arXiv subjects

Andrea Cimatti

Publications and source records attributed to Andrea Cimatti.

101 records · Page 6Linked to original sources

arXiv2000

UV Spectropolarimetry of Narrow-line Radio Galaxies

We present the results of UV spectropolarimetry (2000 - 3000A) and far-UV spectroscopy (1500 - 2000A) of two low-redshift narrow-line radio galaxies (NLRGs) taken with the Faint Object Spectrograph onboard the Hubble Space Telescope (HST). Spectropolarimetry of several NLRGs has shown that, by the presence of broad permitted lines in polarized flux spectrum, they have hidden quasars seen through scattered light. Imaging polarimetry has shown that NLRGs including our targets often have large scattering regions of a few kpc to >~10 kpc scale. This has posed a problem about the nature of the scatterers in these radio galaxies. Their polarized continuum has the spectral index similar to or no bluer than that of quasars, which favors electrons as the dominant scattering particles. The large scattering region size, however, favors dust scattering, because of its higher scattering efficiency compared to electrons. In this paper, we investigate the polarized flux spectrum over a wide wavelength range, combining our UV data with previous optical/infrared polarimetry data. We infer that the scattering would be often caused by opaque dust clouds in the NLRGs and this would be a part of the reason for the apparently grey scattering. In the high-redshift radio galaxies, these opaque clouds could be the proto-galactic subunits inferred to be seen in the HST images. However, we still cannot rule out the possibility of electron scattering, which could imply the existence of a large gas mass surrounding these radio galaxies.

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arXiv2000

Co Detection of the Extremely Red Galaxy HR10

CO J = 5 - 4 and J = 2 - 1 emission lines were detected towards the extremely red galaxy (ERG) HR10 (J164502+4626.4) at z=1.44. The CO intensities imply a molecular gas mass M(H2) of 1.6 x 10^{11} h^{-2}_50 solar masses, and, combined with the intensity of the dust continuum, a gas-to-dust mass ratio around 200-400 (assuming galactic values for the conversion factors). The peak of the CO lines are at the same redshift as the [O II]3727 line, but blue- shifted by 430 km s^{-1} from the Halpha line. These CO detections confirm the previous results that HR10 is a highly obscured object with a large thermal far-infrared luminosity and a high star-formation rate. The overall properties of HR10 (CO detection, L_FIR to L'_CO ratio, and FIR to radio flux ratio) clearly favour the hypothesis that its extreme characteristics are related to star-formation processes rather than to a hidden AGN.

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arXiv1999

Extremely Red Galaxies

Preliminary results of a project aiming at unveiling the nature of the extremely red galaxies (ERGs) (objects with colours R-K>6 and I-K>5) found in deep optical-NIR surveys are presented. Very little is known about these objects, the critical issue being whether they are old ellipticals at z>1 or distant star-forming galaxies strongly reddened by dust extinction. We expect to shed light onto the unknown nature of these galaxies by completing our three-step project: (1) the construction of two very deep optical/NIR surveys to select ERGs, (2) subsequent VLT/NIR spectroscopy; (3) observations in the sub-mm region with SCUBA at the JCMT and with MPIfRbolo at the IRAM 30m antenna.

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arXiv1999

BeppoSAX Observations of 2 Jy Lobe-dominated Broad-Line Sources. I. 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 energy range 0.1 - 10 keV, reaching ~ 50 keV for one source (Pictor A). The main result from the spectral fits is that all sources have a hard X-ray spectrum with energy index alpha_x ~ 0.75 in the 2 - 10 keV range. This is at variance with the situation at lower energies where these sources exhibit steeper spectra. Spectral breaks ~ 0.5 at 1 - 2 keV characterize in fact 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. 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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arXiv1998

Extremely Red Galaxies: Old or Dusty ?

The nature of the extremely red galaxies is poorly known, the critical issue being whether they are old ellipticals at $z>1.5$ or distant star-forming galaxies strongly reddened by dust extinction. Here we review the little we know about them, and we present recent and preliminary results from our ongoing submillimeter -- millimeter observations.

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arXiv1998

Vigorous star formation hidden by dust in a galaxy at $z=1.4$

Near-infrared surveys have revealed a substantial population of enigmatic faint galaxies with extremely red optical-to-near-infrared colours and with a sky surface density comparable to that of faint quasars. There are two scenarios for these extreme colours: (i) these distant galaxies have formed virtually all their stars at very high redshifts and, due to the absence of recently formed stars, the colours are extremely red and (ii) these distant galaxies contain large amounts of dust, severely reddening the rest-frame UV--optical spectrum. HR10 ($z = 1.44$) is considered the archetype of the extremely red galaxies. Here we report the detection of the continuum emission from HR10 at 850$μ$m and at 1250$μ$m, demonstrating that HR10 is a very dusty galaxy undergoing a major episode of star formation. Our result provides a clear example of a high-redshift galaxy where the star formation rate inferred from the ultraviolet luminosity would be underestimated by a factor up to 1000, and shows that great caution should be used to infer the global star formation history of the Universe from optical observations only.

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arXiv1998

The UV radiation from $z\sim2.5$ radio galaxies: Keck spectropolarimetry of 4C 23.56 and 4C 00.54

We present the results of deep spectropolarimetry of two powerful radio galaxies at $z\sim2.5$ (4C 00.54 and 4C 23.56) obtained with the W.M. Keck II 10m telescope, aimed at studying the relative contribution of the stellar and non-stellar components to the ultraviolet continuum. Both galaxies show strong linear polarization of the continuum between rest-frame $\sim$1300-2000~Å, and the orientation of the electric vector is perpendicular to the main axis of the UV continuum. In this sense, our objects are like most 3C radio galaxies at $z\sim1$. The total flux spectra of 4C 00.54 and 4C 23.56 do not show the strong P-Cygni absorption features or the photospheric absorption lines expected when the UV continuum is dominated by young and massive stars. The only features detected can be ascribed to interstellar absorptions by SiII, CII and OI. Our results are similar to those for 3C radio galaxies at lower $z$, suggesting that the UV continuum of powerful radio galaxies at $z\sim2.5$ is still dominated by non-stellar radiation, and that young massive stars do not contribute more than $\approx$50% to the total continuum flux at 1500~Å.

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arXiv1997

Dust in high-z radio-loud AGN

We present continuum observations of a small sample of high-redshift, radio-loud AGN (radio galaxies and quasars) aimed at the detection of thermal emission from dust. Seven AGN were observed with IRAM and SEST at 1.25mm; two of them, the radio galaxies 1243+036 ($z \sim 3.6$) and MG1019+0535 ($z \sim 2.8$) were also observed at 0.8mm with the JCMT submillimetre telescope. Additional VLA observations were obtained in order to derive the spectral shape of the synchrotron radiation of MG1019+0535 at high radio frequencies. MG1019+0535 and TX0211$-$122 were expected to contain a large amount of dust based on their depleted Ly$α$ emission. The observations suggest a clear 1.25-mm flux density excess over the synchrotron radiation spectrum of MG1019+0535, suggesting the presence of thermal emission from dust in this radio galaxy, whereas the observations of TX0211$-$122 were not sensitive enough to meaningfully constrain its dust content. On the other hand, our observations of 1243+036 provide a stringent upper limit on the total dust mass of $<10^8$ M$_{\odot}$. Finally, we find that the spectra of the radio-loud quasars in our sample ($2 < z < 4.5$) steepen between rest-frame radio and the far-infrared. We discuss the main implications of our results, concentrating on the dusty radio galaxy, MG1019+0535.

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arXiv1997

Anisotropic [OIII] emission in radio loud AGN

We present the results of spectropolarimetry of a sample of 7 powerful radio galaxies and 2 quasars with $0.07<z<0.35$ obtained to detect possible anisotropies in the [OIII] line emission, which could explain the higher [OIII] luminosity of quasars than that of radio galaxies within the framework of the Unified Model of radio-loud AGN. We detect polarized [OIII] in 4 radio galaxies, consistent with the possibility that a considerable fraction ($\sim 20%$) of the observed [OIII] emission is scattered in a similar way to the hidden nuclear continuum. However, small but detectable rotation between the polarization direction of the line and of the continuum in two radio galaxies shows that the geometry of the emitting regions can be different.

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arXiv1997

On the Dust Extinction in High-z Galaxies and the Case of Extremely Red Objects

We present the preliminary results of Monte Carlo simulations aimed to investigate the effects of realistic dust extinction (absorption + scattering) on the colours of high-z galaxies. In this paper, we concentrate on the case of spheroidal galaxies, and we obtain attenuation curves in the range 0.1-1.2$μ$m for different dust spatial distributions, and for a range of values of the dust optical depth, geometrical thickness and inclination of the dust disk. We find that the resultant curves are strongly dependent on the dust geometrical distribution and optical depth. A serendipitous finding is that the strength of the 2200 Åabsorption feature depends not only on the optical depth, but also on the dust geometrical distribution. As a first application, we test our results on two high-z galaxies with extremely red colours (HR10 and HR14) in order to infer clues on their ages, dust content and dust spatial distribution. We confirm that HR10 must be a very dusty galaxy, and we suggest that its stellar component should be strongly embedded in the dust in order to reproduce the observed extremely red colours.

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arXiv1996

On the Origin of the UV Continuum Emission from the High Redshift Radio Galaxy 3C256

We report spectropolarimetric observations obtained with the W.M. Keck Telescope of the high redshift z=1.824 aligned radio galaxy 3C256. Our observations confirm that the spatially extended UV continuum emission from this galaxy is polarized (P_V = 10.9% +/- 0.9%) with the electric vector perpendicular to the aligned radio and optical major axes (PA = 48.0 +/- 2.4 degrees). This strongly suggests that a significant fraction of the rest frame UV continuum emission from the galaxy is not starlight, but is instead scattered light from a powerful AGN which is hidden from our direct view. The narrow emission lines, including MgII, are unpolarized. The percentage polarization of the continuum emission and the polarization position angle are roughly constant as a function of wavelength. The present data do not permit us to discriminate between cool electrons and dust as the origin of the scattering. But scattering by hot (T > 10^7 K) electrons cannot be the dominant process, since such a population would overproduce X-ray emission. A large population of cooler T ~ 10^4 K electrons could be responsible for both the line emission and the scattered light, but would require that the dust to gas ratio in the scattering cones is 0.001 times smaller than that in our Galaxy and would imply that a large fraction of the baryonic mass in the galaxy is in the ionized component of its ISM. Dust scattering is more efficient, but would result in detectable extinction of the emission line spectrum unless the dust distribution is more highly clumped than the line emitting gas. Finally, we detect a strong (EW = 12A) broad (FWHM=6500 km/s) absorption line centered at 1477A. We conclude that the most likely explanation is that the absorption is due to a high velocity BAL cloud near the nucleus of 3C256.

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