A first [CII] view of high-z quiescent galaxies

Abstract

We present ALMA detections (or stringent upper limits) of the [CII] 158 $\hat{I}¼m$ emission line and underlying dust continuum from five massive quenched galaxies (QGs) at 2<z<4.7. We find extreme variations in the molecular gas fractions (${\mathrm{f}}_{\mathrm{g}}={\mathrm{M}}_{\mathrm{mol}}/{\mathrm{M}}_{\star }$), spanning 0.1%-25%, if a standard $\hat{\mathrm{I}}{\pm }_{[\mathrm{CII}]}$ applies. We attempt a first empirical calibration of $\hat{\mathrm{I}}{\pm }_{[\mathrm{CII}]}$ with respect to dust continuum in a $z=2$ lensed QG and with respect to CO(3-2) in a $z=3.1$ QG, finding no evidence of strong deviations from the standard value. Dust continuum measurements, coupled with JWST/MIRI fluxes, suggest higher dust temperatures compared to expectations from $z<2$ QGs, reaching $T_d\sim 40-50K$ in two galaxies. Coupled with remarkably high total infrared luminosities (LIR) not explained by observed JWST colors not by energy balance based on literature dust extinction measurements, and with [CII] deficits down to $[\mathrm{CII}]/LIR\sim 2\times 10^{-4}$ typical of (Ultra)Luminous Infrared Galaxies, our findings point to additional dust-heating mechanisms other than dust-absorbed stellar radiation. Surprisingly, JWST/NIRCam and ALMA imaging reveal widespread disturbed stellar morphologies and offsets/tails in dust and gas, indicative of ongoing interactions. While larger samples are needed to assess how common these features are in high-z QGs, these findings support a merger-driven origin for the phenomenology observed in these systems, with key similarities with respect to local post-starburst galaxies where low-velocity shocks and turbulence also inject energy into the residual ISM.

20 pages, 19 figures, 3 Tables. Submitted to A&A. Comments are welcome