Research
My research area includes cosmology, galaxies, and astroparticle physics. Below are short introductions to my work.
Highest energy cosmic-ray leptons
The cosmic-ray leptons (electrons and positrons) have many mysterious features. We introduce a question that have received little attension: where does their spectrum end? We outline the physics that may determine the end of leptonic spectrum, and discuss how it can be useful to probe all-spectrum cosmic rays, high-energy gamma-ray sources, and dark matters (Sudoh&Becom, PRD, 2023c)
Where are "PeVatrons"?
Somewhere in the Milky Way there is a class of energetic astrophysical sources that accelerate hadrons to PeV energies -- "PeVatrons" -- but they are not identified yet. They in principle should produce gamma rays and neutrinos. While many gamma-ray sources are observed near a PeV, no high-energy neutrino source are found in the Milky Way. Why?
We quantify present constraints and future prospects by introducing a "n-tau" plane. Optimistically, PeVatrons are good multi-messenger sources that emit gamma rays and neutrinos, and the future neutrino telescopes with improved sensitivities would soon find them. However, we also point out a pessimistic scenario -- PeVatrons might be thin in gas so that hadrons escape without producing detectable emission, making their identifications hard even for next-generation neutrino telescopes! (Sudoh & Beacom, PRD, 2023a)
We also studied the potential of neutrino shower events for discovering PeVatrons, finding that they can be important for extended sources (Sudoh & Beacom, PRD, 2023b)
Centaurus A
Recent observations indicate that the very-high-energy emission from nearby radio galaxy Centaurus A comes from its kiloparsec-scale jet away from the core. We study the implications of these findings, especially on physical conditions and particle acceleration. (Sudoh+20, ApJ Letter)
Millisecond Pulsars and the Radio-SFR Correlation
The close correlation between galactic radio luminosities and star-formation rates has been well known for decades. However, measurements identify peculiar radio excess in low-SFR and high-mass galaxies. We suggest that cosmic-ray e+- injected by millisecond pulsars may dominate nonthermal radio emission in these quiescent galaxies and explain the excess. (Sudoh+21 PRD)
Microquasars
Microquasar SS433 is a peculiar Galactic binary system. In particular, it launches a pair of jets, which extends to tens of parsecs. We examined the production of very high energy particles in the jets of SS433, and show that these jets are accelerating particles at extremely high efficiency. (Sudoh+20, ApJ).
TeV Halos / Gamma-ray Sources
HAWC collaboration discovered extended TeV gamma-ray emission around nearby pulsars, which looks distinct from what is classically defined as a pulsar wind nebula. We showed that this new source class, named "TeV Halos", can be important in the TeV gamma-ray sky. We also show that TeV halo observations will be a probe of pulsar properties at birth (Sudoh+19, PRD).
In a more recent work, we argue that many of very-high-energy (> 56 TeV) sources reported by HAWC are likely leptonic and powered by pulsars. In this scenario, young pulsars produce a hard-spectrum e+- with high-efficiency, which radiate gamma rays in a region of slow diffusion and ISM-like magnetic fields, all of which are similar to the properties of TeV halos around middle-aged pulsars. (Sudoh+21, JCAP)
IceCube Neutrinos
The origin of TeV-PeV neutrinos detected by IceCube remains unclear. Star-forming galaxies (SFGs) are one of the proposed sources. We studied the contribution from SFGs across cosmic time (Sudoh+18, PASJ). We constructed a new model of gamma-ray and neutrino emission from SFGs, which can reproduce gamma-ray observations of nearby SFGs. We combine this with a realistic model of galaxy formation, finding that the contributions from SFGs to the IceCube diffuse neutrino is subdominant.
Cosmological Constant
The cosmological constant is one of the biggest mystery in modern cosmology and physics. In particular, it is more than about 60 orders of magnitude smaller than theoretical expectation for an unknown reason. We tested an anthropic explanation for this with a model of cosmological galaxy formation (Sudoh+17, MNRAS), and, in subsequent work we further considered the Galactic habitability (Totani+19, AstroBio). We find that the anthropic principle could be a viable option to explain the observed value of the cosmological constant.
