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

Abstract

Present-day multi-wavelength deep imaging surveys allow to characterise the outskirts of galaxies with unprecedented precision. Taking advantage of this situation, we define a new physically motivated measurement of size for galaxies based on the expected location of the gas density threshold for star formation. Employing both theoretical and observational arguments, we use the stellar mass density contour at 1 M pc −2 as a proxy for this density threshold for star formation. This choice makes our size definition operative. With this new size measure, the intrinsic scatter of the global stellar mass (M ) -size relation (explored over five orders of magnitude in stellar mass) decreases to ∼0.06 dex. This value is 2.5 times smaller than the scatter measured using the effective radius (∼0.15 dex) and between 1.5 and 1.8 times smaller than those using other traditional size indicators such as R 23.5,i (∼0.09 dex), the Holmberg radius R H (∼0.09 dex) and the half-mass radius R e,M (∼0.11 dex). Moreover, galaxies with 10 7 M < M < 10 11 M increase monotonically in size following a power-law with a slope very close to 1/3, equivalent to an average stellar mass 3D density of ∼4.5×10 −3 M pc −3 for galaxies within this mass range. Galaxies with M >10 11 M show a different slope with stellar mass, which is suggestive of a larger gas density threshold for star formation at the epoch when their star formation peaks.

The artemis simulations: stellar haloes of Milky Way-mass galaxies

“…Similar to the break in the r-band profiles in Fig. 6, the up-turn in colours is plausibly the result of a transition from a disc-dominated region (e.g., Trujillo et al 2020) to the in situ-dominated stellar halo. The up-turn happens at slightly larger radii in the simulations, which may be due to the simulations having somewhat flatter metallicity gradients compared to the observations (see Fig.…”

“…This figure shows, again, very good agreement with the observations. The observed light profiles display breaks around 10-20 kpc, which plausibly demarcates the transition from the disc to the stellar halo component (e.g., Trujillo et al 2020; note that the curves are total surface brightness profiles, not just the bulge+halo component). We also show the stacked SDSS DR9 r-band profile of D'Souza et al (2014) for Milky Way-mass galaxies (selected purely on stellar mass with no morphological criterion, as in the case of ARTEMIS), which is in excellent agreement with the simulations.…”

Stellar masses, sizes, and radial profiles for 465 nearby early-type galaxies: an extension to the Spitzer Survey of Stellar Structure in Galaxies (S$^{4}$G)

“…Recently, the mass-size relation for galaxies generally has come under additional scrutiny due to the recent rediscovery of sometimes abundant populations of extremely extended but low-mass galaxies in clusters, originally identified by Binggeli et al (1985) in the Virgo Cluster, now broadly referred to as ultra-diffuse galaxies (e.g., van Dokkum et al 2015;Koda et al 2015;Mihos et al 2015;Iodice et al 2020). Depending on the size metric used, such galaxies either appear as extreme outliers in the masssize relation, or show little difference from other dwarf galaxies (Chamba et al 2020), leading to the argument that a more physically motivated definition of galaxy size (based on, e.g., star formation thresholds) is called for (Trujillo et al 2020, and references therein).…”

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

“…size is also mediated by the halo concentration (e.g., Desmond 2017;Jiang et al 2019;Zanisi et al 2020). Moreover, other definitions of galaxy sizes, such as 80 (Miller et al 2019) or 1 (Trujillo et al 2020), have been proposed to correlate to ℎ equally well or even better than effective radius. We will discuss these models in Appendix E.…”

“…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.…”