For a long time, our understanding of the Universe\u2019s first galaxies leaned heavily on theory. The light from that age only reached us after travelling for billions of years, and on the way, it was obscured and stretched into the infrared. Clues about the first galaxies are hidden in that messy light. Now that we have the James Webb Space Telescope and its powerful infrared capabilities, we\u2019ve seen further into the past\u2014and with more clarity\u2014than ever before. <\/p>\n
The JWST has imaged some of the very first galaxies, leading to a flood of new insights and challenging questions. But it can\u2019t see individual stars.<\/p>\n
How can astronomers detect their impact on the Universe\u2019s first galaxies?<\/p>\n
<\/p>\n Stars are powerful, dynamic objects that wield a potent force. They can fuse atoms together into entirely new elements, an act called nucleosynthesis. Supernovae are especially effective at this, as their powerful explosions unleash a maelstrom of energy and matter and spread it back out into the Universe. <\/p>\n Supernovae have been around since the Universe\u2019s early days. The first stars in the Universe are called Population III stars, and they were extremely massive stars. Massive stars are the ones that explode as supernovae, so there must have been an inordinately high number of supernovae among the Population III stars. <\/p>\n New research examines how all of these supernovae must have affected their host galaxies. The paper \u201cHow Population III Supernovae Determined the Properties of the First Galaxies\u201d has been accepted for publication by the Astrophysical Journal. The lead author is Ke-Jung Chen from the Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan. <\/p>\n Stellar metallicity is at the core of this work. When the Universe began, it was comprised of primordial hydrogen, helium, and only trace amounts of lithium and beryllium. If you check your periodical table, these are the first four elements. Elements heavier than hydrogen and helium are called \u201cmetals\u201d in astronomy, and metallicity in the Universe increases over time due to stellar nucleosynthesis. <\/p>\n But hydrogen dominated the Universe then as it does now. Only once the first stars formed and then exploded did other elements start to play a role.<\/p>\n \u201cThe birth of primordial (Pop III) stars at z ~ 20 ~ 25 marked the end of the cosmic dark ages and the onset of the first galaxy and supermassive black hole (SMBH) formation,\u201d the authors of the new paper write. But their role as creators of astronomical metals is at the heart of this research.<\/p>\n The researchers used computer hydrodynamical simulations to examine how Pop III stars shaped early galaxies. They looked at core-collapse supernovae (CCSNe), pair-instability supernovae (PISNe), and Hypernovae (HNe.)<\/p>\n Stars can only form from cold, dense gas. When gas is too hot, it simply isn\u2019t dense enough to collapse into protostellar cores. The researchers found that when Pop III stars exploded as supernovae, they produced metals and spread them into the surrounding gas. The metals cooled the star-forming gas quickly, leading to faster formation of more stars. \u201cOur findings indicate that SNRs from a top-heavy Pop III IMF <initial mass function> produce more metals, leading to more efficient gas cooling and earlier Pop II star formation in the first galaxies.\u201d<\/p>\n The simulations showed that the supernova remnants (SNR) from the Pop III SN fall towards the center of the dark matter haloes they reside in. \u201cThese Pop III SNRs and the primordial gas are dragged by the halo gravity toward its center,\u201d the authors explain. These SNRs sometimes collide and produce turbulent flows. The turbulence mixes the gas and the metals from the SN and \u201ccreates filamentary structures that soon form into dense clumps due to the self-gravity and metal cooling of the gas.\u201d <\/p>\n This leads to more star formation, though at this point, they\u2019re still Pop III stars. These aren\u2019t enriched by the earlier Pop III supernovae and are still made of primordial gas. Some of these later Pop III stars form before the initial ones reach the center of the halo. That creates a complicated situation.<\/p>\n The second round of Pop III stars then \u201cimpose strong radiative and SN feedback before the initial Pop III SNRs reach the halo center,\u201d the authors write. <\/p>\n The Pop III stars heat the surrounding gas with their powerful UV radiation, as shown in the figure above, inhibiting star formation. But they\u2019re massive stars, and they don\u2019t live very long. Once they explode, they spread metals out into their surroundings, which can cool gas and trigger more star formation. \u201cAfter its short lifetime of about 2.0 Myr, the star dies as a PI SN, and its shock heats the gas to high temperatures (> 105 K) and ejects a large mass of metals that enhance cooling and promotes a transition to Pop II SF,\u201d the authors explain. <\/p>\n This is where the Pop III stars shaped the earliest galaxies. By injecting metals into the clouds of star-forming gas, they cooled the gas. The cooling fragmented the clouds of star-forming gas, making the following generation of Pop II stars less massive. \u201cDue to the effective metal cooling, the mass scale of these Pop II stars shifted to a low mass end and formed in a cluster, as shown in the right panel of Figure 6.\u201d<\/p>\n Pop III stars existed mostly in dark matter haloes. However, the research shows how they shaped the succeeding Pop II stars, which populated the early galaxies. One question astronomers have faced regarding the first galaxies is whether they were filled with extremely metal-poor (EMP) Pop II stars. But this research shows otherwise. \u201cWe thus find that EMP stars were not typical of most primitive galaxies,\u201d the authors conclude.<\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/02\/Pop-2-low-mass-star-clusters.png\")
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