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: An undergraduate astrophysics student at UC Berkeley has found three unusual galaxies—cataloged as Shapiro Dwarf Galaxies I, II and III. Each galaxy is far from other galaxies and has stopped making stars, a combination that is extremely rare.
The discovery, published in The Astrophysical Journal, provides compelling observational evidence of a long-held astronomical theory. "Dwarf galaxies exist both in isolation and as satellites around larger galaxies like our own Milky Way," said Julian Shapiro, the study's sole author, who conducted his research while still in high school. "In isolation, these galaxies are expected to be actively forming stars, as there are no larger galaxies to strip their star-forming gas, but the galaxies I discovered break this rule.
"My paper argues that they may be among the first resolved 'backsplash' galaxy candidates, or galaxies that once passed close to a large neighbor, whose pressure removed their gas, before being flung outward." Usually, small dwarf galaxies found in isolated, low-density regions of deep space still contain cold gas that allows them to produce stars. Conversely, dwarf galaxies that have stopped forming stars—or have become "quenched"—are usually found in dense environments or near larger host galaxies, where strong hydrodynamic and gravitational forces siphon off their gas. The three dwarf galaxies Shapiro discovered present a puzzle: They are both isolated and quenched.
Astrophysicists have been attempting to explain what causes these two traits to coexist. Shapiro believes the three dwarf galaxies once plunged through the gravitational well of a massive host galaxy—with Messier 101, known as the Pinwheel Galaxy, being the primary candidate—where environmental forces violently stripped away their star-forming gas before flinging them out into deep space. Alternatively, Shapiro Dwarf Galaxy II lies close enough to spiral galaxy NGC 5585 to possibly be a faint satellite rather than a backsplash galaxy.
The standard cosmological model, referred to as lambda-cold dark matter (ΛCDM), predicts that backsplash galaxies should be common, but identifying examples in the local universe has proven elusive. Shapiro's findings show that the standard physics model likely holds true even in milder cosmic neighborhoods while offering individual candidates to be confirmed through follow-up observations. The paper also lends credence to the hypothesis that external forces must be responsible for stripping isolated, quenched galaxies of their star formation ability.
Shapiro used simulations to determine where backsplash dwarf galaxies were most likely to exist near a Milky Way–like galaxy and where the telescope archives could detect them. His paper proposes new methods for predicting the frequency of these galaxies. "Building the foundational knowledge necessary to detect the galaxies, develop modeling and analysis code, and analyze the results in comparison to our cosmological model was a challenge," said Shapiro.
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