排序方式： 共有43条查询结果，搜索用时 15 毫秒
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J. Fritz † A. Franceschini E. Hatziminaoglou 《Monthly notices of the Royal Astronomical Society》2006,366(3):767-786
We describe improved modelling of the emission by dust in a toroidal-like structure heated by a central illuminating source within active galactic nuclei (AGNs). We have chosen a simple but realistic torus geometry, a flared disc, and a dust grain distribution function including a full range of grain sizes. The optical depth within the torus is computed in detail taking into account the different sublimation temperatures of the silicate and graphite grains, which solves previously reported inconsistencies in the silicate emission feature in type 1 AGNs. We exploit this model to study the spectral energy distributions (SEDs) of 58 extragalactic (both type 1 and type 2) sources using archival optical and infrared data. We find that both AGN and starburst contributions are often required to reproduce the observed SEDs, although in a few cases they are very well fitted by a pure AGN component. The AGN contribution to the far-infrared luminosity is found to be higher in type 1 sources, with all the type 2 requiring a substantial contribution from a circumnuclear starburst. Our results appear in agreement with the AGN unified scheme, because the distributions of key parameters of the torus models turn out to be compatible for type 1 and type 2 AGNs. Further support to the unification concept comes from comparison with medium-resolution infrared spectra of type 1 AGNs by the Spitzer observatory, showing evidence for a moderate silicate emission around 10 μm, which our code reproduces. From our analysis we infer accretion flows in the inner nucleus of local AGNs characterized by high equatorial optical depths ( AV ≃ 100) , moderate sizes ( R max < 100 pc) and very high covering factors ( f ≃ 80 per cent) on average. 相似文献
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Giulia Rodighiero Alberto Franceschini Giovanni Fasano 《Monthly notices of the Royal Astronomical Society》2001,324(2):491-497
We have extended our previous analysis of morphologically selected elliptical and S0 galaxies in the Hubble Deep Field (HDF) North to include Hubble Space Telescope ( HST ) data in the HDF South and the HDFS–NICMOS areas. Our final sample amounts to 69 E/S0 galaxies with K <20.15 over an area of 11 arcmin2 . Although a moderately small number over a modest sky area, this sample benefits from the best imaging and photometric data available on high-redshift galaxies. Multi-waveband photometry allows us to estimate with good accuracy the redshifts for the majority of these galaxies, which lack a spectroscopic measure. We confirm our previous findings that massive E/S0s tend to disappear from flux-limited samples at z >1.4. This adds to the evidence that the rest-frame colours and spectral energy distributions (SEDs) of the numerous objects found at 0.8< z <1.2 are inconsistent with a very high redshift of formation for the bulk of stars, while they are more consistent with protracted (either continuous or episodic) star formation down to z ≤1. These results based on high-quality imaging on a small field can be complemented with data from colour-selected extremely red objects (EROs) on much larger sky areas: our claimed demise of E/S0s going from z =1 to z =1.5 is paralleled by a similarly fast decrease in the areal density of EROs when the colour limit is changed from ( R − K )=5 to ( R − K )=6 (corresponding to z ≃1 and z ≃1.3 respectively). Altogether, the redshift interval from 1 to 2 seems to correspond to a very active phase for the assembly of massive E/S0 galaxies in the field, and also probably one where a substantial fraction of their stars are formed. 相似文献
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Gian Luigi Granato Laura Silva Luigi Danese Giulia Rodighiero Alberto Franceschini Gianni Fasano Alessandro Bressan 《Astrophysics and Space Science》2001,276(2-4):973-978
We summarize our modelling of galaxy photometric evolution (the GRASIL code). By including the effects of dust grains and
PAH molecules in a two-phase clumpy medium, where clumps are associated with star-forming regions, we reproduce the observed
UV to radio SEDs of galaxies with star formation rates from zero to several hundred M
⊙ yr-1.GRASIL is a powerful tool for investigating star formation, the initial mass function and the supernova rate in nearby starbursts
and normal galaxies, as well as for predicting the evolution of luminosity functions of different types of galaxies at wavelengths
covering six decades. It may be interfaced with any device to provide 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 ≳ 1.3, we suggest a sequence of dust-enshrouded merger-driven starbursts in the first few Gyr of galaxy lifetimes. We are
at present working on a complementary sample of late-type objects selected in a similar way.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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A. Franceschini S. Vercellone & A. C. Fabian 《Monthly notices of the Royal Astronomical Society》1998,297(3):817-824
Using recently published estimates — based on high spatial resolution spectroscopy — of the mass M BH of nuclear black holes for a sample of nearby galaxies, we explore the dependence of galaxy nucleus emissivity at various wavelengths on M BH . We confirm an almost linear scaling of the black hole mass with the baryonic mass of the host spheroidal galaxy. A remarkably tight relationship is also found with both nuclear and total radio centimetric flux, with a very steep dependence of the radio flux on M BH ( P ∝ M 2.5 BH ). The high-frequency radio power is thus a very good tracer of a supermassive black hole, and a good estimator of its mass. This, together with the lack of significant correlations with the low-energy X-ray and far-IR flux, supports the view that advection-dominated accretion is ruling the energy output in the low accretion rate regime. Using the tight dependence of total radio power on M BH and the rich statistics of radio emission of galaxies, we derive an estimate of the mass function of remnants in the nearby Universe. This is compared with current models of quasar and active galactic nucleus (AGN) activity and of the origin of the hard X-ray background (HXRB). As for the former, continuous long-lived AGN activity is excluded by the present data with high significance, whereas the assumption of a short-lived, possibly recurrent, activity pattern gives remarkable agreement. The presently estimated black hole mass function also implies that the HXRB has been produced by a numerous population (∼ 10−2 Mpc−3 ) of moderately massive ( M BH ∼ 107 M⊙) black holes. 相似文献
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Andreas Efstathiou Seb Oliver Michael Rowan-Robinson C. Surace T. Sumner P. Héraudeau M. J. D. Linden-Vørnle D. Rigopoulou S. Serjeant R. G. Mann C. J. Cesarsky L. Danese A. Franceschini R. Genzel A. Lawrence D. Lemke R. G. McMahon G. Miley J.-L. Puget B. Rocca-Volmerange 《Monthly notices of the Royal Astronomical Society》2000,319(4):1169-1177
We present results and source counts at 90 μm extracted from the preliminary analysis of the European Large Area ISO Survey (ELAIS). The survey covered about 12 deg2 of the sky in four main areas and was carried out with the ISOPHOT instrument onboard the Infrared Space Observatory ( ISO ). The survey is at least an order of magnitude deeper than the IRAS 100-μm survey and is expected to provide constraints on the formation and evolution of galaxies. The majority of the detected sources are associated with galaxies on optical images. In some cases the optical associations are interacting pairs or small groups of galaxies, suggesting that the sample may include a significant fraction of luminous infrared galaxies. The source counts extracted from a reliable subset of the detected sources are in agreement with strongly evolving models of the starburst galaxy population. 相似文献
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Stephen Serjeant † Alberto Carramiñana † Eduardo Gonzáles-Solares Phillipe Héraudeau Raúl Mújica † Ismael Perez-Fournon Nicola Sedgwick † Michael Rowan-Robinson Alberto Franceschini Thomas Babbedge Carlos del Burgo Paolo Ciliegi reas Efstathiou Fabio La Franca Carlotta Gruppioni David Hughes Carlo Lari Seb Oliver Francesca Pozzi Manfred Stickel Mattia Vaccari 《Monthly notices of the Royal Astronomical Society》2004,355(3):813-818
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G. Rodighiero D. Fadda A. Franceschini C. Lari 《Monthly notices of the Royal Astronomical Society》2005,357(2):449-460
We present the optical identifications of a 95-μm ISOPHOT sample in the Lockman hole over an area of approximately half a deg2 . The Rodighiero et al. catalogue includes 36 sources, making up a complete flux-limited sample for S 95 μm ≥ 100 mJy . Reliable sources were detected, with decreasing but well-controlled completeness, down to S 95 μm ≃ 20 mJy . We have combined mid-infrared (IR) and radio catalogues in this area to identify the potential optical counterparts of the far-IR sources. We found 14 radio and 13 15-μm associations, 10 of which have both associations. For the 11 sources with spectroscopic redshift, we have performed a spectrophotometric analysis of the observed spectral energy distributions (SEDs). Four of these 95-μm sources have been classified as faint IR (FIR) galaxies ( L FIR < 1. e 11 L⊙ ) , six as luminous IR galaxies (LIRGs) and only one as an ultraluminous IR galaxy (ULIRG). We have discussed the redshift distribution of these objects, comparing our results with evolutionary model predictions 95 and 175 μm. Given their moderate distances (the bulk of the closest spectroscopically identified objects lying at z < 0.2 ), their luminosities and star formation rates (SFR; median value ∼ 10 M⊙ yr−1 ), the sources unveiled by ISOPHOT at 95 μm seem to correspond to the low redshift ( z < 0.3) FIRBACK 175-μm population, composed of dusty, star-forming galaxies with moderate SFRs. We computed and compared different SFR estimators, and found that the SF derived from the bolometric IR luminosity is well correlated with that computed from the radio and mid-IR fluxes. 相似文献