Hubble Space TelescopeImaging of Bipolar Nuclear Shells in the Disturbed Virgo Cluster Galaxy NGC 4438

We present broadband and narrowband Hubble Space Telescope images of thecentral region of the heavily disturbed Virgo cluster galaxy NGC 4438 (Arp120),whose nucleus has been described as a type 1 LINER or dwarf Seyfert. NarrowbandH-alpha and [NII] HST images reveal striking bipolar shell features, 1 kpc inprojected length from end-to-end, which are likely the result of an outflowfrom the nuclear region experiencing a strong interaction with the ISM. Whilethese outflow shells share similarities with those in some other starburst orAGN galaxies, these in NGC 4438 are notable because NGC 4438 harbors neither aluminous circumnuclear starburst nor a luminous AGN. The shells appear to beclosed at their outer ends, suggesting that the outflow in NGC 4438 isdynamically younger than those in some other galaxies. The radio continuumemission is strongly enhanced near the outer ends of the shells, suggestingworking surfaces arising from collimated nuclear outflows which have impactedand shocked the surrounding ISM. The 2 shells are quite different, as thenorthwestern (NW) shell is luminous and compact, while the southeastern (SE)shell is 2.5 times longer and much fainter, in both optical emission lines andthe non-thermal radio continuum. The differences between the 2 shells may beattributed to a difference in ISM density on the 2 sides of the nuclear disk.Such an ISM asymmetry exists on larger scales in this heavily disturbed galaxy.At the base of the outflow is a nuclear source, which is the highest surfacebrightness source in the galaxy at optical wavelengths, We discuss whether theoutflow is powered by a low luminosity AGN or a compact nuclear starburst.