When observed, a galaxy has an age and a past history. Modelling the observed luminosity from its stars, gas and dust with the Pégase evolution model*, allows one to estimate the mass of the supernovae that have already exploded and created dense stellar remnants, black holes or neutron stars. This modelling takes into account the history of star formation and that of metal ejecta, which determine the dust absorption and emission in the mid and far infrared, revealed by the Herschel and Spitzer satellite data.

The main result is that the accumulated mass of stellar black holes (and neutron stars) reaches a few billions of solar masses. This value is huge, and moreover of the same order of magnitude as that of the supermassive black hole hosted in the active galaxy center, and measured independently. Hence the idea that stellar black holes, created by past supernovae explosions, migrate towards the galaxy core, thus explaining the growth of supermassive black holes. 

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* Pégase evolution model: Pégase  is a code to compute the spectral evolution of galaxies.  The evolution of the stars, gas and metals is followed  for a law of star formation and a stellar initial mass function.  The stellar evolutionary tracks extend from the main sequence to the white dwarf stage.  The emission of the gas in HII regions is also taken into account.    The main improvement in version 2 is the use of evolutionary tracks of different metallicities  (from 10-4 to 5×solar). The effect of extinction by dust is also modelled  using a radiative transfer code.

Picture: Artist rendition of a radio galaxy containing a supermassive black hole in its center credit: ESO/M. Kornmesser

The Medusa Nebula, also known as Abell 21, is an old planetary nebula some 1,500 light-years away in the constellation Gemini. The nebula is associated with a dramatic transformation and shows a final stage in the evolution of low mass stars like the sun, as they transform themselves from red giants to white dwarf stars and in the process shrug off their outer layers. 
credit: European Southern Observatory, VLT

CR7 -  The brightest Galaxy in the early Universe

These massive, brilliant, and previously purely theoretical objects were the creators of the first heavy elements in history - the elements necessary to forge the stars around us today, the planets that orbit them, and life as we know it. 

Astronomy Photo of the Day: 7/01/15 — NGC 2367

Using the power of the Wide Field Imager (WFI) Camera mounted to the ESO’s 2.2 Meter Telescope, astronomers have taken a stunning new image of a cluster of stars—all of which, sit approximately 7,000 light-years from Earth toward the constellation of Canis Major.

Called NGC 2367, the stars within this open cluster are believed to be around 5 million years old—So young, they burn brightly and intensely, with their blue-tinged hue adding yet another color to an already impressive palette. It doesn’t hurt that the stars have set surrounding hydrogen gas alight, which gives the region its red glow.

Learn more here: http://bit.ly/1NvvNjP

Image Credit: ESO/G. Beccari