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Constraining the Local IGM/CGM Diffuse Emission with HST+COS

Presentation #116.04 in the session “LSS, Intergalactic Medium, and QSO Absorption Line Systems”.

Published onJun 18, 2021
Constraining the Local IGM/CGM Diffuse Emission with HST+COS

We are attempting to detect local (z < 0.48) diffuse emission from known UV IGM/CGM absorbers. Our motivation to measure the diffuse FUV background emission is to characterize the cosmologically significant, “lukewarm” (103 < T < 106 K) IGM. We attempt to measure this diffuse extragalactic emission, primarily in Lyα, He II, and OVI, by co-adding > 2000 HST+COS (Cosmic Origins Spectrograph) medium resolution FUV spectra (G130M and G160M), of 127 sightlines, in 2D. This represents a total equivalent HST exposure time of ~ 100 Million seconds or > 4.20 YEARS of HST time.

The COS aperture is not filled by the target but also samples the sky above and below the spectrum. We search for emission associated with detected absorbers (0.5-8 h75 kpc off the line-of-site), being careful to characterize the detector background, scattered light, and aperture vignetting properties. Geocoronal emission, particularly Lyα (1216Å), completely fills the sky, and therefore the COS aperture. We use this emission to characterize the extraction regions necessary to search for diffuse emission, not only from Lyα, but from other transitions such as OVI, CIV, SiII,  and NII.

Scientific interests at low-z, include:

•Lyα escape fractions from Galaxies

•CGM cooling streams

•IGM temperatures and ionization 

•Large-scale baryon distributions

•UV background constraint


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