Cosmic-ray ionization of low-excitation lines in active galactic nuclei and starburst galaxies

¹ Section of Astrophysics, Astronomy & Mechanics, Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR, 15784 Athens, Greece
² Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, E–44001 Teruel, Spain
³ Istituto di Astrofisica e Planetologia Spaziali (INAF–IAPS), Via Fosso del Cavaliere 100, I–00133 Roma, Italy

^⋆ Corresponding author; evkoutso@phys.uoa.gr

Received: 13 September 2024
Accepted: 25 November 2024

Abstract

Cosmic rays (CRs) can significantly impact dense molecular clouds in galaxies, heating the interstellar medium (ISM) and altering its chemistry, ionization, and thermal properties. Their influence is particularly relevant in environments with high CR rates, such as starburst galaxies with supernova remnants or jets and outflows in active galactic nuclei (AGN). CRs also transfer substantial energy to the ionized phase of the ISM far from the ionization source, preventing gas cooling and driving large-scale winds. In this work, we use CLOUDY photoionization models to investigate the effect of CRs on nebular gas which is an area of study that remains relatively under-explored, mainly focusing on cold molecular gas. Our models cover a broad range of density (1 to 10⁴ cm⁻³), ionization parameter (−3.5 ≤ log U ≤ −1.5), and CR ionization rate (10⁻¹⁶ s⁻¹ to 10⁻¹² s⁻¹). These are compared to VLT/MUSE observations of two prototypical AGN, Centaurus A (radio-loud) and NGC 1068 (radio-quiet), and the starburst NGC 253. We find that high CR rates (≳10⁻¹³ s⁻¹) typical of AGN and strong starburst galaxies can significantly alter the thermal structure of the ionized gas by forming a deep secondary low-ionization layer beyond the photoionization-dominated region. This enhances emission from low-ionization transitions, such as [N II]λ6584 Å, [S II]λλ6716,6731 Å, and [O I]λ6300 Å, affecting classical line-ratio diagnostics, metallicity, and ionization estimates. Unlike pure photoionization models, AGN simulations with high CR ionization rates reproduce the Seyfert loci in Baldwin, Phillips, and Terlevich (BPT) diagrams without requiring supersolar metallicities for the narrow-line region. Additionally, star-formation simulations with high CR ionization rates can explain line ratios in the LINER domain. We propose new maximum starburst boundaries for BPT diagrams in order to distinguish regions dominated by AGN photoionization from those that could be explained by star formation in conjunction with high CR ionization rates.