arXiv subjects

Sourav Mitra

Publications and source records attributed to Sourav Mitra.

40 records · Page 3Linked to original sources

arXiv2011

Formation rates of Dark Matter Haloes

We derive an estimate of the rate of formation of dark matter halos per unit volume as a function of the halo mass and redshift of formation. Analytical estimates of the number density of dark matter halos are useful in modeling several cosmological phenomena. We use the excursion set formalism for computing the formation rate of dark matter halos. We use an approach that allows us to differentiate between major and minor mergers, as this is a pertinent issue for semi-analytic models of galaxy formation. We compute the formation rate for the Press-Schechter and the Sheth-Tormen mass function. We show that the formation rate computed in this manner is positive at all scales. We comment on the Sasaki formalism where negative halo formation rates are obtained. Our estimates compare very well with N-Body simulations for a variety of models. We also discuss the halo survival probability and the formation redshift distributions using our method.

astro-ph.CO↗

arXiv2011

Data-constrained reionization and its effects on cosmological parameters

We perform an analysis of the recent WMAP7 data considering physically motivated and viable reionization scenarios with the aim of assessing their effects on cosmological parameter determinations. The main novelties are: (i) the combination of CMB data with astrophysical results from quasar absorption line experiments; (ii) the joint variation of both the cosmological and astrophysical [governing the evolution of the free electron fraction x_e(z)] parameters. Including a realistic, data-constrained reionization history in the analysis induces appreciable changes in the cosmological parameter values deduced through a standard WMAP7 analysis. Particularly noteworthy are the variations in Omega_bh^2 = 0.02258^{+0.00057}_{-0.00056} (WMAP7) vs. Omega_bh^2$ = 0.02183\pm 0.00054 (WMAP7 + ASTRO), and the new constraints for the scalar spectral index, for which WMAP7 + ASTRO excludes the Harrison-Zel'dovich value n_s=1 at more than 3σ. Finally, the e.s. optical depth value is considerably decreased with respect to the standard WMAP7, i.e. τ_{e}=0.080\pm 0.012. We conclude that the inclusion of astrophysical datasets, allowing to robustly constrain the reionization history, in the extraction procedure of cosmological parameters leads to relatively important differences in the final determination of their values.

astro-ph.CO↗

arXiv2011

Joint QSO-CMB constraints on reionization history

Based on the work by Mitra, Choudhury & Ferrara (2010), we obtain model-independent constraints on reionization from cosmic microwave background (CMB) and QSO absorption line data by decomposing the function N_{ion}(z) (the number of photons entering the IGM per baryon in collapsed objects) into its principal components. The main addition in this work is that for the CMB data set, we explicitly include the angular power spectra C_l for TT, TE and EE modes in our analysis which seem to contain somewhat more information than taking the electron scattering optical depth tau_{el} as a single data point. Using Markov Chain Monte Carlo methods, we find that all the quantities related to reionization can be severely constrained at z < 6 whereas a broad range of reionization histories at z > 6 are still permitted by the current data sets. With currently available data from WMAP7, we constrain 0.080 < tau_{el} < 0.112 (95% CL) and also conclude that reionization is 50% complete between 9.0 < z(Q_{HII} = 0.5) < 11.8 (95% CL) and is 99% complete between 5.8 < z(Q_{HII} = 0.99) < 10.4 (95% CL). With the forthcoming PLANCK data on large-scale polarization (ignoring effect of foregrounds), the z > 6 constraints will be improved considerably, e.g., the 2-sigma error on tau_{el} will be reduced to 0.009 and the uncertainties on z(Q_{HII} = 0.5) and z(Q_{HII} = 0.99) would be \sim 1 and 3 (95% CL), respectively. For more stringent constraints on reionization at z > 6, one has to rely on data sets other than CMB. Our method will be useful in such case since it can be used for non-parametric reconstruction of reionization history with arbitrary data sets.

astro-ph.CO↗

arXiv2010

Reionization constraints using Principal Component Analysis

Using a semi-analytical model developed by Choudhury & Ferrara (2005) we study the observational constraints on reionization via a principal component analysis (PCA). Assuming that reionization at z>6 is primarily driven by stellar sources, we decompose the unknown function N_{ion}(z), representing the number of photons in the IGM per baryon in collapsed objects, into its principal components and constrain the latter using the photoionization rate obtained from Ly-alpha forest Gunn-Peterson optical depth, the WMAP7 electron scattering optical depth and the redshift distribution of Lyman-limit systems at z \sim 3.5. The main findings of our analysis are: (i) It is sufficient to model N_{ion}(z) over the redshift range 2 6 are relatively loose. (iii) The weak constraints on N_{ion}(z) at z>6 do not allow to disentangle different feedback models with present data. There is a clear indication that N_{ion}(z) must increase at z>6, thus ruling out reionization by a single stellar population with non-evolving IMF, and/or star-forming efficiency, and/or photon escape fraction. The data allows for non-monotonic N_{ion}(z) which may contain sharp features around z \sim 7. (iv) The PCA implies that reionization must be 99% completed between 5.8 6 constraints will be significantly improved.

astro-ph.CO↗