Keeping the Universe ionized: photoheating and the clumping factor of the high-redshift intergalactic medium

By A. Pawlik, Joop Schaye, E. Scherpenzeel. 2008

222 citations recorded by the source corpus.

The critical star formation rate (SFR) density required to keep the intergalactic hydro- gen ionized depends crucially on the average rate of recombinations in the intergalac- tic medium (IGM). This rate is proportional to the clumping factor C ≡� ρ 2� IGM/�ρ b� 2 , where ρb and �ρ bare the local and cosmic mean baryon density, respectively, and the brackets �� IGM indicate spatial averaging over the recombining gas in the IGM. We per- form a suite of cosmological smoothed particle hydrodynamic simulations that include ra- diative cooling to calculate the volume-weighted clumping factor of the IGM at redshifts z ≥ 6. We focus on the effect of photoionization heating by a uniform ultraviolet background and find that photoheating strongly reduces the clumping factor because the increased pressure support smoothes out small-scale density fluctuations. Photoionization heating is often said to provide a negative feedback on the re-ionization of the IGM because it suppresses the cosmic SFR by boiling the gas out of low-mass haloes. However, because of the reduction of the clumping factor it also makes it easier to keep the IGM ionized. Photoheating therefore also provides a positive feedback which, while known to exist, has received much less attention. We demonstrate that this positive feedback is in fact very strong. Using conservative assumptions, we find that if the IGM was reheated at z 9, the observed population of star-forming galaxies at z ≈ 6 may be sufficient to keep the IGM ionized, provided that the fraction of ionizing photons that escape the star-forming regions to ionize the IGM is larger than ∼0.2.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/db758cbf7d79c3a46a4915e5a964321d8ee1ea1b

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Citations in the corpus

REIONIZATION AFTER PLANCK: THE DERIVED GROWTH OF THE COSMIC IONIZING EMISSIVITY NOW MATCHES THE GROWTH OF THE GALAXY UV LUMINOSITY DENSITY (2015, background): “8 In Figure 3, evolution in the ionizing emissivity is presented, allowing for different values of the clumping factor CH II and also supposing that the clumping factor evolves in redshift as z1 43 1.71+ - as found by Pawlik et al. (2009) in sophisticated hydrodynamical simulations.”
NEW CONSTRAINTS ON COSMIC REIONIZATION FROM THE 2012 HUBBLE ULTRA DEEP FIELD CAMPAIGN (2013, background): “Simulations suggest that the clumping factor of IGM gas is CHII ≈ 1− 6 at the redshifts of interest (e.g., Sokasian et al. 2003; Iliev et al. 2006; Pawlik et al. 2009; Shull et al. 2012; Finlator et al. 2012).”
Inhomogeneous recombinations during cosmic reionization (2014, methodology): “Although this is not exactly the case (Pawlik et al. 2009), this assumption is relatively safe since the response time-scale is shorter than the typical extension of inhomogeneous reionization.”
THE EVOLUTION OF THE GALAXY REST-FRAME ULTRAVIOLET LUMINOSITY FUNCTION OVER THE FIRST TWO BILLION YEARS (2014, background): “The clumping factor is primarily constrained theoretically, but most studies agree that it is low (<6) at high redshift (e.g., Faucher-Giguère et al. 2008; Pawlik et al. 2009; McQuinn et al. 2011; Finlator et al. 2012).”
New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization (2013, background): “This effect, known as “Jeans smoothing”, is particularly significant during hydrogen reionization, when the temperature of the gas is increased by orders of magnitude (e.g., Pawlik et al. 2009).”
21 cm cosmology in the 21st century (2011, background): “consistently perform a full hydrodynamical simulation of reionization, since thermal feedback can modify the gas density distribution [47].”
Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionization (2016, methodology): “…it is desirable to measure the escape fraction at a radius large enough that it can be combined with estimates of the clumping factor from large-scale simulations (e.g., Pawlik et al. 2009; Finlator et al. 2012; Shull et al. 2012; So et al. 2014) to study the reionisation history of the universe.”
THE STELLAR MASS DENSITY AND SPECIFIC STAR FORMATION RATE OF THE UNIVERSE AT z ∼ 7 (2009, unknown): “…becomes: ρ crit SFR(z) ≈ 0.04 fesc ( 1 + z 8 )3 ( C 30 )( Ωb h 2 70 0.0463 )2 M⊙ yr −1 Mpc−3 (1) At z = 7, for an escape fraction fesc = 0.1 (e.g. Shapley et al. 2006), and for a clumping factor C = 30 (but see Pawlik et al. 2009 who suggest C ∼ 6) the value of of ρcritSFR = 0.4 M⊙ yr −1 Mpc−3.”
Concordance models of reionization: implications for faint galaxies and escape fraction evolution (2012, background): “Although some ambiguity is inherent in this definition, recent studies suggest that values CHII = 1−3 are appropriate during the epoch of reionization (e.g., Pawlik et al. 2009; Shull et al. 2011; McQuinn et al. 2011).”
THE GALAXY LUMINOSITY FUNCTION DURING THE REIONIZATION EPOCH (2010, methodology): “We adopt fesc & 0.2, consistent with the very blue UV slope of small z = 7 galaxies (Bouwens et al. 2010b), and C ∼ 5 (Bolton & Haehnelt 2007; Pawlik et al. 2009).”
Cosmic Dawn (CoDa): the first radiation-hydrodynamics simulation of reionization and galaxy formation in the Local Universe (2015, background): “First of all, the temperature map shows that gas filaments after reionization (or after reionization is completed locally) are rather thick, as also seen for instance in Pawlik et al. (2009), although using a uniform background, and in Wise et al. (2014).”
ARE ULTRA-FAINT GALAXIES AT z = 6–8 RESPONSIBLE FOR COSMIC REIONIZATION? COMBINED CONSTRAINTS FROM THE HUBBLE FRONTIER FIELDS CLUSTERS AND PARALLELS (2015, background): “Assuming standard values for the ionizing conversion factor ξion and the clumping factor CHii = 3, the ionizing budget of galaxies would be sufficient to maintain the IGM ionized by z ∼ 7 provided the ionizing escape fraction from galaxies is greater than 10%.”
ON THE STELLAR POPULATIONS AND EVOLUTION OF STAR-FORMING GALAXIES AT 6.3 < z ⩽ 8.6 (2009, background): “Following Madau et al. (1999) and Pawlik et al. (2009), we find the required luminosity density at 1500 Å needed to maintain an ionized universe is ρUV = 1.25×1025 ǫ53−1 ( 1+ z 8 )3( Ωbh270 0.0463 )2 C fesc , (5) in units of erg s−1 Hz−1 Mpc−3, whereΩb is the cosmic baryon density andh70 is the…”