Constraining the primordial magnetic field with dwarf galaxy simulations

Abstract

Using a set of cosmological hydro-dynamical simulations, we constrained the properties of primordial magnetic fields by studying their impact on the formation and evolution of dwarf galaxies. We performed a large set of simulations (8 dark matter only and 72 chemo-hydrodynamical) including primordial magnetic fields through the extra density fluctuations they induce at small length scales (k ≥ 10 h Mpc −1 ) in the matter power spectrum. Our sample of dwarfs include 9 systems selected out of the initial (3.4 Mpc/h) 3 parent box, re-simulated from z = 200 to z = 0 using a zoom-in technique and including the physics of baryons. We explored a large variety of primordial magnetic fields with strength B λ ranging from 0.05 to 0.50 nG and magnetic energy spectrum slopes n B from −2.9 to −2.1. Strong magnetic fields characterized by a high amplitude (B λ = 0.50, 0.20 nG with n B = −2.9) or by a steep initial power spectrum slope (n B = −2.1, −2.4, with B λ = 0.05 nG) induce perturbations in the mass scales from 10 7 to 10 9 M . In this context emerging galaxies see their star formation rate strongly boosted. They become more luminous and metal rich than their counterparts without primordial magnetic fields. Such strong fields are ruled out by their inability to reproduce the observed scaling relations of dwarf galaxies. They predict dwarf galaxies to be at the origin of an unrealistically early reionization of the Universe and also overproduce luminous satellites in the Local Group. Weaker magnetic fields impacting the primordial density field at corresponding masses 10 6 M , produce a large number of mini dark halos orbiting the dwarfs, however out of reach for current lensing observations. This study allows for the first time to constrain the properties of primordial magnetic fields based on realistic cosmological simulations of dwarf galaxies.

Introducing SPHINX-MHD: The Impact of Primordial Magnetic Fields on the First Galaxies, Reionization, and the Global 21cm Signal

“…This effect, combined with the tendency of strong magnetic fields to delay star formation in haloes, translates to a small decrease in the average stellar mass at halo masses < 10 8 M in the top panel of Figure 12. Similar behaviour was observed in the zoom-in simulations of Sanati et al (2020) at 𝑧 = 0 where data from the Local Group was used to constrain both the strength and slope of the PMF.…”

“…The B14 simulation is used for comparison to other simulations in the literature that explored primordial seed fields of this value (e.g., Pillepich et al 2018). Finally the B11 simulations represent scenarios where the magnetic field is likely to have a dynamic impact on galaxy structure (e.g., Martin-Alvarez et al 2020b), and where the impact of the primordial seed field on the matter power spectrum can modify structure formation and potentially the reionization history (Shaw & Lewis 2012;Pandey et al 2015;Sanati et al 2020). We describe below in detail the numerical methods for each of these simulations.…”

“…Because PMFs can impact both the ISM of galaxies as well as the dark matter halo mass function, we expect that the history of reionization and the properties of the intergalactic medium can change depending on the characteristics of the PMF. Pandey et al (2015) and Sanati et al (2020) have already demonstrated that the history of reionization can be used as a constraint on the properties of the PMFs using analytical models and zoom-in simulations, respectively. Furthermore, for strong enough PMFs, we expect to be able to observe their signatures in the properties of the Ly𝛼 forest (Chongchitnan & Meiksin 2014).…”

“…This modification to the matter power spectrum is coincidentally in the region of 𝑘-space that predominantly affects the formation of dwarf galaxies which, as discussed earlier, are the primary candidates to be the dominant source of reionization. Pandey et al (2015); Sanati et al (2020) have shown that the reionization history can significantly change depending on the assumptions regarding PMFs. Hence, the observed reionization history itself provides a constraint on the properties of the PMFs (Sanati et al 2020).…”

“…Pandey et al (2015); Sanati et al (2020) have shown that the reionization history can significantly change depending on the assumptions regarding PMFs. Hence, the observed reionization history itself provides a constraint on the properties of the PMFs (Sanati et al 2020). Similarly, the Ly𝛼 forest is currently our best constraint on the tail-end of reionization (Fan et al 2006; Kulkarni et al 2019) and the effective optical depth is sen-sitive to the presence of sub-n𝐺 PMFs (e.g., Pandey & Sethi 2013;Chongchitnan & Meiksin 2014).…”