The broad H-alpha, [O III] line wings in stellar supercluster A of NGC2363 and the turbulent mixing layer hypothesis

TL;DR

This study investigates the origin of broad emission line wings in supercluster A of NGC2363, proposing turbulent mixing layers caused by high-velocity cluster winds as a plausible explanation, and compares model predictions with observations.

Contribution

It introduces a turbulent mixing layer model driven by high-velocity winds to explain broad emission line wings in an extragalactic HII region.

Findings

A wind velocity of ~4300 km/s is needed to reproduce faint line wings.

A slower wind of 3500 km/s can fit the broad component better.

The model predicts no broad component for [N II] and [S II] lines.

Abstract

Context. SuperclusterA in the extragalactic HII region NGC2363 is remarkable for the hypersonic gas seen as faint extended broad emission lines with a full width zero intensity of 7000km/s. Aims. We explore the possibility that the observed broad profiles are the result of the interaction of a high velocity cluster wind with dense photoionized clumps. Methods. The geometry considered is that of near static photoionized condensations at the surface of which turbulent mixing layers arise as a result of the interaction with the hot wind. The approximative treatment of turbulence is carried out using the mixing length approach of Canto & Raga. The code mappings Ic is used to derive the mean quantities describing the flow and to compute the line emissivities within the turbulent layers. The velocity projection in three dimensions of the line sources is carried out analytically. Results. A fast entraining wind of up to ~4300km/s appears to be required to reproduce the faint wings of the broad H-alpha and [O III] profiles. A slower wind of 3500km/s, however, can still reproduce the bulk of the broad component and does provide a better fit than an ad hoc Gaussian profile. Conclusions. Radial acceleration in 3D (away from supercluster A) of the emission gas provides a reasonable first order fit to the broad line component. No broad component is predicted for the [N II] and [S II] lines, as observed. The wind velocity required is uncomfortably high and alternative processes that would provide comparable constant acceleration of the emission gas up to 4000km/s might have to be considered.