The Diffuse Ionised Gas in NGC 4449

Abstract

AbstractIt is now well established that there is a diffuse ionised medium which extends far beyond the disks of spirals. The mechanism responsible for the ionisation is still an issue of debate, and the method of measuring the total luminosity, independently of the observational set-up, has to be defined. Here we analyse the case of the Magellanic irregular NGC 4449, using Fabry–Perot bidimensional mapping of the Hα emission line, at the 4·2 m William Herschel Telescope (WHT) telescope on La Palma, Canary Islands. The total spatial coverage of 80×80 arcsec2 allows most of the star forming regions to be sampled at 0·26 arcsec/pixel spatial resolution. Using 3D spectroscopy, it is possible to define the individual star-forming regions and to produce a luminosity versus size diagram. This can be used to estimate directly the number of photons leaking out from those regions exceeding the thickness of the galactic disc. In the case of NGC 4449 it is shown that the large population of GEHRs can be the source of UV photons ionising the diffuse ionised gas (DIG) of the galaxy, and the logL–R diagram can be used to quantify the number of photons leaking out of the material surrounding the star forming regions. In the case of NGC 4449 it is found that 54% of the total luminosity of the giant star-forming regions is lost to the DIG. This implies a contribution of 3·12×1040 erg s−1 (1 erg s−1 = 10−7 J s−1) which amounts to 30% of the galaxy total Hα luminosity.