Color Gradients along the Quiescent Galaxy Sequence: Clues to Quenching and Structural Growth

Abstract

This Letter examines how the sizes, structures, and color gradients of galaxies change along the quiescent sequence. Our sample consists of ∼400 quiescent galaxies at 1.0 ≤ z ≤ 2.5 and in three CANDELS fields. We exploit deep multi-band Hubble Space Telescope imaging to derive accurate mass profiles and color gradients, then use an empirical calibration from rest-frame UVJ colors to estimate galaxy ages. We find that—contrary to previous results—the youngest quiescent galaxies are not significantly smaller than older quiescent galaxies at fixed stellar mass. These “post-starburst” galaxies only appear smaller in half-light radii because they have systematically flatter color gradients. The strength of color gradients in quiescent galaxies is a clear function of age, with older galaxies exhibiting stronger negative color gradients (i.e., redder centers). Furthermore, we find that the central mass surface density Σ1 is independent of age at fixed stellar mass, and only weakly depends on redshift. This finding implies that the central mass profiles of quiescent galaxies do not significantly change with age; however, we find that older quiescent galaxies have additional mass at large radii. Our results support the idea that building a massive core is a necessary requirement for quenching beyond z = 1, and indicate that post-starburst galaxies are the result of a rapid quenching process that requires structural change. Furthermore, our observed color gradient and mass profile evolution supports a scenario where quiescent galaxies grow inside-out via minor mergers.

SQuIGGLE: Studying Quenching in Intermediate-z Galaxies -- Gas, AnguLar Momentum, and Evolution

“…Our preliminary study of six SQuIGG LE galaxies shows that they have flat age gradients as probed by Hδ (Setton et al 2020). In the absence of strong radial dust or metallicity gradients, these flat age gradients are consistent with the flat color gradients that Maltby et al (2018) and Suess et al (2020) find for post-starburst galaxies. Setton et al (2020) find that these six poststarburst galaxies have young light-weighted ages at all radii, implying that star formation shut off uniformly throughout the galaxy.…”

“…We note that at z ∼ 0.7, the diameter of the SDSS/BOSS fiber is larger than the effective radius of these compact post-starburst galaxies and thus the SDSS spectrum includes the majority of the light from each galaxy; our median aperture correction is a factor of 1.3. Maltby et al (2018), Suess et al (2020), andSetton et al (2020) find that post-starburst galaxies have flat color gradients across similar spatial scales as the SDSS fiber, indicating that the aperture-corrected SDSS spectra are likely accurate reflections of the total integrated stellar light from these galaxies.…”

“…Some previous studies have found that post-starburst galaxies have extremely compact sizes, and may be smaller than their older quiescent counterparts (e.g., Whitaker et al 2012a;Yano et al 2016;Almaini et al 2017). Suess et al (2020) suggests that these size differences may be primarily caused by the effects of radial color gradients (see also Maltby et al 2018;Setton et al 2020). In either case, differences in the sizes and/or color gradients of post-starburst galaxies and older quiescent galaxies could provide clues both to the formation mechanisms for post-starburst galaxies and the processes contributing to evolution along the quiescent sequence.…”

“…A similar spectral shape indentification approach was also used by Kriek et al (2010), using synthetic restframe colors instead of PCA-based "supercolors". Finally, some studies have selected post-starburst galaxies based on their location in the U V J plane (e.g., Whitaker et al 2012a;Belli et al 2019;Suess et al 2020; see also Akins et al 2021). These different methods of selecting post-starburst galaxies produce samples that are relatively similar, but do not fully overlap; we will explore the differences in these sample selection methods in more detail later in this paper.…”

Searching for local counterparts of high-redshift post-starburst galaxies in integral field unit spectroscopic surveys of nearby galaxies

“…Many studies on field post-starburst galaxies at z 0.5 found them having smaller half-light radii (R e ) in restframe optical wavlengths than quiescent galaxies of similar stellar masses (Whitaker et al 2012;Belli et al 2015;Yano et al 2016;Almaini et al 2017;Wu et al 2018;Belli et al 2019), contrary to the typical correlation between the half-light radii and the stellar ages of quiescent galaxies (Wu et al 2018). They also have flatter color gradient than older quiescent galaxies (Suess et al 2020). The light and color profiles can be explained by a centrally-concentrated star-formation activity happend before quenching (Wu et al 2020).…”