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Multiscale Structure in Cold H I Steven J. Gibson National Astronomy and Ionosphere Center Collaborators ● Ken Nordsieck – UW-Madison ● Mark Holdaway – NRAO-Tucson ● Russ Taylor, Jeroen Stil – U. Calgary ● Chris Brunt - U. Exeter ● Peter Dewdney, Lloyd Higgs - DRAO Big The History of the Universe Bang (condensed) Galactic Infall Stellar Mass Loss Diffuse Disruption Molecular Stars ISM Clouds Star Condensation Formation Stellar Evolution Area of Interest Stellar Remnants Planets People 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, polarimetry) ● Nearby (130 pc), well-lit nebula (good view of small-scale structure ● 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 = 150,000! 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 fine-scale angular and velocity structure. CGPS Sample HISA 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 Filament 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 . . . Summary ● 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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