Incidence of HI 21-cm absorption in strong FeII systems at $0.5<z<1.5$

Abstract

We present the results from our search for HI 21-cm absorption in a sample of 16 strong FeII systems ($W_{\mathrm{r}}$(MgII $\lambda 2796$) $\ge 1.0$ Å and $W_{\mathrm{r}}$(FeII $\lambda 2600$) or $W_{\mathrm{FeII}}$ $\ge 1$ Å) at $0.5<z<1.5$ using the Giant Metrewave Radio Telescope and the Green Bank Telescope. We report six new HI 21-cm absorption detections from our sample, which have increased the known number of detections in strong MgII systems at this redshift range by $\sim 50$%. Combining our measurements with those in the literature, we find that the detection rate of HI 21-cm absorption increases with $W_{\mathrm{FeII}}$, being four times higher in systems with $W_{\mathrm{FeII}}$ $\ge 1$ Å compared to systems with $W_{\mathrm{FeII}}$ $<1$ Å. The $N$(HI) associated with the HI 21-cm absorbers would be $\ge 2\times 10^{20}$ cm${}^{-2}$, assuming a spin temperature of $\sim 500$ K (based on HI 21-cm absorption measurements of damped Lyman-$\alpha$ systems at this redshift range) and unit covering factor. We find that HI 21-cm absorption arises on an average in systems with stronger metal absorption. We also find that quasars with HI 21-cm absorption detected towards them have systematically higher $E(B-V)$ values than those which do not. Further, by comparing the velocity widths of HI 21-cm absorption lines detected in absorption- and galaxy-selected samples, we find that they show an increasing trend (significant at $3.8\sigma$) with redshift at $z<3.5$, which could imply that the absorption originates from more massive galaxy haloes at high-$z$. Increasing the number of HI 21-cm absorption detections at these redshifts is important to confirm various trends noted here with higher statistical significance.

17 pages, 10 figures, 8 tables, accepted for publication in MNRAS

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