Prospects of newly detecting nearby star-forming galaxies by the Cherenkov Telescope Array

TL;DR

This paper evaluates the potential of the Cherenkov Telescope Array to detect gamma-ray emissions from nearby star-forming galaxies, using a model based on physical galaxy properties to predict their detectability.

Contribution

The study applies a physical model to predict gamma-ray emissions from undetected nearby galaxies, identifying promising candidates for CTA detection and highlighting the need for advanced models beyond calorimetric assumptions.

Findings

NGC 5236, M33, NGC 6946, and IC 342 are prime CTA detection candidates.

NGC 1482 may be detectable despite low GeV flux, due to its proximity to the calorimetric limit.

Most galaxies' TeV fluxes are lower than simple SFR-based estimates, indicating the need for more complex models.

Abstract

Prospects of the Cherenkov Telescope Array (CTA) for the study of very high energy gamma-ray emission from nearby star-forming galaxies are investigated. In the previous work, we constructed a model to calculate luminosity and energy spectrum of pion-decay gamma-ray emission produced by cosmic-ray interaction with the interstellar medium (ISM), from four physical quantities of galaxies [star formation rate (SFR), gas mass, stellar mass, and effective radius]. The model is in good agreement with the observed GeV--TeV emission of several nearby galaxies. Applying this model to nearby galaxies that are not yet detected in TeV (mainly from the KINGFISH catalog), their hadronic gamma-ray luminosities and spectra are predicted. We identify galaxies of the highest chance of detection by CTA, including NGC 5236, M33, NGC 6946, and IC 342. Concerning gamma-ray spectra, NGC 1482 is particularly interesting because our model predicts that this galaxy is close to the calorimetric limit and its gamma-ray spectral index in GeV--TeV is close to that of cosmic-ray protons injected into ISM. Therefore this galaxy may be detectable by CTA even though its GeV flux is below the {\it Fermi} Large Area Telescope sensitivity limit. In the TeV regime, most galaxies are not in the calorimetric limit, and the predicted TeV flux is lower than that assuming a simple relation between the TeV luminosity and SFR of M82 and NGC 253, typically by a factor of 15. This means that a more sophisticated model beyond the calorimetric limit assumption is necessary to study TeV emission from star-forming galaxies.