---
title: "Pea galaxy"
chunk: 5/11
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"
---

In a presentation to the AAS Meeting #229 in January 2017, Matt Brorby and Philip Kaaret describe the observations of two GPs and their x-ray emission. Using both space telescope programs Chandra GO: 16400764 and Hubble GO: 13940, they examine Luminous Compact Galaxies, both GPs, J0842+1150 and SHOC 486. They conclude: i) These are the first x-ray observations of GPs. ii) The two GPs studied are the first test of Lx-SFR-Z planar relation and that they are consistent with this. iii) Low-metallicity galaxies exhibit enhanced x-ray emission relative to normal metallicity starforming galaxies. iv) GPs are useful for predictions of X-ray output in the early universe.
In March 2017, Yang et al. published a paper in the ApJ called: "Lyα and UV Sizes of Green Pea Galaxies". The authors studied the Lyman-alpha (LyA) escape in a statistical sample of 43 GPs with HST/COS LyA spectra, taken from 6 HST programs. Their conclusions include: i) Using GPs that cover the whole ranges of dust extinction and metallicity, they find about two-thirds are strong LyA emitters. This confirms that GPs generally are "the best analogs of high-z (redshift) Lyman-alpha Emitters (LAEs) in the nearby universe." ii) The authors find many correlations regarding the dependence of LyA escape on galactic properties, such as dust extinction and metallicity. iii) The single shell radiative transfer model can reproduce most LyA profiles of GPs. iv) An empirical linear relation between the LyA escape fraction, dust extinction and the LyA red peak velocity.
In August 2017, Yang et al. published a study in the ApJ called: "Lyα profile, dust, and prediction of Lyα escape fraction in Green Pea Galaxies". The authors state that GPs are nearby analogues of high-redshift Lyman-alpha (LyA)- emitting galaxies. Using spectral data from the HST-COS MAST archive, 24 GPs were studied for their LyA escape and the spatial profiles of LyA and UV continuum emissions. Results include: i) Having compared LyA and UV sizes from the 2D spectra and 1D spatial profiles, it is found that most GPs show more extended LyA emission than the UV continuum. ii) 8 GPs had their spatial profiles of LyA photons at blueshifted and redshifted velocities compared. iii) The LyA escape fraction was compared with the size ration of LyA to UV. It was found that GPs that have LyA escape fractions greater than 10% "tend to have more compact LyA morphology".
In October 2017, Lofthouse et al. published a study in MNRAS named:  The authors used integral field spectroscopy, from the SWIFT and Palm 3K instruments, to perform a spatially-resolved spectroscopic analysis of four GPs, numbered 1,2,4 and 5. Among the results are that GPs 1 & 2 are rotationally-supported (they have a rotating centre), while GPs 4 & 5 are dispersion-dominated systems. GPs 1 & 2 show morphologies indicative of ongoing or mergers. However, GPs 4 & 5 show no signs of recent interactions and have similar star-forming rates. This indicates mergers are not "a necessary requirement for driving the high star formation in these types of galaxies".
In December 2017, authors Jaskot et al. published a paper in the ApJ Letters titled:"Kinematics and Optical Depth in the Green Peas: Suppressed Superwinds in Candidate LyC Emitters". Within the paper, they say that current thinking describes how superwinds clear neutral gas away from young starburst galaxies, which in turn regulates the escape of Lyman Continuum photons from star-forming galaxies. Models predict however that in the most extreme compact starbursts, those superwinds may not launch. The authors explore the role of outflows in generating low optical depth in GPs, using observations from the Hubble Space Telescope. They compare the kinematics of ultraviolet absorption and with Lyman alpha escape fraction, Lyman alpha peak separation or low-ionisation absorption. The most extreme GPs show the slowest velocities, which "are consistent with models for suppressed superwinds, which suggests that outflows may not be the only cause of LyC escape from galaxies."

=== 2021–2024 ===
In this study using images of Peas taken as part of the Zoogems project, Leonardo Clarke et al. examine PG content to find out about the different ages of the stars and find that, while the central star-forming clusters were up to 500 million years old, there are stars, possibly the host galaxy stars, which are older and are thought to be more than 1 billion years old. Peas have been intensively studied as they are the only population that has hydrogen-ionizing radiation escaping in large amounts and so are substitutes for the earliest galaxies. Yet Clarke et al. argue the substantial presence of old stars would not have been possible at the earliest stages of the first galaxies. The mix of old and new stars within Pea galaxies could create different gravitational conditions which might influence galactic winds and element retention. Their conclusions imply that Pea galaxies are not real analogs of the galaxies responsible for the Epoch of Reionisation.

This study from January 2023 uses Early Release Observations from the James Webb Space Telescope to analyse the Near Infrared Spectrograph of three galaxies at a redshift of z~8 to determine their metallicities, gas temperatures and ionisation. Using robust measurement procedures, the scientists compare the abundances and emission-line ratios to a nearby sample of Green Pea galaxies. JWST data shows further similarities between these GPs and the three high-redshift galaxies. These three galaxies show a compact morphology typical of emission-line-dominated galaxies at all redshifts and based on similarities with GPs, "it is likely that these are the first rest-optical spectra of galaxies that are actively driving cosmological reionization".