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Date and Time of the Query: 2019-07-20 T04:47:10 PDT
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For refcode 1988ApJ...335..644C:
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Copyright by American Astronomical Society. Reproduced by permission
1988ApJ...335..644C PHYSICAL CONDITIONS IN H II GALAXIES ALISON CAMPBELL Space Telescope Science Institute Received 1988 February 4; accepted 1988 June 3 ABSTRACT The temperature of the ionizing radiation, T_ion_, and the mean nebular ionization parameter, U^bar^, are measured for a sample of 45 H II galaxies with estimated oxygen abundances in the range -4.9 <~ log O/H <~ -3.6. A simple, Stromgren sphere model fits very well all but three of the objects. The sensitivity of emission-line strengths to T_ion_ and U^bar^ is illustrated for several familiar forbidden-line ratios. The data show that the maximum value of T_ion_ at a particular abundance increases smoothly with decreasing O/H, from ~ 45,500 K at a true log O/H of -3.6 to ~ 63,000 K at log O/H = -4.4. The maximum value of U^bar^ at each abundance also shows a shallow inverse trend with O/H, varying from ~9 x 10^-3^ at a true log O/H of -3.7 to ~ 3 x 10^-2^ at log O/H = -4.4. Both T_ion_, and especially U^bar^ show large scatter at a given abundance, interpreted as differences in nebular parameters and in the age and mass of the ionizing cluster. The fact that T_ion_ varies systematically with abundance rules out differences in nebular geometry as the primary cause of the line ratio gradients observed among H II galaxies and, by analogy, giant H II regions in spiral galaxies. The usual method of oxygen abundance determination, via the electron temperature in the O^++^ region, T_e_(O^++^), underestimates O/H by an amount which depends on T_ion_, U^bar^ and the true O/H; the error can be larger than 0.3 in the log. Below log O/H ~ -4.0, T_e_(O^++^) is equally sensitive to variations in T_ion_ and U^bar^; unless the ionization parameter is known, T_ion_ cannot be accurately estimated from T_e_(O^++^) at low abundance. Subject headings: galaxies: abundances - galaxies: structure - nebulae: H II regions
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