The Main Sequence at z ∼ 1.3 Contains a Sizable Fraction of Galaxies with Compact Star Formation Sizes: A New Population of Early Post-starbursts?

Puglisi, A.; Daddi, E.; Liu, D.; Bournaud, F.; Silverman, J. D.; Circosta, C.; Calabrò, A.; Aravena, M.; Cibinel, A.; Dannerbauer, H.; Delvecchio, I.; Elbaz, D.; Gao, Y.; Gobat, R.; Jin, S.; Le Floc’h, E.; Magdis, G. E.; Mancini, C.; Riechers, D. A.; Rodighiero, G.; Sargent, M.; Valentino, F.; Zanisi, L.
Bibliographical reference

The Astrophysical Journal Letters, Volume 877, Issue 2, article id. L23, 6 pp. (2019).

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6
2019
Number of authors
23
IAC number of authors
1
Citations
54
Refereed citations
52
Description
Atacama Large Millimeter/submillimeter Array (ALMA) measurements for 93 Herschel-selected galaxies at 1.1 ≤ z ≤ 1.7 in COSMOS reveal a sizable (>29%) population with compact star formation (SF) sizes, lying on average >×3.6 below the optical stellar mass (M ⋆)–size relation of disks. This sample widely spans the star-forming main sequence (MS), having 108 ≤ M ⋆ ≤ 1011.5 M ⊙ and 20 ≤ star formation rate (SFR) ≤ 680 M ⊙ yr‑1. The 32 size measurements and 61 upper limits are measured on ALMA images that combine observations of CO(5–4), CO(4–3), CO(2–1), and λ obs ∼ 1.1–1.3 mm continuum, all tracing the star-forming molecular gas. These compact galaxies have instead normally extended K band sizes, suggesting strong specific SFR gradients. Compact galaxies comprise the 50 ± 18% of MS galaxies at M ⋆ > 1011 M ⊙. This is not expected in standard bimodal scenarios, where MS galaxies are mostly steadily growing extended disks. We suggest that compact MS objects are early post-starburst galaxies in which the merger-driven boost of SF has subsided. They retain their compact SF size until either further gas accretion restores premerger galaxy-wide SF, or until becoming quenched. The fraction of merger-affected SF inside the MS seems thus larger than anticipated and might reach ∼50% at the highest M ⋆. The presence of large galaxies above the MS demonstrates an overall poor correlation between galaxy SF size and specific SFR.
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