Research Statement:

Areas of my research are cosmology, dark matter, structure formation in the Universe, intergalactic medium, galaxy formation and reionization - mainly, semi-analytic and analytic modeling of cosmological reionization and galaxy formation. A brief summary of my research work is given below:

• Model-independent Constraints on Reionization

Reionization is a complex process whereby hydrogen (and helium) in the Universe is ionized by the radiation from first luminous sources. Theoretically, the importance of the reionization lies in its close coupling with the formation of first cosmic structures. Also observationally, the reionization era represents a phase of the Universe which is yet to be probed. In the past few years, the understanding of reionization process has become increasingly sophisticated in both the observational and theoretical communities, thanks to the availability of good quality data related to reionization. However, recent studies suggest that reionization process is too complex to be described as a sudden process, in fact observations suggest that the reionization occurred somewhere between redshift z ∼ 6−15. Using our semi-analytical model of reionization, we study the observational constraints on reionization via a principal component analysis (PCA), a sophisticated data analysis technique. The advantage of this approach is that it provides constraints on reionization in a model–independent manner. We compare our models with different observations and find that the models we developed can match a wide variety of observational data sets. For example, the figure below is taken from one of our recent paper, where we show the limits on the fraction of neutral hydrogen gas (primarily hydrogen) as a function of redshift and compare that with various existing observations (data points with errorbars). The different color-shaded regions correspond to the different reionization models.

• Galaxies in Equilibrium: A New Type of Galaxy Formation Model

The formation and evolution of galaxies is governed by a vast number of interrelated physical processes that span many decades in physical and temporal scale. Such complexity makes this area of astrophysics extraordinarily rich and interesting, driven by the accelerating pace of observational data that has statistically characterized the multi-wavelength properties of galaxies from locally out to high redshifts. Galaxies strongly self-regulate their growth via energetic feedback from stars, supernovae, and black holes, but these processes are among the least understood aspects of galaxy formation theory. We present an analytic galaxy evolution model that directly constrains such feedback processes from observed galaxy scaling relations. The equilibrium model, which is broadly valid for star-forming central galaxies that dominate cosmic star formation, is based on the ansatz that galaxies live in a slowly-evolving equilibrium between inflows, outflows, and star formation. Using a Bayesian Monte Carlo Markov chain (MCMC) approach, we constrain our model to match observed galaxy scaling relations between stellar mass and halo mass, star formation rate, and metallicity from redshifts 0 to 2. Our results suggest that this simple analytic framework captures the basic physical processes required to model the mean evolution of stars and metals in galaxies, despite not incorporating many canonical ingredients of galaxy formation models such as merging or disk formation. Very recently, we also explore the merging of an extended version of the equilibrium model into a machine learning framework. We are now able to recover more properties than the analytic formalism alone can provide, creating a high-speed hydrodynamic simulation emulator that populates galactic dark matter haloes in N-body simulations with baryonic properties. We demonstrate that this novel hybrid system enables the fast completion of dark matter-only information by mimicking the properties of a full hydrodynamic suite to a reasonable degree, and discuss the advantages and disadvantages of hybrid versus machine learning-only frameworks. In doing so, we offer an acceleration of commonly deployed simulations in cosmology. Below are some of the figures taken from that work (see the full paper for details).

List of Publications:

1. Chatterjee, Atrideb; Choudhury, Tirthankar Roy; Mitra, Sourav, CosmoReionMC: A package for estimating cosmological and astrophysical parameters using CMB, Lyman-α absorption and global 21 cm data, MNRAS (2021, in press), arXiv:2101.11088

2. Moews, Ben; Davé, Romeel; Mitra, Sourav; Hassan, Sultan; Cui, Weiguang, Hybrid analytic and machine-learned baryonic property insertion into galactic dark matter haloes, MNRAS (2021, in press), arXiv:2012.05820

3. Kar, Arpan; Mukhopadhyaya, Biswarup; Tingay, Steven; McKinley, Ben; Haverkorn, Marijke; McSweeney, Sam; Hurley-Walker, Natasha; Mitra, Sourav; Choudhury, Tirthankar Roy, Dark matter annihilation in Centauri: Astrophysical implications derived from the MWA radio data, Physics of the Dark Universe, 30, 100689, (2020), arXiv:2005.11962

4. Kar, Arpan; Mitra, Sourav; Mukhopadhyaya, Biswarup; Choudhury, Tirthankar Roy; Tingay, Steven, Constraints on dark matter annihilation in dwarf spheroidal galaxies from low frequency radio observations, Phys. Rev. D, 100, 043002, (2019), arXiv:1907.00979

5. Kar, Arpan; Mitra, Sourav; Mukhopadhyaya, Biswarup; Choudhury, Tirthankar Roy, Heavy dark matter particle annihilation in dwarf spheroidal galaxies: radio signals at the SKA telescope, Phys. Rev. D, 101, 023015, (2020), arXiv:1905.11426

6. Mitra, Sourav; Park, Chan-Gyung; Choudhury, Tirthankar Roy; Ratra, Bharat, First study of reionization in tilted flat and untilted non-flat dynamical dark energy inflation models, MNRAS, 487, 5118, (2019), arXiv:1901.09927

7. Kar, Arpan; Mitra, Sourav; Mukhopadhyaya, Biswarup; Choudhury, Tirthankar Roy, Can SKA–Phase 1 go much beyond the LHC in supersymmetry search?, Phys. Rev. D, 99, 021302 (2019), arXiv:1808.05793

8. Mitra, Sourav; Choudhury, Tirthankar Roy; Ratra, Bharat, First study of reionization in the Planck 2015 normalized closed ΛCDM inflation model, MNRAS, 479, 4566, (2018), arXiv:1712.00018

9. Kar, Arpan; Mitra, Sourav; Mukhopadhyaya, Biswarup; Choudhury, Tirthankar Roy, Do astrophysical data disfavour the minimal supersymmetric standard model?, (2017), arXiv:1711.09069

10. Kar, Arpan; Mitra, Sourav; Mukhopadhyaya, Biswarup; Choudhury, Tirthankar Roy, Dark matter, extraterrestrial gamma-rays and the MSSM: a viability study, JCAP, 02, 045, (2018), arXiv:1711.11477

11. Hassan, Sultan; Davé, Romeel; Mitra, Sourav; Finlator, Kristian; Ciardi, Benedetta; Santos, Mario G., Constraining the contribution of active galactic nuclei to reionisation, MNRAS, 473, 227, (2018), arXiv:1705.05398

12. Mitra, Sourav; Choudhury, Tirthankar Roy; Ferrara, Andrea, Cosmic reionization after Planck II: contribution from quasars, MNRAS, 473, 1416, (2018), arXiv:1606.02719

13. Mitra, Sourav; Davé, Romeel; Simha, Vimal, Finlator, Kristian, Equilibrium model prediction for the scatter in the star-forming main sequence, MNRAS, 464, 2766, (2017), arXiv:1606.07436

14. Mitra, Sourav; Choudhury, Tirthankar Roy; Ferrara, Andrea, Cosmic reionization after Planck, MNRAS Letter, 454L, 76, (2015), arXiv:1505.05507

15. Mitra, Sourav; Davé, Romeel; Finlator, Kristian, Equilibrium Model Constraints on Baryon Cycling Across Cosmic Time, MNRAS 452, 1184, (2015), arXiv:1411.1157

16. Mitra, Sourav; Ferrara, Andrea; Choudhury, Tirthankar Roy, The escape fraction of ionizing photons from high-redshift galaxies from data-constrained reionization models, MNRAS Letter, 428L, 1, (2013), arXiv:1207.3803

17. Pandolfi, S.; Ferrara, A.; Choudhury, Tirthankar Roy; Melchiorri, A.; Mitra, Sourav, Data-constrained reionization and its effects on cosmological parameters, Phys. Rev. D, 84, 123522, (2011), arXiv:1111.3570

18. Guha Sarkar, Tapomoy; Mitra, Sourav; Majumdar, Suman; Choudhury, Tirthankar Roy, Constraining large scale HI bias using redshifted 21-cm signal from the post-reionization epoch, MNRAS 421, 3570, (2012), arXiv:1109.5552

19. Mitra, Sourav; Choudhury, Tirthankar Roy; Ferrara, Andrea, Joint quasar-cosmic microwave background constraints on reionization history, MNRAS 419, 1480, (2012), arXiv:1106.4034

20. Mitra, Sourav; Kulkarni, Girish; Bagla, J. S.; Yadav, Jaswant K., Formation rates of Dark Matter Haloes, BASI, 39, 563, (2011), arXiv:1103.5828

21. Mitra, Sourav; Choudhury, Tirthankar Roy; Ferrara, Andrea, Reionization constraints using principal component analysis, MNRAS 413, 1569, (2011), arXiv:1011.2213