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

¹ INAF – Osservatorio Astronomico di Padova Via dell’Osservatorio 5 35122 Padova, Italy
² Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova Via Marzolo 8 35131 Padova, Italy
³ INAF, Istituto di Radioastronomia Via Piero Gobetti 101 40129 Bologna, Italy
⁴ Department of Physics and Astronomy, Colby College Waterville ME 04901, USA
⁵ Department of Physics and Astronomy, The Johns Hopkins University 3400 N Charles St. Baltimore MD 21218, USA
⁶ INAF – Osservatorio Astronomico di Roma Via di Frascati 33 00078 Monte Porzio Catone, Italy
⁷ Space Telescope Science Institute (STScI) 3700 San Martin Drive Baltimore MD 21218, USA
⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio Via Gobetti 93/3 40129 Bologna, Italy
⁹ Physics & Astronomy Department, University of Louisville 40292 KY Louisville, USA
¹⁰ Center for Astrophysics | Harvard & Smithsonian 60 Garden St Cambridge MA 02138, USA
¹¹ Black Hole Initiative, Harvard University 20 Garden St Cambridge MA 02138, USA
¹² Centro de Astrobiología (CAB), CSIC-INTA Ctra. de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain
¹³ Astrophysics Science Division, NASA Goddard Space Flight Center 8800 Greenbelt Rd Greenbelt MD 20771, USA
¹⁴ Department of Astronomy, The University of Texas at Austin Austin TX, USA
¹⁵ NSF’s National Optical-Infrared Astronomy Research Laboratory 950 N. Cherry Ave. Tucson AZ 85719, USA
¹⁶ Cosmic Frontier Center, The University of Texas at Austin Austin TX, USA
¹⁷ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology 84 Lomb Memorial Drive Rochester NY 14623, USA
¹⁸ Department of Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
¹⁹ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University College Station TX 77843-4242, USA
²⁰ ESA/AURA Space Telescope Science Institute, USA
²¹ Department of Physics and Astronomy, University of Kansas Lawrence KS 66045, USA
²² Department of Astronomy, Indiana University 727 East Third Street Bloomington IN 47405, USA

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Received: 19 August 2025
Accepted: 15 December 2025

Abstract

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 log₁₀(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 10⁷ 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.