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The chemical enrichment in low-ionization nuclear emission-line regions (LINERs) is still an issue with spatial resolution spectroscopic data due to the lack of studies and the uncertainties in the nature of their ionizing source, despite being the most abundant type of active galaxies in the nearby Universe. Considering different scenarios for the ionizing source (hot old stellar populations, active galactic nuclei (AGN) or inefficient accretion disks), we analyze the implications of these assumptions to constrain the chemical content of the gas-phase interstellar medium (ISM). We used a sample of 105 galaxies from Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, whose nuclear central spaxels show LINER-like emission. For each scenario considered, we built a grid of photoionization models (4928 models for each considered ionizing source) which are later used in the open-source code HII-CHI-Mistry, allowing us to estimate chemical abundance ratios such as 12+log(O/H) or log(N/O) and constrain the ionization parameter that characterize the ionized ISM in those galaxies. We obtain that oxygen abundances in the nuclear region of LINER-like galaxies spread over a wide range 8.08 < 12+log(O/H) < 8.89, with a median solar value (in agreement with previous studies) if AGN models are considered. Nevertheless, the derived nitrogen-to-oxygen ratio is much less affected by the assumptions on the ionizing source, and point towards suprasolar values (log(N/O) = -0.69). By comparing the different analyzed scenarios, we show that if hot old stellar populations were responsible of the ionization of the ISM a complex picture (such as outflows and/or inflows scaling with galaxy chemical abundance) would be needed to explain the chemical enrichment history, whereas the assumption of AGN activity is compatible with the standard scenario found in most galaxies., Comment: 24 pages, 13 figures, 9 tables (5 as supplemented material). Accepted for publication in A&A

This contribution summarizes the main activities and objectives of the outreach project Astroaccesible, whose main aim is to carry the teaching and diffusion of astronomy among all kinds of collectives, focusing on blind and visually impaired (BVI) people. This project is led by a blind astronomer and aims to use a variety of resources based on different sensory channels, avoiding limiting the transmission of concepts to visual perception. This principle favors inclusion and benefits everyone, as the information is not presented using just one channel. This strategy is especially convenient for the nowadays typical data acquisition, where a variety of sources of information, not solely based on the collection of different spectral domains of electromagnetic radiation, is used. Moreover, the study of new multi-messenger astronomy could be much better understood using a multi-messenger teaching approach, favoring inclusion, motivation, and creativity., Comment: To appear as a proceeding of the 13th Cosmic-Ray International Studies and Multi-messenger Astroparticle Conference held on June 2024 in Trapani (Sicily, Italy). 8 pages and 4 figures

The infrared (IR) range is extremely useful in the context of chemical abundance studies of the gas-phase interstellar medium (ISM) due to the large variety of ionic species traced in this regime, the negligible effects from dust attenuation or temperature stratification, and the amount of data that has been and will be released in the coming years. Taking advantage of available IR emission lines, we analysed the chemical content of the gas-phase ISM in a sample of 131 Star-Forming Galaxies (SFGs) and 73 Active Galactic Nuclei (AGNs). Particularly, we derived the chemical content via their total oxygen abundance in combination with nitrogen and sulfur abundances, and with the ionisation parameter. We used a new version of the code HII-CHI-Mistry-IR v3.1 which allows us to estimate log(N/O), 12+log(O/H), log(U), and, for the first time, 12+log(S/H) from IR emission lines, which can be applied to both SFGs and AGNs. We tested that the estimations from this new version, that only considers sulfur lines for the derivation of sulfur abundances, are compatible with previous studies. While most of the SFGs and AGNs show solar log(N/O) abundances, we found a large spread in the log(S/O) relative abundances. Specifically, we found extremely low log(S/O) values (1/10th solar) in some SFGs and AGNs with solar-like oxygen abundances. This result warns against the use of optical and IR sulfur emission lines to estimate oxygen abundances when no prior estimation of log(S/O) is provided., Comment: Accepted for publication in A&A. 16 pages, 11 figures, 4 electronic tables

Extreme Emission Line Galaxies (EELGs) stand as remarkable objects due to their extremely metal poor environment and intense star formation. Considered as local analogues of high-redshift galaxies in the peak of their star-forming activity, they offer insights into conditions prevalent during the early Universe. Assessment of their stellar and gas properties is, therefore, of critical importance, which requires the assembly of a considerable sample, comprehending a broad redshift range. The Javalambre-Physics of the Accelerating Universe Astrophysical Survey (JPAS) plays a significant role in assembling such a sample, encompassing approximately 8000 deg2 and employing 54 narrow-band optical filters. The present work describes the development and subsequent application of the tools that will be employed in the forthcoming JPAS spectrophotometric data, allowing for the massive and automated characterization of EELGs that are expected to be identified. This fully automated pipeline (requiring only the object coordinates from users) constructs Spectral Energy Distributions (SEDs) by retrieving virtually all the available multi-wavelength photometric data archives, employs SED fitting tools and identifies optical emission lines. It was applied to the sample of extreme line emitters identified in the miniJPAS Survey, and its derived physical properties such as stellar mass and age, coupled with fundamental relations, mirror results obtained through spectral modeling of SDSS spectra. Thorough testing using galaxies with documented photometric measurements across different wavelengths confirmed the pipeline's accuracy, demonstrating its capability for automated analysis of sources with varying characteristics, spanning brightness, morphology, and redshifts. The modular nature of this pipeline facilitates any addition from the user., Comment: 15 pages, 9 Figures, accepted for publication in MNRAS

The build up of heavy elements and the stellar mass assembly are fundamental processes in the formation and evolution of galaxies. Although they have been extensively studied through observations and simulations, the key elements that govern these processes, such as gas accretion and outflows, are not fully understood. This is especially true for luminous and massive galaxies, which usually suffer strong feedback in the form of massive outflows, and large-scale gas accretion triggered by galaxy interactions. For a sample of 77 luminous infrared (IR) galaxies, we derive chemical abundances using new diagnostics based on nebular IR lines, which peer through the dusty medium of these objects and allow us to include the obscured metals in our abundance determinations. In contrast to optical-based studies, our analysis reveals that most luminous IR galaxies remain close to the mass-metallicity relation. Nevertheless, four galaxies with extreme star-formation rates ($> 60$M$_{\odot }$yr$^{-1}$) in their late merger stages show heavily depressed metallicities of 12+log(O/H) $\sim 7.7$--$8.1$ along with solar-like N/O ratios, indicative of gas mixing processes affecting their chemical composition. This evidence suggests the action of a massive infall of metal-poor gas in a short phase during the late merger stages, eventually followed by a rapid enrichment. These results challenge the classical gas equilibrium scenario usually applied to main-sequence galaxies, suggesting that the chemical enrichment and stellar-mass growth in luminous IR galaxies are regulated by different processes., Comment: Submitted to Nature Astronomy. 19 pages, 4 figures

We present an adapted version of the code HII-CHI-Mistry-UV (P\'erez-Montero & Amor\'in 2017) to derive chemical abundances from emission lines in the ultraviolet, for use in narrow line regions (NLR) of Active Galactic Nuclei (AGN). We evaluate different ultraviolet emission line ratios and how different assumptions about the models, including the presence of dust grains, the shape of the incident spectral energy distribution, or the thickness of the gas envelope around the central source, may affect the final estimates as a function of the set of emission lines used. We compare our results with other published recipes for deriving abundances using the same emission lines and show that deriving the carbon-to-oxygen abundance ratio using CIII] $\lambda$ 1909 \r{A} and OIII] $\lambda$ 1665 \r{A} emission lines is a robust indicator of the metal content in AGN that is nearly independent of the model assumptions, similar to the case of star-forming regions. Moreover, we show that a prior determination of C/O allows for a much more precise determination of the total oxygen abundance using carbon UV lines, as opposed to assuming an arbitrary relationship between O/H and C/O, which can lead to non-negligible discrepancies., Comment: Accepted for publication in MNRAS. 16 pages and 10 figures. Tables 2 and 3 will be completely available as supplementary data and submitted to CDS. These tables will be temporarily shared upon request

We explore the so-called softness diagram -- whose main function is to provide the hardness of the ionizing radiation in star-forming regions -- in order to check whether hot and old low-mass evolved stars (HOLMES) are significant contributors to the ionization within star-forming regions, as suggested by previous MaNGA data analyses. We used the code HCm-Teff to derive both the ionization parameter and the equivalent effective temperature (T*), adopting models of massive stars and planetary nebulae (PNe), and exploring different sets of emission lines in the softness diagram to figure out the main causes of the observed differences in the softness parameter in the MaNGA and CHAOS star-forming region samples. We find that the fraction of regions with a resulting T* > 60 kK, which are supposedly ionised by sources harder than massive stars, is considerably larger in the MaNGA (66%) than in the CHAOS (20%) sample when the [SII] $\lambda\lambda$ 6716,6731 emission lines are used in the softness diagram. However, the respective fractions of regions in this regime for both samples are considerably reduced (20% in MaNGA and 10% in CHAOS) when the [NII] emission line at $\lambda$ 6584 is used instead. This may indicate that diffuse ionised gas (DIG) contamination in the lower resolution MaNGA data is responsible for artificially increasing the measured T* as opposed to there being a predominant role of HOLMES in the HII regions., Comment: Accepted for publication by A&A. 7 pages, 6 figures

Future and on-going infrared and radio observatories such as JWST, METIS or ALMA will increase the amount of rest-frame IR spectroscopic data for galaxies by several orders of magnitude. While studies of the chemical composition of the ISM based on optical observations have been widely spread over decades for SFG and, more recently, for AGN, similar studies need to be performed using IR data. This regime can be especially useful in the case of AGN given that it is less affected by temperature and dust extinction, traces higher ionic species and can also provide robust estimations of the chemical abundance ratio N/O. We present a new tool based on a bayesian-like methodology to estimate chemical abundances from IR emission lines in AGN. We use a sample of 58 AGN with IR spectroscopic data retrieved from the literature to probe the validity of our method. The estimations of the chemical abundances based on IR lines in our sample are later compared with the corresponding abundances derived from the optical emission lines in the same objects. HII-CHI-Mistry-IR takes advantage of photoionization models, characterized by the chemical abundance ratios O/H and N/O and the ionization parameter $U$, to compare their predicted emission-line fluxes with a set of observed values. Instead of matching single emission lines, the code uses some specific emission-line ratios sensitive to the above free parameters. We report mainly solar and also subsolar abundances for O/H in the nuclear region for our sample of AGN, whereas N/O clusters around solar values. We find a discrepancy between the chemical abundances derived from IR and optical emission lines, being the latter higher than the former. This discrepancy, also reported by previous studies of the composition of the ISM in AGN from IR observations, is independent from the gas density or the incident radiation field to the gas., Comment: 20 pages, 15 figures, 8 tables. Accepted for publication in A&A

Astroaccesible is an outreach project hosted by the Instituto de Astrof\'I sica de Andaluc\'I a - CSIC and leaded by a blind astronomer aimed at the teaching and popularisation of astronomy and astrophysics among all kind of disabled and non-disabled people. Among the different strategies followed to transmit information to blind and partially sighted people, audio description is one of the most accesible and popular in the case of films and museums, but it has not been yet widely incorporated for the description of astronomical images. In this contribution we introduce {"The Universe in words", which are a series of videos describing images of some of the most popular objects in the Messier catalogue. These audio descriptions do not only have a clear inclusive aspect, but also imply a better and deeper understanding of the represented images for everybody. This is one of the most important aspects of using inclusive resources, as they also clearly improve the efficiency of the transmission process for all kind of public. These videos can also be used as supplementary material in of in-person activities and as a complement to other kind of materials, such as sonifications or models of the same or similar type of astronomical objects., Comment: 4 pages. To appear as conference contribution to the 2nd IAU Workshop Beyond the Common Senses held on 2021

CosMOnic (COSmos harMONIC) is a sonification project with a triple purpose: analysis (by means of sounds) of any type of data, source of inspiration for artistic creations, and pedagogical and dissemination purposes. In this contribution we present the work recently produced by CosMonic in the latter field, creating specific cases for the inclusive astronomy dissemination project AstroAccesible for blind and partially sighted people, but also aimed at a general public that wants to understand astrophysics in an alternative format. For this project, CosMonic seeks to create simple astronomical cases in their acoustic dimension in order to be easily understood. CosMonic's philosophy for these sonifications can be summarized in a simple metaphor: painting graphs with sounds. Sonification is a powerful tool that helps to enhance visual information. Therefore, CosMonic accompanies its audios with animations, using complementary methods to reach a general public. In addition to provide some cases created by CosMonic for inclusive astronomy, we also share our experience with different audiences, as well as suggest some ideas for a better use of sonification in (global) inclusive outreach., Comment: 5 pages, 2nd Workshop on Astronomy Beyond the Common Senses for Accessibility and Inclusion, 17-18 November 2021 (virtual). To be appeared in the Proceedings (digital version) of the Revista Mexicana de Astronom\'ia y Astrof\'isica (Serie de Conferencias)

Extreme emission line galaxies (EELGs) are a notable galaxy genus, ultimately being regarded as local prototypes of early galaxies at the cosmic noon. Robust characterisation of their stellar content, however, is hindered by the exceptionally high nebular emission present in their optical spectroscopic data. This study is dedicated into recovering the stellar properties of a sample of 414 EELGs as observed by the SDSS Survey. Such is achieved by means of the spectral synthesis code FADO, which self-consistently considers the stellar and nebular emission in an optical spectrum. Additionally, a comparative analysis was carried on, by further processing the EELGs sample with the purely stellar spectral synthesis code Starlight, and by extending the analysis to a sample of 697 normal star-forming galaxies, expected to be less affected by nebular contribution. We find that, for both galaxy samples, stellar mass and mean age estimates by Starlight are systematically biased towards higher values, and that an adequate determination of the physical and evolutionary properties of EELGs via spectral synthesis is only possible when nebular continuum emission is taken into account. Moreover, the differences between the two population synthesis codes can be ascribed to the degree of star-formation activity through the specific star-formation rate and the sum of the flux of the most prominent emission lines. As expected, on the basis of the theoretical framework, our results emphasise the importance of considering the nebular emission while performing spectral synthesis, even for galaxies hosting typical levels of star-formation activity., Comment: 14 pages, 13 figures, accepted for publication in A&A

We use a sample of 536 \hii\ regions located in nearby spirals, with an homogeneous determination of their $T_e$-based abundances, to obtain new empirical calibrations of the N2O2, N2S2, O3N2, and N2 strong-line indices to estimate the nitrogen-to-oxygen abundance ratio when auroral lines are not detected. All indices are strongly correlated with the $T_e$-based $\log$(N/O) for our \hii\ region sample, even more strongly than with $12+\log$(O/H). N2O2 is the most strongly correlated index, and the best fit to the $\log$(N/O)-N2O2 relation is obtained with a second-order polynomial. The derived relation has a low dispersion ({\em rms}$<$0.09~dex), being valid in the range $-1.74 < $ N2O2 $< 0.62$ (or $-1.81 < $ $\log$(N/O) $< -0.13$). We have compared our calibration with previous ones and have discussed the differences between them in terms of the nature of the objects used as calibrators., Comment: 14 pages, 11 figures, accepted for publication in MNRAS

We present new and archival SOFIA FIFI-LS far-IR spectroscopic observations of 25 local galaxies of either the [OIII]52um and/or the [NIII]57um lines. Including other 31 galaxies from Herschel-PACS, we discuss a local sample of 47 galaxies, including HII region, luminous IR, low-metallicity dwarf and Seyfert galaxies. Analyzing the mid- to far-IR fine-structure lines of this sample, we assess the metallicity and compare with the optical spectroscopy estimates. Using the IR, we find a similar O/H--N/O relation to that known in the optical. As opposite, we find systematically lower N/O IR abundances when compared to the optical determinations, especially at high values of N/O (log(N/O) > -0.8). We explore various hypotheses to account for this difference: (i) difference in ionization structure traced by optical (O+, N+ regions) versus IR lines (O++, N++ regions); (ii) contamination of diffuse ionized gas affecting the optical lines used to compute the N/O abundance; (iii) dust obscuration affecting the optical-based determinations. However, we have not found any correlation of the Delta(N/O)= (N/O)_OPT-(N/O)_IR with either ionization, or electron density, or optical extinction. We speculatively suggest that accretion of metal-poor gas from the circumgalactic medium could provide an explanation for this difference, because the rapid decrease of total abundances during infall is followed by a N/O ratio decrease due to primary production of young - possibly embedded - massive stars, are preferentially traced by the IR diagnostics, while optical diagnostics would better trace the secondary production, when both N/O and O/H abundance ratios will increase., Comment: Accepted in the AstrophysicalJournal on the 7th November 2021

We investigate radiation hardness within a representative sample of 67 nearby (0.02 $\lesssim $z$ \lesssim$0.06) star-forming (SF) galaxies using the integral field spectroscopic data from the MaNGA survey. The softness parameter $\eta$ = $\frac{O^{+}/O^{2+}}{S^{+}/S^{2+}}$ is sensitive to the spectral energy distribution of the ionizing radiation. We study $\eta$ via the observable quantity $\eta\prime$ (=$\frac{[OII]/[OIII]}{[SII][SIII]}$) We analyse the relation between radiation hardness (traced by $\eta$ and $\eta\prime$) and diagnostics sensitive to gas-phase metallicity, electron temperature, density, ionization parameter, effective temperature and age of ionizing populations. It is evident that low metallicity is accompanied by low log $\eta\prime$, i.e. hard radiation field. No direct relation is found between radiation hardness and other nebular parameters though such relations can not be ruled out. We provide empirical relations between log $\rm\eta$ and strong emission line ratios N$_2$, O$_3$N$_2$ and Ar$_3$O$_3$ which will allow future studies of radiation hardness in SF galaxies where weak auroral lines are undetected. We compare the variation of [O III]/[O II] and [S III]/[S II] for MaNGA data with SF galaxies and H II regions within spiral galaxies from literature, and find that the similarity and differences between different data set is mainly due to the metallicity. We find that predictions from photoionizaion models considering young and evolved stellar populations as ionizing sources in good agreement with the MaNGA data. This comparison also suggests that hard radiation fields from hot and old low-mass stars within or around SF regions might significantly contribute to the observed $\eta$ values., Comment: 31 pages, 38 figures, 2 tables, accepted for publication in MNRAS

We estimate chemical abundances and ionization parameters in the nuclear region of a sample of 143 galaxies from the Palomar Spectroscopic Survey, composed by Star-Forming Galaxies (87), Seyferts 2 (16) and LINERs (40) using the \textsc{Hii-Chi-mistry} code. We also study for each spectral type the correlation of the derived quantities with other different properties of the host galaxies, such as morphology, stellar mass, luminosity and mass of their Supermassive Black Holes. The results obtained for Star-Forming Galaxies are used to check the soundness of our methodology. Then, we replicate a similar study for our sample of AGN, distinguishing between Seyferts 2 and LINERs. We report a saturation of Oxygen abundances for the nuclear regions of SFG. The correlations between chemical abundances and their host galaxy properties for SFG are in good agreement with previous studies. We find that Seyferts 2 present slightly higher chemical abundances but this result must be reexamined in larger samples of Seyfert galaxies. In contrast, we obtain lower chemical abundances for LINERs than for SFG. We confirm these relatively lower abundances for another sample of infrared luminous LINERs in the same stellar mass range. Our analysis of AGNs (both LINERs and Seyferts) shows that their host galaxy properties are not correlated with our estimated chemical abundances., Comment: Accepted for publication in MNRAS. 21 pages, 17 figures, 12 tables. Minor change: description of the code (Sec 2.4)

We use integral field spectroscopic (IFS) observations from Gemini North Multi-Object Spectrograph (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 $\sim$ 9 pc, we map the spatial distribution of a variety of physical properties of the ionised gas: internal dust attenuation, kinematics, stellar age, star-formation rate, emission line ratios and chemical abundances. The region of study is found to be photoionised. 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 FOV as well as the integrated data of the H II regions from ten 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., Comment: 24 pages, 25 figures, 7 tables, accepted for publication in MNRAS

In this tutorial it is explained the procedure to analyze an optical emission-line spectrum produced by a nebula ionized by massive star formation. Particularly, it is described the methodology used to derive physical properties, such as electron density and temperature, and the ionic abundances of the most representative elements whose emission lines are present in the optical spectrum. The tutorial is focused on the direct method,based on the measurement of the electron temperature to derive the abundances, given that the ionization and thermal equilibrium of the ionized gas is dominated by the metallicity. The ionization correction factors used to obtain total abundances from the abundances of some of their ions are also given. Finally, some strong-line methods to derive abundances are described. These are used when no estimation of the temperature can be derived, but that can be consistent with the direct method if they are calibrated., Comment: 27 pages, 9 figures. Accepted for publication as invited tutorial in PASP. Equation (3.5) relating t(OII) with RO2 corrected. Equation (4.20) for Ne2+ corrected. Some coefs. of the extinction law have been corrected

We present a new method to derive oxygen and carbon abundances using the ultraviolet (UV) lines emitted by the gas-phase ionised by massive stars. The method is based on the comparison of the nebular emission-line ratios with those predicted by a large grid of photo-ionisation models. Given the large dispersion in the O/H - C/O plane, our method firstly fixes C/O using ratios of appropriate emission lines and, in a second step, calculates O/H and the ionisation 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., Comment: 7 pages, 4 figures. MNRAS in press (DOI: 10.1093/mnras/stx186). Modified to match the published version

We study the physical and chemical properties of a sample of HII regions located at RG >11 kpc and present the radial distribution of abundances towards the Galaxy anticentre. We carried out optical spectroscopic observations of nine HII regions with the WHT. The sample was increased by searching the literature for optical observations of regions towards the Galactic anticentre, re-analysing them to obtain a single sample of 23 objects covering the Galactocentric radius from 11 kpc to 18 kpc to be processed in a homogeneous and consistent manner. Accurate electron densities and temperatures of several ionic species were derived in 13 HII regions. These physical parameters were applied to the spectra to determine direct total chemical abundances. For those regions without direct estimations of temperature, chemical abundances were derived by performing tailor-made photoionisation models and/or by using an empirical relation obtained from radio recombination and optical temperatures. We performed weighted least-squares fits to the distribution of the derived abundances along the Galactocentric distances to study the radial gradients of metallicity. The distributions O/H, N/H, S/H, and Ar/H towards the anticentre can be represented by decreasing linear radial gradients, while in the case of N/O abundances the radial distribution is better fitted with a two-zone model. The He/H radial gradient is presented here for the first time; we find a slope that is not significantly different from zero. The derived gradient for oxygen shows a clear decrease with distance with a slope of -0.053dex/kpc. Although a shallower slope at large Galactocentric distances is suggested by our data, the flattening of the distribution cannot be confirmed and more objects towards the anticentre need to be studied in order to establish the true form of the metallicity gradient., Comment: Accepted for publication in Section 5. Galactic structure, stellar clusters and populations of Astronomy and Astrophysics. The official date of acceptance is 13/09/2016. 19 pages, 15 figures and 10 tables

Here we highlight our recent results from the IFS study of Mrk178, the closest metal-poor WR galaxy, and of IZw18, the most metal-poor star-forming galaxy known in the local Universe. The IFS data of Mrk178 show the importance of aperture effects on the search for WR features, and the extent to which physical variations in the ISM properties can be detected. Our IFS data of IZw18 reveal its entire nebular HeII4686-emitting region, and indicate for the first time that peculiar, very hot (nearly) metal-free ionizing stars (called here PopIII-star siblings) might hold the key to the HeII-ionization in IZw18., Comment: 3 pages, 4 figures, to appear in proceedings of International Workshop on Wolf-Rayet Stars (editors W.-R. Hamann, A. Sander, and H. Todt, publisher Universitat Potsdam)

The derivation of abundances in gaseous nebulae ionised by massive stars using optical collisionally excited emission lines is studied in this work comparing the direct or $T_e$ method with updated grids of photoionisation models covering a wide range of input conditions of O/H and N/O abundances and ionisation parameter. The abundances in a large sample of compiled objects with at least one auroral line are re-derived and later compared with the $\chi^2$ weighted-mean abundances from the models. The agreement between the abundances using the two methods both for O/H and N/O is excellent with no additional assumptions about the geometry or physics governing the HII regions. Although very inaccurate model-based O/H are obtained when no auroral lines are considered, this can be overcome assuming empirical laws between O/H, log $U$, and N/O to constrain the considered models. In this way, for 12+log(O/H) $>$ 8.0, a precision better than 0.1dex consistent with the direct method is attained. For very low-$Z$, models give higher O/H values and a high dispersion, possibly owing to the contamination of the low-excitation emission-lines. However, in this regime, the auroral lines are usually well-detected. The use of this procedure, in a publicly available script, HII-CHI-mistry}, leads to the derivation of abundances in faint/high redshift objects consistent with the direct method based on CELs., Comment: 14 pages, 9 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society. Some figures and text have been replaced to match published version

We use optical integral field spectroscopy and 8 and 24 micron mid-IR observations of the giant HII region NGC 588 in the disc of M33 as input and constraints for two-dimensional tailor-made photoionisation models. Two different geometrical approaches are followed for the modelling structure: i) Each spatial element of the emitting gas is studied individually using models which assume that the ionisation structure is complete in each element to look for azimuthal variations across gas and dust. ii) A single model is considered, and the two-dimensional structure of the gas and the dust are assumed to be due to the projection of an emitting sphere onto the sky. The models in both assumptions reproduce the radial profiles of Hbeta surface brightness, the observed number of ionising photons, and the strong optical emission-line relative intensities. The first approach produces a constant-density matter-bounded thin shell of variable thickness and dust-to-gas ratio, while the second gives place to a radiation-bounded thick shell sphere of decreasing particle density. However, the radial profile of the 8/24 microns IR ratio, depending on the gas and dust geometry, only fits well when the thick-shell model is used. The resulting dust-to-gas mass ratio, which was obtained empirically from the derived dust mass using data from Spitzer, also has a better fit using the thick-shell solution. In both approaches, models support the chemical homogeneity, and the ionisation-parameter radial decrease, These results must be taken with care in view of the very low extinction values that are derived from the IR, as compared to that derived from the Balmer decrement. Besides, the IR can be possibly contaminated with the emission from a cloud of diffuse gas and dust above the plane of the galaxy detected at 250 micron Herschel image., Comment: 16 pages, 14 figures. Accepted for publication in Astronomy & Astrophysics. Fig. 1 included

We investigate the spatial distribution of chemical abundances in a sample of low metallicity Wolf-Rayet (WR) galaxies selected from the SDSS. We used the integral field spectroscopy technique in the optical spectral range (3700-6850 AA) with PMAS attached to the CAHA 3.5 m telescope. Our statistical analysis of the spatial distributions of O/H and N/O, as derived using the direct method or strong-line parameters consistent with it, indicates that metallicity is homogeneous in five out of the six analysed objects in scales of the order of several kpc. Only in the object WR404, a gradient of metallicity is found in the direction of the low surface brightness tail. In contrast,we found an overabundance of N/O in spatial scales of the order of hundreds of pc associated with or close to the positions of the WR stars in 4 out of the 6 galaxies. We exclude possible hydrodynamical causes, such as the metal-poor gas inflow, for this local pollution by means of the analysis of the mass-metallicity relation (MZR) and mass-nitrogen-to-oxygen relation (MNOR) for the WR galaxies catalogued in the SDSS., Comment: Accepted by "Advances in Astronomy" for the special issue "Metals in 3D: A cosmic view from the integral field spectroscopy". 19 pages, 6 figures. Bold sentences corrected

We employed observational spectroscopic data of star-forming regions compiled from the literature and photoionization models to analyse the neon ionic abundances obtained using both optical and mid-infrared emission-lines. Comparing Ne++/H+ ionic abundances from distinct methods, we found that, in average, the abundances obtained via IR emission-lines are higher than those obtained via optical lines by a factor of 4. Photoionization models with abundance variations along the radius of the hypothetical nebula provide a possible explanation for a large part of the difference between ionic abundances via optical and infrared emission-lines. Ionization Correction Factor (ICF) for the neon is obtained from direct determinations of ionic fractions using infrared emission-lines. A constant Ne/O ratio (logNe/O \approx -0.70) for a large range of metallicity, independently of the ICF used to compute the neon total abundance is derived., Comment: 17 pages, 14 figures, accepted by MNRAS

Deep, high resolution spectroscopic observations have been obtained for six compact, strongly star-forming galaxies at redshift z~0.1-0.3, most of them also known as Green Peas. Remarkably, these galaxies show complex emission-line profiles in the spectral region including H\alpha, [NII]$\lambda\lambda 6548, 6584$ and [SII]$\lambda\lambda 6717, 6731$, consisting of the superposition of different kinematical components on a spatial extent of few kpc: a very broad line emission underlying more than one narrower component. For at least two of the observed galaxies some of these multiple components are resolved spatially in their 2D-spectra, whereas for another one a faint detached H\alpha\ blob lacking stellar continuum is detected at the same recessional velocity ~7 kpc away from the galaxy. The individual narrower H\alpha\ components show high intrinsic velocity dispersion (\sigma ~30-80 km s$^{-1}$), suggesting together with unsharped masking HST images that star formation proceeds in an ensemble of several compact and turbulent clumps, with relative velocities of up to ~500 km s$^{-1}$. The broad underlying H\alpha\ components indicate in all cases large expansion velocities (full width zero intensity FWZI $\ge$ 1000 km s$^{-1}$) and very high luminosities (up to ~10$^{42}$ erg s$^{-1}$), probably showing the imprint of energetic outflows from SNe. These intriguing results underline the importance of Green Peas for studying the assembly of low-mass galaxies near and far., Comment: 11 pages, 3 figures, and 1 Table. Accepted for publication in ApJ Letters

We present a detailed auto-consistent study of the nearest blue compact dwarf galaxy NGC 6789 by means of optical and UV archive photometry data and optical long-slit ISIS-WHT spectroscopy observations of the five brightest star-forming knots. The analysis of the spectra in all knots allowed the derivation of ionic chemical abundances of oxygen, nitrogen, sulphur, argon and neon using measures of both the high- and low-excitation electron temperatures, leading to the conclusion that NGC 6789 is chemically homogeneous with low values of the abundance of oxygen in the range 12+log(O/H) = 7.80-7.93, but presenting at the same time higher values of the nitrogen-to-oxygen ratio than expected for its metal regime. We used archival HST/WFPC2 F555W and F814W observations of NGC 6789 to perform a photometric study of the colour-magnitude diagram (CMD) of the resolved stellar populations and derive its star formation history (SFH), which is compatible with the presence of different young and old stellar populations whose metallicities do not necessarily increase with age. We fit the observed optical spectrum in all the five knots using the STARLIGHT code and a combination of single stellar populations following the SFH obtained from the CMD. We compare the resulting stellar masses and the relative fractions of the ionising populations with a non-constrained SFH case. The properties of the younger populations were obtained using CLOUDY photoionisation models, giving similar ages in all the knots in the range 3-6 Myr and the estimation of the dust absorption factor, which correlates with the observed GALEX FUV-NUV colour indices. The total photometric extinction and dust-absorption corrected H\alpha\ fluxes were finally used to derive the star formation rates., Comment: 18 pages, 6 figures, 10 tables; accepted by MNRAS

We investigate the excitation mechanisms of near-infrared [Fe ii] and H2 emission lines observed in Active Galactic Nuclei (AGNs). We built a photoionization model grid considering a two-component continuum, one accounts for the Big Bump component peaking at 1Ryd and another represents the X-ray source that dominates the continuum emission at high energies. Photoionization models considering as ionizing source a spectral energy distribution obtained from photometric data of the Sy 2 Mrk 1066 taken from the literature were considered. Results of these models were compared with a large sample of observational long-slit and Integral field Unit (IFU) spectroscopy data of the nuclear region for a sample of active objects. We found that the correlation between the observational [Fe ii]{\lambda}1.2570 {\mu}m/Pa{\beta} vs. H2{\lambda}2.1218 {\mu}m/Br{\gamma} is well reproduced by our models as well as the relationships that involve the H2 emission line ratios observed in the spectroscopic data.We conclude that the heating by X-rays produced by active nuclei can be considered a common and very important mechanism of excitation of [Fe ii] and H2., Comment: 10 pages, 9 figures, accepted by MNRAS

The softness parameter is based on the relative intensity of several optical emission lines emitted by the gas ionized by young massive star clusters and can be used to derive the equivalent effective temperature (T*) in those objects whose stellar population cannot be resolved. This method has several uncertainties due to the disagreement between different synthesis model atmospheres but it is robust to study the relative variations between objects. Following the 2D photoionization models of the giant HII region NGC 595 (P\'erez-Montero et al. 2011) we show that the determination of T* with the softness parameter is also robust in different regions of a same object with large variations in the geometry of the gas and in the dust-to-gas ratio., Comment: Proceeding for the Workshop "Star clusters and Associations: A RIA Workshop on Gaia", held in Granada from 23rd to 27th May, 2011. 6 pages and 4 figures

We review recent results on the oxygen and nitrogen chemical abundances in extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3 recently named to as "Green Pea" galaxies. These galaxies are genuine metal-poor galaxies ($\sim$ one fifth solar) with N/O ratios unusually high for galaxies of the same metallicity. In combination with their known general properties, i.e., size, stellar mass and star-formation rate, these findings suggest that these objects could be experiencing a short and extreme phase in their evolution. The possible action of both recent and massive inflow of gas, as well as stellar feedback mechanisms are discussed here as main drivers of the starburst activity and their oxygen and nitrogen abundances., Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon, September 2010, Springer Verlag, in press

We analyze the reliability of oxygen abundances and ionization parameters obtained from different diagnostic diagrams. For this, we compiled from the literature observational emission line intensities and oxygen abundance of 446 star-forming regions whose O/H abundance was determined by direct estimation of electron temperature. The abundances compiled were compared with the values calculated in this work using different diagnostic diagrams in combination with results from a grid of photoionization models. We found that the [\ion{O}{iii}]/[\ion{O}{ii}] vs. [\ion{N}{ii}]/[\ion{O}{ii}], [\ion{O}{iii}]/H$\beta$ vs. [\ion{N}{ii}]/[\ion{O}{ii}], and ([\ion{O}{iii}]/H$\beta$)/([\ion{N}{ii}]/H$\alpha$) vs. [\ion{S}{ii}]/[\ion{S}{iii}] diagnostic diagrams give O/H values close to the $T_{\rm e}$-method, with differences of about 0.04 dex and dispersion of about 0.3 dex. Similar results were obtained by detailed models but with a dispersion of 0.08 dex. The origin of the dispersion found in the use of diagnostic diagrams is probably due to differences between the real N/O-O/H relation of the sample and the one assumed in the models. This is confirmed by the use of detailed models that do not have a fixed N/O-O/H relation. We found no correlation between ionization parameter and the metallicity for the objects of our sample. We conclude that the combination of two line ratio predicted by photoionization models, one sensitive to the metallicity and another sensitive to the ionization parameter, which takes into account the physical conditions of star-forming regions, gives O/H estimates close to the values derived using direct detections of electron temperatures., Comment: 12 pages, 9 figures, accepted by MNRAS

We analyze high signal-to-noise spectrophotometric observations acquired simultaneously with TWIN, a double-arm spectrograph, from 3400 to 10400 \AA of three star-forming regions in the HII galaxy SDSS J165712.75+321141.4. We have measured four line temperatures: Te([OIII]), Te([SIII]), Te([OII]), and Te([SII]), with high precision, rms errors of order 2%, 5%, 6% and 6%, respectively, for the brightest region, and slightly worse for the other two. The temperature measurements allowed the direct derivation of ionic abundances of oxygen, sulphur, nitrogen, neon and argon. We have computed CLOUDY tailor-made models which reproduce the O2+ measured thermal and ionic structures within the errors in the three knots, with deviations of only 0.1 dex in the case of O+ and S2+ ionic abundances. In the case of the electron temperature and the ionic abundances of S+/H+, we find major discrepancies which could be consequence of the presence of colder diffuse gas. The star formation history derived using STARLIGHT shows a similar age distribution of the ionizing population among the three star-forming regions. This fact suggests a similar evolutionary history which is probably related to a process of interaction with a companion galaxy that triggered the star formation in the different regions almost at the same time. The hardness of the radiation field mapped through the use of the softness parameter $\eta$ is the same within the observational errors for all three regions, implying that the equivalent effective temperature of the radiation fields are very similar for all the studied regions of the galaxy, in spite of some small differences in the ionization state of different elements., Comment: 18 pages, 10 figures, accepted by MNRAS

We present a set of photoionization models that reproduce simultaneously the observed optical and mid-infrared spatial distribution of the HII region NGC595 in the disk of M33 using the code CLOUDY. Both optical (PMAS-Integral Field Spectroscopy) and mid-infrared (8 mi and 24 mi bands from Spitzer) data provide enough spatial resolution to model in a novel approach the inner structure of the HII region. We define a set of elliptical annular regions around the central ionizing cluster with an uniformity in their observed properties and consider each annulus as an independent thin shell structure. For the first time our models fit the relative surface brightness profiles in both the optical (Halpha, [OII], [OIII]) and the mid-infrared emissions (8 mi and 24 mi), under the assumption of a uniform metallicity (12+log(O/H) = 8.45; Esteban et al. 2009) and an age for the stellar cluster of 4.5 Myr (Malumuth et al. 1996). Our models also reproduce the observed uniformity of the R23 parameter and the increase of the [OII]/[OIII] ratio due to the decrease of the ionization parameter. The variation of the Halpha profile is explained in terms of the differences of the occupied volume (the product of filling factor and total volume of the shell) in a matter-bounded geometry, which also allows to reproduce the observed pattern of the extinction. The 8 mi/24 mi ratio is low (ranging between 0.04 and 0.4) because it is dominated by the surviving of small dust grains in the HII region, while the PAHs emit more weakly because they cannot be formed in these thin HII gas shells. The ratio is also well fitted in our models by assuming a dust-to-gas ratio in each annulus compatible with the integrated estimate for the whole HII region after the 70 mi, and 160 mi Spitzer observations., Comment: Accepted for publication in MNRAS, 9 pages, 17 figures

We present a self-consistent study of the stellar populations and the ionized gas in a sample of 10 HII 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 program STARLIGHT in order to quantify the contribution of the underlying stellar population to EW(Hbeta), which amounts to about 10% for most of the objects. We then studied the Wolf-Rayet stellar populations detected in seven of the galaxies. The presence of these populations and the corrected EW(Hbeta) values 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 strongest emission lines were used as input parameters to compute tailored models with the photoionization code CLOUDY. Our models are able to y reproduce their thermal and ionization structure as deduced from their collisionally excited emission lines and, hence, no abundance discrepancy factors are implied for this kind of objects. Only the electron temperature of S+ is overestimated by the models, pointing to the possible presence of outer shells of diffuse gas in these objects. 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., Comment: 20 pages, 12 figures, MNRAS (in press)

We carry out a study of the hardness of the radiation of ionizing clusters in HII regions. Firstly, we explore the applicability of the softness parameter, 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 both in 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 HII 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 HII regions, it looks that their existence is related to the mass and type of the galaxies and, hence, to the roperties of the entire disc., Comment: To appear as a letter in MNRAS. 5 pages, 4 figures and 2 tables

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 enrichement that we can find in the Universe. We first construct the N/O vs. O/H diagram and we 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 [NII] emission lines, such as N2 and O3N2. We demonstrate that these parameters underestimate the metallicity at low N/O ratios and viceversa. We investigate also the effect of the N/O ratio on different diagnostic diagrams used to discriminate narrow-line AGNs from star forming regions, such as the [OIII]/Hbeta vs. [NII}]/Halpha, and we 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 SDSS. 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 in the high metallicity regime., Comment: 12 pages, 13 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society

We carried out long-slit spectroscopic observations of the star forming knots along the polar ring of the dwarf galaxy IIZw71 in the spectral range 3500 - 10000 angstroms taken with the William erschel Telescope (WHT). The spectroscopic observations were complemented with available photometry of the galaxy in the narrow Halpha filter. We measured the rotation curve of the ring, from which we infer a ratio M/L_B = 3.9 inside the star forming ring. We measured the auroral [OIII] line in the two brightest knots, allowing us to measure oxygen, sulphur, nitrogen, argon and neon chemical abundances following the direct method. Different empirical calibrators were used to estimate the oxygen abundance in the two faintest knots. The metallicities obtained are very similar for all the knots, but lower than previously reported in the literature from integrated spectra. The N/O abundance, as derived from the N2O2 parameter, is remarkably constant over the ring, indicating that local polution processes are not conspicuous. Using synthetic stellar populations (SSPs) calculated with the code STARLIGHT, we studied the age distribution of the stellar populations in each knot, finding that in all of them there is a combination of a very young population with less than 10 Myr, responsible for the ionisation of the gas, with other populations older than 100 Myr, probably responsible for the chemical evolution of the knots. The small differences in metallicity and the age distributions among the different knots are indicative of a common chemical evolution, probably related to the process of interaction with the companion galaxy IIZw70., Comment: 11 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics, language edited, typos corrected

We present high signal-to-noise spectrophotometric observations of seven luminous HII galaxies. The observations have been made with the use of a double-arm spectrograph which provides spectra with a wide wavelength coverage, from 3400 to 10400\AA free of second order effects, of exactly the same region of a given galaxy. These observations are analysed applying a methodology designed to obtain accurate elemental abundances of oxygen, sulphur, nitrogen, neon, argon and iron in the ionized gas. Four electron temperatures and one electron density are derived from the observed forbidden line ratios using the five-level atom approximation. For our best objects errors of 1% in t_e([OIII]), 3% in t_e([OII]) and 5% in t_e([SIII]) are achieved with a resulting accuracy of 7% in total oxygen abundances, O/H. The ionisation structure of the nebulae can be mapped by the theoretical oxygen and sulphur ionic ratios, on the one side, and the corresponding observed emission line ratios, on the other -- the \eta and \eta' plots --. The combination of both is shown to provide a means to test photo-ionisation model sequences currently applied to derive elemental abundances in HII galaxies., Comment: 24 pages, 8 figures, accepted by MNRAS

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 neon and argon. 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 disks of some nearby spirals. We study the relation between the intensities of the emission lines of [NeIII] at 3869 \AA and [OIII] at 4959 \AA and 5007 \AA. 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 [OII], [NeIII] and Hdelta emission lines to derive metallicities for galaxies at high z., Comment: 12 pages, 12 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

We present a detailed photoionisation model of the brightest knot of star format ion in the blue compact dwarf galaxy Mrk 209. The model reproduces the intensiti es of main lines emitted by the ionised 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 ionising source. The features of Wolf-Rayet stars observed in the spectrum of the object, along with its ionising properties, lead to different solutions for the ages and characteristics of the ionising stellar populations.The found solutions are compatible with either a composite population of two ionising 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 ionisation 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 amout of dust was required in order to reproduce correctly the measured electron temperatures. This dust implies a gas/dust ratio identical to that in the Milky Way, and a fraction of absorbed photons of f = 0.58. 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 ionisation 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., Comment: 13 pages, 14 figures, in press in MNRAS, LaTeX: typos added, references removed

We propose a methodology to perform a self-consistent analysis of the physical properties of the emitting gas of HII galaxies adequate to the data that can be obtained with the XXI century technology. This methodology requires the production and calibration of empirical relations between the different line temperatures that should superseed currently used ones based on very simple, and poorly tested, photo-ionization model sequences. As a first step to reach these goals we have obtained simultaneous blue to far red longslit spectra with the William Herschel Telescope (WHT) of three compact HII galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2) spectral catalog using the INAOE Virtual Observatory superserver. Our spectra cover the range from 3200 to 10500 \AA, including the Balmer jump, the [OII]3727,29 \AA lines, the [SIII]9069,9532 \AA doublet as well as various weak auroral lines such as [OIII]4363 \AA and [SIII]6312 \AA. For the three objects we have measured at least four line temperatures: T([OIII]), T([SIII]), T([OII]) and T([SII]) 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%. These accuracies are expected to improve as better calibrations based on more precise measurements, both on electron temperatures and densities, are produced. ..., Comment: 22 pages, 13 figures and 11 tables. Accepted by MNRAS

We have derived an empirical calibration of the abundance of S/H as a function of the S23 parameter, defined using the bright sulphur lines of [SII] and SIII]. Contrary to what is the case for the widely used O$_{23}$ parameter, the calibration remains single valued up to the abundance values observed in the disk HII regions. The calibration is based on a large sample of nebulae for which direct determinations of electron temperatures exist and the sulphur chemical abundances can be directly derived. ICFs, as derived from the [SIV] 10.52 $\mu$ emission line (ISO observations), are shown to be well reproduced by Barker's formula for a value of alpha = 2.5. At any rate, only about 30% of the objects in the sample require ICFs larger than 1.2. The use of the proposed calibration opens the possibility of performing abundance analysis with red to IR spectroscopic data using S/H as a metallicity tracer., Comment: 9 pages, 5 figures. Accepted for publication by Astronomy & Astrophysics

We present a new analysis of the main empirical calibrators of oxygen abundance for ionised gas nebulae. With that aim we have compiled an extensive sample of objects with emission line data including the near IR [SIII] 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 homogeneus 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 photo-ionisation 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., Comment: 16 pages, 26 figures, accepted for publication in MNRAS

We present a study of the stellar populations and gas physical conditions in Circumnuclear Star Forming Regions (CNSFRs) based on spectrophotometric data, including the 7000 angstroms to 1 micron spectral range, which is of great interest to the whole diagnostic of the ionized gas. The properties of CNSFRs as a class are inferred from a comparison with those of other star forming emission-line objects: low excitation disc HII regions and HII galaxies., Comment: 6 pages, 4 eps figures. Proceeding for the conference "Starbursts: From 30 Doradus to Lyman Break Galaxies", held in Cambridge (UK) in September, 2004