Dwarf Galaxy Questions Current Galaxy Formation Models

CAUP Astronomer Polychronis Papaderos, along with his colleague Goeran Oestlin (Oskar Klein Center, U. Stockholm), used the Hubble Space Telescope (HST) to get extremely accurate observations of the I Zw 18 galaxy. Their research led to the conclusion that this enigmatic blue compact dwarf might force astronomers to review current galaxy formation models.

I Zw 18 is one of the most studied dwarf galaxies, because among those that have strong star forming activity, it's one of the poorest in heavy elements. Besides, its proximity to the Earth, combined with a total exposure time of nearly 3 days, gave the researchers data with unprecedented resolution and sensitivity.

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. This 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.

Papaderos, a Greek astronomer working in Portugal, comments that: "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. 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.

This study showed that galaxies undergoing starbursts, similar to I Zw 18, might not obey this rule. 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 galaxy's 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.

Media Contacts:
Ricardo Reis / Pedro Mondim
+351 22 608 98 36 / +351 22 608 98 35
ricardo.reis@astro.up.pt / pedro.mondim@astro.up.pt

Science Contact:
Polychronis Papaderos
+351 22 608 98 39

The article "I Zw 18 as morphological paradigm for rapidly assembling
high-z galaxies" by Papaderos et al. was published in Astronomy &

The Centro de Astrofisica da Universidade do Porto (CAUP, http://www.astro.up.pt) is a private, non-profit, scientific and technical association, recognized as of public utility. It is the largest astronomy research institute in Portugal and since 2000, it has been evaluated as Excellent by international panels, organized under the auspices of the national science foundation (FCT). Among its statutory objectives is the support and promotion of Astronomy, through research, education at the graduate and undergraduate levels, science outreach and popularization of astronomy. The long-term research strategy of CAUP is the assembly of strong research teams on origin and evolution of stars and planets and galaxies and observational cosmology.

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