Relation between polycyclic aromatic hydrocarbon, Br$\alpha$ and infrared luminosity of local galaxies observed with AKARI

TL;DR

This study creates a catalogue of PAH 3.3 μm, Brα, and IR luminosity for 412 local galaxies, revealing how their ratios vary with IR luminosity and exploring potential physical causes.

Contribution

It provides the first comprehensive catalogue linking PAH, Brα, and IR luminosities in local galaxies, analyzing their interrelations and dependencies.

Findings

L(PAH 3.3 μm)/L(IR) is constant below 10^11 L_sun and decreases above

L(Brα)/L(IR) decreases with increasing L(IR)

Ratios are weakly dependent on galaxy type and dust temperature

Abstract

We produce a catalogue of polycyclic aromatic hydrocarbon (PAH) 3.3 $\mu$m, Br$\alpha$ and infrared luminosity ($L$(IR)) of 412 local galaxies, and investigate a relation between these physical parameters. We measure the PAH 3.3 $\mu$m and Br$\alpha$ flux using AKARI 2-5 $\mu$m spectra and the $L$(IR) using the AKARI-all-sky-survey data. The $L$(IR) and redshift ranges of our sample are $L$(IR)=$10^{9.7-12.8}$L$_\odot$ and $z_{\rm spec}=0.002-0.3$, respectively. We found that the ratio of $L$(PAH 3.3 $\mu$m) to $L$(IR) is constant at $L$(IR) $<$ $10^{11} \rm L_\odot$ whereas it decreases with the $L$(IR) at higher $L$(IR). Also, the ratio of $L$(Br$\alpha$) to $L$(IR) decreases with the $L$(IR). The both $L$(PAH)/$L$(IR) and $L$(Br$\alpha$)/$L$(IR) ratios are not strongly dependent on galaxy type and dust temperature. The relative weakness of the two ratios could be attributed to destruction of PAH, a lack of UV photons exciting PAH molecules or ionising hydrogen gas, extremely high dust attenuation, or active galactic nucleus contribution to the $L$(IR). Although we cannot determine the cause of the decreases of the luminosity ratios, a clear correlation between them implies that they are related with each other. The catalogue presented in our work will be available at the AKARI archive web page.