The Stacked Lyman-Alpha Emission Profile from the Circum-Galactic Medium of z~2 Quasars

Abstract

In the context of the FLASHLIGHT survey, we obtained deep narrow band images of 15 $z\sim 2$ quasars with GMOS on Gemini-South in an effort to measure Ly$\alpha$ emission from circum- and inter-galactic gas on scales of hundreds of kpc from the central quasar. We do not detect bright giant Ly$\alpha$ nebulae (SB~10${}^{-17}$ erg s${}^{-1}$ cm${}^{-2}$ arcsec${}^{-2}$ at distances >50 kpc) around any of our sources, although we routinely ($\simeq 47$%) detect smaller scale <50 kpc Ly$\alpha$ emission at this SB level emerging from either the extended narrow emission line regions powered by the quasars or by star-formation in their host galaxies. We stack our 15 deep images to study the average extended Ly$\alpha$ surface brightness profile around $z\sim 2$ quasars, carefully PSF-subtracting the unresolved emission component and paying close attention to sources of systematic error. Our analysis, which achieves an unprecedented depth, reveals a surface brightness of SB${}_{\mathrm{Ly}\alpha }\sim 10^{-19}$ erg s${}^{-1}$ cm${}^{-2}$ arcsec${}^{-2}$ at $\sim 200$ kpc, with a $2.3\sigma$ detection of Ly$\alpha$ emission at SB${}_{\mathrm{Ly}\alpha }=(5.5\pm 3.1)\times 10^{-20}$ erg s${}^{-1}$ cm${}^{-2}$ arcsec${}^{-2}$ within an annulus spanning 50 kpc <R< 500 kpc from the quasars. Assuming this Ly$\alpha$ emission is powered by fluorescence from highly ionized gas illuminated by the bright central quasar, we deduce an average volume density of $n_{\mathrm{H}}=0.6\times 10^{-2}$ cm${}^{-3}$ on these large scales. Our results are in broad agreement with the densities suggested by cosmological hydrodynamical simulations of massive ($M\simeq 10^{12.5}{M}_{\odot }$) quasar hosts, however they indicate that the typical quasars at these redshifts are surrounded by gas that is a factor of ~100 times less dense than the (~1 cm${}^{-3}$) gas responsible for the giant bright Ly$\alpha$ nebulae around quasars recently discovered by our group.

22 pages, 13 figures, 1 table, submitted to ApJ

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