Shedding light on galaxy rotation secrets

Spiral galaxies are found to be strongly rotating, with
an angular momentum higher by a factor of about 5 than
ellipticals. What is the origin of such a difference? Credit:
Wikimedia Common

The dichotomy concerns the so-called angular momentum (per
unit mass) that in physics is a measure of size and rotation
velocity. Spiral galaxies are found to be strongly rotating,
with an angular momentum higher by a factor of about five
than ellipticals. What is the origin of such a difference? An
international research team investigated the issue in a study
just published in the Astrophysical Journal. The team
was led by SISSA Ph.D. student JingJing Shi under the
supervision of Prof. Andrea Lapi and Luigi Danese, and in
collaboration with Prof. Huiyuan Wang from USTC (Hefei) and
Dr. Claudia Mancuso from IRA-INAF (Bologna). The researchers
inferred from observations the amount of gas fallen into the
central region of a developing galaxy, where most of the star
formation takes places.

The outcome is that in elliptical galaxies, only about 40
percent of the available gas fell into that central region.
More relevantly, this gas fueling star formation was
characterized by a rather low . This is in stark contrast with
the conditions found in spirals, where most of the gas that
ends up in stars has an appreciably higher angular momentum.
The researchers have traced the dichotomy in the angular
momentum of spiral and elliptical galaxies to their different
formation histories. Elliptical galaxies form most of their
stars in a fast collapse in which angular momentum is
dissipated. This process is likely stopped early on by powerful
gas outflows from supernova explosions, stellar winds and
possibly even from the central supermassive black hole. For
spirals, on the other hand, the gas fell slowly, conserving its
angular momentum, and stars formed steadily along a timescale
comparable to the age of the universe.

“Until recent years, in the paradigm of and evolution, were thought to have formed
by the merging of stellar disks in the distant universe. Along
this line, their angular momentum was thought to be the result
of dissipative processes during such merging events,” the
researchers write. Recently, this paradigm had been challenged
by far-infrared/sub-millimeter observations brought about by
the advent of space observatories like Herschel and
ground-based interferometers like the Atacama Large Millimeter
Array (ALMA).

These observations have the power of penetrating through
interstellar dust thus unveiling the star formation processes
in the very distant, dusty galaxies that constituted the
progenitors of local ellipticals. “The net outcome from these
observations is that the populating present-day
ellipticals are mainly formed in a fast dissipative collapse in
the central regions of dusty star-forming . After a short timescale of less than 1
billion years, the has been quenched by powerful gas
outflows.” Despite this change of perspective, the origin of
the low angular momentum observed in local ellipticals remained
unclear.

“This study reconciles the low angular momentum observed in
present-day ellipticals with the new paradigm emerging from
Herschel and ALMA observations of their progenitors,” conclude
the scientists. “We demonstrated that the low angular of ellipticals is mainly originated by
nature in the central regions during the early galaxy formation
process, and not nurtured substantially by the environment via
merging events, as envisaged in previous theories.”

Explore further:
The shapes of
galaxies

More information: J. Shi et al. Angular Momentum of
Early- and Late-type Galaxies: Nature or Nurture?, The
Astrophysical Journal
(2017). DOI: 10.3847/1538-4357/aa7893

Journal reference: Astrophysical
Journal

Provided by:
International School of Advanced Studies (SISSA)

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