Obscured Star Formation in the Dwarf Galaxy DDO 43? A Comparative UV-IR Analysis

Abstract

We present a study of recent star formation in the dwarf irregular galaxy DDO 43 using GALEX FUV and WISE NIR imaging. We identify regions of elevated FUV flux, indicating unobscured star-forming activity across much of the galaxy. To further characterize the stellar content, we compare the FUV fluxes to archival WISE W1 and W2 infrared data across 56 regions of interest. A general correlation is found between the FUV and infrared fluxes, suggesting spatially coherent star formation throughout the galaxy. A few regions, however, show elevated infrared fluxes but little or no UV emission, potentially indicating localized, dust-obscured star formation.

Figures (4)

• Figure 1: WISE W1 image (3.4 $\mu$m) showing the positions of the four foreground stars (A–D) used for photometric comparison. The galaxy center lies slightly southeast of star A. The selected stars broadly sample the surroundings of the galaxy, allowing for background estimation across different local conditions.
• Figure 2: W1 (left), W2 (middle), and FUV (right) images with overlaid 10$\times$10 arcsec$^2$ sampling boxes. Red frames indicate W2-bright, UV-faint regions (SE1-3), corresponding to dust-enshrouded star-forming clumps in the southeastern quadrant. The blue frame marks a W1-bright, W2- and UV-faint region in the north (N1), likely dominated by an evolved stellar population.
• Figure 3: Locally normalized intensity contour maps for W1 (left) and W2 (right). The W1 image shows a regular, centrally concentrated profile, while the W2 emission is more asymmetric, with an extended structure toward the southeast. A faint vertical artifact in W2 (running northwest--southeast) passes through the galaxy and may visually enhance the apparent separation of the southeastern clumps. Taking this into account, the overall morphology still supports the interpretation of localized dust-obscured star formation.
• Figure 4: Correlation between GALEX FUV surface brightness and WISE W1 (left) and W2 (right). Points are color-coded by the W1$-$W2 color index (redder values indicate warmer dust emission). Pearson and Spearman correlation coefficients are shown above each panel. While the correlation is stronger in W1, consistent with relatively unobscured star formation, the W2--FUV plot reveals several outliers: three W2-bright, UV-dark red points (circled in red) and one blue point with low W2 and no UV emission (circled in blue). Excluding these, the W2--FUV correlation would likely be comparable to that of W1. For reference, unreddened main-sequence stars typically have W1$-$W2 $\approx$ 0 mag, since their spectra follow the Rayleigh--Jeans tail in this wavelength regime. Normal star-forming galaxies, on the other hand, generally fall in the range W1$-$W2 $\approx$ 0--0.5 mag Stern2012, which provides context for assessing the unusually red colors of the W2-bright regions.