BL Lacertae object

A BL Lacertae object or BL Lac object is a special kind of very bright area at the center of some galaxies, called an active galactic nucleus. These objects are named after the first one discovered, BL Lacertae. What makes BL Lac objects different from other bright galaxy centers is that their brightness can change very quickly and by large amounts, and they show a special kind of light called optical polarization.

Because of these unusual properties, the first BL Lac object was thought to be a variable star at first. Unlike some brighter galaxy centers called quasars, BL Lac objects do not have clear lines in their spectra, making them harder to study and understand at first.

Scientists now believe that BL Lac objects are related to powerful streams of energy, called relativistic jets, shooting out from the center of their galaxies in a way that points right at us. These objects are thought to be similar to a type of galaxy called low-power radio galaxies, but we see them differently because of how their jets are aimed. BL Lac objects are a type of object called a blazar, which means they have these special jets pointing toward us.

Some well-known BL Lac objects include BL Lacertae itself, OJ 287, AP Librae, PKS 2155-304, PKS 0521-365, Markarian 421, 3C 371, W Comae Berenices, ON 325 and Markarian 501. These objects help scientists learn more about the powerful processes happening at the centers of galaxies.

Host galaxies

After scientists found these special objects, they noticed they were surrounded by a faint, cloudy area. Using better tools in the late 1970s, they learned more about this area. The first pictures of one of these objects, called PKS 0548-322, showed it was part of a huge round galaxy with a very bright center.

Later, in the year 2000, many pictures from the Hubble Space Telescope were taken of 132 of these objects. The pictures showed that two-thirds of them had galaxies around them. Most of these galaxies were round and very bright, similar to the brightest galaxies found in clusters of galaxies.

History

In 1968, an astronomer named John L. Schmitt noticed something unusual about a star-like object called BL Lac. Soon after, scientists observed that its brightness changed and that its light had a special property called polarization. In 1972, another scientist named Peter Albert Strittmatter suggested grouping these special objects into their own category and added four more to the list. By 1976, there were 30 known objects of this type.

Later, in 2017, a very high energy particle called a neutrino was detected by the IceCube project, and it seemed to come from a BL Lac object named TXS 0506+056.