Pierini, D. and Giodini, S. and Finoguenov, A. and Böhringer, H. and D'Onghia, E. and Pratt, G. W. and Démoclès, J. and Pannella, M. and Zibetti, S. and Braglia, F. G. and Verdugo, M. and Ziparo, F. and Koekemoer, A. M. and Salvato, M. (2011) Two fossil groups of galaxies at z ≈ 0.4 in the Cosmic Evolution Survey: accelerated stellar-mass build-up, different progenitors. Monthly Notices of the Royal Astronomical Society, 417 (4). pp. 2927-2937. ISSN 0035-8711 http://resolver.caltech.edu/CaltechAUTHORS:20120105-111108015
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We report on two fossil groups of galaxies at redshifts z= 0.425 and 0.372 discovered in the Cosmic Evolution Survey (COSMOS) area. Selected as X-ray extended sources, they have total masses (M_(200)) equal to 1.9(±0.41) × 10^(13) and 9.5(±0.42) × 10^(13) M_⊙, respectively, as obtained from a recent X-ray luminosity–mass scaling relation. The lower mass system appears isolated, whereas the other sits in a well-known large-scale structure (LSS) populated by 27 other X-ray emitting groups. The identification as fossil is based on the i-band photometry of all the galaxies with a photometric redshift consistent with that of the group at the 2σ confidence level and within a projected groupcentric distance equal to 0.5R_(200), and i_(AB)≤ 22.5 mag limited spectroscopy. Both fossil groups exhibit high stellar-to-total mass ratios compared to all the X-ray selected groups of similar mass at 0.3 ≤z≤ 0.5 in the COSMOS. At variance with the composite galaxy stellar mass functions (GSMFs) of similarly massive systems, both fossil group GSMFs are dominated by passively evolving galaxies down to M^(stars)∼ 10^(10) M_⊙ (according to the galaxy broad-band spectral energy distributions). The relative lack of star-forming galaxies with 10^(10) ≤ M^(stars) ≤ 10^(11) M_⊙ is confirmed by the galaxy distribution in the b−r versus i colour–magnitude diagram. Hence, the two fossil groups appear as more mature than the coeval, similarly massive groups. Their overall star formation activity ended rapidly after an accelerated build up of the total stellar mass; no significant infall of galaxies with M^(stars) ≥ 10^(10) M_⊙ took place in the last 3 to 6 Gyr. This similarity holds although the two fossil groups are embedded in two very different density environments of the LSS, which suggests that their galaxy populations were shaped by processes that do not depend on the LSS. However, their progenitors may do so. We discuss why the late merging of a compact group is favoured over the early assembly as a formation scenario for the isolated, low-mass fossil group.
|Additional Information:||© 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Accepted 2011 July 14. Received 2011 July 14; in original form 2011 January 21. Article first published online: 8 Sep 2011. We thank the referee, Paul Eigenthaler, for a careful reading of the original version of the paper and the useful comments that improved the presentation of the paper. DP acknowledges the kind and fruitful hospitality at the Max-Planck-Institut für extraterrestrische Physik (MPE).|
|Subject Keywords:||galaxies: evolution; galaxies: groups: general; galaxies: luminosity function, mass function; X-rays: galaxies: clusters|
|Official Citation:||Pierini, D., Giodini, S., Finoguenov, A., Böhringer, H., D’Onghia, E., Pratt, G. W., Démoclès, J., Pannella, M., Zibetti, S., Braglia, F. G., Verdugo, M., Ziparo, F., Koekemoer, A. M., Salvato, M. and the COSMOS Collaboration (2011), Two fossil groups of galaxies at z≈ 0.4 in the Cosmic Evolution Survey: accelerated stellar-mass build-up, different progenitors. Monthly Notices of the Royal Astronomical Society, 417: 2927–2937. doi: 10.1111/j.1365-2966.2011.19454.x|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||05 Jan 2012 21:04|
|Last Modified:||05 Jan 2012 21:04|
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