Formation of the Nearby Void catalog and a sample of Galaxies residing in them

The purpose of this work is to form a catalog of Nearby Voids and galaxies residing in them. Voids are elements of the cosmic web that are visible in the matter distribution; they occupy more than 75% of the Universe volume but still contain 15% of its mass only. Observational properties of voids and their galaxies are important for comparison with the predictions of the modern cosmological models of the formation and evolution of galaxies and their structures. Particularly, in the models there are indications on possible differences between the properties of void galaxies and of those located in a denser environment.

Due to the observational selection effects, the previous works in the field dealt with quite remote voids (D=80-200 Mpc) and their brighter galaxies (with the blue absolute magnitudes M_B in the range of [-16, -20]). Properties of low-mass dwarfs in voids are little-known and studied for only one nearby void. To study the least massive void galaxies and the fine structure of voids, the sample in the nearby neighborhood is required. However, formation of such a sample is complicated due to relatively large contribution of so-called peculiar velocities into the Hubble flow velocities in the nearby Universe. To a first approximation, we managed to resolve this issue in our paper.

The result of the present work is the catalog of 25 voids located at distances up to 25 Mpc from our Galaxy along the whole celestial sphere. The voids are limited by «big» galaxies that in the near IR range have the absolute magnitudes M_K brighter than -22.0 mag (which corresponds to 20 billion solar luminosities in K-band). The largest void sizes range from 14 to 35 Mpc. Inside the voids, there are 1354 galaxies with the absolute magnitudes in the blue filter M_B in the range of [-8, -19.5] and the median value M_B of about -15. Of them, 1088 objects are «inner» void galaxies, with distances to «big» galaxies of greater than 2 Mpc. The galaxy catalog includes their basic parameters (coordinates, distances, apparent brightness, and absolute magnitudes) as well as their finding charts. The paper briefly describes the statistical properties of galaxies in Nearby Voids including the morphology and fraction of early-type galaxies (elliptical and similar to them), clustering, and the interstellar gas content.

The present catalog is the data-base for studying the properties of low-mass galaxies in voids and for comparing their properties with theoretical models as well as for studying the substructure of voids which, within the modern paradigm including the Dark Matter, should depend on the properties of the Dark Matter. Interestingly, about 200 galaxies in the Nearby Voids fall into the Local Volume (i.e., the region within a distance of 11 Mpc from us); this gives us additional opportunities to study them in detail via their individual resolved stars. Moreover, we show potential opportunities of the use the sample in searching for unusual youngest dwarfs in the nearby Universe.

Figures 2 and 3 show the 3D images of several described Nearby Voids in the supergalactic coordinates SGX, SGY, and SGZ.

Fig.1. Distributions of all the galaxies in the Nearby Voids and the subsample of «inner» galaxies (the dashed area in the histograms). Left to right: 1) the distribution of absolute magnitudes (luminosities) in the B band for all 1247 galaxies with the known M_B; 2) the distribution of distances (in Mpc) from the Local Group for all 1354 Nearby Void galaxies: 3) the distributions of DNN - distances (in Mpc) to the nearest big galaxy for all 1354 galaxies in the Nearby Voids.

Published: Pustilnik S.A., Tepliakova A.L., Makarov D.I. Void galaxies in the nearby Universe. I. Sample description, 2019, MNRAS, 482, 4329-4345 (https://arxiv.org/abs/1812.02785)