arXiv subjects

A. Cimatti

Publications and source records attributed to A. Cimatti.

549 records · Page 31Linked to original sources

arXiv1999

The Star Formation History of Ellipticals from the Fossil Evidence

The current evidence about the age of stellar populations in elliptical galaxies is reviewed. The case for the bulk of stars in galactic spheroids (ellipticals and bulges) having formed at high redshift (z>~3) is now compelling, both for cluster and well as field galaxies. Whether the assembly of ellipticals is deferred to lower redshifts compared to the formation of their stars remains controversial, and we mention ongoing observational programs that are designed to solve the issue. We present preliminary results of a pilot project aimed at ascertain the nature of extremely red galaxies (ERGs) with near-IR spectroscopy at the VLT.

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arXiv1999

New Clues on the Nature of Extremely Red Galaxies

We present near-infrared VLT-UT1+ISAAC spectroscopy of a sample of 9 extremely red galaxies (ERGs) with R-K>5 and K<19.0. Neither strong emission lines (F_lim<1-5 10^{-16} erg s^{-1}cm^{-2}) nor continuum breaks are detected. From near-infrared spectrophotometry, complemented with broad-band optical and near-IR photometry, we estimate ``spectro-photometric'' redshifts to be in the range of 0.8 0.8. We discuss the general implications of our findings in relation with the problem of the formation of early type galaxies.

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arXiv1999

GRB 990510: linearly polarized radiation from a fireball

Models for gamma-ray burst afterglows envisage an hyper-relativistic fireball that is decelerated in the ambient medium around the explosion site. This interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large-scale ordered magnetic field, synchrotron radiation can be highly polarized. The optical transient associated with GRB 990510 was observed ~18.5 hr after the event and linear polarization in the R band was measured at a level of 1.7 +/- 0.2 %. This is the first detection of linear polarization in the optical afterglow of a gamma-ray burst. We exclude that this polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma-ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.

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arXiv1999

An Atlas of Monte Carlo Models of Dust Extinction in Galaxies for Cosmological Applications

We present an extensive study of the radiative transfer in dusty galaxies based on Monte Carlo simulations. The main output of these simulations are the attenuation curves ${\cal A}_λ$ (i.e. the ratio between the observed, dust extinguished, total intensity to the intrinsic unextinguished one of the galaxy as a function of wavelength). We have explored the dependence of ${\cal A}_λ$ on a conspicuous set of quantities (Hubble type, inclination, dust optical thickness, dust distribution and extinction properties) for a large wavelength interval, ranging from 1250Åto the K band, thus finally providing a comprehensive atlas of dust extinction in galaxies, which is electronically available. This study is particularly suitable for inclusion into galaxy formation evolution models and to directly interpret observational data on high redshift galaxies.

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arXiv1999

The NV/CIV ratio in high redshift radio galaxies

Deep Keck spectropolarimetry has been obtained of a sample of powerful radio galaxies with $z \sim 2.5 \pm 1$. In addition to a clear 2200Ådust scattering/extinction signature and an inverse correlation between the degree of continuum polarization and the Lyman-αemission line strength (relative to C IV), we find an intriguing positive correlation between the N~V/C IV ratio and the polarization. The line ratio, which varies by almost a dex amongst objects having a very similar ionization state, is most likely to indicate an abundance ratio. We speculate that the correlation with polarization represents a connection between nitrogen enhancement and dust production following a major starburst event connected with the birth of the quasar.

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arXiv1998

Dust in 3C324

The results of a deep submillimetre observation using SCUBA of the powerful radio galaxy 3C324, at redshift z=1.206, are presented. At 850 microns, emission from the location of the host radio galaxy is marginally detected at the 4.2 sigma level, 3.01 +/- 0.72 mJy, but there is no detection of emission at 450 microns to a 3 sigma limit of 21 mJy. A new 32 GHz radio observation using the Effelsberg 100m telescope confirms that the sub-millimetre signal is not associated with synchrotron emission. These observations indicate that both the mass of warm dust within 3C324, and the star formation rate, lie up to an order of magnitude below the values recently determined for radio galaxies at z = 3 to 4. The results are compared with dust masses and star formation rates derived in other ways for 3C324.

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arXiv1998

BeppoSAX Observations of 2 Jy Lobe-dominated Broad-Line Sources: the Discovery of a Hard X-ray Component

We present new BeppoSAX LECS, MECS, and PDS observations of five lobe-dominated, broad-line active galactic nuclei selected from the 2 Jy sample of southern radio sources. These include three radio quasars and two broad-line radio galaxies. ROSAT PSPC data, available for all the objects, are also used to better constrain the spectral shape in the soft X-ray band. The collected data cover the 0.1 - 10 keV energy range, reaching 40 keV for one source. Detailed spectral fitting shows that all sources have a flat hard X-ray spectrum with energy index alpha_x ~ 0.75 in the 2 - 10 keV energy range. This is a new result, which is at variance with the situation at lower energies where these sources exhibit steeper spectra. Spectral breaks ~0.5 at 1 - 2 keV characterize the overall X-ray spectra of our objects. The flat, high-energy slope is very similar to that displayed by flat-spectrum/core-dominated quasars, which suggests that the same emission mechanism (most likely inverse Compton) produces the hard X-ray spectra in both classes. Contrary to the optical evidence for some of our sources, no absorption above the Galactic value is found in our sample. Finally, a (weak) thermal component is also present at low energies in the two broad-line radio galaxies included in our study.

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arXiv1997

Spectropolarimetry of the Circinus Galaxy

High quality 4500-6800 A spectropolarimetric observations of the Circinus galaxy are reported. These show polarized and relatively broad (FWHM~3300 km/s) Halpha (as well as marginal Hbeta) arising from a <3" (<60 pc) region centered on the nucleus and coincident with the the peak of the narrow line region. Broad Halpha might also be present ~8" (160 pc) SE from the nucleus and close to the dust lane, but the result is only marginal and should be verified by higher s/n data. The continuum is dominated by stellar emission with <3% contribution from the scattered AGN continuum, and all the light (i.e. all the stellar continuum and narrow lines) is polarized, most probably by transmission through and scattering by dust in the disk. The data are compatible with a simple model where the scattered broad lines and AGN emission are quite polarized (P~25%) but account for only 1% of the observed continuum. The scattered broad Halpha has an equivalent width EW~400 A, a flux similar to the narrow Halpha and a luminosity ~5 10^5 Lo. Assuming an efficiency of 1% for the BLR mirror, the intrinsic luminosity of the broad Halpha is roughly 5 10^7 Lo and ~0.5% of the FIR luminosity, a ratio similar to those found in type 1 Seyferts. Specific prediction for future observations are also presented.

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arXiv1995

Intrinsic Interstellar Absorption in two Radio Galaxies at z=0.8

We have found a strong absorption feature at 2598Å~ in the spectra of two distant radio galaxies, which we interpret as due to interstellar FeII. We discuss this interpretation and the possibility of obtaining important information on the evolution of the ISM in galaxies, to be compared with the results of the study of intervening and associated absorptions in quasars.

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