Volume 597, January 2017
|Number of page(s)||12|
|Published online||12 January 2017|
Anisotropy in the all-sky distribution of galaxy morphological types
1 Argelander Institut für Astronomie der Universität Bonn Auf dem Hügel 71 53121 Bonn Germany
2 Helmholtz-Institut fuer Strahlen und Kernphysik, Nussallee 14-16, 53115 Bonn, Germany
3 Charles University in Prague, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
Received: 26 July 2016
Accepted: 19 September 2016
We present the first study of the isotropy of the all-sky 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 use a hemispherical comparison method where the sky is divided into two opposite hemispheres and the abundance distribution of the morphological types, T, are compared using the Kolmogorov-Smirnov (KS) test. By pointing the axis of symmetry of the hemisphere pairs to different directions in the sky, 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 observed hemispherical asymmetry for the three distance ranges is aligned with the celestial equator at the 97.1−99.8% confidence level and with the ecliptic at 94.6−97.6%, 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 ≤ 15 mag is applied to these samples, the galaxies within 100 Mpc show no significant anisotropy after randomization of T. However, the direction of the asymmetry in the samples within 150 and 200 Mpc and the same magnitude limit is found to be within an angular separation of 32 degrees from (l,b) = (123.7,24.6) with a 97.2% and 99.9% confidence level, 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 morphologies 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 the exact source of this anisotropy.
Key words: galaxies: structure / galaxies: statistics / galaxies: abundances / cosmology: observations / methods: data analysis
© ESO, 2017
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