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The Ancient Star Formation History of the Extremely Low-Mass Galaxy Leo P: Insights into a Post-Reionization Pause in Star Formation


Core Concepts
The star formation history of the extremely low-mass, isolated galaxy Leo P shows early star formation followed by a pause subsequent to the epoch of reionization, and then a later re-ignition of star formation. This pattern is rarely produced in simulations and has important implications for the contribution of low-mass galaxies to the UV photon budget at intermediate redshifts.
Abstract

The study presents the ancient star formation history (SFH) of the extremely low-mass, isolated galaxy Leo P, using deep near-infrared imaging from the James Webb Space Telescope (JWST). The key findings are:

  1. The SFH of Leo P shows early star formation, followed by a pause subsequent to the epoch of reionization, and then a later re-ignition of star formation. This pattern is very similar to the SFHs of other dwarf galaxies in the "transition zone" between quenched very low-mass galaxies and more massive galaxies, but is rarely produced in simulations.

  2. The lifetime SFH reveals that Leo P's stellar mass at the epoch of reionization was in the range that is normally associated with being totally quenched. The extended pause in star formation from z ∼5 −1 has important implications for the contribution of low-mass galaxies to the UV photon budget at intermediate redshifts.

  3. Observing in two NIRCam short wavelength filters (F090W and F150W) is superior to observing in a combination of a short and a long wavelength filter (F277W) for accurately measuring the ancient SFH, due to the higher sensitivity and angular resolution of the shorter wavelength data.

The study demonstrates the power of JWST to probe the ancient SFHs of low-mass galaxies outside the Local Group, providing crucial insights into the impact of reionization and the role of environment on the evolution of these systems.

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Stats
The galaxy Leo P has a distance of 1.62 ± 0.15 Mpc, a stellar mass of 2.7 ± 0.4 × 10^5 M_sun, and a gas mass of 8.1 × 10^5 M_sun. The 50% completeness limits of the JWST NIRCam observations are F090W = 29.21 mag (0.76 M_sun), F150W = 28.85 mag (0.76 M_sun), and F277W = 28.55 mag (0.78 M_sun).
Quotes
"The star formation history of Leo P shows early star formation followed by a pause subsequent to the epoch of reionization which is then later followed by a re-ignition of star formation." "The lifetime SFH reveals that Leo P's stellar mass at the epoch of reionization was in the range that is normally associated with being totally quenched." "Observing in two NIRCam short wavelength filters (F090W and F150W) is superior to observing in a combination of a short and a long wavelength filter (F277W) for accurately measuring the ancient SFH."

Deeper Inquiries

How do the star formation histories of other isolated, low-mass galaxies outside the Local Group compare to the pattern observed in Leo P?

The star formation histories (SFHs) of isolated, low-mass galaxies outside the Local Group exhibit notable similarities to the pattern observed in Leo P, particularly in the context of the "transition zone" between quenched very low-mass galaxies and more massive galaxies. For instance, galaxies such as Pegasus W, Aquarius, and Leo A have shown extended periods of quiescence post-reionization, followed by a re-ignition of star formation. These patterns suggest that while many low-mass galaxies are expected to be quenched by reionization, some, like Leo P, have managed to sustain star formation despite their low mass. In contrast to the SFHs of Local Group galaxies, which are often influenced by environmental factors such as tidal interactions and ram pressure stripping, the SFHs of isolated galaxies like Leo P are less affected by such processes. This isolation allows for a clearer understanding of the intrinsic star formation processes and the impact of reionization. The SFH of Leo P, characterized by early star formation, a significant pause during the epoch of reionization, and a subsequent resurgence, aligns with the observed trends in other isolated low-mass galaxies, indicating a potential commonality in their evolutionary paths.

What physical mechanisms could explain the extended pause in star formation seen in Leo P and other transition zone dwarf galaxies?

The extended pause in star formation observed in Leo P and other transition zone dwarf galaxies can be attributed to several interrelated physical mechanisms. One primary factor is the impact of reionization, which is theorized to have quenched star formation in low-mass galaxies by heating the surrounding gas and preventing it from cooling and collapsing to form new stars. In galaxies with low stellar masses (M∗ < 10^5 M⊙), the shallow gravitational potential wells make it difficult for them to retain gas against the energetic feedback from reionization and stellar processes. Additionally, stellar feedback mechanisms, such as supernova explosions and radiation pressure from massive stars, can further disrupt the interstellar medium (ISM) and inhibit star formation. In the case of Leo P, the galaxy's low metallicity (3% Z⊙) may also play a role, as metal-poor environments can lead to less efficient cooling of gas, thereby prolonging the quiescent phase. Moreover, the isolation of Leo P suggests that it has not experienced significant external perturbations that could trigger star formation, allowing it to maintain a prolonged period of inactivity. This combination of internal and external factors contributes to the observed SFH patterns in Leo P and similar dwarf galaxies, highlighting the complex interplay between reionization, stellar feedback, and environmental influences in shaping their evolutionary trajectories.

What insights can the ancient star formation histories of low-mass galaxies provide about the role of reionization in shaping the early universe?

The ancient star formation histories of low-mass galaxies, such as Leo P, provide critical insights into the role of reionization in shaping the early universe. By reconstructing the SFHs of these galaxies, researchers can discern the timing and impact of reionization on star formation processes. The observed pattern of early star formation followed by a significant pause during the epoch of reionization suggests that low-mass galaxies were profoundly affected by this cosmic event. These SFHs indicate that many low-mass galaxies experienced a cessation of star formation due to the heating and ionization of their gas reservoirs, which aligns with theoretical predictions regarding the effects of reionization. The fact that some galaxies, like Leo P, managed to reignite star formation after this pause implies that reionization did not completely quench all low-mass galaxies, but rather created a complex landscape where some could continue to evolve. Furthermore, the contribution of low-mass galaxies to the UV photon budget at intermediate redshifts can be better understood through their SFHs. The extended periods of inactivity in these galaxies suggest that their role in the reionization process may have been less significant than previously thought, as they were unable to contribute to the ionizing radiation needed to sustain the reionization of the universe. Overall, the study of ancient SFHs in low-mass galaxies enhances our understanding of the interplay between galaxy formation, reionization, and the evolution of the early universe, providing a more nuanced view of cosmic history.
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