud Naomi McClure-Griffiths CSIRO Australia Telescope National Facility ATNF Science Symposium 30 April 2007

  2. HI Morphological Structure • What is the structure of the cold neutral gas? • McKee & Ostriker (1977) isotropic cold clouds? • Or sheets and filaments (Heiles 1997)? • The structure of the cold neutral medium (CNM) is particularly difficult to study • CNM is generally studied by measuring HI absorption towards continuum sources • While good for studying the temperature, density, etc. of the gas, this 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) McKee & Ostriker (1977)

  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 McClure-Griffiths et al. (2006)

  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 McClure-Griffiths et al. (2006)

  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 (e.g. Montgomery et al. 1995) • 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 • Use the Chandrasekhar-Fermi (1953) method to estimate B • C-F gives a B ~ 60 G • Errors of a factor of two expected 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 energy density must exceed the kinetic energy density of the filaments • Kinetic energy density • Magnetic energy density • For M > K, B > 30 G

  10. Filamentary Structure in Cold Gas • The structure of the R-C cloud is far from isotropic! • These high aspect ratio are reminiscent of the filaments suggested by Heiles (1997) to explain “tiny scale atomic structure” • Filamentary structure also seen in molecular clouds, i.e Taurus,  Oph, Orion • These molecular clouds often show magnetic fields aligned with or perpendicular to the filaments (e.g. Goldsmith et al 2005, Matthews & Wilson 2000) • Is all cold HI structured more like molecular gas than warm HI? • Or only when the magnetic field is strong? Filamentary structure Taurus cloud 13CO emission Mizuno et al (1995)

  11. Conclusions • The Riegel-Crutcher cloud is a cold cloud observed in HI self-absorption • The cloud is made up of very thin filaments with aspect ratios of ~200:1 • 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 • The cloud structure appears determined by the magnetic field • The field strength is higher than that in most CNM regions • Field strength similar to that seen in molecular clouds of comparable density • But in those the field is expected to be gravitationally compressed • What role do magnetic fields play in general in producing the CNM structure?

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