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Register a new userThe Photon Underproduction Crisis
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We examine the statistics of the low-redshift Lyman-alpha forest from smoothed particle hydrodynamic simulations in light of recent improvements in the estimated evolution of the cosmic ultraviolet background (UVB) and recent observations from the Cosmic Origins Spectrograph (COS). We find that the value of the metagalactic photoionization rate required by our simulations to match the observed properties of the low-redshift Lyman-alpha forest is a factor of 5 larger than the value predicted by state-of-the art models for the evolution of this quantity. This mismatch results in the mean flux decrement of the Lyman-alpha forest being underpredicted by at least a factor of 2 (a 10-sigma discrepancy with observations) and a column density distribution of Lyman-alpha forest absorbers systematically and significantly elevated compared to observations over nearly two decades in column density. We examine potential resolutions to this mismatch and find that either conventional sources of ionizing photons (galaxies and quasars) must be significantly elevated relative to current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.
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We examine the impact of black hole jet feedback on the properties of the low-redshift intergalactic medium (IGM) in the SIMBA simulation, with a focus on the Ly$alpha$ forest mean flux decrement $D_A$. Without jet feedback, we confirm the Photon Underproduction Crisis (PUC) in which $Gamma_{rm HI}$ at $z=0$ must be increased by $times6$ over the Haardt & Madau value in order to match the observed $D_{A}$. Turning on jet feedback lowers this discrepancy to $simtimes 2.5$, and additionally using the recent Faucher-Gigu`ere background mostly resolves the PUC, along with producing a flux probability distribution function in accord with observations. The PUC becomes apparent at late epochs ($z lesssim 1$) where the jet and no-jet simulations diverge; at higher redshifts SIMBA reproduces the observed $D_{A}$ with no adjustment, with or without jets. The main impact of jet feedback is to lower the cosmic baryon fraction in the diffuse IGM from 39% to 16% at $z=0$, while increasing the warm-hot intergalactic medium (WHIM) baryon fraction from 30% to 70%; the lowering of the diffuse IGM content directly translates into a lowering of $D_{A}$ by a similar factor. Comparing to the older MUFASA simulation that employs different quenching feedback but is otherwise similar to SIMBA, MUFASA matches $D_{A}$ less well than SIMBA, suggesting that low-redshift measurements of $D_{A}$ and $Gamma_{rm HI}$ could provide constraints on feedback mechanisms. Our results suggest that widespread IGM heating at late times is a plausible solution to the PUC, and that SIMBAs jet AGN feedback model, included to quench massive galaxies, approximately yields this required heating.
Recent suggestions of a photon underproduction crisis (Kollmeier etal 2014) have generated concern over the intensity and spectrum of ionizing photons in the metagalactic ultraviolet background (UVB). The balance of hydrogen photoionization and recombination determines the opacity of the low-redshift intergalactic medium (IGM). We calibrate the hydrogen photoionization rate ($Gamma_{rm H}$) by comparing {it Hubble Space Telescope} spectroscopic surveys of the low-redshift column density distribution of HI absorbers and the observed ($z < 0.4$) mean Lya flux decrement, $D_A = (0.014)(1+z)^{2.2}$, to new cosmological simulations. The distribution, $f(N_{rm HI}, z) equiv d^2 {cal N} / d(log N_{rm HI}) dz$, is consistent with an increased UVB that includes contributions from both quasars and galaxies. Our recommended fit, $Gamma_{rm H}(z) = (4.6 times 10^{-14}$ s$^{-1})(1+z)^{4.4}$ for $0 < z < 0.47$, corresponds to unidirectional LyC photon flux $Phi_0 approx 5700$~cm$^{-2}$~s$^{-1}$ at $z = 0$. This flux agrees with observed IGM metal ionization ratios (CIII/CIV and SiIII/SiIV) and suggests a 25-30% contribution of Lya absorbers to the cosmic baryon inventory. The primary uncertainties in the low-redshift UVB are the contribution from massive stars in galaxies and the LyC escape fraction ($f_{rm esc}$), a highly directional quantity that is difficult to constrain statistically. We suggest that both quasars and low-mass starburst galaxies are important contributors to the ionizing UVB at $z < 2$. Their additional ionizing flux would resolve any crisis in photon underproduction.
The small-scale crisis, discrepancies between observations and N-body simulations, may imply suppressed matter fluctuations on subgalactic distance scales. Such a suppression could be caused by some early-universe mechanism (e.g., broken scale invariance during inflation), leading to a modification of the primordial power spectrum at the onset of the radiation-domination era. Alternatively, it may be due to nontrivial dark-matter properties (e.g., new dark-matter interactions or warm dark matter) that affect the matter power spectrum at late times, during radiation domination, after the perturbations re-enter the horizon. We show that early- and late-time suppression mechanisms can be distinguished by measurement of the $mu$ distortion to the frequency spectrum of the cosmic microwave background. This is because the $mu$ distortion is suppressed, if the power suppression is primordial, relative to the value expected from the dissipation of standard nearly scale-invariant fluctuations. We emphasize that the standard prediction of the $mu$ distortion remains unchanged in late-time scenarios even if the dark-matter effects occur before or during the era (redshifts $5times 10^4 lesssim z lesssim 2times 10^6$) at which $mu$ distortions are generated.
The so-called textit{China crisis}, well documented in textit{History of the IAU} by Adriaan Blaauw and in textit{Under the Same Starry Sky: History of the IAU} by Chengqi Fu and Shuhua Ye, refers to the withdrawal in 1960 of the Peoples Republic of China (PRC) from the Union. The crisis stemmed from the admission by the IAU, amidst strong protest from PRC and some other member countries, of the Republic of China (ROC) to the Union, creating the so-called `textit{Two Chinas} -- or `textit{One China, one Taiwan} problem. The crisis directly led to the absence of mainland Chinese astronomers from the stage of international collaborations and exchanges, and was only solved two decades later. The solution, accepted by all the parties involved, is that China is to have two adhering organizations, with mainland China astronomers represented by the Chinese Astronomical Society located in Nanjing (China Nanjing) and China Taiwan astronomers represented by the Academia Sinica located in Taipei (China Taipei). The denominations `textit{China Nanjing} and `textit{China Taipei} represent the IAU official resolution and should be used in all IAU events. The China crisis, probably the most serious one in IAU history, was a painful lesson in the 100-year development of the Union. Yet, with its eventual solution, the Union has emerged stronger, upholding its spirit of promoting astronomical development through international collaboration of astronomers from all regions and countries, regardless of the political systems, religion, ethnicity, gender or level of astronomical development.
Astronomers in CANDELS outline changes for the academic system to promote a smooth transition for junior scientists from academia to industry.
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