The Differential Assembly History of the Centers and Outskirts of Main Sequence Galaxies at $z\sim 2.3$

Abstract

We present a study of spatially-resolved star formation histories (SFHs) for 60 $z\sim 2.3$ main-sequence, star-forming galaxies selected from the MOSDEF spectroscopic survey in the GOODS-N field, with median stellar mass $\log (M_{\star }/M_{\odot })=9.75$ and spanning the range $8.6<\log (M_{\star }/M_{\odot })<11.5$. Photometry is decomposed into a central and outer spatial component using observed $z_{F850LP}-H_{F160W}$ colors. The Prospector code is used to model spectral energy distributions for the center, outskirt, and integrated galaxy using HST/ACS and WFC3, Spitzer/IRAC, and ground-based photometry, with additional constraints on gas-phase metallicity and spectroscopic redshift from MOSDEF spectroscopy. For the low-resolution bands, spatially-resolved photometry is determined with an iterative approach. The reconstructed SFHs indicate that the majority of galaxies with $\log (M_{\star }/M_{\odot })<10.5$ are observed while their central regions undergo relatively recent ($<100$ Myr) bursts of star formation, while the outskirts have a smooth, quasi-steady SFH that gently increases towards the redshift of observation. The enhanced star formation activity of the central parts is broadly consistent with the idea that it is produced by highly dissipative gas compaction and accretion. The wide range of central densities and sizes observed in the sample suggests that for the selected galaxies such a process has started but is still far from being completed. The implication would be that selecting star-forming galaxies at cosmic noon frequently includes systems in an "evolved" evolutionary phase where the centers have recently started a burst of star formation activity that will likely initiate inside-out quenching in the next several hundred million years.

28 pages, 21 figures. Accepted for publication in ApJ

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