The far-UV Interstellar Radiation Field in Galactic Disks: Numerical and Analytic Models. (arXiv:2008.00009v1 [astro-ph.GA])

The far-UV Interstellar Radiation Field in Galactic Disks: Numerical and Analytic Models. (arXiv:2008.00009v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bialy_S/0/1/0/all/0/1">Shmuel Bialy</a>

The intensity of the far-ultraviolet (FUV; 6-13.6 eV) interstellar radiation
field (ISRF) in galaxies determines the thermal and chemical evolution of the
neutral interstellar gas and is key for interpreting extragalactic observations
and for theories of star-formation. We run a series of galactic disk models and
derive the FUV ISRF intensity as a function of the dust-to-gas ratio,
star-formation rate density, gas density, scale radius, and observer position.
We develop an analytic formula for the median FUV ISRF flux. We identify two
dimensionless numbers in the problem: the inter-source dust opacity,
$tau_{star}$, which measures the importance of dust absorption, and the
galactic contrast, $X$, which is proportional to the galaxy disk size. These
parameters encapsulate the dependence on all of the physical parameters. We
find that there exists a critical $tau_{rm star, crit}$, or equivalently a
critical dust-to-gas ratio, $Z_{d,{rm crit}}’ approx 0.01-0.1$ the Milky Way
value, at which the ISRF changes behavior. For $Z’_d>Z_{d,{rm crit}}’$ the
ISRF is limited by dust absorption. With decreasing $Z’_d$, the ISRF intensity
increases as more sources contribute to the flux. For $Z’_d < Z_{d,{rm
crit}}’$ the ISRF saturates as the disk becomes optically thin. We find that
the ISRF per star-formation rate density in low metallicity systems, such as
dwarf and high redshift galaxies, is higher by up to a factor of 3-6 compared
to their Milky-Way counterparts. We discuss implications to the potential
mechanisms that regulate star-formation in low metallicity galaxies.

The intensity of the far-ultraviolet (FUV; 6-13.6 eV) interstellar radiation
field (ISRF) in galaxies determines the thermal and chemical evolution of the
neutral interstellar gas and is key for interpreting extragalactic observations
and for theories of star-formation. We run a series of galactic disk models and
derive the FUV ISRF intensity as a function of the dust-to-gas ratio,
star-formation rate density, gas density, scale radius, and observer position.
We develop an analytic formula for the median FUV ISRF flux. We identify two
dimensionless numbers in the problem: the inter-source dust opacity,
$tau_{star}$, which measures the importance of dust absorption, and the
galactic contrast, $X$, which is proportional to the galaxy disk size. These
parameters encapsulate the dependence on all of the physical parameters. We
find that there exists a critical $tau_{rm star, crit}$, or equivalently a
critical dust-to-gas ratio, $Z_{d,{rm crit}}’ approx 0.01-0.1$ the Milky Way
value, at which the ISRF changes behavior. For $Z’_d>Z_{d,{rm crit}}’$ the
ISRF is limited by dust absorption. With decreasing $Z’_d$, the ISRF intensity
increases as more sources contribute to the flux. For $Z’_d < Z_{d,{rm
crit}}’$ the ISRF saturates as the disk becomes optically thin. We find that
the ISRF per star-formation rate density in low metallicity systems, such as
dwarf and high redshift galaxies, is higher by up to a factor of 3-6 compared
to their Milky-Way counterparts. We discuss implications to the potential
mechanisms that regulate star-formation in low metallicity galaxies.

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