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Title:
Z-Spec: A broadband spectrometer for millimeter-wave astrophysics---Instrument development and results of a molecular line survey of nearby star-forming galaxy NGC 253
Authors:
Earle, Lieko
Affiliation:
AA(University of Colorado at Boulder)
Publication:
Proquest Dissertations And Theses 2008. Section 0051, Part 0606 147 pages; [Ph.D. dissertation].United States -- Colorado: University of Colorado at Boulder; 2008. Publication Number: AAT 3337091. Source: DAI-B 69/11, May 2009
Publication Date:
00/2008
Origin:
UMI
Keywords:
Cryogenic, Millimeter-wave, NGC 253, Spectroscopy, Star formation, Waveguides
Comment:
Publication Number: AAT 3337091; ISBN: 9780549917007; Advisor: Glenn, Jason; Committee members: Darling, Jeremy, Horanyi, Mihaly, Lehnert, Konrad, Rogers, Charles
Bibliographic Code:
2008PhDT.........1E

Abstract

We have built Z-Spec, a broadband spectrometer for millimeter-wave astrophysics. The instrument's instantaneous bandwidth of 185-305 GHz covers the entire 1 millimeter atmospheric transmission window with a moderate resolving power ( R = n/Dn) of 250-350. The spectrometer employs a novel architecture called WaFIRS (Waveguide Far-Infrared Spectrometer) which confines the light propagation for a curved diffraction grating within a parallel-plate waveguide, resulting in a minimum mechanical envelope. An array of 160 silicon- nitride micromesh bolometers is cooled to 60 mK via an adiabatic demagnetization refrigerator (ADR) backed by a closed-cycle 3 He- 4 He sorption pump refrigerator. Z-Spec's compact design serves as a concept demonstration for a future far-infrared spectrometer aboard a cold telescope in space. Routine observations with Z-Spec from the Caltech Submillimeter Observatory on Mauna Kea have been conducted since April 2006, and the instrument currently achieves good sensitivities that are within a factor of two of the photon background limit set by the atmosphere and telescope.

Z-Spec's primary science objectives are to determine the redshifts of faint submillimeter galaxies using the 12 CO rotational ladder, and to conduct systematic line surveys of local galaxies. The millimeter waveband hosts low- to mid- J rotational transitions for several molecular species which trace the dense interstellar gas associated with active star formation. Z-Spec's bandwidth offers a unique advantage over the traditional single-dish heterodyne approach: the spectral lines and the corresponding continua are all observed simultaneously, greatly reducing relative uncertainties in flux calibration and line-to-continuum ratios.

The starburst galaxy NGC 253 was observed with Z-Spec in November 2006, for a total integration of 3.49 hours over two nights. NGC 253 is one of the brightest neighbors outside the Local Group of galaxies and its compact nuclear region is a site of prodigious star-formation. Twenty-one transitions in 13 species were detected at greater than 3s, including the well-known density tracers HCO + , HCN, HNC, and three transitions of CS. The results are compared with large-velocity-gradient (LVG) radiative-transfer simulations and the implications for the physical conditions of the gas in the starburst core are discussed.

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