---
title: "Pea galaxy"
chunk: 10/11
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"
---

== GTC-OSIRIS spectrophotometry ==
In February 2012, authors R. Amorin et al. published a paper titled "The star formation history and metal content of the "Green Peas". New detailed GTC-OSIRIS spectrophotometry of three galaxies" in which they presented the findings of observations carried out using the Gran Telescopio Canarias at the Roque de los Muchachos Observatory. They gather deep broad-band imaging and long-slit spectroscopy of 3 GPs using high-precision equipment.
Their findings show that the three GPs display relatively low extinction, low oxygen abundances and high nitrogen-to-oxygen ratios. Also reported are the clear signatures of Wolf–Rayet stars, of which a population are found (between ~800 and ~1200). A combination of population and evolutionary synthesis models strongly suggest a formation history dominated by starbursts. These models show that these three GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as these models imply, they are old galaxies having formed most of their stellar mass several billion years ago. The presence of old stars has been spectroscopically verified in one of the three galaxies by the detection of Magnesium. Surface photometry, using data from the Hubble Space Telescope archive, indicates that the three GPs possess an exponential low surface brightness envelope (see Low-surface-brightness galaxy). This suggests that GPs are identifiable with major episodes in the assembly history of local Blue Compact Dwarf galaxies.
The three galaxies are (using SDSS references):

587724199349387411
587729155743875234
587731187273892048

== Comparison to luminous compact galaxies ==
In February 2011, Izotov et al. published a paper titled "Green Pea Galaxies and Cohorts: Luminous Compact Emission-line Galaxies in the Sloan Digital Sky Survey", examining the GPs and comparing these to a larger set of 803 Luminous Compact Galaxies (LCGs). They use a different set of selection criteria from Cardamone et al. These are: a) a high extinction-corrected luminosity > 3×1040 ergs/s of the hydrogen beta emission line; (see hydrogen spectral series) b) a high equivalent width greater than 5 nm; c) a strong [OIII] wavelength at the 436.3 nm emission line allowing accurate abundance determination; d) a compact structure on SDSS images; and e) an absence of obvious active galactic nuclei spectroscopic features.
Its conclusions (shortened) are:

The selected galaxies have redshifts between 0.02 and 0.63, a range equal or greater than a factor of 2 when compared to the GPs. They find the properties of LCGs and GPs are similar to Blue Compact Dwarf galaxies. Explaining how the colours of emission-line galaxies change with distance using SDSS, they conclude that GPs are just subsamples within a narrow redshift range of their larger LCG sample.
Although there were no upper limits on the hydrogen beta luminosities, it was found that there was a 'self-regulating' mechanism which bound the LCGs to a limit of ~3×1042 ergs/s.
In the [OIII] wavelength 500.7 nm ratio to hydrogen beta vs. [NII] wavelength 658.3 nm ratio to hydrogen alpha, LCGs occupy the region, in the diagnostic diagram, of star-forming galaxies with the highest excitation. However, some active galactic nuclei also lie in this region in the diagnostic diagram.
The oxygen abundances 12 + log O/H in LCGs are in the range 7.6–8.4 with a median value of ~8.11, confirming Amorin et al.'s analysis of a subset of GPs. This range of oxygen abundances is typical of nearby lower-luminosity Blue Compact Dwarfs. These results show that the original Cardamone et al. median oxygen abundance of 12 + log O/H = ~8.7 is overestimated, as a different, empirical method was originally used, rather than the direct method by Amorin et al. and Izotov et al. There is no dependence of oxygen abundance on redshift.
In the luminosity-metallicity diagram (fig. 8 in paper), LCGs are shifted by ~2 magnitudes brighter when compared to nearby emission-line galaxies. LCGs form a common luminosity-metallicity relation, as for the most actively star-forming galaxies. Some LCGs have oxygen abundances and luminosities similar to Lyman-break galaxies (LBGs), despite much lower redshifts, thus enabling the study of LBGs through LCGs.

== Radio detection ==

=== Chakraborti et al. ===
In February 2012, authors Sayan et al. published a paper titled "Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies" which deals with the magnetic properties of the GPs. In it, they describe observations which have produced some unexpected results raising puzzling questions about the origin and evolution of magnetism in young galaxies. The ages are estimated from looking at the star formation that the GPs currently have ongoing and then estimating the age of the most recent starburst. GPs are very young galaxies, with models of the observed stellar populations indicating that they are around 10^8 (one hundred million) years old (1/100 the age of the Milky Way). There is some question as to whether the GPs all started from the same starburst or if multiple starbursts went on (much older stellar populations are hidden as we can't see the light from these).
Using data from the Giant Metrewave Radio Telescope (GMRT) and archive observations from the Karl G. Jansky Very Large Array (VLA), Chakraborti et al. produced a set of results which are based around the VLA FIRST detection of stacked flux from 32 GPs and three 3-hour low-frequency observations from the GMRT which targeted the three most promising candidates which had expected fluxes at the milli-Jansky (mJy) level.
Chakraborti et al. find that the three GPs observed by the GMRT have a magnetic field of B~39 μG, and more generally a figure of greater than B~30μG for all the GPs. This is compared to a figure of B~5μG for the Milky Way. The present understanding is of magnetic field growth based on the amplification of seed fields by dynamo theory and its action over a galaxy's lifetime. The observations of GPs challenge that thinking.