arXiv subjects

Karlie A. Noon

Publications and source records attributed to Karlie A. Noon.

4 recordsLinked to original sources

arXiv2026

Modelling the non-equilibrium chemistry of the Milky Way's cold nuclear wind

Cold atomic and molecular gas are commonly observed in the winds of both external galaxies and the Milky Way, yet the survival and origin of these cool phases within hot galactic winds is poorly understood. To help gain insight into these problems, we carry out time-dependent chemical modelling of cool clouds in the Milky Way's nuclear wind, which possess unusual molecularto-atomic hydrogen ratios that are inconsistent with both disc values and predictions from chemical equilibrium models. We confirm that CO and Hi emission comparable to that in the observed nuclear wind clouds cannot be produced by gas in chemical equilibrium, but that such conditions can be produced in a molecule-dominated cloud that has had its atomic envelope rapidly removed and has not yet reached a new chemical equilibrium. Clouds in this state harbour large reservoirs of molecular gas and consequently have anomalously large CO-to-H2 conversion factors, suggesting that the masses of the observed clouds may be significantly larger than suggested by earlier analyses assuming disc-like conversions. These findings provide a new framework for interpreting cold gas in galactic winds, providing strong evidence that cold outflows can originate from the galactic disc molecular clouds that survive acceleration into the wind but lose their diffuse atomic envelopes in the process, and suggesting that the Milky Way's nuclear outflow may be more heavily mass-loaded than previously thought.

astro-ph.GA↗

arXiv2026

A survey of molecular clouds in the Galactic center's outflow

The nucleus of the Milky Way is known to drive a large-scale, multiphase galactic outflow, with gas phases ranging from the hot highly-ionized to the cold molecular component. In this work, we present the first systematic search for molecules in the Milky Way wind. We use the Atacama Pathfinder EXperiment (APEX) to observe the 12CO(2-1) emission line in 19 fields centered on previously known high-velocity atomic hydrogen (HI) clouds associated with the outflow. Over 200 CO clumps are detected within 16 different HI clouds. These clumps have typical radii of 1 - 3 parsec, high velocity dispersions of 1 - 6 km/s and molecular gas masses ranging from a few to several hundred solar masses. Molecular clumps in the wind sit on the low-mass end of the mass - size relation of regular molecular clouds, but are far displaced from the mass (or size) - linewidth relation, being generally more turbulent and showing high internal pressures. Nearly 90% of the clumps are gravitationally unbound with virial parameters >> 10 - 100, indicating that these structures are either being disrupted or they must be confined by external pressure from the surrounding hot medium. While the observed properties of CO clumps do not seem to evolve clearly with latitude, we find that molecular gas is not detected in any of the 6 HI clouds with projected distances over 1 kpc from the Galactic Center, suggesting the existence of a maximum timescale of ~ 3 Myr for the dissociation of molecular gas within the wind. Overall, current observations in the Galactic center support a scenario in which a hot wind entrains cold gas clouds from the disk, driving their progressive transformation from molecular to atomic and ultimately ionized gas through stripping, turbulence, and dissociation.

astro-ph.GA↗

arXiv2024

Turbulence statistics of HI clouds entrained in the Milky Way's nuclear wind

The interstellar medium (ISM) is ubiquitously turbulent across many physically distinct environments within the Galaxy. Turbulence is key in controlling the structure and dynamics of the ISM, regulating star formation, and transporting metals within the Galaxy. We present the first observational measurements of turbulence in neutral hydrogen entrained in the hot nuclear wind of the Milky Way. Using recent MeerKAT observations of two extra-planar HI clouds above (gal. lat.$\,\sim7.0^{\circ}$) and below (gal. lat.$\,\sim-3.9^{\circ}$) the Galactic disc, we analyse centroid velocity and column density maps to estimate the velocity dispersion ($σ_{v,\mathrm{3D}}$), the turbulent sonic Mach number ($\mathcal{M}$), the volume density dispersion ($σ_{ρ/ρ_0}$), and the turbulence driving parameter ($b$). We also present a new prescription for estimating the spatial temperature variations of HI in the presence of related molecular gas. We measure these turbulence quantities on the global scale of each cloud, but also spatially map their variation across the plane-of-sky extent of each cloud by using a roving kernel method. We find that the two clouds share very similar characteristics of their internal turbulence, despite their varying latitudes. Both clouds are in the sub-to-trans-sonic Mach regime, and have primarily compressively-driven ($b\sim1$) turbulence. Given that there is no known active star-formation present in these clouds, this may be indicative of the way the cloud-wind interaction injects energy into the entrained atomic material on parsec scales.

astro-ph.GA↗

arXiv2023

Direct observations of the atomic-molecular phase transition in the Milky Way's nuclear wind

Hundreds of high-velocity atomic gas clouds exist above and below the Galactic Centre, with some containing a molecular component. However, the origin of these clouds in the Milky Way's wind is unclear. This paper presents new high-resolution MeerKAT observations of three atomic gas clouds and studies the relationship between the atomic and molecular phases at $\sim 1$ pc scales. The clouds' atomic hydrogen column densities, $N_{\mathrm{HI}}$, are less than a $\mbox{few}\times 10^{20}$ cm$^{-2}$, but the two clouds closest to the Galactic Centre nonetheless have detectable CO emission. This implies the presence of H$_{2}$ at levels of $N_{\mathrm{HI}}$ at least a factor of ten lower than in the typical Galactic interstellar medium. For the cloud closest to the Galactic Centre, there is little correlation between the $N_{\mathrm{HI}}$ and the probability that it will harbour detectable CO emissions. In contrast, for the intermediate cloud, detectable CO is heavily biased toward the highest values of $N_{\mathrm{HI}}$. The cloud most distant from the Galactic Centre has no detectable CO at similar $N_{\mathrm{HI}}$ values. Moreover, we find that the two clouds with detectable CO are too molecule-rich to be in chemical equilibrium, given the depths of their atomic shielding layers, which suggests a scenario whereby these clouds consist of pre-existing molecular gas from the disc that the Galactic wind has swept up, and that is dissociating into atomic hydrogen as it flows away from the Galaxy. We estimate that entrained molecular material of this type has a $\sim \mathrm{few}-10$ Myr lifetime before photodissociating.

astro-ph.GA↗