Galaxy Distribution Systems as Fractals

TL;DR

This study investigates whether the large-scale distribution of galaxies can be described as a fractal system, analyzing multiple surveys across different redshifts to determine the fractal dimension and its evolution.

Contribution

It introduces a relativistic framework to estimate galaxy fractal dimensions across surveys and redshifts, revealing a decrease in fractal dimension at higher redshifts.

Findings

Fractal dimensions decrease with increasing redshift, especially beyond z=1.

Blue galaxies show consistent fractal dimensions across redshifts.

Red galaxies exhibit smaller fractal dimensions at higher redshifts.

Abstract

This work tests if the large-scale galaxy distribution can be characterized as a fractal system. The $\Lambda$CDM cosmology with $H_0=(70\pm 5)$ km/s/Mpc is adopted to study the UltraVISTA DR1, COSMOS2015 and SPLASH surveys, alongside the number density equations of these galaxy distribution systems as fractals with dimension D. The relativistic distance definitions $d_L$, $d_Z$ and $d_G$ are used to estimate the galaxy number densities in the redshift interval $0.1 \leq z \leq 4$ at volume limited subsamples. Applying the appropriate relations for the description of galaxy fractal structures with single dimension $D$ in the relativistic settings to these surveys datasets it is possible to state that for $z<1$ the UltraVISTA DR1 galaxies presented an average of $D=(1.58\pm 0.20)$, the COSMOS2015 galaxies produced $D=(1.39\pm 0.19)$ and the SPLASH galaxies generated $D=(1.00\pm 0.12)$. For $1 \leq z \leq 4$ the dimensions respectively decreased to $D=(0.59\pm 0.28)$, $D=0.54^{+0.27}_{-0.26}$ and $D=0.83^{+0.36}_{-0.37}$. These results are robust under the Hubble constant uncertainty assumed here. Analysis of blue and red galaxies subsamples in the COSMOS2015 and SPLASH surveys show that the fractal dimensions of blue galaxies present essentially no alteration from the values above, although the ones for the red galaxies changed mostly to smaller values, meaning that D may be assumed as a more intrinsic property of the distribution of objects in the Universe, thus allowing for the fractal dimension to be used as a tool to study different populations of galaxies. All results confirm the decades old theoretical prediction of a decrease in the fractal dimension for $z>1$ suggesting that either there are yet unclear observational biases causing such decrease in the fractal dimension, or the galaxy clustering was possibly more sparse and the universe void dominated in a not too distant past.