Lyman-$\alpha$ photons through rotating outflows

Abstract

Outflows and rotation are two ubiquitous kinematic features in the gas kinematics of galaxies. Here we introduce a semi-analytic model to quantify how rotating outflows impact the morphology of the Lyman-$\alpha$ emission line. The model is contrasted against Monte Carlo radiative transfer simulations of outflowing gas with additional solid body rotation. We explore a range of neutral Hydrogen optical depth of $10^5\le {\tau }_{\mathrm{H}}\le 10^7$, rotational velocity $0\le v_{\mathrm{rot}}/{\mathrm{km}\mathrm{s}}^{-1}\le 100$ and outflow velocity $0\le v_{\mathrm{out}}/{\mathrm{km}\mathrm{s}}^{-1}\le 50$. We find three consequences of rotation. First, it introduces a dependency with viewing angle; second it broadens the line and third it increases the flux at the line's center. For all viewing angles, the semi-analytic model reproduces the radiative transfer results for the line width and flux change at the line's center within a $7\%$ and $50\%$ precision for an optical depth of ${\tau }_{\mathrm{H}}=10^5$, respectively, and within $2\%$ and $1\%$ for an optical depth of ${\tau }_{\mathrm{H}}=10^7$. Using this model we also show that the peaks of integrated spectra taken from opposite sides of an edge-on rotating gas distribution should have a separation of $\frac{1}{2}{v}_{\mathrm{rot}}$. The semi-analytic model presented here is a convenient tool to introduce rotational kinematics as a post-processing step of idealized Monte Carlo simulations; it provides a framework to interpret \lya spectra in systems where rotation is expected or directly measured through kinematic maps.

9 pages, 7 figures. Submitted to MNRAS

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• Resonant-line radiative transfer within power-law density profiles
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