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Hubble Space Telescope Imaging of Bipolar Nuclear Shells in the Disturbed Virgo Cluster Galaxy NGC 4438 1

We present broadband and narrowband Hubble Space Telescope images of the central region of the heavily disturbed Virgo cluster galaxy NGC 4438 (Arp120), whose nucleus has been described as a LINER or dwarf Seyfert. Narrowband Hα and [NII] HST images reveal striking bipolar shell features, 1 kpc in projected length from end-to-end, which are likely the result of outflowing jets from an active nucleus. The 2 shells are quite different, as the western shell is luminous and compact, while the eastern shell is faint and 2.5 times longer, in both optical emission lines and the non-thermal radio continuum. The differences between the 2 shells may be attributed to a difference in ISM density on the 2 sides of the nuclear disk. The radio continuum emission is strongly enhanced near the outer ends of the shells, suggesting working surfaces in which collimated nuclear jets have impacted and shocked the surrounding ISM. At the base of the outflow is a nuclear source, which is the highest surface brightness source in the galaxy at optical wavelengths, with FWHM=0.3′′=25 pc, MB=-13, a modest Hα luminosity of 5×1038 erg s−1, and an Hα/[NII] ratio of 1.6±0.4. Although the nuclear source has an Hα/[NII] ratio consistent with an HII region, there is very little massive star formation occuring in the central kpc. The collimation of the southeastern shell, the strongly enhanced radio continuum emission at the outer ends of the shells, the broad line component, and the total energy of the shells are more easily explained by jets associated with a nuclear black hole than a compact nuclear starburst. We suggest that the observed activity in the center of NGC 4438 is due to a low luminosity AGN experiencing a strong jet-ISM interaction. Several properties of the circumnuclear region appear to be influenced by the global disturbance to the host galaxy. Subject headings: galaxies: active — galaxies: clusters: individual (Virgo) — galaxies: interactions — galaxies: ISM — galaxies: jets — galaxies: nuclei — galaxies: peculiar