Spatial-Kinematic Absorption Models of the Circumgalactic Medium. II. Ionized Gas Phases and Absorption Lines

TL;DR

This work generalizes spatial-kinematic absorption models (SKAMs) for the circumgalactic medium by adding a detailed, parameterized treatment of gas phases and ionization to the four-structure CGM geometry (halo, disk/EPG, bi-polar wind, and extended planar accretion). It combines a rigorous geometric set-up with analytic kinematic prescriptions and Cloudy-based ionization calculations to produce HI and metal-line absorption profiles along arbitrary quasar sightlines, enabling forward modeling of complex multiphase absorbers. The paper provides fiducial parameterizations, illustrative examples, and a public Fortran 95 code with a GUI prototype to explore viewing angles, impact parameters, and the contributions of each CGM component to observed spectra. This framework offers a versatile, teaching-friendly platform for interpreting quasar absorption lines, testing baryon-cycle scenarios, and guiding future multiphase and clumpy extensions. The work highlights both the practical utility of SKAMs and the need for cautious treatment of ionization physics and self-consistency with global galaxy/CGM dynamics.

Abstract

In this two-paper series, we present a straightforward mathematical model for synthesizing quasar absorption line profiles from sight lines through idealized, spatial-kinematic models of the circumgalactic medium (CGM) and their host galaxies. In Paper I, we developed the spatial components of the galaxy/CGM structures (disk, halo, wind, accretion) and their 3D velocity fields. We derived the formalism for arbitrary observed orientation of the galaxy/CGM model and quasar line of sight positioning. In this paper, following a brief review of Paper I, we present the formalism for populating the galaxy/CGM structures with multiphase photoionized and collisionally ionized gas and for generating HI and metal-line absorption profiles. Example absorption line systems through a fiducial galaxy/CGM model are presented. These flexible spatial-kinematic absorption models (SKAMs) can be directly applied to and/or easily modified/expanded for studying individual or ensembles of observed absorption line systems, for exploring various competing theoretical scenarios of the baryon cycle as studied through quasar absorption line systems, and/or serving as pedagogical tools for developing physical intuition. We briefly describe a SKAM GUI that is in early stages of development.

Figures (10)

• Figure 1: Schematic of the baryon cycle in the CGM of a typical spiral galaxy. The prominent processes occurring throughout the CGM are outflowing winds directed along the galactic poles and accreting IGM/filaments converging into an extended planar region. Some fraction of the outflowing stellar-driven wind, a result of stellar feedback, can escape the galaxy, whereas some fraction can be recycling back into the accreting inflow. (original image credit: Zheng Cai, Tsinghua University; annotations added)
• Figure 2: The galaxy coordinate system $G$$(x,y,z)$ and the observer coordinate system $O$$(x_o,y_o,z_o)$. The LOS is parallel to the $x_o$ axis and has unit vector ${\hat{\hbox{\bf s}}}= - {\hat{\hbox{\bf e}}_{x_o}}$. In $O$, the LOS intersects the sky plane at $P_1 = P_{\hbox{\tiny O}}(0,R_\perp \cos \gamma, R_\perp \sin \gamma)= P_{\hbox{\tiny G}}(X_0,Y_0,Z_0)$, where $R_\perp$ is the impact parameter and $\gamma$ is the sky position angle. The observer is positioned at $P_{\hbox{\tiny O}}(+\infty, R_\perp \cos \gamma, R_\perp \sin \gamma)$, while the quasar is at $P_{\hbox{\tiny O}}(-\infty, R_\perp \cos \gamma, R_\perp \sin \gamma)$.
• Figure 3: (a) A cross-sectional schematic (not to scale) of the galaxy/CGM structures in the galaxy frame, $G$. The spherical "halo" has radius $R_{\hbox{\tiny CGM}}$. The hyperboloidal wind has opening angle $\Theta_w$, base radius $\rho_{w,0}$, and maximum extent $R_w$. The cylindrical disk has axial radius $\rho_d$ and height $h_d$. The planar accretion fills the void of the hyperbola with accretion radius $\rho_{a,0}$, maximum extent $R_a$, opening angle complement $\Theta_a$. (b, c) A schematic of a spatial-kinematic model and LOS in the observer frame, $O$. The galaxy is obliquely inclined with $\alpha,\beta = 65^\circ, 60^\circ$, which yields inclination $i=68^\circ$. The quasar is placed at $R_\perp = 50$ kpc with position angle $\gamma = 65^\circ$, which yields $\Phi = 57^\circ$. The wind parameters are $\rho_{w,0}=10$ kpc, $\Theta_w=40^\circ$, and $R_w=R_{\rm vir}$, where $R_{\rm vir}= 200$ kpc. The accretion parameters are $\rho_{a,0}= 23$ kpc, $\Theta_w=20^\circ$, and $R_a=R_{\rm vir}$. The disk parameters are $\rho_d=25$ kpc, and $h_d=5$ kpc. (left) The sky plane showing the observer perspective. (right) The side view showing where LOS is probing the wind and accretion structure (thicker portion of the LOS).
• Figure 4: (a) The Disk/EPG kinematic model with exponential scale height governing lagging halo kinematics. The gray shading corresponds to the range of observed $dV_\upphi/dz$. (b) The cross section of a wind kinematic model for hyperboloid velocity trajectories, illustrate as arrows. (c) The accretion kinematic model of Keplerian infall from infinity. The illustrated $z$ component is the maximum, $|V_z(r,z_{\rm max})|$. (top) For $e=1$ describing the parabolic trajectory. (bottom) For $e=2$ describing a hyperbolic trajectory.
• Figure 5: Examples of enhanced wind models with the highest wind speeds along the wind axis ($\rho/\rho_w(z)=0$) and the slowest speeds on the wind surface ($\rho/\rho_w(z)=1$). (a) Gaussian enhanced wind profiles. (b) Even powered polynomial wind profiles. (c) Truncated cosine wind profiles (that become Gaussian-like as $p$ is increased. (d) A truncated cosine model with $p=1$ and scaling $w$ that emulates a wind wall of thickness $w\rho_w(z)$