[Paper Review] Dust in the Wheel: The Cartwheel Galaxy in the Mid-IR

V. Charmandaris, O. Laurent+6 more|arXiv (Cornell University)|Oct 19, 1998

Astronomy and Astrophysical Research3 citations

TL;DR

This study presents mid-infrared ISOCAM observations of the Cartwheel galaxy at 6.7 μm and 15 μm, revealing strong, asymmetric dust emission in the outer star-forming ring and unexpected bright mid-IR emission from its inner ring, spokes, and nucleus—indicating significant heating by weak star formation activity despite low Hα emission. The 15 μm to 6.7 μm flux ratio of 5.2 is the highest in the sample, highlighting extreme dust content and thermal properties in the ring structure.

ABSTRACT

We present mid-infrared images at $6.7 μm$ and $15 μm$ 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$α$ region of the ring. The 15 $μm$ to 6.7 $μm$ 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$α$ emission had been reported. At 6.7 $μm$, this emission is stronger than the one from the outer star forming ring, and at 15 $μm$, it represents 40% of the emission from the outer ring. The H$α$ to mid-IR flux ratios from the inner regions are consistent with the heating of grains from weak star formation activity.

Motivation & Objective

• To map the mid-infrared dust distribution in the Cartwheel galaxy, a prototypical collisional ring galaxy.
• To investigate the origin and spatial distribution of mid-infrared emission in the outer star-forming ring and inner regions.
• To determine whether the observed mid-IR emission correlates with star formation activity, especially in regions with low Hα emission.
• To quantify the dust thermal properties via flux ratios, particularly the 15 μm to 6.7 μm flux ratio, across different galactic components.

Proposed method

• Mid-infrared imaging was conducted using the ISOCAM instrument on the Infrared Space Observatory (ISO) at 6.7 μm and 15 μm.
• Spatially resolved flux measurements were extracted from the outer star-forming ring, inner ring, spokes, and nucleus.
• Flux ratios (15 μm / 6.7 μm) were calculated to infer dust temperature and emission characteristics.
• Hα flux data from previous observations were compared with mid-IR fluxes to assess the link between star formation and dust heating.
• Azimuthal variations in mid-IR emission were analyzed to identify regions of enhanced dust activity.

Experimental results

Research questions

• RQ1 What is the spatial distribution of hot dust in the Cartwheel galaxy’s outer star-forming ring and inner structures?
• RQ2 Why is the 15 μm to 6.7 μm flux ratio in the Cartwheel galaxy significantly higher than in other galaxies in the sample?
• RQ3 What is the origin of the strong mid-infrared emission detected in the inner ring, spokes, and nucleus, despite weak Hα emission?
• RQ4 How does the mid-infrared emission correlate with star formation activity in different galactic components?
• RQ5 What do the azimuthal variations in mid-IR emission reveal about the localized heating of dust in the ring?

Key findings

• The mid-infrared emission in the outer star-forming ring exhibits strong azimuthal variation, peaking at the most active Hα region.
• The 15 μm to 6.7 μm flux ratio of 5.2 is the highest observed in the sample, indicating extreme dust content or high-temperature emission.
• Significant mid-infrared emission is detected from the inner ring, spokes, and nucleus—regions previously reported to have only low-level Hα emission.
• At 6.7 μm, the inner region's mid-IR emission exceeds that of the outer ring, indicating strong dust heating in the central regions.
• At 15 μm, the inner region contributes 40% of the total emission from the outer ring, highlighting its dominant role in mid-IR output.
• The Hα to mid-IR flux ratios in the inner regions are consistent with dust heating from weak star formation activity.