Are ultra-diffuse galaxies Milky Way-sized?

Abstract

Now almost 70 years since its introduction, the effective or half-light radius has become a very popular choice for characterising galaxy size. However, the effective radius measures the concentration of light within galaxies and thus does not capture the intuitive definition of size which is related to the edge or boundary of objects. For this reason, we aim to demonstrate the undesirable consequence of using the effective radius to draw conclusions about the nature of faint 'ultra-diffuse galaxies' (UDGs) when compared to dwarfs and Milky Way-like galaxies. Instead of the effective radius, we use a measure of galaxy size based on the location of the gas density threshold required for star formation. Compared to the effective radius, this physically motivated definition places the sizes much closer to the boundary of a galaxy. Therefore, considering the sizes and stellar mass density profiles of UDGs and regular dwarfs, we find that the UDGs have sizes that are within the size range of dwarfs. We also show that currently known UDGs do not have sizes comparable to Milky Way-like objects. We find that, on average, UDGs are ten times smaller in extension than Milky Way-like galaxies. These results show that the use of size estimators sensitive to the concentration of light can lead to misleading results.

A physically motivated definition for the size of galaxies in an era of ultradeep imaging

“…In this regard, if anything, our measure for the size of galaxies could lead to slightly larger sizes where the efficiency of forming stars is higher than what we assume in this work (see Appendix A). On the contrary, if the star formation were very inefficient (as may well be the case for dwarf galaxies), our measure of size will be biased towards smaller sizes (see Chamba et al 2020). In Appendix B, we discuss the use of alternative proxies to locate the radial location of the gas density threshold for star formation.…”

“…For this reason, and as we will show in this work, galaxies with the same extension can have very different effective radii. This is not a minor issue and has serious consequences when one wants to address or infer the nature of galaxies (Chamba et al 2020). In addition, the R e of a galaxy can vary significantly with wavelength (see e.g.…”

The evolution of compact massive quiescent and star-forming galaxies derived from the Re–Rh and Mstar–Mh relations

“…Using the publicly available catalog of 1 measurements from Trujillo et al ( 2020) and Chamba et al (2020) we find that 1 ≈ 7.8 , which implies that the normalization of the K13 relation in eq. E2,…”

“…Therefore, we expect ,80 ≈ 4.2 . Trujillo et al (2020) and Chamba et al (2020) used deep imaging of local galaxies to define 1 as the radius that encloses the region within a physically-motivated mass surface density of 1M pc −2 (see also Sánchez Almeida 2020). Trujillo et al (2020) found that the scatter in the 1 − star relation is of the order of only ≈ 0.06 dex across five orders of magnitude, including the regime of MGs for which the relation, which is linear at lower masses, breaks.…”

“…Lastly, we would like to highlight the fact that different size definitions provide different pieces of information: while is tight to the concentration of the light profile (see Chamba et al 2020), 80 probes also the outer regions of the galaxy. Likewise, 1 has been proposed based on the gas mass density threshold required to initiate star formation.…”