Elena Terlevich

The star formation history of Seyfert 2 nuclei

We present a study of the stellar populations in the central ∼200 pc of a large and homogeneous sample comprising 79 nearby galaxies, most of which are Seyfert 2s. The star formation history of these nuclei is reconstructed by means of state-of-the-art population synthesis modelling of their spectra in the 3500‐5200 A interval. A quasar-like featureless continuum (FC) is added to the models to account for possible scattered light from a hidden active galactic nucleus (AGN). We find the following. (1) The star formation history of Seyfert 2 nuclei is remarkably heterogeneous: young starbursts, intermediate-age and old stellar populations all appear in significant and widely varying proportions. (2) A significant fraction of the nuclei show a strong FC component, but this FC is not always an indication of a hidden AGN: it can also betray the presence of a young, dusty starburst. (3) We detect weak broad Hβ emission in several Seyfert 2s after cleaning the observed spectrum by subtracting the synthesis model. These are most likely the weak scattered lines from the hidden broad-line region envisaged in the unified model, given that in most of these cases independent spectropolarimetry data find a hidden Seyfert 1. (4) The FC strengths obtained by the spectral decomposition are substantially larger for the Seyfert 2s which present evidence of broad lines, implying that the scattered nonstellar continuum is also detected. (5) There is no correlation between the star formation in the nucleus and either the central or overall morphology of the parent galaxies. Ke yw ords: galaxies: active ‐ galaxies: Seyfert ‐ galaxies: statistics ‐ galaxies: stellar content.

Lyα Emission in Starbursts: Implications for Galaxies at High Redshift*

We present the results of a high resolution UV 2-D spectroscopic survey of star forming galaxies observed with HST-STIS. Our main aim was to map the Lyman alpha profiles to learn about the gas kinematics and its relation with the escape of Lyman alpha photons and to detect extended Lyman alpha emission due to scattering in gaseous halos. We have combined our data with previously obtained UV spectroscopy on other three star-forming galaxies. We find that the P-Cygni profile is spatially extended, smooth and spans several kiloparsecs covering a region much larger than the starburst itself. We propose a scenario whereby an expanding super-shell is generated by the interaction of the combined stellar winds and supernova ejecta from the young starbursts, with an extended low density halo. The variety of observed Lyman alpha profiles both in our sample and in high redshift starbursts is explained as phases in the time evolution of the super-shell expanding into the disk and halo of the host galaxy. The observed shapes, widths and velocities are in excellent agreement with the super-shell scenario predictions and represent a time sequence. We confirm that among the many intrinsic parameters of a star forming region that can affect the properties of the observed Lyman alpha profiles, velocity and density distributions of neutral gas along the line of sight are by far the dominant ones, while the amount of dust will determine the intensity of the emission line, if any.

The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations

We present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III 1666 A and C III 1909 A in our spectra, enabling us to determine C(+2)/O(+2) with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of carbon from intermediate-mass stars. Our results also suggest that it may not be appropiate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver & Woosleys best estimate for the 12C(alpha, gamma) 16O nuclear reaction rate, assuming negligible contanmination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III 1750 A multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma uppr limits on the 1750 A feature indicate that the N(+2)/O(+2) ratios in these objects are not significantly larger than the N(+)/O(+) ratios measured from optical spectra. This behavior is consistent with predictions of photionization models, although better detections of N III are needed to confirm the results.

The evolution of superbubbles and the detection of Lyα in star‐forming galaxies

The detection of Lyemission in star-forming galaxies in different shapes and in- tensities (always smaller than predicted for case B recombination) has puzzled the astronomical community for more than a decade. Here we use two dimensional cal- culations to follow the evolution of superbubbles and of the H II regions generated by the output of UV photons from massive stars. We show the impact caused by massive star formation in the ISM of different galaxies and we look at the conditions required to detect Lyemission from a nuclear H II region, and the variety of profiles that may be expected as a function of time.

Violent star formation in NGC 2363

This is an electronic version of an article published in The Astrophysical Journal. Gonzalez-Delgado, R.M. et al. Violent star formation in NGC 2363. The Astrophysical Journal 437 (1994): 239-261

Carbon in Spiral Galaxies from HUBBLE SPACE TELESCOPE Spectroscopy

We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O≈-0.8 at log O/H=-4.0 to log C/O≈-0.1 at log O/H=-3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, confirming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemical-evolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much flatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important effect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

An Empirical Calibration of Star Formation Rate Estimators

ABSTRA C T The observational determination of the behaviour of the star formation rate (SFR) with lookback time or redshift has two main weaknesses: (i) the large uncertainty of the dust/extinction corrections, and (ii) that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. Most frequently, the luminosity of the Ha emission line, that of the forbidden line [O II] l3727 and that of the far-ultraviolet continuum are used with low-, intermediate- and high-redshift galaxies, respectively. To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using Ha ,[ OII] l3727 A ˚ , ultraviolet (UV) and far-infrared (FIR) luminosities [SFR(Ha), SFR(O II), SFR(UV) and SFR(FIR), respectively] of a sample comprising the 31 nearby star-forming galaxies that have high-quality photometric data in the UV, optical and FIR. We review the different ‘standard’ methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(Ha) and SFR(FIR), both SFR(O II) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law. We show that the excess in the SFR(O II) and SFR(UV) is mainly due to an overestimation of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(O II) and SFR(UV) in line with the SFR(FIR), and simultaneously reduces the internal scatter of the SFR estimations. Based on these results, we have derived ‘unbiased’ SFR expressions for the SFR(UV), SFR(O II) and SFR(Ha). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys. The main results are that the use of the unbiased SFR estimators brings into agreement the results of all surveys. Particularly important is the agreement achieved for the SFR derived from the FIR/millimetre and optical/UV surveys. The ‘unbiased’ star formation history of the Universe shows a steep rise in the SFR from za 0t oza 1 with SFR/O1a zU 4:5 , followed by a decline for z . 2 where SFR/O1a zU 21:5 . Galaxy formation models tend to have a much flatter slope from za 0t o za 1.

NEW DIAGNOSTIC METHODS FOR EMISSION-LINE GALAXIES IN DEEP SURVEYS

ABSTRACT We present new quantitative classification methods for emission-line galaxies,which are specially designed to be used in deep galaxy redshift surveys. A good seg-regation between starbursts and active galactic nuclei, i.e. Seyferts 2 and LINERs, isobtained from diagnostic diagrams involving the [O II]λ3727 ˚A, [Ne III]λ3869 ˚A, Hβand [O III]λ5007 ˚A relative intensities or the [O II]λ3727 ˚A and Hβ equivalent widths.Furthermore, the colour index of the continuum underlying [O II]λ3727 ˚A and Hβprovides an additional separation parameter between the two types of emission-linegalaxies.We have applied the equivalent widths method to the 0 < z ≤0.3 emission-linegalaxies of the Canada-France Redshift Survey. Our results are in very good agreementwith those obtained using the standard diagnostic diagrams including all the strongoptical emission-line intensity ratios.Key words: surveys – H II regions – galaxies: active – galaxies: starburst – galaxies:Seyfert 1 INTRODUCTIONDeep redshift surveys have produced in the past few years arelatively large number of optical/near UV spectra of galax-ies at redshifts z < 0.7 − 0.8 (e.g., Dressler & Gunn 1983;Couch & Sharples 1987; Broadhurst, Ellis & Shanks 1988;Lavery & Henry 1988; Colless et al. 1990; Colless et al.1993;Songaila et al. 1994; Glazebrook et al. 1995; Le F`evre etal. 1995). A large percentage of galaxies in these deep sur-veys present narrow emission lines. The analysis of the spec-trum of these distant emission-line galaxies (hereafter ELGs)should provide important information about their intrinsicproperties (stellar population, rate of star formation, metal-licity, etc.) and the evolution of these parameters with in-creasing look-back time. But a central problem is that verylittle is known about the nature, either H II galaxies or ac-tive galaxies⋆, of the ELGs discovered in these deep surveys.

Group, field and isolated early-type galaxies¿ II. Global trends from nuclear data

We have derived ages, metallicities and enhanced-element ratios [α/Fe] for a sample of 83 early-type galaxies essentially in groups, the field or isolated objects. The stellar-population properties derived for each galaxy correspond to the nuclear re/8 aperture extraction. The median age found for Es is 5.8±0.6 Gyr and the average metallicity is +0.37±0.03 dex. For S0s, the median age is 3.0±0.6 Gyr and [Z/H] = 0.53±0.04 dex. We compare the distribution of our galaxies in the Hβ-[MgFe] diagram with Fornax galaxies. Our elliptical galaxies are 3‐4 Gyr younger than Es in the Fornax cluster. We find that the galaxies lie in a plane defined by [Z/H] = 0.99 log σ 0 − 0.46 log(age) − 1.60, or in linear terms Z ∝ σ 0 × (age) −0.5 . More massive (larger σ 0) and older galaxies present, on average, large [α/Fe] values, and therefore must have undergone shorter star-formation time-scales. Comparing group against field/isolated galaxies, it is not clear that environment plays an important role in determining their stellar-population history. In particular, our isolated galaxies show ages differing by more than 8 Gyr. Finally we explore our large spectral coverage to derive log (O/H) metallicity from the Hα and N II λ6584 and compare it with model-dependent [Z/H]. We find that the O/H abundances are similar for all galaxies, and we can interpret it as if most chemical evolution has already finished in these galaxies. Ke yw ords: galaxies: abundances ‐ galaxies: elliptical and lenticular, cD ‐ galaxies: evolution ‐g alaxies: nuclei ‐ galaxies: stellar content.

A comprehensive study of reported high-metallicity giant H ii regions — I. Detailed abundance analysis

We present long-slit observations in the optical and near infrared of fourteen HII regions in the spiral galaxies: NGC 628, NGC 925, NGC 1232 and NGC 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios have been obtained and for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of 12+log(O/H) = 8.95 ± 0.20 and constitutes, to the best of our knowledge, the first high metallicity HII region for which accurate line temperatures, and hence elemental abundances, have been derived. For the rest of the regions no line temperature measurements could be made and the metallicity has been determined by means of both detailed photoionisation modelling and the sulphur abundance parameter S23. Only one of these regions shows values of O23 and S23 implying a solar or oversolar metallicity. According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. The two of them fit the trends previously found in other high metallicity HII regions, i.e. N/O and S/O abundance ratios seem to be higher and lower than solar respectively.