Anisotropy in the all-sky distribution of galaxy morphological types

TL;DR

This study investigates the distribution of galaxy morphological types across the sky, revealing significant hemispherical asymmetries aligned with celestial coordinates, suggesting potential biases in galaxy classification or new anisotropic features in the universe.

Contribution

First analysis of galaxy morphological type distribution for isotropy in the Local Universe using a large dataset, revealing significant hemispherical asymmetries aligned with celestial coordinates.

Findings

Significant hemispherical asymmetry in galaxy types within 200 Mpc.

Asymmetry aligned with Celestial Equator and Ecliptic.

No significant anisotropy in a magnitude-limited sample within 100 Mpc.

Abstract

We present the first study of the isotropy of the distribution of morphological types of galaxies in the Local Universe out to around 200 Mpc using more than 60,000 galaxies from the HyperLeda database. We divide the sky into two opposite hemispheres and compare the abundance distribution of the morphological types, $T$, using the Kolmogorov-Smirnov (KS) test. This is repeated for different directions in the sky and the KS statistic as a function of sky coordinates is obtained. For three samples of galaxies within around 100, 150, and 200 Mpc, we find a significant hemispherical asymmetry with a vanishingly small chance of occurring in an isotropic distribution. Astonishingly, regardless of this extreme significance, the hemispherical asymmetry is aligned with the Celestial Equator at the 97.1-99.8% and with the Ecliptic at the 94.6-97.6% confidence levels, estimated using a Monte Carlo analysis. Shifting $T$ values randomly within their uncertainties has a negligible effect on this result. When a magnitude limit of $B\leq 15$ mag is applied, the sample within 100 Mpc shows no significant anisotropy after random shifting of $T$. However, the direction of the asymmetry in the samples within 150 and 200 Mpc and $B\leq 15$ mag is found to be within an angular separation of 32 degrees from $(l,b)=(123.7, 24.6)$ with 97.2% and 99.9% confidence levels, respectively. This direction is only 2.6 degrees away from the Celestial North Pole. Unless the Local Universe has a significant anisotropic distribution of galaxy types aligned with the orientation or the orbit of the Earth (which would be a challenge for the Cosmological Principle), our results show that there seems to be a systematic bias in the classification of galaxy morphological types between the data from the Northern and the Southern Equatorial sky. Further studies are absolutely needed to find out the exact source of this anisotropy.