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The Impact of Hubble Space Telescope Thermal Variations on Diffuse Light Measurements in the SKYSURF VI Project


Core Concepts
Variations in the Hubble Space Telescope's (HST) temperature, particularly in its pick-off mirror and main mirrors, introduce a systematic uncertainty in diffuse light measurements, but these variations are small and result in updated diffuse light limits of 21, 32, and 25 nW m−2 sr−1 in the F125W, F140W, and F160W filters, respectively.
Abstract

Bibliographic Information:

McIntyre, I. A., Carleton, T., O’Brien, R., Windhorst, R. A., Caddy, S., Cohen, S. H., Jansen, R. A., MacKenty, J., & Kenyon, S. J. (2024). SKYSURF VI: The Impact of Thermal Variations of HST on Background Light Estimates. arXiv. https://doi.org/10.48550/arXiv.2407.12290v2

Research Objective:

This research paper investigates the impact of thermal variations in the Hubble Space Telescope (HST) on measurements of diffuse light, aiming to improve the accuracy of extragalactic background light (EBL) and diffuse light estimations from the SKYSURF project.

Methodology:

The authors analyzed archival WFC3/IR engineering data from HST, focusing on temperature variations in different components, particularly the pick-off mirror, as a function of time, orbital phase, Earth-Sun distance, and year. They developed an empirical model to describe these variations and used it to correct diffuse light measurements from the SKYSURF project.

Key Findings:

  • The pick-off mirror temperature (TPOM) varies by less than 1 Kelvin throughout HST's orbit and in relation to time since Earth occultation.
  • TPOM exhibits a yearly variation of approximately 1 Kelvin, correlating with the Earth-Sun distance, being cooler when Earth is farthest from the Sun.
  • Empirical analysis suggests a ±3.5K variation in the primary and secondary mirror temperatures over a year, likely due to their direct exposure to Earth's radiation.
  • After correcting for thermal background variations, the updated diffuse light limits are 21, 32, and 25 nW m−2 sr−1 in the F125W, F140W, and F160W filters, respectively.

Main Conclusions:

  • While HST's thermal variations introduce a systematic uncertainty in diffuse light measurements, these variations are relatively small and well-characterized.
  • The updated diffuse light limits, after accounting for thermal variations, are lower than previous estimations, refining the constraints on the EBL.
  • The remaining diffuse sky signal observed in SKYSURF data might be attributable to a faint Zodiacal light source, potentially originating from icy dust in the inner Solar System.

Significance:

This research significantly contributes to the field of observational cosmology by improving the accuracy of diffuse light measurements, which are crucial for understanding the EBL and the history of galaxy formation. The findings have implications for interpreting observations from future space telescopes, such as the James Webb Space Telescope (JWST).

Limitations and Future Research:

  • The study primarily focuses on the WFC3/IR instrument on HST. Further investigation into thermal variations in other instruments is necessary.
  • The exact nature of the remaining diffuse sky signal requires further investigation, potentially through modeling the Zodiacal light contribution from icy dust.
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Stats
The pick-off mirror temperature (TPOM) changes by less than one Kelvin. The primary and secondary mirror temperatures vary by ±3.5K over a year. The updated diffuse light limits are 21 nW m−2 sr−1, 32 nW m−2 sr−1, and 25 nW m−2 sr−1 in the F125W, F140W, and F160W filters, respectively. Conservative lower limits on diffuse emission are 12 nW m−2 sr−1, 20 nW m−2 sr−1, and 2 nW m−2 sr−1 in F125W, F140W, and F160W, respectively.
Quotes
"The thermal background changes by less than one Kelvin in the WFC3 pick-off mirror, one of the most important contributors to the thermal background." "Based on this improved modeling, we provide new upper limits on the level of diffuse light of 21 nW m−2 sr−1, 32 nW m−2 sr−1, and 25 nW m−2 sr−1 for F125W, F140W, and F160W."

Deeper Inquiries

How might the James Webb Space Telescope (JWST), with its advanced thermal control systems, further refine our understanding of diffuse light and the EBL?

The James Webb Space Telescope (JWST), with its advanced thermal control systems and significantly lower operating temperature compared to HST, offers a unique opportunity to refine our understanding of diffuse light and the EBL. Here's how: Reduced Thermal Background: JWST's passively cooled optics and instruments, operating at cryogenic temperatures around 40 K, result in a drastically reduced thermal background compared to HST. This significantly minimizes the contamination from the telescope's own thermal emission, allowing for more precise measurements of faint, diffuse light. Wider Wavelength Coverage: JWST's broader wavelength coverage, extending from the near-infrared to the mid-infrared, enables it to observe a wider range of astrophysical sources contributing to the EBL. This includes cooler and more distant galaxies that were challenging to observe with HST. Higher Sensitivity and Resolution: JWST's larger primary mirror and advanced instruments provide significantly higher sensitivity and angular resolution than HST. This allows for the detection of fainter sources and the resolution of finer spatial structures in the diffuse light background, potentially revealing contributions from previously unresolved sources like intra-halo light or faint dwarf galaxies. By leveraging these capabilities, JWST can provide crucial data to: Constrain the EBL more accurately: JWST's reduced thermal background and higher sensitivity will enable more precise measurements of the EBL across a wider wavelength range, providing tighter constraints on the integrated light from galaxies and potentially revealing the presence of any excess light from other sources. Characterize the Zodiacal Light: JWST's observations can help better characterize the Zodiacal light, the diffuse light scattered by interplanetary dust. This is crucial for accurately subtracting the Zodiacal foreground and isolating the EBL signal. Investigate the nature of the diffuse light excess: If the excess diffuse light observed by SKYSURF is confirmed by JWST, its observations can help determine the origin of this excess, whether it arises from faint, unresolved galaxies, intra-halo light, or other sources. JWST's observations, in conjunction with the SKYSURF findings, will be instrumental in advancing our understanding of the diffuse light background and its implications for cosmology and galaxy evolution.

Could there be other, yet unconsidered, sources of systematic error in diffuse light measurements beyond instrumental thermal variations?

While instrumental thermal variations are a significant source of systematic error in diffuse light measurements, other potential sources need careful consideration: Stray Light: Scattered light from bright sources outside the field of view, or even within the instrument itself, can contaminate the measurements. This is particularly challenging to model and remove, especially for faint, diffuse backgrounds. Flat Fielding Imperfections: Variations in the sensitivity of the detector pixels, even after flat-fielding corrections, can introduce uncertainties. These imperfections can be challenging to characterize fully and may vary over time. Unknown or Variable Foregrounds: Unaccounted for or poorly understood foreground emissions, such as faint Galactic cirrus or emission from the interplanetary medium, can contaminate the measurements. Data Reduction and Analysis Techniques: Subtle biases or uncertainties introduced during the data reduction and analysis process, such as background subtraction methods or photometric calibration, can affect the final results. Temporal Variations in Known Foregrounds: The Zodiacal light and DGL are not static and can exhibit temporal variations due to factors like the solar cycle or the motion of the Solar System through the interstellar medium. Accurately accounting for these variations is crucial. Addressing these potential sources of systematic error requires: Careful Instrument Design and Calibration: Minimizing stray light and characterizing detector imperfections are crucial aspects of instrument design and calibration. Improved Foreground Modeling: Developing more accurate models of known foregrounds and searching for potential unknown foregrounds is essential. Robust Data Analysis Techniques: Employing robust data analysis techniques that minimize biases and uncertainties, including careful background subtraction and photometric calibration, is crucial. Multi-wavelength Observations: Observing the diffuse light background across a wide wavelength range using different instruments and telescopes can help identify and disentangle various foreground and background components. By meticulously addressing these potential sources of systematic error, we can improve the accuracy and reliability of diffuse light measurements, leading to a more complete understanding of the EBL and its cosmological implications.

If the remaining diffuse sky signal is indeed from icy dust in the inner Solar System, what does this tell us about the early Solar System's formation and evolution?

The confirmation of a diffuse sky signal originating from icy dust in the inner Solar System would provide intriguing insights into the early Solar System's formation and evolution: Transportation of Icy Material: The presence of icy dust in the inner Solar System, where temperatures are typically too high for ice to survive, suggests a mechanism for transporting icy material from the outer Solar System inwards. This could be linked to the early migration of giant planets, scattering icy planetesimals and dust into the inner regions. Cometary Activity and Evolution: Icy dust in the inner Solar System could be remnants of cometary activity. As comets approach the Sun, they sublimate, releasing dust and gas. The observed signal could indicate a population of comets with orbits that bring them relatively close to the Sun, contributing to the icy dust population. Collisional Processes: Collisions between asteroids and other small bodies in the inner Solar System can generate dust. If a significant fraction of these collisions involve icy bodies, they could contribute to the observed icy dust signal. Early Solar System Dynamics: The distribution and properties of the icy dust can provide clues about the early dynamical evolution of the Solar System. The presence and characteristics of this dust can constrain models of planetary migration, planetesimal scattering, and the overall evolution of the dust disk from which planets formed. Further investigation of this icy dust population, including its spatial distribution, size distribution, and composition, would be crucial to refine our understanding of its origin and implications for the early Solar System. This could involve: Spectroscopic Observations: Analyzing the spectrum of the diffuse light can reveal the composition of the icy dust, providing clues about its origin and formation conditions. Modeling Efforts: Developing detailed dynamical models of the early Solar System, incorporating planetary migration and planetesimal scattering, can help reproduce the observed distribution of icy dust and constrain the processes responsible for its presence. Comparison with Other Observations: Comparing the properties of the icy dust with observations of comets, asteroids, and interplanetary dust particles can provide further insights into its origin and evolution. Confirming and characterizing this icy dust population would not only refine our understanding of the diffuse sky background but also provide valuable constraints on models of Solar System formation and evolution, shedding light on the processes that shaped our planetary system.
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