NGC4438 and its environment at radio wavelengths

TL;DR

This study uses multi-frequency radio and HI observations to analyze the disturbed galaxy NGC4438 in the Virgo cluster, revealing an AGN-driven radio structure, extended HI features, and complex interactions with its environment.

Contribution

It provides new high-resolution radio and HI data that clarify the nature of radio lobes as AGN-driven and details the galaxy's extended HI structures and kinematics.

Findings

Detection of an inverted-spectrum radio nucleus indicating AGN activity.

Identification of an extended HI tail of ~50 kpc possibly associated with the galaxy.

Observation of a displaced HI structure with systematic rotation.

Abstract

We present multi-frequency, radio-continuum and HI observations of NGC4438, the highly-disturbed, active galaxy in the Virgo cluster, with the VLA and the GMRT. High-resolution observations 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 arise due to an AGN 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 HI observations show an elongated structure which is displaced by ~4.1 kpc on the western side of NGC4438 and has a size of ~9.8 kpc and a mass of 1.8*10^8 M_sun. The velocity field suggests systematic rotation. We also detect HI emission from the disk of the galaxy with a mass of 1.2*10^8 M_sun. We detect a faint HI tail towards the north of NGC4438 close to a stellar tail seen in deep optical observations. This HI tail has a total extent of ~50 kpc and a mass of 1.4*10^8 M_sun if it is at the distance of NGC4438. The velocity of the HI tail is ~-10 km/s similar to that of HI 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 the light of different models.