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 Cold H I
     Steven J. Gibson
National Astronomy and Ionosphere Center
●   Ken Nordsieck – UW-Madison
●   Mark Holdaway – NRAO-Tucson
●   Russ Taylor, Jeroen Stil – U. Calgary
●   Chris Brunt - U. Exeter
●   Peter Dewdney, Lloyd Higgs - DRAO
                  The History of the Universe
  Bang                         (condensed)

 Infall                         Stellar Mass Loss

   Diffuse      Disruption   Molecular                          Stars
    ISM                       Clouds
             Condensation                           Formation        Stellar

           Area of Interest
                                                    Stellar Remnants
                                                    Cell Phones
            Why Study Cold H I ?

●   Abundant ISM phase
●   Traces quiescent gas (needed for star formation)
●   Exhibits intricate small-scale structure
●   Relationship with molecular hydrogen
●   Radiative transfer probes Galactic structure
                The Utility of Dust
●   Challenge – observed power spectra are red, so very
    small stuff hard to see, especially in HI
●   Approach -- try other CNM tracers, like dust! HI and
    dust should be well mixed in the CNM, and dust is traced
    by continuum (absorbed, scattered, or thermally
    radiated), so more detectable. Still, should compare the
    two where possible to check their agreement.
●   In the Galactic plane this may not work, and we must try
    other things like HISA. But first consider HI emission
    and dust structure in a nearby cloud.
The Pleiades Cluster – Optical Image by Robert Gendler
           Pleiades Reflection Nebula

    Relatively unconfused sightline (IRAS, E(B-V), Na I absorption,
●   Nearby (130 pc), well-lit nebula (good view of small-scale
●   Abundant structure is already known in many ISM tracers,
    including optical filaments
●   Additional evidence for structure implied by derived dust
    scattering properties in the UV (a ~ 0.4, g ~ 0.8), which are at odds
    with most models unless the dust is clumpy
●   Chance cloud/cluster collision – opportunity to see random CNM
    sample lit by passing stars (perhaps shaped in part by the
    interaction, but only in part)
Extended Pleiades Nebulosity - Image by Russell
                            Burrell Schmidt 0.6m Mosaic
                                      (log intensity scale)

Gibson & Nordsieck (2003)
IRAS 100 um, Log Scale
Larger IRAS View
Burrell-Schmidt 0.6m Mosaic
          (log intensity scale)
Burrell-Schmidt 0.6m Mosaic
          (log intensity scale)
Single Burrell-Schmidt Field
Single Burrell-Schmidt Field
Close-up of nebulosity East of Merope - WIYN 3.5m

                                       Merope (23 Tau)
Close-up of nebulosity East of Merope - WIYN 3.5m

                                       Merope (23 Tau)
     IC 349, Barnard’s Merope Nebula
HST Planetary Camera (Herbig & Simon 2000)
IRAS 100 um, Log Scale
IRAS 100 um, Log Scale
Burrell-Schmidt 0.6m Mosaic
          (log intensity scale)
      VLA D-array mosaic
      + Green Bank 43m
      H I 21cm emission
      V(LSR) = -1.3 km/s

Gibson, Holdaway
& Nordsieck (1995)
      VLA D-array mosaic
      + Green Bank 43m
      H I 21cm emission
      V(LSR) = +10 km/s

Gibson, Holdaway
& Nordsieck (1995)
    HI Filaments : Cylinders or Sheets?
●   VLA beam-scale = 60” ~ 0.035 pc
●   dT ~ 30 K, FWHM ~ 6 km/s
●   So NHI(tau<<1) ~ 3.5e+20 cm^-2
●   If line-of-sight thickness = angular diameter,
    then n ~ 3000 cm^-3; for T=50 K, n*T =
    HI Filaments : Cylinders or Sheets?
●   PCNM(therm)/k ~ 4000 (``standard’’)
●   PCNM(turbulence) /k ~ 20,000 (Heiles 1997)
●   PCNM(HSEQ)/k ~ 28,000 (Boulares & Cox 1990)
●   P(therm) => elongation factor of 38 or T=1.3 K
●   P(turb) => elongation factor of 7.5 or T=6.7 K
●   P(HSEQ) => elongation factor 5.4 or T=9.3 K
VLA D-array mosaic
+ Green Bank 43m
H I 21cm emission
V(LSR) = +10 km/s
    How Else Can CNM Be Imaged?
●   Dust and Emission are both useful for targets
    away from the Galactic plane.
●   What about down in the disk, where most
    material is found?
●   Try absorption, using velocity to discriminate
    distance and to probe Galactic kinematics
A Very Nearby Edge-On Spiral
Galactic H I 21 cm Line Emission

 Leiden-Dwingeloo Northern Sky Survey (Hartmann & Burton 1997)
        A Closer View of Galactic H I
                  (Small Single-Dish Radio Telescope;
                 one velocity plane in the Perseus Arm)

25-m Radio Telescope, Dwingeloo (0.5 degree beam)
Netherlands Foundation for Radio Astronomy
        A Closer View of Galactic H I
                  (Radio Interferometer Synthesis Array;
                  one velocity plane in the Perseus Arm)

7-element Interferometer, Penticton (1 arcminute beam)
Dominion Radio Astrophysical Observatory                 Canadian Galactic Plane Survey
equivalent diameter equals 600m
                                                                     (Taylor et al. 2003)
H I Self-Absorption (HISA)

                             Has both
      Dark Optical and Radio Clouds
●   Both HISA and classical optical dark clouds trace
    cold gas in the ISM.
●   Optical dark clouds can be found on many scales.
●   With synthesis imaging, we see that HISA also
    exists on a range of scales.
●   How small does it go?
Coalsack Dust Cloud           Perseus HISA Complex

Distance ~ 600 pc              Distance ~ 2000 pc
Angular size ~ 7 x 4 deg2      Angular size ~ 3 x 2 deg2
Physical size ~ 75 x 45 pc2    Physical size ~ 105 x 70 pc2
B92 Dark Cloud                  MK2 HISA Complex

Distance ~ 3000 pc              Distance ~ 100 - 1000 pc ?
Angular size ~ 15 x 9 arcmin2   Angular size ~ (3 deg)2
Physical size ~ 14 x 8 pc2      Physical size ~ (5 - 50 pc)2 ?
MK2 Complex + Northern Coalsack

     CGPS HISA    CfA 12CO
B86 (Ink Spot Nebula)           Perseus HISA Globule

Distance ~ 1700 pc
Angular size ~ 5' x 3'
Physical size ~ 2.5 x 1.5 pc2

                                Distance ~ 2000 pc
                                Angular size ~ 1.5' x 1.3'
                                Physical size ~ 1.0 x 0.8 pc2
B33 (Horsehead Nebula)                       IC 2944 (Thackeray's Globules)

                                   Local HISA

Distance ~ 500 pc
Angular size ~ 5'                                         Distance ~ 1800 pc
Physical size ~ 0.7 pc                                    Angular size ~ < 40”
                                                          Physical size ~ < 0.3 pc

                         Distance ~ 100 - 500 pc
                         Angular size ~ 1' x 5'
                         Physical size ~ 0.03-0.1 x 0.13-0.25 pc
What about HISA Power Spectrum?
●   HISA is easier to see at high angular resolution –
    is PS different than emission?
    –   Need for background and general detectability may
        require this
    –   But perhaps there is astrophysical reason as well?
●   Unfortunately untangling detection biases is
    tricky – method of identifying and extracting
    HISA must be taken into account
HISA Identification Algorithm
HISA vs HIE power spectra?
              H I Emission
  Log Power

                         H I Self-Absorption

               Log Spatial Frequency

              Stay tuned . . .
●   CNM structure can be traced in a variety of ways over a
    large range in angular and linear scale.
●   Optical reflection nebulosity shows a power-law power
    spectrum of index -2.8 over 5 orders of scale magnitude.
●   IRAS structure tracks this same law but less well.
●   VLA HI may also follow this but more work is required
    to verify the result.
●   HISA shows cold gas in the disk over large areas but
    with considerably intricate structure, often directly
    analogous to optical dark clouds.
●   The HISA power spectrum would be very useful if its
    biases can be overcome. Work is ongoing.

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