arXiv subjects

R. Terlevich

Publications and source records attributed to R. Terlevich.

At least 55 records · Page 3Linked to original sources

arXiv2004

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 type 2 Seyferts. The star-formation history of these nuclei is reconstructed by means of state-of-the art population synthesis modeling of their spectra in the 3500--5200 A interval. A QSO-like featureless continuum (FC) is added to the models to account for possible scattered light from a hidden AGN. We find that: (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 Hbeta 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 finds 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 non-stellar 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.

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arXiv2004

Computer science approach to the stellar fabric of violent starforming regions in AGN

In order to analyse the large numbers of Seyfert galaxy spectra available at present, we are testing new techniques to derive their physical parameters fastly and accurately. We present an experiment on such a new technique to segregate old and young stellar populations in galactic spectra using machine learning methods. We used an ensemble of classifiers, each classifier in the ensemble specializes in young or old populations and was trained with locally weighted regression and tested using ten-fold cross-validation. Since the relevant information concentrates in certain regions of the spectra we used the method of sequential floating backward selection offline for feature selection. Very interestingly, the application to Seyfert galaxies proved that this technique is very insensitive to the dilution by the Active Galactic Nucleus (AGN) continuum. Comparing with exhaustive search we concluded that both methods are similar in terms of accuracy but the machine learning method is faster by about two orders of magnitude.

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arXiv2004

The nearest extreme starburst: bubbles, young star clusters, and outflow in NGC 3256

In this Work we report, for the extreme starburst in the IR merger NGC 3256: (i) The detection of 4 galactic bubbles, associated with SN explosions.(ii) The first analysis of the spatial distribution of young star clusters (YSC) candidates.(iii) The kinematic study of the ionized gas in the core of of the main optical nucleus, performed with HST STIS spectra. The shape of the rotation curve and the emission line profile could be explained by the presence in the core of YSC with outflow.

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arXiv2001

An Empirical Calibration of Star Formation Rate Estimators

(Abridged) The observational determination of the behaviour of the star formation rate (SFR) with look-back time or redshift has two main weaknesses: 1- the large uncertainty of the dust/extinction corrections, and 2- that systematic errors may be introduced by the fact that the SFR is estimated using different methods at different redshifts. To assess the possible systematic differences among the different SFR estimators and the role of dust, we have compared SFR estimates using H$α$, SFR(H$α$), [OII]$λ$3727Å, SFR(OII), UV, SFR(UV) and FIR, SFR(FIR) luminosities of a sample comprising the 31 nearby star forming galaxies having high quality photometric data in the UV, optical and FIR. We review the different "standard" methods for the estimation of the SFR and find that while the standard method provides good agreement between SFR(H$α$) and SFR(FIR), both SFR(OII) and SFR(UV) are systematically higher than SFR(FIR), irrespective of the extinction law. We show that the excess in the SFR(OII) and SFR(UV) is mainly due to an overestimate of the extinction resulting from the effect of underlying stellar Balmer absorptions in the measured emission line fluxes. Taking this effect into consideration in the determination of the extinction brings the SFR(OII) and SFR(UV) in line with the SFR(FIR) and simultaneously reduces the internal scatter of the SFR estimations. Based on these results we have derived "unbiased" SFR expressions for the SFR(UV), SFR(OII) and SFR(H$α$). We have used these estimators to recompute the SFR history of the Universe using the results of published surveys.

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arXiv2001

The ionising cluster of 30 Doradus.IV. Stellar kinematics

On the basis of multislit spectroscopy of 180 stars in the ionising cluster of 30 Doradus we present reliable radial velocities for 55 stars. We calculate a radial velocity dispersion of ~35 km/s for the cluster and we analyse the possible influence of spectroscopic binaries in this rather large velocity dispersion. We use numerical simulations to show that the observations are consistent with the hypothesis that all the stars in the cluster are binaries, and the total mass of the cluster is ~5E+5 solar masses. A simple test shows only marginal evidence for dynamical mass segregation which if present is most likely not due to dynamical relaxation.

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arXiv2001

Narrowband CCD Photometry of Giant HII Regions

We have obtained accurate CCD narrow band H_beta and H_alpha photometry of Giant HII Regions in M 33, NGC 6822 and M 101. Comparison with previous determinations of emission line fluxes show large discrepancies; their probable origins are discussed. Combining our new photometric data with global velocity dispersion (sigma) derived from emission line widths we have reviewed the L(H_beta)-sigma relation. A reanalysis of the properties of the GEHRs included in our sample shows that age spread and the superposition of components in multiple regions introduce a considerable spread in the regression. Combining the information available in the literature regarding ages of the associated clusters, evolutionary footprints on the interstellar medium, and kinematical properties of the knots that build up the multiple GEHRs, we have found that a subsample - which we refer to as young and single GEHRs - do follow a tight relation in the L-sigma plane.

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arXiv2000

The X-ray Spectrum and Light Curve of Supernova 1995N

We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) supernova SN 1995N with the ROSAT and ASCA satellites. The January 1998 ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung (kT~10 keV, N_H~6e20 cm^-2) or power-law (alpha~1.7, N_H~1e21 cm^-2) model. The X-ray light curve shows evidence for significant flux evolution between August 1996 and January 1998: the count rate from the source decreased by 30% between our August 1996 and August 1997 ROSAT observations, and the X-ray luminosity most likely increased by a factor of ~2 between our August 1997 ROSAT and January 1998 ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity, L_X~1e41 erg/sec, places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.

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arXiv2000

Extreme Galactic-Winds and Starburst in IR Mergers and IR QSOs

We report -as a part of a long-term study of mergers and IR QSOs- detailed spectroscopic evidences for outflow (OF) and/or Wolf Rayet features in: (i) low velocity OF in the ongoing mergers NGC 4038/39 and IRAS 23128-5919; (ii) extreme velocity OF (EVOF) in the QSOs IRAS 01003-2238 and IRAS 13218+0552; (iii) OF and EVOF in a complete sample of ultra-luminous IR galaxies/QSOs ("The IRAS 1 Jy MKO-KPNO Survey", of 118 objects). We found EVOF in IRAS 11119+3257, 14394+5332, 15130+1958 and 15462-0450. The OF components detected in these objects were mainly associated to starburst processes: i.e., to galactic-winds generated in multiple type II SN explosions and massive stars. The EVOF were detected in objects with strong starburst plus obscured IR QSOs; which suggest that interaction of both processes could generate EVOF. In addition, we analyze the presence of Wolf Rayet features in the large sample of Bright PG-QSOs (Boroson and Green 1992), and nearby mergers and galactic-wind galaxies. We found clear WR features in the Fe II QSOs (type I): PG 1244+026, 1444+407, 1448+273, 1535+547; and in the IR merger Arp 220. HST archive images of IR+BAL QSOs show in practically all of these objects "arc or shell" features probably associated to galactic-winds (i.e., to multiple type II SN explosions) and/or merger processes. Finally, we discuss the presence of extreme starburst and galactic wind as a possible evolutive link between IR merger and IR QSOs; where the relation between mergers and extreme starburst (with powerful galactic-winds) plays in important role, in the evolution of galaxies.

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arXiv2000

Stellar indices and kinematics in Seyfert 1 nuclei

We present spectra of 6 type 1 Seyfert galaxies, 2 Seyfert 2, a starburst galaxy and a compact narrow line radiogalaxy, taken in two spectral ranges centered around the near--IR CaII triplet (CaT) (at ~8600 Angstroms), and the Mgb stellar feature at 5180 Angstroms. We measured the equivalent width (EWs) of these features and the Fe52 and Fe53 spectral indices. We found that the strength of the CaT in type 1 Seyfert galaxies with prominent central point sources, is larger than what would be expected from the observed strength of the blue indices. This could be explained by the presence of red supergiants in the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these galaxies could also be diluted by the strong FeII multiplets that can be seen in their spectra. We have also measured the stellar and gas velocity dispersions of the galaxies in the sample. The stellar velocity dispersions were measured using both, the Mgb and CaT stellar features. The velocity dispersion of the gas in the narrow line region (NLR) was measured using the strong emission lines [OIII] 5007, 4959 and [SIII] 9069. We compare the gas and star velocity dispersions and find that both magnitudes are correlated in Seyfert galaxies. Most of the Seyfert 1 we observe have stellar velocity dispersion somehow greater than that of the gas in the NLR.

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arXiv1999

Luminous Infrared Galaxies. III. Multiple Merger, Extended Massive Star Formation, Galactic-Wind and Nuclear-Inflow in NGC 3256

We find in a detailed morphological study (at 15 pc resolution) that the extended massive star formation process, shows: (i) extended triple asymmetrical spiral arms structure (r = 5 kpc); and (ii) the spiral arms emanate from three different nuclei. The main optical nucleus shows a small spiral-disk (r = 500 pc) which is a continuation of the external one and reach the very nucleus. And this very nucleus shows blue elongate structure (63 pc x 30 pc), and luminous blue star cluster properties. We study the kinematics of this system and present a detailed Halpha velocity field for the central region (r = 5 kpc). In the main optical nucleus we found a clear "outflow component" associated to galactic-winds and a "inflow radial motion" (in the spiral-disk nuclear structure, r = 700 pc). In addition we detected the outflow component in the central and external regions (r < 5-6 kpc), with a very wide opening angle of 140. We found that the mean value of the inflow region (at PA = 80) is practically perpendicular to the axis of the bipolar outflow (at PA = 160). We analyze in detail the physical conditions in the giant H II regions located in the asymmetric spiral arms, the two main optical nuclei, and the outflow component. In the central region (r = 5-6 kpc) we detected that the nuclear starburst and the extended giant H II regions have very similar properties. We suggest that the interaction between dynamical effects, the galactic--wind (outflow), low-energy cosmic rays, and the molecular+ionized gas (probably in the inflow phase) could be the possible mechanism that generate the "similar extended properties in the massive star formation, at scale of 5-6 kpc!". We analyze for NGC3256 the possible evolution from luminous IR galaxy to QSOs.

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arXiv1999

Optical and X-ray Variability in The Least Luminous AGN, NGC4395

We report the detection of optical and X-ray variability in the least luminous known Seyfert galaxy, NGC4395. The featureless continuum changed by a factor of 2 in 6 months, which is typical of more luminous AGN. The largest variation was seen at shorter wavelengths, so that the spectrum becomes `harder' during higher activity states. In a one week optical broad band monitoring program, a 20% change was seen between successive nights. In a 1 month period the spectral shape changed from a power law with spectral index alpha ~0 (characteristic of quasars) to a spectral index alpha ~2 (as observed in other dwarf AGN). ROSAT HRI and PSPC archive data show a variable X-ray source coincident with the galactic nucleus. A change in X-ray flux by a factor \~2 in 15 days has been observed. When compared with more luminous AGN, NGC4395 appears to be very X-ray quiet. The hardness ratio obtained from the PSPC data suggests that the spectrum could be absorbed. We also report the discovery of weak CaIIK absorption, suggesting the presence of a young stellar cluster providing of the order of 10% of the blue light. Using HST UV archive data, together with the optical and X-ray observations, we examine the spectral energy distribution for NGC4395 and discuss the physical conditions implied by the nuclear activity under the standard AGN model. The observations can be explained by either an accreting massive black hole emitting at about 10^(-3) L_(Edd) or by a single old compact SNR with an age of 50 to 500 yr generated by a small nuclear starburst.

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arXiv1998

The Lyman-alpha emission in local Star-Forming Galaxies: Scenario and Connection with Primeval Galaxies

We review the Lyan alpha emission in local star-forming galaxies. In most cases as already shown by the IUE, the emission is absent or much weaker than expected. This occurs because Lyman alpha photons can be resonantly scattered by the neutral gas and destroyed by even very low amounts of dust. However new Hubble Space Telescope observations (HST) indicate that other factors such as the velocity structure of the gas play a crucial role. Gas flows are likely to occur as powered by the kinetic energy released via stellar winds and supernova. We propose a scenario based on the hydrodynamics of superbubbles powered by massive bursts of star formation that naturally accounts for the variety of Lyman alpha line detections in star-forming galaxies. We caution with the attempts to derive the co-moving star formation rate at high redshift from Lyman alpha emission searches.

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arXiv1998

HST study of Lyman-alpha emission in star-forming galaxies: the effect of neutral gas flows

We present high dispersion HST GHRS UV spectroscopic observations of 8 HII galaxies covering a wide range of metallicities and physical properties. We have found Lyalpha emission in 4 galaxies with blueshifted absorption features, leading to PCygni like profiles in 3 of them. In all these objects the OI and S II absorption lines are also blueshifted with respect to the ionized gas, indicating that the neutral gas is outflowing in these galaxies with velocities up to 200 km/s.The rest of the sample shows broad damped Lyalpha absorption profiles centered at the wavelength corresponding to the redshift of the H II emitting gas. We therefore find that the velocity structure of the neutral gas in these galaxies is the driving factor that determines the detectability of Lyalpha in emission. Relatively small column densities of neutral gas with even very small dust content would destroy the Lyalpha emission if this gas is static with respect to the ionized region where Lyalpha photons originate. The situation changes dramatically when most of the neutral gas is velocity-shifted with respect to the ionized regions because resonant scattering by neutral hydrogen will be most efficient at wavelengths shorter than the Lyalpha emission, allowing the Lyalpha photons to escape (at least partially). This mechanism complements the effect of porosity in the neutral interstellar medium discussed by other authors. The anisotropy of these gas flows and their dependence on the intrinsic properties of the violent star-forming episodes might explain the apparent lack of correlation between metallicity and the frequency of occurence and strength of Lyalpha emission in star-forming galaxies. Attempts to derive the comoving star-formation rate at high redshifts from Lyalpha emission searches are highly questionable.

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arXiv1997

Patterns and Coincidences in the Light Curves of Active Galaxies

We draw attention to some intriguing coincident patterns in the optical light curves of NGC 4151 and NGC 5548. Inspecting the light curves of these two classic Seyferts, we find evidence for at least five occurrences of a similar sequence of variations. This pattern consists of a ~ 2 yr long sequence of shorter flares, with a total energy of 3--5 10^{50} ergs in the B-band. We predict that the on-going monitoring campain on NGC 5548 should soon reveal more occurrences of this general pattern if it is truly recurrent. We speculate on the possibility that this pattern is associated with a fundamental/universal `unit' of variability in active galaxies, and discuss a possible connection with the variability properties of QSOs.

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arXiv1997

A high excitation HII region in the faint dwarf elliptical galaxy A0951+68

We present the results of BVRI imaging and optical spectroscopy of the dwarf galaxy A0951+68. The images reveal that, although this galaxy is classified as a dwarf elliptical, it has some properties that are similar to dwarf irregular galaxies. It contains two bright knots of emission, one of which is red and unresolved and the other blue and resolved. The blue knot also shows a high excitation emission line spectrum. The observed line ratios indicate that this is an HII region, although with some line ratios that are border-line with those in AGN. The emission line luminosity is consistent with ionisation by a single, very luminous O star, or several smaller O stars, but the extended blue light in the knot shows that this has occurred as part of a substantial recent star formation event. We find that the metal abundance, while low compared to typical large galaxies, actually seems to be high for such a low luminosity dwarf. The position of A0951 in the literature is incorrect and we provide the correct value.

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arXiv1997

A New Superwind Wolf-Rayet Galaxy Mrk 1259

We report the discovery of a starburst-driven wind (superwind) from the starburst nucleus galaxy Mrk 1259. The estimated number ratio of Wolf-Rayet (WR) to O stars amounts to ~0.09. While the nuclear emission-line region is due to usual photoionization by massive stars, the circumnuclear emission-line regions show anomalous line ratios that can be due to cooling shocks. Since the host galaxy seems to be a face-on disk galaxy and the excitation conditions of the circumnuclear emission-line regions show the spatial symmetry, we consider that we are seeing the superwind nearly from a pole-on view. Cooling shock models may explain the observed emission line ratios of the circumnuclear regions although a factor of 2 overabundance of nitrogen is necessary. All these suggest that the high-mass enhanced starburst occurred ~5X10^6 years ago in the nuclear region of Mrk 1259.

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arXiv1996

UV HST Spectroscopy of Star-Forming Galaxies

HST spectroscopical observations of 8 HII galaxies are reported. Ly alpha emission was detected in 4 of them. We find that the velocity structure of the gas is the main determining factor for the escape of the Ly alpha photons, and not the abundance of dust. The rest of the sample shows broad damped Ly alpha absorption attributed to large HI column densities, that is static with respect to the emitted Ly alpha photons, emerging from the HII regions. The star-forming galaxies IZW18 and even more SBS0335-052 may have extremely metal deficient HI clouds, the latter with [O/H] as low as -7.2.

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arXiv1996

The QSO variability-luminosity-redshift relation

The relationship between variability, luminosity and redshift in the South Galactic Pole QSO sample is examined in an effort to disentangle the effects of luminosity and redshift in the amplitude of the optical variations. The anticorrelation between variability and luminosity found by other authors is confirmed. Our analysis also supports claims that variability increases with redshift, most likely due to an anticorrelation between variability and wavelength. In particular, our parametric fits show that the QSO variability-wavelength relation is consistent with that observed in low-luminosity nearby active galactic nuclei. The results are used to constrain Poissonian-type models. We find that if QSO variability results from a random superposition of pulses, then the individual events must have B-band energies between $\sim 10^{50}$ and a few times $10^{51}$ erg and time-scales of $\sim 2$ yr. Generalized Poissonian models in which the pulse energy and lifetime scale with luminosity are also discussed.

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