Cospatial Star Formation and Supermassive Black Hole Growth in $z \sim 3$ Galaxies: Evidence for In-situ Co-evolution

TL;DR

This study uses high-resolution radio and submillimeter observations to show that supermassive black hole growth and intense star formation occur in the same compact regions of galaxies at redshift 3, supporting in-situ co-evolution.

Contribution

It provides the first sub-kiloparsec localization of SMBH growth relative to star-forming regions in high-redshift galaxies, demonstrating co-spatial growth evidence.

Findings

SMBHs are located within compact, heavily obscured star-forming regions.

Star formation rates of 100-1200 M_sun/yr in central kpc regions.

Concurrent growth of SMBH and stellar mass supports in-situ co-evolution.

Abstract

We present a sub-kpc localization of the sites of supermassive black hole (SMBH) growth in three active galactic nuclei (AGN) at $z \sim 3$ in relation to the regions of intense star formation in their hosts. These AGNs are selected from Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations in the HUDF and COSMOS, with the centimetric radio emission tracing both star formation and AGN, and the sub/millimeter emission by dust tracing nearly pure star formation. We require radio emission to be $\geqslant5\times$ more luminous than the level associated with the sub/millimeter star formation to ensure that the radio emission is AGN-dominated, thereby allowing localization of the AGN and star formation independently. In all three galaxies, the AGN are located within the compact regions of gas-rich, heavily obscured, intense nuclear star formation, with $R_e = 0.4-1.1$ kpc and average star formation rates of $\simeq100-1200$ $M_\odot$yr$^{-1}$. If the current episode of star formation continues at such a rate over the stellar mass doubling time of their hosts, $\simeq 0.2$ Gyr, the newly formed stellar mass will be of the order of $10^{11}$ $M_\odot$ within the central kpc region, concurrently and cospatially with significant growth of the SMBH. This is consistent with a picture of in-situ galactic bulge and SMBH formation. This work demonstrates the unique complementarity of VLA and ALMA observations to unambiguously pinpoint the locations of AGN and star formation down to $\simeq30$ mas, corresponding to $\simeq 230$ pc at $z = 3$.