Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Planetesimals inevitably bear the signatures of their natal environment, preserving in their composition a record of the metallicity of their system's original gas and dust, albeit one altered by the formation processes. When planetesimals are dispersed from their system of origin, this record is carried with them. As each star is likely to contribute at least 10¹² interstellar objects (ISOs), the Galaxy's drifting population of ISOs provides an overview of the properties of its stellar population through time. Using the EAGLE cosmological simulations and models of protoplanetary formation, our modeling predicts an ISO population with a bimodal distribution in their water mass fraction: objects formed in low-metallicity, typically older, systems have a higher water fraction than their counterparts formed in high-metallicity protoplanetary disks, and these water-rich objects comprise the majority of the population. Both detected ISOs seem to belong to the lower water fraction population; these results suggest they come from recently formed systems. We show that the population of ISOs in galaxies with different star formation histories will have different proportions of objects with high and low water fractions. This work suggests that it is possible that the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time will detect a large enough population of ISOs to place useful constraints on models of protoplanetary disks, as well as galactic structure and evolution.

Abstract:

We present deep ALMA dust continuum observations for a sample of luminous (MUV < −22) star-forming galaxies at z ≃ 7. We detect five of the six sources in the far-infrared (FIR), providing key constraints on the obscured star-formation rate (SFR) and the infrared-excess-β (IRX–β) relation without the need for stacking. Despite the galaxies showing blue rest-frame UV slopes (β ≃ −2) we find that 35–75 percent of the total SFR is obscured. We find the IRX–β relation derived for these z ≃ 7 sources is consistent with that found for local star-burst galaxies. Using our relatively high-resolution (FWHM ≃ 0.7 arcsec) observations we identify a diversity of dust morphologies in the sample. We find both compact emission that appears offset relative to the unobscured components and extended dust emission that is co-spatial with the rest-frame UV light. In the majority of the sources we detect strong rest-frame UV colour gradients (with up to Δβ ≃ 0.7–1.4) as probed by the multi-band UltraVISTA ground-based data. The observed redder colours are spatially correlated with the location of the FIR detection. Our results show that even in bright Lyman-break galaxies at z ≃ 7 the peak of the star-formation is typically hosted by the fainter, redder, regions in the rest-frame UV, which have an obscured fraction of fobs ≥ 0.8. As well as demonstrating the importance of dust obscured star-formation within the Epoch of Reionization, these observations provide an exciting taster of the rich spatially resolved datasets that will be obtained from JWST and high-resolution ALMA follow-up at these redshifts.