arXiv subjects

Robert Antonucci

Publications and source records attributed to Robert Antonucci.

62 records · Page 4Linked to original sources

arXiv1998

Below the Lyman Edge: UV Polarimetry of Quasars

The Lyman edge at 912 Åis an important diagnostic region for studying quasi-stellar objects (QSOs). In particular, it reveals a great deal about the physical conditions within the atmospheres of accretion disks, a ubiquitous component of QSO theories. A robust prediction of accretion disk models is a significant polarization due to electron scattering just longward (in wavelength) of the Lyman edge because of the wavelength dependence of the Hydrogen absorption opacity. Observations of the Lyman edge regions of QSOs have shown scant evidence for the predicted features-few QSOs show the broad, partial Lyman edges expected to be common according to most theories, and none show the high polarizations expected longward of the Lyman edge. Still, polarization spectra of a small number of QSOs have shown a rising polarization (up to 20%) at wavelengths shortward of the Lyman edge. We have now doubled our sample of intermediate-redshift QSOs observed with the HST/FOS spectropolarimeter to determine the amount of polarization on both sides of the Lyman limit. For this new sample of six objects, polarizations are low and mostly consistent with zero below the Lyman edge. Another important result of the new data is that it strengthens the conclusion that quasars are generally not polarized significantly just longward of the Lyman edge at $\sim 1000 Å. There is no significant statistical wavelength dependence to the polarization longward of the Lyman edge indicating that simple plane-parallel atmospheres with scattering-dominated opacity are not significant sources of UV flux in quasars.

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arXiv1998

Spectropolarimetry of 3CR 68.1: A Highly Inclined Quasar

We present Keck spectropolarimetry of the highly polarized radio-loud quasar 3CR 68.1 (z=1.228, V=19). The polarization increases from 5 in the red (4000 A rest-frame) to >10% in the blue (1900 A rest-frame). The broad emission lines are polarized the same as the continuum, which shows that 3CR 68.1 is not a blazar as it has sometimes been regarded in the past. We also present measurements of the emission lines and a strong, blueshifted, associated absorption line system, as well as a detection at the emission-line redshift of Ca II K absorption, presumably from stars in the host galaxy. 3CR 68.1 belongs to an observationally rare class of highly polarized quasars that are neither blazars nor partially obscured radio-quiet QSOs. Taking into account 3CR 68.1's other unusual properties, such as its extremely red spectral energy distribution and its extreme lobe dominance, we explain our spectropolarimetric results in terms of unified models. We argue that we have a dusty, highly inclined view of 3CR 68.1, with reddened scattered (polarized) quasar light diluted by even more dust-reddened quasar light reaching us directly from the nucleus.

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arXiv1997

Detection of CO(3-2) Emission at z=2.64 from the Gravitationally Lensed Quasar MG 0414+0534

We have detected CO(3-2) line emission from the gravitationally lensed quasar MG 0414+0534 at redshift 2.64, using the IRAM Plateau de Bure Interferometer. The line is broad, with Delta v_FWHM = 580 km/s. The velocity-integrated CO flux is comparable to, but somewhat smaller than, that of IRAS F10214+4724 and the Cloverleaf quasar (H1413+117), both of which are at similar redshifts. The lensed components A1+A2 and B were resolved, and separate spectra are presented for each. We also observed the unlensed radio quiet quasar PG 1634+706 at z=1.33, finding no significant CO emission.

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arXiv1997

Spectropolarimetry of FIRST BAL QSOs

We present Keck spectropolarimetry of two rare low-ionization broad absorption line (BAL) QSOs, FIRST J084044.5+363328 and FIRST J155633.8+351758, that also exhibit narrow absorption lines from metastable excited levels of Fe II (``Iron Lo-BALs''). These QSOs were discovered in optical follow-ups to a deep radio survey; FIRST J155633.8+351758 is radio-loud, the first BAL QSO so identified. FIRST J084044.5+363328 is highly polarized and exhibits many features found in other BAL QSOs, and the polarization and its position angle vary in a complicated manner across the metastable Fe II absorption lines, suggesting that more than one mechanism is at work, or that the system geometry is complex. FIRST J155633.8+351758 may be the most highly polarized BAL QSO known, and exhibits other unusual polarization properties compared to other highly polarized BAL QSOs. Currently available data cannot yet discriminate among the possible lines of sight to BAL QSOs (edge-on, pole-on, or random).

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arXiv1997

Triggered Star Formation in a Massive Galaxy at z=3.8: 4C41.17

Spectropolarimetric observations obtained with the W. M. Keck Telescope of the z=3.8 radio galaxy 4C41.17 show that the UV continuum emission from this galaxy, which is aligned with the radio axis, is unpolarized (P[2sigma] < 2.4%). This implies that scattered AGN light, which is generally the dominant contributor to the rest-frame UV emission in z~1 radio galaxies, is unlikely to be a major component of the UV flux from 4C41.17. The spectrum shows absorption lines that are similar to those detected in the spectra of the recently discovered population of star forming galaxies at z~2-3. A galaxian outflow may contribute partially to the low ionization absorption lines; however, the high velocity wings of the high ionization lines are unlikely to be dominated by a galaxian wind since the implied outflow mass is very large. The detection of stellar absorption lines, the shape of the SiIV profile, the unpolarized continuum, the inability of any AGN-related processes to account for the UV flux, and the similarity of the UV continuum spectra of 4C41.17 and the nearby starburst region NGC 1741B1 suggest that the UV light in 4C41.17 is dominated by young stars. If so, the implied star-formation rate is roughly 140-1100Msun/yr. We discuss the possibility that star formation in 4C41.17 was triggered by the radio source. Our data are consistent with the hypothesis that 4C41.17 is undergoing its major epoch of star formation at z~4, and that by z~1 it will have evolved to have spectral and morphological properties similar to those observed in known z~1 powerful radio galaxies.

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arXiv1997

Spectropolarimetry of FIRST 0840+3633

We present Keck spectropolarimetry of a rare ``Iron Lo-BALQSO,'' FIRST 0840+3633. The continuum is 4% polarized near 2000 angstroms rest-frame, but falls to 2% at longer wavelengths, and maintains a relatively constant position angle of 50 degrees. The emission lines are unpolarized. The polarization increases up to 8% in the low-ionization absorption troughs of Mg II 2800 and Al III 1860. The polarization and its position angle vary in a complicated manner across the metastable Fe II absorption lines, suggesting that more than one mechanism is at work or that the system geometry is complex.

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arXiv1997

Multiple CO Transitions, CI, and HCN from the Cloverleaf Quasar

New millimeter-wavelength emission lines are reported for the Cloverleaf (H1413+117), a gravitationally lensed quasar at redshift 2.56. High signal-to-noise ratio spectra of four lines in CO (J=3-2, J=4-3, J=5-4, and J=7-6) have been obtained and are modeled using an escape probability formalism. The line brightness temperatures are flat or rising between CO(3-2) and CO(4-3) and then drop to the CO(5-4) and CO(7-6) lines; this falloff suggests that the optical depths in the CO lines are modest (tau_{4-3} < 3), and that the gas is relatively warm (T ~ 100 K) and dense (n_H2 > 3 X 10^3 cm^{-3}). The neutral carbon fine structure line CI(3P1-3P0) was detected with moderate SNR on two separate occasions, and appears secure. The HCN(4-3) line is somewhat more tentative, as it was observed only once and detected with moderate SNR. However, its brightness ratio of 0.25 relative to the CO(4-3) line is very similar to the ratio seen in luminous infrared galaxies. The detectability of the Cloverleaf in molecular and neutral atomic transitions is largely due to high emissivity of the gas and magnification from gravitational lensing, rather than an extremely large mass of gas.

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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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