[Paper Review] The Infuence of Omega_baryon on High-Redshift Structure

Jeffrey P. Gardner, Neal Katz+2 more|Digital Access to Scholarship at Harvard (DASH) (Harvard University)|Jul 16, 2001

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses hydrodynamic simulations to isolate the effects of varying the baryon density parameter $\Omega_{\rm baryon}$ on high-redshift structure formation. It finds that higher $\Omega_{\rm baryon}$ enhances gas cooling, increasing condensed gas and stellar masses in galaxies, star formation rates, and high-column-density absorption (DLA, Lyman limit), while the Lyman-alpha forest remains insensitive to $\Omega_{\rm baryon}$ when the UV background is adjusted to match observed opacity.

ABSTRACT

We analyze high-redshift structure in three hydrodynamic simulations that have identical initial conditions and cosmological parameters and differ only in the value of the baryon density parameter, Omega_b=0.02, 0.05, 0.125. Increasing Omega_b does not change the fraction of baryons in the diffuse (unshocked) phase of the intergalactic medium, but it increases cooling rates and therefore transfers some baryons from the shocked intergalactic phase to the condensed phase associated with galaxies. Predictions of Lyman-alpha forest absorption are almost unaffected by changes of Omega_b provided that the UV background intensity is adjusted so that the mean opacity of the forest matches the observed value. The required UV background intensity scales as Omega_b^1.7, and the higher photoionization rate increases the gas temperature in low density regions. Damped Lyman-alpha absorption and Lyman limit absorption both increase with increasing Omega_b, though the impact is stronger for damped absorption and is weaker at z=4 than at z=2-3. The mass of cold gas and stars in high-redshift galaxies increases faster than Omega_b but slower than Omega_b^2, and the global star formation rate scales approximately as Omega_b^1.5. In the higher Omega_b models, the fraction of baryonic material within the virial radius of dark matter halos is usually higher than the universal fraction, indicating that gas dynamics and cooling can lead to over-representation of baryons in virialized systems. On the whole, our results imply a fairly intuitive picture of the influence of Omega_b on high-redshift structure, and we provide scalings that can be used to estimate the impact of Omega_b uncertainties on the predictions of hydrodynamic simulations.

Motivation & Objective

• To isolate the impact of $\Omega_{\rm baryon}$ on high-redshift cosmic structure formation by varying only this parameter in identical cosmological simulations.
• To quantify how changes in $\Omega_{\rm baryon}$ affect the phase structure of the intergalactic medium (diffuse, shocked, condensed phases).
• To assess the sensitivity of observable diagnostics—Lyman-alpha forest, damped Lyman-alpha absorption, Lyman limit systems, and star formation rates—to variations in $\Omega_{\rm baryon}$.
• To derive scaling laws for key astrophysical quantities as functions of $\Omega_{\rm baryon}$ to estimate uncertainties in hydrodynamic simulations due to $\Omega_{\rm baryon}$ uncertainty.
• To evaluate whether the Lyman-alpha forest remains a robust probe of cosmology when $\Omega_{\rm baryon}$ varies, provided the UV background is adjusted to match observed mean opacity.

Proposed method

• Performs three hydrodynamic simulations with identical initial conditions and cosmological parameters, differing only in $\Omega_{\rm baryon}$: 0.02, 0.05, and 0.125.
• Uses a standard cold dark matter model with $\Omega_m = 1$, $h = 0.5$, $\sigma_8 = 0.7$, and a 11.11 $h^{-1}$ Mpc periodic box with $64^3$ gas and dark matter particles.
• Applies gravitational softening of 5 $h^{-1}$ comoving kpc and includes radiative cooling, heating from a UV background, and star formation with feedback.
• Analyzes the phase structure of the intergalactic medium by classifying gas into diffuse (unshocked), shocked (hot IGM), and condensed (galaxy-associated) phases based on density and temperature.
• Synthesizes Lyman-alpha forest, damped Lyman-alpha, and Lyman limit absorption spectra from the simulations and compares them across $\Omega_{\rm baryon}$ values.
• Adjusts the UV background intensity across simulations to match the observed mean Lyman-alpha forest opacity, enabling a fair comparison of absorption features.

Experimental results

Research questions

• RQ1 How does increasing $\Omega_{\rm baryon}$ affect the phase distribution of baryons in the intergalactic medium at high redshift?
• RQ2 How sensitive are predictions of the Lyman-alpha forest to changes in $\Omega_{\rm baryon}$ when the UV background is tuned to match the observed mean opacity?
• RQ3 How does $\Omega_{\rm baryon}$ influence the mass of cold gas and stars in high-redshift galaxies, and what is the scaling of these masses with $\Omega_{\rm baryon}$?
• RQ4 How does $\Omega_{\rm baryon}$ affect the incidence of damped Lyman-alpha and Lyman limit absorption systems, and does this dependence vary with redshift?
• RQ5 To what extent do dark matter halos in higher $\Omega_{\rm baryon}$ models over-represent baryons compared to the universal fraction?

Key findings

• The fraction of baryons in the diffuse (unshocked) phase of the intergalactic medium is largely unaffected by changes in $\Omega_{\rm baryon}$.
• Higher $\Omega_{\rm baryon}$ increases cooling rates, shifting gas from the shocked IGM phase to the condensed phase associated with galaxies.
• The required UV background intensity to match the observed mean Lyman-alpha forest opacity scales approximately as $\Omega_{\rm baryon}^{1.7}$, with higher photoionization rates increasing gas temperature in low-density regions.
• Damped Lyman-alpha absorption increases more strongly with $\Omega_{\rm baryon}$ than Lyman limit absorption, and the dependence is stronger at $z=2-3$ than at $z=4$.
• The mass of cold gas and stars in high-redshift galaxies scales superlinearly with $\Omega_{\rm baryon}$, with a power-law index between 1 and 2, and the global star formation rate scales approximately as $\Omega_{\rm baryon}^{1.5}$.
• In higher $\Omega_{\rm baryon}$ models, the fraction of baryons within dark matter halo virial radii exceeds the universal fraction, indicating over-representation of baryons in virialized systems due to cooling and gas dynamics.