Possible evolution of the circum-galactic medium around QSOs with QSO age and cosmic time revealed by Ly$\hat{I}\pm$ halos

Abstract

We first present new Subaru narrow-band observations of the Ly$\hat{I}\pm$ halo around the quasi-stellar object (QSO) CFHQ J232908$-$030158 at $z=6.42$, which appears the most luminous and extended halo at $z>5$ ($L_{Ly\hat{I}\pm }=9.8\times 10^{43}$ erg s${}^{-1}$ within $37$ pkpc diameter). Then, combining these measurements with available data in the literature, we find two different evolutions of QSOs' Ly$\hat{I}\pm$ halos. First is a possible short-term evolution with QSO age seen in four $z>6$ QSOs. We find the anti-correlation between the Ly$\hat{I}\pm$ halo scales with QSOs' IR luminosity, with J2329-0301's halo being the brightest and largest. It indicates that ionizing photons escape more easily out to circum-galactic regions when host galaxies are less dusty. We also find a positive correlation between IR luminosity and black hole mass ($M_{\text{BH}}$). Given $M_{\text{BH}}$ as an indicator of QSO age, we propose a hypothesis that a large Ly$\hat{I}\pm$ halo mainly exists around QSOs in the young phase of their activity due to a small amount of dust. The second is an evolution with cosmic time seen over $z\sim 2-5$. We find the increase of surface brightness toward lower-redshift with a similar growth rate to that of dark matter halos (DHs) which evolve to $M_{\text{DH}}=10^{12}-10^{13}$ M${}_{\odot }$ at $z=2$. The extent of Ly$\hat{I}\pm$ halos is also found to increase at a rate scaling with the virial radius of growing DHs, $r_{\text{vir}}\propto M_{\text{DH}}^{1/3}$($1+z$)${}^{-1}$. These increases are consistent with a scenario that the CGM around QSOs evolves in mass and size keeping pace with hosting DHs.

16 pages, 7 figures, published by MNRAS