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J. D. P. Kenney, E. E. Yale (Yale University)
We present 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 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 outflowing jets from an active nucleus. The LINER-like line ratios detected in ground-based spectra originate in the outflow shells, one of which is the highest luminosity source in the central kpc. 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. On large scales, NGC~4438 is already known to have a much denser ISM on the western side of the disk, due to its interaction. 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 barely resolved nuclear source, which is the highest surface brightness source in the galaxy at optical wavelengths. Although the nuclear source has an H\alpha/[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, and the broad line component are more easily explained by jets associated with a nuclear black hole than a compact nuclear starburst. Several properties of the circumnuclear region appear to be influenced by the global disturbance to the host galaxy NGC~4438.
This work has been supported by Space Telescope grant GO-06791.