The uncertain interstellar medium of high-redshift quiescent galaxies: Impact of methodology

Abstract

How much gas and dust is contained in high-redshift quiescent galaxies (QGs) is currently an open question with relatively few and contradictory answers, as well as important implications for our understanding of the nature of star formation quenching processes at cosmic noon. Here we revisit far-infrared (FIR) observations of the REQUIEM-ALMA sample of six z = 1.6 − 3.2 QGs strongly lensed by intermediate-redshift galaxy clusters. We measured their continuum emission using priors obtained from high resolution near-infrared (NIR) imaging, as opposed to focusing on point-source extraction, converted it into dust masses using a FIR dust emission model derived from statistical samples of QGs, and compared the results to those of the reference work. We find that, while at least the most massive sample galaxy is indeed dust-poor, the picture is much more nuanced than previously reported. In particular, these more conservative constraints remain consistent with high dust fractions in early QGs. We find that these measurements are very sensitive to the adopted extraction method and conversion factors: the use of an extended light model to fit the FIR emission increases the flux of detections by up to 50% and the upper limit by up to a factor 6. Adding the FIR-to-dust conversion, this amounts to an order of magnitude difference in dust fraction, casting doubts on the power of these data to discriminate between star formation quenching scenarios. Unless these are identified by other means, mapping the dust and gas in high-redshift QGs will continue to require somewhat costly observations.

The large molecular gas fraction of post-starburst galaxies at z > 1

“…The far-infrared continuum dust emission has been used as a proxy of the cold gas content of post-SB and quiescent galaxies up to z ∼ 2 (e.g. Gobat et al 2018Gobat et al , 2022Michałowski et al 2019 ;Whitaker et al 2021 ). Ho we ver such studies yield contradictory results, mainly depending on the method used to estimate the dust and in turn gas mass of galaxies.…”

“…Such discrepancies might be due to uncertainties on the photometric redshift of galaxies entering the stacking, the coarse resolution of the observations and hence blending of quiescent and star-forming sources (which affect the first method), uncertainties on the lensing model and magnification of lensed targets especially in case of extended emission (which affect the second method). Moreo v er uncertainties about the dust-to-gas conversion factor, dust temperature, and compactness of the emission might also be responsible for such discrepancies (Gobat et al 2022 ).…”