The debiased morphological transformation of galaxies since Z=3 in CANDELS.

Abstract

To trace the morphological evolution of galaxies one first needs to correctly quantify the morphological mixture at different epochs. Morphological quantitative measurements and visual-like classifications are, however, susceptible to biases such as cosmological surface brightness dimming (CSBD): the measured surface brightness of a galaxy decays with redshift as (1 + z) −4 . However, the effect of CSBD on morphology has not been thoroughly discussed in literature. To investigate the impact of CSBD on morphological classifications, we employ artificial redshifting techniques on a sample of 206 galaxies in the GOODS-S field, spanning redshifts from z = 0.2 to z = 3.0. We compare the visual classifications and morphological coefficients (G and M20) obtained from the original and simulated images. Subsequently, we develop two correction methods to mitigate the effects of CSBD. The first approach involves calculating correction percentages for false-positives and false-negatives in the visual classifications, while the second method is based on tracking the shifts of objects in the G − M20 parameter space with increasing redshift. Both correction methods are then applied to a CANDELS sample. Our findings reveal that CSBD, low resolution and signal-to-noise significantly biases visual morphological classifications beyond z > 1.0. Specifically, we observe an overestimation of the fractions of spheroids and irregular galaxies by up to 20% and 60%, respectively, while the fractions of early and late-type disks are underestimated by up to 40%. However, we find that morphological coefficients are not significantly affected by CSBD. Furthermore, we find that galaxies tend to increase their asymmetry and G value, and decrease their M20 and half-radius toward high-redshift, thus exhibiting a more diffuse, asymmetric, and less clumpy appearance at higher redshifts. We validate the consistency of our correction methods by applying them to the observed morphological fractions in the IllustrisTNG-50 sample, yielding similar results to those obtained from the CANDELS sample. This suggests that the TNG simulation effectively reproduces galaxy evolution in terms of morphology. We propose two potential confusion channels in morphological classifications due to CSBD: galaxies may be misclassified as spheroids or as irregulars. Additionally, we analyze the morphological fractions of star-forming and quiescent subsamples as a function of redshift and stellar mass. Our investigation of the quiescent subsample reveals a decreasing trend in the fraction of late-type disk galaxies with cosmic time, while the fractions of quiescent early-type disks and spheroid galaxies. Star-forming late-type galaxies are also found to decrease at the exchange of an increasing irregular fraction. Our results imply that late-type-disks are transforming into star-forming irregulars or into quiescent spheroids/early-type disks. We suggest that merger events are responsible for the morphological transformation of late-type disks and that the increase in bulge mass is driving the star-formation quenching.