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TwitterA&A 482, 21-42 (2008)
DOI: 10.1051/0004-6361:20078739
GMASS ultradeep spectroscopy of galaxies at z ~ 2
II. Superdense passive galaxies: how did they form and evolve?
A. Cimatti1, P. Cassata2, L. Pozzetti3, J. Kurk4, M. Mignoli3, A. Renzini5, E. Daddi6, M. Bolzonella3, M. Brusa7, G. Rodighiero8, M. Dickinson9, A. Franceschini8, G. Zamorani3, S. Berta6, P. Rosati10, and C. Halliday111 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: a.cimatti@unibo.it
2 Laboratoire d'Astrophysique de Marseille, OAMP, UMR6110, CNRS-Université de Provence Aix-Marseille I, BP 8, 13376 Marseille Cedex 12, France
3 INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
4 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
5 INAF - Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
6 CEA - Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette Cedex, France
7 Max Planck Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
8 Università di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
9 NOAO - Tucson, 950 North Cherry Avenue, Tucson, AZ 85719, USA
10 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
11 INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
(Received 26 September 2007 / Accepted 28 December 2007)
Abstract
Aims. The aim of this work is to investigate the physical, structural and
evolutionary properties of old, passive galaxies at z > 1.4 and to
place new constraints on massive galaxy formation and evolution.
Methods. We combine ultradeep optical spectroscopy from the GMASS project
(Galaxy Mass Assembly ultradeep Spectroscopic Survey) with
GOODS multi-band (optical to mid-infrared) photometry and HST imaging
to study a sample of spectroscopically identified passive galaxies
at
1.39 < z < 1.99 selected from Spitzer Space Selescope imaging
at 4.5 
m.
Results. A stacked spectrum with an equivalent integration time of ~500 h was obtained and compared with libraries of synthetic stellar
population spectra. The stacked spectrum is publicly released.
The spectral and photometric SED properties indicate very weak or absent
star formation, moderately old stellar ages of 
1 Gyr
(for solar metallicity) and stellar masses in the range of
1010-11 
, thus implying that the major star formation and assembly
processes for these galaxies occurred at z > 2. No X-ray emission was
found neither from individual galaxies nor from a stacking analysis
of the sample. Only one galaxy shows a marginal detection at 24 m.
These galaxies have morphologies that are predominantly compact and spheroidal.
However, their sizes (
1 kpc) are much smaller than those
of spheroids in the present-day Universe. Their stellar mass surface
densities are consequently higher by 1 dex if compared to
spheroids at z0 with the same mass. Their rest-frame B-band
surface brightness scales with the effective radius, but the offset
with respect to the surface brightness of the local Kormendy relation is
too large to be explained by simple passive evolution.
At 
, a larger fraction
of passive galaxies follows the z0 size-mass relation.
Superdense relics
with 1 kpc are extremely rare at z0 with respect
to z > 1, and absent if < 1 kpc. Because of the similar sizes and
mass densities, we suggest that the superdense passive galaxies at
1 < z < 2 are the remnants of the powerful starbursts occurring in
submillimeter-selected galaxies at z > 2. The results are compared
with theoretical models and the main implications discussed in the framework
of massive galaxy formation and evolution.
Key words: cosmology: observations -- galaxies: distances and redshifts -- galaxies: elliptical and lenticular, cD -- galaxies: evolution -- galaxies: formation -- galaxies: high-redshift
© ESO 2008