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C. Chiosi

Publications and source records attributed to C. Chiosi.

At least 37 records · Page 2Linked to original sources
arXiv2010

Orbital evolution of the Carina dwarf galaxy and self-consistent star formation history determination

We present a new study of the evolution of the Carina dwarf galaxy that includes a simultaneous derivation of its orbit and star formation history. The structure of the galaxy is constrained through orbital parameters derived from the observed distance, proper motions, radial velocity and star formation history. The different orbits admitted by the large proper motion errors are investigated in relation to the tidal force exerted by an external potential representing the Milky Way (MW). Our analysis is performed with the aid of fully consistent N-body simulations that are able to follow the dynamics and the stellar evolution of the dwarf system in order to determine self-consistently the star formation history of Carina. We find a star formation history characterized by several bursts, partially matching the observational expectation. We find also compatible results between dynamical projected quantities and the observational constraints. The possibility of a past interaction between Carina and the Magellanic Clouds is also separately considered and deemed unlikely.

astro-ph.CO↗
arXiv2009

GALEX UV properties of the polar ring galaxy MCG-05-07-001 and the shell galaxies NGC 1210 and NGC 5329

Systems of shells and polar rings in early-type galaxies are considered "bona fide" tracers of mass accretion and/or mergers. Their high frequency in low density environments suggests that such episodes could drive the evolution of at least a fraction of the early-type galaxy population. Their UV emission is crucial to test whether these galaxies host ongoing/recent star formation. We used far and near ultraviolet, optical, near infrared images, HI maps, and line strength indices to investigate the nuclear and outer regions of the galaxies as well as the regions where fine structures are present. The GALEX Near (NUV) and Far UV (FUV) images of MCG-05-07-001 and NGC 1210 show complex tidal tails and debris structures. The UV morphology of both galaxies appears so different from the optical one that the early-type classification may not apply. In both GALEX bands the polar ring of MCG-05-07-001 is the dominant feature, whereas an extended tidal tail dominates the FUV bands of NGC 1210. In MCG-05-07-001 and NGC 1210 there is a strong correlation between structures detected in the FUV and NUV bands and in HI. NGC 5329 does not show evidence of shells in the UV. We try to constrain the age of the accretion episode or merger which gave rise to the shells and polar rings with the aid of composite stellar populations that take the presence of dust into account. The presence of HI in both MCG-05-07-001 and NGC 1210 argues in favour of wet mergers. Models suggest the presence of very young stellar populations in MCG-05-07-001: the observations could be explained in the framework of a conspicuous burst of star formation that occurred <=1 Gyr ago and involved a large fraction of the galaxy mass. Our models suggest that also the nuclei of NGC 1210 and NGC 5329 could have been rejuvenated by an accretion episode about 2-4 Gyr ago. (abridged)

astro-ph.CO↗
arXiv2008

XMM-Newton X-ray and Optical Monitor far UV observations of NGC 7070A and ESO 2400100 shell galaxies

We present XMM-Newton X-ray observations of two shell galaxies, NGC 7070A and ESO 2400100, and far UV observations obtained with the Optical Monitor for these and for an additional shell galaxy, NGC 474, for which we also have near and far UV data from GALEX. We aim at gaining insight on the overall evolution traced by their star formation history and by their hot gas content. The X-ray and the far UV data are used to derive their X-ray spatial and spectral characteristics and their UV luminosity profiles. We use models developed ad hoc to investigate the age of the last episode of star formation from the (UV - optical) colors and line strength indices. The X-ray spatial and spectral analysis show significant differences in the two objects. A low luminosity nuclear source is the dominant component in NGC 7070A log L_X=41.7 erg s^{-1} in the 2-10 keV band. In ESO 2400100, the X-ray emission is due to a low temperature plasma with a contribution from the collective emission of individual sources. In the Optical Monitor image ESO 2400100 shows a double nucleus, one bluer than the other. This probably results from a very recent star formation event in the northern nuclear region. The extension of the UV emission is consistent with the optical extent for all galaxies, at different degrees of significance in different filters. The presence of the double nucleus, corroborated by the (UV - optical) colors and line strength indices analysis, suggests that ESO 2400100 is accreting a faint companion. We explore the evolution of the X-ray luminosity during accretion processes with time. We discuss the link between the presence of gas and age, since gas is detected either before coalescence or several Gyr (>3) after (Abridged).

astro-ph↗
arXiv2008

The GALEX UV emission in shell galaxies

Shell galaxies are widely considered the debris of recent accretion/merging episodes. Their high frequency in low density environment suggests that such episodes could be among the driver of the early-type galaxy secular evolution. We present far and near UV (FUV and NUV respectively hereafter) GALEX photometric properties of a sample of shell galaxies.

astro-ph↗
arXiv2007

The GALEX UV emission in shell galaxies: tracing galaxy "rejuvenation" episodes

We present the GALEX far FUV and near NUV ultraviolet imaging of three nearby shell galaxies, namely NGC 2865, NGC 5018 and NGC 7135. The system of shells and fine structures visible in the optical is detected in the NUV image of NGC 2865 and in both NUV and FUV images of NGC 7135. The NUV image of NGC 5018 does not present shell structures. We detect absorption features in the nuclear region of all three galaxies. NGC 2865 has a nearly flat colour profile with (FUV-NUV)~2 throughout the whole galaxy. NGC 7135 is blue in the center (FUV-NUV)~0 and as red as (FUV-NUV)~1.5 in the outskirts, including the faint shell-like feature. We investigate the ability of the nuclear GALEX (FUV-NUV) colour to provide information about rejuvenation phenomena in the stellar populations of the shell galaxies. To this aim, we derive from theory the relationship between the Mg2, Hbeta, HgammaA, HdeltaA Lick line-strength indices and the (FUV-NUV) colour. We extend the study to a sample of early-type galaxies with emission lines in their optical spectra (Annibali et al. 2007). In the index vs.(FUV-NUV) colour diagrams, most of the galaxies are well explained by passively evolving SSPs. On the average, ages and metallicities of the galaxies in our sample estimated from optical line-strength indices are consistent with those inferred from the (FUV-NUV) colour. In general, all the colours but for (FUV-NUV) and (FUV-V), become nearly age insensitive when 1-2 Gyr have elapsed from the last star forming event. Considering composite stellar population models with a recent burst of star formation, we suggest that the position of the NGC 7135 and NGC 2865 nuclei in the (FUV-NUV)-Hbeta plane could be explained in term of a recent rejuvenation episode. (Abridged)

astro-ph↗
arXiv2006

New response functions for absorption-line indices from high-resolution spectra

Basing on the huge library of 1-A resolution spectra calculated by Munari et al. over a large range of logT, log g, [Fe/H] and both for solar and a-enhanced abundance ratios [a/Fe], we present theoretical absorption-line indices on the Lick system. First we derive the so-called response functions (RFs) of Tripicco & Bell for a wide range of logT, log g, [Fe/H] and [a/Fe]=+0.4 dex. The RFs are commonly used to correct indices with solar [a/Fe] ratios to indices with [a/Fe]>0. Not only the RFs vary with the type of star but also with the metallicity. Secondly, with the aid of this and the fitting functions (FFs) of Worthey et al., we derive the indices for single stellar populations and compare them with those obtained by previous authors, e.g. Tantalo & Chiosi. The new RFs not only supersede the old ones by Tripicco & Bell, but also show that Hb increases with the degree of enhancement in agreement with the results by Tantalo & Chiosi. The new indices for single stellar populations are used to derive with aid of the recursive Minimum Distance method the age, metallicity and degree of enhancement of a sample of Galactic Globular Clusters for which these key parameters have been independently derived from the Colour-Magnitude Diagram and/or spectroscopic studies. The agreement is remarkably good.

astro-ph↗
arXiv2005

Modelling galaxy spectra in presence of interstellar dust. I. The model of ISM and the library of dusty SSPs

In this paper, the first of a series of two devoted to modelling the spectra of galaxies of different morphological type in presence of dust, we present our description of the dust both in the diffuse ISM and the molecular clouds. Our model for the dust takes into account three components, i.e. graphite, silicates and polycyclic aromatic hydrocarbons. We consider and adapt to our aims two prescriptions for the size distribution of the dust grains and two models for the emission of the dusty ISM. We cross-check the emission and extinction models of the ISM by calculating the extinction curves and the emission for the typical environments of the Milky Way and the Large and Small Magellanic Clouds and by comparing the results with the observational data. The final model we have adopted is an hybrid one which stems from combining the analysis of Guhathakurta et al (1989) for the emission of graphite and silicates and Puget et al. (1985) for the PAH emission, and using the distribution law of Weingartner et al. (2001a) and the ionization model for PAHs of Weingartner et al. (2001b). We apply the model to calculate the SED of SSPs of different age and chemical composition, which may be severely affected by dust in young, massive stars while they are still embedded in their parental MCs. We use the "Ray Tracing" method to solve the problem of radiative transfer and to calculate libraries of SSP SEDs. Particular care is paid to model the contribution from PAHs, introducing different abundances of C in the population of very small carbonaceous grains (VSGs) and different ionization states in PAHs. The SEDs of young SSPs are then compared with observational data of star forming regions of four local galaxies successfully reproducing their SEDs from the UV-optical regions to the mid and far infrared.

astro-ph↗
arXiv2004

Measuring age, metallicity and abundance ratios from absorption line indices

In this study we present detailed calculations of absorption line indices on the Lick System based on the stellar models by Salasnich et al. (2000) calculated with enhanced mix of alpha-elements. Using the so-called Response Functions (RFs) of Tripicco & Bell (1995, TB95), we calculate the indices for SSPs of different age, metallicity and enhancement. We made use of the triplet Hb, Mgb and , and Minimum-Distance Method proposed by Trager et al. (2000, TFWG00) to estimate the age, metallicity and enhancement degree for the galaxies of the Gonzalez (1993) sample, and compare the results with those TFWG00 and Thomas et al (2003). Since very large differences are found, in particular as far as the age is concerned, we analyze in a great detail all possible sources of disagreement, going from the stellar models and SSPs to many technical details of the procedure to calculate the indices, and finally the pattern of chemical elements (especially when alpha-enhanced mixtures are adopted). The key issue of the analysis is that at given metallicity Z and enhancement factor, the specific abundance ratios [Xel/Fe] adopted for some elements (e.g. O, Mg, Ti, and likely others) dominate the scene because with the TB95 RFs they may strongly affect indices like Hb and the age in turn. Finally we have drawn some remarks on the interpretation of the distribution of early-type galaxies in popular two-indices planes, like Hb vs. [MgFe]. We argue that part of the scatter along the Hb axis observed in this plane could be attributed instead of the age, the current explanation, to a spread both in the degree of enhancement and some abundance ratios. The main conclusion of this study is that deriving ages, metallicities and degree of enhancement from line indices is a cumbersome affair whose results are still uncertain.

astro-ph↗
arXiv2004

Star formation history in early-type galaxies. I. The line absorption indices diagnostics

The bottom line of this study is (i) to check whether normal (quiescent) and interacting EGs have a different behavior in the popular diagnostic planes such as Hb vs. [MgFe] (and others); (ii) to seek whether the signature of mergers and/or interactions may mirror in some specific changes of the indices that could un-equivocally hint for burts of stellar activity; (iii) to evaluate the intensity of those bursts or secondary episodes of star formation; (iv) to explore whether other alternatives can exist, i.e. distinct from obvious ones resting on large age range. The data show that both normal, field and interacting galaxies have the same scattered but smooth distribution in the Hb vs. [MgFe] diagnostic plane. There are two interpretations for this: (1) EGs span larges ranges of ages and metallicities. (2) The bulk population of stars is old but subsequent episodes of star formation scatter the object in the diagnostic planes. Both these interpretation are not able to reproduce the observed distribution. The above schemes are too demanding because of the many ad hoc ingredients that have to be introduced. Furthermore they neglect the observationally grounded hint that the stellar content of EGs is likely enhanced in alfa-elements. We propose here a new scheme, in which the bulk dispersion of galaxies in the index-index plane is caused by a different mean degree of enhancement. The possibility that EGs span large ranges of [alpha/Fe] and metallicities but narrow ranges of ages for the bulk population favors the monolithic scheme.

astro-ph↗
arXiv2004

Evolution of planetary nebulae II. Population effects on the bright cut-off of the PNLF

We investigate the bright cut-off of the [OIII]l5007 planetary nebula luminosity function (PNLF), that has been suggested as a powerful extragalactic distance indicator. Theoretical PNLFs are constructed via Monte-Carlo simulations of populations of PNe, whose individual properties are described with the aid of recent PN synthetic models (Marigo et al. 2001), coupled to a detailed photoionisation code (CLOUDY). The basic dependences of the cut-off magnitude M* are then discussed. We find that: (i) In galaxies with recent or ongoing star formation, the modelled PNLF present M* values between -4 and -5, very close to the observationally-calibrated value for the LMC. (ii) In these galaxies, the PNLF cut-off is produced by PNe with progenitor masses of about 2.5 Msun, while less massive stars give origin to fainter PNe. As a consequence M* is expected to depend strongly on the age of the last burst of star formation, dimming by as much as 5 mag as we go from young to 10-Gyr old populations. (iii) Rather than on the initial metallicity of a stellar population, M* depends on the actual [O/H] of the observed PNe, a quantity that may differ significantly from the initial value (due to dredge-up episodes), especially in young and intermediate-age PN populations. (iv) Also the transition time from the end of AGB to the PN phase, and the nuclear-burning properties (i.e. H- or He-burning) of the central stars introduce non-negligible effects on M*. The strongest indication derived from the present calculations is a serious difficulty to explain the age-invariance of the cut-off brightness over an extended interval, say from 1 to 13 Gyr, that observations of PNLFs in galaxies of late-to-early type seem to suggest.

astro-ph↗
arXiv2004

Theoretical isochrones in several photometric systems II. The Sloan Digital Sky Survey ugriz system

Following Paper I, we provide extended tables of bolometric corrections, extinction coefficients, stellar isochrones, and integrated magnitudes and colours of single-burst stellar populations, for the Sloan Digital Sky Survey (SDSS) ugriz photometric system. They are tested on comparisons with DR1 data for a few stellar systems, namely the Palomar 5 and NGC 2419 globular clusters and the Draco dSph galaxy.

astro-ph↗
arXiv2003

Can a "standard" Initial Mass Function explain the metal enrichment in clusters of galaxies?

It is frequently debated in literature whether a "standard" Initial Mass Function (IMF) - meaning an IMF of the kind usually adopted to explain the chemical evolution in the local Solar Neighbourhood - can account for the observed metal enrichment and Iron Mass-to-Light Ratio in clusters of galaxies. We address this problem by means of straightforward estimates that should hold independently of the details of chemical evolution models. It is crucial to compute self-consistently the amount of mass and metals locked-up in stars, by accounting for the stellar mass-to-light ratio predicted by a given IMF. It becomes then clear that a "standard" Solar Neighbourhood IMF cannot provide enough metals to account for the observed chemical properties in clusters: clusters of galaxies and the local environment must be characterized by different IMFs. Alternatively, if we require the IMF to be universal, in order to explain clusters such an IMF must be much more efficient in metal production than usually estimated for the Solar Vicinity. In this case, substantial loss of metals is required from the Solar Neighbourhood and from disc galaxies in general. This "non-standard" scenario for the local chemical evolution would challenge our present understanding of the Milky Way and of disc galaxy formation.

astro-ph↗
arXiv2003

Chemical evolution of the intra-cluster medium

The high metallicity of the intra-cluster medium (ICM) is generally interpreted on the base of the galactic wind scenario for elliptical galaxies. In this framework, we develop a toy-model to follow the chemical evolution of the ICM, formulated in analogy to chemical models for individual galaxies. The model computes the galaxy formation history (GFH) of cluster galaxies, connecting the final luminosity function (LF) to the corresponding metal enrichment history of the ICM. The observed LF can be reproduced with a smooth, Madau-plot like GFH peaking at z~ 1-2, plus a "burst" of formation of dwarf galaxies at high redshift. The model is used to test the response of the predicted metal content and abundance evolution of the ICM to varying input galactic models. The chemical enrichment is computed from "galactic yields" based on models of elliptical galaxies with a variable initial mass function (IMF), favouring the formation of massive stars at high redshift and/or in more massive galaxies. For a given final galactic luminosity, these model ellipticals eject into the ICM a larger quantity of gas and of metals than do standard models based on the Salpeter IMF. However, a scenario in which the IMF varies with redshift as a consequence of the effect of the the cosmic background temperature on the Jeans mass scale, appears to be too mild to account for the observed metal production in clusters. The high iron-mass-to-luminosity-ratio of the ICM can be reproduced only by assuming a more dramatic variation of the typical stellar mass, in line with other recent findings. The mass in the wind-ejected gas is predicted to exceed the mass in galaxies by a factor of 1.5-2 and to constitute roughly half of the intra-cluster gas.

astro-ph↗
arXiv2003

The red tail of carbon stars in the LMC: Models meet 2MASS and DENIS observations

Carbon stars are known to exhibit systematically redder near-infrared colours with respect to M-type stars. In the near-infrared colour-magnitude diagrams provided by the 2MASS and DENIS surveys, the LMC C-type stars draw a striking red tail, well separated from the sequences of O-rich giants. So far, this conspicuous feature has been absent from any set of available isochrones, even the few existing ones that include the TP-AGB evolution of low- and intermediate-mass stars. To investigate such issue we simulate the complete 2MASS Ks vs.(J-Ks) data towards the LMC by means of a population synthesis approach, that relies on extended libraries of published stellar evolutionary tracks, including the TP-AGB phase. The simulations provide quite a detailed description of the several vertical fingers and inclined sequences seen in 2MASS data, due to both Galactic foreground and LMC O-rich stars. Instead, as mentioned, the red tail of C-stars sets a major difficulty: we find that TP-AGB models with solar-scaled molecular opacities, the usual assumption of existing AGB calculations, do not succeed in reproducing this feature. Our tests indicate that the main reason for this failure should not be ascribed to empirical Teff - (J-K) transformations for C-type stars. Instead, the discrepancy is simply removed by adopting new evolutionary models that account for the changes in molecular opacities as AGB stars get enriched in carbon via the third dredge-up (Marigo 2002). In fact, simulations that adopt these models are able to reproduce, for the first time, the red tail of C-stars in near-infrared CMDs. Finally, we point out that these simulations also provide useful indications about the efficiency of the third dredge-up process, and the pulsation modes of long-period variables.

astro-ph↗
arXiv2002

Kinematics of the disk population in the GAIA era

GAIA data will create a precise 3-dimensional map of the Galaxy, providing positional information, radial velocities, luminosity, temperature and chemical composition of a representative sample of stars. Here we present a new implementation of the Padova Galaxy Model, where kinematics simulations are included. A few examples of application to GAIA science are discussed, in particular concerning radial velocities determinations of the thin and thick disk.

astro-ph↗
arXiv2002

Zero-metallicity stars II. Evolution of very massive objects with mass loss

We present evolutionary models of zero-metallicity very massive objects, with initial masses in the range 120 Msun -- 1000 Msun, covering their quiescent evolution up to central carbon ignition. In the attempt of exploring the possible occurrence of mass loss by stellar winds, calculations are carried out with recently-developed formalisms for the mass-loss rates driven by radiation pressure (Kudritzki 2002) and stellar rotation (Maeder & Meynet 2000).The study completes the previous analysis by Marigo et al. (2001) on the constant-mass evolution of primordial stars. Our results indicate that radiation pressure (assuming a minimum metallicity Z = 10^{-4} Zsun)is not an efficient driving force of mass loss, except for very massive stars with M >= 750 Msun. On the other hand, stellar rotation might play a crucial role in triggering powerful stellar winds, once the (Omega-Gamma)-limit is approached. However, this critical condition of intense mass loss can be maintained just for short, as the loss of angular momentum due to mass ejection quickly leads to the spinning down of the star. As by-product to the present work, the wind chemical yields from massive zero-metallicity stars are presented. The helium and metal enrichments, and the resulting Delta(Y)/Delta(Z) ratio are briefly discussed.

astro-ph↗
arXiv2002

Testing intermediate-age stellar evolution models with VLT photometry of LMC clusters. III. Padova results

The color-magnitude diagrams (CMDs) of three intermediate-age LMC clusters, NGC 2173, SL556 and NGC2155 are analyzed to determine their age and metallicity basing on Padova stellar models. Synthetic CMDs are compared with cluster data. The best match is obtained using two fitting functions based on star counts in the different bins of the cluster CMD. Two different criteria are used. One of them takes into account the uncertainties in the color of the red clump stars. Given the uncertainties on the experimental values of the clusters metallicity, we provide a set of acceptable solutions. They define the correspondent values of metallicity, age, reddening and distance modulus (for the assumed IMF). The comparison with Padova models suggests for NGC 2173 a prolonged star formation (spanning a period of about 0.3 Gyr), beginning 1.7 Gyr and ending 1.4 Gyr ago. The metallicity Z is in the range 0.0016 $-$ 0.003. Contrary to what suggested for NGC 2173 a period of extended star formation was not required to fit the SL 556 and the NGC 2155 observations. For SL 556 an age of 2.0 Gyr is obtained. The metallicity value is in the range 0.002 $-$ 0.004, depending on the adopted comparison criterium. The derived age for NGC 2155 is 2.8 Gyr and its metallicity Z is in the range 0.002 $-$ 0.003. The CMD features of this cluster suggest that a more efficient overshoot should be adopted in the evolutionary models.

astro-ph↗