AR_final file_2018-19

Majidul Rahaman, and Siddharth Malu. Kanan Kumar Datta Impact of inhomogeneous CMB heating of gas on the HI 21-cm signal during dark ages Observations of redshifted HI 21-cm signal appear to be the most promising probe of the cosmic dark ages. The signal carries information about the thermal state along with density distribution of the intergalactic medium. The Cosmic Microwave Background Radiation (CMBR), through its inter- action with charged particles, plays a major role in determining the kinetic and spin temperature of neutral hydrogen (HI) gas in the IGM during dark ages. Spatially fluctuating ionization fraction, which is caused by inhomogeneous recombinations, causes heat transfer from the CMBR to the IGM gas inhomogeneous. We revisit impact of this in- homogeneous heat transfer on spatial fluctuations in the observed HI 21-cm signal over a large red- shift range during dark ages. Our study shows that the effect negatively impacts fluctuations in the HI spin temperature and results in enhanced HI 21-cm power spectrum. We find that the effect is partic- ularly important during the transition of the gas kinetic temperature being coupled to the CMBR to fully decoupled from it, i.e., in the redshift range 30 < z < 300. It is found that, on the average the HI power spectrum P T b ( k, z ) is enhanced by ∼ 4%, ∼ 10% , ∼ 20% and ∼ 30% at redshifts 60, 90, 140 and 200 respectively at k = 0 . 1Mpc − 1 . The ef- fect becomes even more significant at lower values of k 2 k /k 2 due to reduced dominance of the peculiar velocity. It is observed that the power spectrum is enhanced by ∼ 49% and ∼ 93% at redshifts 140 and 200 respectively at k = 0 . 1Mpc − 1 for k 2 k /k 2 = 0 . This enhancement has a weak k-mode depen- dence. This study has been done in collaboration with Sioree Ansar, and Dhruba Dutta Chowdhury. A method to determine the evolution history of the mean neutral hydrogen fraction The light-cone (LC) effect imprints the cosmolog- ical evolution along the frequency axis, which is the line of sight ∼ (LoS) direction of the red- shifted 21-cm signal T b (ˆ n , ν ). The effect is par- ticularly pronounced during the Epoch of Reion- ization (EoR) when the mean hydrogen neu- tral fraction x HI ( ν ) falls rapidly as the uni- verse evolves. The multi-frequency angular power spectrum (MAPS) C ℓ ( ν 1 , ν 2 ) quantifies the entire second-order statistics of T b (ˆ n , ν ) considering both the systematic variation along ν due to the cos- mological evolution and also the statistically ho- mogeneous and isotropic fluctuations along all the three spatial directions encoded in ˆ n and ν . Here we propose a simple model where the systematic frequency ( ν 1 , ν 2 ) dependence of C ℓ ( ν 1 , ν 2 ) arises entirely due to the evolution of x HI (( ν ). If valid, this provides a new method to observationally de- termine the reionization history. Considering a LC simulation of the EoR 21-cm signal, we use the di- agonal elements ( ν 1 = ν 2 ) of C ℓ ( ν 1 , ν 2 ) to validate our model. We demonstrate that it is possible to recover the reionization history across the entire observational bandwidth provided we have the val- ues x HI (( ν c ) at a single frequency ν c as an external input. This work has been done in collaboration with Rajesh Mondal, Somnath Bharadwaj, Ilian Iliev, Suman Majumdar, Abinash K. Shah, et al. Sukanta Deb Geometry of the Large Magellanic Cloud using mul- tiwavelength photometry of classical Cepheids We determine the geometrical and viewing angle parameters of the Large Magellanic Cloud (LMC) using the Leavitt law based on a sample of more than, 3,500 common classical Cepheids (FU and FO) in optical (V, I), near-infrared (JHKs), and mid-infrared ([3.6] µm and [4.5] µm ) photometric bands. Statistical reddening and distance modu- lus free from the effect of reddening to each of the individual Cepheids are obtained using the simulta- neous multiband fit to the apparent distance mod- uli from the analysis of the resulting Leavitt laws in these seven photometric bands. A reddening map of the LMC obtained from the analysis shows good agreement with the other maps available in the literature. Extinction-free distance measure- ments along with the information of the equato- rial coordinates ( α, δ ) for individual stars are used to obtain the corresponding Cartesian coordinates with respect to the plane of the sky. By fitting a plane solution of the form z = f(x, y) to the ob- served three-dimensional distribution, the following viewing angle parameters of the LMC are obtained: inclination angle i = 25 0 · 110 ± 0 0 · 365, and posi- tion angle of line of nodes θ lon = 154 0 · 702 ± 1 0 · 378. On the other hand, modelling the observed three- dimensional distribution of the Cepheids as a triax- ( 185 )