Winding Back the Clock: Recent Star Formation Histories of Massive Quiescent Galaxies Are Consistent With Their Rapid Number Density Evolution Since $mathbf{zsim7}$
Yunchong Zhang, Zhiyuan Ji, Rachel Bezanson, Christina C. Williams, Gabriel Brammer, Aidan P. Cloonan, Anna de Graaff, Jenny E. Greene, Michaela Hirschmann, Christian Kragh Jespersen, Gourav Khullar, Claudia del P. Lagos, Joel Leja, Michael V. Maseda, Ian McConachie, Pascal A. Oesch, Sedona H. Price, David J. Setton, Katherine A. Suess, Katherine E. Whitaker
arXiv:2604.05024v1 Announce Type: new
Abstract: Massive quiescent galaxies have been identified out to $zsim7$ in early JWST data in a substantial excess ($rm gtrsim 1,dex$ at $z>4$) of number densities from most theoretical predictions. We investigate whether the number densities implied by the star formation histories of quiescent galaxies at $27$. For this work, we rely on stellar population synthesis modeling of JWST NIRCam photometry (from CEERS and PRIMER) and NIRSpec/PRISM spectra of massive ($rm M_{*} > 10^{10.5}M_{odot}$) quiescent galaxies in the RUBIES survey. We infer their star-formation histories through Bayesian spectro-photometric fitting with Prospector, exploring the sensitivity of our results to stellar libraries and SFH priors. For each source, we compute a timescale over which it would be identified as quiescent — leveraging the recent and most robust SFH timescale — and deduce the number density of the quiescent population at previous epochs. These reconstructed number densities are then compared to existing observational constraints, including a new measurement from the PANORAMIC pure parallel survey, whose wide-area and independent sightlines reduce sensitivity to cosmic variance. We find striking agreement between reconstructed and observed number densities up to $zsim7$, a self-consistency that lends credence to stellar population synthesis modeling of distant quiescent galaxies. Furthermore, by connecting the recent ($rm sim 1,Gyr$) star-formation histories and number densities of quiescent galaxies and their implied progenitors, we reinforce the known tension between observations and model predictions at $3arXiv:2604.05024v1 Announce Type: new
Abstract: Massive quiescent galaxies have been identified out to $zsim7$ in early JWST data in a substantial excess ($rm gtrsim 1,dex$ at $z>4$) of number densities from most theoretical predictions. We investigate whether the number densities implied by the star formation histories of quiescent galaxies at $27$. For this work, we rely on stellar population synthesis modeling of JWST NIRCam photometry (from CEERS and PRIMER) and NIRSpec/PRISM spectra of massive ($rm M_{*} > 10^{10.5}M_{odot}$) quiescent galaxies in the RUBIES survey. We infer their star-formation histories through Bayesian spectro-photometric fitting with Prospector, exploring the sensitivity of our results to stellar libraries and SFH priors. For each source, we compute a timescale over which it would be identified as quiescent — leveraging the recent and most robust SFH timescale — and deduce the number density of the quiescent population at previous epochs. These reconstructed number densities are then compared to existing observational constraints, including a new measurement from the PANORAMIC pure parallel survey, whose wide-area and independent sightlines reduce sensitivity to cosmic variance. We find striking agreement between reconstructed and observed number densities up to $zsim7$, a self-consistency that lends credence to stellar population synthesis modeling of distant quiescent galaxies. Furthermore, by connecting the recent ($rm sim 1,Gyr$) star-formation histories and number densities of quiescent galaxies and their implied progenitors, we reinforce the known tension between observations and model predictions at $3