arXiv subjects

Yuvraj Harsha Sreedhar

Publications and source records attributed to Yuvraj Harsha Sreedhar.

4 recordsLinked to original sources

arXiv2016

In-orbit Performance of UVIT on ASTROSAT

We present the in-orbit performance and the first results from the ultra-violet Imaging telescope (UVIT) on ASTROSAT. UVIT consists of two identical 38cm coaligned telescopes, one for the FUV channel (130-180nm) and the other for the NUV (200-300nm) and VIS (320-550nm) channels, with a field of view of 28 $arcmin$. The FUV and the NUV detectors are operated in the high gain photon counting mode whereas the VIS detector is operated in the low gain integration mode. The FUV and NUV channels have filters and gratings, whereas the VIS channel has filters. The ASTROSAT was launched on 28th September 2015. The performance verification of UVIT was carried out after the opening of the UVIT doors on 30th November 2015, till the end of March 2016 within the allotted time of 50 days for calibration. All the on-board systems were found to be working satisfactorily. During the PV phase, the UVIT observed several calibration sources to characterise the instrument and a few objects to demonstrate the capability of the UVIT. The resolution of the UVIT was found to be about 1.4 - 1.7 $arcsec$ in the FUV and NUV. The sensitivity in various filters were calibrated using standard stars (white dwarfs), to estimate the zero-point magnitudes as well as the flux conversion factor. The gratings were also calibrated to estimate their resolution as well as effective area. The sensitivity of the filters were found to be reduced up to 15\% with respect to the ground calibrations. The sensitivity variation is monitored on a monthly basis. UVIT is all set to roll out science results with its imaging capability with good resolution and large field of view, capability to sample the UV spectral region using different filters and capability to perform variability studies in the UV.

astro-ph.IM↗

arXiv2014

Red population of Abell 1314 : A Rest-Frame Narrowband Photometric Evolutionary Analysis

Red sequence galaxies form with an intense burst of star formation in the early universe to evolve passively into massive, metal rich, old galaxies at z ~ 0. But Abell 1314 (z=0.034) is found to host almost all red sequence galaxy members -- identified using the mz index, classified using the Principle Component Analysis technique and SDSS colour correlations -- some of which show properties of low-mass, star forming, and metal rich galaxies. The variably spread Intra-Cluster Medium (ICM) near the core forms a vital part in influencing the evolution of these members. To study their evolution, I correlated different parameters of the rest-frame narrowband photometry and the derived luminosity-weighted mean Single Stellar Population model ages and metallicities. The study finds the member galaxies evolve differently in three different sections of the cluster: 1. the region of < 200 kpc hosts passively evolving old, massive systems which accumulate mass by dry, minor mergers, 2. the zone between 200-500 kpc shows stripped systems (or in the process of being gas stripped) by ram pressure with moderate star formation history, 3. the outer regions (> 500 kpc) show low-mass red objects with blue, star forming Butcher-Oemler galaxy like colours. This sort of environmental condition is known to harbour hybrid systems, like, the pseudo bulges, blue sequence E/S0 and Butcher-Oemler like satellite cluster galaxies. Overall, the cluster is found to be poor, quiescent with galaxies to have formed by the monolithic structure formation in the early universe and are now evolving with mergers and gas stripping processes by ram pressure.

astro-ph.GA↗

arXiv2014

Comparative studies of Population Synthesis Models in the frame work of modified Strömgren filters

Evolutionary models form a vital part of stellar population research to understand their evolution, but despite their long history of development, they often misrepresent and misinterpret the properties of stellar population observed through broadband and spectroscopic measurements. With the growing numbers of these synthesis models, model comparison becomes an important analysis to choose a suitable model for upgrade. Along with the model comparison, we reinvestigate the technique of modified Strömgren photometry to measure reliable parameter-sensitive colours and estimate precise model ages and metallicities. The assessment of Rakos/Schulz models with GALEV and Worthey's Lick/IDS model find smaller colour variation: Δ(uz-vz) $\leq$ 0.056, Δ(bz-yz) $\leq$ -0.05 and Δ(vz-yz) $\leq$ 0.061. The study conveys a good agreement of GALEV models with the modified Strömgren colours but with poor UV model predictions with the observed globular cluster data, while the spectroscopic models perform badly due to the use of older isochrone and stellar spectral libraries with inaccurate/insufficient knowledge of various stellar phases and their treatment. Overall, the assessment finds modified Strömgren photometry well suited to study different types stellar populations by mitigating the effects of the age-metallicity degeneracy.

astro-ph.GA↗

arXiv2012

Ages and Metallicities of Cluster Galaxies in A779 using Modified Strömgren Photometry

In the quest for the formation and evolution of galaxy clusters, Rakos and co-workers introduced a spectrophotometric method using the modified Strömgren photometry. But with the considerable debate toward the project's abilities, we re-introduce the system after a thorough testing of repeatability of colors and reproducibility of the ages and metallicities for six common galaxies in the three A779 data sets. A fair agreement has been found between the modified Strömgren and Strömgren filter systems to produce similar colors (with the precision of 0.09 mag in (uz-vz), 0.02 mag in (bz-yz), and 0.03 mag in (vz-vz)), ages and metallicities (with the uncertainty of 0.36 Gyr and 0.04 dex from the PCA and 0.44 Gyr and 0.2 dex using the GALEV models). We infer that the technique is able to relieve the age-metallicity degeneracy by separating the age effects from the metallicity effects, but still unable to completely break. We further extend this paper to re-study the evolution of galaxies in the low mass, dynamically poor A779 cluster by correlating the luminosity (mass), density, radial distance with the estimated age, metallicity, and the star formation history. Our results distinctly show the bimodality of the young, low-mass, metal-poor population with the mean age of 6.7 Gyr (\pm 0.5 Gyr) and the old, high-mass, metal-rich galaxies with the mean age of 9 Gyr (\pm 0.5 Gyr). The method also observes the color evolution of the blue cluster galaxies to red, and the downsizing phenomenon. Our analysis shows that the modified Strömgren photometry is very well suited for studying low- and intermediate-z clusters, as it is capable of observing deeper with better spatial resolution at spectroscopic redshift limits, and the narrowband filters estimate the age and metallicity with lesser uncertainties compared to other methods that study stellar population scenarios.

astro-ph.GA↗