FAR-UV EMISSION PROPERTIES OF FR1 RADIO GALAXIES

Abstract

ABSTRACT The power mechanism and accretion geometry for low-power FR 1 radio galaxies are poorly understood in comparison to those for Seyfert galaxies and QSOs. In this paper, we use the diagnostic power of the Lyα recombination line observed using the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST) to investigate the accretion flows in three well-known, nearby FR 1s: M87, NGC 4696, and Hydra A. The Lyα emission line’s luminosity, velocity structure, and the limited knowledge of its spatial extent provided by COS are used to assess conditions within a few parsecs of the supermassive black hole in these radio-mode active galactic nuclei. We observe strong Lyα emission in all three objects with total luminosity similar to that seen in BL Lacertae objects. M87 shows a complicated emission-line profile in Lyα, which varies spatially across the COS aperture and possibly temporally over several epochs of observation. In both NGC 4696 and M87, the Lyα luminosities ∼1040 erg s−1 are closely consistent with the observed strength of the ionizing continuum in Case B recombination theory and with the assumption of a near-unity covering factor. It is possible that the Lyα-emitting clouds are ionized largely by beamed radiation associated with the jets. Long-slit UV spectroscopy can be used to test this hypothesis. Hydra A and the several BL Lac objects studied in this and previous papers have Lyα luminosities larger than M87 but their extrapolated, nonthermal continua are so luminous that they overpredict the observed strength of Lyα, a clear indicator of relativistic beaming in our direction. Given their substantial space density (∼4 × 10−3 Mpc−3), the unbeamed Lyman continuum radiation of FR 1s may make a substantial minority contribution (∼10%) to the local UV background if all FR 1s are similar to M87 in ionizing flux level.