Not just PAH_3.3: Why galaxies turn red in the near-infrared

¹ INAF – Osservatorio astronomico di Padova, Vicolo Osservatorio 5, I-35122 Padova, Italy
² Department of Physics and Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA
³ IPAC, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁴ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monteporzio Catone, Italy
⁵ Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro 2, 00185 Roma, Italy
⁶ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁷ Instituto de Radioastronomia y Astrofisica, UNAM, Campus Morelia, AP 3-72, CP 58089, Mexico
⁸ School of Physics, The University of Melbourne, VIC 3010, Australia
⁹ Australian Research Council Centre of Excellence for All-Sky Astrophysics in 3-Dimensions, Melbourne, VIC 3000, Australia
¹⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹¹ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, China
¹² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹³ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China

^⋆ Corresponding author; benedetta.vulcani@inaf.it

Received: 25 October 2024
Accepted: 6 December 2024

Abstract

We measured the spectral properties of a sample of 20 galaxies at z ∼ 0.35 selected for having surprisingly red JWST/NIRCAM F200W-F444W colors. Of these, 19 galaxies were observed with JWST/NIRSpec in the PRISM configuration, while the remaining galaxy was observed with the high-resolution gratings. Of the 20 galaxies in our sample, 17 exhibit strong 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission (equivalent width (EW) (PAH_3.3) ≥ 0.03 μm). In these galaxies, the strength of the color excess does not depend on environment and correlates with EW(PAH_3.3). Nonetheless, the presence of the PAH_3.3 alone cannot fully explain the color excess, as an EW of ∼0.1 μm 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 PAH_3.3 EW and flux correlate with the Hα EW and flux, suggesting that they are produced by the same mechanism. Five of the galaxies of our sample showing PAH_3.3 would be classified as passive based on broadband rest frame colors ((B-V) and/or UVJ diagrams) and are hence “faux passive”. Of these, three galaxies have a significantly lower EW(PAH_3.3) given their color and also have low EW(Hα), and we tentatively conclude that this behavior is due to the presence of an active galactic nucleus. The three galaxies with no PAH_3.3 in emission have passive spectra, as do the eight 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 star formation rate (SFR) from PAH_3.3 is a factor of 3.5 higher than the SFR obtained from Hα, suggesting that these galaxies are characterized by significant amounts of dust.