22 research outputs found
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 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 -minimization, we find that the observed UV luminosity
functions of LBGs at 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 . 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- 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 > 5 can be reproduced only when we assume that nearly all LBGs are Lyman emitters. Thus, it appears that 4 < z < 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 > 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 > 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