Thin, Cold Strands of Hydrogen in the Riegel-

Crutcher Cloud

Naomi McClure-GriffithsAustralia Telescope National Facility, CSIRO

SINS meeting 21 May 2006

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

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

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

Interpolating the Background

Widths <0.07 pc, length ~17 pc

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

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

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

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

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?