Galaxies
64 158 CATALOGUEDGravitationally bound systems of stars, gas and dark matter — the structures the universe organises itself into.
Reading the Hubble type
E is elliptical, S is spiral, SB is barred spiral, Irr is irregular. The trailing letter grades how tightly the arms wind: Sa is tight, Sc is open.
Why distances are uncertain
Beyond the local group, distance is inferred from redshift, which needs an expansion rate to convert. That rate is measured two ways that disagree — Planck's 67.4 km/s/Mpc and SH0ES's 73.04 — so every redshift distance here carries ±4% that no telescope can remove until that argument ends. This catalogue divides by the round number between them, H0 = 70, and it does the dividing itself for every galaxy: the redshifts come from several surveys, and where one published distances under an expansion rate of its own, the conversion was re-run rather than the number copied, so that two galaxies in one cluster are not 3.5% apart for a reason that has nothing to do with either. The error bar beside the number carries that ±4%, and adds the galaxy's own motion through space to it.
Photometric and spectroscopic redshifts
A spectroscopic redshift is measured: the lines in the spectrum are where they are, and the number is good to a few per cent. A photometric redshift is fitted — the galaxy's colours through a handful of filters, matched against templates — and GLADE+, which supplies most of the galaxies here, gives it a standard deviation of 20% of itself. The distance says which one it came from, and the error bar beside a photometric one is several times wider than a spectroscopic neighbour's for that reason and not because the galaxy is further away.