Star formation peaked during the Cosmic Noon, which spanned from 10 to 12 billion years ago. During Cosmic Noon, star formation was 10 to 100 times greater than it is now.
New research shows that a particular class of galaxy was experiencing its first intense burst of star formation during this time.
Were these galaxies the progenitors of galaxies like the Milky Way?
Since the Cosmic Noon was a time of intense star formation, it was also a time of peak galaxy development as they converted gas to stars at a spectacular rate.
In the new research, scientists focused on ancient galaxies called Lyman Alpha Emitters, or LAEs. Lyman Alpha (Lyα) is a hydrogen line emission in the UV.
The Lyman Alpha line indicates active star formation when young stars emit intense UV that ionizes hydrogen. When the hydrogen recombines and neutralizes, the electron falls to its ground state and emits the Lyα line.
LAEs are likely the progenitors of galaxies like our own Milky Way. They have low masses and are young, only about 200 million to 600 million years old. Their star formation rate (SFR) is the highest among galaxies.
LAEs are puzzling in many ways, and may be even brighter than observed. These galaxies hold many clues to how galaxies evolve.
The new research is titled “ODIN: Star Formation Histories Reveal Formative Starbursts Experienced by Lyα-emitting Galaxies at Cosmic Noon.”
It’s published in The Astrophysical Journal Letters and the lead author is Nicole Firestone, an NSF grad student at the Department of Physics and Astronomy at Rutgers University.
In a talk, Firestone called LAEs “the most profound beacons of the high-redshift Universe.” She also says that because of their low line-of-sight dust content and high SFRs, the LAEs shine very brightly.
“Because of this feature, LAEs are relatively easily detectable from the ground, which makes them absolutely fantastic probes of distant galaxy populations.”
LAEs could hold one of the keys to understanding how galaxies evolve. Astronomers have been uncertain if LAEs are young galaxies undergoing their first burst of star formation, or older galaxies resuming star formation after being inactive for a time. ODIN in the study’s title refers to the One-hundred-deg^2 DECam Imaging in Narrowbands survey. It’s an ongoing survey that so far has detected 74 LAEs.
The research team used machine learning to examine the light from the 74 LAEs. They were able to determine the SFR as a function of time. They were able to trace the histories for each of the LAEs.
“LAE SFHs can be characterized by three archetypes: first burst at the time of observation, dominant burst showing the highest SFR at the time of observation but with at least one significant star formation episode in the past, and nondominant burst describing the case where the highest SFR occurred in the past,” the authors explain in their paper.
It’s frequently assumed that LAEs are undergoing their first major burst of star formation at the time they’re observed, due to their brightness. This work tests that assumption.
“We find that a strong majority (67%) of our LAE SFHs (star formation histories) align with the frequently assumed archetype of a first major star formation burst, with at most modest star formation rates (SFRs) in the past.” However, the rest of the LAEs SFHs exhibit significant star formation in the past, and some exhibit ongoing rapid star formation.
Overall, the researchers found that 95% of the LAEs are experiencing bursts of star formation that the authors call the dominant bursts. Only 5% experienced this in the past. The results show that LAEs are indeed young galaxies experiencing their first burst of star formation.
“For the very first time, we have been able to definitively show that most LAEs are experiencing their first major starburst at the time of observation and only have very young stars,” Firestone said.
“LAEs have been identified as progenitors of typical present-day galaxies like our own Milky Way,” said lead author Firestone in a press release. “Now that we know when they first formed their stars, we have discovered our own galaxy’s ‘origin story,’ unlocking one of the mysteries of creation.”
Astronomers are intensely interested in the Milky Way’s history. The JWST stunned us by finding surprisingly massive and well-structured spiral galaxies in the early universe, which is counter to theory.
The powerful space telescope has heralded a new wave of interest in galaxy formation and evolution. If LAEs are indeed the precursors to galaxies like ours, the finding will feed the interest in the Milky Way’s history.
“Until now, it remained an open question whether we had looked far enough back in time to find the starting points for the Milky Way and galaxies like it,” said Eric Gawiser, also from the Department of Physics and Astronomy at Rutgers. He also led the research team behind the new finding.
“Now we know the answer to that question is ‘Yes!’”
Written by Evan Gough/Universe Today.
