Probing the star formation history using the redshift evolution of luminosity functions

We present a self-consistent, semi-analytical LCDM model of star formation and reionization. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the optical depth to reionization, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3<z<6 for a reasonable range of model parameters. While simple photoionization feedback produces the correct shape of LF at z = 6, for $z = 3$ we need additional feedback that suppresses star formation in halos with 10<log(M/M_\odot)<11. Models with prolonged continuous star formation activities are preferred over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z~6. The halo number density evolution from the standard LCDM structure formation model that fits LF up to z=6 is consistent with the upper limits on z~7 LF and source counts at 8<z<12 obtained from the HUDF observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6<z<10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the IMF, reddening correction and the mode of star formation at z>8. Molecular cooled halos, which may be important for reionizing the universe, are not detectable in the present deep field observations. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the LF at 6<z< 12.Accurately measuring the LF at high z can be used to understand the nature of star formation in the dark ages and probe the history of reionization. (Abridged)Comment: This version exactly matches with the published version in MNRA

Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function

The form of the halo mass function is a basic ingredient in any semi-analytical galaxy formation model. We study the existing forms of the mass functions in the literature and compare their predictions for semi-analytical galaxy formation models. Two methods are used in the literature to compute the net formation rate of halos, one by simply taking the derivative of the halo mass function and the other using the prescription due to Sasaki (1994). For the Press-Schechter (PS) mass function, we compare various model predictions, using these two methods. However, as the Sasaki formalism cannot be easily generalized for other mass functions, we use the derivative while comparing model predictions of different mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function differs from those using any other existing mass function, like Sheth-Tormen (ST) mass function.In particular the reionization efficiency of molecular cooled halos has to be substantially reduced when one uses the ST and other mass functions obtained from the simulation instead of the PS mass function. Using $\chi^2$-minimization, we find that the observed UV luminosity functions of LBGs at $3.0\le z\le 7.4$ are better reproduced by models using the ST mass function compared to models that use the PS mass function. On the other hand, the volume filling factor of the metals expelled from the galaxies through supernovae driven outflows differs very little between models with different mass functions. It depends on the way we treat merging outflows. We also show that the porosity weighted average quantities related to the outflow are not very sensitive to the differences in the halo mass function.Comment: Accepted for publication in New Astronom

Impact of cosmic rays on the global 21-cm signal during cosmic dawn

It is extremely important to understand the processes through which the thermal state of the inter-galactic medium (IGM) evolved in the early universe in order to study the evolution of HI 21-cm signal during cosmic dawn. Here, we consider the heating of the IGM due to cosmic ray protons generated by the supernovae from both early Pop III and Pop II stars. The low energy cosmic ray protons from Pop III supernovae can escape from minihalos and heat the IGM via collision and ionization of hydrogen. Furthermore, high energy protons generated in Pop II supernovae can escape the hosting halos and heat the IGM via magnetosonic Alfv\'en waves. We show that the heating due to these cosmic ray particles can significantly impact the IGM temperature and hence the global 21-cm signal at $z\sim 14-18$. The depth, location, and duration of the 21-cm absorption profile are highly dependent on the efficiencies of cosmic ray heating. In particular, the EDGES signal can be well fitted by the cosmic ray heating along with the Lyman-$\alpha$ coupling, and the dark matter-baryon interaction that we consider to achieve a `colder IGM background'. Further, we argue that the properties of cosmic rays and the nature of first generation of stars could be constrained by accurately measuring the global 21-cm absorption signal during the cosmic dawn.Comment: 15 pages, 9 figures, accepted for publication in MNRA

Understanding the redshift evolution of the luminosity functions of Lyman-α emitters

We present a semi-analytical model of star formation which explains simultaneously the observed ultraviolet (UV) luminosity function (LF) of high-redshift Lyman break galaxies (LBGs) and LFs of Lyman emitters. We consider both models that use the Press-Schechter (PS) and Sheth-Tormen (ST) halo mass functions to calculate the abundances of dark matter haloes. The Lyman α LFs at z ≾ 4 are well reproduced with only ≾ 10 per cent of the LBGs emitting Lyman lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed LF at z &gt; 5 can be reproduced only when we assume that nearly all LBGs are Lyman emitters. Thus, it appears that 4 &lt; z &lt; 5 marks the epoch when a clear change occurs in the physical properties of the high-redshift galaxies. As Lyman α escape depends on dust and gas kinematics of the interstellar medium (ISM), this could mean that on an average the ISM at z &gt; 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman photons. At z &gt; 5, the observed Lyman α LF are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high-redshift galaxies. In particular, up to z= 6.5, we do not see the effect of evolving intergalactic medium opacity on the Lyman α escape from these galaxies