Three dwarf spheroidal galaxies found to rotate

Date:
July 27, 2021
Source:
Instituto de Astrofi'sica de Canarias (IAC)
Summary:
Astrophysicists have discovered the presence of transverse rotation
(in the plane of the sky) in three dwarf spheroidal galaxies, a very
faint type of galaxies and difficult to observe, which are orbiting
round the Milky Way; this helps to trace their evolutionary history.



FULL STORY ==========================================================================
An international team of astrophysicists from the Instituto de
Astrofi'sica de Canarias (IAC), the University of La Laguna (ULL)
and the Space Telescope Science Institute (STScI, USA) has discovered
the presence of transverse rotation (in the plane of the sky) in three
dwarf spheroidal galaxies, a very faint type of galaxies and difficult
to observe, which are orbiting round the Milky Way; this helps to trace
their evolutionary history. The finding was made using the most recent
data from the GAIA satellite of the European Space Agency. The results
of the study have just been published in the journal Monthly Notices of
the Royal Astronomical Society (MNRAS).


========================================================================== Dwarf galaxies have a particular interest for cosmology. The standard cosmological model suggests that this type of galaxies was the first
to form.

Many of them, the majority, have been destroyed and cannibalized by
large galaxies such as the Milky Way. However, those that remain can be
studied and contain valuable information about the early Universe.

One subclass of dwarf galaxies are the dwarf spheroidals. They are very diffuse, with low luminosity, they contain large proportions of dark
matter and little or no gas. Since their discovery they have been deeply studied. However, their internal kinematics are still little known,
due to the technical difficulties needed for their detailed study.

Various previous studies have shown that the dwarf spheroidals do not
have patterns of internal rotation, but their stars move on random orbits predominantly towards and away from the centre of the galaxy. But the
galaxies within the other major sub-class of dwarfs, the irregulars, have
large quantites of gas, and in some cases do have internal rotation. These differences suggest a different origin for the two types of dwarfs,
or to a very different evolutionary history in which interactions with
large galaxies, in our case with the Milky Way, have played a crucial
role in eliminating the internal rotation of the spheroidals.

To carry out their present research, the team of astrophysicists form
the IAC and the STScI have used the latest data from ESA's Gaia to study
the internal kinematics of six dwarf spheroidal galaxies, satellites
of the Milky Way, and have discovered the presence of transverse
rotation (in the plane of the sky) in three of them: Carina, Fornax,
and Sculptor. These are the first detections of this type of rotation in
dwarf spheroidal galaxies, except for the Sagittarius spheroidal, which
is strongly distorted by the gravitational potential of the Milky Way,
and is therefore not representative of its type.

"The importance of this result is because, in general, the internal
kinematics of galaxies, in this case their rotation, is an important
tracer of their evolutionary history, and of the conditions in which the
system was formed," explains Alberto Manuel Marti'nez-Garci'a, doctoral
student at the IAC and the ULL, and first author of the article.

"Although the standard model of cosmology assumes that the dwarf galaxies
were the first to form, it is not clear if they are simple systems or
whether those we observe are formed by the agglomeration of other even
simpler systems, smaller and older. The presence of rotation suggests the second option. It also suggests a common origin for all dwarf galaxies,
those that are at present rich in gas (the irregulars) and those which
are not (the spheroidals)," explains Andre's del Pino, researcher at
the STScI and a co-author of the article.

"The Gaia satellite has revolutionised our knowledge of the Milky Way and
its neighbourhood, giving us very precise measurements of the positions
and motions of almost two thousand million stars. Although the data
from Gaia are used mainly to study our Galaxy, this ESA mission has
also opened a new window on the study of the satellite galaxies of the
Milky Way, giving specific access to their internal kinematics," says
Antonio Aparicio, a researcher at the IAC and the ULL and a co-author
of the article.

Even so, according to the researchers, studies based on Gaia data entail
many technical difficulties. In the first place, one must determine which
of the stars in the database really belong to the satellite galaxies,
and which to the Milky Way itself, as the latter tend to contaminate the sample. The problem is that although the data to be analysed are limited
to the region and the angular size of the spheroidal under study, which
is the equivalent of one quarter of the angular diameter of the Moon,
the vast majority of the stars detected in this area belong to the Milky
Way and therefore indeed contaminate the sample.

In addition, the distance of the spheroidals studied, which is up to
some half a million light-years, and the low intrinsic luminosity of
their stars, imply that the measurements are affected by a considerable
level of noise. For all these reasons the analysis of the data requires
a thorough filtration and a deep analysis of the different observational parameters to be able to reach reliable conclusions.

========================================================================== Story Source: Materials provided by Instituto_de_Astrofi'sica_de_Canarias_(IAC). Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Alberto Manuel Marti'nez-Garci'a, Andre's del Pino, Antonio
Aparicio,
Roeland P van der Marel, Laura L Watkins. Internal rotation of
Milky Way dwarf spheroidal satellites with Gaia Early Data Release
3. Monthly Notices of the Royal Astronomical Society, 2021; 505
(4): 5884 DOI: 10.1093/mnras/stab1568 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210727121247.htm

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