Researchers from the University of Michigan have uncovered a key factor behind the higher rates of star formation in less evolved dwarf galaxies compared to massive ones. The less evolved dwarf galaxies experience a 10-million-year delay in expelling gas from their environments, enabling star-forming regions to retain gas and dust.
Dwarf Galaxies' Unique Star Formation Dynamics
The findings of the study, titled "Delayed Massive-star Mechanical Feedback at Low Metallicity" published in the Astrophysical Journal, reports that the prolonged retention period allows for the coalescence and evolution of more stars.
Massive stars collapse into black holes in dwarf galaxies instead of exploding as supernovae, a phenomenon that differs from more evolved, polluted galaxies like the Milky Way. In the latter, the likelihood of massive stars exploding leads to a collective superwind, expelling gas and dust and halting star formation.
University of Michigan researchers, led by undergraduate researcher Michelle Jecmen, have identified the 10-million-year delay using the Hubble tuning fork diagram, classifying galaxies based on Edwin Hubble's model, to illustrate the correlation between metallicity and star-forming capabilities.
Massive well-evolved galaxies at the handle of the tuning fork, having converted all their gas into stars, contrast with less evolved smaller galaxies at the fork's end, exhibiting robust star-forming regions.
Sally Oey, the study's senior author and a U-M astronomer, explained that dwarf galaxies face challenges in halting star formation due to their difficulty in dispelling gas. The 10-million-year delay, as proposed by Jecmen, provides astronomers with a unique opportunity to observe scenarios akin to the cosmic dawn, a post-Big Bang period.
During this period of relative quiet, gas clumps together in a phenomenon termed the "picket fence" model in which they form gaps through which ultraviolet (UV) radiation escapes.
Jecmen stressed the importance of studying low-metallicity dwarf galaxies with abundant UV radiation, offering a glimpse into conditions resembling the cosmic dawn and contributing to a better understanding of the early universe. This research sheds light on the intricate interplay between metallicity, superwinds, and star formation in different types of galaxies.
Catastrophic Cooling and Cosmic Dawn Insights in Mrk 71
In another study, titled "Nebular C ivλ1550 Imaging of the Metal-poor Starburst Mrk 71: Direct Evidence of Catastrophic Cooling" published in the Astrophysical Journal Letters, the team examined Mrk 71, a region in a nearby dwarf galaxy located 10 million light years away, using the Hubble Space Telescope.
This study aimed to validate the scenario proposed by Michelle Jecmen in a prior study. The team employed a new technique with the Hubble Space Telescope, utilizing a filter set designed to observe the light of triply ionized carbon.
In galaxies with numerous supernova explosions, such as more evolved ones, the explosions heat the gas within star clusters to extremely high temperatures. This hot superwind, as it expands, forces the remaining gas out of the star clusters.
In contrast, in low metallicity environments like Mrk 71, where supernova events are less frequent, the energy within the region is radiated away, preventing the formation of a superwind.
The use of specialized filters revealed a diffuse glow of ionized carbon in Mrk 71, signifying the dissipation of energy and the absence of a hot superwind, allowing dense gas to endure in the environment.
Oey and Jecmen propose that these findings could significantly enhance comprehension of galaxies observed during the cosmic dawn, particularly through instruments like the James Webb Space Telescope, underscoring the need for ongoing exploration to fully grasp the broader implications of their research.
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