The effects of the small-scale DM power on the cosmological neutral hydrogen (HI) distribution at high redshifts

Abstract

JCAP04(2016)012 ou rnal of C osmology and A strop article P hysics An IOP and SISSA journal J The effects of the small-scale DM power on the cosmological neutral hydrogen (H i ) distribution at high redshifts Abir Sarkar, a,b Rajesh Mondal, c,d Subinoy Das, e Shiv.K. Sethi, a Somnath Bharadwaj c,d and David J.E. Marsh f a Department of Astronomy And Astrophysics, Raman Research Institue, C.V. Raman Avenue, Bangalore, India b Department of Physics, Indian Institue of Science, C.V. Raman Avenue, Bangalore, India c Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India d Centre for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur 721302, India e Indian Institue of Astrophysics, 100 Feet Rd, Madiwala, Bangalore, India f Department of Physics, King’s College London, Strand, London, WC2R 2LS, United Kingdom E-mail: abir@rri.res.in, rm@phy.iitkgp.ernet.in, subinoy@iiap.res.in, sethi@rri.res.in, somnath@phy.iitkgp.ernet.in, david.marsh@kcl.ac.uk Received December 17, 2015 Revised February 24, 2016 Accepted March 29, 2016 Published April 7, 2016 Abstract. The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter models — Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models — where the matter power spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their parameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range 2 < z < 5. We model the theoretical predictions of the models using CDM-like N-body simulations with modified initial conditions, and generate reionization fields using an excursion set model. The N-body approximation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict an increase in the HI power spectrum from the epoch JCAP04(2016)012 of reionization by a factor between 2–10 for a range of scales 0 . 1 < k < 4 Mpc − 1 . Assuming a fiducial model where a neutral hydrogen fraction ̄ x HI = 0 . 5 must be achieved by z = 8, the reionization process allows us to put approximate bounds on the redshift of dark matter formation z f > 4 × 10 5 (for LFDM) and the axion mass m a > 2 . 6 × 10 − 23 eV (for ULA). The comparison of the collapsed mass fraction inferred from damped Lyman- α observations to the theoretical predictions of our models lead to the weaker bounds: z f > 2 × 10 5 and m a > 10 − 23 eV. These bounds are consistent with other constraints in the literature using different observables; we briefly discuss how these bounds compare with possible constraints from the observation of luminosity function of galaxies at high redshifts. In the case of ULAs, these constraints are also consistent with a solution to the cusp-core problem of CDM.