---
title: "Pea galaxy"
chunk: 9/11
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"
---

 
Color selection was by using the difference in the levels of three optical filters, in order to capture these color limits: u−r ≤ 2.5 (1), r−i ≤ −0.2 (2), r−z ≤ 0.5 (3), g−r ≥ r−i + 0.5 (4), u−r ≥ 2.5 (r−z) (5).
If the diagram on the right (one of two in the paper) is looked at, the effectiveness of this color selection can be seen. The color-color diagram shows ~100 GPs (green crosses), 10,000 comparison galaxies (red points) and 9,500 comparison quasar (purple stars) at similar redshifts to the GPs. The black lines show how these figures are on the diagram.
Comparing a GP to the Milky Way can be useful when trying to visualize these star-forming rates. An average GP has a mass of ~3,200 million M☉ (~3,200 million solar masses). The Milky Way (MW) is a spiral galaxy and has a mass of ~1,125,000, million M☉ (~1,125,000 million solar masses). So the MW has the mass of ~390 GPs.
Research has shown that the MW converts ~2 M☉/yr (~2 solar masses per year) worth of interstellar medium into stars. An average GP converts ~10 M☉/yr (~10 solar masses) of interstellar gas into stars, which is ~5 times the rate of the MW.
One of the original ways of recognizing GPs, before SQL programming was involved, was because of a discrepancy about how the SDSS labels them within Skyserver. Out of the 251 of the original GP sample that were identified by the SDSS spectroscopic pipeline as having galaxy spectra, only 7 were targeted by the SDSS spectral fibre allocation as galaxies i.e. 244 were not.

== Later studies ==
In June 2010, authors R. Amorín et al. published a paper in ApJ letters titled "On the Oxygen and Nitrogen Chemical Abundances and the Evolution of the "Green Pea" Galaxies", which disputes the metallicities calculated in the original Cardamone et al. GPs paper Amorin et al. use a different methodology from Cardamone et al. to produce metallicity values more than one fifth (20%) of the previous values (about 20% solar or one fifth solar) for the 80 'starburst' GPs. These mean values are log[O/H]+12~8.05, which shows a clear offset of 0.65dex between the two papers' values. For these 80 GPs, Amorin et al., using a direct method, rather than strong-line methods as used in Cardamone et al., calculate physical properties, as well as oxygen and nitrogen ionic abundances. These metals pollute hydrogen and helium, which make up the majority of the substances present in galaxies. As these metals are produced in supernovae, the more recent a galaxy is, the fewer metals it would have. As GPs are in the nearby, or recent, Universe, they should have more metals than galaxies at an earlier time.

Amorin et al. find that the amount of metals, including the abundance of nitrogen, is different from normal values and that GPs are not consistent with the mass-metallicity relation, as concluded by Cardamone et al. This analysis indicates that GPs can be considered as genuine metal-poor galaxies. They then argue that this oxygen under-abundance is due to a recent interaction-induced inflow of gas, possibly coupled with a selective metal-rich gas loss driven by supernovae winds and that this can explain their findings. This further suggests that GPs are likely very short-lived as the intense star formation in them would quickly enrich the gas.

In May 2011, R.Amorin et al. published a conference proceeding paper titled "Unveiling the Nature of the "Green Pea" galaxies". In it they review recent scientific results and announcing a forthcoming paper on their recent observations at the Gran Telescopio Canarias. They conclude that GPs are a genuine population of metal-poor, luminous and very compact starburst galaxies. Amongst the data, five graphs illustrate the findings they have made. Amorin et al. use masses calculated by Izotov, rather than by Cardamone. The metallicities that Amorin et al. use agree with Izotov's findings, or vice versa, rather than Cardamone's.
The first graph (on the left; fig.1 in paper) plots the nitrogen/oxygen vs. oxygen/hydrogen abundance ratio. The 2D histogram of SDSS star forming galaxies is shown in logarithmic scale while the GPs are indicated by circles. This shows that GPs are metal-poor.

The second graph (on the right; fig.2 in paper) plots O/H vs. stellar mass. The 2D histogram of SDSS SFGs is shown in logarithmic scale and their best likelihood fit is shown by a black solid line. The subset of 62 GPs are indicated by circles and their best linear fit is shown by a dashed line. For comparison we also show the quadratic fit presented in Amorin et al. 2010 for the full sample of 80 GPs. SFGs at z ≥ 2 by Erb et al. are also shown by asterisks for comparison.

The third graph (on the left; fig.3 in paper) plots N/O vs. stellar mass. Symbols as in fig.1.

The fourth graph (on the right; fig.4 in paper) plots O/H vs. B-band (rest-frame) absolute magnitude. The meaning of symbols is indicated. Distances used in computing (extinction corrected) absolute magnitudes were, in all cases, calculated using spectroscopic redshifts and the same cosmological parameters. The dashed line indicates the fit to the HII galaxies in the Luminosity-Metallicity Relationship (MZR) given by Lee et al. 2004.
The fifth graph (on the left; fig.5 in paper) plots gas mass fraction vs. metallicity. Different lines correspond to closed-box models at different yields, as indicated in the legend. Open and filled circles are GPs that are above and below the fit to their MZR. Diamonds are values for the same Wolf-Rayet galaxies as in Fig. 4.