The Ly$\alpha$ emission from high-$z$ galaxies hosting strong Damped Ly$\alpha$ systems

Abstract

We study the average Ly$\alpha$ emission associated with high-$z$ strong (log $N$(H I) $\ge$ 21) damped Ly$\alpha$ systems (DLAs). We report Ly$\alpha$ luminosities ($L_{\rm Ly\alpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(r-i)$ colours of QSOs and rest equivalent width of Si II$\lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{\rm Ly\alpha}$$< 10^{41} (3\sigma)\ \rm erg\ s^{-1}$ with a $2.8\sigma$ level detection of Ly$\alpha$ emission in the red part of the DLA trough. The $L_{\rm Ly\alpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $\geq 3\sigma$, redshifted by about 300-400 $\rm km\ s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$\alpha$ emitters. A clear signature of a double-hump Ly$\alpha$ profile is seen when we consider $W_{1526} \ge 0.4$ \AA\ and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$\alpha$ escape. The measured Ly$\alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$\alpha$ halos seen around LBGs could also explain our measurements.