Tracking Galaxy Evolution Through Low-Frequency Radio Continuum Observations using SKA and Citizen-Science Research using Multi-Wavelength Data

We present a brief review of progress in the understanding of general spiral and elliptical galaxies, through merger, star formation and AGN activities. With reference to case studies performed with the GMRT, we highlight the unique aspects of studying galaxies in the radio wavelengths where powerful quasars and bright radio galaxies are traditionally the dominating subjects. Though AGN or quasar activity is extremely energetic, it is extremely short-lived. This justify focussing on transitional galaxies to find relic-evidences of the immediate past AGN-feedback which decide the future course of evolution of a galaxy. Relic radio lobes can be best detected in low frequency observations with the GMRT, LOFAR and in future SKA. The age of these relic radio plasma can be as old as a few hundred Myr. There is a huge gap between this and what is found in optical bands. The very first relic-evidences of a past quasar activity (Hanny’s Voorwerp) was discovered in 2007 by a Galaxy Zoo citizen-scientist, a school teacher, in the optical bands. This relic is around a few tens of thousand years old. More discoveries needed to match these time-scales with star formation time-scales in AGN host galaxies to better understand black hole galaxy co-evolution process via feedback-driven quenching of star formation. It is now well-accepted that discovery and characterization of such faint fuzzy relic features can be more efficiently done by human eye than a machine. Radio interferometry images are more complicated than optical and need the citizen-scientists to be trained. RAD@home, the only Indian citizen-science research project in astronomy, analysing TIFR GMRT Sky Survey (TGSS) 150 MHz data and observing from the Giant Meterwave Radio Telescope (GMRT), was launched in April 2013. Unique, zero-infrastructure zero-funded design of RAD@home as a collaboratory of 69 trained e-astronomers is briefly described. Some of the new-found objects like episodic radio galaxies, radio-jet and companion galaxy interaction, radio galaxy bent by motion of the intra-filament medium in a Mpc-scale galaxy filament etc. are briefly presented as demonstration of its potential. Citizen-science has not only opened up a new way for astronomy research but also possibly the only promising way to extract maximum science out of the Big Data in the SKA-era. This possibly can convert the Big Data problem into a prospect. Citizen-science can contribute to the knowledge creation in never-seen-before speed and in approach. As it is based on internet, it can provide an equal opportunity of academic-growth to people even in the under-developed regions where we always need to put our optical and radio telescopes. This can liberate the research-activity of city-based research-institutes out of the four brick walls and alleviate various socio-economic and geo-political constraints on growth of citizens educated in undergraduate-level science but located in remote areas.     

Determination of the mean H<Emphasis Type="SmallCaps">i</Emphasis> absorption of the intergalactic medium

In recent years, the Lyman-α forest in quasar spectra has been used, together with N-body simulations, to determine the underlying matter distribution in the intergalactic medium (IGM). One of the key parameters to be known in order to compare observations and numerical simulations is the mean HI absorption in the IGM. To derive the latter, one has first to fit the quasar continuum. We have observed 20 high redshift and highly luminous QSOs (m _V ≤ 17.5 and 2.40 ≤ z _em ≤ 3.91) at intermediate spectral resolution, with either EMMI (ESO Multi-Mode Instrument) on the ESO-NTT telescope or CARELEC at the OHP (Observatoire de Haute-Provence), and applied different methods of determining the QSO continuum to this QSO sample. We have measured the amount of absorption, known as the flux decrement, DA, in the Lyman-α forest for these different methods and compared the results. In addition, we have compared DA values measured along the same lines of sight observed at high and intermediate spectral resolutions. We discuss the systematics resulting from the use of automatic continuum fitting methods.     

Generation of a Near Infra-Red Guide Star Catalog for Thirty-Meter Telescope Observations

The requirements for the production of a near Infra-Red Guide Star Catalog (IRGSC) for Thirty Meter Telescope (TMT) observations are identified and presented. A methodology to compute the expected J band magnitude of stellar sources from their optical (g, r, i) magnitudes is developed. The computed and observed J magnitudes of sources in three test fields are compared and the methodology developed is found to be satisfactory for the magnitude range, J_Vega = 16–22 mag. From this analysis, we found that for the production of final TMT IRGSC (with a limiting magnitude of J_Vega = 22 mag), we need g, r, i bands optical data which go up to i _AB ~ 23 mag. Fine tuning of the methodology developed, such as using Spectral Energy Distribution (SED) template fitting for optimal classification of stars in the fainter end, incorporating spectral libraries in the model, to reduce the scatter, and modification of the existing colour–temperature relation to increase the source density are planned for the subsequent phase of this work.     

In-orbit Performance of UVIT and First Results

The performance of the ultraviolet telescope (UVIT) on-board AstroSat is reported. The performance in orbit is also compared with estimates made from the calibrations done on the ground. The sensitivity is found to be within \(\sim \)15% of the estimates, and the spatial resolution in the NUV is found to exceed significantly the design value of \(1.8^{\prime \prime }\) and it is marginally better in the FUV. Images obtained from UVIT are presented to illustrate the details revealed by the high spatial resolution. The potential of multi-band observations in the ultraviolet with high spatial resolution is illustrated by some results.     

Long-term optical photometric monitoring of the quasar SDSS J153259.96−003944.1

We present results from an observation of the luminous cluster of galaxies Abell 2204 using the Chandra X-ray Observatory. We show the core of the cluster has a complex morphological structure, made up of a high-density core  ( n _e∼ 0.2 cm⁻³)  with flat surface brightness, a surrounding central plateau, a tail-like feature, wrapping around to the east, and an unusual radio source. A temperature map and deprojected profile shows that the temperature rises steeply outside these regions, until around ∼100 kpc where it drops, then rises again. Abundance maps and profiles show that there is a corresponding increase in abundance at the same radius as where the temperature drops. In addition, there are two cold fronts at radii of ∼28 and 54.5 kpc. The disturbed morphology indicates that the cluster core may have undergone a merger. However, despite this disruption, the mean radiative cooling time in the centre is short (∼230 Myr) and the morphology is regular on large scales.     

Detection of IMBHs from microlensing in globular clusters

Globular clusters have been alternatively predicted to host intermediate-mass black holes (IMBHs) or nearly impossible to form and retain them in their centres. Over the last decade enough theoretical and observational evidence have accumulated to believe that many galactic globular clusters may host IMBHs in their centres, just like galaxies do. The well-established correlations between the supermassive black holes and their host galaxies do suggest that, in extrapolation, globular clusters (GCs) follow the same relations. Most of the attempts in search of the central black holes (BHs) are not direct and present enormous observational difficulties due to the crowding of stars in the GC cores. Here we propose a new method of detection of the central BH – the microlensing of the cluster stars by the central BH. If the core of the cluster is resolved, the direct determination of the lensing curve and lensing system parameters are possible; if unresolved, the differential imaging technique can be applied. We calculate the optical depth to central BH microlensing for a selected list of Galactic GCs and estimate the average time duration of the events. We present the observational strategy and discuss the detectability of microlensing events using a 2-m class telescope.