GALAXIES IN VOIDS
VOIDS: A DEFINITION OF THE PROBLEM
Galaxy is the basic unit in extragalactic astronomy with a characteristic length of 20 kpc, that typically belongs to a group or cluster with a diameter of 50 kpc to 5 Mpc which in turn is located in a supercluster with a diameter of 50 Mpc or larger. Vioid is a large region of the universe containing few or no galaxies. The first of these voids to be discovered lies in the direction of Bootes. Voids take up about 98% of the volume of the universe, with galaxies concentrated in the thin walls that surround them. But this does not necessarily mean that the voids are empty. On the contrary, it is possible that they contain most of the mass of the universe in the form of dark matter. Superclusters and voids are often discussed together because they are identified and studied on a common data base and it is likely that they share a common origin and represent two complementary effects of related evolutionary processes.
The structure of superclusters, the material entities that make up the contiguous shell, was reviewed by Oort (1983) and the voids - by Rood (1988). Voids are not immediately recognized in the surface distribution of galaxies because they are superimposed by background and especially by foreground galaxies. Voids are recognized only after Doppler velocities have been measured for statistically homogeneous samples of galaxies in selected solid angles of the sky, which provides direct information on the three-dimensional distribution of galaxies.
There are two ways to study an individual void observationally:
a) the void itself can be probed to detect galaxies within it;
b) the structure and content of the contiguous shell of superclusters surrounding the void can be studied.
Coma and Hercules are amongst the famous known voids. They are found to be practically devoid of matter, whereas Bootes and Perseus-Pisces are not empty. The Bootes Void is one of the largest known regions of low density. Several dozens of emission line galaxies are known to lie within its boundaries up to now. Absorption features have also been detected, indicating the presence of older stellar populations within some of the galaxies. At least 25% of the void galaxies have large H_alpha luminosities compared to the most luminous normal galaxies.
The early studies of the surface distribution of galaxies featured three different models:
a) clusters of galaxies;
b) large and rich superclusters of galaxies;
c) groups and clusters of galaxies in a uniform general field.
To discriminate between these models, we need to know the three-dimensional distribution of galaxies in large homogeneous samples.
Chincarini & Rood, 1970, pointed out the redshift segregate into several groups in Hercules constellation. They hypothesized that galaxies occur in groups and that the apparent two dimensional distribution of galaxies is a result of a superposition of such groups - model (a).
Oort (1983) pointed out a large void in Hercules from the analysis of data from 1976 - 1981. In fact “void” is a natural abbreviation of “a region devoid of galaxies”.
Bahckal N. A., 1995, represents the Sloan Digital Sky Survey. The idea of SDSS is to provide a complete imaging and spectroscopic survey of the high-latitude northern sky. The 2D survey should image the sky in five colors and contains nearly 5x10^7 galaxies to approx. 23 mag. The spectroscopic survey was meant to obtain spectra of the brightest 10^6 galaxies, 10^5 quasars, and 10^3 rich clusters of galaxies (to approx. 18.3-19.3 mag). The survey should identify a complete sample of several thousands of rich clusters of galaxies, both in 2D and 3D - the largest automated sample yet available.
Lindner U. (1996) investigated the distribution of normal (faint) galaxies and blue compact galaxies (BCGs) in voids by analyzing their distribution as a function of the distance from the void centers and by employing nearest neighbour statistics between objects of various subsamples. They find that galaxies in voids defined by brighter galaxies tend to be concentrated close to the walls of the voids in a hierarchical manner, similar to the behaviour of brighter galaxies. In this regard BCGs behave similarly to normal dwarf galaxies.
Kuhn B. et al. (1977) presented the results of a search for intrinsically faint galaxies towards three regions with known voids and the Hercules supercluster. They pointed out that no clear indication of a void-population was found.
Petrov G., A. Kniazev and J. Fried (2004) (a larger PDF format could be found here), presented photometry and morphology of faint galaxies in the direction of 1600+18 in Hercules void. Coordinates of approx. 1800 faint galaxies per one square degree centered at 1600+18 (1950) (Hercules void), and m(B), diameters, position angles and morphological classification were presented. The distribution of the magnitudes of the galaxies in this direction was compared to “Log Normal” and “Gauss” ones and to similar results from SDDS studies of galaxies. Major axes luminosity profiles were analysed. Some candidates for primeval galaxies - low surface brightness galaxies were detected in the direction of the void.
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OUR OBSERVATIONS AND INVESTIGATION OF HERCULES AND OTHER VOIDS
G. T. Petrov, 2004