1808.03654
How low does it go? Too few Galactic satellites with standard reionization quenching
Graus, et al
A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than V_peak ~20 km/s, rendering the majority of haloes below this scale completely dark. Use a suite of cosmological zoom simulations of MW-like haloes that include central MW disk galaxy potentials to investigate the relationship between sub haloes and ultra faint galaxies. Find that there are far too few sub haloes within 50 kpc of the MW that had V_peak > 20 km/s to account for the number of ultra faint galaxies already known within that volume today. In order to match the observed count, must populate sub haloes down to V_peak~6 km/s with ultra faint dwarfs. The required haloes have peak viral temperatures as low as 1500 K, well below the atomic H cooling limit of 1e4 K. Allowing for the possibility that the LMC contributes several of the satellites within 50 kpc could potentially raise this threshold to 10 km/s (4000 K), still below the atomic cooling limit and for below the nominal reioinization threshold.
1808.04083
Measuring the duration of last scattering
Hadzhiyska, Spergel
The CMB fluctuations effectively measure the basic properties of the universe during the recombination epoch. CMB measurements fix the distance to the surface of last scatter, the sound horizon of the baryon-photon fluid and the fraction of the energy density in relativistic species. Show that the microwave background observations can also very effectively constrain the thickness of the last scattering surface, which is directly related to the ratio of the small-scale E_mode polarization signal to the small-scale temperature signal. The current cosmological data enables a 0.1% measurement of the thickness of the surface of fast scatter: 19±0.065 Mpc. This constraint is relatively model-independent, so it can provide a new metric for systematic errors and an independent test of the LCDM model. On the other hand, it is sensitive to models which affect the reionization history of the universe such as models with annihilating dark matter and varying fundamental constants (e.g., the fine-structure constant, alpha_EM, and electron rest mass, m_e) and as such can be used as a viable tool to constrain them.
1808.04367
New constraints one IGM thermal evolution from the Ly{\alpha} forest power spectrum
Walther, et al
Determine the thermal evolution of the IGM over 3 Gyr of cosmic time 1.8<z<5.4 by comparing measurements of the Ly_alpha forest power spectrum to a suite of ~70 hydro sims. Conduct Bayesian inference of IGM thermal parameters using an end-to-end forward modeling framework whereby mock spectra generated form the simulation grid are used to build a custom emulator which interpolates the PS between thermal grid points. The temperature at mean density T0 rises steadily from T0 ~ 6000 K at z=5.4, peaks at 14000 K at z~3.4, and decreases at lower z reading T0 ~ 7000K by z~1.8. This evolution provides conclusive evidence for photoionization heating resulting from the reioinization of He II, as well as the subsequent cooling of the IGM due to the expansion of the Universe after all reioinization events are complete. The results are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the PS, the combination of high precision BOSS measurements of large-scale modes (k <~ 0.02 s/km) with the recent determination of the small-scale power, the large grid of models, and the careful statistical analysis allow breaking of the well known degeneracy between the temperature at mean density T0 and the slope of the temperature density relation gamma that has plagued previous analyses. At the highest redshifts z>=5, infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Ly_alpha forest by free streaming of warm dark matter.
1808.04384
The impact of wrong assumptions in BAO reconstruction
Sherwin, White
The process of density field reconstruction enhances the statistical power of distance scale measurements using BAO. During the process a fiducial cosmology is assumed in order to convert sky coordinates and redshifts into distances; fiducial bias and z-space-distortion parameters are also assumed in the procedure. Analytically assess the impact of incorrect cosmology and bias assumptions on the post-reconstruction PS using low-order Lagrangian perturbation theory, deriving general expressions for the incorrectly reconstructed spectra. Find that the BAO peak location appears to shift only by a negligible amount due to wrong assumptions made during reconstruction. However, the shape of the BAO peak and the quadrupole amplitude can be affected by such errors (at the percet- and five-percet-level respectively), which potentially could cause small biases in parameter inference for future surveys; outline solutions to such complications.
How low does it go? Too few Galactic satellites with standard reionization quenching
Graus, et al
A standard prediction of galaxy formation theory is that the ionizing background suppresses galaxy formation in haloes with peak circular velocities smaller than V_peak ~20 km/s, rendering the majority of haloes below this scale completely dark. Use a suite of cosmological zoom simulations of MW-like haloes that include central MW disk galaxy potentials to investigate the relationship between sub haloes and ultra faint galaxies. Find that there are far too few sub haloes within 50 kpc of the MW that had V_peak > 20 km/s to account for the number of ultra faint galaxies already known within that volume today. In order to match the observed count, must populate sub haloes down to V_peak~6 km/s with ultra faint dwarfs. The required haloes have peak viral temperatures as low as 1500 K, well below the atomic H cooling limit of 1e4 K. Allowing for the possibility that the LMC contributes several of the satellites within 50 kpc could potentially raise this threshold to 10 km/s (4000 K), still below the atomic cooling limit and for below the nominal reioinization threshold.
1808.04083
Measuring the duration of last scattering
Hadzhiyska, Spergel
The CMB fluctuations effectively measure the basic properties of the universe during the recombination epoch. CMB measurements fix the distance to the surface of last scatter, the sound horizon of the baryon-photon fluid and the fraction of the energy density in relativistic species. Show that the microwave background observations can also very effectively constrain the thickness of the last scattering surface, which is directly related to the ratio of the small-scale E_mode polarization signal to the small-scale temperature signal. The current cosmological data enables a 0.1% measurement of the thickness of the surface of fast scatter: 19±0.065 Mpc. This constraint is relatively model-independent, so it can provide a new metric for systematic errors and an independent test of the LCDM model. On the other hand, it is sensitive to models which affect the reionization history of the universe such as models with annihilating dark matter and varying fundamental constants (e.g., the fine-structure constant, alpha_EM, and electron rest mass, m_e) and as such can be used as a viable tool to constrain them.
1808.04367
New constraints one IGM thermal evolution from the Ly{\alpha} forest power spectrum
Walther, et al
Determine the thermal evolution of the IGM over 3 Gyr of cosmic time 1.8<z<5.4 by comparing measurements of the Ly_alpha forest power spectrum to a suite of ~70 hydro sims. Conduct Bayesian inference of IGM thermal parameters using an end-to-end forward modeling framework whereby mock spectra generated form the simulation grid are used to build a custom emulator which interpolates the PS between thermal grid points. The temperature at mean density T0 rises steadily from T0 ~ 6000 K at z=5.4, peaks at 14000 K at z~3.4, and decreases at lower z reading T0 ~ 7000K by z~1.8. This evolution provides conclusive evidence for photoionization heating resulting from the reioinization of He II, as well as the subsequent cooling of the IGM due to the expansion of the Universe after all reioinization events are complete. The results are broadly consistent with previous measurements of thermal evolution based on a variety of approaches, but the sensitivity of the PS, the combination of high precision BOSS measurements of large-scale modes (k <~ 0.02 s/km) with the recent determination of the small-scale power, the large grid of models, and the careful statistical analysis allow breaking of the well known degeneracy between the temperature at mean density T0 and the slope of the temperature density relation gamma that has plagued previous analyses. At the highest redshifts z>=5, infer lower temperatures than expected from the standard picture of IGM thermal evolution leaving little room for additional smoothing of the Ly_alpha forest by free streaming of warm dark matter.
1808.04384
The impact of wrong assumptions in BAO reconstruction
Sherwin, White
The process of density field reconstruction enhances the statistical power of distance scale measurements using BAO. During the process a fiducial cosmology is assumed in order to convert sky coordinates and redshifts into distances; fiducial bias and z-space-distortion parameters are also assumed in the procedure. Analytically assess the impact of incorrect cosmology and bias assumptions on the post-reconstruction PS using low-order Lagrangian perturbation theory, deriving general expressions for the incorrectly reconstructed spectra. Find that the BAO peak location appears to shift only by a negligible amount due to wrong assumptions made during reconstruction. However, the shape of the BAO peak and the quadrupole amplitude can be affected by such errors (at the percet- and five-percet-level respectively), which potentially could cause small biases in parameter inference for future surveys; outline solutions to such complications.
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