Harvard Astronomy 201b

Detecting the phases of the ISM: wild ideas

In Journal Club, Journal Club 2013 on March 7, 2013 at 7:45 pm

These are the ideas reported by the student groups in our 7 March discussion of McKee & Ostriker (1977).

Group 1

  • Use line ratios to discriminate between CNM/WNM
  • Use SNR light echoes to determine dust 3D structure

Group 2

  • The key is to disentangle v_thermal from v_bulk
  • If the velocity dispersion is dominated by thermal motion, then v ~ m^1/2, and therefore different species should have different line widths. If instead you’re looking at a cold cloud with a velocity distribution dominated by bulk motion, the line width will be independent of the mass of the species.
  • Use different distributions along a line of sight. If the implied temperature variance changes as a function of distance along the line of sight, then it may be bulk velocity. if it doesn’t, you’re looking at thermal velocity.
  • Use face on spiral
  • Use dust content as proxy, since it will only sublimate at high temperatures
  • Use hyperfine lines kin a transition which have different optical depth to look into different shells of a cloud

Group 3

  • conduct a large survey of 21 cm emission
    • goal: see whether WNM is a distinct phase, or just the product of several CNM clouds moving quickly
    • method: try to distinguish between line widths and line shifts
    • need very high-resolution spectrograph!
  • look for dust towards a region that might be WNM
    • dust sublimates at ~2000 K
    • so, dust would exist if the region is actually CNM, but not if it’s WNM
  • probe a cloud in a “layered” approach
    • test the proposed structure of CNM core with WNM, WIM in successive layers
    • observe at a range of wavelengths that probe different layers (perhaps at a range of wavelengths that become optically thick at different layers)
  • collaborate with other alien astronomers
    • get different lines of sight!

Group 4

  • 21-cm observations at different galactic latitudes
  • count clouds in a volume to get a filling factor (use CO or IR emission from dust)
  • look at other galaxies
  • look for bubbles along the line of sight
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