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The Primordial Helium Abundance: No Problem

The Primordial Helium Abundance: No Problem. Evan Skillman (University of Minnesota) Based on Aver, Olive, & Skillman (2010, 2011, 2012) Presented at “Lithium in the Cosmos” Institut d’Astrophysique de Paris February 28, 2012. Motivation. From Izotov & Thuan (2010):

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The Primordial Helium Abundance: No Problem

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  1. The Primordial Helium Abundance:No Problem Evan Skillman (University of Minnesota) Based on Aver, Olive, & Skillman (2010, 2011, 2012) Presented at “Lithium in the Cosmos” Institut d’Astrophysique de Paris February 28, 2012

  2. Motivation From Izotov & Thuan (2010): From 93 spectra of 86 low-metallicity extragalactic H II regions, they find the best value to be Yp = 0.2565 ± 0.0010 (stat.) ± 0.0050 (syst.). This value is higher at the 2σ level than the value given by standard big bang nucleosynthesis. The effective number of light neutrino species N ν is equal to 3.68+0.80-0.70 (2σ) and 3.80+0.80-0.70 (2σ) for a neutron lifetime τ n equal to 885.4 ± 0.9 s and 878.5 ± 0.8 s, respectively, compared to the experimental value of 2.993 ± 0.011.

  3. Evidence for Non-Standard BBN, Izotov & Thuan 2010

  4. The History of Primordial Helium Measurements

  5. Table 7 from Peimbert, Luridiana, & Peimbert 2007

  6. Because of the degeneracies in solutions for the physical conditions in the HII regions, it is absolutely essential to use Monte Carlo analyses in order to estimate the true uncertainties on individual observations.

  7. The Dominant Underlying Degeneracy

  8. The Underlying Degeneracy

  9. In Aver, Olive, & Skillman (2010) We explored: • new He I emissivities • “integrating” the H I and He I lines in a minimization • better treatment of underlying absorption • solving for H I collisional excitation • Only a small number of high quality spectra were analyzed, resulting in a relatively large uncertainty in the predicted value of Yp.

  10. A Representative Regression

  11. The History of Primordial Helium Measurements

  12. In Aver, Olive, & Skillman (2011) We explored improvements allowed by adopting a Markov Chain Monte Carlo analysis: • Our MCMC method is superior to previous implementations (free from biases due to non-physical parameter space). • The MCMC analysis allows a detailed exploration of degeneracies, and, in particular, a false minimum at large values of optical depth in the He I emission lines. • Introducing the [O III] temperature as a prior, in a very conservative manner, produces negligible bias and effectively eliminates the false minima occurring at large optical depth.

  13. In Aver, Olive, & Skillman (2012) We explore improvements allowed by adopting a Markov Chain Monte Carlo analysis: • We analyze the large dataset of Izotov, Thuan, and Stasinska (2007). • We draw attention to the use of c2 as a measure of the quality of the solution for individual spectra and adopt this measure in order to cut the sample. • We emphasize the importance of the He I 4026 emission line for its sensitivity to underlying absorption. • The final dataset, after cuts, exhibits improved consistency.

  14. The importance of including He I 4026

  15. How MCMC allows cuts on quality of solution

  16. How MCMC allows cuts on quality of solution

  17. Quality cuts eliminate less likely solutions

  18. Results • 14 of the original 93 spectra are used in the final analysis • The regression yields a value of Yp = 0.2534 +/- 0.0083, which is in agreement with the WMAP prediction of 0.2487 +/- 0.0002, to be contrasted to the value of 0.2565 ± 0.0010 (stat.) ± 0.0050 (syst.) from IT2010.

  19. Resulting final sample and regression

  20. Results in perspective

  21. Summary • Only a fraction of spectra analyzed to date are suitably high quality for non-parametric analysis. • Primordial He is not in a crisis, yet. • There is still room for improvement on the observational side.

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