A Z=1.82 Analog of local ultra-massive elliptical galaxies
Résumé
We present observations of a very massive galaxy at $z$=1.82 which show that its morphology, size, velocity dispersion and stellar population properties that are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. These findings are based on a deep optical rest-frame spectrum obtained with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru telescope of a high-$z$ passive galaxy candidate (pBzK) from the COSMOS field, for which we accurately measure its redshift of $z$=1.8230 and obtain an upper limit on its velocity dispersion $\sigma$$_ \star$<326 km/s$^{-1}$. By detailed stellar population modeling of both the galaxy broad-band SED and the rest-frame optical spectrum we derive a star-formation-weighted age and formation redshift of $t_{sf}$$\simeq$1-2 Gyr and $z_{form}$$\simeq$2.5-4, and a stellar mass of $M$$_ \star$$\simeq$(3-4)x10$^{11}$ $M \odot$. This is in agreement with a virial mass limit of $M_{vir}$<7x10$^{11}$$M_\odot$, derived from the measured $\sigma_\star$ value and stellar half-light radius, as well as with the dynamical mass limit based on the Jeans equations. In contrast with previously reported super-dense passive galaxies at $z$$\sim$2, the present galaxy at $z$=1.82 appears to have both size and velocity dispersion similar to early-type galaxies in the local Universe with similar stellar mass. This suggests that $z$$\sim$2 massive and passive galaxies may exhibit a wide range of properties, then possibly following quite different evolutionary histories from $z$$\sim$2 to z=0.
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