Effective temperature of ionizing stars of extragalactic HII regions

The effective temperature (Teff) of the radiation field of the ionizing star(s) of a large sample of extragalactic HII regions was estimated using the R = log([O II] (λλ3726 + 29)/[O III] λ5007) index. We used a grid of photoionization models to calibrate the Teff-R relation finding that it has a strong dependence with the ionizing parameter, while it shows a weak direct dependence with the metallicity (variations in Z imply variations in U) of both the stellar atmosphere of the ionizing star and the gas phase of the HII region. Since the R index varies slightly with the Teff for values larger than 40 kK, the R index can be used to derive the Teff in the 30-40 kK range. A large fraction of the ionization parameter variation is due to differences in the temperature of the ionizing stars and then the use of the (relatively) low Teff dependent S2 = [S II] (λλ6717 + 31)/Hα emission-line ratio to derive the ionization parameter is preferable over others in the literature. We propose linear metallicity dependent relationships between S2 and U. Teff and metallicity estimations for a sample of 865 HII regions, whose emission-line intensities were compiled from the literature, do not show any Teff-Z correlation. On the other hand, it seems to be hints of the presence of an anticorrelation between Teff-U.We found that the majority of the studied HII regions (~87 per cent) present Teff values in the range between 37 and 40 kK, with an average value of 38.5(±1) kK. We also studied the variation of Teff as a function of the galactocentric distance for 14 spiral galaxies. Our results are in agreement with the idea of the existence of positive Teff gradients along the disc of spiral galaxies.Instituto de Astrofísica de La Plat

Effective temperature of ionizing stars of extragalactic HII regions

The effective temperature (Teff) of the radiation field of the ionizing star(s) of a large sample of extragalactic HII regions was estimated using the R = log([O II] (λλ3726 + 29)/[O III] λ5007) index. We used a grid of photoionization models to calibrate the Teff-R relation finding that it has a strong dependence with the ionizing parameter, while it shows a weak direct dependence with the metallicity (variations in Z imply variations in U) of both the stellar atmosphere of the ionizing star and the gas phase of the HII region. Since the R index varies slightly with the Teff for values larger than 40 kK, the R index can be used to derive the Teff in the 30-40 kK range. A large fraction of the ionization parameter variation is due to differences in the temperature of the ionizing stars and then the use of the (relatively) low Teff dependent S2 = [S II] (λλ6717 + 31)/Hα emission-line ratio to derive the ionization parameter is preferable over others in the literature. We propose linear metallicity dependent relationships between S2 and U. Teff and metallicity estimations for a sample of 865 HII regions, whose emission-line intensities were compiled from the literature, do not show any Teff-Z correlation. On the other hand, it seems to be hints of the presence of an anticorrelation between Teff-U.We found that the majority of the studied HII regions (~87 per cent) present Teff values in the range between 37 and 40 kK, with an average value of 38.5(±1) kK. We also studied the variation of Teff as a function of the galactocentric distance for 14 spiral galaxies. Our results are in agreement with the idea of the existence of positive Teff gradients along the disc of spiral galaxies.Instituto de Astrofísica de La Plat

New metallicity calibration for Seyfert 2 galaxies based on the N2O2 index

We derive a new relation between the metallicity of Seyfert 2 active galactic nuclei (AGNs) and the intensity of the narrow emission-lines ratio N2O2 = log([N II] λ6584/[O II] λ3727). The calibration of this relation was performed by determining the metallicity (Z) of a sample of 58 AGNs through a diagram containing the observational data and the results of a grid of photoionization models obtained with the CLOUDY code. We find the new Z/Z⊙–N2O2 relation using the obtained metallicity values and the corresponding observational emission-line intensities for each object of the sample. Estimations derived through the use of this new calibration indicate that the narrow-line regions of Seyfert 2 galaxies exhibit a large range of metallicities (0.3 ≲ Z/Z⊙ ≲ 2.0), with a median value Z ≈ Z⊙. Regarding the possible existence of correlations between the luminosity L(Hβ), the electron density and the colour excess E(B − V) with the metallicity in this kind of objects, we do not find correlations between them.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

Metallicity evolution of AGNs from UV emission lines based on a new index

We analysed the evolution of the metallicity of the gas with the redshift for a sample of AGNs in a very wide redshift range (0 < z < 4) using ultraviolet emission lines from the narrow-line regions (NLRs) and photoionization models. The new index C43 = log[(C IV+C III])/He II] is suggested as a metallicity indicator for AGNs. Based on this indicator, we confirmed the no metallicity evolution of NLRs with the redshift pointed out by previous works. We found that metallicity of AGNs shows similar evolution than the one predicted by cosmic semianalytic models of galaxy formation set within the cold dark matter merging hierarchy (for z ≲ 3).Our results predict amean metallicity for local objects in agreement with the solar value (12+log(O/H)=8.69). This value is about the same that themaximum oxygen abundance value derived for the central parts of local spiral galaxies. Very low metallicity log (Z/Z⊙) ≈ -0.8 for some objects in the range 1.5<z<3 is derived.Facultad de Ciencias Astronómicas y Geofísica

The circumnuclear environment of the peculiar galaxy NGC 3310

Gas and star velocity dispersions have been derived for eight circumnuclear star-forming regions (CNSFRs) and the nucleus of the spiral galaxy NGC 3310 using high-resolution spectroscopy in the blue and far red. Stellar velocity dispersions have been obtained from the Ca ii triplet in the near-IR, using cross-correlation techniques, while gas velocity dispersions have been measured by Gaussian fits to the Hβ λ4861 Å and [O iii] λ5007 Å emission lines.The CNSFR stellar velocity dispersions range from 31 to 73 km s-1. These values, together with the sizes measured on archival Hubble Space Telescope images, yield upper limits to the dynamical masses for the individual star clusters between 1.8 and 7.1 × 106 M⊙, for the whole CNSFR between 2 × 107 and 1.4 × 108 M⊙, and 5.3 × 107 M⊙, for the nucleus inside the inner 14.2 pc. The masses of the ionizing stellar population responsible for the H ii region gaseous emission have been derived from their published Hα luminosities and are found to be between 8.7 × 105 and 2.1 × 106 M⊙ for the star-forming regions and 2.1 × 105 M⊙ for the galaxy nucleus; they therefore constitute between 1 and 7 per cent of the total dynamical mass.The ionized gas kinematics is complex; two different kinematical components seem to be present as evidenced by different linewidths and Doppler shifts.Facultad de Ciencias Astronómicas y Geofísica

On the derivation of dynamical masses of the stellar clusters in the circumnuclear region of NGC 2903

Gas and star velocity dispersions have been derived for four circumnuclear star-forming regions (CNSFRs) and the nucleus of the spiral galaxy NGC 2903 using high-resolution spectroscopy in the blue and far red. Stellar velocity dispersions have been obtained from the Ca ii triplet lines at λλ 8494, 8542, 8662 Å, using cross-correlation techniques, while gas velocity dispersions have been measured by Gaussian fits to the Hβ λ 4861 Å line. The CNSFRs, with sizes of about 100 to 150 pc in diameter, show a complex structure at the Hubble Space Telescope (HST) resolution, with a good number of subclusters with linear diameters between 3 and 8 pc. Their stellar velocity dispersions range from 39 to 67 km s-1. These values, together with the sizes measured on archival HST images, yield upper limits to the dynamical masses for the individual star clusters between 1.8 and 8.7 × 106 M⊙ and upper limits to the masses for the whole CNSFR between 4.9 × 106 and 4.3 × 107 M⊙. The masses of the ionizing stellar population responsible for the H ii region gaseous emission have been derived from their published Hα luminosities and are found to be between 1.9 and 2.5 × 10 6 M⊙ for the star-forming regions, and 2.1 × 105 M⊙ for the galaxy nucleus, therefore constituting between 1 and 4 per cent of the total dynamical mass. In the CNSFR, star and gas velocity dispersions are found to differ by about 20 km s-1 with the Hβ lines being narrower than both the stellar lines and the [O iii] λ 5007 Å lines. The ionized gas kinematics are complex; two different kinematical components seem to be present as evidenced by different widths and Doppler shifts. The line profiles in the spectra of the galaxy nucleus, however, are consistent with the presence of a single component with radial velocity and velocity dispersion close to those measured for the stellar absorption lines. The presence and reach of two distinct components in the emission lines in ionized regions and the influence that this fact could have on the observed line ratios are of major interest for several reasons, among others, the classification of the activity in the central regions of galaxies, the inferences about the nature of the source of ionization for the two components and the derivation of the gaseous chemical abundances.Facultad de Ciencias Astronómicas y Geofísica

The circumnuclear environment of the peculiar galaxy NGC 3310

Gas and star velocity dispersions have been derived for eight circumnuclear star-forming regions (CNSFRs) and the nucleus of the spiral galaxy NGC 3310 using high-resolution spectroscopy in the blue and far red. Stellar velocity dispersions have been obtained from the Ca ii triplet in the near-IR, using cross-correlation techniques, while gas velocity dispersions have been measured by Gaussian fits to the Hβ λ4861 Å and [O iii] λ5007 Å emission lines.The CNSFR stellar velocity dispersions range from 31 to 73 km s-1. These values, together with the sizes measured on archival Hubble Space Telescope images, yield upper limits to the dynamical masses for the individual star clusters between 1.8 and 7.1 × 106 M⊙, for the whole CNSFR between 2 × 107 and 1.4 × 108 M⊙, and 5.3 × 107 M⊙, for the nucleus inside the inner 14.2 pc. The masses of the ionizing stellar population responsible for the H ii region gaseous emission have been derived from their published Hα luminosities and are found to be between 8.7 × 105 and 2.1 × 106 M⊙ for the star-forming regions and 2.1 × 105 M⊙ for the galaxy nucleus; they therefore constitute between 1 and 7 per cent of the total dynamical mass.The ionized gas kinematics is complex; two different kinematical components seem to be present as evidenced by different linewidths and Doppler shifts.Facultad de Ciencias Astronómicas y Geofísica

Sulphur abundance determinations in star-forming regions : I. Ionization correction factor

In this work, we used a grid of photoionization models combined with stellar population synthesis models to derive reliable ionization correction factors (ICFs) for the sulphur in star-forming regions. These models cover a large range of nebular parameters and yielding ionic abundances in consonance with those derived through optical and infrared observational data of star-forming regions. From our theoretical ICFs, we suggested an α value of 3.27 ± 0.01 in the classical Stasińska formulae. We compared the total sulphur abundance in the gas phase of a large sample of objects by using our theoretical ICF and other approaches. In average, the differences between the determinations via the use of the different ICFs considered are similar to the uncertainties in the S/H estimations. Nevertheless, we noted that for some objects it could reach up to about 0.3 dex for the low-metallicity regime. Despite of the large scatter of the points, we found a trend of S/O ratio to decrease with the metallicity, independently of the ICF used to compute the sulphur total abundance.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Sulphur abundance determinations in star-forming regions : I. Ionization correction factor

In this work, we used a grid of photoionization models combined with stellar population synthesis models to derive reliable ionization correction factors (ICFs) for the sulphur in star-forming regions. These models cover a large range of nebular parameters and yielding ionic abundances in consonance with those derived through optical and infrared observational data of star-forming regions. From our theoretical ICFs, we suggested an α value of 3.27 ± 0.01 in the classical Stasińska formulae. We compared the total sulphur abundance in the gas phase of a large sample of objects by using our theoretical ICF and other approaches. In average, the differences between the determinations via the use of the different ICFs considered are similar to the uncertainties in the S/H estimations. Nevertheless, we noted that for some objects it could reach up to about 0.3 dex for the low-metallicity regime. Despite of the large scatter of the points, we found a trend of S/O ratio to decrease with the metallicity, independently of the ICF used to compute the sulphur total abundance.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Implications of the kinematical structure of circumnuclear star-forming regions on their derived properties

We reviewthe results of high-dispersion spectroscopy of 17 circumnuclear star-forming regions (CNSFRs) in 3 nearby early spiral galaxies, NGC 2903, NGC 3310 and NGC 3351. We findthat single-Gaussian fitting to the Hβ and [O III]λ5007 Å line profiles results in velocity dispersions around 32 and 52 km s-1, respectively, while the IR Ca II triplet cross-correlation technique provides stellar velocity dispersion values close to 50 km s-1. Even though multiple kinematical components are present, the relation between gas velocity dispersion and Balmer emission line luminosity (L-σ relation) reproduces the correlation for disc giant HII regions albeit with a larger scatter. The scatter in the L-σ relation is considerably reduced when theoretical evolutionary corrections are applied suggesting that an age range is present in the sample of CNSFRs. To analyse the observed complex profiles, we performed multiple Gaussian component fits to the Hβ and [O III]λ5007 Å lines obtaining optimal fits with two Gaussians of different widths. These best fits indicate that the narrower component has average velocity dispersion close to 23 km s-1 while the broader component shows average values in the range 50-60 km s-1 for both lines, close to the observed stellar velocity dispersions. The fluxes of the broad and narrow Hβ components are similar. This is not the case for [O III]λ5007 Å for which the broad components have higher fluxes than the narrow ones, thus producing a clear segregation in their [OIII]/Hβ ratios. We suggest a possible scenario for understanding the behaviour of CNSFRs in the L-σ and σgas-σ* diagrams involving aninner gaseous disc responsible for the narrow component of the emission lines. Our main conclusion is that the presence of different kinematical components with similar total fluxes in the emission line spectrum of CNSFRs raises important doubts regarding the properties of the ionized gas derived from global line ratios obtained with low-resolution spectroscopy in star-forming regions in the central regions of early-type galaxies. Given the ubiquity of these star-forming systems, ionized gas analyses should be done preferably from high-dispersion spectra with high spatial resolution.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat