CCD imaging of NGC 4861 : ionized gas properties and stellar formation

Abstract

This work undertakes the study of the galaxy NGC 4861, using imaging centered at the strong lines Hα, Hβ, [O III] λ5007 and nearby continua. Integrated, absolute fluxes of 28 H II regions, and the region known as the bright knot, have been measured showing correlations between the equivalent width of Hβ and the excitation index log([O III]/Hβ). An excitation gradient has been detected along the chain of H II regions extending northwards from the bright knot, which is unusual for irregular galaxies like NGC 4861. We suggest that this gradient is due to different stages of evolution, or ages, of the H II regions. The continuum fluxes of eight emissionless associations and lhe continuum colors of the companion galaxy IC 3961 have also been measured, and the presence of ionized gas cannot be detected. The age and mass of the stellar population and ionizing stars of the bright knot were subsequently estimated. lt was also verified that this region is the most important burst of star formation in NGC 4861.