Scientists have discovered a relatively rapid change in state in six galaxies at once: in a few months, their nucleus sharply increased activity. Such a transformation is associated with a sharp change in the rate of fall of matter onto central supermassive black holes in these objects. Such transitions are rare, but differ from those already known, so the authors proposed to separate them into a separate class.
There is an extensive classification of galaxies, including galaxies with active galactic nucleus (AGNs), which are visible due to the powerful radiation of the central region, and not because of the stellar population. AGNs are distinguished by spectral characteristics, and from a theoretical point of view, this difference is determined by two parameters: the angle between the line of sight and the galactic plane, as well as the rate of accretion onto the central black hole.
Historically, there have been approaches that have attempted to reduce the entire variety of observational manifestations of AGN exclusively to orientation. However, in recent years, more and more changing their “appearance” galaxies have been discovered that do not fit into such models, since such large structures cannot noticeably rotate in a few years.
Typically, such changes consist in the appearance (or disappearance) of either continuous radiation in a continuum that cannot be explained by stars, or regions with wide emission lines. Such regions should arise in the immediate vicinity of the central black hole, since the Doppler effect is responsible for the expansion of the line, and for a large spread of velocities it is necessary to move quickly. There are already a lot of similar cases of type change, and they are usually associated with a sharp change in the rate of accretion on a monthly scale.
Astronomers from the USA, the Netherlands, Sweden and Taiwan, led by Sarah Frederick from the University of Maryland at College Park, described the transitions of six LINER class galaxies at once (Low-ionization nuclear emission-line region) in the active state of AGN for nine months. In four cases, broad lines appeared in the radiation of galaxies, that is, they actually became quasars, and in two objects turned into active Type I Seyfert galaxies.
The LINER type is very common: it is estimated that about one third of all galaxies in the local vicinity belong to it. In the scientific community, however, there is no consensus either regarding their classification as AGN, nor regarding the radiation mechanism: according to one theory, this is a manifestation of moderate activity of the nucleus, and according to another theory, a high rate of star formation and a lot of young bright stars are responsible for this.
In the case of Seyfert galaxies, scientists have previously paid attention to changes in the spectrum (usually a transition between types I and II occurs), but the conversion from LINER is observed very rarely, even despite the prevalence of this type of galaxy. In the framework of the new work, astronomers wanted to understand the transitions between Seyfert types, but they found the transformations that were distinguished by spectral properties from all other known cases, which turned out to be much more interesting.
The discovery was made thanks to the new Zwicky Transient Facility survey instrument (ZFT – an automatic installation at the Palomar Observatory in California, which began regular observations in March 2018. “It’s amazing that any galaxy can change on a human time scale. These measurements occur much faster than we can explain with current theories about quasars, ” says Frederick.“ New research will be needed to understand what could affect the rate of accretion in the galaxy and lead to such rapid changes. ” “The processes involved must be exceptional and impressive.”
Recently, astronomers have identified a new type of “cold quasars” that managed to fill the gap in the evolution of galaxies. Scientists also suggested that they found the furthest hidden quasar and figured out the radiation of images of a six-fold lensed quasar.