Dust in the Wheel: The Cartwheel Galaxy in the Mid-IR

TL;DR

This study presents mid-infrared imaging of the Cartwheel galaxy, revealing detailed dust distribution, significant inner region emission, and variations in star-forming activity, providing new insights into its structure and star formation processes.

Contribution

First mid-infrared images of the Cartwheel galaxy revealing dust distribution and unexpected inner region emission, advancing understanding of collisional ring galaxy characteristics.

Findings

High 15μm/6.7μm flux ratio of 5.2 among sample galaxies

Inner regions show stronger mid-IR emission than previously detected in Hα

Mid-IR emission correlates with weak star formation activity in inner regions

Abstract

We present mid-infrared images at $6.7 \mu m$ and $15 \mu m$ of ``The Cartwheel'' (AM~0035-33), the prototypical collisional ring galaxy. The observations, taken with ISOCAM, reveal the distribution of hot dust in the galaxy and its two companions in the north-east. The intensity of the mid-IR emission from the outer star forming ring of the Cartwheel shows considerable azimuthal variation and peaks at the most active H$\alpha$ region of the ring. The 15 $\mu m$ to 6.7 $\mu m$ flux ratio of 5.2 is the highest among all the galaxies of our sample. A surprising result of our observations is the discovery of significant emission from the inner regions of the galaxy, including the inner ring, spokes and nucleus, where previously only low level H$\alpha$ emission had been reported. At 6.7 $\mu m$, this emission is stronger than the one from the outer star forming ring, and at 15 $\mu m$, it represents 40% of the emission from the outer ring. The H$\alpha$ to mid-IR flux ratios from the inner regions are consistent with the heating of grains from weak star formation activity.