The Nuclear Ionized Gas in the Radio Galaxy M84 (NGC 4374)

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Abstract
We present optical images of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C 272.1) obtained with the Wide Field/Planetary Camera 2 aboard the Hubble Space Telescope. Our three images cover the Hα + [N II] emission lines as well as the V and I continuum bands. Analysis of these images confirms that the Hα + [N II] emission in the central 5'' (410 pc) is elongated along position angle (P.A.) ≈ 72°, which is roughly parallel to two nuclear dust lanes. Our high-resolution images reveal that the Hα + [N II] emission has three components, namely, a nuclear gas disk, an "ionization cone," and outer filaments. The nuclear disk of ionized gas has diameter ≈ 1'' = 82 pc and major axis P.A. ≈ 58° ± 6°. On an angular scale of 05, the major axis of this nuclear gas disk is consistent with that of the dust. However, the minor axis of the gas disk (P.A. ≈ 148°) is tilted with respect to that of the filamentary Hα + [N II] emission at distances greater than 2'' from the nucleus; the minor axis of this larger scale gas is roughly aligned with the axis of the kiloparsec-scale radio jets (P.A. ≈ 170°). The ionization cone (whose apex is offset by ≈ 03 south of the nucleus) extends 2'' from the nucleus along the axis of the southern radio jet. This feature is similar to the ionization cones seen in some Seyfert nuclei, which are also aligned with the radio axes.
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