Find the few galaxies floating in a cosmic ocean of nothingness.
Four “bright” members of the Boötes Void are around magnitude 15. Each field measures 5' on a side. Credit: digital sky studio
I don't normally write much about deep, “holy grail” objects, but in the case of the Boötes Void I'll make an exception. Specifically, I'm wondering what the smallest telescope is that would be needed to see the galaxies in the Boötes Void. If that sounds like an oxymoron (seeing something in a vacuum), it's not.
Originally known as the Big Nothing, the Boötes Void (pronounced “boo-OH-teez”) is an immense volume of space without galaxies. Robert Kirshner, currently executive director of the planned Thirty meter telescope, and his colleagues discovered this massive nothing in 1981 while mapping galaxies to help determine the large-scale structure of the universe. Galaxies are visible components of this large-scale structure, which we now know is a vast cosmic network. They gather in groups, clusters and superclusters and, under the influence of dark matter, organize themselves into twisted, threadlike structures called filaments. Between the filaments are spaces containing few galaxies, as if they were on the surface of cosmic bubbles.
The Boötes Void is one of the largest known voids or voids, classified as a supervoid and spanning about 300 million light years. In this amount of space, astronomers would normally expect to see a few thousand galaxies. But to date, Boötes Void has only revealed a couple handfuls.
As seen in the two-dimensional sky dome, the void is a circle about 25° across and centered at RA 14h50m, 46°0′ Dec., very near the sixth-magnitude star 38 Boötis. On many star maps, you’ll see some two dozen or more galaxies within this circle that shine at magnitude 13 or brighter, including the great Pinwheel Galaxy (M101) and its companions in Ursa Major, along with the magnificent Splinter Galaxy (NGC 5907) in Draco. But all of these objects are relatively nearby, within a couple hundred million light-years of Earth, while the Boötes Void lies much farther away, about 700 million light-years away.
Despite its distance, the Void contains several galaxies within the range of amateur-sized telescopes. Below you can find descriptions of four galaxies in the Boötes Void that are among the brightest and most condensed. Their descriptions and magnitudes (ranging from 15 to 16) come from the 1977 study “Photometry of Galaxies in the Boötes Void” in The Astronomical Journal.
The objects I have selected have bright cores, so those using powerful telescopes will have to look for a near-stellar object. On the other hand, these galaxies should be a piece of cake for those using astronomical telescopes or larger telescopes.
1. The brightest member of the Void is Markarian 845 (RA 15h07.7m, Dec. 51°27.2′). This nearly edge-on (northwest to southeast) system has a magnitude of 14.8 and a length of about 1′, and a very bright nucleus. Some observers can detect a knot-like feature south of the nucleus, outside the elongated disk, although it is not known whether this feature belongs to the galaxy. The northwestern segment of the disk also shows some irregularities that may be indicative of a spiral structure.
2. CG 692 (RA 15h21.1m, Dec 50°40.3′) is the brighter member of a pair of interacting galaxies. This magnitude 15.2 spiral appears elongated, measures about 0.7′ in length, and is oriented northeast to southwest. CG 692 comprises a complex, bright nuclear region and a warped spiral arm north of the bright bulge, suspected to have resulted from the interaction with its companion Seyfert 1 (CG 693) to the immediate southeast. This companion is a nearly face-to-face spiral with S-shaped arms emanating from opposite sides of the nucleus.
3. CG 538 (RA 14h46.4m, Dec. 43°49.9′) is a compact magnitude 15.3 system with a bright elliptical core in a tight elliptical disk with faint traces of asymmetric spiral structure: two short arms to the north and a faint spiral that arches to the southeast. Visual observers will likely look for a near-stellar system only 0.5′ in diameter.
4. I Zwicky 81 (RA 14h08.2m, Dec 48°51.7′) is a largely featureless magnitude 15.3 disk galaxy. Only the bright lenticular nuclear region is generally well resolved in images. However, Palomar Sky Survey images reveal mere traces of a faint disk. So this target is a definite point source for visual observers.
Remember that galaxy magnitudes may not be entirely reliable at the faint end, so some may appear visually brighter than indicated. I suspect Markarian 845 can be seen even with a decent 10-inch telescope in very dark skies. As always, be sure to send notes about what you see or don't see to sjomeara31@gmail.com.
