MUNA Lunch Talk:

Alan Aversa and AJ Heroux

University of Arizona, University of Wisconsin - Whitewater, and NRAO

Probably "Searching for the Youngest Star Clusters", and "Formaldehyde Densitometry of External Galaxies"..

August 13

12:10PM, Room 230, NRAO, Edgemont Road


Aversa: Globular star clusters were formed prodigiously in the primordial universe and are the most ancient objects known, almost as old as the universe itself. As such, they are the only relics of the early universe that can be used to probe the detailed astrophysical processes that took place at the time of galaxy assembly. By studying relatively nearby nascent globular clusters with radio light, we can penetrate their birth cocoons and witness their formation.

Heroux: This project aims to measure H2CO in mainly starburst galaxies to derive the spatial density in these objects. Formaldehyde (H2CO) is a proven tracer of the high density environs of molecular clouds. It is ubiquitous: H2CO is associated with 80% of the HII regions surveyed by Downes etal. (1980), and possesses a large number of observationally accessible transitions from cm to far-infrared wavelengths. Because H2CO is a slightly asymmetric rotor molecule, each rotational energy level is split by this asymmetry into two energy levels. Therefore, the energy levels must be designated by a total angular momentum quantum number, J, the projection of J along the symmetry axis for a limiting prolate symmetric top, K-1, and the projection of J along the symmetry axis for a limiting oblate symmetric top, K+1. This splitting leads to two basic types of transitions: the high-frequency ΔJ = 1, ΔK-1= 0, ΔK+1= -1 "P-branch" transitions and the lower-frequency ΔJ = 0, ΔK-1= 0, ΔK+1= +- 1 "Q-branch" transitions, popularly known as the "K-doublet" transitions. The P-branch transitions are only seen in emission in regions where n(H2) ≥ 105 cm-3. The excitation of the K-doublet transitions, though, is not so simple. For n(H2) ≤ 105 cm-3, the lower energy states of the 110 → 111 through 514 → 515 K-doublet transitions become overpopulated due to a collisional selection effect (Evans etal (1975); Garrison etal (1975)). This overpopulation cools the J ≤ 5 K-doublets to excitation temperatures lower than that of the cosmic microwave background, causing them to appear in absorption. For n(H2) ≥ 105.5 cm-3, this collisional pump is quenched and the J ≤ 5 K-doublets are then seen in emission over a wide range of kinetic temperatures and abundances. For structurally-simple molecular clouds, this density-selective property of the K-doublet transitions allows for a simple and definite identification of the highest densities. This project aims to measure the spatial density in starburst and AGN nuclei of mainly nearby galaxies to accurately derive the spatial density in these environments.