Form 8

Research Result Report
ICRR Inter-University Research Program 2022
Research Subject: Understanding the GeV-TeV signatures of star-forming galaxies inthe extragalactic gamma-ray background with CTA
Principal Investigator: Dr Ellis Richard Owen
Participating Researchers: Prof Albert Kong (NTHU, Taiwan), Prof Masahiro
Teshima (ICRR)
Project summary: During the cosmic ‘high noon’ (at redshifts of z~2-3), physicalconditions and processes in galaxies differ dramatically from those in the localUniverse. In particular, cosmic ray (CR) interactions, their associated production ofparticles and radiation, and their deposition of momentum play an essential role inshaping the evolution of galaxies (Owen et al. 2018, 2019b) as well as producing
energetic cosmic backgrounds - including a component of the extra-galactic gammaray background, EGB (e.g. Lamastra et al. 2017). Understanding the activities andinteractions of CRs in young star-forming galaxies (SFGs) is important to uncover thedetails of feedback processes influencing their evolution, and spatial analyses of nearfuture EGB surveys will soon offer an important new opportunity to do this: thecontinued operation of the Fermi-LAT space telescope will reach exposure times
sufficient to provide suitable data in the next three years (Owen et al. 2020b), andthis will be complemented by the Key Science Projects (KSPs) of the upcoming
Cherenkov Telescope Array, CTA (see Science with the Cherenkov Telescope Array,
CTA Consortium 2017, in particular, KSP 8: Extragalactic Survey).
This project was initiated in FY2020. Prior to FY2022, work was completed toconstruct tentative source sub-population models of SFGs, informed by public resultsfrom large-scale numerical simulations (EAGLE and IllustrisTNG). These wereadapted into a computational code (initially constructed during FY2020, see resultsin Owen et al. 2021). Modeling work was then completed to investigate signatures ofSFG sub-populations in FY2021 using an extension of this code (Owen et al. 2022).
In FY2022, continuation of the project put focus on developing the technical details
of SFG prototype models. This was particularly intended to support more meaningfulpredictions of the EGB signatures imprinted by SFGs in the TeV energy range, which
are most relevant to future observations to be conducted with CTA. Certain physicalprocesses (reprocessing of gamma-rays by infrared dust emission in galaxies, and CRtransport) had previously been included in the model framework in an approximatefashion. This was tolerable when modeling the EGB at GeV energies, where theimpact is marginal. However, proper modeling of the EGB at TeV energies required arefined treatment of these effects. This is because EGB predictions are more strongly
dependent on the detailed model configuration at these higher energies.

Form 8During FY2022, three specific objectives were identified:(1) Upgrade TeV prototype emission models for SFGs, accounting for theirevolutionary progression, the distribution and properties of their interstellar dust
and its associated re-processed infrared radiation field.

Achievement: Work is progressing to augment the existing computational framework.
An advanced model accounting for an improved treatment of CR transport has been
constructed. Detailed 1-D, 2-D and 3-D MHD simulation codes have been developed
to properly capture spectrally-resolved CR transport and flow interactions,accounting for hadronic interactions and multiple CR species. The code has been
produced as bespoke modules for the adaptive mesh refinement code, FLASH. Tests
have been completed, and production runs will begin in mid-FY2023. This will
properly capture the essential aspects of the evolutionary progression of EGB
prototype galaxy models, and its impacts on the CR distribution and gamma-ray
luminosity from prototype SFGs. Alongside this, theoretical models for dust
refinement have been investigated, and a new collaborative initiative has begun with
Dr Hiroyuki Hirashita (ASIAA & Osaka University) to develop an advanced
treatment of dust evolution under CR irradiation, and its effects on gamma-ray
escape in SFGs. Some of these model extensions mark substantial refinements of thephysical treatment of SFGs, and will advance the EGB prototype code beyond thecurrent cutting edge of the field. These developments are expected to be completed
towards the end of FY2023.
(2) Introduce a prototype CR transport model for SFGs, to properly assess the spectral
containment fraction of CRs within a SFG.

Achievement: The model framework has been constructed. A detailed parameter
study is underway with various flow configurations and galaxy halo masses.
Dedicated computational time on national facilities has been acquired to support
numerical modeling and parameter studies (0.2 million cpu-hours) into FY 2023.
(3) Interface refined TeV prototype emission models with the existing computationalframework to determine the key features of the EGB anisotropy signatures atenergies from 0.01 GeV to 10s of TeV that will be accessible with CTA.

Achievement: This component of the project envisioned substantial interaction withexperts at ICRR to gain a detailed understanding of the CTA instrument
specifications such that meaningful model predictions could be constructed. Due tothe pandemic, an extended visit planned for several weeks was not possible. Instead,
only 3 short-term visits could be completed (costing 80,000 JPY). A carry-over requestof the un-used 220,000 JPY was approved. This will be used to support a longer
planned visiting period during FY 2023 so this aspect of the project can proceed.
No. 2022i-F-003