Chemical abundances and ionizing clusters of H ii regions in the LINER galaxy NGC 4258

Abstract

We present long‐slit observations in the optical and near‐infrared of eight H ii regions in the spiral galaxy NGC 4258. Six of the observed regions are located in the south‐east inner spiral arms, and the other two are isolated in the northern outer arms. A detailed analysis of the physical conditions of the gas has been performed. For two of the regions, an electron temperature has been derived from the [S iii] λ6312 line. For the rest, an empirical calibration based on the red and near‐infrared sulphur lines has been used. The oxygen abundances derived by both methods are found to be significantly lower (by a factor of 2) than previously derived by using empirical calibrations based on the optical oxygen lines. In the brightest region, 74C, the observation of a prominent feature caused by Wolf–Rayet (WR) stars provides an excellent constraint over some properties of the ionizing clusters. In the light of the current evolutionary synthesis models, no consistent solution is found to explain at the same time both the WR feature characteristics and the emission‐line spectrum of this region. In principle, the presence of WR stars could lead to large temperature fluctuations and also to a hardening of the ionizing radiation. None of these effects is found in region 74C, for which the electron temperatures found from the [S iii] λ6312 line and the Paschen discontinuity at 8200 Å are equal within the errors, and the effective temperature of the ionizing radiation is estimated at around 35 300 K. Both more observations of confirmed high‐metallicity regions and a finer metallicity grid for the evolutionary synthesis models are needed in order to understand the ionizing populations of H ii regions.

SCG0018‐4854: A young and dynamic compact group

“…This value is consistent with what we observe in the nuclear region of NGC 92, which is classified as a LINER. Further out, the [NI]/Hβ ratio assumes values of the order of a few 10 −2 , similar to those observed for HII regions in other LINER galaxies (e.g., Díaz et al 2000). These [NI] lines are even observed in the presence of relatively weak radiation fields, as in the partially ionized zones of the Orion Nebula, and fluorescence is thought to play an important role in their excitation (Bautista 1999).…”

Integral field spectroscopy of a sample of nearby galaxies

“…This parameter is directly related to the fraction of very young stars (∼10 Myr), and can be used to estimate the aging process of the ionizing population; (ii) A V , the dust attenuation derived from the Hα/Hβ Balmer decrement. To derive it the extinction law by Cardelli et al (1989) was assumed, with R V = 3.1, and the theoretical value for the unobscured line ratio for case B recombination of Hα/Hβ = 2.86, for T e = 10 000 K and n e = 100 cm −3 (Osterbrock 1989 (Díaz et al 2000); (v) the oxygen abundance, 12 + log (O/H), derived using the O3N2 indicator defined by Pettini & Pagel (2004), that estimates the gas enrichment; (vi) r HII , the Strömgren radius of the H ii regions (Osterbrock 1989), i.e., a hint of the size of the H ii regions based on pressure equilibrium considerations, and (vii) n e , the electron density derived from the [S ii] doublet line ratio, i.e., a proxy to the density of the ionized gas (e.g., Osterbrock 1989). Table 7 shows the mean values and standard deviations of the considered parameters for the H ii regions of each galaxy, together with the mean value for all the galaxies.…”

Type Ia supernova host galaxies as seen with IFU spectroscopy

“…The ionization maps of the galaxies were computed from the ratio of the [O ii] λ3727 and [O iii] λ5007 lines using the relation of Díaz et al (2000).…”