Anisotropy of gas Anisotropy of gas random motions random motions in galactic disks in galactic disks Laurent Chemin Laurent Chemin Universidad de Antofagasta (Chile) Françoise Combes Françoise Combes Observatoire de Paris-LERMA (F) Erwin de Blok Erwin de Blok ASTRON (NL)
Gas velocity dispersion in disks Gas velocity dispersion in disks Many applications for gas velocity dispersion Many applications for gas velocity dispersion ✔ Gas pressure, mass distribution (Dalcanton+2010, Oh et al. 2015, Genzel+2017) ✔ Noncircular motions (e.g. Kuzio de Naray+2006) ✔ Disk thickness, vertical equilibrium (e.g. van der Kruit 1981, Combes+1997) isotropy of gas dispersion ellipsoid is always always assumed σ assumed σ R =σ Φ =σ z isotropy of gas dispersion ellipsoid is R =σ Φ =σ z i= inclination = σ σ los (observed line-of-sight dispersion) = los (observed line-of-sight dispersion)
Gas velocity dispersion in disks Gas velocity dispersion in disks Higher σ los Higher σ los Lower σ los Lower σ los Spiral and ring-like features in dispersion maps Lower σ los Lower σ los Signs of anisotropy Higher σ los Higher σ los THINGS velocity dispersion map of NGC2903 (Walter+08) THINGS velocity dispersion map of NGC2903
Objectives Objectives Measure the dispersion ellipsoid of gas: Measure the dispersion ellipsoid of gas: z σ R ,σ Φ ,σ z σ R ,σ Φ ,σ Assess the assumption of isotropy Assess the assumption of isotropy Study the properties of ellipsoid & anisotropy Study the properties of ellipsoid & anisotropy
Data Data NGC3521 NGC3031 NGC2976 NGC2903 The HI Nearby Galaxy Survey (Walter+08) The HI Nearby Galaxy Survey NGC2841 17 galaxies NGC4736 (de Blok+08) 17 galaxies NGC2366 NGC3198 σ instrumental = 2.6-5.2 km/s σ instrumental = 2.6-5.2 km/s DDO154 NGC3621 Angular resolution = 150-800 pc Angular resolution = 150-800 pc NGC5055 NGC7793 NGC925 NGC7331 NGC6946 NGC925 NGC6946 THINGS velocity dispersion maps THINGS velocity dispersion maps
Testing the isotropy assumption Testing the isotropy assumption Give the fits the best chance Give the fits the best chance to find σ σ R = σ Φ = σ z = σ los to find (R) = σ (R) = σ (R) = σ (R) R (R) Φ (R) z (R) los (R) 2 +σ Φ 2 ) 1/2 σ z = 0.71σ P = 0.71(σ R Fixed σ R = σ los Free σ Φ
Testing the isotropy assumption Testing the isotropy assumption NGC925 NGC925 σ R σ P σ Φ σ tot = (σ R 2 +σ Φ 2 +σ z 2 ) 1/2 σ z Anisotropy parameters β →1 more radial orbits β = 0 isotropy β Φ < 0 more tangential orbits
Testing the isotropy assumption Testing the isotropy assumption Closest galaxies to isotropy Closest galaxies to isotropy DDO154 (i=65°, v max =45 km/s) σ R σ Φ σ z σ P NGC6946 σ total (i=33°, v max =200 km/s) Isotropy is never verified Isotropy is never verified
Anisotropic random motions Anisotropic random motions NGC3521 σ z = 0.5σ P σ R σ Φ (hydrodynamical simulations, Agertz+ 2009) σ z σ P Free σ R and σ Φ σ total
Anisotropic random motions Anisotropic random motions Identification of two classes of disks Identification of two classes of disks More tangential orbits More radial orbits More radial orbits More tangential orbits NGC3621 NGC3031
Anisotropic random motions Anisotropic random motions Anisotropy vs mass Anisotropy vs mass More radial More tangential No clear trend No clear trend
Anisotropic random motions Anisotropic random motions Disagreement with theory Disagreement with theory (epicyclic approximation) (epicyclic approximation) (Binney & Tremaine 2008) NGC3521
Anisotropy of σ σ gas in galactic disks Anisotropy of gas in galactic disks Summary & Future work Summary & Future work σ R ≠ σ σ Φ ≠ σ σ z σ R ≠ Φ ≠ z HI disks have more radial radial/ /tangential tangential orbits orbits HI disks have more Analysis and implications of anisotropy Analysis and implications of anisotropy ✔ Increase galaxy sample(s) Increase galaxy sample(s) ✔ Atomic vs molecular vs ionized gas Atomic vs molecular vs ionized gas ✔ Dwarf galaxies Dwarf galaxies DDO154 thickness density of dark matter disk (articles in preparation)
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