arXiv subjects

A. Bressan

Publications and source records attributed to A. Bressan.

At least 199 records · Page 11Linked to original sources

arXiv2003

Dust and Nebular Emission. I. Models for Normal Galaxies

We present a model for nebular emission in star forming galaxies, which takes into account the effects of dust reprocessing. The nebular emissions have been computed with CLOUDY and then included into GRASIL, our spectrophotometric code specifically developed for dusty galaxies. The interface between nebular emission and population synthesis is based on a set of pre-computed HII region emission models covering a wide range of physical quantities. Concerning the extinction properties of normal star forming galaxies, we are able to interpret the observed lack of correlation between the attenuation measured at Halpha and in the UV band as a consequence of age selective extinction. We also find that, for these galaxies with modest SFR, the ratio FIR/UV provides the best constraints on the UV attenuation. Our model also allows to deal with different SFR estimators in a consistent way, from the UV to radio wavelengths, and to discuss the uncertainties arising from the different physical conditions encountered in star forming galaxies. We provide our best estimates of SFR/luminosity calibrations, together with their expected range of variation. It results that SFR derived through Halpha, even when corrected for extinction using the Balmer decrement, is affected by important uncertainties due to age selective extinction. Another remarkable result is that SFR from UV luminosity corrected by means of the ratio FIR/UV has a small uncertainty. Finally, our model provides a calibration of SFR from radio luminosity; we are also able to reproduce the observed FIR/radio ratio.

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arXiv2002

Spectro-Polarimetric search for hidden AGNs in four southern Ultraluminous Infrared Galaxies

We report on a spectro-polarimetric analysis of four southern Ultra-Luminous Infrared Galaxies (ULIRGs), aimed at constraining the presence of hidden broad AGN lines. For IRAS 19254--7245 ({\sl The Superantennae}) we find evidence for a significant level of polarized light in the H$α$ line with FWHM$>$2300 km/s. Some degree of polarization is also detected in IRAS 20551-4250, though with lower significance. In the two other sources (IRAS 20100--4156 and IRAS 22491--1808) no polarized signals are detected. Although it is unclear from the present data if the origin of polarization is due to reflected light from an AGN or more simply to dichroic transmission by a dust slab, we find interesting correlation between the presence of polarized components in the optical spectra and independent evidence for AGN emissions in hard X-rays and the far-IR.

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arXiv2002

Modeling the Radio to X-ray SED of Galaxies

Our multi-wavelength model GRASIL for the SED of galaxies is described, in particular the recent extension to the radio and X-ray range. With our model we can study different aspects of galaxy evolution by exploiting all available spectral observations, where different emission components dominate.

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arXiv2002

Some Results on the Boundary Control of Systems of Conservation Laws

This note is concerned with the study of the initial boundary value problem for systems of conservation laws from the point of view of control theory, where the initial data is fixed and the boundary data are regarded as control functions. We first consider the problem of controllability at a fixed time for genuinely nonlinear Temple class systems, and present a description of the set of attainable configurations of the corresponding solutions in terms of suitable Oleinik-type estimates. We next present a result concerning the asymptotic stabilization near a constant state for general $n\times n$ systems. Finally we show with an example that in general one cannot achieve exact controllability to a constant state in finite time.

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arXiv2002

Theoretical isochrones in several photometric systems I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets

We provide tables of theoretical isochrones in several photometric systems. To this aim, the following steps are followed: (1) First, we re-write the formalism for converting synthetic stellar spectra into tables of bolometric corrections. The resulting formulas can be applied to any photometric system, provided that the zero-points are specified by means of either ABmag, STmag, VEGAmag, or a standard star system that includes well-known spectrophotometric standards. Interstellar absorption can be considered in a self-consistent way. (2) We assemble an extended and updated library of stellar intrinsic spectra. It is mostly based on non-overshooting ATLAS9 models, suitably extended to both low and high effective temperatures. This offers an excellent coverage of the parameter space of Teff, logg, and [M/H]. We briefly discuss the main uncertainties and points still deserving more improvement. (3) From the spectral library, we derive tables of bolometric corrections for Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey systems (this latter consisting on the WFI, EMMI, and SOFI filter sets). (4) These tables are used to convert several sets of Padova isochrones into the corresponding absolute magnitudes and colours, thus providing a useful database for several astrophysical applications. All data files are made available in electronic form.

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arXiv2002

On Core Collapse Supernovae in Normal and in Seyfert Galaxies

This paper estimates the relative frequency of different types of core-collapse supernovae, in terms of the ratio f between the number of type Ib--Ic and of type II supernovae. We estimate f independently for all normal and Seyfert galaxies whose radial velocity is <=14000 km/s, and which had at least one supernova event recorded in the Asiago catalogue from January 1986 to August 2000. We find that the ratio f is approx. 0.23+/-0.05 in normal galaxies. This value is consistent with constant star formation rate and with a Salpeter Initial Mass Function and average binary rate approx. 50 %. On the contrary, Seyfert galaxies exceed the ratio f in normal galaxies by a factor approx. 4 at a confidence level >= 2 sigma. A caveat is that the numbers for Seyferts are still small (6 type Ib-Ic and 6 type II supernovae discovered as yet). Assumed real, this excess of type Ib and Ic with respect to type II supernovae, may indicate a burst of star formation of young age (<= 20 Myr), a high incidence of binary systems in the inner regions (r <= 0.4 R25) of Seyfert galaxies, or a top-loaded mass function.

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arXiv2001

Recent star formation in very luminous infrared galaxies

Among star forming galaxies, a spectral combination of a strong Hdelta line in absorption and a moderate [OII] emission has been suggested to be a useful method to identify dusty starburst galaxies at any redshift, on the basis of optical data alone. On one side it has been shown that such a spectral particularity is indeed suggestive of obscured starburst galaxies but, on the other, the degeneracy of the optical spectrum has hindered any quantitative estimate of the star formation and extinction during the burst. The optical spectrum by itself, even complemented with the information on the far-IR flux, is not enough to identify univocal evolutionary patterns. We discuss in the following whether it is possible to reduce these uncertainties by extending the spectral analyses to the near infrared spectral region.

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arXiv2001

FIR and Radio emission in star forming galaxies

We examine the tight correlation observed between the far infrared (FIR) and radio emission in normal and star burst galaxies. We show that significant deviations from the average relation are to be expected in young star burst or post star burst galaxies, due to the different fading times of FIR and Radio power.

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arXiv2000

Modelling the Extinction Properties of Galaxies

Recently (Granato, Lacey, Silva et al. 2000, astro-ph/0001308) we have combined our spectrophotometric galaxy evolution code which includes dust reprocessing (GRASIL, Silva et al. 1998) with semi-analytical galaxy formation models (GALFORM, Cole et al. 1999). One of the most characteristic features of the former is that the dust is divided in two main phases: molecular cloud complexes, where stars are assumed to be born, and the diffuse interstellar medium. As a consequence, stellar populations of different ages have different geometrical relationships with the two phases, which is essential in understanding several observed properties of galaxies, in particular those undergoing major episodes of star formation at any redshift. Indeed, our merged GRASIL+GALFORM model reproduces fairly well the SEDs of normal spirals and starbursts from the far-UV to the sub-mm and their internal extinction properties. In particular in the model the observed starburst attenuation law (Calzetti 1999) is accounted for as an effect of geometry of stars and dust, and has nothing to do with the optical properties of dust grains.

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arXiv2000

The infrared side of galaxy formation. I. The local universe in the semi-analytical framework

We present a new evolutionary model for the far-UV to sub-mm properties of the galaxy population. This combines a semi-analytic galaxy formation model based on hierarchical clustering (GALFORM) with a spectro-photometric code which includes dust reprocessing (GRASIL). The former provides the star formation and metal enrichment histories, together with the gas mass and various geometrical parameters, for a representative sample of galaxies formed in different density environments. These quantities allow us to model the SEDs of galaxies, taking into account stellar emission and also dust extinction and re-emission. Two phases are considered for the dust: molecular cloud complexes, where stars are assumed to be born, and the diffuse interstellar medium. The model includes both galaxies forming stars quiescently in disks, and starbursts triggered by galaxy mergers. We test our models against the observed spectro-photometric properties of galaxies in the local Universe. The models reproduce fairly well the SEDs of normal spirals and starbursts, and their internal extinction properties. The starbursts follow the observed relationship between the FIR to UV luminosity ratio and the slope of the UV continuum. They also reproduce the observed starburst attenuation law (Calzetti et al 99). This result is remarkable, because we use a dust mixture which reproduces the Milky Way extinction law. It suggests that the observed attenuation law is related to the geometry of the stars and dust. We compute galaxy luminosity functions over our wide range of wavelengths, which turn out to be in good agreement with observational data. The UV continuum turns out to be a poor star formation indicator for our models, whilst the infrared luminosity is much more reliable.

astro-ph↗

arXiv2000

Zero metallicity stellar sources and the reionization epoch

We reconsider the problem of the cosmological reionization due to stellar sources. Using a method similar to that developed by Haiman & Loeb (1997), we investigate the effect of changing the stellar models and the stellar spectra adopted for deriving the ionizing photon production rate. In particular, we study the consequences of adopting zero metallicity stars, which is the natural choice for the first stellar populations. We construct young isochrones representative of Population III stars from existing sets of evolutionary models (Forieri 1982; Cassisi & Castellani 1993) and calculate a suitable library of zero metallicity model atmospheres. The number of ionizing photons emitted by such a zero metal population is about 40% higher than that produced by standard metal poor isochrones. We find that adopting suitable zero metallicity models modifies the reionization epoch. However the latter is still largely affected by current uncertainties in other important physical processes such as the efficiency of the star formation and the fraction of escaping UV photons.

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arXiv1999

Star formation history of early-type galaxies in low density environments

In this paper we analyze the line-strength indices in the Lick-system measured by Longhetti et al. (1998a, b) for a sample of 51 early-type galaxies located in low density environments (LDE) and showing signatures of fine structures and/or interactions. The sample contains 21 shell-galaxies and 30 members of interacting pairs. Firstly we perform a preliminary comparison between three different sources of calibrations of the line strength indices, namely Buzzoni et al. (1992, 1994), Worthey (1992), Worthey et al. (1994) and Idiart et al. (1995), derived from stars with different effective temperature, gravity, and metallicity. Secondly, we discuss the properties of the galaxies in our sample by comparing them both with theoretical Single Stellar Populations (SSPs) and the {\sl normal} galaxies of the González (1993: G93) sample. The analysis is performed by means of several diagnostic planes. In the H$β$ vs. [MgFe] plane, which is perhaps best suited to infer the age of the stellar populations, the peculiar galaxies in our sample show nearly the same distribution of the {\sl normal} galaxies in the G93 sample. There is however a number of peculiar galaxies with much stronger H$β$. Does this mean that the scatter in the H$β$ vs. [MgFe] plane, of normal, shell- and pair-galaxies has a common origin, perhaps a secondary episode of star formation? We suggest that, owing to their apparent {\sl youth}, shell- and pair-galaxies should have experienced at least one interaction event after their formation. The explanation comes natural for shell- and pair-galaxies where the signatures of interactions are evident. It is more intrigued in {\sl normal} galaxies (perhaps other causes may concur).

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arXiv1999

Modeling Dust on Galactic SED: Application to Semi-Analytical Galaxy Formation Models

We present the basic features and preliminary results of the interface between our spectro-photometric model GRASIL (that calculates galactic SED from the UV to the sub-mm with a detailed computation of dust extinction and thermal reemission) with the semi-analytical galaxy formation model GALFORM (that computes galaxy formation and evolution in the hierarchical scenario, providing the star formation history as an input to our model). With these two models we are able to synthetize simulated samples of a few thousands galaxies suited for statistical studies of galaxy properties to investigate on galaxy formation and evolution. We find good agreement with the available data of SED and luminosity functions.

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arXiv1999

New photometric models of galactic evolution applied to the HDF

We summarize our modelling of galaxy photometric evolution (GRASIL code). By including the effects of dust grains and PAHs molecules in a two phases clumpy medium, where clumps are associated to star forming regions, we reproduce the observed UV to radio SEDs of galaxies with star formation rates from zero to several hundreds $M_\odot/yr$. GRASIL is a powerful tool to investigate the star formation, the initial mass function and the supernovae rate in nearby starbursts and normal galaxies, as well as to predict the evolution of luminosity functions of different types of galaxies at wavelengths covering six decades. It may be interfaced with any device providing the star formation and metallicity histories of a galaxy. As an application, we have investigated the properties of early-type galaxies in the HDF, tracking the contribution of this population to the cosmic star-formation history, which has a broad peak between z=1.5 and 4. To explain the absence of objects at z $\gsim$ 1.3, we suggest a sequence of dust-enshrouded merging-driven starbursts in the first few Gyrs of galaxies lifetime. We are at present working on a complementary sample of late type objects selected in a similar way.

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arXiv1999

Star Formation History of Early-Type Galaxies in Low Density Environments V. Blue line-strength indices for the nuclear region

We analyze the star formation properties of a sample of 21 shell galaxies and 30 early-type galaxies members of interacting pairs, located in low density environments (Longhetti et al 1998a, 1998b). The study is based on new models developed to interpret the information coming from `blue' H$δ$/FeI, H+K(CaII) and \D4000 line-strength indices proposed by Rose (1984; 1985) and Hamilton (1985). We find that the last star forming event that occurred in the nuclear region of shell galaxies is statistically old (from 0.1 up to several Gyr) with respect to the corresponding one in the sub-sample of pair galaxies (<0.1 Gyr or even ongoing star formation). If the stellar activity is somehow related to the formation of shells, as predicted by several dynamical models of galaxy interaction, shells have to be considered long lasting structures. Since pair members show evidence of very recent star formation, we suggest that either large reservoirs of gas have to be present to maintain active star formation, if these galaxies are on periodic orbits, or most of the pair members in the present sample are experiencing unbound encounters.

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arXiv1998

Early-Type Galaxies in the Hubble Deep Field: The Star Formation History

We have investigated the properties of a complete K-band selected sample of 35 elliptical and S0 galaxies brighter than K=20.15 in the Hubble Deep Field, as representative of the field galaxy population. This sample has been derived from deep K-band image by the KPNO-IRIM camera, by applying a rigorous morphological classification scheme based on quantitative analyses of the surface brightness profiles. The broad-band spectra of the sample galaxies allow us to date their dominant stellar populations. The majority of bright early-types in this field are found at redshifts z<1.3 to display colors indicative of a fairly wide range of ages (typically 1.5 to 3 Gyrs). We find that the major episodes of star-formation building up typical M_star galaxies have taken place during a wide redshift interval 1 1.3, which should be detectable during the luminous star-formation phase expected to happen at these redshifts. Obvious solutions are a) that the merging events imply perturbed morphologies which prevent selecting them by our morphological classification filter, or b) that a dust-polluted ISM obscures the (either continuous or episodic) events of star-formation. We conclude that the likely solution is a combination thereof, i.e. a set of dust-enshrouded merging-driven starbursts occurring during the first few Gyrs of the galaxy's lifetime.

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arXiv1998

Spectro-Photometric Evolution of Elliptical Galaxies. III. Infall models with gradients in mass and star formation

In this study we present a simple model of elliptical galaxies aimed at interpreting the gradients in colours and narrow band indices observed across these systems. Salient features of the model are the gradients in mass density and star formation and infall of primordial gas aimed at simulating the collapse of a galaxy into the potential well of dark matter. Adopting a multi-zone model we follow in detail the history of star formation, gas consumption, and chemical enrichment of the galaxy and also allow for the occurrence of galactic winds according to the classical supernova (and stellar winds) energy deposit. The outline of the model, the time scale of gas accretion and rate of star formation as a function of the galacto-centric distance in particular, seek to closely mimic the results from Tree-SPH dynamical models. Although same specific ingredients of the model can be questioned from many points of view (of which we are well aware) the model predictions have to be considered as a gross tool for exploring the consequences of different receipts of gas accretion and star formation in which the simple one-zone scheme is abandoned. With the aid of this model we discuss the observational data on the gradients in metallicity, colour, and narrow band indices across elliptical galaxies.

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arXiv1998

Ages and Metallicities in Elliptical Galaxies from the H_beta, , and Mg2 Diagnostics

Systematic variations in the line strength indices H_beta, Mg2, and are observed across elliptical galaxies and from one object to another. Furthermore arguments are given for an enhancement of Mg (\alfa in general) with respect to Fe toward the center of these galaxies. In this study we have investigated the ability of the H_beta, Mg2 and diagnostics to assess the metallicity, [Mg/Fe] ratios, and ages of elliptical galaxies. To this aim, first we derive basic calibrations for the variations dH_beta, dMg2 and d as a function of Dlog(t), Dlog(Z/Zo), and D[Mg/Fe]. Second, we analyze how the difference dH_beta, dMg2, and d between the external and central values translates into D[Mg/Fe], Dlog(Z/Zo), and Dlog(t). Third, we explore the variation from galaxy to galaxy of the nuclear values of H_beta, Mg2, and limited to a sub-sample of the Gonzales (1993) list. The differences dH_beta, dMg2, and d are converted into the differences Dlog(t), Dlog(Z/Zo), and D[Mg/Fe]. Various correlations among the age, metallicity, and enhancement variations are explored. In particular we thoroughly examine the relationship Dlog(t)-Mv. It is found that a sort of age limit is likely to exist in this plane, traced by galaxies with mild or no sign of rejuvenation. In these objects, the duration of the star forming activity is likely to have increased at decreasing galactic mass. The results are discussed in the context of current theories of galaxy formation and evolution. i.e. merger and isolation. Finally, the suggestion is advanced that models with time and space dependent IMF might be able to alleviate some of the difficulties encountered by the standard SN-driven galactic wind model.

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