This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers using data from the Euclid space telescope have discovered an exceptionally faint stellar system near the Fornax dwarf galaxy. Called Fornax-7, it appears to contain only about 170 times the mass of the sun in stars.
The discovery paper was posted to the arXiv preprint server on Sept. 13. The lambda cold dark matter model of the universe predicts that dark matter halos of any mass should host their own smaller subhalos. This is well established for galaxies with masses similar to the Milky Way's, which have many dwarf satellites, but it has never been confirmed for hosts much less massive than the Milky Way.
The current record-holder is the Large Magellanic Cloud (LMC), which has candidate ultra-faint companions. Finding a satellite around something as small as the Fornax dwarf spheroidal galaxy (Fornax dSph), roughly 100 times less massive than the LMC, would extend "the observed hierarchical formation of satellite systems nearly two orders of magnitude below the LMC," researchers write in the paper. Fornax is a dwarf galaxy that orbits the Milky Way, roughly 470,000 light-years away.
It hosts six known globular clusters. Five of them are more than 10 billion years old but are spread far from the galaxy's center. Given their mass and age, calculations predict that such clusters should have migrated toward the center by now.
This "timing problem" has fueled debate over whether Fornax's dark matter halo is a diffuse, low-density "core" rather than the dense central "cusp" predicted by standard cold dark matter models. The team, led by Teymoor Saifollahi of the Observatory of Strasbourg, serendipitously spotted a tiny, extremely faint stellar system called Fornax-7 in new Euclid satellite images. The system is very close to Fornax dSph.
To pin down Fornax-7's properties, the team first built a color-magnitude diagram from its six brightest, most reliable member stars, filtering out foreground Milky Way stars using Gaia motion data and color cuts. Six stars are not enough to establish the system's age, metal content and distance, so the researchers simulated thousands of possible stellar populations and compared each with the data to find the best fit. They then conducted more calculations to check whether Fornax-7 behaves like a star cluster or a dark matter-dominated galaxy.
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