Selections from 2016: Counting Galaxies in the Observable Universe

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

The Evolution of Galaxy Number Density at z < 8 and Its Implications

Published October 2016


Main takeaway:

How many galaxies are there in the observable universe? The latest estimate is approximately 2 trillion, according to a study led by Christopher Conselice (University of Nottingham, UK). The authors produced this estimate by using observations of the number of galaxies in recent deep-field surveys by Hubble and other telescopes, and then extrapolating this number to account for small and faint galaxies that we aren’t able to see.

Why it’s interesting:

The original Hubble Deep Field study from the mid-1990s provided the basis for our previous working estimate of the number of galaxies the universe contains, which was around 120 billion. The new estimate from Conselice and collaborators therefore suggests that there are a factor of ten more galaxies in the universe than we previously thought!

What to expect from observations:

Right now we only have the capability to see roughly 10% of these 2 trillion galaxies. But future observatories like the James Webb Space Telescope will be able to pick out many more distant galaxies than what we’ve found so far, helping us to understand how these galaxies formed in the early universe.


Christopher J. Conselice et al 2016 ApJ 830 83. doi:10.3847/0004-637X/830/2/83