Infrared spectroscopy of HII regions and starburst galaxies

• Main Author: Doherty, Ruth Mary
• Published: University of Edinburgh 1994
• Subjects: 520
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.649605

Observations of UCHII regions shown that HII region models with electron densities of n<I><SUB>e</SUB></I> = 10<SUP>4</SUP>cm<SUP>-3</SUP>, electron temperatures in the range T<I><SUB>e</SUB></I> = 5,000-10,000K and a Galactic helium abundance can reproduce the observed HeI 2.058<I>μ</I>m/Br<I>γ</I> ratios in these objects, subject to one condition. As the electron temperature is increased from T<I><SUB>e</SUB></I>=5,000K, microturbulence is incorporated into the model which counteracts the increase in the HeI 2.058<I>μ</I>m/Br<I>γ</I> ratio with electron temperature. Microturbulent velocities of the order ~20kms<SUP>-1</SUP> are required at T<I><SUB>e</SUB></I>=10,000K, in agreement with observed radio recombination line and high resolution HeI and HI measurements. Lower HeI 2.058<I>μ</I>m/Br<I>γ</I> ratios are generally found in larger objects classed as compact HII regions, consistent with lower electron densities or lower stellar effective temperatures, or a combination of both effects in such objects. Alternative electron densities have been calculated from radio continuum measurements and provide evidence to suggest that electron densities are slightly lower in most of the compact HII regions than in the UCHII regions. Detailed density gradient modelling of the HeI 2.058<I>μ</I>m/Br<I>γ</I> ratio has been performed, which reveals the need for realistic density distributions and explicit integration over the volume of the source when attempting to model specific HII regions. Accurate integrated electron densities or density gradients act as an alternative to microturbulence to bring theoretical HeI 2.058<I>μ</I>m/Br<I>γ</I> ratios into agreement with observations for electron temperature models higher than T<I><SUB>e</SUB></I> = 5,000K. Finally, a large sample of starburst galaxies is analysed. The HeI 2.058<I>μ</I>m/Br<I>γ</I> ratios are much lower than found in compact or UCHII regions consistent with the proposal that the integrated HeI 2.058<I>μ</I>m/Br<I>γ</I> emission is generally dominated by low density giant HII regions present in these galaxies. Effective temperatures derived from the HeI 2.058<I>μ</I>m/Br<I>γ</I> ratio are consistent with those estimated from FIR fine structure lines. From consideration of new models of the HeI 2.058<I>μ</I>m/Br<I>γ</I> ratio, the combination of optical HeI/Hβ data with HeI 2.058<I>μ</I>m/Br<I>γ</I> observations extends the sensitivity of these model ratios to a wider range of effective temperatures.