Wolf-Rayet Features Research Articles

Integral field spectroscopy of H ii region complexes: the outer disc of NGC 6946

Integral field spectroscopy obtained with the Potsdam Multi-Aperture Spectrophotometer Fiber Package (PPak) and the 3.5-m telescope at the Calar Alto Observatory has been used to study an outer H ii region complex in the well-studied galaxy NGC 6946. This technique provides detailed maps of the region in different emission lines, yielding spatially resolved information about the physical properties of the gas. The configuration was chosen to cover the whole spectrum from 3600 up to 10 000 Å, allowing the measurement of the near-infrared [S iii] lines. We selected four luminous knots to perform a detailed integrated spectroscopic analysis of these structures and of the whole PPak field of view (FOV). For all the knots the electron density has been found to be very similar and below 100 cm−3. The [O iii] electron temperature was measured in knots A, B, C and in the integrated PPak field, and was found to be around 8000 K. The temperatures of [O ii] and [S iii] were estimated in the four cases. The elemental abundances computed from the ‘direct method’ are typical of high-metallicity disc H ii regions, with a mean value of 12+log(O/H) = 8.65, comparable to what has been found in this galaxy by other authors for regions at similar galactocentric distance. Therefore a remarkable abundance uniformity is found despite the different excitations found throughout the nebula. However, due to the quality of the data, the electron temperatures and metallicities obtained have associated errors comparable to the typical dispersion found in empirical calibrations. Wolf–Rayet (WR) features have been detected in three of the knots, leading to a derived total number of WR stars of 125, 22 and 5 for knots A, C and B, respectively. The ratios of the numbers of WR to O stars are consistent with the prediction of Starburst99 for individual bursts with an age about 4 Myr. Knot D, with no WR features, shows weak Hα emission, low excitation and the lowest Hβ equivalent width, all of which points to a more evolved state. The integrated spectrum of the whole PPak FOV shows high excitation and a relatively evolved age that does not correspond to the individual knot evolutionary stages. Some effects associated with the loss of spatial resolution are also evidenced by the higher ionizing temperature that is deduced from the η′ parameter measured in the integrated PPak spectrum with respect to that of the individual knots.

A study of the interplay between ionized gas and star clusters in the central region of NGC 5253 with 2D spectroscopy

ABRIDGED: A detailed 2D study of the central region of NGC5253 has been performed to characterize the stellar and ionized gas structure as well as the extinction distribution, physical properties and kinematics of the ionized gas in the central ~210pc x 130pc. We utilized optical integral field spectroscopy (IFS) data obtained with FLAMES. A detailed extinction map for the ionized gas in NGC5253 shows that the largest extinction is associated with the prominent Giant HII region. There is an offset of ~0.5" between the peak of the optical continuum and the extinction peak in agreement with findings in the infrared. We found that stars suffer less extinction than gas by a factor of 0.33. The [SII]l6717/[SII]l6731 map shows an electron density (N_e) gradient declining from the peak of emission in Ha (790cm^-3) outwards, while the argon line ratio traces areas with $N_e~4200 - 6200cm^(-3). The area polluted with extra nitrogen, as deduced from the excess [NII]/Ha, extends up to distances of 3.3" (~60pc) from the maximum pollution, which is offset by ~1.5" from the peak of continuum emission. Wolf-Rayet features are distributed in an irregular pattern over a larger area (~100pc x 100pc) and associated with young stellar clusters. We measured He^+ abundances over most of the field of view and values of He^++/H^+<~0.0005 in localized areas which do not coincide, in general, with the areas presenting W-R emission or extra nitrogen. The line profiles are complex. Up to three emission components were needed to reproduce them. One of them, associated with the giant HII region, presents supersonic widths and [NII] and [SII] emission lines shifted up to 40km/s with respect to Ha. Similarly, one of the narrow components presents offsets in the [NII] line of <~20km/s. This is the first time that maps with such velocity offsets for a starburst galaxy have been presented.

The Wolf-Rayet features and mass–metallicity relation of long-duration gamma-ray burst host galaxies

Aims. We have gathered optical spectra of 8 long-duration GRB host galaxies selected from the archival data of VLT/FORS2. We investigated whether or not Wolf-Rayet (WR) stars can be detected in these GRB host galaxies. We also tried to estimate the physical properties of GRB host galaxies, such as metallicity. Methods. We identified the WR features in these spectra by fitting the WR bumps and WR emission lines in blue and red bumps. We also identified the subtypes of the WR stars, and estimated the numbers of stars in each subtype, then calculated the WR/O star ratios. The (O/H) abundances of GRB hosts were estimated from both the electron temperature (Te) and the metallicity-sensitive strong-line ratio (R23), for which we have broken the R23 degeneracy. We compared the environments of long-duration GRB host galaxies with those of other galaxies in terms of their luminosity (stellar mass)-metallicity relations (LZ, MZ). Results. We detected the presence of WR stars in 5 GRB host galaxies having spectra with relatively high signal-to-noise ratios (S/N). In the comparison of LZ, MZ relations, it shows that GRB hosts have lower metallicities than other samples with comparable luminosity and stellar mass. The presence of WR stars and the observed high WR/O star ratio, together with low metallicity, support the core-collapsar model and implie the first stage of star formation in the hosted regions of GRBs.

A Very Large Telescope imaging and spectroscopic survey of the Wolf-Rayet population in NGC 7793★

We present a VLT/FORS1 imaging and spectroscopic survey of the Wolf-Rayet (WR) population in the Sculptor group spiral galaxy NGC 7793. We identify 74 emission line candidates from archival narrow-band imaging, from which 39 were observed with the Multi Object Spectroscopy (MOS) mode of FORS1. 85% of these sources displayed WR features. Additional slits were used to observe HII regions, enabling an estimate of the metallicity gradient of NGC 7793 using strong line calibrations, from which a central oxygen content of log (O/H) + 12 = 8.6 was obtained, falling to 8.25 at R_25. We have estimated WR populations using a calibration of line luminosities of Large Magellanic Cloud stars, revealing ~27 WN and ~25 WC stars from 29 sources spectroscopically observed. Photometric properties of the remaining candidates suggest an additional ~27 WN and ~8 WC stars. A comparison with the WR census of the LMC suggests that our imaging survey has identified 80% of WN stars and 90% for the WC subclass. Allowing for incompleteness, NGC 7793 hosts ~105 WR stars for which N(WC)/N(WN)~0.5. From our spectroscopy of HII regions in NGC 7793, we revise the global Halpha star formation rate of Kennicutt et al. upward by 50% to 0.45 M_sun/yr. This allows us to obtain N(WR)/N(O)~0.018, which is somewhat lower than that resulting from the WR census by Schild et al. of another Sculptor group spiral NGC 300, whose global physical properties are similar to NGC 7793. Finally, we also report the fortuitous detection of a bright (m_V = 20.8 mag) background quasar Q2358-32 at z~2.02 resulting from CIV 1548-51 redshifted to the 4684 passband.

The AGN Properties of the Starburst Galaxy NGC 7582

NGC 7582 was identified as a starburst galaxy in the optical (Veron et al. 1981) but its X-ray emission is typical of a Seyfert 1 galaxy (Ward et al. 1978). We analyzed a datacube on this object obtained with the GMOS-IFU on the Gemini-South telescope. After a subtraction of the stellar component using the starlight code (Cid Fernandes et al. 2005), we looked for optical signatures of the AGN. We detected a broad Hα component (Figure 1) in the source where Bianchi et al. (2007) identified the AGN in an HST optical image. We also found a broad Hβ feature (Figure 2), but its emission reveals a extended source. We suggest that it is the light of the AGN scattered in the ionization cone. We propose that NGC 7582 is a Seyfert 1 galaxy. A number of other “hot-spots” and Wolf–Rayet features were also identified.

A SPECTROSCOPIC CENSUS OF THE M82 STELLAR CLUSTER POPULATION,

We present a spectroscopic study of the stellar cluster population of M82, the archetype starburst galaxy, based primarily on new Gemini-North multi-object spectroscopy of 49 star clusters. These observations constitute the largest to date spectroscopic data set of extragalactic young clusters, giving virtually continuous coverage across the galaxy; we use these data to deduce information about the clusters as well as the M82 post-starburst disk and nuclear starburst environments. Spectroscopic age dating places clusters in the nucleus and disk between ( 7, 15) and ( 30, 270) Myr, with distribution peaks at similar to 10 and 140 Myr, respectively. We find cluster radial velocities ( RVs) in the range v(R) is an element of (-160, 220) km s(-1) ( with respect to the galaxy center), and line-of-sight Na I D interstellar absorption line velocities v(R)(Na I D) is an element of (-75, 200) km s(-1), in many cases entirely decoupled from the clusters. As the disk cluster RVs lie on the flat part of the galaxy rotation curve, we conclude that they comprise a regularly orbiting system. Our observations suggest that the largest part of the population was created as a result of the close encounter with M81 similar to 220 Myr ago. Clusters in the nucleus are found in solid body rotation on the bar. The possible detection of Wolf-Rayet features in their spectra indicates that cluster formation continues in the central starburst zone. We also report the potential discovery of two old populous clusters in the halo of M82, aged greater than or similar to 8 Gyr. Using these measurements and simple dynamical considerations, we derive a toy model for the invisible physical structure of the galaxy, and confirm the existence of two dominant spiral arms.

NGC 2770: A SUPERNOVA Ib FACTORY?

NGC 2770 has been the host of three supernovae of Type Ib during the last 10 years, SN 1999eh, SN 2007uy and SN 2008D. SN 2008D attracted special attention due to the serendipitous discovery of an associated X-ray transient. In this paper, we study the properties of NGC 2770 and specifically the three SN sites to investigate whether this galaxy is in any way peculiar to cause a high frequency of SNe Ib. We model the global SED of the galaxy from broadband data and derive a star-formation and SN rate comparable to the values of the Milky Way. We further study the galaxy using longslit spectroscopy covering the major axis and the three SN sites. From the spectroscopic study we find subsolar metallicities for the SN sites, a high extinction and a moderate star-formation rate. In a high resolution spectrum, we also detect diffuse interstellar bands in the line-of-sight towards SN 2008. A comparison of NGC 2770 to the global properties of a galaxy sample with high SN occurance (at least 3 SN in the last 100 years) suggests that NGC 2770 is not particularly destined to produce such an enhancement of observed SNe observed. Its properties are also very different from gamma-ray burst host galaxies. Statistical considerations on SN Ib detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe detected in 10 years. The high number of rare Ib SNe in this galaxy is therefore likely to be a coincidence rather than special properties of the galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is highly starforming, has a very low mass and one of the lowest metallicities detected in the nearby universe as derived from longslit spectroscopy. In the most metal poor part, we even detect Wolf-Rayet features, against the current models of WR stars which require high metallicities.

THE PLANETARY NEBULA POPULATION OF M33 AND ITS METALLICITY GRADIENT: A LOOK INTO THE GALAXY'S DISTANT PAST

The Planetary Nebula (PN) population of M33 is studied via multi-fiber spectroscopy with Hectospec at the MMT. In this paper we present the spectra of 102 PNe, whereas plasma diagnostic and chemical abundances were performed on the 93 PNe where the necessary diagnostic lines were measured. About 20% of the PNe are compatible with being Type I; the rest of the sample is the progeny of an old disk stellar population, with main sequence masses M<3M${_\odot}$ and ages t$>$0.3 Gyr. By studying the elemental abundances of the PNe in the M33 disk we were able to infer that: (1) there is a tight correlation between O/H and Ne/H, broadly excluding the evolution of oxygen; (2) the average abundances of the $\alpha$-elements are consistent with those of \hii regions, indicating a negligible global enrichment in the disk of M33 from the epoch of the formation of the PN progenitors to the present time; (3) the radial oxygen gradient across the M33 disk has a slope of -0.031$\pm$0.013 dex kpc$^{-1}$, in agreement, within the errors, with the corresponding gradient derived from HII regions. Our observations do not seem to imply that the metallicity gradient across the M33 disk has flattened considerably with time. We report also the discovery of a PN with Wolf-Rayet features, PN039, belonging the class of late [WC] stars

Galaxies with Wolf-Rayet signatures in the low-redshift Universe

Context. The availability of large spectroscopic datasets has opened up the possibility of constructing large samples of rare objects in a systematic manner. Aims. The goal of this study is to analyse the properties of galaxies showing Wolf-Rayet features in their optical spectrum using spectra from the Sloan Digital Sky Survey Release 6. With this unprecedentedly large sample we aim to constrain the properties of the Wolf-Rayet phase and its impact on the surrounding interstellar medium. Methods. We carried out very careful continuum subtraction on all galaxies with equivalent widths of Hβ > 2 A in emission and identify Wolf-Rayet features using a mixture of automatic and visual classification. We combined this with spectroscopic and photometric information from the SDSS and derive metal abundances using a number of methods. Results. We find a total of 570 galaxies with significant Wolf-Rayet (WR) features and a further 1115 potential candidates, several times more than even the largest heterogeneously assembled catalogues. We discuss in detail the properties of galaxies showing Wolf-Rayet features with a focus on their empirical properties. We are able to accurately quantify the incidence of Wolf-Rayet galaxies with metal abundance and show that the likelihood of otherwise similar galaxies showing Wolf-Rayet features increases with increasing metallicity, but that WR features are found in galaxies of a wide range in morphology. The large sample allows us to show explicitly that there are systematic differences in the metal abundances of WR and non-WR galaxies. The most striking result is that, below EW(Hβ) = 100 A, Wolf-Rayet galaxies show an elevated N/O relative to non-WR galaxies. We interpret this as a rapid enrichment of the ISM from WR winds. We also show that the model predictions for WR features strongly disagree with the observations at low metallicity; while they do agree quite well with the data at solar abundances. We discuss possible reasons for this and show that models incorporating binary evolution reproduce the low-metallicity results reasonably well. Finally we combine the WR sample with a sample of galaxies with nebular He II λ4686 to show that, at 12 + log O/H < 8, the main sources of He II ionising photons appears to be O stars, arguing for a less dense stellar wind at these metallicities, while at higher abundances WN stars might increasingly dominate the ionisation budget.

The Eclipsing Cataclysmic Variable Lanning 386: Dwarf Nova, SW Sextantis Star, or Both?

ABSTRACTWe present photometry and spectroscopy of the suspected cataclysmic variable (CV) Lanning 386. We confirm that it is a CV, and observe deep eclipses, from which we determine the orbital period Porb to be 0.1640517 ± 0.0000001 days (= 3.94 h). Photometric monitoring over two observing seasons shows a very active system with frequent outbursts of variable amplitude, up to ∼2 mag. The spectrum in quiescence is typical of dwarf novae, but in its high state the system shows strong He II emission and a broad C IV Wolf-Rayet feature. This is unusual for dwarf novae in outburst and indicates a high excitation. In its high state the system shows some features reminiscent of an SW Sextantis-type CV, but lacks others. We discuss the classification of this puzzling object.

The nature of the Wolf-Rayet galaxy Mrk 209 from photoionization models

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 6.13 × 10−3, 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 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.

High-quality spectrophotometry of the planetary nebula in the Fornax dSph

We present results of NTT spectroscopy of the one known planetary nebula (PN) in the dwarf spheroidal galaxy Fornax, a gas-deficient Local Group galaxy that stopped its star formation activity a few hundred million years ago. We detected the [OIII] 4363 line with a signal-to-noise ratio of ~22. For the first time we detected the weak [SII] 6717,6731 lines (I(6717+6731) ~0.01 I(Hbeta)), determined the electron number density (Ne(SII) = 750 cm^-3), and calculated O, N, Ne, Ar, S, Cl, Fe, He and C abundances. The abundance analysis presented here is based on the direct calculation of the electron temperature Te and yields an oxygen abundance of 12+log(O/H) = 8.28+/-0.02. The analysis of the O, Ne, Ar and S abundances shows that the original ISM oxygen abundance was 0.27+/-0.10 dex lower and that third-dredge-up self-pollution in oxygen took place. The blue spectrum shows weak Wolf-Rayet features, and the progenitor star is classified as a weak emission-line star. Four of the five PNe in dwarf spheroidal galaxies are now known to show WR wind features. Overall, the metallicity of the progenitor of the PN fits in well with stellar spectroscopic abundances derived in previous studies as well as with the stellar age-metallicity relation of Fornax.

The Stellar Populations of Seyfert 2 Nuclei

We present a preliminary analysis of the stellar populations in the central parsecs of a sample of 22 Seyfert 2 galaxies, based on a careful separation of nebular emission and stellar light in high-spatial resolution HST-STIS spectra. 10% of the surveyed nuclei display stellar populations of intermediate age, ∼1-2 Gyr old, whereas the remaining targets appear to be evenly split between objects showing only very old stellar populations and nuclei requiring also an additional blue featureless component, which we characterise by means of very young, few-Myr-old stars. The small fraction of stellar population of intermediate age seems to argue against the presence of such a young component, however, since the short lifetime of O-stars would imply recurrent star-formation episodes and the build-up over the last 1-2 Gyr of a detectable intermediate-age population. Additionally, a correlation between the luminosity of such a blue component and the emission from highly-ionised species, together with the general absence of Wolf-Rayet features, further suggests that the featureless continuum arises from the central engine rather than from star-forming regions. We discuss our results in the framework of the unification paradigm and of models for star formation close to supermassive black holes.

The massive star population in the giant H II region Tol 89 in NGC 5398

We present new high spectral resolution Very Large Telescope (VLT)/UV–Visual Echelle Spectrograph (UVES) spectroscopy and archival Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) imaging and spectroscopy of the giant H ii region Tol 89 in NGC 5398. From optical and ultraviolet (UV) HST images, we find that the star-forming complex as a whole contains at least seven young compact massive clusters. We resolve the two brightest optical knots, A and B, into five individual young massive clusters along our slit, A1–4 and B1, respectively. From UV spectral modelling using the starburst99 code of Leitherer et al., and nebular Hβ equivalent widths in the optical, we derive ages that are consistent with the formation of two separate burst events, of ∼4 ± 1 Myr and <3 Myr for knots A (A1–4) and B (B1), respectively. A Large Magellanic Cloud (LMC) metallicity is measured for both knots from a nebular line analysis, while nebular He ii 4686 is observed in knot B and perhaps in knot A. We detect underlying broad wings on the strongest nebular emission lines indicating velocities up to 600 km s−1. From UV and optical spectroscopy, we estimate that there are ∼95 early WN stars and ∼35 early WC stars in Tol 89-A, using empirical template spectra of LMC Wolf–Rayet (WR) stars from Crowther & Hadfield, with the WC population confined to cluster A2. Remarkably, we also detect a small number of approximately three mid WNs in the smallest (mass) cluster in Tol 89-A, A4, whose spectral energy output in the UV is entirely dominated by the WN stars. From the strength of nebular Hβ, we obtain N(O) ∼ 690 and 2800 for knots A and B, respectively, which implies N(WR)/N(O) ∼ 0.2 for knot A. We also employ a complementary approach using starburst99 models, in which the O star content is inferred from the stellar continuum, and the WR population is obtained from spectral synthesis of optical WR features using the grids from Smith et al. We find reasonable agreement between the two methods for the O star content and the N(WR)/N(O) ratio but find that the WR subtype distribution is in error in the starburst99 models, with far too few WN stars being predicted. We attribute this failure to the neglect of rotational mixing in evolutionary models. Our various modelling approaches allow us to measure the cluster masses. We identify A1 as a super star cluster (SSC) candidate with a mass of ∼1–2 × 105 M⊙. A total mass of ∼6 × 105 M⊙ is inferred for the ionizing sources within Tol 89-B.

An exceptional population of late-type WC stars in the metal-rich spiral galaxy M 83

We have surveyed the metal-rich spiral galaxy M 83 (NGC 5236) for its Wolf-Rayet population using VLT-FORS2 narrow-band imaging and follow-up spectroscopy. From a total of 280 candidates identified using He  λ4686 imaging, Multi Object Spectroscopy of 198 sources was carried out, revealing 132 objects containing bona-fide Wolf-Rayet features. From this sample, an exceptional W-R content of ∼1030 is inferred, with N(WC)/N(WN) ∼ 1.3, continuing the trend to larger values at higher metallicity amongst Local Group galaxies. More dramatic is the dominance of late-type WC stars in M 83 with N(WC8−9)/N(WC4−7) = 9 which we attribute to the sensitivity of the classification line C  λ5696 to mass-loss, providing the strength of WC winds scale with metallicity. One young massive compact cluster, #74 in our catalogue, hosts 20% of the entire galactic population, namely ∼180 late WC stars and ∼50 late WN stars.

Wolf-Rayet features in active galactic nuclei

It was suggested by Terlevich & Melnick (1985) that many nuclei classified as Seyfert 2s could be associated with WR stars recently formed in a powerful starburst. Since then, the signature of WR stars has been claimed to be observed in several Seyfert 2s and LINERs. Goncalves et al. (1999) have shown that a small number of Seyfert 2s have very weak [N ii] 6548,6584 lines, suggesting that these objects could be partly ionized by WR stars. We have obtained high S/N spectra of such “Weak-[N ii] Seyferts” to investigate if WR stars can be observed in galaxies without any other evidence for a starburst nebulosity. Amongst other findings, we have detected the presence of emission features such as Si ii 5056,6347 and Ni ii 7380. Other lines, such as [Ar iv] 4711,4740 and [Fe iii] 4658 are possibly present, mimicking the Bowen blend. The detection of WR stars in AGN poses interesting questions concerning the link between nuclear activity and star formation, providing another angle to the study of the Starburst–AGN connection. However, one must be very careful in interpreting the available data; for the time being, the role played by WR stars in these AGN remains an open debate.

Infrared spectroscopy of the Seyfert 2 galaxy Mrk 1210

Mrk 1210 is an Sa galaxy at z = 0.01350. Its optical spectrum displays a strong featureless continuum and Wolf-Rayet features within the central 200 pc (Storchi-Bergmann et al. 1998), evidencing the presence of a circumnuclear starburst. Here, we present SpeX nearinfrared (NIR) spectroscopy in the interval 0.8− 2.4 μm, combined with optical (3700A-7400A) observations, aimed at studying the physical properties of the nuclear and extended gas. Kinematics derived from the line profiles, extinction affecting the emitting gas, temperature and density indicators of the NLR, and excitation sources for the molecular and low-ionization gas are discussed. The presence of high-ionization as well as low-ionization and molecular lines in the optical and NIR spectrum suggests a strong competition between the starburst component and the ionized gas from the central engine. This makes Mrk 1210 an ideal target to study the interplay between starburst and nuclear activity in galaxies. 1. Observational results and kinematics The NIR spectrum of Mrk1210 (see Figure 1) is dominated by permitted H i and He ii lines, and forbidden lines of [S ii], [S iii] and [Fe ii]. There are also present highly ionized lines of [Sviii], [S ix], and [Six], as well as numerous H2 lines. Extended [S iii] and He i emission is found up to a distance of 500 pc from the nucleus. The analysis of the emission line profiles, both allowed and forbidden, shows a narrow (FWHM ∼ 900 km/s) line on top of a broad (FWHM ∼ 1600 km/s) blue-shifted component (∆V ∼ 120 km/s). The latter seems to be associated to a nuclear outflow, most probably located in the NLR instead of the BLR. This result rules out the presence of a hidden BLR claimed to be present in previous NIR observations of this object (Veilleux et al. 1997). 2. Reddening The numerous H i and forbidden lines in the spectra of Mrk 1210 allowed us to map the intrinsic extinction affecting the NLR by means of indicators such as the Pashen decrement and the [Fe ii] 1.257μm/1.64μm ratio. The extinction law of Cardelli, Clayton, & Mathis (1989) was used for this purpose. The results reveal a dusty AGN while the extended regions are reddening free. In the inner 200 pc we found an E(B − V ) = 0.5 from the H i lines ratios, except for Hα/Hβ from which we found E(B − V ) = 0.3, indicating differences in dust columns. From the [Fe ii] ratio 1.257μm/1.64μm a large extinction value, E(B− V ) = 1.5 was derived. This supports the hypothesis that the [Fe ii] lines are formed in a separate region, different from the classical NLR. At 250 pc from the nucleus, no extinction is found from our data. 3. Electron Density and Temperature Electronic densities and temperatures of the nuclear gas in Mrk 1210 were determined by means of several diagnostic line ratios. The results are summarized in Table 1. Notice

A mid-infrared spectroscopic survey of starburst galaxies: Excitation and abundances

We present spectroscopy of MIR emission lines in twelve starburst regions, located in eleven starburst galaxies, for which a significant number of lines between 2.38 and 45micron were observed with the ISO Short Wavelength Spectrometer, with the intention of providing a reference resource for MIR spectra of starburst galaxies. The observation apertures were centred on actively star forming regions, including those which are inaccessible at optical wavelengths due to high levels of obscuration. We use this data set, which includes fine structure and hydrogen recombination lines, to investigate excitation and to derive gas phase abundances of neon, argon, and sulphur of the starburst galaxies. The derived Ne abundances span approximately an order of magnitude, up to values of ~3 times solar. The excitation ratios measured from the Ne and Ar lines correlate well with each other (positively) and with abundances (negatively). Both in excitation and abundance, a separation of objects with visible Wolf-Rayet features (high excitation, low abundance) is noted from those without (low excitation, high abundance). For a given abundance, the starbursts are of relatively lower excitation than a comparative sample of HII regions, possibly due to ageing stellar populations. By considering the abundance ratios of S with Ne and Ar we find that, in our higher metallicity systems, S is relatively underabundant by a factor of ~3. We discuss the origin of this deficit and favour depletion of S onto dust grains as a likely explanation. This weakness of the MIR fine structure lines of S has ramifications for future infrared missions such as SIRTF and Herschel since it indicates that the S lines are less favourable tracers of star formation than is suggested by nebular models which do not consider this effect.

The Massive Stellar Content in the Starburst NGC 3049: A Test for Hot‐Star Models

The objective of this work is twofold. First, we seek evidence for or against the depletion of massive stars in metal-rich starbursts. A second, equally important goal is to perform a consistency test of the latest generation of starburst models in such a high-metallicity environment. We have obtained high spatial resolution ultraviolet and optical STIS spectroscopy and imaging of the metal-rich nuclear starburst in NGC 3049. The stellar continuum and the absorption-line spectrum in the ultraviolet are used to constrain the massive stellar population. The strong, blueshifted stellar lines of C IV and Si IV detected in the UV spectra indicate a metal-rich, compact, massive (~106 M☉) cluster of age 3-4 Myr emitting the UV-optical continuum. We find strong evidence against a depletion of massive stars in this metal-rich cluster. The derived age and the upper mass-limit cutoff of the initial mass function are also consistent with the detection of Wolf-Rayet (W-R) features at optical wavelengths. As a second, independent constraint on the massive stellar content, the nebular emission-line spectrum is modeled with photoionization codes using stellar spectra from evolutionary synthesis models. The morphology of the nuclear starburst of NGC 3049 from the STIS images indicates a simple geometry for the nebular emission-line region. However, the nebular lines are badly reproduced by 3-4 Myr instantaneous bursts, as required by the UV line spectrum, when unblanketed W-R and/or Kurucz stellar atmospheres are used. The corresponding number of photons above 24 and 54 eV in the synthetic models is too high in comparison with values suggested by the observed line ratios. Since the ionizing spectrum in this regime is dominated by emission from W-R stars, this discrepancy between observations and models is most likely the result of incorrect assumptions about the W-R stars. Thus, we conclude that the nebular spectrum of high-metallicity starbursts is poorly reproduced by models for W-R-dominated populations. However, the new model set of Smith et al. with blanketed W-R and O atmospheres and adjusted W-R temperatures predicts a softer far-UV radiation field, providing a better match to the data.

VLT observations of metal-rich extra galactic HIIregions

We have obtained high quality FORS1/VLT optical spectra of 85 disk H ii regions in the nearby spiral galaxies NGC 3351, NGC 3521, NGC 4254, NGC 4303, and NGC 4321. Our sample of metal-rich H ii regions with metallicities close to solar and higher reveal the presence of Wolf-Rayet (WR) stars in 27 objects from the blue WR bump (~4680 Å) and 15 additional candidate WR regions. This provides for the first time a large set of metal-rich WR regions. Approximately half (14) of the WR regions also show broad C iv λ5808 emission attributed to WR stars of the WC subtype. The simultaneous detection of C iii λ5696 emission in 8 of them allows us to determine an average late WC subtype compatible with expectations for high metallicities. Combined with literature data, the metallicity trends of WR features and the WC/WN number ratio are discussed. The WR regions show quite clear trends between their observed WR features and the Hβ emission line. Detailed synthesis models are presented to understand/interpret these observations. In contrast with earlier studies of low metallicity WR galaxies, both and are here found to be smaller than “standard” predictions from appropriate evolutionary synthesis models at corresponding metallicities. Various possibilities which could explain this discrepancy are discussed. The most likely solution is found with an improved prescription to predict the line emission from WN stars in synthesis models. The availability of a fairly large sample of metal-rich WR regions allows us to improve existing estimates of the upper mass cut-off of the IMF in a robust way and independently of detailed modeling: from the observed maximum Hβ equivalent width of the WR regions we derive a lower limit for Mup of 60–90 in the case of a Salpeter slope and larger values for steeper IMF slopes. This constitutes a lower limit on Mup as all observational effects known to affect potentially the Hβ equivalent width can only reduce the observed . From our direct probe of the massive star content we conclude that there is at present no evidence for systematic variations of the upper mass cut-off of the IMF in metal-rich environments, in contrast to some claims based on indirect nebular diagnostics.