MessageToEagle.com - Astronomers from Center for Astrophysics of the University of Porto, Portugal
and Oskar Klein Centre, Stockholm University, Sweden come up with new findings regarding one of the most studied
objects - the dwarf galaxy I Zw 18.
CAUP Astronomer Polychronis Papaderos, and Göran Östlin (Oskar Klein Center, U. Stokholm), used the Hubble Space
Telescope (HST) to get extremely accurate observations of this galaxy and data with unprecedented resolution and sensitivity.
The results led the scientists to the conclusion that this enigmatic blue compact dwarf might force astronomers to review
current galaxy formation models and much of what is known about galaxy formation and evolution might need substantial revision.
Composite image of 3 filters, in visible and infrared light, of the I Zw 18 region. Gas sorrounding the two stellar cores is clearly visible. In the upper right corner, you can also see the blue dwarf I Zw 18 C.
Credit: Polychronys Papaderos (CAUP)/Göran Östlin (Oskar Klein Center, U. Stockholm)
The dwarf galaxy I Zw 18 is, one of the poorest in heavy elements, located only 59 million light years away.
It's among those that have strong star forming activity.
Analysis of these data revealed an extended gas halo surrounding this galaxy, 16 times larger than the
star component of the galaxy, and without any stars.
This halo is the result of huge amounts of energy generated by the starburst this galaxy is going through.
The energy heats and disturbs I Zw 18's cold gas, which ends up emitting an amount of light comparable to what's
being emitted by the stellar component.
This emission is designated nebular emission.
"This is ground-breaking work because it provides the first observational proof that, in the early Universe,
young galaxies that underwent starbursts must have been surrounded by a huge halo of nebular emission," Papaderos said.
"This extended nebular halo results from the cumulative energetic output from thousands of massive stars exploding
as supernovae, shortly after their formation."
So far, in distant galaxies where it's not possible to reach resolutions high enough in order to distinguish between
nebular and star emission, it was assumed that the gas occupied the same region as the stars and stars were responsible
for emitting most of the light.
The study showed that galaxies undergoing starbursts, similar to I Zw 18, might not obey this rule.
I Zw 18 region, with stellar distribution contours. Credit: Polychronys Papaderos (CAUP)/Göran Östlin
(Oskar Klein Center, U. Stockholm)
This result might lead to substantial corrections in a lot of the work being developed in cosmology and extragalactic
astronomy. An example is the estimate of star mass in a galaxy, which is calculated from the galaxies total luminosity.
But, as these results shows, up to 50% of that luminosity might originate in nebular, and not star, emission.
Another result from this research shows that, according to Papaderos, "the distribution of nebular emission might
be misinterpreted as a stellar disk. These galaxies, still in early stages of formation, might thus be wrongly
classified as fully formed galaxies" (such as spirals or ellipticals), a classification mistake that might have
happened in many past studies to determine galaxy evolution in the early Universe.
These results are also of importance for our understanding of galaxy formation, because the team concluded that
I Zw 18 is extremely young, with most stars younger than 1 billion years.
So this galaxy is currently undergoing the dominant phase of its formation, much like the ones formed shortly after the Big Bang.
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