Primordial magnetic fields (PMFs) are one of the plausible candidates for the
origin of the observed large-scale magnetic fields. While many proposals have
been made for the generation mechanism of PMFs by earlier studies, it remains a
subject of debate. In this paper, to obtain new insights into PMFs, we focus on
the intrinsic alignments (IAs) of galaxies induced by the vector and tensor
modes of the anisotropic stress of PMFs. The long-wavelength vector and tensor
modes locally induce the tidal gravitational fields, leading to the
characteristic distortions of the intrinsic ellipticity of galaxies. We
investigate the shear E- and B-mode power spectra induced by the magnetic
vector and tensor modes in the three-dimensional space, assuming the
combination of galaxy imaging and galaxy redshift surveys. We find that the
magnetic tensor mode dominates both the E- and B-mode spectra. In particular,
the B-mode spectrum induced by the magnetic tensor mode plays a crucial role in
constraining the amplitude of PMFs, even in the presence of the non-magnetic
scalar contribution to the B-mode spectrum arising from the one-loop effect. In
future galaxy redshift surveys, such as Euclid and Square Kilometre Array, the
minimum detectable value reaches $sim 30 , rm nG$, which can potentially get
even smaller in proportion to the number of observed galaxies and reach $sim
mathcal{O}(1 , {rm nG})$. Measuring the IAs of galaxies would be a potential
probe for PMFs in future galaxy surveys.
Primordial magnetic fields (PMFs) are a leading contender for the origin of the large-scale magnetic fields that we observe. However, the mechanism by which PMFs are generated is still a matter of debate. In this paper, we focus on the intrinsic alignments (IAs) of galaxies that are induced by the anisotropic stress of PMFs. By studying the distortions in the ellipticity of galaxies caused by the tidal gravitational fields induced by PMFs, we can gain new insights into these magnetic fields.
We examine the shear E- and B-mode power spectra that are induced by the vector and tensor modes of the PMFs in three-dimensional space. We assume a combination of galaxy imaging and galaxy redshift surveys in order to make these measurements. Our results show that the magnetic tensor mode dominates both the E- and B-mode spectra. This means that the B-mode spectrum induced by the magnetic tensor mode is particularly important for constraining the amplitude of PMFs.
In future galaxy redshift surveys, such as Euclid and Square Kilometre Array, it will be possible to detect PMFs with a minimum value of approximately 30 nG (nanogauss). This detection threshold can potentially be even smaller as the number of observed galaxies increases, reaching the order of 1 nG (nanogauss). Therefore, measuring the intrinsic alignments of galaxies could serve as a potential probe for PMFs in future galaxy surveys.
Future Roadmap: Challenges and Opportunities
Looking ahead, there are several challenges and opportunities on the horizon for studying PMFs through galaxy surveys:
1. Improving Measurement Sensitivity
The minimum detectable value for PMFs is expected to decrease as the number of observed galaxies increases. This presents an opportunity to potentially reach sensitivities on the order of 1 nG. However, improving measurement sensitivity will require advancements in data collection, analysis techniques, and instrumentation.
2. Understanding the Non-Magnetic Scalar Contribution
In our study, we find that the B-mode spectrum induced by the magnetic tensor mode is crucial for constraining PMFs. However, it is important to consider and understand the contribution of non-magnetic scalar effects to the B-mode spectrum. Further research is needed to accurately separate the contributions from different sources and mitigate any potential biases.
3. Utilizing Advanced Galaxy Surveys
The future galaxy surveys such as Euclid and Square Kilometre Array will provide valuable data for studying PMFs. These surveys will allow for the observation of a large number of galaxies, potentially improving the sensitivity for PMF detection. Leveraging the capabilities of these surveys will be essential in advancing our understanding of PMFs.
4. Broadening the Investigation
While our study focuses on the intrinsic alignments of galaxies induced by PMFs, there are many other aspects that can be explored to gain further insights. Investigating other observable effects, such as statistical correlations between galaxies and cosmic microwave background radiation, can provide additional constraints on PMFs.
In summary, the study of primordial magnetic fields through galaxy surveys holds great potential for advancing our understanding of the origin and properties of these magnetic fields. Improving measurement sensitivity, understanding non-magnetic scalar contributions, utilizing advanced surveys, and broadening the investigation are key components of the future roadmap in this field.