Morphological evolution ofz ~ 1 galaxies from deepK-band AO imaging in the COSMOS deep field

Abstract

Context. We present the results of an imaging program of distant galaxies (z ∼ 0.8) at high spatial resolution (∼0.1 �� ) aiming at studying their morphological evolution. We observed 7 fields of 1 � × 1 � with the NACO Adaptive Optics system (VLT) in Ks (2.16 µm) band with typical V ∼ 14 guide stars and 3 h integration time per field. Observed fields are selected within the COSMOS survey area, in which multi-wavelength photometric and spectroscopic observations are ongoing. High angular-resolution K-band data have the advantage of probing old stellar populations in the rest-frame, enabling a determination of galaxy morphological types unaffected by recent star formation, which are more closely linked to the underlying mass than classical optical morphology studies (HST). Adaptive optics on ground based telescopes is the only method today for obtaining such a high resolution in the K-band, but it suffers from limitations since only small fields are observable and long integration times are necessary. Aims. In this paper we show that reliable results can be obtained and establish a first basis for larger observing programs. Methods. We analyze the morphologies by means of B/ D( bulge/disk) decomposition with GIM2D and C-A (concentrationasymmetry) estimators for 79 galaxies with magnitudes between Ks = 17−23 and classify them into three main morphological types (late type, early type and irregulars). Automated and objective classification allows precise error estimation. Simulations and comparisons with seeing-limited (CFHT/Megacam) and space (HST/ACS) data are carried out to evaluate the accuracy of adaptive optics-based observations for morphological purposes. Results. We obtain the first estimate of the distribution of galaxy types at redshift z ∼ 1 as measured from the near infrared at high spatial resolution. We show that galaxy parameters (disk scale length, bulge effective radius, and bulge fraction) can be estimated with a random error lower than 20% for the bulge fraction up to Ks = 19 (AB = 21) and that classification into the three main morphological types can be done up to Ks = 20 (AB = 22) with at least 70% of the identifications correct. We used the known photometric redshifts to obtain a redshift distribution over 2 redshift bins (z < 0.8, 0.8 < z < 1.5) for each morphological type.