O/H-N/O: the curious case of NGC 4670

Abstract

We use integral field spectroscopic (IFS) observations from Gemini Multi-Object Spectrograph North (GMOS-N) of a group of four H ii regions and the surrounding gas in the central region of the blue compact dwarf (BCD) galaxy NGC 4670. At spatial scales of ∼9 pc, we map the spatial distribution of a variety of physical properties of the ionized gas: internal dust attenuation, kinematics, stellar age, star formation rate, emission-line ratios, and chemical abundances. The region of study is found to be photoionized. Using the robust direct Te method, we estimate metallicity, nitrogen-to-oxygen ratio, and helium abundance of the four H ii regions. The same parameters are also mapped for the entire region using the HII-CHI-mistry code. We find that log(N/O) is increased in the region where the Wolf–Rayet bump is detected. The region coincides with the continuum region, around which we detect a slight increase in He abundance. We estimate the number of WC4, WN2-4, and WN7-9 stars from the integrated spectrum of WR bump region. We study the relation between log(N/O) and 12 + log(O/H) using the spatially resolved data of the field of view as well as the integrated data of the H ii regions from 10 BCDs. We find an unexpected negative trend between N/O and metallicity. Several scenarios are explored to explain this trend, including nitrogen enrichment, and variations in star formation efficiency via chemical evolution models.