Abstract

Contributed Talk - Splinter Galaxies

Friday, 11 September 2026, 14:00   (MW-0250)

The CIB as a window into star formation in the early Universe

Sylvia Adscheid
Argelander-Institute for Astronomie, University of Bonn

In order to understand the formation of stars across cosmic time, it is crucial to study the environments from which they originate. The interstellar medium of highly star-forming galaxies is particularly interesting in this endeavour, as these galaxies evidently offer ideal conditions for the formation of great quantities of stars in comparatively short times. Representative samples of such highly star-forming galaxies are required to allow a systematic and unbiased view of this population and their modes of star formation. One way to do this is through the cosmic infrared background (CIB), which is the integrated stellar and dust emission from earlier cosmic times, and which serves as a beacon for highly star-forming galaxies. The elusive far-infrared (FIR) part of the CIB deserves particular attention, as it reveals the dust-obscured part of star-formation activity, which is not visible in rest-frame UV-to-optical wavelengths. In this talk, I will present the results from our work on a large sample of 260 ALMA-detected galaxies from the A3COSMOS project. These constitute a representative galaxy sample, used to infer (sub)mm number counts, making up between ∼4 and ∼40% of the CIB in four different ALMA bands. With this sample, we are able to dissect part of the FIR/(sub)mm CIB into individual sources and characterise its origin, also making use of near-infrared (NIR) imaging from the JWST COSMOS-Web and JADES programmes, and SED-fitting using CIGALE. We find that the morphologies of most galaxies in our sample appear irregular in rest-frame UV and optical wavelengths, and disk-like in the rest-frame NIR, indicating that these are predominantly highly dust-obscured, normal star-forming galaxies, as opposed to merger-driven starburst galaxies. Our galaxies are located at high redshift (median z ∼ 2.5) and have high stellar masses, sitting in massive haloes, requiring fast and efficient stellar build-up at early cosmic times to explain their existence. Our findings reveal that a large part of the stars in the early Universe are born in dust-obscured regions of massive star-forming galaxies, which show hints at a potentially increased cosmic star-formation efficiency at early cosmic times, close to theoretical expectations for the conversion of baryons into stars.