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Abstarct: We present a new tool for the analysis of the optical emission lines of the gas in the Narrow Line Region (NLR) around Active Galactic Nuclei (AGNs). This new tool can be used in large samples of objects in a consistent way using different sets of optical emission-lines taking into the account possible variations from the O/H - N/O relation. The code compares certain observed emission-line ratios with the predictions from a large grid of photoionization models calculated under the most usual conditions in the NLR of AGNs to calculate the total oxygen abundance, nitrogen-to-oxygen ratio and ionization parameter. We applied our method to a sample of Seyfert 2 galaxies with optical emission-line fluxes from the literature. Our results confirm the high metallicity of the objects of the sample and provide consistent values with the direct method. The usage of models to calculate precise ICFs is mandatory when only optical emission lines are available to derive chemical abundances using the direct method in NLRs of AGN. [ABSTRACT FROM AUTHOR]

In this work we explore accurate new ways to derive the ionization parameter (U) and the equivalent effective temperature (T *) in H  ii regions using emission-line intensities from the ionized gas. The so-called softness parameter (η), based on [O  ii ], [O  iii ], [S  ii ], and [S  iii ], has been proposed to estimate the hardening of the ionizing incident field of radiation, but the simplest relation of this parameter with T * also depends on U and metallicity (Z). Here we provide a Bayesian-like code (HCm-Teff) that compares the observed emission lines of η with the predictions of a large grid of photoionization models giving precise estimations of both U and T * when Z is known. We also study the radial variation of these parameters in well-studied disc galaxies observed by the CHAOS collaboration. Our results indicate that the observed radial decreasing of η can be attributed to a radial hardening of T * across galactic discs as in NGC 628 and NGC 5457. On the other hand NGC 5194, which presents a positive slope of the fitting of the softness parameter, has a flat slope in T *. On the contrary the three galaxies do not seem to present large radial variations of the ionization parameter. When we inspect a larger sample of galaxies, we observe steeper radial variations of T * in less bright and later-type galaxies, mimicking a similar trend observed for Z, but the studied sample should be enlarged to obtain more statistically significant conclusions. [ABSTRACT FROM AUTHOR]

We present a new method to derive oxygen and carbon abundances using the ultraviolet (UV) lines emitted by the gas phase ionized by massive stars. The method is based on the comparison of the nebular emission-line ratios with those predicted by a large grid of photoionization models. Given the large dispersion in the O/H-C/O plane, our method first fixes C/O using ratios of appropriate emission lines and in a second step, calculates O/H and the ionization parameter from carbon lines in the UV. We find abundances totally consistent with those provided by the direct method when we apply this method to a sample of objects with an empirical determination of the electron temperature using optical emission lines. The proposed methodology appears as a powerful tool for systematic studies of nebular abundances in star-forming galaxies at high redshift. [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 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 propose a methodology to perform a self-consistent analysis of the physical properties of the emitting gas of H ii galaxies adequate to the data that can be obtained with the 21st century technology. This methodology requires the production and calibration of empirical relations between the different line temperatures that should supersede currently used ones based on very simple, and poorly tested, photoionization model sequences. As a first step to reach these goals, we have obtained simultaneous blue to far red long-slit spectra with the William Herschel Telescope (WHT) of three compact H ii galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2) spectral catalogue using the INAOE Virtual Observatory superserver. Our spectra cover the range from 3200 to 10 500 Å, including the Balmer jump, the [O ii] lines, the [S iii] λλ 9069, 9532 Å doublet as well as various weak auroral lines such as [O iii] λ 4363 Å and [S iii] λ 6312 Å. For the three objects, we have measured at least four line temperatures, T([O iii]), T([S iii]), T([O ii]) and T([S ii]), and the Balmer continuum temperature T(Bac). These measurements and a careful and realistic treatment of the observational errors yield total oxygen abundances with accuracies between 5 and 9 per cent. These accuracies are expected to improve as better calibrations based on more precise measurements, both on electron temperatures and densities, are produced. We have compared our obtained spectra with those downloaded from the SDSS DR3 finding a satisfactory agreement. The analysis of these spectra yields values of line temperatures and elemental ionic and total abundances which are in general agreement with those derived from the WHT spectra, although for most quantities they can only be taken as estimates since, due to the lack of direct measurements of the required lines, theoretical models had to be used whose uncertainties are impossible to quantify. The ionization structure found for the observed objects from the O+/O2+ and S+/S2+ ratios points to high values of the ionizing radiation, as traced by the values of the ‘softness parameter’η which is less than 1 for the three objects. The use of line temperatures derived from T([O iii]) based on current photoionization models yields for the two highest excitation objects, much higher values of η which would imply lower ionizing temperatures. This is, however, inconsistent with the ionization structure as probed by the measured emission-line intensities. Finally, we have measured the T(Bac) for the three observed objects and derived temperature fluctuations. Only for one of the objects, the temperature fluctuation is significant and could lead to higher oxygen abundances by about 0.20 dex. [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]

Hii regions in galaxy disks can be used as a powerful tool to trace the radial distribution of several of their properties and shed some light on the different relevant processes on galaxy formation and evolution. Among the properties that can be extracted from the study of the ionized gas are the metallicity, the excitation and the hardness of the ionizing field of radiation. In this contribution we focus on the determination of both the ionization parameter (U) and the effective temperature of the ionizing clusters (T) by means of a bayesian-like comparison between the observed relative fluxes of several emission-lines with the predictions from a set of photoionization models. We also show the implications that the use of our method has for the study of the radial variation of both U and T in some very well-studied disk galaxies of the Local Universe. [ABSTRACT FROM AUTHOR]