Beyond the Burst: How Host Galaxies Shape Fast Radio Bursts

Join us as we explore the latest research into Fast Radio Bursts (FRBs), mysterious, intense pulses of radio emission lasting only milliseconds. These cosmic phenomena are not just fleeting signals; they are powerful probes of the ionized gas across the universe and valuable tools for cosmological studies. In this episode, we delve into an investigation of FRB properties and their host galaxies, aiming to understand how the environment surrounding an FRB influences its observed characteristics.

What we discuss:

• The Phenomenon of Scattering: Learn how FRBs' paths through ionized media cause "scattering," a frequency-dependent broadening of their pulse profiles. This scattering is thought to primarily originate within their host galaxies.

• Key Correlations Found with FRB Scattering:

◦ Compactness and Stellar Surface Density: The study found a highly significant positive correlation between an FRB's scattering timescale (τ) and the stellar surface density (or compactness) of its host galaxy. This suggests that more compact (denser) host galaxies may contain more ionized content, leading to greater scattering of the FRB signal.

◦ Mass-Weighted Age: A highly significant positive correlation was also found between scattering timescale and the mass-weighted age of stars in the host galaxy. This implies that older stellar populations might contribute to increased scattering, though it's not driven by the overall galaxy mass.

◦ Gas-Phase Metallicity: There's a weakly significant positive correlation between scattering timescale and the gas-phase metallicity. Higher metallicity gas could mean more ionizing photons and electrons within the galaxy, contributing to scattering. This might be connected to compactness and mass-weighted age, as these properties can also correlate with metallicity.

• Surprising Absences of Correlation for Scattering:

◦ The study found no correlation between FRB scattering and host galaxy stellar mass or star formation rate.

◦ Crucially, there was no correlation found with the galaxy's inclination angle or optical disc axis ratio (b/a) for scattering. This finding challenges previous suggestions of an inclination bias in FRB detection.

• Rotation Measure and Galaxy Orientation:

◦ A strong anti-correlation was identified between the absolute Faraday rotation measure (|RMex|) of an FRB and the optical disc axis ratio (b/a) of its host galaxy. This means that FRBs from more edge-on galaxies tend to exhibit greater rotation measures, likely because the signal travels through a larger amount of the galaxy's magnetic field.

◦ The absence of other strong correlations for RM suggests the immediate environment of the FRB progenitor might play a significant role in determining RM, but the host galaxy's orientation is still important.

• Polarization Insights:

◦ While some weak correlations were seen for circular polarization fractions, these were often driven by single outlier datapoints and are not considered broadly significant across the sample. No strongly significant correlations were found for linear polarization.

• The Modest Sample Size: The researchers emphasize that while several correlations are statistically robust, the sample size is still relatively modest. Further high-time resolution FRB detections and detailed host galaxy follow-up are essential to confirm these initial findings.

Source Article:

• Glowacki, M., Bera, A., James, C. W., et al. (2020). An investigation into correlations between FRB and host galaxy properties. Cambridge Large Two, 1–21.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: ICRAR