The evolution of the low-density H i intergalactic medium from z = 3.6 to 0: data, transmitted flux, and H i column density,,

Abstract

We present a new, uniform analysis of the H i transmitted flux (F) and H i column density ($N_{\mathrm{H\,{\small I}}}$) distribution in the low-density IGM as a function of redshift z for 0 < z < 3.6 using 55 HST/COS FUV (Δz = 7.2 at z < 0.5), five HST/STIS + COS NUV (Δz = 1.3 at z ∼ 1) and 24 VLT/UVES, and Keck/HIRES (Δz = 11.6 at 1.7 < z < 3.6) AGN spectra. We performed a consistent, uniform Voigt profile analysis to combine spectra taken with different instruments, to reduce systematics and to remove metal-line contamination. We confirm previously known conclusions on firmer quantitative grounds in particular by improving the measurements at z ∼ 1. Two flux statistics at 0 < F < 1, the mean H i flux and the flux probability distribution function (PDF), show that considerable evolution occurs from z = 3.6 to z = 1.5, after which it slows down to become effectively stable for z < 0.5. However, there are large sightline variations. For the H i column density distribution function (CDDF, f ∝ $N_{\rm H\,{\small I}}^{-\beta }$) at $\log (N_{\mathrm{H\,{\small I}}}/1\, {\mathrm{cm}^{-2}})$ ∈ [13.5, 16.0], β increases as z decreases from β = 1.60 at z ∼ 3.4 to β = 1.82 at z ∼ 0.1. The CDDF shape at lower redshifts can be reproduced by a small amount of clockwise rotation of a higher-z CDDF with a slightly larger CDDF normalization. The absorption line number per z (dn/dz) shows a similar evolutionary break at z ∼ 1.5 as seen in the flux statistics. High-$N_{\mathrm{H\,{\small I}}}$ absorbers evolve more rapidly than low-$N_{\mathrm{H\,{\small I}}}$ absorbers to decrease in number or cross-section with time. The individual dn/dz shows a large scatter at a given z. The scatter increases towards lower z, possibly caused by a stronger clustering at lower z.

Limits on non-canonical heating and turbulence in the intergalactic medium from the low redshift Lyman-alpha forest

“…The photoionisation rate required to match the observed CDDF 3 , Γ CDDF HI , is a factor of 0.4-0.6 times the P19 UV background model value at z = 0.1, suggesting that the P19 model may overproduce the UV background 3 We choose to match the CDDF over a limited range in N HI , rather than matching the mean or distribution of the Lyα forest transmission, as the CDDF is less susceptible to systematic uncertainties associated with signalto-noise and the uncertain properties of high density gas in the CGM. Nevertheless, we note the mean transmission associated with the rescaled CDDF -given in Table 1 -is in excellent agreement with F = 0.983 ± 0.009 at z = 0.08 from Kim et al (2021) at z ∼ 0.1. The simulations with a higher fraction of hot gas, either from additional physics (StrongAGN and Blazar) or from artificially increased photoheating rates (e.g.…”

“…In order to approximately forward model the COS data, all the simulated spectra are convolved with the COS line spread function 1 at 1341 Å, for central wavelength G130M/1327 at lifetime position LP1. The spectra are then rebinned onto pixels of width 0.02991 Å (i.e 3 times the COS binning of 0.00997 Å following Kim et al (2021)) and a Gaussian distributed signal-to-noise of S/N = 30 per 19 km s −1 resolution element (i.e. S/N ∼ 17.7 per pixel) is added.…”

The Low-redshift Lyα Forest as a Constraint for Models of AGN Feedback

“…Our study demonstrates again the key role that COS HST UV spectroscopy plays in our understanding of the low-z IGM. Given the time-sensitive state of HST's remaining mission, a focus on UV absorption now is critical to provide further constraints on the physical state of IGM gas covering the Lyα transition at 0.4 < z < 1.7, where the Lyα forest statistics transition from dark matter/UVB dominated to baryonic feedback/UVB dominated (see also Kim et al 2020). Bolton et al (2022) recently suggested that in order for the simulated and observed diffuse IGM Lyα b distribution to match, unresolved turbulent broadening at the ratio of b turb /b therm = 0.73 is needed.…”

“…However, observing the Lyα forest at z 2 is substantially more challenging than at higher redshifts as it requires state-of-the-art spectrographs above Earth's atmosphere to reach the rest-frame far-ultraviolet (FUV) band (Danforth et al 2016;Gurvich et al 2017;Viel et al 2017;Christiansen et al 2020). Recent observations with the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope have enabled studies of the low-redshift Lyα forest in great statistical detail and with high sensitivity (Meiring et al 2011;Danforth et al 2016;Kim et al 2020). In particular, Danforth et al (2016, henceforth D16) built on the heritage of many previous low-z IGM absorber catalogs from HST/FOS (Bahcall et al 1996;Jannuzi et al 1998;Weymann et al 1998), HST/GHRS (Penton et al 2000), FUSE (Danforth & Shull 2005), and HST/STIS (Penton et al 2004;Lehner et al 2007;Tripp et al 2008;Danforth et al 2010;Tilton et al 2012).…”

HeII Ly$α$ Transmission Spikes and Absorption Troughs in Eight High-resolution Spectra Probing the End of HeII Reionization

“…At redshifts 𝑧 H I < 0.2, corresponding to 𝑧 = 3.0-3.8 for He II, there are on average 5 H I absorbers with log 𝑁 H I = 14-17(Kim et al 2021) per line of sight. Each of these absorbers has an optical depth at the Ly𝛼 line center 𝜏 H I > 1.7 and full width at half maximum ∼ 0.3 Å after accounting for the COS line-spread-function.…”