A small thread about the sizes of galaxies. The Tenerife group is proposing a different definition of radius, based on a fixed stellar surface density - see arxiv.org/abs/2001.02689 . For a given M/L ratio this is identical to the "isophotal" radius, widely used until the 1990s. The reason why this measure has been mostly abandoned is that it mixes sizes with luminosities: an isophotal (or iso-density) radius is not really a size, but answers the question "what part of a galaxy is above a given surface brightness".

@asborlaff @mireiamontesq @PeterYoachim @astro_delgado Absolutely! I have tremendous respect for the amateur community. They use different techniques though, and there's (understandable) variation among their images - these are all deep ones I could find. That's why we only compared to published (prof or amateur) images in the paper. @asborlaff @mireiamontesq @PeterYoachim @astro_delgado Several clearly agree with the one from @astro_delgado . One exotic explanation is that the 2nd loop is 100% 3727 emission, and some of the detectors are UV sensitive. That doesn't explain why features in the first loop are at a different location though (see Fig 5 in our paper).

We did another little (very little) paper on the dark matter-deficient galaxies in the NGC1052 group:
arxiv.org/abs/1902.02807…
We were in the process of writing a reply to a referee report and then realized it was so fun that we might as well write it up as a Research Note. 1/9 One of the few "ways out" for the weird, apparently dark matter-less galaxies NGC1052-DF2 and NGC1052-DF4 is to place them at a much closer distance. The only way to make this work is to have them be satellites of the spiral galaxy NGC1042. 2/9