Mini-voids in the Local Volume

A.Tikhonov, St. Petersburg Univ. I.Karachentsev, SAO RAS

RUSSIA

Abstract

We consider a sphere of 7.5 Mpc radius, which contains 355 galaxies with accurately measured distances, to detect the nearest empty volumes. Using a simple void detection algorithm, we found six large (mini)voids in Aquila, Eridanus, Leo, Vela, Cepheus and Octans, each of more than 30 Mpc3. Besides them, 24 middle-size "bubbles" of more than 5 Mpc3 volume are detected, as well as 52 small "pores".

The six largest mini-voids occupy 58% of the considered volume. Addition of the bubbles and pores to them increases the total empty volume up to 75% and 81%, respectively.

The detected local voids look like oblong potatoes with typical axial ratios b/a = 0.75 and c/a = 0.62 (in the triaxial ellipsoide approximation). A correlation Gamma-function of the Local Volume galaxies follows a power low with a formally calculated fractal dimension D = 1.5.

We found that galaxies surrounding the local minivoids do not differ significantly from other nearby galaxies on their luminosity, but have appreciably higher hydrogen mass-to-luminosity ratio and also higher star formation rate. We recognize an effect of local expansion of typical minivoid to be ΔH/H0 ≈ (25 ± 15)%.

Distribution of 355 nearby galaxies within 7.5 Mpc

Illustration of action of the void-finding algorithm in a case of 2D point-like distribution. Seed spheres and voids growing from them are shown. The numerals indicate the order of detection of voids from large seed spheres to smaller ones.

Distribution of six large mini-voids within the sphere of radius 7.5 Mpc

Distribution of two nearest bubbles (N 20 and 18) and four pores within 3 Mpc

Directions of major axes of 30 voids

The void volumes vs. their ranks (left), and the correlation Gamma-function for 355 galaxies in the

Local Volume

Distribution of nearby galaxies (dots) and voids (circles) within 25 Mpc

The same distribution, but in the Galactic coordinates

3D distribution of galaxies (dots) and large voids within 25 Mpc

The Local (Tully) void

Galaxies surrounding the Local (Tully) void