Interpreting the relation between the gamma-ray and infrared luminosities of star-forming galaxies. (arXiv:1902.09654v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_Y/0/1/0/all/0/1">Yi Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Peng_F/0/1/0/all/0/1">Fang-Kun Peng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_X/0/1/0/all/0/1">Xiang-Yu Wang</a>
It has been found that there is a quasi-linear scaling relationship between
the gamma-ray luminosity in GeV energies and the total infrared luminosity of
star-forming galaxies, i.e. $L_{gamma}propto L_{rm IR}^{alpha}$ with
$alphasimeq 1$. However, the origin of this linear slope is not well
understood. Although extreme starburst galaxies can be regarded as calorimeters
for hadronic cosmic ray interaction and thus a quasi-linear scaling may hold,
it may not be the case for low star-formation-rate (SFR) galaxies, as the
majority of cosmic rays in these galaxies are expected to escape. We calculate
the gamma-ray production efficiency in star-forming galaxies by considering
realistic galaxy properties, such as the gas density and galactic wind velocity
in star-forming galaxies. We find that the slope for the relation between
gamma-ray luminosity and the infrared luminosity gets steeper for low infrared
luminosity galaxies, i.e. $alpharightarrow 1.6$, due to increasingly lower
efficiency for the production of gamma-ray emission. We further find that the
measured data of the gamma-ray luminosity is compatible with such a steepening.
The steepening in the slope suggests that cosmic-ray escape is very important
in low-SFR galaxies.
It has been found that there is a quasi-linear scaling relationship between
the gamma-ray luminosity in GeV energies and the total infrared luminosity of
star-forming galaxies, i.e. $L_{gamma}propto L_{rm IR}^{alpha}$ with
$alphasimeq 1$. However, the origin of this linear slope is not well
understood. Although extreme starburst galaxies can be regarded as calorimeters
for hadronic cosmic ray interaction and thus a quasi-linear scaling may hold,
it may not be the case for low star-formation-rate (SFR) galaxies, as the
majority of cosmic rays in these galaxies are expected to escape. We calculate
the gamma-ray production efficiency in star-forming galaxies by considering
realistic galaxy properties, such as the gas density and galactic wind velocity
in star-forming galaxies. We find that the slope for the relation between
gamma-ray luminosity and the infrared luminosity gets steeper for low infrared
luminosity galaxies, i.e. $alpharightarrow 1.6$, due to increasingly lower
efficiency for the production of gamma-ray emission. We further find that the
measured data of the gamma-ray luminosity is compatible with such a steepening.
The steepening in the slope suggests that cosmic-ray escape is very important
in low-SFR galaxies.
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