Large-scale Interstellar Medium Research Articles

Escaping the maze: a statistical subgrid model for cloud-scale density structures in the interstellar medium

ABSTRACT The interstellar medium (ISM) is a turbulent, highly structured multiphase medium. State-of-the-art cosmological simulations of the formation of galactic discs usually lack the resolution to accurately resolve those multiphase structures. However, small-scale density structures play an important role in the life cycle of the ISM, and determine the fraction of cold, dense gas, the amount of star formation, and the amount of radiation and momentum leakage from cloud-embedded sources. Here, we derive a statistical model to calculate the unresolved small-scale ISM density structure from coarse-grained, volume-averaged quantities such as the gas clumping factor, $\mathcal {C}$, and mean density 〈ρ〉V. Assuming that the large-scale ISM density is statistically isotropic, we derive a relation between the three-dimensional clumping factor, $\mathcal {C}_\rho$, and the clumping factor of the 4$\pi$ column density distribution on the cloud surface, $\mathcal {C}_\Sigma$, and find $\mathcal {C}_\Sigma =\mathcal {C}_\rho ^{2/3}$. Applying our model to calculate the covering fraction, i.e. the 4$\pi$ sky distribution of optically thick sightlines around sources inside interstellar gas clouds, we demonstrate that small-scale density structures lead to significant differences at fixed physical ISM density. Our model predicts that gas clumping increases the covering fraction by up to 30 per cent at low ISM densities compared to a uniform medium. On the other hand, at larger ISM densities, gas clumping suppresses the covering fraction and leads to increased scatter such that covering fractions can span a range from 20 per cent to 100 per cent at fixed ISM density. All data and example code are publicly available at GitHub.

Magnetohydrodynamic Particle-in-cell Simulations of the Cosmic-Ray Streaming Instability: Linear Growth and Quasi-linear Evolution

Abstract The gyro-resonant cosmic-ray (CR) streaming instability is believed to play a crucial role in CR transport, leading to the growth of Alfvén waves at small scales that scatter CRs, and impacts the interaction of CRs with the interstellar medium (ISM) on large scales. However, extreme scale separation (λ ≪ pc), low CR number density (n CR/n ISM ∼ 10−9), and weak CR anisotropy (∼v A/c) pose strong challenges for proper numerical studies of this instability on a microphysical level. Employing the recently developed magnetohydrodynamic particle-in-cell method, which has unique advantages to alleviate these issues, we conduct 1D simulations that quantitatively demonstrate the growth and saturation of the instability in the parameter regime consistent with realistic CR streaming in the large-scale ISM. Our implementation of the δf method dramatically reduces Poisson noise and enables us to accurately capture wave growth over a broad spectrum equally shared between left- and right-handed Alfvén modes. We are also able to accurately follow the quasi-linear diffusion of CRs subsequent to wave growth, which is achieved by employing phase randomization across periodic boundaries. Full isotropization of the CRs in the wave frame requires the pitch angles of most CRs to efficiently cross 90° and can be captured in simulations with relatively high wave amplitude and/or spatial resolution. We attribute this crossing to nonlinear wave–particle interaction (rather than mirror reflection) by investigating individual CR trajectories. We anticipate that our methodology will open up opportunities for future investigations that incorporate additional physics.

A Widespread, Clumpy Starburst in the Isolated Ongoing Dwarf Galaxy Merger dm1647+21

Abstract Interactions between pairs of isolated dwarf galaxies provide a critical window into low-mass hierarchical, gas-dominated galaxy assembly and the build-up of stellar mass in low-metallicity systems. We present the first Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE) optical integral field unit (IFU) observations of the interacting dwarf pair dm1647+21 selected from the TiNy Titans survey. The Hα emission is widespread and corresponds to a total unobscured star formation rate (SFR) of 0.44 M ⊙ yr−1, which is 2.7 times higher than the SFR inferred from Sloan Digital Sky Survey (SDSS) data. The implied specific SFR (sSFR) for the system is elevated by more than an order of magnitude above non-interacting dwarfs in the same mass range. This increase is dominated by the lower-mass galaxy, which has a sSFR enhancement of >50. Examining the spatially resolved maps of classic optical line diagnostics, we find that the interstellar medium (ISM) excitation can be fully explained by star formation. The velocity field of the ionized gas is not consistent with simple rotation. Dynamical simulations indicate that the irregular velocity field and the stellar structure is consistent with the identification of this system as an ongoing interaction between two dwarf galaxies. The widespread, clumpy enhancements in the star formation in this system point to important differences in the effect of mergers on dwarf galaxies, compared to massive galaxies; rather than the funneling of gas to the nucleus and giving rise to a nuclear starburst, starbursts in low-mass galaxy mergers may be triggered by large-scale ISM compression, and thus may be more distributed.