Empirical estimation of host galaxy dispersion measure toward well-localized fast radio bursts

Context. Fast radio bursts (FRBs) are very energetic pulses in the radio wavelengths that have an unknown physical origin. They can be used to study the intergalactic medium thanks to their dispersion measure (DM). The DM has several contributions that can be measured (or estimated), including the contribution from the host galaxy itself, DM_host. The DM_host is generally difficult to measure, thus limiting the use of FRBs as cosmological probes and our understanding of their physical origin(s).

Aims. In this work we empirically estimated DM_host for a sample of 12 galaxy hosts of well-localized FRBs at 0.11 < z < 0.53 using a direct method based solely on the properties of the host galaxies themselves, referred to as DM_host^direct. We also explored possible correlations between DM_host and some key global properties of galaxies.

Methods. We used VLT/MUSE observations of the FRB hosts to estimate our empirical DM_host^direct. The method relies on estimating the DM contribution of both the FRB host galaxy' s interstellar medium (DM_host^ISM) and its halo (DM_host^halo) separately. For comparison purposes, we also provide an alternative indirect method for estimating DM_host based on the Macquart relation (DM_host^Macquart).

Results. We find an average DM_host = 80 ± 11 pc cm^-3 with a standard deviation of 38 pc cm^-3 (in the rest frame) using our direct method, with a systematic uncertainty of ∼30%. This is larger than the typically used value of 50 pc cm^{' 3} but consistent within the uncertainties. We report positive correlations between DM_host and both the stellar masses and the star formation rates of their hosts galaxies. In contrast, we do not find any strong correlation between DM_host and the redshift nor the projected distances to the center of the FRB hosts. Finally, we do not find any strong correlation between DM_host^direct and DM_host^Macquart, although the average values of the two are consistent within the uncertainties.

Conclusions. Our reported correlations between DM_host^direct and stellar masses and/or the star formation rates of the galaxies could be used in future studies to improve the priors used in establishing DM_host for individual FRBs. Similarly, such correlations and the lack of a strong redshift evolution can be used to constrain models for the progenitor of FRBs, for example by comparing them with theoretical models. However, the lack of correlation between DM_host^direct and DM_host^direct indicates that there may be contributions to the DM of FRBs not included in our DM_host^direct modeling, for example large DMs from the immediate environment of the FRB progenitor and/or intervening large-scale structures not accounted for in DM_host^Macquart.