The development of replicated optical integral field spectrographs and their application to the study of Lyman-alpha emission at moderate redshifts

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2015-08

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Chonis, Taylor Steven

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Abstract

In the upcoming era of extremely large ground-based astronomical telescopes, the design of wide-field spectroscopic survey instrumentation has become increasingly complex due to the linear growth of instrument pupil size with telescope diameter for a constant spectral resolving power. The upcoming Visible Integral field Replicable Unit Spectrograph (VIRUS), a baseline array of 150 copies of a simple integral field spectrograph that will be fed by 33,600 optical fibers on the upgraded Hobby-Eberly Telescope (HET) at McDonald Observatory, represents one of the first uses of large-scale replication to break the relationship between instrument pupil size and telescope diameter. By dividing the telescope's field of view between a large number of smaller and more manageable instruments, the total information grasp of a traditional monolithic survey spectrograph can be achieved at a fraction of the cost and engineering complexity. To highlight the power of this method, VIRUS will execute the HET Dark Energy Experiment (HETDEX) and survey ~420 square degrees of sky to an emission line flux limit of ~1e-17 erg/s/cm^2 to detect ~1e6 Lyman-alpha emitting galaxies (LAEs) as probes of large-scale structure at redshifts of 1.9<z<3.5. HETDEX will precisely measure the evolution of dark energy at that epoch, and will simultaneously amass an LAE sample that will be unprecedented for extragalactic astrophysics at the redshifts of interest.

Large-scale replication has clear advantages to increasing the total information grasp of a spectrograph, but there are also challenges. In this dissertation, two of these challenges with respect to VIRUS are detailed. First, the VIRUS cryogenic system is discussed, specifically the design and tests of a novel thermal connector and internal camera croygenic components that link the 150 charge-coupled device detectors to the instrument's liquid nitrogen distribution system. Second, the design, testing, and mass production of the suite of volume phase holographic (VPH) diffraction gratings for VIRUS is presented, which highlights the challenge and success associated with producing of a very large number of highly customized optical elements whose performance is crucial to meeting the efficiency requirements of the spectrograph system.

To accommodate VIRUS, the HET is undergoing a substantial wide-field upgrade to increase its field of view to 22 arcminutes in diameter. The previous HET facility Low Resolution Spectrograph (LRS), which was directly fed by the telescope's previous spherical aberration corrector, must be removed from the prime focus instrument package as a result of the telescope upgrades and instead be fiber-coupled to the telescope focal plane. For a similar cost as modifying LRS to accommodate these changes, a new second generation instrument (LRS2) will be based on the VIRUS unit spectrograph. The design, operational concept, construction, and laboratory testing and characterization of LRS2 is the primary focus of this dissertation, which highlights the benefits of leveraging the large engineering investment, economies of scale, and laboratory and observatory infrastructure associated with the massively replicated VIRUS instrument. LRS2 will provide integral field spectroscopy for a seeing-limited field of 12 arcseconds by 6 arcseconds. The multiplexed VIRUS framework facilitates broad wavelength coverage from 370 nm to 1.0 micron spread between two dual-channel spectrographs at a moderate spectral resolving power of 2000. The design departures from VIRUS are presented, including the novel integral field unit, VPH grism dispersers, and various optical changes for accommodating the broadband wavelength coverage. Laboratory testing has verified that LRS2 largely meets its image quality specification and is nearly ready for delivery to the HET where its final verification and validation tasks will be executed. LRS2 will enable the continuation of most legacy LRS science programs and provide improved capability for future investigations.

To investigate the physical nature of the galaxy population used to trace large-scale structure by HETDEX, two studies of the spectrally resolved hydrogen Lyman-alpha emission of 2.0<z<3.5 LAEs from the HETDEX Pilot Survey are included. The first compares the Lyman-alpha emission of three LAEs to simulated spectra of resonant Lyman-alpha line transfer through an expanding shell of neutral hydrogen gas, and concludes that high luminosity LAEs primarily have low neutral hydrogen column densities. The second introduces a sample of 38 star-forming LAEs to investigate how the parameters describing the Lyman-alpha emission depend on various physical properties of the emitting galaxies. Low column density of neutral hydrogen is again suggested to be the primary regulator of Lyman-alpha emission in these galaxies, but contributions from other physical mechanisms (such as galactic-scale outflows) must be invoked to better explain the observed correlations. The presented data set is one of the most uniform and comprehensive to which future advanced Lyman-alpha radiative transfer modeling efforts can be compared. Looking forward, a sensitivity model for LRS2 is presented, and a case is made for conducting one of the first systematic integral field surveys of extended Lyman-alpha emission at moderate redshifts. In three hours of exposure time on the HET, LRS2 can reach an average Lyman-alpha surface brightness of ~7.8e-18 erg/s/cm^2/arcsecond^2 at a signal to noise ratio of 5, which is competitive with similar future instruments on other 10 meter-class telescopes. This science case illustrates the power of LRS2 as a workhorse survey follow-up instrument for HETDEX.

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