HELM’s deep: Highly extincted low-mass galaxies seen by JWST

The dust content of star-forming galaxies is generally positively correlated with their stellar mass. However, some recent James Webb Space Telescope (JWST) studies have shown the existence of a population of dwarf galaxies with an unexpectedly large dust attenuation. Using Cosmic Evolution Early Release Science Survey (CEERS) data, we identified a sample of 1361 highly extincted low-mass (HELM) galaxies, defined as dwarf galaxies ( M * ≤ 10 8.5 ) with A V > 1 mag or more massive galaxies with an exceptionally high dust attenuation given their stellar mass (i.e. A V > 1.6 log 10 ( M * /M ⊙ )−12.6). The selection was performed using the multi-parameter distribution obtained through a comprehensive spectral energy distribution fitting analysis, based on optical to near-infrared data. After the exclusion of possible contaminants, such as brown dwarfs, little red dots, and high- z ( z > 8.5) and ultra-high- z ( z > 15) galaxies, the sample mainly includes sources at z < 1, with a tail extending up to z = 7.2. The sample has a median stellar mass of 10 7 M ⊙ and a median dust attenuation of A V = 2 mag. We analysed the morphology, environment, and star formation rate of these sources to investigate the reason behind their large dust attenuation. In particular, HELM sources have sizes (effective radii, R e ) similar to non-dusty dwarf galaxies and no correlation is visible between the axis ratios ( b / a ) and the dust attenuation. These findings indicate that it is unlikely that the large dust attenuation is due to projection effects, but a prolate or disc-on oblate geometry is still possible, at least for a sub-sample of the sources. We have found that the distribution of HELM sources is slightly skewed towards more clustered environments than non-dusty dwarfs and tends to be slightly less star-forming. This finding, if confirmed by spectroscopic follow-up, indicates that HELM sources could be going through some environmental processes, such as galaxy interactions.