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Astrophysicists Find Out Error in the AGN Coordinates Measured by Gaia Space Telescope

Astrophysicists at the Moscow Institute of Physics and Technology, the Lebedev Physical Institute of the Russian Academy of Sciences (LPI RAS), and NASA have discovered an error in the coordinates of active galactic nuclei evaluated by the Gaia space telescope, and contributed toward correcting it.

Gaia, a space observatory of the European Space Agency. (Image credit: ESA)

The study outcomes, reported in The Astrophysical Journal, also turns out to be an independent corroboration of the astrophysical model of such objects.

One of the key results of our work is a new and fairly unexpected way of indirectly studying the optical emission from the central regions of active galactic nuclei. There is a lot that direct optical observations cannot show us. But radio telescopes proved useful in complementing the picture.

Alexander Plavin, Researcher, Relativistic Astrophysics Lab, MIPT

Plavin is also a doctoral student at LPI RAS.

The precision of the coordinates acquired by using Earth-based optical telescopes is largely restricted, whereas orbital observatories like Gaia help overcome this challenge. Gaia was launched in 2013 and receives signals from comparatively remote cosmic sources and extracts their coordinates with excellent precision.

Prior to the launch of Gaia, radio telescope arrays were the ones to measure the most precise coordinates. These are telescope systems with the potential to collect a low-frequency signal—i.e., radio waves—with a modest resolution. Although quite detailed images can be generated this way, the positions of objects in space are ascertained with a slightly lesser precision compared to that of Gaia.

The MIPT-LPI researchers discovered that apart from its superior precision, Gaia is also highly reliable. A comparison of the data from Gaia and the radio telescopes unraveled a systematic error in the orbital observatory’s measurements of a while category of celestial objects, known as active galactic nuclei. Consequently, the most precise space maps are those that are dependent on orbital observations supported by Earth-based telescopes, whose radio data allow correction of the coordinates.

An active galactic nucleus is a very bright and compact region at a galaxy’s center. The AGNs’ emission spectra are different from those of the stars, which leads to the doubt of what object is at the center. The existing consensus is that AGNs host black holes that absorb the matter of their host galaxies. Apart from including the bright nucleus, the galactic disk, and a dust cloud surrounding it, these systems could also have robust outflows of matter called jets. Based on the nature of the jet, an AGN may be categorized as a blazar, a quasar, or the like.

We hypothesized that the jet may be responsible for the systematic error in the coordinates of the active galactic nuclei measured by Gaia. This indeed proved to be the case. It turned out that if an object has a sufficiently long jet, Gaia perceives the source to be much farther along the jet direction than the radio telescopes.

Yuri Kovalev, Heads, Astrophysics Labs, MIPT and LPI RAS

It is not possible to consider this effect as random since the offset was in the direction of the jet, and a statistically significant error was noticed for only the AGNs with the longest “tails.” That is, AGNs whose jets were several orders of magnitude larger compared to the size of the galaxies themselves. The offset magnitude was comparable to the length of the jets.

Moreover, from 2018, Gaia has also been sending information on the visible “colors” of the galaxies. This has allowed the scientists to ascertain the individual coordinates and contributions of different parts of the galaxy to the emission spectrum: the source, jet, disk, and stars. It is was proven that the coordinate shifts were primarily because of the fact that the jets were long and the accretion disks were small. At the same time, the measurement of stellar emission has a near-zero effect on the accuracy with which a galaxy’s position is determined.

With these outcomes, the authors draw a conclusion that astrophysical effects associated with long jets could confuse the Gaia orbital observatory. In other words, it cannot be considered as a completely credible independent source of data related to quasar coordinates. In order to acquire better data, it is necessary for the space telescope to be supported with ground-based radio observations.

In the future, by combining observation results, we can see the structure of the central disk-jet system in a quasar in minute detail—with subparsec resolution [where a parsec is an astronomical unit of distance equal to about 3¼ light years]. Direct optical telescope observations yield no such images, yet we can get them!

Alexander Plavin, Researcher, Relativistic Astrophysics Lab, MIPT

The study outcomes are independent evidence supporting the unified AGN model. It accounts for the behavior of different types of AGNs regarding their orientation in space with respect to the observer, and not with respect to their inner workings.

The ability to accurately measure the positions of celestial objects outside the Milky Way galaxy is significant from a practical standpoint: It is their positions that turn out to be the best reference for the most punctual coordinate systems, such as those underlying GPS and its Russian counterpart GLONASS.

The Russian Science Foundation funded this research.

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