The Highly Polarized Hidden Quasar IRAS 09104+4109: A Double-lobed Radio Source in a Rich Cluster
Abstract
IRAS 09104+4109 is the second most luminous active galactic nucleus (AGN) so far discovered by IRA S, with Lbol ∼ 1012.6 h-2 Lsun. Kleinmann et al. have shown it to have a strong Seyfert 2 emission-line spectrum (z = 0.442), with a range of excitations and very large equivalent widths but no broad lines. They suggested that this V ∼ 19 cD galaxy hides a luminous QSO that ionizes the narrow-line region and heats dust within ∼130 pc of the central engine. Here we further investigate the geometry and environment of this extreme AGN by means of broad-band polarimetry, spectropolarimetry, new spectroscopy, and radio imaging. The observed polarization is very high and strongly wavelength-dependent, increasing from 4% at 7500 Å to 21 % at 3600 Å. The position angle is wavelength-independent with ±5°. The optical spectrum is dominated by unpolarized narrow-line emission and starlight. After correcting for dilution by these unpolarized components, we show that the remaining spectrum is highly polarized (∼18%), and, within the uncertainties, this polarization is wavelength-independent. Spectrophotometry shows an ultraviolet excess and broad Mg II λ2798 emission that provide strong evidence that we are viewing a luminous quasar obscured in direct light but visible in light polarized by scattering. The polarization position angle is perpendicular to the position angle of an extended high-ionization [O III] emission region discovered by Kleinmann et al., so we postulate that this extended emission is ionized by the same collimated nuclear continuum source that gives rise to the scattered light. Assuming that the polarization arises in an optically thin scattering cone, we derive an opening angle less than 39° and inclination ∼35°, and compare this geometry with the dust covering factor implied by the infrared luminosity.
If the observed wavelength dependence of polarization for IRAS 09104+4109 is typical of radio galaxies, it can explain the trend of increasing polarization with increasing redshift found by Tadhunter et al., strongly supporting their suggestion that this trend is the result of decreasing dilution of scattered light by unpolarized starlight, toward the shorter wavelengths. IRAS 09104+4109 is also unusual in being the most powerful radio source associated with an IRAS-discovered object, and our new radio images show classic linear core and double-lobed structure. IRAS 09104+4109 is also a dominant cD galaxy in a rich, flattened galaxy cluster of intermediate redshift whose major axis is perpendicular to the radio jet. These facts suggest that this object may provide a clue to the relation between galaxy environment and strong radio jets.- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 1993
- DOI:
- 10.1086/173145
- Bibcode:
- 1993ApJ...415...82H
- Keywords:
-
- GALAXIES: ACTIVE;
- GALAXIES: CLUSTERING;
- GALAXIES: INDIVIDUAL ALPHANUMERIC: IRAS 09104+4109;
- GALAXIES: ELLIPTICAL AND LENTICULAR;
- CD;
- GALAXIES: QUASARS: GENERAL;
- POLARIZATION;
- RADIO CONTINUUM: GALAXIES