NGC 4438 and its environment at radio wavelengths

Hota, Ananda and Saikia, DJ and Irwin, Judith A (2007) NGC 4438 and its environment at radio wavelengths. In: Monthly Notices of the Royal Astronomical Society, 380 (3). pp. 1009-1022.

We present multifrequency radio continuum and H i observations of NGC 4438, the highly disturbed, active galaxy in the Virgo cluster, with the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT). High-resolution observations of the central 1 kpc with the VLA at 4860 and 8460 MHz show the presence of an inverted-spectrum radio nucleus located between the highly asymmetric lobes of radio emission. This demonstrates that these lobes which are seen in radio continuum, Hα and X-ray wavelengths and are located at \sim 230 and 730 pc from the nucleus arise due to an active galactic nucleus rather than a compact nuclear starburst. The low-frequency radio continuum observations made with the GMRT detect the extended emission on the western side of the galaxy whose spectral index is flatter at higher frequencies and suggests that it is a mixture of thermal and non-thermal emission. The $H_I$ observations show an elongated structure which is displaced by \sim 4.1 kpc on the western side of NGC 4438 and has a size of \sim 9.8 kpc and a mass of 1.8 \times 108 Mo. The velocity field suggests systematic rotation, consistent with earlier observations. These observations also detect $H_I$ emission from the disc of the galaxy with a mass of $1.2 \times 10^8 M \bigodot$. We detect a faint $H_I$ tail towards the north of NGC 4438 close to a stellar tail seen earlier in deep optical observations. This $H_I$ tail has a total extent of \sim 50 kpc and a mass of 1.4 \times 108 Mo if it is at the distance of NGC 4438. The velocity of the H i tail is $\sim −10 km s^{-1}$ similar to that of H i emission from IC 3355, but the possibility that the tail could be foreground Galactic emission cannot be ruled out. We discuss the different structures in light of different models which have been suggested for this disturbed galaxy, namely ram pressure stripping, tidal and interstellar medium (ISM)–ISM interactions.