PDF《compact?》

arXiv:0909.

3088v1 [astro-ph.CO]

16 Sep

2009 Mon. Not. R. Astron. Soc. 000, 1C9 (2002) Printed

3 October

2018 (MN L A TEX style ?le v2.2) High-redshift elliptical galaxies: are they (all) really compact? C. Mancini1? ,E. Daddi2 , A. Renzini1 , F. Salmi2 , H. J. McCracken3 , A. Cimatti4 , M. Onodera2 , M. Salvato5 , A. M. Koekemoer6 , H. Aussel2 , E. Le Floc'

h 2,7 , C. Willott8 1Osservatorio Astronomico di Padova (INAF-OAPD), Vicolo dell'

Osservatorio 5, I-35122, Padova Italy 2CEA-Saclay,DSM/DAPNIA/Service d'

Astrophysique,

91191 Gif-Sur Yvette Cedex, France 3Institut d'

Astrophysique de Paris, UMR7095, Universit? e Pierre et Marie Curie,

98 bis Boulevard Arago,

75014 Paris, France 4Dipartimento di Astronomia, Universit` a di Bologna, via Ranzani 1, I-40127 Bologna, Italy 5California Institute of Technology, 105-24 Robinson,

1200 E. California Blvd., Pasadena CA 91125, USA 6Space Telescope Science Institute,

3700 San Martin Drive, Baltimore, MD 21218, USA 7Institute for Astronomy, Univ. of Hawaii,

2680 Woodlawn Drive, Honolulu, HI 96822, USA 8Herzberg Institute of Astrophysics, National Research Council,

5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada

3 October

2018 ABSTRACT We investigate the properties of

12 ultra-massive passively evolving early type galaxies (ETGs) at zphot >

1.4 in the COSMOS

2 deg2 ?eld. These

12 ETGs were selected as pBzKs, have accurate 1.4 zphot 1.7, high S? ersic index pro?les typical of ellipticals, no detection at 24?m, resulting in a complete ETG sample at M? >

2.5 *

1011 M⊙ (Chabrier IMF). Contrary to previous claims, the half light radii estimated in very high S/N imaging data from HST+ACS are found to be large for most of the sample, consistent with local ellipticals. If the high redshift ETGs with M? <

2.5 *

1011 M⊙ are really small in size and compact as reported in previous studies, our result may suggest a downsizing scenario, whereby the most massive ETGs reach their ?nal structure earlier and faster than lower mass ones. However, simulating galaxies with morphological properties ?xed to those of local ETGs with the same stellar mass show that the few compact galaxies that we still recover in our sample can be understood in term of ?uctuations due to noise preventing the recovery of the extended low surface brightness halos in the light pro?le. Such halos, typical of S? ersic pro?les, extending even up to

40 kpc, are indeed seen in our sample. Key words: galaxies:evolution C galaxies:formation Cgalaxies: high redshift C galax- ies: elliptical and lenticular, cD.

1 INTRODUCTION Understanding the formation processes of galactic spheroids, i.e., bulges and early-type galaxies (ETG), remains a cen- tral theme in the context of galaxy formation and evolu- tion. The discovery of a widespread population of passively evolving ETGs at z >

1.4 proved that quenching of star formation in massive galaxies is well under way by z ?

2 (Glazebrook et al. 2004;

Cimatti et al. 2004). Further studies of high redshift ETGs then revealed an unexpected property of a sizable fraction of them: many such ETGs appear to have much smaller e?ective radii (Re, by factors ?

2 to ? 4) with respect to ETGs of comparable stellar mass (M?) in the local Universe (Daddi et al. 2005, D05), a result then con?rmed by several other studies (e.g. ? E-mail:chiara.mancini@oapd.inaf.it Trujillo et al. 2006, 2007;

Longhetti et al. 2007;

Zirm et al. 2007;

McGrath et al. 2008;

Toft et al. 2007;