Blue Compact Dwarf Galaxy I Zwicky 18
The first galaxies were formed some 13.3 billion years ago, mainly
composed of hydrogen and helium, the primary elements that emerged from
the Big Bang. Their study to date has been technically very challenging
due to their great distance from us, but the observation of analogous
galaxies in our vicinity has turned out to be an excellent shortcut.
"Dwarf galaxy IZw18 is the least abundant in metals (in astrophysics,
elements heavier than hydrogen and helium) in the nearby universe, and
one of the most akin to the primeval galaxies. Its study therefore
allows us to catch glimpses of the conditions that prevailed in the
primordial universe”, says Carolina Kehrig, researcher at the Institute
of Astrophysics of Andalusia in charge of the study that analyses the
properties of IZw18.
The study has found a very large region in this small galaxy of ionized
helium, which tends to be more frequent in very distant galaxies with
low presence of metals. The ionization of helium implies the presence of
objects emitting a radiation intense enough to knock electrons off the
helium atoms. "In this study we propose a new interpretation of the
origin of this radiation in galaxy IZw18, a subject which is still
enigmatic”, Kehrig says.
Using the PMAS integral field spectrograph of the 3.5 meter telescope
at the Calar Alto Observatory (CAHA), researchers have obtained the
first detailed map of this region of IZw18 and have analyzed possible
ionizing sources.
Conventional sources of ionization, such as Wolf-Rayet stars – very
massive and with very violent stellar winds – or shocks generated by
remnants of supernovae, cannot provide the energy necessary to explain
the halo of ionized helium present on IZw18, so researchers considered
other possibilities.
"Our data point to the fact that extremely hot stars, such as
supermassive stars with low metal content or massive stars practically
devoid of metals may hold the key to the enigma of the excitement of
helium on IZw18, even though the existence of these stars has not yet
been confirmed by observations on any galaxy”, says Carolina Kehrig
(IAA-CSIC).
We would be talking about very hot stars analogous to first generation
stars (known as Population III stars) which, according to theoretical
models, would be composed only of hydrogen and helium and could be
hundreds of times more massive than the Sun. These stars are believed to
have played a decisive role in the “reionization” of the universe,
during which period the first stars and galaxies became visible and
which is still little known.
This study shows how it is possible to extract information about the history of the universe within our own galactic vicinity.
Reference:
C. Kehrig, J.M. Vilchez, E. Perez-Montero, J. Iglesias-Paramo, J. Brinchmann, D. Kunth, F. Durret, F.M. Bayo "The extended HeIIλ4686-emitting region in IZw18: clues for peculiar ionizing sources". Astrophysical Journal Letters. DOI: 10.1088/2041-8205/801/2/L28
Contact:
Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll@iaa.es - 958230532
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll@iaa.es - 958230532
