arXiv subjects

Robin Shelton

Publications and source records attributed to Robin Shelton.

4 recordsLinked to original sources

arXiv2025

Demystifying Strange O VI Line Widths with Hydrodynamic Simulations

We present a survey of O VI line widths obtained from hydrodynamic simulations that model the mixing between High Velocity Clouds and the circumgalactic medium and track the non-equilibrium ionization populations of the ions. We run 10 simulations with various physical conditions of the clouds and ambient environments, so that our results can be compared to observations of various cloud environments. Synthetic spectra are created for the simulated sight lines that contain O VI. The range of our Doppler broadening parameter, $b$, is from $\sim$6 km s$^{-1}$ to $\sim$107 km s$^{-1}$. We calculate the thermal and non-thermal contributions to the $b$ values. Both $b$ and the thermal contribution to $b$ can be substantially less than the CIE value. These narrow line widths are due to the time delay in recombination from O VI to O V in mixing regions, which results in substantial amounts of O VI at temperatures below the 2.9 $\times$ 10$^{5}$ K CIE temperature. Our results show that non equilibrium collisional ionization and mixing can produce the narrow line widths that are seen in multiple observations.

astro-ph.GA↗

arXiv2024

The Observed O VI is Just the Tip of the Iceberg: Estimating the Hidden Material in Circumgalactic and Intergalactic Clouds

This paper proposes a new method for estimating the total quantity of material in moving circumgalactic and intergalactic clouds from O VI measurements. We simulate high-velocity clouds (HVCs) with the FLASH hydrodynamic code and track the ionization and recombination of all ionization levels of oxygen as a function of time. We calculate the O VI/oxygen ratio ($f_{\rm O VI}$) in our dynamic NEI clouds, finding that it differs significantly from that in static gas. We find that O VI exists in cool, medium, and hot gas in the clouds. As such, it traces all of the hydrogen rather than merely the ionized hydrogen. The total quantity of hydrogen along a typical observed line of sight through a cloud can be estimated from the observed O VI column density, metallicity, and our $f_{\rm O VI}$. We provide the simulations' $f_{\rm O VI}$, a prescription for finding $f_{\rm O VI}$ for observed dynamic clouds, and a methodology for calculating the total hydrogen column density from this $f_{\rm O VI}$ and an observed O VI column density. As examples, we use our $f_{\rm O VI}$ to estimate the total hydrogen column densities along various observed sight lines through two HVCs, Complex C and the Magellanic Stream, finding that these clouds contain more material than the previous lower limits. We also extend this analysis to {low-redshift} intergalactic O VI clouds, finding that they contain several times more baryonic material than previously thought and therefore may account for a significant fraction of the Universe's baryons.

astro-ph.GA↗

arXiv2018

An Intermediate Velocity HI Cloud Falling to the Galactic Disk; Possible Evidence for Low Metallicity HI Gas Originated Outside the Galactic Disk

We found that an intermediate velocity cloud (IVC) IVC 86-36 in HI 21 cm emission shows a head-tail distribution toward the Galactic plane with marked parallel filamentary streamers, which is extended over 40 degrees in the sky. The distance of IVC 86-36 is constrained to be less than ~3 kpc from absorption of a background star as determined from opticalspectroscopy. There is a bridge feature in velocity between the IVC and the local ISM with velocity separation of ~50 km s-1, which may indicate dynamical interaction of the IVC with the disk. If the interaction is correct, the distance estimate d of the IVC ranges from 200 pc to 3 kpc, and the mass of the IVC head is estimated to be 7X103(d/1kpc)2Msol. The IVC shares similar properties to the Smith cloud located at 12 kpc, including the head-tail distribution, streamers, and bridge feature, while the mass of the IVC is less than ~0.1 of the Smith cloud. A comparison between the Hi and the Planck/IRAS dust emission indicates that the dust emission of IVC 86-36 is not detectable in spite of its HI column density of 2X10^20 cm-2, indicating low metalicity of IVC 86-36 by a factor of ~< 0.2 as compared with the solar neighbor. We conclude that IVC 86-38 is an infalling cloud which likely originated in the low-metallicity environment of the Galactic halo or the Magellanic system.

astro-ph.GA↗

arXiv2000

Far Ultraviolet Spectroscopic Explorer Observations of the Supernova Remnant N49 in the Large Magellanic Cloud

We report a Far Ultraviolet Spectroscopic Explorer satellite observation of the supernova remnant N49 in the Large Magellanic Cloud, covering the 905 -- 1187 A spectral region. A 30'' square aperture was used, resulting in a velocity resolution of ~100 km/s. The purpose of the observation was to examine several bright emission lines expected from earlier work and to demonstrate diffuse source sensitivity by searching for faint lines never seen previously in extragalactic supernova remnant UV spectra. Both goals were accomplished. Strong emission lines of O VI 1031.9 A, 1037.6 A and C III 977.0 A were seen, Doppler broadened to +/- 225 km/s and with centroids red-shifted to 350 km/s, consistent with the LMC. Superimposed on the emission lines are absorptions by C III and O VI 1031.9 at +260 km/s, which are attributed to warm and hot gas (respectively) in the LMC. The O VI 1037.6 A line is more severely affected by overlying interstellar and H2 absorption from both the LMC and our galaxy. N III 989.8 A is not seen, but models indicate overlying absorption severely attenuates this line. A number of faint lines from hot gas have also been detected, many of which have never been seen in an extragalactic supernova remnant spectrum.

astro-ph↗