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

TL;DR

This study investigates the occurrence of HI 21-cm absorption in strong FeII systems at redshifts 0.5 to 1.5, revealing increased detection rates linked to metal absorption strength and galaxy properties, with implications for galaxy halo evolution.

Contribution

The paper reports six new HI 21-cm detections in strong FeII systems, doubling the known detections at these redshifts, and analyzes the correlation between absorption features, metal strength, and galaxy characteristics.

Findings

Detection rate of HI 21-cm absorption increases with FeII equivalent width.

HI column densities are estimated to be ≥ 2×10^{20} cm^{-2} assuming typical spin temperatures.

Velocity widths of HI 21-cm lines increase with redshift, suggesting more massive galaxy halos at higher z.

Abstract

We present the results from our search for HI 21-cm absorption in a sample of 16 strong FeII systems ($W_{\rm r}$(MgII $\lambda2796$) $\ge1.0$ \AA\ and $W_{\rm r}$(FeII $\lambda2600$) or $W_{\rm FeII}$ $\ge1$ \AA) 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 $\sim50$%. Combining our measurements with those in the literature, we find that the detection rate of HI 21-cm absorption increases with $W_{\rm FeII}$, being four times higher in systems with $W_{\rm FeII}$ $\ge1$ \AA\ compared to systems with $W_{\rm FeII}$ $<1$ \AA. The $N$(HI) associated with the HI 21-cm absorbers would be $\ge 2 \times 10^{20}$ cm$^{-2}$, assuming a spin temperature of $\sim500$ 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.