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

A. Puglisi, E. Daddi, D. Liu, F. Bournaud, J. D. Silverman, C. Circosta, A. Calabro, M. Aravena, A. Cibinel, H. Dannerbauer, I. Delvecchio, D. Elbaz, Y. Gao, R. Gobat, S. Jin, E. Le Floc'h, G. E. Magdis, C. Mancini, D. A. Riechers, G. RodighieroM. Sargent, F. Valentino, L. Zanist

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Abstract

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 sstarf)–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.
Original languageEnglish
Article numberL23
JournalAstrophysical Journal Letters
Volume877
Issue number2
Number of pages6
ISSN2041-8205
DOIs
Publication statusPublished - 2019

Keywords

  • Galaxies: evolution
  • Galaxies: high-redshift
  • Galaxies: interactions
  • Galaxies: ISM
  • Galaxies: star formation

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