The Local Void is a vast, empty region of space, lying adjacent to the Local Group.[3] [4] Discovered by Brent Tully and Rick Fisher in 1987,[5] the Local Void is now known to be composed of three separate sectors, separated by bridges of "wispy filaments". The precise extent of the void is unknown, but it is at least 45 Mpc (150 million light-years) across,[6] and possibly 150 to 300 Mpc.[7] [8] The Local Void appears to have significantly fewer galaxies than expected from standard cosmology.[9]
Voids are affected by the way gravity causes matter in the universe to "clump together", herding galaxies into clusters and chains, which are separated by regions mostly devoid of galaxies, yet the exact mechanisms are subject to scientific debate.[3]
Astronomers have previously noticed that the Milky Way sits in a large, flat array of galaxies called the Local Sheet, which bounds the Local Void.[3] The Local Void extends approximately 60Mpc, beginning at the edge of the Local Group.[10] It is believed that the distance from Earth to the centre of the Local Void must be at least .[4]
The size of the Local Void was calculated due to an isolated dwarf galaxy known as ESO 461-36 located inside it. The bigger and emptier the void, the weaker its gravity, and the faster the dwarf should be fleeing the void towards concentrations of matter, yet discrepancies give room for competing theories.[4] Dark energy has been suggested as one alternative explanation for the speedy expulsion of the dwarf galaxy.[3]
An earlier "Hubble Bubble" model, based on measured velocities of Type 1a supernovae, proposed a relative void centred on the Milky Way. Recent analysis of that data, however, suggested that interstellar dust had resulted in misleading measurements.[11]
Several authors have shown that the local universe up to 300 Mpc from the Milky Way is less dense than surrounding areas – by 15–50%. This has been called the Local Void or Local Hole.[7] [8] Some media reports have dubbed it the KBC Void,[12] although this name has not been taken up in other publications.
Scientists believe that the Local Void is growing and that the Local Sheet, which makes up one wall of the void, is rushing away from the void's centre at 260km/s.[13] Concentrations of matter normally pull together, creating a larger void where matter is rushing away. The Local Void is surrounded uniformly by matter in all directions, except for one sector in which there is nothing, which has the effect of taking more matter away from that sector. The effect on the nearby galaxy is astonishingly large.[4] The Milky Way's velocity away from the Local Void is 600000mph.[3]
Several void galaxies have been found within the Local Void. These include:
| Galaxy | Image | Constellation | Declination | Distance | Right ascension | Notes | |
|---|---|---|---|---|---|---|---|
| Pisces A | Rowspan=2 | +10° 48′ 47.01″ | 18.4 Mly (5.64 Mpc) | 00h 14m 46.000s | The two void dwarf galaxies located in the Local Void and are in the Pisces constellation. Pisces A is 18.4 million light-years (5.64 megaparsecs) away and Pisces B is 30 million light-years (9.2 megaparsecs) away. The galaxies were discovered with the WIYN Observatory. About 100 million years ago, they started moving out of the void and into the local filament zone and denser gaseous environment. This sparked off a doubling of the rate of star formation.[14] | ||
| Pisces B | +11° 07′ 18.22″ | 30 Mly (9.2 Mpc) | 01h 19m 11.700s | ||||
| NGC 7077 | Aquarius | 02° 24′ 51″ | 56 Mly (17.2 Mpc) | 21h 29m 59.6s | A lenticular blue compact dwarf galaxy located about 56 million light-years away from Earth in the constellation Aquarius. Discovered by astronomer Albert Marth on 11 August 1863, the galaxy lies within the Local Void.[15] | ||
| NGC 6503 | Draco | +70° 08′ 39.587″ | 13.05 ± 0.33 Mly (4.0 ± 0.1 Mpc) | 17h 49m 26.4207s | A field[16] dwarf spiral galaxy located at the edge of a region of space called the Local Void. The dwarf galaxy spans 30,000 light-years and lies approximately 17 million light-years away in the constellation of Draco (the Dragon). This spiral galaxy is especially colorful where bright red regions of gas can be seen scattered through its spiral arms. Bright blue regions contain stars that are forming. Dark brown dust areas are in the galaxy's arms and center. It has one known satellite galaxy, known as KK 242. With a stellar mass of about 3 million solar masses, KK 242 is on the border between a dwarf irregular galaxy (dIrr) and a dwarf spheroidal galaxy (dSph).[17] | ||
| NGC 6789 | +63° 58′ 23″ | 12 Mly (3.6 Mpc) | 19h 16m 41s | An irregular galaxy in the constellation Draco. It was discovered by Lewis Swift on 30 August 1883. It is located within the Local Void, a region of space with far fewer galaxies than its surroundings. It is the nearest blue compact dwarf (BCD) galaxy to the Milky Way. It is chemically homogeneous and relatively metal-poor.[18] |