Spectrophotometric Analysis of HII Regions in the Galaxy NGC 4258

Abstract

We report long-slit observations corresponding to 8 HII regions in the spiral galaxy NGC4258. Four of them have been identified as 74C, 69C, 5N(A) and 5N(B) (Courtés et al. 1993). The observations were obtained at the WHT. Electron temperatures for two of the regions, 74C and 69C, were derived from the weak auroral [SIII] λ 6312 line and the nebular lines at λλ 9069, 9532 Å. Values of 8600 ± 400 K and 6700 db 300 K respectively are found. A prominent feature corresponding to WR stars is observed in region 74C. We obtain for the HeII line luminosity: L(HeII λ 4686) = (6.6 ± 0.6) x 10³⁷ erg s^-1. The non-detection of NV λλ 4604, 4620 Å, suggests that the WR stars are of late N or intermediate type. Using the calibration of Vacca & Conti (1992), we find 38 ± 5 WN stars in this region. We have tried to evaluate possible temperature fluctuations due to the presence of WR stars (Pérez 1997) by measuring the Paschen discontinuity at 8200 Å. Our measurements imply an electron temperature of 8000 ± 700 K and therefore, despite the presence of WR stars, no important fluctuations exist in this region. We have calculated the elemental abundances for regions 74C and 69C following standard methods (e.g. Pagel et al. 1992). The oxygen abundances derived are 0.2 and 0.5 times solar respectively. The ionization parameters found are lower than -2.50 in all cases. Finally we have found T _eff around 35,000K by using single star (Mihalas NLTE stellar atmospheres) photoionization models (CLOUDY. Ferland 1996).