The cosmic microwave background, originating some half a million years after the Big Bang, signified that cosmic hydrogen gas has cooled enough to no longer be ionized. However, about a billion years later, most of the intergalactic gas was ionized again, as it is today. Turning hydrogen back into its ionized form is the last major phase transition of the universe. Thanks to the HST and now JWST, we think that the intense radiation from massive stars in the first galaxies was responsible for this "reionization". However, the energetic photons (Lyman continuum (LyC) photons) required to do this normally get absorbed by the clouds where stars form, so a big open question is how did these photons "escaped" into intergalactic space?
Unfortunately, this process of LyC escape cannot be studied directly at high redshift, because LyC photons get absorbed by trace amounts of neutral hydrogen lying between us and the high-redshift galaxy. Luckily, we can learn a lot about LyC escape by observing the low-redshift analogs of early galaxies - the so-called Green Peas. These extremely compact starbursts were first found only in 2009, because they are quite rare and because, being so compact, they are difficult to weed out from tons of similarly looking stars and distant quasars. Some of the Green Peas were indeed found to leak a significant fraction of LyC photons, so they proved indispensable for studying the processes of the early universe.
In a recent paper "Highly Efficient Identification of Extreme Emission-line Galaxies in the Local Universe: >8000 New Green Pea Candidates at 0.12 < z < 0.36", published in the Astrophysical Journal Supplement Series and led by IU's recent post-bac researcher Heather Samonski, a new method was presented that identified about 10 times as many Green Peas as were previously known. This should form a much richer, newer sample for future investigations of LyC escape. Furthermore, the new sample contains a large number of Peas that appear as two compact galaxies merging (pictured), which was not seen in previous Green Pea samples. This in turn may help us understand what triggers the intense starburst in Green Peas, which is also a puzzle.
This new, "all-sky" study was based on archival data. It complements the more focused efforts of IU's SFACT team to discover, using WIYN, a wider range of Green Peas in terms of mass. WIYN's Hydra spectroscopy will actually be utilized for the confirmation of some of these 8000 new Green Peas, similarly to how it is used to validate SFACT ones.
This project was kindly supported by the John and A-Lan Reynolds Post-Baccalaureate Fellowship.
Caption: Examples of the "Extended Peas." Extended Peas are rare among the known Green Peas. Many of the Extended Peas appear in higher resolution Subaru images as mergers or as close pairs (right columns), which is less obvious in SDSS images (left columns)
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