---
title: "Pea galaxy"
chunk: 7/11
source: "https://en.wikipedia.org/wiki/Pea_galaxy"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:15:28.328105+00:00"
instance: "kb-cron"
---

== J0925+1403 and LyC leakage ==
In January 2016, a letter was published in the journal Nature called: "Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy" by authors Y.I. Izotov et al. which was a result of observations carried out using the COS aboard the HST. The abstract states: "One of the key questions in observational cosmology is the identification of the sources responsible for ionisation of the Universe after the cosmic Dark Ages". It also states: "Here we present far-ultraviolet observations of a nearby low-mass star-forming galaxy, J0925+1403, selected for its compactness and high excitation... The galaxy is 'leaking' ionising radiation, with an escape fraction of 7.8%." These levels of radiation are thought to be similar to those of the first galaxies in the universe, which emerged in a time known as reionization. These findings have led the research team to conclude that J0925 can ionise intergalactic material up to 40 times its own stellar mass.
GP J0925 is thought to be similar to the most distant, and thus earliest, galaxies in the universe and has been shown to 'leak' LyC. It is about 3 billion light years away (redshift z=0.301), or approximately 75% of the current age of the universe. Co-author Trinh Thuan said that i) The finding is significant because it gives us a good place to look for probing the reionization phenomenon, which took place early in the formation of the universe that became the universe we have today, ii) As we make additional observations using Hubble, we expect to gain a much better understanding of the way photons are ejected from this type of galaxy, and the specific galaxy types driving cosmic reionization, and iii) These are crucial observations in the process of stepping back in time to the early universe.

=== LyC detection in J1152+3400, J1333+6246, J1442-0209, J1503+3644 ===
In October 2016, a study was published in the MNRAS entitled: "Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies". Its authors are Y. I. Izotov et al. The abstract states: "Following our first detection reported in Izotov et al. (2016) [as above], we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST)".
This study contains the methods and findings from Izotov et al. 2016 (a) which concentrated on one galaxy, whereas the above paper, Izotov et al. 2016 (b) has findings for four galaxies, all of which have LyC leakage. When compared with other known local galaxies that leak LyC, as listed in this article, Izotov et al. 2016 (a & b) doubled the numbers of known leakers.

== Lyman alpha emission ==

In May 2015, authors Alaina Henry et al. published a paper titled: "Lyα Emission from Green Peas: The Role of Circumgalactic Gas Density, Covering, and Kinematics".  The motivation of this work was to understand why some galaxies have Lyα emission, while others don't. A host of physical conditions in galaxies regulate the output of this spectral feature; hence, understanding its emission is fundamentally important for understanding how galaxies form and how they impact their intergalactic surroundings.
Henry et al. hypothesized that, since the GPs seem more like galaxies at redshift=z>2, and Lyα is common at these redshifts, that Lyα would be common in the GPs as well. Observations with the HST using the COS, as in 'Description', proved this to be true for a sample of 10 GPs. The spectra, shown here to the right, indicate resonant scattering of Lyα photons that are emitted near zero velocity. The wealth of data existing on the GPs, combined with the COS spectra, allowed Henry et al. to explore the physical mechanisms that regulate the Lyα output. These authors concluded that variations in the amount of neutral hydrogen gas, which scatters Lyα photons, are the cause of a factor of 10 difference in Lyα output in their sample.
The spectrum of GP J1219 (an image of which is in 'Description') shows its very strong flux measurements when compared to other 9 GPs. Indeed, only GP J1214 has a value approaching that of J1219. Note also the double peaks in some GPs and the velocity values of the emissions, indicating the inflow and outflow of matter in the GPs.

== Papers by A. Jaskot and M.S. Oey ==
In April 2013, authors A. Jaskot and M. Oey published a paper in ApJ titled "The Origin and Optical Depth of Ionizing Radiation in the "Green Pea" Galaxies". Six "extreme" GPs are studied. Using these, they endeavour to narrow down the list of possibilities about what is producing the UV-radiation and the substantial amounts of high-energy photon that might be escaping from the GPs. Through trying to observe these photons in nearby galaxies such as the GPs, our understanding of how galaxies behaved in the early Universe might well be revolutionised. It is reported that the GPs are exciting candidates to help astronomers understand a major milestone in the development of the cosmos 13 billion years ago, during the epoch of reionization.
In February 2014, authors A. Jaskot and M. Oey published a conference report titled "The Origin and Optical Depth of Ionizing Photons in the Green Pea Galaxies". This will appear in "Massive Young Star Clusters Near and Far: From the Milky Way to Reionization", based on the 2013 Guillermo Haro Conference. In the publication, Jaskot and Oey write: "We are currently analyzing observations from IMACS and MagE on the Magellan Telescopes and COS and ACS on Hubble Space Telescope (HST) to distinguish between WR (Wolf-Rayet star) and the shock ionization scenarios and confirm the GPs' optical depths.  The absence of WR features in the deeper IMACS spectra tentatively supports the shock scenario, although the detection limits do not yet definitively rule out the WR photoionization hypothesis."

== Physics from the Cardamone 2009 paper ==