Radio Bursts Reveal Limits to Hydrogen Gas Dynamics in Distant Galaxies

Scientists are now utilising fast radio bursts to constrain the evolution of the HI spin temperature, a crucial indicator of gas dynamics in distant galaxies. Hugh Roxburgh, Marcin Glowacki, and Apurba Bera, all from the International Centre for Radio Astronomy Research at Curtin University, alongside Clancy James, present a proof of concept utilising the bright FRB 20211127I detected by ASKAP to establish upper and lower limits on the integrated optical depth and HI excitation temperature respectively. This research is significant because it demonstrates a novel method for probing the interstellar medium of host galaxies at cosmological distances, offering insights into gas phase balance and thermal processes. Although current constraints are limited, the authors show that future observations with existing and planned telescopes like MeerKAT, ASKAP, and DSA, as well as utilising the sensitivity of FAST, could unlock the potential for measuring HI absorption signals and addressing key challenges in understanding FRB origins and host galaxy properties.

This work demonstrates a novel approach to understanding how galaxies evolve by utilising the unique properties of FRBs, intense bursts of radio waves originating from beyond our galaxy.

While no absorption signal has yet been definitively detected within an FRB, this research establishes a proof of concept, opening a new avenue for investigating the interstellar medium at cosmological distances. The study centres on the potential of FRBs to encode subtle absorption signals from atomic hydrogen gas as the radio waves traverse the interstellar medium of their host galaxies.

By combining these signals with detailed maps of HI emission, radiation emitted by hydrogen gas, researchers can effectively measure the HI excitation temperature, or Tspin. This temperature characterises the balance between the cold, dense, and warm, diffuse phases of neutral gas, providing insights into the underlying physical conditions that govern star formation and galactic evolution.

The team analysed FRB 20211127I, a particularly bright and narrow burst detected by the ASKAP telescope, to assess the feasibility of this technique. Through this analysis, they established an upper limit of 33 kilometres per second for the integrated optical depth of the signal, and subsequently determined a lower limit of 26 Kelvin for Tspin, based on HI emission data obtained with the MeerKAT telescope.

Although this initial test case offers limited constraints, the research highlights the potential of future observations with current and next-generation telescopes. Specifically, narrow, non-repeating FRBs with sufficient brightness, observed with facilities like MeerKAT, ASKAP, and the DSA telescope, could probe integrated optical depths below 5 kilometres per second.

Furthermore, the incredible sensitivity of the FAST telescope offers a promising route to stacking signals from numerous bursts emitted by hyperactive repeaters, potentially enabling the direct measurement of HI absorption and, therefore, Tspin. This innovative approach promises to address several longstanding challenges in FRB science, providing a crucial physical anchor for pinpointing the location of bursts within their host galaxies.

By disentangling the contribution of the host galaxy to the dispersion and scattering of the FRB signal, scientists can gain a clearer understanding of the burst’s origin and propagation. Ultimately, this work demonstrates that HI absorption measurements using FRBs can provide a powerful new tool for studying the evolution of galaxies and the processes that drive star formation across cosmic time.

Mapping Neutral Hydrogen to Constrain Fast Radio Burst Environments

A 3-hour MeerKAT L-band observation underpinned this work, enabling detailed scrutiny of the host galaxy of FRB 20211127I for hydrogen (HI) emission. This high-resolution mapping provided a crucial reference dataset against which to search for potential HI absorption signatures within the fast radio burst signal itself.

The chosen frequency range centred on the 21-centimetre line, the characteristic emission wavelength of neutral hydrogen, allowing for precise measurements of gas dynamics at cosmological distances. By combining FRB data with HI emission maps, the research aimed to constrain the HI excitation temperature, a key parameter characterising the balance of gas phases within galaxies.

The methodology leveraged the unique properties of FRBs as compact background sources, simplifying the analysis of absorption features. Unlike traditional quasar-based absorption studies, FRBs are assumed to have a covering fraction of unity, meaning the entire source is obscured by any intervening HI gas, thus eliminating a significant variable in the optical depth calculation.

Pulse-averaged spectra of FRB 20211127I were analysed to establish an upper limit of 33km s−1 on the integrated optical depth, representing the total amount of HI absorption along the line of sight. This measurement was then cross-referenced with the MeerKAT HI emission data to derive a lower limit of 26 K for the excitation temperature.

This study innovatively applies a technique traditionally used for probing intervening gas clouds to the study of FRB host galaxies. The approach relies on measuring the optical depth of the HI absorption line, directly linked to the population ratio of atoms in different hyperfine spin states, and thus to the gas’ kinetic temperature.

While the initial test case with FRB 20211127I yielded limited constraints, the research demonstrates the feasibility of this method and identifies the observational parameters needed to unlock its full potential. Specifically, the work highlights that narrow, non-repeating FRBs with fluences exceeding 20, 70, and 150 Jy ms, observed with current-generation telescopes like MeerKAT, ASKAP, and DSA, can probe optical depths below 5km s−1.

Fast Radio Burst Absorption Constrains Interstellar Hydrogen Gas Properties

An upper limit of 33km s−1 was established for the integrated optical depth of the observed fast radio burst, FRB 20211127I. This measurement, derived from pulse-averaged spectra, represents the amount of radio wave absorption by hydrogen gas along the signal’s path. Subsequent analysis, utilising a 3hr MeerKAT L-band observation of the host galaxy, yielded a lower limit of 26 K for the spin temperature, denoted as Tspin.

Tspin characterises the balance between different phases of gas and the thermal processes within galactic environments, providing insight into the interstellar medium. The column density of neutral hydrogen, NHI, was calculated to be 1.43 ×1021cm−2, based on a 50km s−1 integration width matching the assumed absorption profile.

Combining this column density with the optical depth limit allows for the determination of Tspin, revealing the minimum temperature of the gas responsible for the absorption. While this initial test case provides a relatively unremarkable result, the derived Tspin value is consistent with expectations for a non-detection.

Typical lower limits for cold neutral medium cloud temperatures in the Milky Way are around 80 K, making the current 26 K limit less informative. However, the research demonstrates the viability of this technique for future observations. The narrow bandwidth and high signal-to-noise ratio of FRB 20211127I, with a fluence of 35 Jy ms, facilitated this initial assessment.

Narrow, non-repeating FRBs with fluences exceeding 20, 70, and 150 Jy ms, observed with MeerKAT, ASKAP, and DSA telescopes respectively, are predicted to probe integrated optical depths below 5km s−1. Furthermore, stacking thousands of bursts from hyperactive repeaters using the FAST telescope offers a promising pathway to measure HI absorption and, consequently, Tspin.

The Bigger Picture

Scientists are beginning to use fast radio bursts, brief, intense pulses of radio waves, not just as beacons from distant galaxies, but as probes of the material lying between us and them. For years, the challenge has been to extract meaningful information from these fleeting signals, often obscured by the intervening interstellar medium.

This work demonstrates a potential pathway to do just that, by searching for subtle absorption features within FRB signals caused by hydrogen gas. While a definitive detection remains elusive, the team has established a crucial benchmark and identified the observational capabilities needed to make it happen.

The significance extends beyond simply pinpointing the location of FRBs within their host galaxies, a long-standing problem. Measuring the properties of interstellar hydrogen at cosmological distances offers a unique window into galactic environments, revealing details about gas dynamics and thermal processes that are otherwise inaccessible.

It’s a clever repurposing of a transient signal to study the persistent medium it travels through. However, the current limits on detectable absorption are still relatively high, requiring either exceptionally bright bursts or the stacking of many signals from repeating sources. The reliance on a specific subset of FRBs, narrow, non-repeating events, also introduces a potential bias.

Future progress hinges on the next generation of radio telescopes, particularly those with increased sensitivity and wider fields of view, to capture a larger sample of bursts and push the boundaries of detection. The prospect of combining these observations with detailed mapping of hydrogen emission lines promises a more complete picture of the interstellar ecosystems shaping these enigmatic signals.