Seminars and Colloquia at ESO Garching and on the campus

We present new and archival ALMA observations of two strongly lensed dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope Survey, SPT0418-47 (z = 4.225) and SPT2147-50 (z = 3.760). We study the [CII], CO(7-6), [CI](2-1), and, in SPT0418-47, p-H2O emission, which along with the underlying dust continuum are routinely used as tracers of gas mass and/or star-formation rate (SFR). We perform a pixel-by-pixel analysis of both sources in the image plane to study the resolved Kennicutt-Schmidt relation, finding generally good agreement between the slopes of the SFR vs. gas mass surface densities using the different tracers. Using lens modeling methods, we find the the dust emission is more compact than the line emission in both sources, with CO(7-6) and [CI](2-1) similar in extent and [CII] the most extended, reminiscent of recent findings of extended [CII] spatial distributions in galaxies at similar cosmic epochs. We develop the [CI](2-1)/CO(7-6) flux density ratio as an observable proxy for the gas depletion timescale, which can be applied to large samples of DSFGs, in lieu of more detailed inferences of the this timescale which require multi-wavelength observations. Furthermore, the extended [CII] emission in both sources, compared to the total continuum and line emission, suggests that [CII], used in recent years as a molecular gas mass and SFR tracer in high-redshift galaxies, may not always be a suitable tracer of these physical quantities.

Two of the biggest questions in astronomy are "how did we get here?" and "are we alone?" These questions relate in part to the composition of planets, which is largely determined by the composition of the solids in planetary nurseries within protoplanetary disks. These solids begin as ice-mantled dust grains that grow in dense molecular clouds prior to the collapse of protostars. Astrochemical comparison of gaseous isotope ratios in clouds, protostars, and comets suggests that some cold, interstellar ices are directly "inherited" from the molecular cloud by some Solar System bodies. However, icy grains may be thermally processed during the formation of the first protostar and disk in the first 100,000 years, due to the intrinsic temperature gradient close to the protostar and to variable accretion outbursts that heat the infalling solids. Thermal processing should lead to various degrees of fiery "reset" through sublimation and re-condensation of the disk ice and even silicates. There is evidence for reset in the meteoritic record, with the oldest re-condensed minerals marking the t=0 moment of the Solar System planet formation timeline. Discovering which ice and dust species survive is critical to understanding when and where planets can form in these disks, along with the possibility of them becoming habitable.

The James Webb Space Telescope is uniquely suited to answer this question, as it is capable of sensitively detecting all major and many minor ice and silicate species in the near- and mid-infrared. I will synthesize results from several JWST programs, including the ERS program Ice Age (http://jwst-iceage.org/), revealing how solid state chemistry evolves from the dark regions of molecular clouds to planet-forming regions of disks. We see early chemical pathways to mixed complex ices and CO2 in the cloud, and icy grains in the cloud have also grown, which may promote grain survival. However, JWST has also seen evidence for total thermal destruction of even silicates towards some protostars. These data identify for the first time an example of total sublimation of rocky dust grains and subsequent re-condensation, which marks the t=0 moment of planet formation according to meteoriticists. This validates the view from the meteoritics community that reset regulates the composition within the rocky-planet-forming regions of 100,000 year old disks. By 2-3 million years old, in the outer regions of mature disks, the situation appears to have reversed. There the distribution and absorption band profile of CO ice in protoplanetary disks suggests that it has become trapped in the CO2 ice matrix on the dust grains, indicating that inheritance becomes important at certain times and disk radii. I end by discussing which future observations and models are needed to understand how the interplay of reset and inheritance over time impacts the evolution of planetary compositions as they form and evolve.

The chemical composition of planets is largely inherited from that of their natal protoplanetary disks. In recent years, the characterization of disk chemistry has advanced significantly. (Sub-)millimeter interferometers such as ALMA have enabled the detection of emission lines from a wide range of molecular species—including deuterated and organic molecules—and revealed their radial and vertical distributions within disks. Meanwhile, JWST has begun to uncover the composition of disk ices.

In this seminar, I will review the chemical evolution of planet-forming disks from the earliest protostellar stages to the emergence of planetary systems, highlighting how accretion and ejection processes, as well as environmental effects, shape their chemistry. I will focus in particular on complex organic and deuterated molecules, which serve as key tracers for reconstructing our chemical heritage through comparisons with the pristine bodies of the Solar System.

Finally, I will discuss how the upcoming SKA Observatory (SKAO) will open new observational frontiers in this field by enabling the detection of emission lines from heavier molecules in planet-forming regions.