The S⁴G Early Type Galaxy Extension: ETG Scaling Relations with Spitzer IRAC Imaging

The Spitzer Survey of Stellar Structure in Galaxies (S⁴G) was a highly successful near-infrared (NIR) imaging survey of 2352 local Universe galaxies, done with the Spitzer Space Telescope’s Infrared Array Camera (IRAC) in both the 3.6 μm and 4.5 μm bandpasses that are nearly direct tracers of stellar mass and suffer very little from the effects of dust. The survey results have been critically important for enhancing our understanding of the evolution of galactic structures such as bulges, bars, and disks (both thick and thin). The results have also improved our understanding of the evolutionary links between the different morphological classes of galaxies, such as S0s and spirals. However, the initial survey selected targets based on radio-derived spectroscopic redshifts, thereby biasing it toward gas-rich galaxies. A follow-up survey was conducted to rectify this bias, targeting an additional 465 early-type galaxies (ETGs) within about 40 Mpc. We now make these newer, fully processed ETG data publicly available, alongside a detailed photometric analysis. We derive a number of different NIR ETG scaling relations, finding generally good agreement with past attempts at stellar-mass scaling relations using visible light. Two relations show remarkably little scatter: the stellar-mass–isophotal-radius relation (using the μ_3.6 = 25.5 AB mag arcsec^-2 isophote, equivalent to ≈ 3.5 M(solar) pc^-2 in stellar mass surface density), and the relation between the total stellar mass and the mean stellar mass surface density within a 1 kpc radius. The former relation has recently sparked new interest, given its potential connection to star formation threshold radii, which may limit the growth of galaxies in a notably consistent way. The latter relation is less discussed, but, especially when combined with the same relation for late-type galaxies (LTGs), which has only a touch more scatter, showcases the remarkable self-similarity of galaxy cores, implying that the central kpc of most galaxies changes very little over cosmic time, even when galaxies undergo the type of catastrophic morphological transformations that may be involved in the transition from an LTG to an ETG.