HELM's deep: Highly Extincted Low-Mass galaxies seen by JWST

L. Bisigello, G. Gandolfi, A. Grazian, G. Rodighiero, G. Girardi, A. Renzini, A. Vietri, E. McGrath, B. Holwerda, Abdurro'uf, M. Castellano, M. Giulietti, C. Gruppioni, N. Hathi, A. M. Koekemoer, R. Lucas, F. Pacucci, P. G. Pérez-González, L. Y. A. Yung, P. Arrabal Haro, B. E. Backhaus, M. Bagley, M. Dickinson, S. Finkelstein, J. Kartaltepe, A. Kirkpatrick, C. Papovich, N. Pirzkal

First listed 2025-12-16 | Last updated 2025-12-16

Abstract

The dust content of star-forming galaxies is generally positively correlated with their stellar mass. However, some recent JWST studies have shown the existence of a population of dwarf galaxies with an unexpectedly large dust attenuation. Using the 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 Av>1mag or more massive galaxies with an exceptionally high dust attenuation given their stellar mass (i.e., $Av>1.6log_{10}(M_*/Mo)-12.6$). The selection is performed using the multiparameter distribution obtained through a comprehensive spectral energy distribution fitting analysis, based on optical to near-infrared data. After excluding possible contaminants, like brown dwarfs, little red dots, 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 $10^7$ Mo and a median dust attenuation of Av=2mag. We analysed the morphology, environment and star-formation rate of these sources to investigate the reason of their large dust attenuation. In particular, HELM sources have sizes (effective radii, Re) similar to non-dusty dwarf galaxies and no correlation is visible between the axis ratios (b/a) and the dust attenuation. This findings indicate that it is unlikely that the large dust attenuation is due to projection effects, but a prolate or a disk-on oblate geometry are still possible, at least for a subsample of the sources. We have found that the distribution of HELM sources is slightly skewed toward more clustered environments than non-dusty dwarfs and tend 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.

Short digest

Using CEERS imaging, the authors assemble 1361 highly extincted low-mass (HELM) systems via posterior-weighted SED fits, defining dwarfs with M* < 10^8.5 Msun and Av > 1 mag or objects exceeding an Av–M* threshold. After removing contaminants—including brown dwarfs, little red dots, and z > 8.5 ultra-high‑z lookalikes—the sample is mostly at z < 1 (tail to z = 7.2) with median M* ≈ 10^7 Msun and Av ≈ 2 mag. HELM galaxies have Re comparable to non‑dusty dwarfs and show no Av–(b/a) trend, arguing against simple projection, while a prolate or disk‑on‑oblate geometry remains plausible. They sit in slightly more clustered environments and are mildly less star‑forming, hinting—pending spectroscopic confirmation—that interactions may drive their high attenuation.

Key figures to inspect

• Figure 1: Inspect the Av–M* plane to see how HELM objects sit well above the McLure (2018) relation and the exact red selection line; compare against literature dusty samples and note the spectroscopically confirmed HELM point for scale.
• Figure 2: Check how the HELM selection is performed on the full posterior (not point medians) and how pre/post quality and contaminant cuts reshape the number-density side panels—useful for judging sample purity and boundary cases.
• Figure 3: Read off the normalized redshift and stellar‑mass distributions; confirm the z < 1 dominance, the tail to z ≈ 7.2, and compare medians across HELM subsamples to understand where dusty dwarfs concentrate in M*–z space.
• Figure 4: Examine the HELM fraction versus redshift, and versus redshift at fixed mass, to quantify how common dusty dwarfs are relative to CEERS peers and whether incidence rises in specific z or mass bins.