A dust component 2 kpc above the plane in NGC 891

TL;DR

This study investigates the asymmetry in NGC 891's halo, attributing it mainly to dust attenuation extending up to 2 kpc above the galaxy plane, and quantifies the dust's optical depth effects across wavelengths.

Contribution

It provides a quantitative model showing dust attenuation as the primary cause of Hα asymmetry in NGC 891, with detailed measurements of dust distribution and optical depth.

Findings

Dust attenuation explains most of the Hα asymmetry.

Intrinsic star formation asymmetry is about 30%.

Dust extends up to 2 kpc above the galaxy plane.

Abstract

The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in H$\alpha$ emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in H$\alpha$ emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. From the 24 $\mu$m emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., $\sim 30%$. The additional asymmetry in H$\alpha$ is modeled as additional symmetric dust attenuation which extends up to $\sim$ 40\arcsec (1.9 kpc) above the plane of the galaxy with a mid-plane value of $\tau$=0.8 and a scale height of 0.5 kpc