Winter Term 2020/21

The interstellar medium (ISM) and its interaction with massive stars are central to galactic evolution. Interstellar gas is known to exist as atomic (HI) clouds and star forming molecular clouds. Recent observations reveal that half of the interstellar gas is in CO-dark molecular gas that has eluded detailed characterization because of lack of suitable tracer. Hence, its physical conditions and relationship to other gas reservoirs are largely unknown. Atomic and CO-dark molecular gas are heated by far-UV photons from massive stars through photo-electrons from large molecules, producing (inter)cloud phases of the ISM and controlling its emission characteristics. Through winds and explosions, stars also stir up the ISM dynamically. This source of turbulent pressure supports clouds & the gas disk against (self)gravity, disrupts molecular clouds, and compresses gas, triggering new star formation.
However, until recently, there was no good tracer for this dynamic interaction. LOFAR has opened up the low frequency sky to efficient surveys, providing a sensitive diagnostic of physical conditions and kinematics of HI and CO-dark molecular gas. Development of sensitive THz heterodyne receiver arrays combined with SOFIA's nimble telescope allow large scale surveys at sub-km/s resolution, probing coupling of radiative and mechanical energy from massive stars to their environment. Centered on these new observing opportunities, I will review our current understanding of the interstellar medium.

The recent discovery of benzonitrile towards the cold, dark molecular cloud TMC-1 with the 100 m Green Bank Telescope sparked interest into understanding the role small aromatic molecules play in the chemistry and physics of the interstellar medium. On the spectrum of molecular complexity, these species are intermediate between the small, several atom large molecules that are routinely as tracers, and the peculiar behemoths like the fullerenes and the ubiquitous polycyclic aromatic hydrocarbons (PAH). Beyond mere chemical curiosities, understanding how these molecules are formed and destroyed is relevant to how a substantial amount of material—in particular carbon—is transported and transformed in the interstellar medium, as well as gas phase synthetic routes to prebiotic molecules.

In this talk, I will present the latest results from the GOTHAM (GBT Observations of TMC-1: Hunting Aromatic Molecules) project, a large scale spectral line survey focusing on characterizing the chemical inventory of TMC-1 in the context of small aromatic molecules.I will discuss new molecular detections from this collaboration, which have been facilitated by new workflows in high resolution laboratory spectroscopy and signal analysis. Finally, I will present new machine learning methods I have developed to automate the discovery of new molecules, with the aim of providing guidance to laboratory, observation, and chemical modeling work in GOTHAM and subsequent campaigns.

Starbursts are a rare phenomenon in the present day universe, but they represent perhaps the most common mode under which stars form and galaxies grow during the z~1-2 peak of cosmic star formation activity. This mode of star formation is not a simple scaling of what happens in typical molecular clouds, but represents a much more efficient manner of converting gas into stars likely through the formation of massive clusters. The resulting feedback in the form of galaxy-scale outflows is, together with AGN, thought to be one of the main forms of regulation of galaxy growth. NGC 253 is the premier local example for a nuclear starburst, and has been targeted with several ALMA observations in order to study how these processes work. I will discuss the properties of the galactic molecular outflow, including our best constraints on the mass and outflow rate, and the
properties of the molecular gas. I will, however, focus on the results from high-resolution observations which reveal a dozen compact structures with properties corresponding to massive young star
clusters and super star clusters (SSCs), most of which are so embedded that are invisible in optical and NIR observations (the exception is a known SSC). Finally, I will present the analysis of our 0.5-pc resolution observations which reveal feedback and disruption on the scales of these clusters, I will discuss the properties of these "cluster-scale" outflows, and I will compare them to theoretical expectations.