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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

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.

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

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 () with Av>1mag or more massive galaxies with an exceptionally high dust attenuation given their stellar mass (i.e., ). 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 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.

Paper Structure

This paper contains 23 sections, 1 equation, 19 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data
  3. Photometric catalogue
  4. SED fitting
  5. Sample selection
  6. Comparison between HELM samples
  7. Stacked photometry
  8. Background removal
  9. Photometric extraction and comparison
  10. On the nature of HELM galaxies
  11. Are we misinterpreting ultra-high-$z$ galaxies?
  12. Are we misinterpreting passive galaxies?
  13. Are HELM galaxies faint little red dots?
  14. Are HELM galaxies more compact than non-dusty dwarfs?
  15. Are HELM galaxies preferentially edge-on?
  16. ...and 8 more sections

Figures (19)

  • Figure 1: The unexpected population of HELM galaxies, as visible in the stellar mass and dust attenuation plane. Black dots are the entire CEERS sample (before any selection), as derived from the first SED fitting run described in Sec. \ref{['sec:SEDfitting']} and considering the median value of both Av and stellar mass. Contour lines show the 20 (yellow) to 90% (dark blue) of the distribution, in steps of 10%. Coloured symbols show other samples of dusty objects from the literature AlcaldePampliega2019PerezGonzalez2023Gentile2024Rodighiero2023Bisigello2023bGandolfi2025Castellano2025ArrabalHaro2023b and the one analysed in this paper (see Sec. \ref{['sec:selection']}). Bisigello2025a is an HELM galaxy confirmed spectroscopically and is also part of the HELM-$2\sigma$ sample, while the source by Castellano2025 has been confirmed spectroscopically, but it is on another field. The green dashed line shows the known relation between dust attenuation and stellar mass McLure2018 derived at $2<z<3$, while the red solid line shows the selection criterion of HELM sources.
  • Figure 2: Stellar mass and dust attenuation plane showing the HELM sources and the HELM selection criterion (red solid line). For the stellar mass and dust attenuation we show the median value of the median outputs of each SED setup. We remind that the HELM selection is performed on the multiparameter probability distribution and not on the point estimates of the physical properties, so the median value shown here can be slightly out of the selection if one of the SED runs has a median output distant from the HELM selection. The two side panels show the number density of the HELM samples before and after the selections of Table \ref{['tab:selections']}.
  • Figure 3: Normalized redshift (top) and stellar mass (bottom) distributions of the HELM samples. Coloured arrows indicate the median of the three samples and are slightly shifted vertically for clarity. In the top panel, we limit the plot to $z<3.5$ for visual clarity, but we show the entire range in the inset for the HELM-$1\sigma$ sample.
  • Figure 4: Fraction of HELM sources, considering all samples, with respect to all galaxies with similar redshift (top) and similar redshift and stellar mass (bottom). We consider only galaxies in the entire CEERS sample with good SED fit (i.e., $\chi^2<20$).
  • Figure 5: Comparison between the stacked rest-frame photometry of the combined HELM samples (top) and the sample of non-dusty dwarfs (bottom) at different redshifts. Stacked photometry is normalised at $2.5\,\mu m$. We show the HELM SED (solid black lines), convolved using a top-hat function over seven data points, in both panels for comparison.
  • ...and 14 more figures