About me

I am an NHFP Einstein Fellow at the University of Chicago. My principal research interest is gravitational lensing and observational cosmology. I work with galaxy-galaxy strong lenses and strongly lensed quasar/supernova systems to measure cosmological parameters such as the Hubble constant and to study properties of elliptical galaxies at the intermediate redshift (\(z\sim0.5\)).

I am the co-convener of the strong lensing topical team within the Rubin Observatory LSST's Dark Energy Science Collaboration. I am a co-PI of the STRIDES collaboration that aims to discover strongly lensed quasars from the Dark Energy Survey and other large-area sky-surveys.

Research Highlights

Time-delay cosmography

In a strong lens creating multiple images of a time-variable background source (quasar or SN), photons from the source that appear in different lensed images travel different cosmological distances around the foreground galaxy. Therefore, the measured time delay between the photon arrivals of the different images allows us to constrain a combination of these cosmological distances, which then provides a direct measurement of \(H_0\). The TDCOSMO collaboration has measured \(H_0\) by analyzing seven lensed quasar systems. Among them, I led the analysis for the system DES J0408\(-\)5354 (left picture) measuring \(H_0\) with 3.9% precision, the most precise single-system measurement of \(H_0\) to date (Shajib et al. 2020).

Improving \(H_0\) precison with resolved kinematics

The seven time-delay analyses from the TDCOSMO collaboration have assumed simple parametric forms for the mass profile, such as the power law, which is a potential systematic if the true mass profile in elliptical galaxies deviates from this assumption. We have developed a methodology to relax this assumption of simply-parametrized mass profiles in the lens galaxy and constrain \(H_0\) and the galaxy mass profile simultaneously using stellar kinematics. Relaxing this assumption increases the \(H_0\) uncertainty from 2% to ~8%. To improve the precision back to \(<\)2%, spatially resolved stellar kinematics is key. Using IFU data from Keck/KCWI (left picture), I have demonstrated for the first time that spatially resolved kinematics provide about seven-fold improvement in \(H_0\) precision over unresolved kinematics (Shajib et al. 2023). I am co-PI of a Cycle 2 JWST program to collect higher-\(S/N\) IFU data for TDCOSMO systems.

Project Dinos: studying galaxy evolution with the largest lens sample

To study the impact of baryonic feedback on elliptical galaxy evolution in the last 7 billion years (\(0.2 \lesssim z \lesssim 1\)) with strong lensing, a large statistical sample is necessary. To achieve this goal, I have formed Project Dinos, a HST archival program. The first paper from this project (Tan et al. 2023, in preparation) will present the lens models of \(\sim\)80 galaxy-galaxy lens systems, the largest such sample to date with uniform, power-law mass models. Joint lensing-dynamics analysis using these state-of-the-art models will constrain the individual distributions of dark matter and baryons to uncover the impact of baryonic feedbacks and mergers by comparing with hydrodynamical simulations.

Efficient lensing-dynamics analysis for any elliptical mass distribution

The 2D integral to compute the deflection angle for an elliptical mass profile is non-analytical in the general case, which lacked an efficient solution for more than three decades. I proposed an efficient method that solves this problem (Shajib 2019). My proposed framework self-consistently unifies the lensing and kinematic analyses of a general elliptical mass distribution.

Integrated Sachs-Wolfe effect

In a dark-energy-dominated universe, the large-scale potential well decays with time. CMB photons gain a little amount of energy after crossing such a decaying potential well, which is the integrated Sachs-Wolfe (ISW) effect. I analyzed WISE and WMAP data to detect the ISW effect signal at \(3.4\sigma\) confidence level (left picture, Shajib & Wright 2016). This was the highest-significance detection of the ISW effect from a single dataset at the time of publication (2016).

Publications

Total 39 refereed/under-review papers: 11 first-author papers, 4 second-author papers, and 24 other co-author papers.

First author publications:
  1. TDCOSMO. XII. Improved Hubble constant measurement from lensing time delays using spatially resolved stellar kinematics of the lens galaxy. A&A, 673, A9, 2023.
  2. Strong Lensing by Galaxies (review article). arXiv:2210.10790, 2022.
  3. LensingETC: a tool to optimize multi-filter imaging campaigns of galaxy-scale strong lensing systems. ApJ, 938, 141, 2022.
  4. TDCOSMO. IX. Systematic comparison between lens modelling software programs: time delay prediction for WGD 2038\(-\)4008. A&A, 667, A123, 2022.
  5. Dark matter haloes of massive elliptical galaxies at \(z\sim0.2\) are well described by the Navarro-Frenk-White profile. MNRAS, 503, 2, 2380-2405, 2021.
  6. High-resolution imaging follow-up of doubly imaged quasars. MNRAS, 503, 2, 1557-1567, 2021.
  7. STRIDES: A 3.9 per cent measurement of the Hubble constant from the strong lens system DES J0408\(-\)5354. MNRAS, 494, 6072--6102, 2020
  8. Unified lensing and kinematic analysis for any elliptical mass profile. MNRAS, stz1796, 2019.
  9. Is every strong lens model unhappy in its own way? Uniform modelling of a sample of 13 quadruply+ imaged quasars. MNRAS, 483, 5649-5671, 2019.
  10. Improving time-delay cosmography with spatially resolved kinematics. MNRAS, 473, 210-226, 2018.
  11. Measurement of the integrated Sachs-Wolfe effect using the AllWISE data release. ApJ, 827:116, 2016.
Visit ADS for my complete publication list.

Outreach

I am the creator of the Astro Bridge program, an initiative aimed at offering research experience to undergraduate students who come from communities or countries with limited access to such opportunities. By providing fully online programs, Astro Bridge aims to engage numerous students in a single project, ensuring access for a large number of participants. As part of a pilot project within this program, I have been mentoring approximately 20 undergraduate students from Bangladesh. Our project focuses on analyzing strong lensing systems, utilizing data obtained from the Hubble Space Telescope.

Please contact me if you are interested in creating such a project with the Astro Bridge template that faciliates efficient mentoring of a large number of students in the same research project.

CV

Find my CV here.