On the diffuse soft X-ray emission from the nuclear region of M51

TL;DR

This study analyzes the diffuse soft X-ray emission in M51's nucleus, revealing spatially distinct components and challenging photoionization models, with implications for understanding galactic nuclear environments.

Contribution

It provides a detailed spatial and spectral analysis of the soft X-ray emission in M51's nucleus, highlighting the distinct origin of the OVII triplet and discussing alternative emission mechanisms.

Findings

OVII triplet is spatially offset from other lines.

Most of the diffuse emission fits a 0.5 keV thermal model.

The OVII triplet distribution challenges photoionization models.

Abstract

We present an analysis of the diffuse soft X-ray emission from the nuclear region of M51 combining both XMM-Newton RGS and Chandra data. Most of the RGS spectrum of M51 can be fitted with a thermal model with a temperature of $\sim0.5$ keV except for the OVII triplet, which is forbidden-line dominated. The Fe L-shell lines peak around the southern cloud, where the OVIII and NVII Lya lines also peak. In contrast, the peak of the OVII forbidden line is about 10$"$ offset from that of the other lines, indicating that it is from a spatially distinct component. The spatial distribution of the OVII triplet mapped by the Chandra data shows that most of the OVII triplet flux is located at faint regions near edges, instead of the southern cloud where other lines peak. This distribution of the OVII triplet is inconsistent with the photoionization model. Other mechanisms that could produce the anomalous OVII triplet, including a recombining plasma and charge exchange X-ray emission, are discussed.