The origin of the H$alpha$ line profiles in simulated disc galaxies. (arXiv:2401.04160v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ejdetjarn_T/0/1/0/all/0/1">Timmy Ejdetj&#xe4;rn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Agertz_O/0/1/0/all/0/1">Oscar Agertz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ostlin_G/0/1/0/all/0/1">G&#xf6;ran &#xd6;stlin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rey_M/0/1/0/all/0/1">Martin P. Rey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Renaud_F/0/1/0/all/0/1">Florent Renaud</a>

Observations of ionised H$alpha$ gas in disc galaxies with high star
formation rates have ubiquitous and significant line broadening with widths
$sigma_{rm Halpha}gtrsim 50-100 {rm km s^{-1}}$. To understand whether
this broadening reflects gas turbulence within the interstellar medium (ISM) of
galactic discs, or arises from off-the-plane emission in mass-loaded galactic
winds, we perform radiation hydrodynamic (RHD) simulations of isolated Milky
Way-mass disc galaxies in a gas-poor (low-redshift) and gas rich
(high-redshift) condition and create mock H$alpha$ emission line profiles. We
find that the vast majority of the ${rm Halpha}$ emission is confined within
the ISM, with extraplanar gas contributing mainly to the extended profile
wings. This substantiates the Halpha emission line as a tracer of mid-plane
disc dynamics. We investigate the relative contribution of diffuse and dense
${rm Halpha}$ emitting gas, corresponding to DIG ($rho lesssim 0.1 {rm
cm^{-3}}$, $Tsim 8 000 {rm K}$) and HII regions ($rho gtrsim 10 {rm
cm^{-3}}$, $Tsim 10 000 {rm K}$), respectively, and find that DIG
contributes $lesssim 10 %$ of the total ${rm L}_{rm Halpha}$. However, the
DIG can reach upwards of $sigma_{rm Halpha} sim 60-80 {rm km s^{-1}}$
while the HII regions are much less turbulent $sigma_{rm Halpha}sim10-40
{rm km s^{-1}}$. This implies that the $sigma_{rm Halpha}$ observed using
the full ${rm Halpha}$ emission line is dependent on the relative ${rm
Halpha}$ contribution from DIG/HII regions and a larger $f_{rm DIG}$ would
shift $sigma_{rm Halpha}$ to higher values. Finally, we show that
$sigma_{rm Halpha}$ evolves, in both the DIG and HII regions, with the
galaxy gas fraction. Our high-redshift equivalent galaxy is roughly twice as
turbulent, except for in the DIG which has a more shallow evolution.

Observations of ionised H$alpha$ gas in disc galaxies with high star
formation rates have ubiquitous and significant line broadening with widths
$sigma_{rm Halpha}gtrsim 50-100 {rm km s^{-1}}$. To understand whether
this broadening reflects gas turbulence within the interstellar medium (ISM) of
galactic discs, or arises from off-the-plane emission in mass-loaded galactic
winds, we perform radiation hydrodynamic (RHD) simulations of isolated Milky
Way-mass disc galaxies in a gas-poor (low-redshift) and gas rich
(high-redshift) condition and create mock H$alpha$ emission line profiles. We
find that the vast majority of the ${rm Halpha}$ emission is confined within
the ISM, with extraplanar gas contributing mainly to the extended profile
wings. This substantiates the Halpha emission line as a tracer of mid-plane
disc dynamics. We investigate the relative contribution of diffuse and dense
${rm Halpha}$ emitting gas, corresponding to DIG ($rho lesssim 0.1 {rm
cm^{-3}}$, $Tsim 8 000 {rm K}$) and HII regions ($rho gtrsim 10 {rm
cm^{-3}}$, $Tsim 10 000 {rm K}$), respectively, and find that DIG
contributes $lesssim 10 %$ of the total ${rm L}_{rm Halpha}$. However, the
DIG can reach upwards of $sigma_{rm Halpha} sim 60-80 {rm km s^{-1}}$
while the HII regions are much less turbulent $sigma_{rm Halpha}sim10-40
{rm km s^{-1}}$. This implies that the $sigma_{rm Halpha}$ observed using
the full ${rm Halpha}$ emission line is dependent on the relative ${rm
Halpha}$ contribution from DIG/HII regions and a larger $f_{rm DIG}$ would
shift $sigma_{rm Halpha}$ to higher values. Finally, we show that
$sigma_{rm Halpha}$ evolves, in both the DIG and HII regions, with the
galaxy gas fraction. Our high-redshift equivalent galaxy is roughly twice as
turbulent, except for in the DIG which has a more shallow evolution.

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