Multi-wavelength morphology and dust emission in low-redshift dwarf galaxies in COSMOS-Web with HST and JWST

TL;DR

This study targets dwarf galaxies with $M_{*}<10^{9}\,M_ extodot$ in low-density environments by combining HST/ACS and JWST/NIRCam+MIRI imaging from COSMOS-Web for $z<0.08$ dwarfs. It assesses dust emission and morphology across rest-frame optical to mid-infrared wavelengths, comparing observed mid-IR fluxes with forward-modelled TNG50 predictions to test ISM and feedback physics. All nine dwarfs are detected in both NIRCam and MIRI, revealing dust and PAH contributions that cause notable morphological changes at longer wavelengths; SEDs indicate substantial non-stellar MIR emission in several targets. The results show broad agreement with simulations and highlight the importance of dust in shaping dwarf galaxy morphology, underscoring the value of JWST+HST multi-wavelength studies and motivating follow-up spectroscopy to constrain dust grain properties and gas kinematics in dwarfs of low-density environments.

Abstract

Low-mass or dwarf galaxies (M$_{\ast}<10^{9}$ M${\odot}$) are abundant in the Universe, yet their formation and evolution remain poorly understood. Their enhanced sensitivity to feedback from star formation and active galactic nuclei (AGN) make them excellent laboratories to test whether feedback prescriptions in cosmological simulations accurately reproduce their interstellar medium (ISM) properties. We present JWST/NIRCam and MIRI imaging of nine dwarf galaxies from COSMOS-Web survey at redshift $z<0.08$, with star formation rates ranging from 0.003-0.3 M${\odot}$ yr$^{-1}$ and stellar masses of log M$_{\ast}\sim8-9$ M$_{\odot}$. The detection rate with both NIRCam and MIRI is 100\%, indicating that these dwarfs possess substantial ISM content. The detected sample includes a roughly equal mix of early-type and late-type dwarfs, suggesting that it is representative of the broader dwarf galaxy population in low-density environments. We find that the observed MIRI flux distributions are comparable to forward-modelled flux distributions of mass-matched simulated galaxies in TNG50. We further conduct a multi-wavelength morphological analysis complementing the JWST NIRCam and MIRI imaging with archival HST/ACS data, employing the CAS (concentration, asymmetry, smoothness) framework. Among the multi-wavelength images, MIRI exhibits the largest variation in CAS parameters, likely due to dust lanes and clumps in several galaxies, also suggested by Spectral Energy Distribution (SED) fitting. This suggests that the dust content in these systems may be higher than those implied by rest-frame optical or near-infrared observations alone. Upcoming UV/optical and mid-infrared spectroscopic follow-up will be critical for constraining the gas kinematics and dust grain properties of dwarf galaxies in low-density environments such as COSMOS.

Figures (12)

• Figure 1: The left panel shows the spatial distribution of the parent sample of dwarf galaxies in the COSMOS field from lazar24. The sources highlighted by red stars are covered by NIRCam and MIRI imaging campaigns of the COSMOS-Web survey. The middle panel shows the location of the dwarf galaxies covered in the COSMOS-Web campaign, compared to the parent lazar24 sample. Most of the dwarf galaxies are located on the main sequence of massive star forming galaxies schreiber15, extrapolated to lower masses. Three galaxies are below the main sequence in the passive galaxy regime. The right panel shows the location of the dwarf galaxies in the colour $(g-i)_{0}$ versus M$_{\ast}$ plane. The grey colour-scale background shows the location of all galaxies from the COSMOS survey (including massive galaxies) and the coloured data points show the different morphological types of dwarf galaxies from lazar24. The dotted line demarcates blue (bottom half) and red (top half) galaxies. The sample presented in this paper, shown as yellow stars show both red and blue dwarfs are detected with the COSMOS-Web imaging campaigns.
• Figure 2: The left, middle and right panels show the HST/ACS F814W, JWST/NIRCam F277W and JWST/MIRI F770W images of three of the dwarf galaxies (from top to bottom: IDs 5, 120 and 232) presented in this paper. The blue contours on the MIRI images in the right panel represent the F814W emission from the HST/ACS images in the left panel. The black horizontal bar shows the physical 1 kpc scale and the black circle in the bottom left shows the PSF (FWHM) of the respective images. The potential emission from the dust becomes clearly visible in the MIRI images, which are otherwise absent in the HST images. See Sect. \ref{['sect4.1']} for further details.
• Figure 3: The left panel shows the location of the TNG50 galaxies (blue contours), along with the parent dwarf galaxy sample (grey circles) and the sample presented in this paper (red stars) in the SFR-M$_{\ast}$ plane. The middle panel shows the flux distributions of the mock IRAC-I4 flux from trcka22 (blue histogram) and the observed MIRI/F770W fluxes in this paper (red histogram outline). The right panel shows the fluxes in the middle panel as a function of stellar mass. The colour scheme in the right panel is the same as the left panel. Further details about the comparison between TNG50 data and MIRI observations are given in Sect. \ref{['sect4.1']}.
• Figure 4: SED fits of targets ID-5 (top panel) and ID-120 (middle panel) and ID232 (bottom panel) using PROSPECTOR. The coloured circles show the different photometric data points from COSMOS2025 shuntov25 corresponding to the following instruments and filters: HSC ($g,r,i,z,y$), UVISTA ($Y, J, H, K$), IRAC (1, 3, 4), HST (F814W), NIRCam (F115W, F150W, F277W, F444W) and MIRI (F770W). The orange curve shows the best-fit SED model. In all the targets, contribution from small and large dust grains and PAHs becomes apparent at mid-infrared wavelengths, the excess emission above stellar model (black body fit shown in grey colour). This is further re-inforced by the multi-wavelength visual analysis of these targets, that already indicated possible presence of dust lanes and dust clumps.
• Figure 5: The top panel shows the Concentration (C, left panel), asymmetry (A, middle panel) and Clumpiness (S, right panel) as a function of the HST/F814W, NIRCam/F277W and MIRI/F770W for the individual targets (different colours and symbols) presented in this paper. The values in each filter have been normalised to the HST/F814W to guide the change in the respective parameters.