Physical Properties of Giant Extragalactic HII Regions

Summary

A discussion of the determination of the “functional parameters” of Giant Extragalactic HII Regions is made for a wide range of physical conditions of the ionized gas. These functional parameters which are found to be the effective temperature of the ionizing radiation, the ionization parameter and the gas metallicity, are then traslated into physical properties of the ionizing star clusters – age, total mass and metal content – through the use of evolutionary models. These models combine evolutionary synthesis techniques with a well-tested photoionization code, CLOUDY, to obtain emission line intensities that can be compared directly with observations. From this comparison some conclusions about the general properties of the ionizing clusters can be extracted.

Introduction

The term Giant Extragalactic HII Region (GEHR) usually refers to outstandingly large and luminous HII regions that are therefore easily observable in external galaxies, mainly on the discs of late-type spirals and in irregulars. Their size, characterized by mean diameters measured on Hα images, can be as large as 1 kpc. Their Hα emission luminosity, which can reach 10⁴¹ erg s⁻¹, translates into a number of ionizing photons betbeen 10⁵⁰ and 10⁵³ s⁻¹. This high number of ionizing photons cannot originate in a single star, which would provide about 10⁴⁹ at most, but requires the presence of a young star cluster.

In the few instances in which GEHR can be spatially resolved, observations reveal a core-halo structure with the core showing detailed substructure.