SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-Sky Surface-Brightness Measurements: I. Survey Overview and Methods

Abstract

We give an overview and describe the rationale, methods, and testing of the Hubble Space Telescope (HST) Archival Legacy project "SKYSURF". SKYSURF uses HST's unique capability as an absolute photometer to measure the ∼0.2-1.7 µm sky surface brightness (SB) from 249,861 WFPC2, ACS, and WFC3 exposures in ∼1400 independent HST fields. SKYSURF's panchromatic dataset is designed to constrain the discrete and diffuse UV to near-IR sky components: Zodiacal Light (ZL; inner Solar System), Kuiper Belt Objects (KBOs; outer Solar System), Diffuse Galactic Light (DGL), and the discrete plus diffuse Extragalactic Background Light (EBL). We outline SKYSURF's methods to: (1) measure sky-SB levels between its detected objects; (2) measure the integrated discrete EBL, most of which comes from AB 17-22 mag galaxies; and (3) estimate how much diffuse light may exist in addition to the extrapolated discrete galaxy counts. Simulations of HST WFC3/IR images with known sky-values and gradients, realistic cosmic ray (CR) distributions, and star plus galaxy counts were processed with nine different algorithms to measure the "Lowest Estimated Sky-SB" (LES) in each image between the discrete objects. The best algorithms recover the inserted LES values within 0.2% when there are no image gradients, and within 0.2-0.4% when there are 5-10% gradients. SKYSURF requires non-standard re-processing of these HST images that includes restoring the lowest sky-level from each visit into each drizzled image. We provide a proof of concept of our methods from the WFC3/IR F125W images, where any residual diffuse light that HST sees in excess of the Kelsall et al.(1998) Zodiacal model prediction does not depend on the total object flux that each image contains. This enables us to present our first SKYSURF results on diffuse light in Carleton et al. (2022).

Toward a Data-driven Model of the Sky from Low Earth Orbit as Observed by the Hubble Space Telescope

“…Moreover, our data are likely not completely free of bad background estimates (see Section 2.2), which will introduce noise into the data. The promising outcome of this work has motivated a more extensive investigation of these issues using essentially the entire HST imaging archive via the SKYSURF Legacy Archival Survey (Windhorst et al 2022). In this pending work, we have a significantly larger sample size that will allow us to explore numerous other factors (e.g., instrumental contributions to the sky, Galactic dust, etc.)…”

“…We conclude with a note that we will extend this study to a much larger data set. We will utilize the full HST legacy archive via the SKYSURF Legacy Archival Survey (Carleton et al 2022;Windhorst et al 2022) for WFC3 and a more extensive ACS data set. This will enable us to extend our work to a more thoroughly sampled parameter space and work toward decoupling sources of stray light from the spatial dependencies of the zodiacal light.…”

“…A number of LEO space telescopes (e.g., SPHEREx, Korngut et al 2018;SkyHopper, 4 MESSIER Lombardo et al 2019), sounding rocket missions (e.g., CIBER, Korngut et al 2013;CIBER2, Korngut et al 2022), and large archival surveys from existing instruments (e.g., SKYSURF, Windhorst et al 2022;Carleton et al 2022) must deal with sources of stray light in order to make a secure measurement of the extragalactic background light (EBL). The EBL consists of all optical/infrared sources over all cosmic time and is considered a valuable benchmark to assess competing theories of structure formation of the universe.…”

The cosmic radio background from 150 MHz to 8.4 GHz and its division into AGN and star-forming galaxy flux

“…In the COB and CIB the long standing factor of ∼ 10 discrepancy (see for example Bernstein et al 2002) has possibly now been resolved by indirect constraints from Very High Energy constraints (e.g., Aharonian et al 2006;Ahnen et al 2016; H. E. S. S. Collaboration et al 2013;Fermi-LAT Collaboration et al 2018) appearing to corroborate the low EBL results from integrated source counts. This also follows from a deeper understanding of the Zodiacal Light from recently revised HST Sky-Surface brightness measurements (Windhorst et al 2022). However some level of tension between the direct and indirect COB does still remain as recent results from the New Horizons instrument, from distances beyond where Zodiacal Light should be significant, still show a factor of ×2 inconsistency with the indirect count and VHE constraints, (Lauer et al 2021(Lauer et al , 2022.…”