Dhanya G. Nair

PhD Thesis :  Global Millimeter VLBI Array Survey of Ultracompact Extragalactic Radio Sources at 86 GHz

PhD Supervisor : Dr. Andrei P. Lobanov

Collaborators : Dr. Thomas P. Krichbaum, Prof. Dr. Eduardo Ros, Prof. Dr. J. Anton Zensus, Dr. Yuri Y. Kovalev, Dr. Sang-Sung Lee, Dr. Florent Mertens, Dr. Yoshiaki Hagiwara, Dr. Michael Bremer, Dr. Michael Lindqvist, Dr. Pablo de Vicente 

PhD Examination : March 08, 2018

The PhD Project : Very Long Baseline Interferometry (VLBI) observations at 86 GHz (~ 3 mm) reach a resolution of about 50 micro arcsec, probing the regions as small as 10³- 10⁴  Schwarzschild radii of the central black hole in active galactic nuclei (AGN) where the acceleration and collimation of relativistic outflows take place. The physical conditions in these regions can be studied by performing 86 GHz VLBI surveys of representative samples of compact extragalactic radio sources.

The primary focus of my research during PhD was to investigate the science on the launching, acceleration and collimation of the relativistic jet plasma in radio sources by conducting a  large global VLBI survey of 162 ultra-compact radio sources at 86 GHz using the Global Millimeter VLBI Array (GMVA) and investigating them on the background of brightness temperature measurements at highest angular resolutions. This GMVA survey at 3 mm is a major breakthrough in the study of AGNs, since 138 sources are imaged for the first time with VLBI at 86 GHz through this survey and contributed an increase of ~ 2 on the total number of AGNs ever imaged with VLBI at 86 GHz.   

We estimated the brightness temperature (T_b) at the jet base (core) and in one or more moving regions (jet components) downstream from the core, using Gaussian model fitting of the visibility data to represent the structure of the observed sources.  A population modeling, assuming that the observed distribution of brightness temperatures can be represented by a single intrinsic brightness temperature, resulted in a good understanding of the intrinsic brightness temperature in par-sec scales of AGNs at 86 GHz. The intrinsic brightness temperature for the jet cores is found to be (3.77±0.14)×10¹¹K, agreeing well with the inverse Compton limit, implying that the inverse Compton losses dominate the emission in the cores. In the nearest jet components, the intrinsic brightness temperature is found to be (1.42±0.19)×10¹¹K, which is slightly higher than the equipartition limit of 5×10¹⁰K expected for these jet regions. For objects with sufficient structural detail detected, the adiabatic energy losses are shown to dominate the observed changes of brightness temperature along the jet. The Gaussian model fit-based estimates of brightness temperatures agree well with the brightness temperature limits made directly from the visibility data.

The brightness temperature measurements from this survey combined with that made from VLBI observations at lower frequencies (2 GHz, 8 GHz and 15 GHz ) show that the T_b  at 86 GHz are systematically lower. We investigated the evolution of T_bwith the absolute distance of the VLBI core from the central engine. From the vicinity of the central engine, the brightness temperature increases slowly (inside 0.01 - 0.5 pc) and then rises with a steeper slope (inside 0.35 - 10 pc) and then slows down again and becomes almost constant (inside 5 - 100 pc). A phenomenological  model is applied to the data and the results indicate that the maximum brightness temperature in the downstream of the jet is (7.96±0.47)×10¹¹K with an initial brightness temperature near the jet base as 3.74×10¹⁰K. This gives a clue that the brightness temperatures on sub-parsec scales are close to the equipartition temperature of 5×10¹⁰K and start to increase on sub-parsec regions, reaching the inverse Compton limit of 10¹²K on parsec scales. The trend on evolution of brightness temperature with the distance from the central engine we observed matches well with the magnetically driven, accelerating jet model, according to which, the mass flux is initially constant in the sub-parsec scale, and then increases, and then the mass flux gets constant again in the outer region. The single intrinsic brightness temperature obtained from population model also falls within the initial and final values of brightness temperatures obtained by modeling the multi frequency measurements of brightness temperatures.

This research is ongoing to better constrain the bulk Lorentz factor and the intrinsic brightness temperature, to distinguish between the acceleration and deceleration scenario for the flow, and to test several alternative acceleration scenarios including hydrodynamic acceleration, acceleration by tangled magnetic field and magnetohydrodynamics acceleration.

About me I come from a small village, Mannar in Kerala, India. Since childhood, I was very much interested in sky observations and reading books about Universe. When I was in high school, I was very much influenced by the book 'A brief history of time' by Stephen Hawkings, that made me to think about the physics behind Universe, beyond my fascination about sky. After higher secondary school education, I did undergraduate in Physics at Mar Thoma College, Mahatma Gandhi University, India. During my bachelor studies, I did an internship at Giant Meter Wave Radio Telescope (GMRT) in Pune, where I got introduced to radio astronomy and interferometry. Later, I moved to Pune to pursue my master degree in Astrophysics at University of Pune, where I got an opportunity to do the masters curriculum in Astrophysics at Inter-University Center for Astronomy and Astrophysics (IUCAA).

I did my master thesis in Extragalactic Astronomy on 'The efficiency of black holes in triggering jets in radio-loud sources as a function of redshift' using the SDSS (DR10) catalog and Very Large Array FIRST Survey at National Centre for Radio Astrophysics (NCRA), TIFR, India. During my masters thesis, I developed a code in Fortran to find the counter parts in large samples from optical and radio catalogs and sampled the radio luminosities in redshift space and found that the fraction of radio loud sources are higher at a redshift space, z > 1 and z < 3, with the optimal combination of black hole spin and accretion rate. Afterwards, I started my PhD as a member of the IMPRS for Astronomy and Astrophysics at the Very Long Baseline Interferometry (VLBI) group at the MPIfR, which was completed by my PhD defense on 8^thMarch 2018 from University of Cologne (Grade : Magna Cum Laude (1.0, Excellent).  After that, I started my post doctorate in the VLBI group at MPIfR.

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