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The deepest space- and ground-based observations find metal-enriched galaxies at cosmic times when the Universe was less than 1 Gyr old. These stellar populations had to be preceded by the metal-free first stars, known as ‘population III’. Recent cosmic microwave background polarization measurements indicate that stars started forming early—when the Universe was ≤200 Myr old. It is now thought that population III stars were significantly more massive than the present metal-rich stellar populations. Although such sources will not be individually detectable by existing or planned telescopes, they would have produced significant cosmic infrared background radiation in the near-infrared, whose fluctuations reflect the conditions in the primordial density field. Here we report a measurement of diffuse flux fluctuations after removing foreground stars and galaxies. The anisotropies exceed the instrument noise and the more local foregrounds; they can be attributed to emission from population III stars, at an era dominated by these objects. Early stars: lifting the veil The most distant and oldest observable stars are in the metal-rich galaxies seen in images such as the Hubble ultra-deep field. The metal — which in cosmology is anything that's not hydrogen or helium — must have come from somewhere and as nucleosynthesis happens in stars, there must have been an earlier population of metal-free stars. No existing or planned telescopes can detect them individually, but evidence of their existence has been found hidden in images obtained by the Infrared Array Camera onboard NASA's Spitzer Space Telescope. After removing foreground stars and galaxies from the image, the tiny fluctuations that remain in the cosmic infrared background are the fossil of emissions from the old metal-free stars.

The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) is a cryogenic balloon-borne instrument that will survey galaxy and star formation history over cosmological timescales. Rather than identifying individual objects, EXCLAIM will be a pathfinder to demonstrate an intensity mapping approach, which measures the cumulative redshifted line emission. EXCLAIM will operate at 420–540 GHz with a spectral resolution R = 512 to measure the integrated CO and [CII] in redshift windows spanning 0 < z < 3.5 . CO and [CII] line emissions are key tracers of the gas phases in the interstellar medium involved in star formation processes. EXCLAIM will shed light on questions such as why the star formation rate declines at z < 2 , despite continued clustering of the dark matter. The instrument will employ an array of six superconducting integrated grating-analog spectrometers ( μ -Spec) coupled to microwave kinetic inductance detectors. Here we present an overview of the EXCLAIM instrument design and status.

Micro-Spec (μ-Spec) is a direct-detection spectrometer which integrates all the components of a diffraction-grating spectrometer onto a ∼10-cm2 chip through the use of superconducting microstrip transmission lines on a single-crystal silicon substrate. A second-generation μ-Spec is being designed to operate with a spectral resolution of 512 in the submillimeter (500-1000 μm, 300-600 GHz) wavelength range, a band of interest for several spectroscopic applications in astrophysics. High altitude balloon missions would provide the first test bed to demonstrate the μ-Spec technology in a space-like environment and would be an economically viable venue for multiple observation campaigns. This work reports on the current status of the instrument design and will provide a brief overview of each instrument subsystem. Particular emphasis will be given to the design of the spectrometer's two-dimensional diffractive region, through which the light of different wavelengths is focused on the detectors along the focal plane. An optimization process is employed to generate geometrical configurations of the diffractive region that satisfy specific requirements on spectrometer size, operating spectral range, and performance. An optical design optimized for balloon missions will be presented in terms of geometric layout, spectral purity, and efficiency.

A large previously unknown population of stars inhabits intergalactic space [Also see Report by Zemcov et al. ] The history of astronomy has largely been concerned with the study of discrete objects: planets, stars, and galaxies. From such observations, we have discovered the nature and evolutionary histories of these objects. It is natural to ask whether these studies provide a comprehensive picture of the evolution of the universe, or whether large numbers of objects too faint to detect individually or intrinsically diffuse sources may be present. On page 732 of this issue, Zemcov et al. ( 1 ) present results from a study of near-infrared background light that reveal that as many as half of all stars have been stripped from galaxies in their many collisions and mergers over the history of the universe. At galactic distances, the stars are faint but can be detected in ensemble through the spatial variations in sky brightness caused by their spatial distributions. It is remarkable that such a major component of the universe could have been hiding in plain sight as an infrared background between the stars and galaxies.

We performed small-scale demonstrations at GSFC of high-resolution Xray TES microcalorimeters read out using a microwave SQUID multiplexer. This work is part of our effort to develop detector and readout technologies for future space-based X-ray instruments such as the microcalorimeter spectrometer envisaged for Lynx, a large mission concept under development for the Astro 2020 Decadal Survey. In this paper we describe our experiment, including details of a recently designed, microwave-optimized low-temperature setup that is thermally anchored to the 55mKstage of our laboratory ADR. Using aROACH2 FPGA at room temperature, we read out pixels of a GSFC-built detector array via a NIST-built multiplexer chip with Nb coplanar waveguide resonators coupled to rf-SQUIDs. The resonators are spaced 6 MHz apart (at ∼ 5.9 GHz) and have quality factors of ∼ 15,000. In our initial demonstration, we used flux-ramp modulation frequencies of 125 kHz to read out 5 pixels simultaneously and achieved spectral resolutions of 2.8-3.1 eV FWHM at 5.9 keV. Our subsequent work is ongoing: to-date we have achieved a median spectral resolution of 3.4 eV FWHM at 5.9 keV while reading out 28 pixels simultaneously with flux-ramp frequencies of 160 kHz. We present the measured system-level noise and maximum slew rates and briefly describe our future development work.

The high-resolution mid-infrared spectrometer instrument will fly onboard the National Aeronautics and Space Administration’s airborne stratospheric observatory for infrared astronomy in 2019. It will provide astronomers with a unique observing window (25–122  μ m ) for exploring the evolution of protoplanetary disks into young solar systems. There are two focal plane detector arrays for the instrument: a high-resolution ( λ / Δ λ = 100 , 000 ) 8 × 16 detector array, with a target noise-equivalent power, NEP ≤ 3 aW / Hz , and a low-resolution ( λ / Δ λ = 600 –19,000) 16 × 64 detector array with a target NEP ≤ 20 aW / Hz . The detectors for both of these arrays are superconducting Mo/Au bilayer transition-edge sensor bolometers on suspended single-crystal silicon membranes. We present detector characterization results for both arrays, including measurements of thermal conductance in comparison with phonon transport models, and measurements of saturation power and noise.

Design of TES microcalorimeters requires a tradeoff between resolution and dynamic range. Often, experimenters will require linearity for the highest energy signals, which requires additional heat capacity be added to the detector. This results in a reduction of low energy resolution in the detector. We derive and demonstrate an algorithm that allows operation far into the nonlinear regime with little loss in spectral resolution. We use a least squares optimal filter that varies with photon energy to accommodate the nonlinearity of the detector and the non-stationarity of the noise. The fitting process we use can be seen as an application of differential geometry. This recognition provides a set of well-developed tools to extend our work to more complex situations. The proper calibration of a nonlinear microcalorimeter requires a source with densely spaced narrow lines. A pulsed laser multi-photon source is used here, and is seen to be a powerful tool for allowing us to develop practical systems with significant detector nonlinearity. The combination of our analysis techniques and the multi-photon laser source create a powerful tool for increasing the performance of future TES microcalorimeters.

We are developing a narrow line calibration source for use with X-ray microcalorimeters. At energies below 300 eV fluorescent lines are intrinsically broad, making calibration of high resolution detectors difficult. This source consists of a 405 nm (3 eV) laser diode coupled to an optical fiber. The diode is pulsed to create approximately one hundred photons in a few microseconds. If the pulses are short compared to the rise time of the detector, they will be detected as single events with a total energy in the soft X-ray range. Poisson fluctuations in photon number per pulse create a comb of X-ray lines with 3 eV spacing, so detectors with energy resolution better than 2 eV are required to resolve the individual lines. Our currently unstabilized diode has a multimode width less than 1 nm, giving a 300 eV event a FWHM less than 0.1 eV. By varying the driving voltage, or pulse width, the source can produce a comb centered on a wide range of energies. The calibration events are produced at precisely known times. This allows continuous calibration of a flight mission without contaminating the observed spectrum and with minimal deadtime.

The design of a two-dimensional spatial beam-combining network employing a parallel-plate superconducting waveguide filled with a monocrystalline silicon dielectric substrate is presented. This component uses arrays of magnetically coupled antenna elements to achieve high coupling efficiency and full sampling of the intensity distribution while avoiding diffractive losses in the multimode waveguide region. These attributes enable the structure’s use in realizing compact far-infrared spectrometers for astrophysical and instrumentation applications. If unterminated, reflections within a finite-sized spatial beam combiner can potentially lead to spurious couplings between elements. A planar meta-material electromagnetic absorber is implemented to control this response within the device. This broadband termination absorbs greater than 0.99 of the power over the 1.7:1 operational band at angles ranging from normal to near-parallel incidence. The design approach, simulations and applications of the spatial power combiner and meta-material termination structure are presented.

Near-infrared array detectors, like the James Webb Space Telescope (JWST) NIRSpec’s Teledyne’s H2RGs, often provide reference pixels and a reference output. These are used to remove correlated noise. Improved reference sampling and subtraction (IRS²) is a statistical technique for using this reference information optimally in a least-squares sense. Compared with the traditional H2RG readout, IRS² uses a different clocking pattern to interleave many more reference pixels into the data than is otherwise possible. Compared with standard reference correction techniques, IRS² subtracts the reference pixels and reference output using a statistically optimized set of frequency-dependent weights. The benefits include somewhat lower noise variance and much less obvious correlated noise. NIRSpec’s IRS² images are cosmetically clean, with less 1/f banding than in traditional data from the same system. This article describes the IRS² clocking pattern and presents the equations needed to use IRS² in systems other than NIRSpec. For NIRSpec, applying these equations is already an option in the calibration pipeline. As an aid to instrument builders, we provide our prototype IRS² calibration software and sample JWST NIRSpec data. The same techniques are applicable to other detector systems, including those based on Teledyne’s H4RG arrays. The H4RG’s interleaved reference pixel readout mode is effectively one IRS² pattern.