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Journal ArticleDOI

Modelling galaxy spectra in presence of interstellar dust – II. From the ultraviolet to the far-infrared

11 Aug 2006-Monthly Notices of the Royal Astronomical Society (Oxford University Press)-Vol. 370, Iss: 3, pp 1454-1478
TL;DR: In this article, the spectral energy distributions (SEDs) of different morphological types of galaxies are derived by using a simple geometrical model for each type of galaxy, based on a robust model of chemical evolution that assumes a suitable prescription for gas infall, initial mass function, star formation rate and stellar ejecta.
Abstract: In this paper, we present spectrophotometric models for galaxies of different morphological type whose spectral energy distributions (SEDs) take into account the effect of dust in absorbing UV-optical light and re-emitting it in the infrared (IR). The models contain three main components: (i) the diffuse interstellar medium (ISM) composed of gas and dust whose emission and extinction properties have already been studied in detail by Piovan et al. (2006), (ii) the large complexes of molecular clouds (MCs) in which new stars are formed and (iii) the stars of any age and chemical composition. The galaxy models stand on a robust model of chemical evolution that assuming a suitable prescription for gas infall, initial mass function, star formation rate and stellar ejecta provides the total amounts of gas and stars present at any age together with their chemical history. The chemical models are taylored in such a way to match the gross properties of galaxies of different morphological type. In order to describe the interaction between stars and ISM in building up the total SED of a galaxy, one has to know the spatial distribution of gas and stars. This is made adopting a simple geometrical model for each type of galaxy. The total gas and star mass provided by the chemical model are distributed over the whole volume by means of suitable density profiles, one for each component and depending on the galaxy type (spheroidal, disk and disk plus bulge). The galaxy is then split in suitable volume elements to each of which the appropriate amounts of stars, MCs and ISM are assigned. Each elemental volume bin is at the same time source of radiation from the stars inside and absorber and emitter of radiation from and to all other volume bins and the ISM in between. They are the elemental seeds to calculate the total SED. Using the results for the properties of the ISM and the Single Stellar Populations (SSPs) presented by Piovan et al. (2006) we derive the SEDs of galaxies of different morphological type. First the technical details of the method are described and the basic relations driving the interaction between the physical components of the galaxy are presented. Second, the main parameters are examined and their effects on the SED of three prototype galaxies (a disk, an elliptical and a starburster) are highlighted. The final part of the paper is devoted to assess the ability of our galaxy models in reproducing the SEDs of a few real galaxies of the Local Universe.

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Citations
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Journal ArticleDOI
TL;DR: The GALEV (Galev Evolutionary Evolutionary Models for Galaxies) model as mentioned in this paper describes the evolution of stellar populations in general, of star clusters as well as of galaxies, both in terms of resolved stellar populations and of integrated light properties over cosmological time-scales of ≥13 Gyr.
Abstract: GALEV (GALaxy EVolution) evolutionary synthesis models describe the evolution of stellar populations in general, of star clusters as well as of galaxies, both in terms of resolved stellar populations and of integrated light properties over cosmological time-scales of ≥13 Gyr from the onset of star formation shortly after the big bang until today. For galaxies, GALEV includes a simultaneous treatment of the chemical evolution of the gas and the spectral evolution of the stellar content, allowing for what we call a chemically consistent treatment: we use input physics (stellar evolutionary tracks, stellar yields and model atmospheres) for a large range of metallicities and consistently account for the increasing initial abundances of successive stellar generations. Here we present the latest version of the GALEV evolutionary synthesis models that are now interactively available at http://www.galev.org. We review the currently used input physics, and also give details on how this physics is implemented in practice. We explain how to use the interactive web interface to generate models for user-defined parameters and also give a range of applications that can be studied using GALEV, ranging from star clusters, undisturbed galaxies of various types E–Sd to starburst and dwarf galaxies, both in the local and the high-redshift Universe.

288 citations

Journal ArticleDOI
TL;DR: In this article, the stellar spectral synthesis code Starburst99, the nebular modeling code MAPPINGS III and a one-dimensional dynamical evolution model of H II regions around massive clusters of young stars were combined to generate improved models of the spectral energy distribution (SED) of starburst galaxies.
Abstract: We combine the stellar spectral synthesis code Starburst99, the nebular modeling code MAPPINGS III and a one-dimensional dynamical evolution model of H II regions around massive clusters of young stars to generate improved models of the spectral energy distribution (SED) of starburst galaxies. We introduce a compactness parameter, , which characterizes the specific intensity of the radiation field at ionization fronts in H II regions and which controls the shape of the far-infrared (IR) dust reemission, often referred to loosely as the dust temperature. We also investigate the effect of metallicity on the overall SED and in particular, on the strength of the polycyclic aromatic hydrocarbon (PAH) features. We provide templates for the mean emission produced by the young compact H II regions, the older (10-100 Myr) stars and for the wavelength-dependent attenuation produced by a foreground screen of the dust used in our model. We demonstrate that these components may be combined to produce a excellent fit to the observed SEDs of star formation-dominated galaxies which are often used as templates (Arp 220 and NGC 6240). This fit extends from the Lyman limit to wavelengths of about 1 mm. The methods presented in both this paper and in the previous papers of this series allow the extraction of the physical parameters of the starburst region (star formation rates, star formation rate history, mean cluster mass, metallicity, dust attenuation, and pressure) from the analysis of the pan-spectral SED.

252 citations

Journal ArticleDOI
TL;DR: In this article, the authors combine far-infrared Herschel photometry from the PACS Evolutionary Probe (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time programs with ancillary datasets in the GOODS-N, COSMOS fields, and it is possible to sample the 8-500μm spectral energy distributions (SEDs) of galaxies with at least 7-10 bands.
Abstract: Combining far-infrared Herschel photometry from the PACS Evolutionary Probe (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time programs with ancillary datasets in the GOODS-N, GOODS-S, and COSMOS fields, it is possible to sample the 8–500 μm spectral energy distributions (SEDs) of galaxies with at least 7–10 bands. Extending to the UV, optical, and near-infrared, the number of bands increases up to 43. We reproduce the distribution of galaxies in a carefully selected restframe ten colors space, based on this rich data-set, using a superposition of multivariate Gaussian modes. We use this model to classify galaxies and build median SEDs of each class, which are then fitted with a modified version of the magphys code that combines stellar light, emission from dust heated by stars and a possible warm dust contribution heated by an active galactic nucleus (AGN). The color distribution of galaxies in each of the considered fields can be well described with the combination of 6–9 classes, spanning a large range of far- to near-infrared luminosity ratios, as well as different strength of the AGN contribution to bolometric luminosities. The defined Gaussian grouping is used to identify rare or odd sources. The zoology of outliers includes Herschel-detected ellipticals, very blue z ~ 1 Ly-break galaxies, quiescent spirals, and torus-dominated AGN with star formation. Out of these groups and outliers, a new template library is assembled, consisting of 32 SEDs describing the intrinsic scatter in the restframe UV-to-submm colors of infrared galaxies. This library is tested against L(IR) estimates with and without Herschel data included, and compared to eightother popular methods often adopted in the literature. When implementing Herschel photometry, these approaches produce L(IR) values consistent with each other within a median absolute deviation of 10–20%, the scatter being dominated more by fine tuning of the codes, rather than by the choice of SED templates. Finally, the library is used to classify 24 μm detected sources in PEP GOODS fields on the basis of AGN content, L(60)/L(100) color and L(160)/L(1.6) luminosity ratio. AGN appear to be distributed in the stellar mass (M_∗) vs. star formation rate (SFR) space along with all other galaxies, regardless of the amount of infrared luminosity they are powering, with the tendency to lie on the high SFR side of the “main sequence”. The incidence of warmer star-forming sources grows for objects with higher specific star formation rates (sSFR), and they tend to populate the “off-sequence” region of the M_∗ − SFR − z space.

203 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the dependence of the total-infrared to UV luminosity ratio method for calculating the UV dust attenuation A(UV) from the age of the underlying stellar populations by using a library of spectral energy distributions for galaxies with different star formation histories.
Abstract: We investigate the dependence of the total-infrared (TIR) to UV luminosity ratio method for calculating the UV dust attenuation A(UV) from the age of the underlying stellar populations by using a library of spectral energy distributions for galaxies with different star formation histories. Our analysis confirms that the TIR/UV vs. A(UV) relation varies significantly with the age of the underlying stellar population: i.e. for the same TIR/UV ratio, systems with low specific star formation rate (SSFR) suffer a lower UV attenuation than starbursts. Using a sample of nearby field and cluster spiral galaxies we show that the use of a standard (i.e. age independent) TIR/UV vs. A(UV) relation leads to a systematic overestimate up to 2 magnitudes of the amount of UV dust attenuation suffered by objects with low SSFR and in particular HI-deficient star forming cluster galaxies. This result points out that the age independent $TIR/UV$ vs. $A(UV)$ relation cannot be used to study the UV properties of large samples of galaxies including low star-forming systems and passive spirals. Therefore we give some simple empirical relations from which the UV attenuation can be estimated taking into account its dependence on the age of the stellar populations, providing a less biased view of UV properties of galaxies.

164 citations

Journal ArticleDOI
TL;DR: In this paper, the authors use the latest Padova isochrones, with detailed modelling of the Thermally Pulsing AGB phase, to update theoretical colour-M/L relations in the optical and NIR and discuss the consequences for the estimated stellar masses in external galaxies.
Abstract: Colour-M/L (mass-to-light) relations are a popular recipe to derive stellar mass in external galaxies. Stellar mass estimates often rely on near infrared (NIR) photometry, considered an optimal tracer since it is little affected by dust and by the "frosting" effect of recent star formation episodes. However, recent literature has highlighted that theoretical estimates of the NIR M/L ratio strongly depend on the modelling of the Asymptotic Giant Branch (AGB) phase. We use the latest Padova isochrones, with detailed modelling of the Thermally Pulsing AGB phase, to update theoretical colour-M/L relations in the optical and NIR and discuss the consequences for the estimated stellar masses in external galaxies. We also discuss the effect of attenuation by interstellar dust on colour-M/L relations in the statistical case of large galaxy samples.

157 citations

References
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Journal ArticleDOI
TL;DR: In this paper, images of Arp 220 from 3.45 to 24.5'' resolution are presented which clearly separate the nucleus into at least two components, and two models are presented, both of which fit the observations.
Abstract: Images of Arp 220 from 3.45 to 24.5 $\mu$m with 0.5'' resolution are presented which clearly separate the nucleus into at least two components. The western component is about three times more luminous than the eastern component, but the silicate absorption in the fainter, eastern component is roughly 50% greater than the absorption in the western component. Each component is marginally resolved. The two components seen at 24.5 $\mu$m are identified with the two radio components. The western source most likely coincides with the high extinction disk previously suggested to exist in Arp 220, while the eastern nucleus is identified with a faint highly reddened source seen in HST 2.2 $\mu$m NICMOS images. The two nuclei together account for essentially all of the measured 24.5 $\mu$m flux density. Two models are presented, both of which fit the observations. In one, the majority of the total luminosity is produced in an extended star formation region and in the other most of the luminosity is produced in the compact but extincted regions associated with the two nuclei seen at 24.5 $\mu$m. In both pictures, substantial luminosity at 100 $\mu$m emerges from a component having a diameter of 2--3'' ($\sim$1kpc).

82 citations

Journal ArticleDOI
TL;DR: In this paper, a point-by-point correlation technique was used to find that the molecular gas is closely associated with the 7micron-emitting dust in the star-forming spiral galaxy NGC6946.
Abstract: We present imaging of molecular gas emission in the star-forming spiral galaxy NGC6946. Our CO(1-0) and CO(3-2) images, made at 22" resolution with the IRAM 30-m and the Heinrich Hertz 10-m radio telescopes, are the most extensive CO observations of this galaxy and are among the most extensive observations of molecular gas in any spiral galaxy. The molecular component in NGC6946 is unusually massive, with a ratio of molecular to atomic Hydrogen of 0.57. A star formation efficiency image for NGC6946 ranges by over two orders of magnitude with highest values found in the northeastern spiral arm, and anticorrelates with the 6cm polarized emission image, which traces the regular part of the magnetic field. We analyse the ISM in NGC6946's disk by making 1-D and 2-D comparisons of images made in several wavebands. A point-by-point correlation technique finds that the molecular gas is closely associated with the 7micron-emitting dust. The high correlation found between the MIR emission and the radio continuum at 6cm cannot be due to dust heating and gas ionization in star-forming regions because the thermal radio emission is less correlated with the MIR than the nonthermal emission. A coupling of magnetic fields to gas clouds is proposed as a possible scenario.

79 citations

Journal ArticleDOI
Abstract: We have re-analyzed the 6-12 µm ISO spectrum of the ultra-luminous infrared galaxy Arp 220 with the conclusion that it is not consistent with that of a scaled up version of a typical starburst. Instead, both template fitting with spectra of the galaxies NGC 4418 and M 83 and with dust models suggest that it is best represented by combinations of a typical starburst component, exhibiting PAH emission features, and a heavily absorbed dust continuum which contributes ∼40% of the 6-12 µm flux and likely dominates the luminosity. Of particular significance relative to previous studies of Arp 220 is the fact that the emission feature at 7.7 µm comprises both PAH emission and a broader component resulting from ice and silicate absorption against a heavily absorbed continuum. Extinction to the PAH emitting source, however, appears to be relatively low. We ten- tatively associate the PAH emitting and heavily dust/ice absorbed components with the diffuse emission region and the two compact nuclei respectively identified by Soifer et al. (2002) in their higher spatial resolution 10 µm study. Both the similarity of the absorbed continuum with that of the embedded Galactic protostars and results of the dust models imply that the embedded source(s) in Arp 220 could be powered by, albeit extremely dense, starburst activity. Due to the high extinction, it is not possible with the available data to exclude that AGN(s) also contribute some or all of the observed luminosity. In this case, however, the upper limit measured for its hard X-ray emission would require Arp 220 to be the most highly obscured AGN known.

77 citations

Journal ArticleDOI
Alberto Buzzoni1
TL;DR: In this article, a new set of evolutionary population synthesis models for template galaxies along the Hubble morphological sequence is presented, which account for the individual evolution of the bulge, disc, and halo components, along with bolometric luminosity and colour evolution.
Abstract: We present here a new set of evolutionary population synthesis models for template galaxies along the Hubble morphological sequence. The models, which account for the individual evolution of the bulge, disc, and halo components, provide basic morphological features, along with bolometric luminosity and colour evolution (including Johnson/Cousins, Gunn g, r, i, and Washington C, M, T1, T2 photometric systems) between 1 and 15 Gyr. The luminosity contribution from residual gas is also evaluated, both in terms of nebular continuum and Balmer-line enhancement. Our theoretical framework relies on the observed colours of present-day galaxies, coupled with a minimal set of physical assumptions related to simple stellar population (SSP) evolution theory, to constrain the overall distinctive properties of galaxies at earlier epochs. A comparison with more elaborate photometric models, and with empirical sets of reference spectral energy distributions (SEDs) for early- and late-type galaxies is accomplished, in order to test output reliability and investigate the internal uncertainty of the models. The match with observed colours of present-day galaxies tightly constrain the stellar birth rate, b, which smoothly increases from E to Im types. The comparison with the observed supernova (SN) rate in low-redshift galaxies shows, as well, a pretty good agreement, and allows us to tune up the inferred star formation activity and the SN and hypernova rates among the different galaxy morphological types. Among others, these results could find useful application also in cosmological studies, given for instance the claimed relationship between hypernova events and gamma-ray bursts. One outstanding feature of the back-in-time evolution model is the prevailing luminosity contribution of the bulge at early epochs. As a consequence, the current morphological look of galaxies might drastically change when moving to larger distances, and we discuss here how sensibly this bias could affect the observation (and the interpretation) of high-redshift surveys. In addition to broad-band colours, the modelling of Balmer line emission in disc-dominated systems shows that striking emission lines, like Hα, can very effectively track stellar birth rate in a galaxy. For these features to be useful age tracers as well, however, one should first assess the real change of b versus time on the basis of supplementary (and physically independent) arguments.

73 citations