A direct constraint on the gas content of a massive, passively evolving elliptical galaxy at z = 1.43

Abstract

Gas and dust in star-forming galaxies at the peak epoch of galaxy assembly are presently the topic of intense study, but little is known about the interstellar medium (ISM) of distant, passively evolving galaxies. We report on a deep 3 mm-band search with IRAM/PdBI for molecular (H$_2$) gas in a massive ($M_{\star}{\sim}6{\times}10^{11}M_{\odot}$) elliptical galaxy at z=1.4277, the first observation of this kind ever attempted. We place a 3$\sigma$ upper limit of 0.32 Jy km/s on the flux of the CO($J$=2$\rightarrow$1) line or $L'_{\rm CO}$$<$8.8$\times$10$^{9}$ K km/s pc$^2$, assuming a disk-like CO-morphology and a circular velocity scaling with the stellar velocity dispersion as in local early-type galaxies (ETGs). This translates to an H$_2$ mass of $<$3.9$\times$10$^{10}$($\alpha_{\rm CO}$/4.4)$M_{\odot}$ or a gas fraction of $\lesssim$6% assuming a Salpeter initial mass function (IMF) and an ISM dominated by H$_2$, as observed in many local, high-mass ellipticals. This low value approaches that of local ETGs, suggesting that the low star formation activity in massive, high-z passive galaxies reflects a true dearth of gas and a lesser role for inhibitive mechanisms like morphological quenching.