The Vertical Dust Structure in Spiral Disks

TL;DR

This study investigates the vertical dust structure in spiral galaxy NGC 891, revealing that observed asymmetries in H-alpha emission are mainly due to dust attenuation rather than intrinsic star formation differences.

Contribution

It provides a quantitative analysis of dust attenuation effects on H-alpha asymmetry, suggesting that the galaxy's intrinsic star formation is nearly symmetric.

Findings

H-alpha asymmetry is largely caused by dust attenuation.

Dust extends up to ~1.9 kpc above the galaxy plane.

Intrinsic star formation asymmetry is about 30%.

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. We have taken a quantitative look at this asymmetry at different wavelengths for the first time. We propose that NGC 891 is intrinsically almost symmetric, as seen in Spitzer observations, and the large asymmetry in H-$\alpha$ emission is mostly due to dust attenuation. We quantify the additional optical depth needed to cause the observed H-$\alpha$ asymmetry. A comparison of 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., approximately 30%. The additional asymmetry in H-alpha is modeled as additional symmetric dust attenuation which extends up to ~ 40'' (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. This observational technique offers the possibility to quantify the effects of vertical ISM disk stability as an explanation for dust lanes in massive galaxies Dalcanton et al. (2004).