CHEMICAL ABUNDANCES AND PROPERTIES OF THE IONIZED GAS IN NGC 1705

We obtained [O III] narrow-band imaging and multi-slit MXU spectroscopy of the blue compact dwarf (BCD) galaxy NGC 1705 with FORS2@VLT to derive chemical abundances of planetary nebulae and H II regions and, more in general, to characterize the properties of the ionized gas. The auroral [O III]lambda 4363 line was detected in all but 1 of the 11 analyzed regions, allowing for a direct estimate of their electron temperature. The only object for which the [O III]lambda 4363 line was not detected is a possible low-ionization PN, the only one detected in our data. For all the other regions, we derived the abundances of nitrogen, oxygen, neon, sulfur, and argon out to similar to 1 kpc from the galaxy center. We detect for the first time in NGC 1705 a negative radial gradient in the oxygen metallicity of -0.24 +/- 0.08 dex kpc(-1). The element abundances are all consistent with values reported in the literature for other samples of dwarf irregular and BCD galaxies. However, the average (central) oxygen abundance, 12+log(O/H)= 7.96 +/- 0.04, is similar to 0.26 dex lower than previous literature estimates for NGC 1705 based on the [O III] l4363 line. From classical emission. line diagnostic diagrams, we exclude a major contribution from shock excitation. On the other hand, the radial behavior of the emission line ratios is consistent with the progressive dilution of radiation with increasing distance from the center of NGC 1705. This suggests that the strongest starburst located within the central similar to 150 pc is responsible for the ionization of the gas out to at least similar to 1 kpc. The gradual dilution of the radiation with increasing distance from the center reflects the gradual and continuous transition from the highly ionized H II regions in the proximity of the major starburst into the diffuse ionized gas.