The nuclear ionized gas in the radio galaxy M84 (NGC 4374)

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 Hobble Space Telescope. Our three images cover the H alpha + [N II] emission lines as well as the V and I continuum bands. Analysis of these images confirms that the H alpha + [N II] emission in the central 5'' (410 pc) is elongated along position angle (P.A.) approximate to 72 degrees, which is roughly parallel to two nuclear dust lanes. Our high-resolution images reveal that the H alpha + [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 approximate to 1'' 82 pc and major axis P.A. approximate to 58 degrees +/- 6 degrees. On an angular scale of 0''.5, 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. approximate to 148 degrees) is tilted with respect to that of the filamentary H alpha + [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. approximate to 170 degrees). The ionization cone (whose apex is offset by approximate to 0.''3 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.