AR-2019-2020

galactic nucleus. Follow up spectroscopy suggested that the foreground galaxy is a typical early- type galaxy at a high redshift of z = 0 . 957 with stellar velocity dispersion σ = 259 ± 56 km s − 1 and the lensed source is identified as an LBG at z = 3 . 403, based on the sharp drop bluewards of Lyα and other absorption features. A simple lens mass model for the system yields an Einstein radius of R Ein = 1 . 23 arcsec and a total mass within the Einstein radius of M Ein = (5 . 550 . 24) × 10 11 M corresponding to a velocity dispersion of σ = 283 ± 3 km s − 1 , which is in good agreement with the value derived spectroscopically. In comparison with other lensed LBGs and typical z ∼ 4 LBG populations, HSC J0904-0102 is unusually compact, an outlier at 2 σ confidence. Together with a previously discovered SuGOHI lens, HSC J1152+0047, which is similarly compact, it is believed that the HSC survey will extend the LBG studies down to smaller galaxy sizes. Lensed quasar search via time variability with the HSC transient survey A new lens search algorithm was developed by Anupreeta More and collaboratos for four-image (quad) lensed quasars based on their time variability. A pipeline simulating multi-epoch images of lensed quasars in cadenced surveys, accounting for quasar variabilities, quasar hosts, lens galaxies, and the PSF variation is constructed. Applying the simulation pipeline to the Hyper-Suprime Cam (HSC) transient survey, an ongoing cadenced survey, HSC-like difference images of the mock lensed quasars are generated. Using the difference images of the mock lensed quasars and other variable objects from the HSC transient survey, their algorithm picks out variable objects as lensed quasar candidates based on their spatial extent in the difference images. The performance of their lens search algorithm is tested on a sample combining the mock lensed quasars and variable objects from the HSC transient survey. The lens search algorithm achieves a high true-positive rate (TPR) of 90.1% and a low false-positive rate (FPR) of 2.3% for the bright quads (the third brightest image brightness m third < 22.0 mag) with wide separation. With a pre-selection on the number of blobs in the difference image, a TPR of 97.6% and a FPR of 2.6% was obtained for the bright quads with wide separation. Even when difference images are only available in one single epoch, the lens search algorithm could still detect the bright quads with wide separation at high TPR and low FPR. Therefore, this algorithm is promising and could find new lensed quasars in ongoing and upcoming cadenced surveys, such as the HSC transient survey and the Large Synoptic Survey Telescope. Extragalactic Astronomy Formation of disk galaxies around z ∼ 2 Understanding the formation of disk galaxies like our Milky Way, Andromeda remains a challenging issue today. It is only in the last few years that cosmological simulations have started producing disk galaxies like ours, which can be followed up via zoom simulations to understand how disks grow over time. However, a number of questions remain to be answered; for example, when did disks start forming? What were the primary physical processes involved? At present, we lack adequate observational evidences to answer any of these questions. Kanak Saha and collaborators show some observational evidences on the epoch of disk formation based on Hubble Space Telescope (HST/WFC3) imaging data. A key component of this work has been a careful decomposition of a galaxy’s light distribution into bulge and disk components for a sample of galaxies with redshifts ranging from z = 1 . 5 − 4 . 0 . Such a decomposition provides us a categorised sample of pure spheroids (or bulge/elliptical like galaxies), pure disks and disk+bulge (two-components) systems (Fig. 9 shows an example of pure disk and two-component disk galaxies at z = 2 . 81). They found that two-component systems have increased from 46% at z > 2 to 70% at z < 2. Pure disks have grown substantially - both in size and mass while pure spheroids didnt evolve much across this redshift range. The same is true for the bulges residing in the two-component systems. They report substantial activity for the disk formation and its growth at around this redshift. This project has been in collaboration with Sonali Sachdeva, Rupjyoti Gogoi, Ajit Kembhavi and Somak Raychaudhury .