In-Situ Spheroid Formation in Distant Submillimeter-Bright Galaxies

TL;DR

This paper demonstrates that spheroids in the universe are formed in situ within the cores of distant, luminous starburst galaxies, supported by ALMA observations and simulations, revealing a key pathway for galaxy evolution.

Contribution

It provides direct observational evidence linking starburst cores to spheroid formation, a connection previously obscured by dust and limited data.

Findings

Spheroids are generated by star formation in galaxy cores.

Most high-redshift luminous galaxies are triaxial rather than disk-shaped.

In-situ spheroid formation occurs via starbursts triggered by interactions and non-co-planar gas streams.

Abstract

The majority of stars in today's Universe reside within spheroids, which are bulges of spiral galaxies and elliptical galaxies. Their formation is still an unsolved problem. Infrared/submm-bright galaxies at high redshifts have long been suspected to be related to spheroids formation. Proving this connection has been hampered so far by heavy dust obscuration when focusing on their stellar emission or by methodologies and limited signal-to-noise ratios when looking at submm wavelengths. Here we show that spheroids are directly generated by star formation within the cores of highly luminous starburst galaxies in the distant Universe. This follows from the ALMA submillimeter surface brightness profiles which deviate significantly from those of exponential disks, and from the skewed-high axis-ratio distribution. The majority of these galaxies are fully triaxial rather than flat disks: the ratio of the shortest to the longest of their three axes is half, on average, and increases with spatial compactness. These observations, supported by simulations, reveal a cosmologically relevant pathway for in-situ spheroid formation through starbursts likely preferentially triggered by interactions (and mergers) acting on galaxies fed by non-co-planar gas accretion streams.