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, DMhost. The DMhost 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 DMhost 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$ _{\mathrm{host}}^{\mathrm{direct}} $. We also explored possible correlations between DMhost and some key global properties of galaxies. Methods. We used VLT/MUSE observations of the FRB hosts to estimate our empirical DM$ _{\mathrm{host}}^{\mathrm{direct}} $. The method relies on estimating the DM contribution of both the FRB host galaxy’s interstellar medium (DM$ _{\mathrm{host}}^{\mathrm{ISM}} $) and its halo (DM$ _{\mathrm{host}}^{\mathrm{halo}} $) separately. For comparison purposes, we also provide an alternative indirect method for estimating DMhost based on the Macquart relation (DM$ _{\mathrm{host}}^{\mathrm{Macquart}} $). Results. We find an average ⟨DMhost⟩ = 80 ± 11 pc cm−3 with a standard deviation of 38 pc cm−3 (in the rest frame) using our direct method, with a systematic uncertainty of ∼30%. This is larger than the typically used value of 50 pc cm−3 but consistent within the uncertainties. We report positive correlations between DMhost and both the stellar masses and the star formation rates of their hosts galaxies. In contrast, we do not find any strong correlation between DMhost and the redshift nor the projected distances to the center of the FRB hosts. Finally, we do not find any strong correlation between DM$ _{\mathrm{host}}^{\mathrm{direct}} $ and DM$ _{\mathrm{host}}^{\mathrm{Macquart}} $, although the average values of the two are consistent within the uncertainties. Conclusions. Our reported correlations between DM$ _{\mathrm{host}}^{\mathrm{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 DMhost 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$ _{\mathrm{host}}^{\mathrm{direct}} $ and DM$ _{\mathrm{host}}^{\mathrm{Macquart}} $ indicates that there may be contributions to the DM of FRBs not included in our DM$ _{\mathrm{host}}^{\mathrm{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$ _{\mathrm{host}}^{\mathrm{Macquart}} $.