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 type 1 LINER or dwarf Seyfert .
Narrowband H \alpha and [ NII ] HST images reveal striking bipolar shell features , 1 kpc in projected length from end-to-end , which are likely the result of an outflow from the nuclear region experiencing a strong interaction with the ISM .
While these outflow shells share similarities with those in some other starburst or AGN galaxies , these in NGC 4438 are notable because NGC 4438 harbors neither a luminous circumnuclear starburst nor a luminous AGN .

The shells appear to be closed at their outer ends , suggesting that the outflow in NGC 4438 is dynamically younger than those in some other galaxies .
The radio continuum emission is strongly enhanced near the outer ends of the shells , suggesting working surfaces arising from collimated nuclear outflows which have impacted and shocked the surrounding ISM .
The 2 shells are quite different , as the northwestern ( NW ) shell is luminous and compact , while the southeastern ( SE ) shell is 2.5 times longer and much fainter , 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 .
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 surface brightness source in the galaxy at optical wavelengths , This source is resolved with a FWHM=0.3 ^ { \prime \prime } =25 pc and has modest luminosities , uncorrected for extinction , of 5 \times 10 ^ { 38 } erg s ^ { -1 } in H \alpha , and M _ { B } =-13 .
We discuss whether the outflow is powered by a low luminosity AGN or a compact nuclear starburst .
The kinetic energy associated with the ionized gas in the shells is \sim 10 ^ { 53 } ergs , which could be produced either by massive star formation or an AGN .
While the NW shell , which contributes most of the flux in most ground-based apertures centered on the nucleus , exhibits LINER-type line ratios , the nucleus has an H \alpha / [ NII ] ratio consistent with an HII region .
Although there appears to be very little massive star formation occuring in the central kpc , the nucleus may contain a partially obscured , young nuclear star cluster .
On the other hand , the collimation of the southeastern shell , and the strongly enhanced radio continuum emission at the outer ends of the shells are more easily explained by jets associated with a nuclear black hole than a compact nuclear starburst , although there is no direct evidence of a jet .
The 2000 km s ^ { -1 } broad line component could be due to an AGN broad line region but might also be due to high velocity gas in the outflow .
Since NGC 4438 has a large bulge , a large nuclear black hole might be expected .