Broadband Millimeter-Wave Spectroscopy with Z-Spec: An Unbiased Molecular-Line Survey of the Starburst Galaxy M82

Citation

Naylor, Bret Justin (2008) Broadband Millimeter-Wave Spectroscopy with Z-Spec: An Unbiased Molecular-Line Survey of the Starburst Galaxy M82. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Y6TP-5E10. https://resolver.caltech.edu/CaltechETD:etd-05282008-104041

Abstract

Z-Spec is a broadband, millimeter-wave, direct-detection spectrometer based on a novel waveguide grating architecture. It is designed to fully cover the wide one-millimeter atmospheric window with a frequency range of 186 to 307 GHz at a resolution of ~ 250. The design goal is the detection of rotational and fine structure lines from the distant population of galaxies that make up the far-infrared background. Its large instantaneous bandwidth enables detection of multiple transitions of carbon monoxide and a relatively rapid determination of redshift. The grating design is based on the Rowland architecture but is much more compact because of the use of a parallel-plate waveguide propagation medium. Z-Spec uses 160 silicon nitride micromesh bolometers and is cooled to less than 100 mK to achieve background-limited performance. Moreover, Z-Spec serves as a technology demonstration of a technique that is uniquely suited to far-infrared extragalactic spectroscopy from space.

Z-Spec's capabilities enable wide-coverage, unbiased line surveys of nearby galaxies. Searches for molecular lines outside our own galaxy have typically been targeted at specific transitions of particular molecules. The brightest sources of extra-galactic molecular lines come from galaxies with physical conditions very different from those found in the Milky Way. Unbiased searches for spectral lines can discover transitions and molecules that are unexpected. A three-pointing study of the prototypical starburst galaxy M82 was performed with Z-Spec at the Caltech Submillimeter Observatory. The survey detected eleven bright lines in the center of the Z-Spec band, of which four had not been previously detected. Two of the new detections are for molecules that had been detected at lower frequencies (HNC 3 - 2 and C₂H 3 - 2) and the other two are potentially transitions of SO₂, for which only upper limits on lower frequency transitions were previously available. Several detected molecules are analyzed using a large-velocity-gradient (LVG) radiative-transfer model and indicate significant regions of high-density molecular gas along the major axis of M82.

Thesis Files