X-Ray Searches for Emission from the WHIM in the Galactic Halo and the Intergalactic Medium

TL;DR

This study used X-ray observations to search for faint emission from the warm-hot intergalactic medium and Galactic halo, but did not detect the expected shadows, providing upper limits and insights into interactions with ambient hot gas.

Contribution

It presents new observational constraints on the diffuse X-ray emission from the WHIM and Galactic halo, challenging previous detections and suggesting interactions with ambient hot gas.

Findings

No X-ray shadows detected in NGC 891 and NGC 5907.

Upper limits above model predictions for diffuse emission.

Brightening of X-ray emission near Magellanic Stream cloud likely due to shock heating.

Abstract

At least 50% of the baryons in the local universe are undetected and predicted to be in a hot dilute phase (1E5-1E7 K) in low and moderate overdensity environments. We searched for the predicted diffuse faint emission through shadowing observations whereby cool foreground gas absorbs more distant diffuse emission. Observations were obtained with Chandra and XMM-Newton. Using the cold gas in two galaxies, NGC 891 and NGC 5907, shadows were not detected and a newer observation of NGC 891 fails to confirm a previously reported X-ray shadow. Our upper limits lie above model predictions. For Local Group studies, we used a cloud in the Magellanic Stream and a compact high velocity cloud to search for a shadow. Instead of a shadow, the X-ray emission was brighter towards the Magellanic Stream cloud and there is a less significant brightness enhancement toward the other cloud also. The brightness enhancement toward the Magellanic Stream cloud is probably due to an interaction with a hot ambient medium that surrounds the Milky Way. We suggest that this interaction drives a shock into the cloud, heating the gas to X-ray emitting temperatures.