SKYSURF IX -- The Cosmic Optical and Infrared Background from Integrated Galaxy Light Measurements

TL;DR

SKYSURF IX presents the most precise optical–NIR integrated galaxy light (IGL) measurement to date by assembling ~83k HST images into ~16.7k mosaics across 17 filters, calibrated against WAVES/DEVILS ground-based data. The study robustly derives IGL and COB values through a careful treatment of extinction, star–galaxy separation, bright-star masking, completeness corrections, and a multi-source error budget (Poisson, cosmic variance, zero-point, and spline-fitting uncertainties). It demonstrates that the COB at 0.1–8 μm is dominated by galaxy light, yielding a COB of $25.15\pm0.49\ \\mathrm{nW\,m^{-2}\,sr^{-1}}$ and, at 0.59 μm, an IGL of $9.07\pm0.35\ \\mathrm{nW\,m^{-2}\,sr^{-1}}$, with an IGL-COB convergence in line with recent VHE and New Horizons measurements. The results significantly tighten prior uncertainties and support a galaxy-origin COB, with future facilities (Euclid, Roman, SphereX, SKA) poised to push these constraints to the percent level across broader wavelengths.

Abstract

As part of the SKYSURF Hubble Space Telescope (HST) Legacy Archival program we present galaxy number counts which yield measurements of the extragalactic background light (EBL) at 15 different wavelengths. We have processed 82,752 HST images across 23 filters into 16,686 mosaics using the same software and processing pipeline throughout. Using 17/23 filters that give reliable galaxy counts, we constrain the integrated galaxy light (IGL) with a 1.5-9\% error between 0.3 and 1.6 $μ$m in combination with 8 bands from WAVES (Wide Area VISTA Extragalactic Survey) and DEVILS (Deep Extragalactic Visible Legacy Survey). While HST was never intended to undertake large area surveys, through extensive quality control and filtering, we were able to extract a reliable and representative sample of fields distributed across the sky. Our final catalogs cover a combined $\approx 19.6 °^2$, with individual filters covering areas ranging from $\approx 0.16-7.0 °^2$. The combination of numerous independent sight-lines and area coverage allows us to reduce cosmic variance uncertainties in deep number counts to 0.06\%-1.8\%. For the first time we are able to establish a measurement of the IGL, $\mathrm{9.07 \pm 0.35 nW m^{-2} sr^{-1}}$, at 0.59 $μ$m using HST data. We obtain a cosmic optical background value of $ 24.45 \pm 0.50 \mathrm{nW m^{-2} sr^{-1}}$. Different techniques used to measure the COB, both directly and indirectly, have recently converged indicating that the COB arises almost exclusively from processes within galaxies. This in combination with the recent values reported from New Horizons and very high energy (VHE) constraints leaves very little room for any diffuse emission coming from outside the Milky Way.

Figures (35)

• Figure 1: An equatorial projection showing the location of the 15,339 mosaics produced during this work. This projection displays all SKYSURF fields before any filtering or field rejection steps are taken. The galactic plane, $\mathrm{\pm 10}$ degrees, is shown as as gray overlay, with the zodiacal plane, $\mathrm{\pm 5}$ degrees, shown in orange.
• Figure 2: A flow chart illustrating the processing steps from MAST query to science ready multi-extension fits files used in this work.
• Figure 3: Footprints from two complex and two simple SKYSURF mosaics.
• Figure 4: An example showing an output produced by a single image from WFC3UVIS. All labeled image dimensions are in units of pixels. (a.) Zoomed-in cut out of the undilated mask produced from the HST DQA extension. (b.) The same zoomed in region after dilation. (c.) The original HST 'SCI' image extension before processing. (d.) The same image after sky-subtraction is performed using ProFound. (e.) The sky map generated from the original image extension. (f.) The inverse variance map generated from the original extension. These maps are used as input for propaneStackWarpInvar as described in Figure \ref{['fig:chart']}.
• Figure 5: The final multi-extension FITS image with all extensions displayed. All labeled image dimensions are in units of pixels. (a.) Sky-Subtracted and ProPane-stacked image extension. (b.) The weight map for the image. Weight values represent the number of unmasked pixels in the stack at each location. (c.) The inverse variance map used as input to ProFound during the object detection stage. (d.) The exposure time map for the field.