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Research Interests

I am an associate professor at the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. Previously, I was at the Kavli Institute for Cosmological Physics at the University of Chicago and part of the Computational Astrophysics Group of Andrey Kravtsov.

For my PhD, I was in the Theory group of the Galaxies and Cosmology section at the Max Planck Institute for Astronomy. I worked with Dr. Frank van den Bosch and my phd thesis focussed on understanding the Galaxy-Dark Matter Connection. Please also have a look at my curriculum vitae.

The reference to the published versions of each of the papers described below can be found on the publications page.

The nature of PS1-10afx

The object PS1-10afx was recently discovered by the Pan-STARRS team as an unusually superluminous supernova, and is very difficult to explain given the current theoretical understanding of supernova. This supernova was too bright given the quick rise and decay of light observed in its light curve. The host redshift has been measured to be 1.3883. In the paper, Extraordinary magnification of the ordinary Type Ia Supernova PS1-10afx, we show that the spectra from this supernova and the shape of its light curve are consistent with it being a Type Ia supernova. The fluxes from the light curve are off by a factor 30 compared to the flux expected from this object given its redshift. We investigate the possibility that this object is gravitationally lensed, and explore constraints on the mass of the intervening object which could cause such an anamolous flux. If the gravitational lensing within this system is confirmed, then it would be the highest magnification SN ever detected. It has implications in the use of SN to perform precision cosmology as such chance alignments can affect the redshift distance relations inferred from SN. The models also show that it may be a low mass halo along the line of sight to the SN, most probably in its vicinity. This result also opens the possibility that the extremely energetic gamma ray bursts detected at high redshifts may actually be gravitationally lensed.

The structure of Milky way dwarf spheroidals

The dwarf spheroidal satellites around the Milky Way are excellent laboratories for testing the cold dark matter paradigm and its predictions on small scales, as they contain very little baryons, and their structure today is dominated by dark matter. It has been recently pointed out that the CDM scenario predicts a number of massive subhalos, whose density structure is inconsistent with the structure observed in the dwarf spheroidals of the Milky Way. We investigated how the lack of baryons in the subhalos, and the enhanced tidal forces due to the presence of the Milky Way disk can affect the predictions of subhalo structure in dark matter simulations. In the paper, Effects of baryon removal on the structure of dwarf spheroidal galaxies, we show how these effects can bring the observed structure of milky way dwarf spheroidals in agreement with predictions of cold dark matter theory.

Are central galaxies special?

Central galaxies are thought to reside atthe minimum of the potential well of their dark matter halos. What perks does this special position come with? In this respect it is important to investigate whether the luminosities of the central galaxies are consistent with statistical draws from the luminosity function of galaxies. In the paper, Magnitude gap statistics and the conditional luminosity function, I predict the luminosity distributions of central and brightest satellite galaxies and the magnitude gap between them, if they are drawn from the conditional luminosity function. I show that it is reasonable to expect that at fixed richness, the more massive halos have larger magnitude gaps.

Pseudo evolution of halo mass

A dark matter halo is commonly defined as a spherical overdensity of matter with respect to a reference density, such as the critical density or the mean matter density of the Universe. Such definitions of halo mass lead to a spurious evolution in the halo's mass even if its physical density profile remains constant over time. This pseudo-evolution in mass is caused by the evolution of the reference density with redshift, and has little connection with the actual physical accretion of mass. This fact has been pointed before, but largely ignored by the community. We quantify the effect of pseudo-evolution of halo massusing density distributions around peaks from numerical simulations, and show that a simple analytical model can be used to describe this pseudo evolution. We also argue that it is important to consider this effect before attaching deep physical meaning to the redshift evolution of various galaxy scaling relations or galaxy cluster scaling relations. Check out the arxiv preprint of the paper: The pseudo evolution of halo mass. We recently extended the analysis to look at cluster scaling relations and how their evolution can be affected due to pseudo-evolution, check out the paper On the origin and evolution of cluster scaling relations.

Cosmological constraints from a combination of clustering and lensing

Galaxies are surrounded by extended halos of dark matter, whose abundance and clustering is sensitive to the cosmological parameters. The abundance of galaxies, their clustering, and the clustering of matter around these galaxies can therefore be used to probe the abundance and clustering of halos, and thus the cosmological parameters of interest. We presented a series of papers in the first week of July regarding this issue. In the first paper (links below), we improved the precision of the halo model, an analytical framework to model the abundance and clustering of galaxies. In the second paper, we presented a Fisher matrix analysis to show that the cosmological parameters such as the matter density, amplitude of matter fluctuations, sum of neutrino masses and the dark energy equation of state can be constrained to a good accuracy using existing data from SDSS. In the third paper, we presented constraints on the matter density parameter and the amplitude of initial density fluctuations in the Universe. To learn more, please have a look at: Cosmological Constraints from a Combination of Galaxy Clustering and Lensing -- I. Theoretical Framework, II. Fisher matrix analysis, III. Application to SDSS data. Some of the results were previously summarized in the conference proceedings The Galaxy-Dark Matter Connection: A Cosmological Perspective.

How accurate is our knowledge of the galaxy bias?

The galaxy distribution in the universe is biased with respect to the overall matter distribution. It is important to quantify the bias of galaxies to be able to use the galaxy distribution to measure the cosmological parameters of the Universe. The large scale bias of galaxies can be measured from the large scale clustering measurements or by using the small scale clustering of galaxies. In this paper, which was recently submitted to the Astrophysical Journal, we compared the measurements of the galaxy bias-luminosity relation obtained by these techniques and show a systematic discrepancy between the two. To learn more please have a look at the preprint How accurate is our knowledge of the galaxy bias?.

Friends-of-friends algorithm

The friends of friends (FOF) is a percolation algorithm which has been routinely used to identify haloes in numerical simulations. We explore the properties of FOF halos, in particular their average overdensity, and the resolution dependence of FOF halo mass in light of results from percolation theory. An improved understanding of the FOF algorithm helps us gain further insight into the universality of the FOF halo mass function. Check out the arxiv preprint of the paper: The overdensity and halo masses of FOF haloes and the universality of the halo mass function. The code to calculate the overdensity of FOF halos as a function of the mass of FOF halos can be downloaded by clicking here.

Halo Mass-Luminosity Relation of Central Galaxies in SDSS

According to our current understanding, galaxies are believed to reside in extended dark matter haloes. The properties of a dark matter halo, in particular its mass, shapes the properties of the galaxy that forms at its center. I am interested in the scaling relations between galaxy properties and halo mass. I have investigated the halo mass-luminosity relationship of galaxies. The kinematics of satellite galaxies can be used to infer the mass of the halo of a galaxy. I have developed methods to reliably obtain the mean and the scatter of the halo mass-luminosity relation of galaxies and applied it to the Sloan Digital Sky Survey. Check the series of papers: Satellite Kinematics:I , Satellite Kinematics:II and Satellite Kinematics:III .

Cosmic Transparency

The Universe is remarkably transparent. Photons from distant cosmic objects travel to us, uninhibited, despite encountering considerable columns of dark matter, hydrogen plasma, hydrogen gas and dust. In a research project, in collaboration with David Hogg and Jo Bovy, we recently obtained a quantitative measure of the transparency of the Universe by testing the Etherington relation using current SNeIa and Baryon acoustic feature measurements. Please have a look at the preprint of the paper on Cosmic Transparency .

Popular science

I have recently started writing about Astronomy in my mother tongue Marathi as a Marathi Astronomy blog. These are Astronomy popularisation articles written about interesting phenomenon explained in an easy to understand level.

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