It is widely known that our planet Earth and the
Solar System itself are embedded in the Milky Way, and it is through
this galaxy that we look out onto the Universe. As it turns out, this
has a larger impact on astrophysical studies than previously thought.
Our Galaxy’s gravitational field and its non-uniformity limit the
accuracy of astrometric observations of distant – extragalactic –
objects. An international group of astrophysicists including a
researcher at the Max Planck Institute for Astrophysics tried to find
out how strong this effect is.
Proper motions, angular sizes, and trigonometric parallaxes
(visible displacements) of astronomical objects such as stars are the
basic parameters for many astrophysical studies. These parameters are
determined by astrometric techniques, and a coordinate system is needed
to calculate, for example, the position or the radial velocity of a
star. All coordinate systems which are currently in use, including the
International Celestial Reference Frame (ICRF), are based on the
coordinates of several hundred “defining” extragalactic sources. Quasars
and distant galaxies are ideal reference points for determining the
celestial reference frame, as their angular movement is very small,
about ten microarcseconds (less than the size of a 1-cent coin on the
Moon).
Astrophysical instrumentation is developing rapidly and it is
expected that the accuracy of radio interferometric observations will
soon reach 1 microarcsecond, and optical observations about 10
microarcseconds. However, with this level of accuracy a new challenge
comes into play that interferes with the observations: the general
theory of relativity and in particular the deflection of a light beam in
a gravitational field.
When a light beam from a distant source passes close to any massive
object, it is slightly deflected by its gravity. This deviation is
typically very small, but if the beam encounters several objects on its
path, the added deviations may become significant. In addition, as the
objects are moving, the beam deflection angle changes with time and the
source coordinates start to “jitter” around their true value.
It is
important to note that this “jittering” effect applies to all distant
sources, including those that are used as reference points for different
coordinate systems. In attempting to improve the accuracy of coordinate
reference systems, in the near future we will reach a limit that cannot
be exceeded by better detection instruments. In fact, the
“gravitational noise” makes it impossible to increase the accuracy of a
coordinate system above a certain level.
The group of researchers now tried to estimate the effect of
gravitational noise on observations. The study relies on extensive
numerical calculations performed by Dr Natalia Lyskova at MPA. She
developed a high-performance parallel code and built two-dimensional
“deviation maps” of the entire sky based on modern models of the
Galactic matter distribution (see figure). The calculations show that
for a reasonable observation time of about ten years, the shift in the
positions of the sources will vary between 3 microarcseconds at high
galactic latitudes up to several dozen microarcseconds close to the
Galactic centre.
Consequently, when the accuracy in absolute astrometry reaches
microarcseconds, the “jittering” effect of the reference source
coordinates due to the Galaxy’s non-stationary gravitational field, will
have to be taken into account. But the scientists also have some good
news: when investigating the properties of this gravitational noise they
were able to demonstrate that the “jittering” effect of the coordinates
can be partially compensated by using mathematical methods.
Note: The team includes researchers from the Astro Space Center of
P.N. Lebedev Physical Institute (Russia), the Space Research Institute
of the RAS (Russia), the Moscow Institute of Physics and Technology
(MIPT), and the Max-Planck-Institut fuer Astrophysik (Germany).
Author
Postdoc
Phone: 2257
Email: nys@mpa-garching.mpg.de
Original Publication
1.
Tatiana I. Larchenkova, Alexander A. Lutovinov, and Natalya S. Lyskova
Influence of the galactic gravitational field on the positional accuracy of extragalactic sources
ApJ, 835, 51
Source / DOI
Postdoc
Phone: 2257
Email: nys@mpa-garching.mpg.de
Original Publication
ApJ, 835, 51
Source / DOI
