Very compact millimeter sizes for composite star-forming/AGN submillimeter galaxies

TL;DR

This study measures the millimeter-wave sizes of 69 submillimeter galaxies and finds that their sizes correlate with the presence of AGN and their evolutionary stage, revealing insights into star formation and black hole growth.

Contribution

It provides the first detailed analysis linking millimeter sizes of SMGs with their AGN activity and evolutionary stage, based on ALMA observations.

Findings

SMGs are above the Eddington limit in size-flux plane.

Extended sizes are associated with star formation or AGN dominance.

Compact sizes are linked to composite star-forming/AGN systems.

Abstract

We report the study of far-IR sizes of submillimeter galaxies (SMGs) in relation to their dust-obscured star formation rate (SFR) and active galactic nuclei (AGN) presence, determined using mid-IR photometry. We determined the millimeter-wave ($\lambda_{\rm obs}=1100 \mu$m) sizes of 69 ALMA-identified SMGs, selected with $\geq10$$\sigma$ confidence on ALMA images ($F_{\rm 1100 \mu m}=1.7$--7.4 mJy). We found that all the SMGs are located above an avoidance region in the millimeter size-flux plane, as expected by the Eddington limit for star formation. In order to understand what drives the different millimeter-wave sizes in SMGs, we investigated the relation between millimeter-wave size and AGN fraction for 25 of our SMGs at $z=1$--3. We found that the SMGs for which the mid-IR emission is dominated by star formation or AGN have extended millimeter-sizes, with respective median $R_{\rm c,e} = 1.6^{+0.34}_{-0.21}$ and 1.5$^{+0.93}_{-0.24}$ kpc. Instead, the SMGs for which the mid-IR emission corresponds to star-forming/AGN composites have more compact millimeter-wave sizes, with median $R_{\rm c,e}=1.0^{+0.20}_{-0.20}$ kpc. The relation between millimeter-wave size and AGN fraction suggests that this size may be related to the evolutionary stage of the SMG. The very compact sizes for composite star-forming/AGN systems could be explained by supermassive black holes growing rapidly during the SMG coalescing, star-formation phase.