Green bean galaxy

These two images show the green bean galaxies J0113+0106 (top) and J1155−0147 (bottom) as seen by the Hyper Suprime-Cam on the Subaru Telescope.

Green bean galaxies (GBGs) are very rare astronomical objects that are thought to be quasar ionization echos.^[1][2][3] They were discovered by Mischa Schirmer and colleagues R. Diaz, K. Holhjem, N.A. Levenson, and C. Winge.^[1] The authors report the discovery of a sample of Seyfert-2 galaxies with ultra-luminous galaxy-wide narrow-line regions (NLRs) at redshifts z=0.2-0.6.^[1]

While examining survey images taken with the 3.6-meter Canada–France–Hawaii Telescope (CFHT) atop 4200-m Mauna Kea, Hawaii, Schirmer noticed a galaxy with unusual colors—strongly peaking in the r filter, suggesting a spectral line.^[1][4] In fact, the color is quite similar to the Green Pea galaxies (GPs), which are compact star-forming galaxies. However, the object which became known as a GBG is much larger.^[1][5]

These galaxies are so rare that there is on average only one in a cube about 1.3 billion light-years across.^[1] They were nicknamed GBGs because of their color and because they are superficially similar to, but larger than, GPs.^[1] The interstellar gas in most GPs is ionized by UV-light from intense star formation, whereas the gas in GBGs is ionized by hard x-rays from an active galactic nucleus (AGN). The scarcity of GBGs indicates that this phenomenon is very rare, and/or very short-lived.^[1]

GBGs are likely related to the object known as Hanny's Voorwerp, another possible quasar ionization echo.^[6] GBGs are substantially different, though, as their luminosities, sizes and gas masses are 10-100 times higher than in other quasar ionisation clouds, for instance the 154 studied in Keel et al. 2012 (nicknamed 'voorwerpjes').^[7] These 'voorwerpjes' are estimated to have bright phases that last between ~20,000 and 200,000 years.^[7]

Possible formation mechanisms are currently under investigation. Likely, the giant gas outflows have been produced during the last stages in the life of super-luminous quasars, which subsequently experienced a rapid shut-down, e.g. due to a process known as AGN feedback.^[8] The escaping X-rays from the former very active quasar state still ionize the gas, causing the ionization echo.