The Low-mass Dwarf Host Galaxy of Nonrepeating FRB 20230708A

Technical Summary: The Low-mass Dwarf Host Galaxy of Nonrepeating FRB 20230708A

Problem and Motivation
Despite thousands of Fast Radio Burst (FRB) detections, the nature of FRB progenitors and their emission mechanisms remains poorly understood. While nearly all identified FRB host galaxies exhibit active star formation, existing samples are drawn from heterogeneous radio surveys and follow-up strategies, introducing potential selection biases that complicate constraints on progenitor models. Specifically, recent studies have suggested a bias against low-metallicity, low-mass galaxies in the non-repeating FRB population, while others propose that faint hosts are exclusively associated with repeating FRBs. To address these uncertainties, the "Fast and Unbiased FRB Host Galaxy" (FURBY) Large Programme was established to create a homogeneous, well-defined sample of FRB hosts with strict selection criteria.

Methodology
This paper presents the initial results of the FURBY program, focusing on the host galaxy of the non-repeating FRB 20230708A. The study utilizes a uniform selection pipeline for FRBs detected by the Australian SKA Pathfinder (ASKAP) via the CRAFT survey. Candidates were filtered based on strict criteria: low Galactic reddening ($E(B-V) < 0.1$), low Galactic dispersion measure ($DM_{ISM}^{MW} < 100$ pc cm$^{-3}$), absence of nearby bright stars, and precise localization ($<0.7''$).

Host galaxy identification employed the Probabilistic Association of Transients to their Hosts (PATH) formalism, requiring a posterior probability $P(O|x) > 0.99$ and an apparent magnitude $m_R < 24$. Spectroscopic follow-up was conducted using the VLT/X-Shooter spectrograph (covering 0.3–2.5 $\mu$m) to obtain high-quality spectra for 12 FRB hosts.

For FRB 20230708A specifically:

• Spectroscopy: Data were reduced using PypeIt. Emission lines were fitted using the Penalized Pixel-Fitting (pPXF) package to determine redshift and gas properties.
• SED Modeling: Stellar population properties were derived using the Bayesian inference code Prospector, jointly fitting VLT photometry, Dark Energy Survey (DES) $griz$ photometry, and the X-Shooter spectrum. The model utilized a Kroupa initial mass function, Kriek & Conroy dust attenuation, and a non-parametric star formation history (SFH).
• Radio Constraints: The location was observed with the Australia Telescope Compact Array (ATCA) across four frequency bands to search for a persistent radio source (PRS).

Key Results
The analysis of FRB 20230708A reveals a host galaxy with the following characteristics:

• Redshift and Luminosity: The host is located at a low redshift of $z = 0.1050 \pm 0.0001$. It is an intrinsically faint dwarf galaxy with an $R$-band magnitude of $m_R = 22.53 \pm 0.02$ and a luminosity of $L \approx 1.6 \times 10^8 L_\odot$ ($\sim 0.01 L^*$). This makes it the lowest-luminosity host identified for a non-repeating FRB to date (by a factor of $\sim 3$) and slightly dimmer than the faintest hosts of repeating FRBs.
• Stellar Mass and Star Formation: The host has a low stellar mass of $\log(M_*/M_\odot) \approx 7.97$ and a low star formation rate (SFR) of $0.04 \pm 0.02 M_\odot$ yr$^{-1}$. The specific star formation rate indicates the galaxy is actively forming stars, with a star formation history showing a steady increase toward the present day.
• Metallicity: The galaxy exhibits very low metallicity, with $12 + \log(O/H)$ constrained between 8.0 and 8.3. This is derived from the non-detection of [N II] lines and the presence of strong [O II] and [O III] emission, placing the host in the star-forming regime of the BPT diagram.
• Persistent Radio Source (PRS): ATCA observations detected a faint, transient source at 7500 MHz in a single epoch, which was not confirmed in follow-up observations. Consequently, the study establishes a deep 3$\sigma$ upper limit on the spectral radio luminosity of $L_{7500 \text{ MHz}} = 5.2 \times 10^{27}$ erg s$^{-1}$ Hz$^{-1}$. This limit is significantly lower (by a factor of 50) than the PRS luminosities observed for repeating FRBs like FRB 20121102A and FRB 20190520B.

Significance and Claims
The discovery of FRB 20230708A's host challenges the developing convention that faint, low-metallicity dwarf galaxies are exclusively associated with repeating FRBs. The authors claim that this finding demonstrates that non-repeating FRBs can also arise in the faintest, metal-poor galaxies of the Universe.

This result has two primary implications:

1. Progenitor Diversity: It suggests that at least one FRB progenitor channel must include stars (or their remnants) formed in very low-metallicity environments. This supports the existence of multiple progenitor channels or formation pathways, rather than a single mechanism tied strictly to high-mass or high-metallicity environments.
2. High-Redshift Prospects: The presence of FRBs in such low-mass, metal-poor systems indicates better prospects for detecting FRBs from the high-redshift Universe, where young, low-mass galaxies proliferate.

The paper emphasizes that the FURBY sample, with its deep imaging and strict selection criteria, reveals a non-negligible fraction ($\sim 8\%$ in this sample) of dwarf galaxy hosts among non-repeating FRBs. This contrasts with previous heterogeneous samples that may have suffered from magnitude-limited biases, potentially missing such faint hosts. The authors conclude that while FRB 20230708A shares a host environment with repeating FRBs, its burst properties (broad pulse, lack of "sad trombone" morphology, low scattering, and low rotation measure) suggest a distinct immediate environment and potentially different progenitor physics compared to known repeating sources.