view Abstract Citations (205) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Sharp Edges to Neutral Hydrogen Disks in Galaxies and the Extragalactic Radiation Field Maloney, Philip Abstract Recent observations of the late-type spiral galaxy NGC 3198 show that the extended atomic hydrogen distribution in this galaxy is sharply truncated at a neutral hydrogen column density of N_HI_ ~ 5 X 10^19^ cm^-2^ (van Gorkom et al. 1993). In this paper I show that this cutoff can be reasonably explained as the result of photoionization of the gas by the extragalactic radiation field. I treat the gaseous disk in plane-parallel geometry, with the vertical equilibrium explicitly calculated for a composite disk/halo model; the gas is assumed to be smoothly distributed and isothermal, with T ~ 10^4^ K. The extragalactic radiation field is taken to be a power law between the Lyman limit and the observed value at 1.5 keV; the actual intensity at the Lyman limit is a free parameter. Models calculated for a range of velocity dispersions and ionizing photon fluxes show similar behavior, with a characteristic column density N_cr_ ~ a few times 10^19^ cm^-2^ at which the ionization fraction sharply increases. This result is not significantly affected by halo eccentricity, but does depend on the clumpiness of the gas. Specific application to NGC 3198 shows that the total hydrogen distribution may be nearly axisymmetric, and that the atomic hydrogen at large radii is not significantly clumped into clouds. The column density at which this transition occurs is not very sensitive to galaxy mass or halo parameters, changing by only a factor of ~3 as the halo surface density {SIGMA}_h_ is varied by a factor of ~30. The numerical results are in good agreement with an analytic estimate of the critical column density; the analytic result shows that N_cr_ is porportional to [φ_i,exsigma_zz_V_A_/{SIGMA}_h_]^1/2^, where V_A_ is the halo asymptotic velocity, σ_zz_ is the gas vertical velocity dispersion, and φ_i,ex_ is the extragalactic ionizing photon flux. Because of the insensitivity to galaxy parameters, this model predicts that all disk galaxies will show sharp edges to their neutral hydrogen distributions at similar column densities, as first discussed by Bochkarev & Sunyaev (1977). It also predicts that objects with atomic hydrogen surface densities below this critical value will be mostly ionized and hence undetectable in the 21 cm line. If the observed H I cutoffs are due to photoionization, the extragalactic ionizing photon flux in the energy range hv = 13.6 ~ 200 eV must be 5 X 10^3^ <~φ_i,ex_ <~ 5 x 10^4^ photons cm^-2^ s^-1^. The emission measure EM of ionized gas at large radii is generally too small to be detectable by emission in Hα at present. Strict limits on or eventual measurement of EM(R) and determination of the neutral gas velocity dispersion will allow a more precise determination of φ_i,ex_. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173055 Bibcode: 1993ApJ...414...41M Keywords: Dark Matter; Diffuse Radiation; Hydrogen Atoms; Neutral Gases; Radiation Distribution; Spiral Galaxies; Galactic Mass; H Alpha Line; Interstellar Gas; Interstellar Matter; Astrophysics; GALAXIES: ISM; COSMOLOGY: DARK MATTER; COSMOLOGY: DIFFUSE RADIATION; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3198; GALAXIES: KINEMATICS AND DYNAMICS full text sources ADS | data products SIMBAD (3) NED (1)