arXiv subjects

Roberto Terlevich

Publications and source records attributed to Roberto Terlevich.

At least 55 records · Page 3Linked to original sources

arXiv2008

Properties of the ionized gas of circumnuclear star-forming regions in early type spirals

(Abbr.) A study of cicumnuclear star-forming regions (CNSFRs) in several early type spirals has been made in order to investigate their main properties: stellar and gas kinematics, dynamical masses, ionising stellar masses, chemical abundances and other properties of the ionised gas. Both high resolution (R$ \sim $20000) and moderate resolution (R ~ 5000) have been used. In some cases these regions, about 100 to 150 pc in size, are seen to be composed of several individual star clusters with sizes between 1.5 and 4.9 pc estimated from Hubble Space Telescope (HST) images. Stellar and gas velocity dispersions are found to differ by about 20 to 30 km/s with the H$β$ emission lines being narrower than both the stellar lines and the [OIII] $λ$ 5007 Ålines. The twice ionized oxygen, on the other hand, shows velocity dispersions comparable to those shown by stars. We have applied the virial theorem to estimate dynamical masses of the clusters, assuming that systems are gravitationally bounded and spherically symmetric, and using previously measured sizes. The measured values of the stellar velocity dispersions yield dynamical masses of the order of 10$^7$ to 10$^8$ solar masses for the whole CNSFRs. ...

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arXiv2007

Precision abundance analysis of bright HII galaxies

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 10400Åfree 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 ηand η' 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.

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arXiv2007

Multiwavelength Study of X-ray Selected Star Forming Galaxies within the Chandra Deep Field South

We have combined multi-wavelength observations of a selected sample of starforming galaxies with galaxy evolution models in order to compare the results obtained for different SFR tracers and to study the effect that the evolution of the starforming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field South (CDFS) and chose the objects that also have UV and IR data from GALEX and GOODS-Spitzer respectively. Our main finding is that there is good agreement between the extinction corrected SFR(UV) and the SFR(X), and we confirm the use of X-ray luminosities as a trustful tracer of recent star formation activity. Nevertheless, at SFR(UV) larger than about 5Msol/year there are several galaxies with an excess of SFR(X) suggesting the presence of an obscured AGN not detected in the optical spectra. We conclude that the IR luminosity is driven by recent star formation even in those galaxies where the SFR(X) is an order of magnitude higher than the SFR(UV) and therefore may harbour an AGN. One object shows SFR(X) much lower than expected based on the SFR(UV); this SFR(X) `deficit' may be due to an early transient phase before most of the massive X-ray binaries were formed. An X-ray deficit could be used to select extremely young bursts in an early phase just after the explosion of the first supernovae associated with massive stars and before the onset of massive X-ray binaries.

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arXiv2007

Kinematics of gas and stars in the circumnuclear starforming ring of NGC 3351

We have measured gas and stellar velocity dispersions in 5 circumnuclear starforming regions (CNSFRs) and the nucleus of the barred spiral galaxy NGC 3351. The stellar dispersions have been obtained from high resolution spectra of the CaT lines at $λλ$ 8494, 8542, 8662 Å, while the gas velocity dispersions have been measured by Gaussian fits to the H$β$ $λ$ 4861 Åline on high dispersion spectra. The CNSFRs, with sizes of about 100 to 150 pc in diameter, are seen to be composed of several individual star clusters with sizes between 1.7 and 4.9 pc on an HST image. Using the stellar velocity dispersions, we have derived dynamical masses for the entire starforming complexes and for the individual star clusters. Values of the stellar velocity dispersions are between 39 and 67 km s$^{-1}$. Dynamical masses for the whole CNSFRs are between 4.9 $\times$ 10$^6$ and 4.3 $\times$ 10$^7$ M$_\odot$ and between 1.8 and 8.7 $\times$ 10$^6$ M$_\odot$ for the individual star clusters. Stellar and gas velocity dispersions are found to differ by about 20 km s$^{-1}$ with the H$β$ lines being narrower than both the stellar lines and the [O{\sc iii}] $λ$ 5007 Ålines. We have found indications for the presence of two different kinematical components in the ionised gas of the regions. The radial velocity curve shows deviation from circular motions for the ionised hydrogen consistent with its infall towards the central regions of the galaxy at a velocity of about 25 km s$^{-1}$. To disentangle the origin of these two components it will be necessary to map these regions with high spectral and spatial resolution and much better S/N in particular for the O$^{2+}$ lines.

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arXiv2006

The temperature and ionization structure of the emitting gas in HII galaxies: Implications for the accuracy of abundance determinations

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 Å, including the Balmer jump, the [OII]3727,29 Ålines, the [SIII]9069,9532 Ådoublet as well as various weak auroral lines such as [OIII]4363 Åand [SIII]6312 Å. 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. ...

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arXiv2006

Evolutive Unification in Composite Active Galactic Nuclei

In this paper we explore an evolutionary Unified scenario involving super massive black hole and starburst with outflow, that seems capable of explaining most of the observational properties of at least part of AGNs. Our suggestion is explored inside the expectations of the Starburst model close associated with the AGN where the NLR, BLR and BAL region are produced in part by the outflow process with shells and in compact supernova remnants. The outflow process in BAL QSOs with extreme IR and Fe II emission is studied. In addition, the Fe II poblem regarding the BLR of AGN is analysed. Neither the correlations between the BAL, IR emission, FeII intensity and the intrinsic properties of the AGN are clearly understood. We suggest here that the behaviour of the BAL, IR and FeII emission in AGNs can be understood inside an evolutionary and composite model for AGNs. In our model, strong BAL systems and Fe II emission are present (and intense) in young IR objects. Orientation/ obscuration effects take the role of a second parameter providing the segregation between Sy1/Sy2 and BLRG/NLRG.

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arXiv2005

Emission Line Properties of Seyfert 2 Nuclei

We present a study of the global properties of Joguet's sample of 65 Seyfert 2 galaxies. We use the population synthesis models of our Paper II to derive `pure' emission-line spectra, and thus explore the statistical properties of the nuclear nebular components and their relation to the stellar populations. We find that the emission line clouds suffer substantially more extinction than the starlight, and confirm the correlations between stellar and nebular velocity dispersions and between emission line luminosity and velocity dispersions. Nuclear luminosities correlate with stellar velocity dispersions. Removing the contribution of young stars to the optical continuum produces a tighter and steeper relation, $L \propto σ_\star^4$, consistent with the Faber-Jackson law. Emission line ratios indicative of the gas excitation such as [OIII]/H$β$ and [OIII]/[OII] are statistically smaller for Seyferts with significant star-formation, implying that ionization by massive stars is responsible for a substantial, and sometimes even a dominant, fraction of the H$β$ and [OII] fluxes. We use our models to constrain the maximum fraction of the ionizing power that can be generated by a hidden AGN. We correlate this fraction with classical indicators of AGN photo-ionization: X-ray luminosity and nebular excitation, but find no significant correlations. Thus, while there is a strong contribution of starbursts to the excitation of the nuclear nebular emission in low-luminosity Seyferts, the contribution of the hidden AGN remains elusive even in hard X-rays. (abridged)

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arXiv2005

An Active Instance-based Machine Learning method for Stellar Population Studies

We have developed a method for fast and accurate stellar population parameters determination in order to apply it to high resolution galaxy spectra. The method is based on an optimization technique that combines active learning with an instance-based machine learning algorithm. We tested the method with the retrieval of the star-formation history and dust content in "synthetic" galaxies with a wide range of S/N ratios. The "synthetic" galaxies where constructed using two different grids of high resolution theoretical population synthesis models. The results of our controlled experiment shows that our method can estimate with good speed and accuracy the parameters of the stellar populations that make up the galaxy even for very low S/N input. For a spectrum with S/N=5 the typical average deviation between the input and fitted spectrum is less than 10**{-5}. Additional improvements are achieved using prior knowledge.

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arXiv2004

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

We have derived ages, metallicities and enhanced-element ratios [alpha/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 corresponds to the nuclear r_e/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 Hbeta-[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 sigma_0 - 0.46 log Age - 1.60. More massive (larger sigma_0) and older galaxies present, on average, large [alpha/Fe] values, and therefore, must have undergone shorter star-formation timescales. 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 in more than 8 Gyr. Finally we explore our large spectral coverage to derive log(O/H) metallicity from the N2 indicator 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 them.

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arXiv2004

Group, field and isolated early-type galaxies I. Observations and nuclear data

This is the first paper of a series on the investigation of stellar population properties and galaxy evolution of an observationally homogeneous sample of early-type galaxies in groups, field and isolated galaxies. We present high S/N long-slit spectroscopy of 86 nearby elliptical and S0 galaxies. 8 of them are isolated, selected according to a rigorous criterion. This survey has the advantage of covering a larger wavelength range than normally found in the literature, including [OIII]5007 and Halpha, both important for emission correction. Among the 86 galaxies with S/N>15 (per resolution element, for r_e/8 central aperture), 57 have their Hbeta-index corrected for emission, average correction is 0.190A in Hbeta; 42 galaxies reveal [OIII]5007 emission, of which 16 also show obvious Halpha emission. Most of the galaxies in the sample do not show obvious signs of disturbances nor tidal features in the morphologies, although 11 belong to the Arp catalogue of peculiar galaxies; only 3 of them (NGC750, NGC751, NGC3226) seem to be strongly interacting. We present the measurement of 25 central line-strength indices calibrated to the Lick/IDS system. Kinematic information is obtained for the sample. We analyse the line-strength index vs velocity dispersion relations for our sample of mainly low density environment galaxies, and compare the slope of the relations with cluster galaxies from the literature. Our main findings are that the index-sigma_0 relations presented for low-density regions are not significantly different from those of cluster E/S0s. The slope of the index-sigma_0 relations does not seem to change for early-type galaxies of different environmental densities, but the scatter of the relations seems larger for group, field and isolated galaxies than for cluster galaxies.

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arXiv2004

Structural parameters of nearby emission-line galaxies

We present the results of an investigation on the main structural properties derived from VRI and Halpha surface photometry of galaxies hosting nuclear emission-line regions (including Seyfert 1, Seyfert 2, LINER and starburst galaxies) as compared with normal galaxies. Our original sample comprises 22 active galaxies, 4 starbursts and 1 normal galaxy and has been extended with several samples obtained from the literature. Bulge and disc parameters, along with B/D relation, have been derived applying an iterative procedure. The resulting parameters have been combined with additional data in order to reach a statistically significant sample. We find some differences in the bulge distribution across the different nuclear types that could imply familes of bulges with different physical properties. Bulge and disc characteristic colours have been defined and derived for our sample and compared with a control sample of early type objects. The results suggest that bulge and disc stellar populations are comparable in normal and active galaxies.

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arXiv2003

Automated Determination of Stellar Population Parameters in Galaxies Using Active Instance-based Learning

In this work we focus on the determination of the relative distributions of young, intermediate-age and old populations of stars in galaxies. Starting from a grid of theoretical population synthesis models we constructed a set of model galaxies with a distribution of ages, metallicities and intrinsic reddening. Using this set we have explored a new fitting method that presents several advantages over conventional methods. We propose an optimization technique that combines active learning with an instance-based machine learning algorithm. Experimental results show that this method can estimate with high speed and accuracy the physical parameters of the stellar populations.

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arXiv2003

Age determination of the nuclear stellar population of Active Galactic Nuclei using Locally Weighted Regression

We present 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. 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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arXiv2003

How old are the HII Galaxies?

Using a novel approach we have reanalized the question of whether the extreme star forming galaxies known as HII galaxies are truly young or rejuvenated old systems. We first present a method of inversion that applies to any monotonic function of time describing the evolution of independent events. We show that, apart from a normalization constant, the ``true'' time dependence can be recovered from the inversion of its probability density function. We applied the inversion method to the observed equivalent width of Hbeta (EW(Hbeta)) distribution for objects in the Terlevich and collaborators Spectrophotometric Catalogue of HII galaxies and found that their global history of star formation behaves much closer to the expectations of a continuos star formation model than to an instantaneous one. On the other hand, when the inversion method is applied to samples within a restricted metallicity range we find that their history of star formation behaves much closer to what the instantaneous model predicts. Our main conclusion is that, globally, the evolution of HII galaxies seems consistent with a succession of short starbursts separated by quiescent periods and that, while the emission lines trace the properties of the present burst, the underlying stellar continuum traces the whole star formation history of the galaxy. Thus, observables like the EW(Hbeta) that combine an emission line flux, i.e. a parameter pertaining to the present burst, with the continuum flux, i.e. a parameter that traces the whole history of star formation, should not be used alone to characterize the present burst.

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arXiv2003

On the Detectability of the SZ Effect of Massive Young Galaxies

The Sunyaev-Zel'dovich (SZ) effect expected to be associated with massive star-formation activity produced during the formation of the most luminous bulges of normal galaxies is discussed. Using 1-D chemohydrodynamical models for spheroidal galaxy evolution we show that during the early epochs of galaxy evolution the gas in massive events of star formation may reach temperatures and densities high enough to produce values of the comptonisation parameter "y" comparable to those present in galaxy clusters. In this scenario, we discuss the possibility of detection of the SZ signature in high redshift starforming galaxies with the next generation of mm telescopes capable of arcsecond resolution and equipped with high sensitivity detectors. We show how millimeter colour--colour diagrams or diagnostic diagrams could be used to distinguish between the dust emission and the SZ effect and suggest the use of simultaneous multifrequency observations to improve the chances of detecting the SZ effect.

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arXiv2001

New light on the search for low metallicity galaxies I. The N2 method

We present a simple metallicity estimator based on the logarithmic [NII]6584 / Halpha ratio, hereafter N2, which we envisage will become very useful for ranking galaxies in a metallicity sequence from redshift survey quality data even for moderately low spectral resolution. We have calibrated the N2 estimator using a compilation of HII galaxies having accurate oxygen abundances, plus photoionization models covering a wide range of abundances. The comparison of models and observations indicates that both primary and secondary nitrogen are important for the relevant range of metallicities. The N2 estimator follows a linear relation with log(O/H) that holds for the whole abundance range covered by the sample, from about 1/50th to twice the Solar value (7.2 < 12+log(O/H) < 9.1). We suggest that the ([SII]6717,6731 / Halpha) ratio (hereafter S2) can also be used as a rough metallicity indicator. Because of its large scatter the S2 estimator will be useful only in systems with very low metallicity, where [NII]6584 is not detected or in low resolution spectra where [NII]6584 is blended with Halpha.

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arXiv2001

Stellar populations in the nuclear regions of nearby radiogalaxies

We present optical spectra of the nuclei of seven luminous (P(178MHz) > 10**25 W Hz**(-1) Sr**(-1)) nearby (z<0.08) radiogalaxies, which mostly correspond to the FR II class. In two cases, Hydra A and 3C 285, the Balmer and 4000A break indices constrain the spectral types and luminosity classes of the stars involved, revealing that the blue spectra are dominated by blue supergiant and/or giant stars. The ages derived for the last burst of star formation in Hydra A are between 7 and 40 Myr, and in 3C 285 about 10 Myr. The rest of the narrow-line radiogalaxies (four) have 4000A break and metallic indices consistent with those of elliptical galaxies. The only broad-line radiogalaxy in our sample, 3C 382, has a strong featureless blue continuum and broad emission lines that dilute the underlying blue stellar spectra. We are able to detect the Ca II triplet in absorption in the seven objects, with good quality data for only four of them. The strengths of the absorptions are similar to those found in normal elliptical galaxies, but these values are both consistent with single stellar populations of ages as derived from the Balmer absorption and break strengths, and, also, with mixed young+old populations.

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arXiv2001

The cosmological evolution of colour gradients in spheroids

The analysis of the four-colour maps of galaxies in the Hubble Deep Field has revealed in the sample of 0.4<z<1 early-type field galaxies the existence of ellipticals with a predominantly old coeval stellar population. However, there is another, unexpected, category of HDF early-type galaxies, in which the galaxy core is significantly bluer than the outer regions. We demonstrate that these colour gradients are predicted by the multi-zone chemodynamical model for the evolution of elliptical galaxies. We suggest that the colour gradient could be used as a chronometer of the evolution of elliptical galaxies: galaxies younger than a few Gyr exhibit cores bluer than the surrounding galaxy, due to ongoing star formation, while more evolved galaxies have redder cores, due to metallicity gradients increasing towards the centre.

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