|
A Pea galaxy, also referred to as a Pea or Green Pea, might be a type of Luminous Blue Compact Galaxy which is undergoing very high rates of star formation. Pea galaxies are so-named because of their small size and greenish appearance in the images taken by the Sloan Digital Sky Survey (SDSS). Pea Galaxies were first discovered in 2007 by the volunteer users within the forum section of the online astronomy project Galaxy Zoo (GZ). == Description == The Pea galaxies, also known as Green Peas (GPs), are compact oxygen-rich emission line galaxies that were discovered at redshifts between ''z'' = 0.112 and 0.360.〔 These low-mass galaxies have an upper size limit generally no bigger than across, and typically they reside in environments less than two-thirds the density of normal galaxy environments.〔 An average GP has a redshift of ''z'' = 0.258, a mass of ~3,200 million (~3,200 million solar masses), a star formation rate of /yr (~10 solar masses a year), an (III ) equivalent width of 69.4 nm and a low metallicity.〔 A GP is purely star-forming, rather than having an Active Galactic Nucleus. They have a strong emission line at the () wavelength of 500.7 nm. (), O++ or doubly ionized oxygen, is a forbidden line of the visual spectrum and is only possible at very low densities.〔 GPs are the least massive and most actively star-forming galaxies in the local universe. A paper published in June 2015 and submitted to the Astrophysical Journal by Huan Yang et al. titled: "Green Pea Galaxies Reveal Secrets of Lyα Escape" reveals current thinking about how many GPs there are. Quoting from that paper: "While the full SDSS data set contains a few tens of thousands of Green Peas, they are not in a category systematically targeted for spectroscopic followup, and only a much smaller sample of about 251 Green Peas have spectra in SDSS DR7 (Cardamone et al. 2009)". "These galaxies would have been normal in the early Universe, but we just don’t see such active galaxies today", said Kevin Schawinski. "Understanding the Green Peas may tell us something about how stars were formed in the early Universe and how galaxies evolve".〔 GPs exist at a time when the Universe was three-quarters of its present age and so are clues as to how galaxy formation took place in the earlier Universe. With the publication of Amorin's GTC paper in February 2012, it is now thought that GPs might be old galaxies having formed most of their stellar mass several billion years ago. Old stars have been spectroscopically confirmed in one of the three galaxies in the study by the presence of Magnesium.〔 Recently in 2014, two separate sources have identified two GPs to be likely Lyman Continuum (LyC)-emitting candidates. This suggests that these two GPs are low-redshift analogs of high-redshift Lyman-alpha and LyC emitters, only two others of which are known:Haro 11 and Tololo-1247-232.〔〔〔 Finding local LyC emitters is crucial to the theories about the early universe and the epoch of reionization.〔〔 These two objects have SDSS DR9 reference numbers: 1237661070336852109 (GP_J1219) and 1237664668421849521. The image to the right shows Pea galaxy GP_J1219.〔 This was taken by a Hubble Space Telescope team whose Principal Investigator was (Alaina Henry ) in early 2014, using the Cosmic Origins Spectrograph and the Near Ultraviolet channel. The scale bar in the image shows 1 arc second (1"), which corresponds to ~10,750 light years at the distance of 2.69 billion light years for GP_J1219. When using the COS Multi-Anode Micro-channel Array, in NUV imaging mode, the detector plate scale is ~40 pixels per arcsecond (0.0235 arcseconds per pixel). As a result of HST Proposal 13744 for Cycle 22, Trinh Thuan from the University of Virginia has been granted observation time. The proposal abstract, titled "Green Peas and diagnostics for Lyman continuum leaking in star-forming dwarf galaxies" states: "Our unique GP sample will allow us to combine for the first time four fundamental tests for LyC leaking in galaxies and validate their usefulness as LyC leaking indicators : 1) direct measurements of the LyC; 2) high ()/() ratios; 3) characteristics of the Lyman alpha line profile; and 4) residual intensities in the low-ionization ISM absorption UV lines". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pea galaxy」の詳細全文を読む スポンサード リンク
|