A robust morphological classification of high-redshift galaxies using support vector machines on seeing limited images. II. Quantifying morphological k-correction in the COSMOS field at 1 < z < 2: Ks band vs. I band
Résumé
Context: Morphology is the most accessible tracer of galaxies physical structure, but its interpretation in the framework of galaxy evolution still remains problematic. Its quantification at high redshift requires deep high-angular resolution imaging, which is why space data (HST) are usually employed. At z > 1, the HST visible cameras however probe the UV flux, which is dominated by the emission of young stars, which could bias the estimated morphologies towards late-type systems.
Aims: In this paper we quantify the effects of this morphological k-correction at 1 < z < 2 by comparing morphologies measured in the K and I-bands in the COSMOS area. The Ks-band data indeed have the advantage of probing old stellar populations in the rest frame for z < 2, enabling determination of galaxy morphological types unaffected by recent star formation.
Methods: In Paper I we presented a new non-parametric method of quantifying morphologies of galaxies on seeing-limited images based on support vector machines. Here we use this method to classify ~50 000 Ks selected galaxies in the COSMOS area observed with WIRCam at CFHT. We use a 10-dimensional volume, including 5 morphological parameters, and other characteristics of galaxies such as luminosity and redshift. The obtained classification is used to investigate the redshift distributions and number counts per morphological type up to z ~ 2 and to compare them to the results obtained with HST/ACS in the I-band on the same objects. We associate to every galaxy with Ks < 21.5 and z < 2 a probability between 0 and 1 of being late-type or early-type. We use this value to assess the accuracy of our classification as a function of physical parameters of the galaxy and to correct for classification errors.
Results: The classification is found to be reliable up to z ~ 2. The mean probability is p ~ 0.8. It decreases with redshift and with size, especially for the early-type population, but remains above p ~ 0.7. The classification globally agrees with the one obtained using HST/ACS for z < 1. Above z ~ 1, the I-band classification tends to find less early-type galaxies than the Ks-band one by a factor ~1.5, which might be a consequence of morphological k-correction effects.
Conclusions: We argue therefore that studies based on I-band HST/ACS classifications at z > 1 could be underestimating the elliptical population. Using our method in a Ks
Aims: In this paper we quantify the effects of this morphological k-correction at 1 < z < 2 by comparing morphologies measured in the K and I-bands in the COSMOS area. The Ks-band data indeed have the advantage of probing old stellar populations in the rest frame for z < 2, enabling determination of galaxy morphological types unaffected by recent star formation.
Methods: In Paper I we presented a new non-parametric method of quantifying morphologies of galaxies on seeing-limited images based on support vector machines. Here we use this method to classify ~50 000 Ks selected galaxies in the COSMOS area observed with WIRCam at CFHT. We use a 10-dimensional volume, including 5 morphological parameters, and other characteristics of galaxies such as luminosity and redshift. The obtained classification is used to investigate the redshift distributions and number counts per morphological type up to z ~ 2 and to compare them to the results obtained with HST/ACS in the I-band on the same objects. We associate to every galaxy with Ks < 21.5 and z < 2 a probability between 0 and 1 of being late-type or early-type. We use this value to assess the accuracy of our classification as a function of physical parameters of the galaxy and to correct for classification errors.
Results: The classification is found to be reliable up to z ~ 2. The mean probability is p ~ 0.8. It decreases with redshift and with size, especially for the early-type population, but remains above p ~ 0.7. The classification globally agrees with the one obtained using HST/ACS for z < 1. Above z ~ 1, the I-band classification tends to find less early-type galaxies than the Ks-band one by a factor ~1.5, which might be a consequence of morphological k-correction effects.
Conclusions: We argue therefore that studies based on I-band HST/ACS classifications at z > 1 could be underestimating the elliptical population. Using our method in a Ks