Three Dimensional Hydrodynamic Simulations of Multiphase Galactic Disks with Star Formation Feedback: II. Synthetic HI 21 cm Line Observations

Abstract

We use three-dimensional numerical hydrodynamic simulations of the turbulent, multiphase atomic interstellar medium (ISM) to construct and analyze synthetic HI 21 cm emission and absorption lines. Our analysis provides detailed tests of 21 cm observables as physical diagnostics of the atomic ISM. In particular, we construct (1) the "observed" spin temperature, $T_{s,obs}(v_{ch})\equiv T_B(v_{ch})/[1-e^{-\tau (v_{ch})}]$, and its optical-depth weighted mean T_s,obs; (2) the absorption-corrected "observed" column density, $N_{H,obs}\propto \int d{v}_{ch}{T}_B(v_{ch})\tau (v_{ch})/[[1-e^{-\tau (v_{ch})}]$; and (3) the "observed" fraction of cold neutral medium (CNM), $f_{c,obs}\equiv T_c/T_{s,obs}$ for T_c the CNM temperature; we compare each observed parameter with true values obtained from line-of-sight (LOS) averages in the simulation. Within individual velocity channels, T_s,obs(v_ch) is within a factor 1.5 of the true value up to $\tau (v_{ch})\approx 10$. As a consequence, N_H,obs and T_s,obs are respectively within 5% and 12% of the true values for 90% and 99% of LOSs. The optically thin approximation significantly underestimates N_H for $\tau >1$. Provided that T_c is constrained, an accurate observational estimate of the CNM mass fraction can be obtained down to 20%. We show that T_s,obs cannot be used to distinguish the relative proportions of warm and thermally-unstable atomic gas, although the presence of thermally-unstable gas can be discerned from 21 cm lines with 200K<$T_{s,obs}(v_{ch})$<1000K. Our mock observations successfully reproduce and explain the observed distribution of the brightness temperature, optical depth, and spin temperature in Roy et al. (2013a). The threshold column density for CNM seen in observations is also reproduced by our mock observations. We explain this observed threshold behavior in terms of vertical equilibrium in the local Milky Way's ISM disk.

34 pages, 12 figures. Accepted for publication in ApJ. For Paper I, see http://arxiv.org/abs/1308.3231

References in corpus (6)

• The VLA Galactic Plane Survey
• The Vertical Structure of the Outer Milky Way HI Disk
• The Spiral Structure of the Outer Milky Way in Hydrogen
• Dark matter in the Milky Way, II. the HI gas distribution as a tracer of the gravitational potential
• Vertical structure of a supernova-driven turbulent magnetized ISM
• Galactic Spiral Shocks with Thermal Instability

Cited by in corpus (13)

• Momentum Injection by Supernovae in the Interstellar Medium
• The KMOS Redshift One Spectroscopic Survey (KROSS): the origin of disk turbulence in z~0.9 star-forming galaxies
• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• Ly-alpha Radiative Transfer: Monte-Carlo Simulation of the Wouthuysen-Field Effect
• Synthetic CO, H2 and HI surveys of the Galactic 2nd Quadrant, and the properties of molecular gas
• Recovering interstellar gas properties with HI spectral lines: A comparison between synthetic spectra and 21-SPONGE
• Radiative Supernova Remnants and Supernova Feedback
• The ACCELERATION programme: I. Cosmology with the redshift drift
• The Nature of Turbulence in the LITTLE THINGS Dwarf Irregular Galaxies
• Neutral hydrogen filaments in interstellar media: Are they physical?
• The MACH HI absorption survey I: Physical conditions of cold atomic gas outside of the Galactic plane