Caracterização de envelopes estendidos de luz em galáxias BCGS

Abstract

In this work, we revisit the problem of morphological classification of BCG galaxies, in order to identify the characteristic signatures of cD-class galaxies. For this, we analyze the r-band surface brightness distributions from the SDSS survey of a sample of 339 BCG galaxies, visually classified by Zhao; Aragón-Salamanca; Conselice (2015). By means of a modeling of the brightness distribution via a Sérsic profile fit, we obtained the residual flux ratio RFF and rotational asymmetry A. In the plane logRFF − η with η = RFF − A, we characterize three families of BCGs (“envelopes”, “transition” and “elliptical" conventional). Analyzing the radial profiles of ellipticity, position angle and brightness of isophots of representative objects, wefindthat the visual classification has no direct correspondence with the families identified in the log RFF−ηplane. Thelightprofilesofthe“envelopes”family are generally consistent with the occurrence of an external structural component described by an exponential law. Objects in the transition family resemble those of the “envelopes” class, except that they show less unequivocal signs of the presence of the envelope and more frequently show signs that the envelope is under development. The typical ellipticities of the different families identified in the logRFF − η plane are compatible with other estimates made in the literature, but the galaxies visually classified as ellipticals are much more circular, suggestive of a classification bias. Furthermore, the logarithmic slope of the radial brightness profiles of galaxies visually classified as cD but which are in the region of the elliptic galaxies in the logRFF −η diagram is almost identical to that shown by cD galaxies as a whole, so these galaxies are compatible with the morphological class D. This suggests that the visual classification is sensitive mainly to the logarithmic slope of the brightness profiles and not to the presence of an external envelope, thus confusing a galaxy D with a galaxy cD. These results show that visual inspection classifications are subject to at least two sources of bias that contaminate samples of cD galaxies, with impacts both on the demography and on studies on the formation and evolution of these objects.