ON THE FORMATION TIMESCALE OF MASSIVE CLUSTER ELLIPTICALS BASED ON DEEP NEAR-INFRARED SPECTROSCOPY AT z {approx} 2
- National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588 (Japan)
- Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Mariesvej 30, DK-2100 Copenhagen (Denmark)
- Department of Physics and Astronomy, Tufts University, Medford, MA 02155 (United States)
- European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei Muenchen (Germany)
- Max-Planck-Institut fuer extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching bei Muenchen (Germany)
- Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720 (United States)
We present improved constraints on the formation timescale of massive cluster galaxies based on rest-frame optical spectra of galaxies in a forming cluster located at z = 2.16. The spectra are obtained with MOIRCS on the Subaru Telescope with an integration time of {approx}7 hr. We achieve accurate redshift measurements by fitting spectral energy distributions using the spectra and broadband photometry simultaneously, allowing us to identify probable cluster members. Clusters at low redshifts are dominated by quiescent galaxies, but we find that quiescent galaxies and star-forming galaxies coexist in this z = 2 system. Interestingly, the quiescent galaxies form a weak red sequence in the process of forming. By stacking the spectra of star-forming galaxies, we observe strong emission lines such as [O II] and [O III] and we obtain a tentative hint of active galactic nucleus activities in these galaxies. On the other hand, the stacked spectrum of the quiescent galaxies reveals a clear 4000 A break with a possible Ca II H+K absorption feature and strong emission lines such as [O II] are absent in the spectrum, confirming the quiescent nature of these galaxies. We then perform detailed spectral analyses of the stacked spectrum, which suggest that these massive quiescent galaxies formed at redshifts between 3 and 4 on a timescale of {approx}< 0.5 Gyr. This short formation timescale is not reproduced in recent numerical simulations. We discuss possible mechanisms for how these galaxies form 10{sup 11} M{sub Sun} stellar mass on a short timescale and become red and quiescent by z = 2.
- OSTI ID:
- 22121801
- Journal Information:
- Astrophysical Journal, Vol. 772, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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