Not just PAH$_{3.3}$: why galaxies turn red in the Near-Infrared

Benedetta Vulcani; Tommaso Treu; Matthew Malkan; Thomas S. -Y Lai; Antonello Calabrò; Marco Castellano; Lorenzo Napolitano; Sara Mascia; Bianca M. Poggianti; Paola Santini; Jacopo Fritz; Benjamin Metha; Ilsang Yoon; Xin Wang

doi:10.1051/0004-6361/202452759

paper

Not just PAH $_{3.3}$ : why galaxies turn red in the Near-Infrared

arXiv:2412.07070 · doi:10.1051/0004-6361/202452759

Abstract

We measure the spectral properties of a sample of 20 galaxies at z~0.35 selected for having surprisingly red JWST/NIRCAM F200W-F444W colors. 19 galaxies were observed with JWST/NIRSpec in the PRISM configuration, while one galaxy was observed with the high resolution gratings. 17/20 galaxies in our sample exhibit strong 3.3 $μ m$ polycyclic aromatic hydrocarbon (PAH $_{3.3}$ ) emission (equivalent width EW(PAH $_{3.3}$ ) $> 0.03 μ m$ ). In these galaxies, the strength of the color excess does not depend on environment and it correlates with EW(PAH $_{3.3}$ ). Nonetheless, the presence of the PAH $_{3.3}$ alone can not fully explain the color excess, as an equivalent width of ~0.1 $μ m$ is able to increase the color of galaxies by only 0.13 mag. A contribution from a hot dust component is required to explain the excess. Both the EW(PAH $_{3.3}$ ) and flux correlate with the H $α$ equivalent width and flux, suggesting that they are produced by the same mechanism. 5/20 galaxies showing PAH would be classified as passive based on broad band rest frame colors ((B-V) and/or UVJ diagrams) and are hence "faux-passive". Of these, 3 galaxies have a significantly lower EW(PAH $_{3.3}$ ) given their color and also have low EW(H $α$ ) and we tentatively conclude this behaviour is due to the presence of an AGN. The three galaxies with no PAH $_{3.3}$ in emission have passive spectra, as do the 8 galaxies in our sample with normal F200W-F444W colors. We therefore conclude that the PAH $_{3.3}$ feature is linked to dust-enshrouded star formation. The dust corrected SFR from PAH $_{3.3}$ is a factor of 3.5 higher than the SFR obtained from H $α$ , suggesting that these galaxies are characterized by significant amounts of dust.

arXiv commentA&A in press

Not just PAH3.3​: why galaxies turn red in the Near-Infrared

Abstract

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Not just PAH $_{3.3}$ : why galaxies turn red in the Near-Infrared