Vertically Extended Neutral Gas in the Massive Edge-on Spiral NGC 5746

Abstract

We present Very Large Array 21-cm observations of the massive edge-on spiral galaxy NGC 5746. This galaxy has recently been reported to have a luminous X-ray halo, which has been taken as evidence of residual hot gas as predicted in galaxy formation scenarios. Such models also predict that some of this gas should undergo thermal instabilities, leading to a population of warm clouds falling onto the disk. If so, then one might expect to find a vertically extended neutral layer. We detect a substantial high-latitude component, but conclude that almost all of its mass of 1.2-1.6 billion solar masses most likely resides in a warp. Four features far from the plane containing about 100 million solar masses are found at velocities distinct from this warp. These clouds may be associated with the expected infall, although an origin in a disk-halo flow cannot be ruled out, except for one feature which is counter-rotating. The warp itself may be a result of infall according to recent models. But clearly this galaxy lacks a massive, lagging neutral halo as found in NGC 891. The disk HI is concentrated into two rings of radii 1.5 and 3 arcminutes. Radial inflow is found in the disk, probably due to the bar in this galaxy. A nearby member of this galaxy group, NGC 5740, is also detected. It shows a prominent one-sided extension which may be the result of ram pressure stripping.