# Extragalactic Background Light: a measurement at 400 nm using dark cloud shadow I. Low surface brightness spectrophotometry in the area of Lynds 1642

## TL;DR

This study measures the Extragalactic Background Light at 400 nm by analyzing the shadow of a dark cloud, using spectrophotometry to separate scattered starlight from the smooth EBL spectrum, improving measurement precision.

## Contribution

It introduces a spectroscopic method to accurately separate scattered starlight from EBL using dark cloud shadows, reducing calibration issues present in previous measurements.

## Key findings

- Achieved a measurement precision of ~0.5 x 10^{-9} erg cm^{-2}s^{-1}sr^{-1}Å^{-1}
- Successfully distinguished EBL from scattered starlight using spectral features
- Demonstrated the effectiveness of the shadow method at 400 nm

## Abstract

We present the method and observations for the measurement of the Extragalactic Background Light (EBL) utilizing the shadowing effect of a dark cloud. We measure the surface brightness difference between the opaque cloud core and its unobscured surroundings. In the difference the large atmospheric and Zodiacal light components are eliminated and the only remaining foreground component is the scattered starlight from the cloud itself. Although much smaller, its separation is the key problem in the method. For its separation we use spectroscopy. While the scattered starlight has the characteristic Fraunhofer lines and 400 nm discontinuity the EBL spectrum is smooth and without these features. Medium resolution spectrophotometry at $\lambda$ = 380 - 580 nm was performed with ${VLT}$/FORS at ESO of the surface brightness in and around the high-galactic-latitude dark cloud Lynds 1642. Besides the spectrum for the core with $A_V \ge 15$ mag, further spectra were obtained for intermediate-opacity cloud positions. They are used as proxy for the spectrum of the impinging starlight spectrum and facilitate the separation of the scattered starlight (cf. Paper II, Mattila et al. 2017b). Our spectra reach a precision of $\sim 0.5$ $10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$ as required to measure an EBL intensity in range of $\sim$1 to a few times $10^{-9}$ erg cm$^{-2}$s$^{-1}$sr$^{-1}$\AA$^{-1}$. Because all surface brightness components are measured using the same equipment the method does not require unusually high absolute calibration accuracy, a condition which has been a problem for some previous EBL projects