Narendra Nath Patra

Visiting Fellow
National Centre for Radio Astrophysics
Tata Institute of Fundamental Research
Savitribai Phule Pune University Campus,
Pune 411 007
Maharashtra, INDIA
Status: Left

Main Research Areas: Extragalactic astronomy; Dynamical modelling of disk galaxies; Phases of the interstellar medium; Astronomical software development etc.


Narendra Nath Patra completed his B.Sc. in Physics (Hons.) from the Ramakrishna Mission Residential College, University of Calcutta, in 2006, and later his M.Sc. (Physics) from the Department of Physics, University of Pune, in 2008. He then joined NCRA-TIFR as a Ph.D student under the guidance of Prof. Jayaram N. Chengalur. After completing his Ph.D in 2015, he joined NCRA-TIFR as a post-doctoral fellow.

Research description:

My research interests include: 1. Phases of the Interstellar Medium. 2. Dynamical modelling of neutral hydrogen disks. 3. Astronomical software development. 4. Neutral hydrogen surveys of faint dwarf galaxies.
Phases of the Interstellar Medium (ISM):
Our theoretical understanding of heating and cooling in the ISM predicts a two-phase neutral medium in pressure equilibrium, consisting of a Cold Neutral Medium (CNM) at kinetic temperatures of 20-200 K and a Warm Neutral Medium (WNM) at kinetic temperatures of 500-5000 K. Extragalactic and Galactic studies revealed that the CNM exists in clumps and the WNM is distributed in more ubiquitous manner in the ISM. A line-of-sight spectrum will have the signature of both the CNM (yielding a narrow line profile) and the WNM (giving a wide line profile). A Gaussian-decomposition technique is traditionally used to decompose a spectrum into these two ISM components. We have developed our own state-of-the art parallel routine 'multigauss' to decompose line-of-sight neutral hydrogen (HI) spectra from external galaxies into multiple Gaussian components and then identify narrow components as arising in the CNM. The brightness temperature of HI emission from the ISM may also be used as a measure of the existence of cold dense CNM. We have independently used the brightness temperatures of line-of-sight HI emission profiles of a sample of dwarf galaxies to identify the CNM in the interstellar medium of these galaxies. Using these two techniques, we detected the CNM in a sample of dwarf galaxies from the FIGGS survey and have studied the connection between the CNM and with star formation in these objects.
Dynamical modelling of disk galaxies:
We have used a three-component (stars, gas, and dark matter) model of galactic disks to understand the vertical structure of disk galaxies. We assume that the stellar and gaseous disks are in hydrostatic equilibrium under mutual gravity. For simplicity, we assume that both the disks are coplanar and axi-symmetric and embedded in the potential of the dark matter halo. For an elemental volume, we then write and solve the equation of hydrostatic equilibrium to get the HI density structure as a function of disk radius and height. With this density distribution, we can build a 3-D model of the galaxy and produce a model observation after inclining the disk to the observed inclination and convolving with the telescope beam. One can then compare the real data to the model results to optimize the various free parameters (e.g. the gas velocity distribution) of the model. The model generation and the optimization is highly compute-intensive. The whole model building and optimization routine were implemented using MPI-based parallel programming and run on NCRA's 32-node IBM Sandy Bridge cluster. We found that our model reproduces the observed spectral properties of observed galaxies quite well. A high velocity dispersion, ~ 22 km/s, was obtained at the centre of a target galaxy. We also find that the HI disks of dwarf galaxies are considerably thicker than those of normal spiral galaxies.
Astronomical software development:
I have developed a number of software programmes to address different scientific problems; these include an automated parallel routine for the gaussian decomposition of HI-21cm spectra into multiple Gaussian components, a dynamical model-building software for modelling galaxy disks, a parallel routine for stacking HI spectra within a specific property bin, etc. I have also developed a number of image plane analysis functions, e.g. to align two images, correct the GMRT primary beam (in CASA), smooth in the spatial dimension, etc.. I am presently heavily involved in the baseline design of the extended Giant Metrewave Radio Telescope (GMRT), a programme under way at NCRA-TIFR to significantly improve the GMRT's performance. This involves a detailed study of the array configuration options for the telescope, to optimize the UV coverage and the science capabilities.
HI in Dwarf Galaxies:
am also involved in an HI-21cm survey of faint irregular galaxies in the nearby Universe. We have carried out observations of 21 nearby galaxies as part of the FIGGS2 survey which is an extension of the earlier FIGGS survey of faint dwarf galaxies. FIGGS data have been used to explore several interesting aspects of dwarf galaxies, ranging from gas dynamics to star formation in the interstellar medium. We have extended the FIGGS survey to even fainter dwarfs, with a median brightness an order of magnitude lower than that of the FIGGS sample. We have detected 15 galaxies in HI-21cm emission in the FIGGS2 survey.

Selected publications:

1. Modelling HI distribution and kinematics in the edge-on dwarf irregular galaxy KK250. (N. N. Patra, A. Banerjee, J. N. Chengalur & A. Begum, 2014, MNRAS, 445, 1424)

2. A slow bar in the dwarf irregular galaxy NGC 3741 (A. Banerjee, N. N. Patra et al. 2013, MNRAS, 434, 1257)

3. Stringent constraints on the HI spin temperature in two z > 3 damped Lyman α systems from redshifted 21 cm absorption studies (N. Roy, S. Mathur, V. Gajjar & N. N. Patra, 2013, MNRAS, 436, L94)

4. The HI column density distribution function in faint dwarf galaxies (N. N. Patra et al. 2012, MNRAS, 429, 1596)

5. First VLF detections of ionospheric disturbances due to Soft Gamma Ray Repeater SGR J1550-5418 and Gamma Ray Burst GRB 090424 (S. Chakrabarti et al. (including N. N. Patra) 2010, InJP, 84, 1461)