Implications of an updated ultraviolet background for the ionization mechanisms of intervening Ne VIII absorbers

Abstract

Ne VIII absorbers seen in QSO spectra are useful tracers of warm ionized gas, when collisional ionization is the dominant ionization process. While photoionization by the ultraviolet background (UVB) is a viable option, it tends to predict large line-of-sight thickness for the absorbing gas. Here, we study the implications of the recently updated UVB at low-z to understand the ionization mechanisms of intervening Ne VIII absorbers. With the updated UVB, one typically needs higher density and metallicity to reproduce the observed ionic column densities under photoionization. Both reduce the inferred line-of-sight thicknesses of the absorbers. We find a critical density of $\ge 5\times 10^{-5}$ cm${}^{-3}$ above which the observed N(Ne VIII)/N(O VI) can be reproduced by pure collisional processes. If the gas is of near solar metallicity (as measured for the low ions) then the cooling timescales will be small (<$10^8$ yrs). Therefore, a continuous injection of heat is required in order to enhance the detectability of the collisionally ionized gas. Using photoionization models we find that in almost all Ne VIII systems the inferred low ion metallicity is near solar or supersolar. If we assume the Ne VIII phase to have similar metallicities then photoionization can reproduce the observed N(Ne VIII)/N(O VI) without the line-of-sight thickness being unreasonably large and avoids cooling issues related to the collisional ionization at these metallicities. However the indication of broad Ly$\alpha$ absorption in a couple of systems, if true, suggests that the Ne VIII phase is distinct from the low ion phase having much lower metallicity.

11 pages, 5 figures, 2 tables. Accepted for publication in MNRAS

References in corpus (14)

• The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium
• The 2013 Release of Cloudy
• Time-Dependent Ionization in Radiatively Cooling Gas
• A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers
• The Low-z Intergalactic Medium. III. HI and Metal Absorbers at z<0.4
• Deuterium Abundance in the Most Metal-Poor Damped Lyman alpha System: Converging on Omega_baryons
• HST-COS Observations of AGN. II. Extended Survey of Ultraviolet Composite Spectra from 159 Active Galactic Nuclei
• The Photon Underproduction Crisis
• Missing Baryons and the Warm-Hot Intergalactic Medium
• A STIS Survey for OVI Absorption Systems at 0.12 < z < 0.5 II.: Physical Conditions of the Ionised Gas
• VoIgt profile Parameter Estimation Routine (VIPER): H I photoionization rate at z<0.5
• Confirming the Detection of an Intergalactic X-ray Absorber Toward PKS 2155-304
• HST/COS detection of a Ne VIII absorber towards PG 1407+265: An unambiguous tracer of collisionally ionized hot gas?
• Constraining the baryon fraction in the Warm Hot Intergalactic Medium at low redshifts with PLANCK data

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• Constraints on QSO emissivity using H I and He II Lyman alpha forest
• The Mass and Absorption Columns of Galactic Gaseous Halos
• The Cosmic Ultraviolet Baryon Survey (CUBS) V: On the Thermodynamic Properties of the Cool Circumgalactic Medium at $z<1$
• COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z < 0.3
• Understanding the strong intervening OVI absorber at z${}_{abs}$ ~0.93 towards PG1206+459
• Detection of Two Intervening Ne VIII Absorbers Probing Warm Gas at z~0.6
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• Role of ionizing background on the statistics of metal absorbers in hydrodynamical simulations
• Multicomponent H2 in DLA at zabs = 2.05: physical conditions through observations and numerical models
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• Role of ionizing background on the non-thermal broadening inferred for the aligned absorbers
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• Physical conditions and redshift evolution of optically thin C III absorbers: Low-z sample