Diffuse Low-Ionization Gas in the Galactic Halo Casts Doubts on $z\simeq 0.03$ WHIM Detections

TL;DR

This study questions previous claims of detecting the Warm Hot Intergalactic Medium at z~0.03, suggesting that those signals are likely due to local galactic low-ionization gas rather than distant intergalactic matter.

Contribution

The paper demonstrates that previous z~0.03 OVII absorption claims are likely misidentifications, providing detailed spectral modeling to refute those detections and refine the expected properties of the WHIM.

Findings

Previous z~0.03 OVII detections are likely misidentified local gas.

Statistical analysis shows no need for z~0.03 WHIM absorption in the spectra.

Upper limits on OVII column density are much lower than earlier reports.

Abstract

In this Letter we demonstrate that the two claims of $z\simeq 0.03$ OVII K$\alpha$ absorption lines from Warm Hot Intergalactic Medium (WHIM) along the lines of sight to the blazars H~2356-309 (Buote et al., 2009; Fang et al., 2010) and Mkn~501 (Ren, Fang \& Buote, 2014) are likely misidentifications of the $z=0$ OII K$\beta$ line produced by a diffuse Low-Ionization Metal Medium in the Galaxy's Interstellar and Circum-Galactic mediums. We perform detailed modeling of all the available high signal-to-noise Chandra LETG and XMM-Newton RGS spectra of H 2356-309 and Mkn 501 and demonstrate that the $z\simeq 0.03$ WHIM absorption along these two sightlines is statistically not required. Our results, however, do not rule out a small contribution from the $z\simeq 0.03$ OVII K$\alpha$ absorber along the line of sight to H~2356-309. In our model the temperature of the putative $z = 0.031$ WHIM filament is T$= 3\times 10^5$ K and the OVII column density is N$_{OV II} \le 4\times 10^{15}$ cm$^{-2}$, twenty times smaller than the OVII column density previously reported, and now more consistent with the expectations from cosmological hydrodynamical simulations.