Integral field spectroscopy : instrumentation and a new window into low-redshift galaxy populations




Indahl, Briana

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Integral field spectrographs (IFS) have transformed studies of low-redshift spatially resolved galaxies as they provide simultaneous spectral coverage over contiguous spatial regions. IFS surveys of galaxies have allowed for observations of the spatially varying physical diagnostics of galaxy disks. Though IFS instruments have been traditionally limited to small fields of view (FOV), on the order of 100 square arcseconds, constraining the extent to which IFS studies can map the most nearby galaxies. Small FOV have also restricted IFS studies of large galaxy samples to pre-selected targets. Scaling up the fields of IFS instruments is challenging due to the limited number of spatial elements that can be imaged on a single CCD. To cost effectively scale up IFS capability in order to observe larger fields, the Visible Integral-field Replicable Unit Spectrograph (VIRUS) utilizes massive replication on an unprecedented 100-fold scale to be able to image ∼35,000 fibers over a ∼22 arcminute FOV with a single observation. VIRUS consists of up to 78 replicable units, each with two integral field spectrograph channels. The VIRUS design takes advantage of large-scale replication of simple units to significantly reduce engineering and production costs of building a facility instrument of this scale. With VIRUS being 156 realizations of the same spectrograph and the first to be replicated on this massive scale, I present analysis that uncovers the statistical variations in performance of these units along with an assessment of cost/tolerance trade offs of scaling up instrument capabilities through massively replicated designs. The VIRUS instrument was designed to conduct the Hobby Eberly Telescope Dark Energy Experiment (HETDEX) which intends to build a sample of nearly a million Lyα emitting galaxies (LAEs) from 1.9<z<3.5 by sampling a 450 square degree area of sky. The HETDEX survey will catalog every emission line source in its survey area, providing an unprecedentedly large sample of low-redshift emission line galaxies (ELGs). To test the VIRUS concept of surveying large areas for ELGs, a prototype instrument, the George and Cynthia Mitchell Spectrograph (commonly referred to as VIRUS-P), was constructed for the 2.7 meter Harlan J. Smith Telescope (HJST). Previous to VIRUS, VIRUS-P, had the largest FOV of any IFS instrument (∼2.89 square arcminutes), which was utilized to test the HETDEX concept via the HETDEX Pilot Survey (HPS), which sampled a 163 square arcminute patch of sky for ELGs. With large samples of low-redshift galaxies being traditionally assembled through photometric selection, HPS and the HETDEX survey provide an opportunity to build samples of continuum-faint ELGs possibly missed by large photometric surveys. This dissertation presents two studies conducted with these ELG samples. In anticipation of the large sample of low-redshift ELGs that HETDEX will find, we first assembled an unbiased sample of 29 galaxies with [O II] λ3727 and/or [O III] λ5007 detections at z < 0.15 from the HPS ELG catalog. We compare the metallicity, stellar mass, and star formation rates (SFRs) for this spectroscopically selected sample to that of the photometrically selected samples of low-redshift galaxies found in the Sloan Digital Sky Survey (SDSS). We find two galaxies that have low metallicity for their mass, falling in a regime that tends to be under-sampled in continuum-based surveys, as their spectra are typically dominated by emission from newly formed stars. With the first HETDEX data release that surveyed the first ∼8 square degrees of the HETDEX field, we specifically search for these low-mass and low-metallicity galaxies. We assemble a sample of 17 galaxies at z ≲ 0.1 with even lower metallicity (7.45 < log(O/H)+12 < 8.12) and stellar masses. We find these galaxies have similar specific star formation rates (sSFRs) as the incredibly rare “blueberry” galaxies found in Yang et al. (2017). These studies both illustrate the power of spectroscopic surveys for finding low mass and metallicity galaxies and reveal that we find a sample of galaxies that are a hybrid between the properties of typical dwarf galaxies and the more extreme blueberry galaxies. Wide field IFS instruments also provide a unique tool for mapping faint surface brightness emission in galaxies to unprecedented extents and sensitivities. This dissertation also presents a VIRUS-P survey of M82’s northern outflow that I conducted with 27 nights of observing. The survey maps the full extent of the northern outflow from the disk to Hα cap at ∼12kpc covering ∼139 square arcminutes. Despite this classic outflow being studied since its discovery in the 1960s, with VIRUS-P we build the most sensitive and extensive map of the warm ionized gas in the northern outflow to date. The survey along with the data reduction and processing are presented along with some of the first science results that map the density profile of the outflow out to unprecedented distances from the disk. Finally, I present the next generation IFS instrument intended to replace VIRUSP on the HJST, VIRUS2. The design of VIRUS2 builds on this previous VIRUS replicated concept to scale up its capabilities of a moderately sized FOV (comparable to VIRUS-P) but much higher spatial and spectral coverage. I present the work I did designing the fiber mapping for the novel VIRUS2 fiber feed that allows VIRUS2 to have cover both a moderately large field and simultaneous spectral coverage over a wide bandpass. This dissertation concludes with how the moderately replicated design of VIRUS2 may provide a road map for cost effectively scaling up instrument capabilities for the next generation of extremely large telescopes (ELTs) and how it will open a new window for studying low-redshift galaxies.



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