Issue |
A&A
Volume 523, November-December 2010
|
|
---|---|---|
Article Number | A13 | |
Number of page(s) | 23 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913020 | |
Published online | 10 November 2010 |
zCOSMOS – 10k-bright spectroscopic sample
The bimodality in the galaxy stellar mass function: exploring its evolution with redshift⋆
1
INAF – Osservatorio Astronomico di Bologna,
via Ranzani 1,
40127
Bologna,
Italy
e-mail: lucia.pozzetti@oabo.inaf.it
2
Institute of Astronomy, Swiss Federal Institute of Technology (ETH
Hönggerberg), 8093
Zürich,
Switzerland
3
Dipartimento di Astronomia, Universitá di Padova,
Padova,
Italy
4
Dipartimento di Astronomia, Universitá degli Studi di
Bologna, via Ranzani
1, 40127
Bologna,
Italy
5
Department of Astronomy, University of
Massachusetts, 710 North Pleasant
Street, Amherst,
MA
01003,
USA
6
Laboratoire d’Astrophysique de Marseille, Université
d’Aix-Marseille, CNRS, 38 rue
Frederic Joliot-Curie, 13388
Marseille Cedex 13,
France
7
INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di
Milano, via Bassini 15,
20133
Milano,
Italy
8
Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de
Toulouse, CNRS, 14 avenue Édouard
Belin, 31400
Toulouse,
France
9
Max Planck Institut für Extraterrestrische Physik,
Giessenbachstrasse,
84571
Garching,
Germany
10
Universitats Sternwarte, Scheinerstrasse 1, 81679
Muenchen,
Germany
11
INAF – Osservatorio Astrofisico di Arcetri,
Largo Enrico Fermi 5,
50125
Firenze,
Italy
12
INAF – Osservatorio Astronomico di Brera,
via Brera 28,
20121
Milano,
Italy
13
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218,
USA
14
Centre de Physique Théorique, Marseille,
Marseille,
France
15
Institut d’Astrophysique de Paris, Université Pierre &
Marie Curie, Paris,
France
16
INAF – Osservatorio Astronomico di Roma,
via di Frascati 33,
00040
Monteporzio Catone,
Italy
17
Spitzer Science Center, Pasadena, CA, USA
18
California Institute of Technology, MC 105-24, 1200 East California Boulevard,
Pasadena, CA
91125,
USA
Received:
30
July
2009
Accepted:
31
May
2010
We present the galaxy stellar mass function (GSMF) to redshift z ≃ 1, based on the analysis of about 8500 galaxies with I < 22.5 (AB mag) over 1.4 deg2, which are part of the zCOSMOS-bright 10k spectroscopic sample. We investigate the total GSMF, as well as the contributions of early- and late-type galaxies (ETGs and LTGs, respectively), defined by different criteria (broad-band spectral energy distribution, morphology, spectral properties, or star formation activities). We unveil a galaxybimodality in the global GSMF, whose shape is more accurately represented by 2 Schechter functions, one linked to the ETG and the other to the LTG populations. For the global population, we confirm a mass-dependent evolution (“mass-assembly downsizing”), i.e., galaxy number density increases with cosmic time by a factor of two between z = 1 and z = 0 for intermediate-to-low mass (log (ℳ/ℳ⊙) ~ 10.5) galaxies but less than 15% for log (ℳ/ℳ⊙) > 11. We find that the GSMF evolution at intermediate-to-low values of ℳ (log (ℳ/ℳ⊙) < 10.6) is mostly explained by the growth in stellar mass driven by smoothly decreasing star formation activities, despite the redder colours predicted in particular at low redshift. The low residual evolution is consistent, on average, with ~0.16 merger per galaxy per Gyr (of which fewer than 0.1 are major), with a hint of a decrease with cosmic time but not a clear dependence on the mass. From the analysis of different galaxy types, we find that ETGs, regardless of the classification method, increase in number density with cosmic time more rapidly with decreasing ℳ, i.e., follow a top-down building history, with a median “building redshift” increasing with mass (z > 1 for log (ℳ/ℳ⊙) > 11), in contrast to hierarchical model predictions. For LTGs, we find that the number density of blue or spiral galaxies with log (ℳ/ℳ⊙) > 10 remains almost constant with cosmic time from z ~1. Instead, the most extreme population of star-forming galaxies (with high specific star formation), at intermediate/high-mass, rapidly decreases in number density with cosmic time. Our data can be interpreted as a combination of different effects. Firstly, we suggest a transformation, driven mainly by SFH, from blue, active, spiral galaxies of intermediate mass to blue quiescent and subsequently (1−2 Gyr after) red, passive types of low specific star formation. We find an indication that the complete morphological transformation, probably driven by dynamical processes, into red spheroidal galaxies, occurred on longer timescales or followed after 1−2 Gyr. A continuous replacement of blue galaxies is expected to be accomplished by low-mass active spirals increasing their stellar mass. We estimate the growth rate in number and mass density of the red galaxies at different redshifts and masses. The corresponding fraction of blue galaxies that, at any given time, is transforming into red galaxies per Gyr, due to the quenching of their SFR, is on average ~25% for log (ℳ/ℳ⊙) < 11. We conclude that the build-up of galaxies and in particular of ETGs follows the same downsizing trend with mass (i.e. occurs earlier for high-mass galaxies) as the formation of their stars and follows the converse of the trend predicted by current SAMs. In this scenario, we expect there to be a negligible evolution of the galaxy baryonic mass function (GBMF) for the global population at all masses and a decrease with cosmic time in the GBMF for the blue galaxy population at intermediate-high masses.
Key words: galaxies: evolution / galaxies: luminosity function, mass function / galaxies: statistics / galaxies: formation
© ESO, 2010
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