---
title: "Pea galaxy"
chunk: 8/11
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"
---

At the time this paper was published, only five Green Peas (GPs) had been imaged by the Hubble Space Telescope (HST). Three of these images reveal GPs to be made up of bright clumps of star formation and low surface density features indicative of recent or ongoing galaxy mergers. These three HST images were imaged as part of a study of local ultraviolet (UV-luminous) galaxies in 2005. Major mergers are frequently sites of active star-formation and to the right a graph is shown that plots specific star formation rate (SFR / Galaxy Mass) against galaxy mass. In this graph, the GPs are compared to the 3003 mergers from the Galaxy Zoo Merger Sample (GZMS). It shows that the GPs have low masses typical of dwarf galaxy and much higher star-forming rates (SFR) when compared to the GZMS. The black, dashed line shows a constant SFR of 10 M☉/yr (~10 solar masses). Most GPs have a SFR between 3 and 30 M☉/yr (between ~3 and ~30 solar masses).

GPs are rare. Of the one million objects that make up GZ's image bank, only 251 GPs were found. After having to discard 148 of these 251 because of atmospheric contamination of their stellar spectra, the 103 that were left, with the highest signal-to-noise ratio, were analyzed further using the classic emission line diagnostic by Baldwin, Phillips and Terlevich which separates starbursts and active galactic nuclei. 80 were found to be starburst galaxies. The graph to the left classifies 103 narrow-line GPs (all with SNR ≥ 3 in the emission lines) as 10 active galactic nuclei (blue diamonds), 13 transition objects (green crosses) and 80 starbursts (red stars). The solid line is: Kewley et al. (2001) maximal starburst contribution (labelled Ke01). The dashed line is: Kauffmann et al. (2003) separating purely star-forming objects from AGN (labelled Ka03).

GPs have a strong [OIII] emission line when compared to the rest of their spectral continuum. In a SDSS spectrum, this shows up as a large peak with [OIII] at the top. The wavelength of [OIII] (500.7 nm) was chosen to determine the luminosities of the GPs using equivalent width (Eq.Wth.). The histogram on the right shows on the horizontal scale the Eq.Wth. of a comparison of 10,000 normal galaxies (marked red), UV-luminous Galaxies (marked blue) and GPs (marked green). As can be seen from the histogram, the Eq.Wth. of the GPs is much larger than normal for even prolific starburst galaxies such as UV-luminous Galaxies.
Within the Cardamone et al. paper, comparisons are made with other compact galaxies, namely Blue Compact Dwarfs Galaxies and UV-luminous Galaxies, at local and much higher distances. The findings show that GPs form a different class of galaxies than Ultra Blue Compact Dwarfs, but may be similar to the most luminous members of the Blue Compact Dwarf Galaxy category. The GPs are also similar to UV-luminous high-redshift galaxies such as Lyman-break Galaxies and Lyman-alpha emitters. It is concluded that if the underlying processes occurring in the GPs are similar to that found in the UV-luminous high-redshift galaxies, the GPs may be the last remnants of a mode of star formation common in the early Universe.

GPs have low interstellar reddening values, as shown in the histogram on the right, with nearly all GPs having E(B−V) ≤ 0.25. The distribution shown indicates that the line-emitting regions of star-forming GPs are not highly reddened, particularly when compared to more typical star-forming or starburst galaxies. This low reddening combined with very high UV luminosity is rare in galaxies in the local Universe and is more typically found in galaxies at higher redshifts.
Cardamone et al. describe GPs as having a low metallicity, but that the oxygen present is highly ionized. The average GP has a metallicity of log[O/H]+12~8.69, which is solar or sub-solar, depending on which set of standard values is used. Although the GPs are in general consistent with the mass-metallicity relation, they depart from it at the highest mass end and thus do not follow the trend. GPs have a range of masses, but a more uniform metallicity than the sample compared against. These metallicities are common in low-mass galaxies such as Peas.

As well as the optical images from the SDSS, measurements from the GALEX survey were used to determine the ultraviolet values. This survey is well matched in depth and area, and 139 of the sampled 251 GPs are found in GALEX Release 4 (G.R.4). For the 56 of the 80 star-forming GPs with GALEX detections, the median luminosity is ~30,000 million 
  
    
      
        
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 (~30,000 million solar luminosities).
When compiling the Cardamone paper, spectral classification was made using Gas And Absorption Line Fitting (GANDALF). This sophisticated computer software was programmed by Marc Sarzi, who helped analyze the SDSS spectra.

== Analysis of the Cardamone 2009 paper ==
These values are from Table 4, pages 16–17 of Cardamone 2009 et al., which shows the 80 GPs that have been analysed here. The long 18-digit numbers are the SDSS DR7 reference numbers.