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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 T e 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. [ABSTRACT FROM AUTHOR]

We present a self-consistent study of the stellar populations and the ionized gas in a sample of 10 H ii galaxies with, at least, four measured electron temperatures and a precise determination of ionic abundances following the ‘direct method’. We fitted the spectral energy distribution of the galaxies using the programstarlight andstarburst99 libraries in order to quantify the contribution of the underlying stellar population to the equivalent width of Hβ[EW(Hβ)], which amounts to about 10 per cent for most of the objects. We then studied the Wolf–Rayet (WR) stellar populations detected in seven of the galaxies. The presence of these populations and the EW(Hβ) values, once corrected for the continuum contribution from underlying stars and ultravoilet dust absorption, indicate that the ionizing stellar populations were created following a continuous star formation episode of 10 Myr duration, hence WR stars may be present in all of objects even if they are not detected in some of them. The derived stellar features, the number of ionizing photons and the relative intensities of the most prominent emission lines were used as input parameters to compute tailored models with the photoionization codecloudy. Our models are able to adequately reproduce the thermal and ionization structure of these galaxies as deduced from their collisionally excited emission lines. This indicates that ionic abundances can be derived following the ‘direct method’ if the thermal structure of the ionized gas is well traced, hence no abundance discrepancy factors are implied for this kind of objects. Only the electron temperature of S+ is overestimated by the models, with the corresponding underestimate of its abundance, pointing to the possible presence of outer shells of diffuse gas in these objects that have not been taken into account in our models. This kind of geometrical effects can affect the determination of the equivalent effective temperature of the ionizing cluster using calibrators which depend on low-excitation emission lines. [ABSTRACT FROM AUTHOR]

We study the radial distribution of oxygen and nitrogen abundances in H ii regions in 350 galaxies observed in the CALIFA survey using the semi-empirical N-sensitive code Hii-Chi-mistry. A separate analysis of N/O when using [N ii] is justified as the dispersion in the O/H-N/O diagram is very large. In fact different O/H and N/O slope distributions and flattening in the outskirts are obtained.On the other hand there is a tight correlation between O/H and N/O when the values of the fittings at the effective radius are considered as representative of each galaxy. [ABSTRACT FROM PUBLISHER]

We carry out a study of the hardness of the radiation of ionizing clusters in H ii regions. First, we explore the applicability of the softness parameter η, originally defined in the optical for pairs of consecutive ionization stages of the same species. With the advent of the infrared space observatories, this definition has been extended to the mid-infrared. We show that the softness parameters, as determined in both the optical and the mid-infrared wavelengths, are sensitive to the effective temperature using a sample of data in both spectral regimes. This is confirmed by comparing the data with a grid of photoionization models, although no complete agreement has been found even for different stellar model atmospheres. Finally, we show that both softness parameters are consistent in the search for radial variations of the hardness of the ionizing radiation of H ii regions in the discs of spiral galaxies. We find a range of trends, from galaxies showing pronounced gradients to those showing very flat ones. Although the detectability and slope of these gradients can be altered by the size and luminosity of the studied H ii regions, it looks that their existence is related to the mass and type of the galaxies and, hence, to the properties of the entire disc. [ABSTRACT FROM AUTHOR]

We study the relation between nitrogen and oxygen abundances as a function of metallicity for a sample of emission-line objects for which a direct measurement of the metallicity has been possible. This sample is representative of the very different conditions in ionization and chemical enrichment that we can find in the Universe. We first construct the N/O versus O/H ratio diagram, and discuss its large dispersion at all metallicity regimes. Using the same sample and a large grid of photoionization models covering very different values of the N/O ratio, we then study the most widely used strong-line calibrators of metallicity based on [N ii] emission lines, such as N2 and O3N2. We demonstrate that these parameters underestimate the metallicity at low N/O ratios and vice versa. We also investigate the effect of the N/O ratio on different diagnostic diagrams used to discriminate narrow-line active galactic nuclei from star-forming regions, such as the [O iii]/Hβ versus [N ii]/Hα, and show that a large fraction of the galaxies catalogued as composite in this diagram can be, in fact, star-forming galaxies with a high value of the N/O ratio. Finally, using strong-line methods sensitive to the N/O abundance ratio, like N2O2 and N2S2, we investigate the relation between this ratio and the stellar mass for the galaxies of the Sloan Digital Sky Survey. We find, as in the case of the mass–metallicity relation, a correlation between these two quantities and a flattening of the relation for the most massive galaxies, which could be a consequence of the enhancement of the dispersion of N/O ratio in the high-metallicity regime. [ABSTRACT FROM AUTHOR]

In this work, we present a study of the strong optical collisional emission lines of Ne and Ar in an heterogeneous sample of ionized gaseous nebulae for which it is possible to derive directly the electron temperature and hence the chemical abundances of Ne and Ar. We calculate using a grid of photoionization models new ionization correction factors for these two elements and we study the behaviour of Ne/O and Ar/O abundance ratios with metallicity. We find a constant value for Ne/O, while there seems to be some evidence for the existence of negative radial gradients of Ar/O over the discs of some nearby spirals. We study the relation between the intensities of the emission lines of [Ne iii] at 3869 Å and [O iii] at 4959 and 5007 Å. This relation can be used in empirical calibrations and diagnostic ratios extending their applicability to bluer wavelengths and therefore to samples of objects at higher redshifts. Finally, we propose a new diagnostic using [O ii], [Ne iii] and Hδ emission lines to derive metallicities for galaxies at high z. [ABSTRACT FROM AUTHOR]

We present a detailed photoionization model of the brightest knot of star formation in the blue compact dwarf galaxy Mrk 209. The model reproduces the intensities of main lines emitted by the ionized gas, resulting in a very good agreement between observed and predicted line temperatures and chemical abundances of the observed ionic species. The model has been calculated using the spectral energy distribution of a massive cluster of recent formation as the ionizing source. The features of Wolf–Rayet (WR) stars observed in the spectrum of the object, along with its ionizing properties, lead to different solutions for the ages and characteristics of the ionizing stellar populations. The found solutions are compatible with either a composite population of two ionizing clusters or a continuous star formation. In the first case, a young cluster, with an age less than 3 Myr, would be responsible for most of the ionization properties while an older cluster, with either 3.2 or 4.2 Myr, would be responsible for the emission of the observed WR features in the spectrum of the knot. In the second case, the duration of the star formation episode is found to be 3.6 Myr. The addition of a certain amount of dust was required in order to reproduce correctly the measured electron temperatures. This dust implies a gas/dust ratio of , identical to that in the Milky Way, and a fraction of absorbed photons of . The taking into account the dust grain physics combined with a thick-shell geometry solves the problem of the heating in this object and allows the fitting of the auroral lines, the line temperatures and the ionization structure of the nebula with a negligible presence of temperature fluctuations, in agreement with the most recent results found for this type of objects from the measurement of the Balmer jump temperature. An analysis of the ionization structure of the gas as well as the calculation of the main ionization correction factors for the unseen ionic stages is presented. [ABSTRACT FROM AUTHOR]

We present a new analysis of the main empirical calibrators of oxygen abundance for ionized gas nebulae. With that aim we have compiled an extensive sample of objects with emission-line data including the near-infrared [S iii] lines and the weak auroral lines which allow for the determination of the gas electron temperature. For all the objects the oxygen abundances have been derived in a homogenous way, using the most recent sets of atomic coefficients and taking into the account the effect of particle density on the temperature of O+. The residuals between directly and empirically derived abundances as a function of abundance have been studied. A grid of photoionization models, covering the range of physical properties of the gas, has been used to explain the origin of the uncertainties affecting each abundance calibrator. The range of validity for each abundance parameter has been identified and its average uncertainty has been quantified. [ABSTRACT FROM AUTHOR]