arXiv subjects

Suman Sarkar

Publications and source records attributed to Suman Sarkar.

At least 37 records · Page 2Linked to original sources

arXiv2022

The maximum extent of the filaments and sheets in the cosmic web: an analysis of the SDSS DR17

Filaments and sheets are striking visual patterns in cosmic web. The maximum extent of these large-scale structures are difficult to determine due to their structural variety and complexity. We construct a volume-limited sample of galaxies in a cubic region from the SDSS, divide it into smaller subcubes and shuffle them around. We quantify the average filamentarity and planarity in the three-dimensional galaxy distribution as a function of the density threshold and compare them with those from the shuffled realizations of the original data. The analysis is repeated for different shuffling lengths by varying the size of the subcubes. The average filamentarity and planarity in the shuffled data show a significant reduction when the shuffling scales are smaller than the maximum size of the genuine filaments and sheets. We observe a statistically significant reduction in these statistical measures even at a shuffling scale of $\sim 130 \,$Mpc, indicating that the filaments and sheets in three dimensions can extend up to this length scale. They may extend to somewhat larger length scales that are missed by our analysis due to the limited size of the SDSS data cube. We expect to determine these length scales by applying this method to deeper and larger surveys in future.

astro-ph.CO↗

arXiv2022

Identification of Grand-design and Flocculent Spirals from SDSS using Convolutional Neural network

Spiral galaxies can be classified into the {\it Grand-designs} and {\it Flocculents} based on the nature of their spiral arms. The {\it Grand-designs} exhibit almost continuous and high contrast spiral arms and are believed to be driven by density waves, while the {\it Flocculents} have patchy and low-contrast spiral features and are primarily stochastic in origin. We train a convolutional neural network (CNN) model to classify spirals into {\it Grand-designs} and {\it Flocculents}, with a testing accuracy of $\mathrm{97.2\%}$. We then use the above model for classifying $\mathrm{1,354}$ new spirals from the SDSS. Out of these, $\mathrm{721}$ were identified as {\it Flocculents}, and the rest as {\it Grand-designs}. We find the median asymptotic rotational velocities of our newly classified {\it Grand-designs} and {\it Flocculents} are $218 \pm 86$ and $145 \pm 67$ respectively, indicating that the {\it Grand-designs} are mostly the high-mass and the {\it Flocculents} the intermediate-mass spirals. This is further corroborated by the observation that the median morphological indices of the {\it Grand-designs} and {\it Flocculents} are $2.6 \pm 1.8$ and $4.7 \pm 1.9$ respectively, implying that the {\it Flocculents} primarily consist of a late-type galaxy population in contrast to the {\it Grand-designs}. Finally, an almost equal fraction of of bars $\sim$ 0.3 in both the classes of spiral galaxies reveals that the presence of a bar component does not regulate the type of spiral arm hosted by a galaxy. Our results may have important implications for formation and evolution of spiral arms in galaxies.

astro-ph.GA↗

arXiv2022

On the origin of red spirals: Does assembly bias play a role?

The formation of the red spirals is a puzzling issue in the standard picture of galaxy formation and evolution. Most studies attribute the colour of the red spirals to different environmental effects. We analyze a volume limited sample from the SDSS to study the roles of small-scale and large-scale environments on the colour of spiral galaxies. We compare the star formation rate, stellar age and stellar mass distributions of the red and blue spirals and find statistically significant differences between them at $99.9\%$ confidence level. The red spirals inhabit significantly denser regions than the blue spirals, explaining some of the observed differences in their physical properties. However, the differences persist in all types of environments, indicating that the local density alone is not sufficient to explain the origin of the red spirals. Using an information theoretic framework, we find a small but non-zero mutual information between the colour of spiral galaxies and their large-scale environment that are statistically significant ($99.9\%$ confidence level) throughout the entire length scale probed. Such correlations between the colour and the large-scale environment of spiral galaxies may result from the assembly bias. Thus both the local environment and the assembly bias may play essential roles in forming the red spirals. The spiral galaxies may have different assembly history across all types of environments. We propose a picture where the differences in the assembly history may produce spiral galaxies with different cold gas content. Such a difference would make some spirals more susceptible to quenching. In all environments, the spirals with high cold gas content could delay the quenching and maintain a blue colour, whereas the spirals with low cold gas fractions would be easily quenched and become red.

astro-ph.GA↗

arXiv2021

The local environment of flat galaxies

The existence of flat or bulgeless galaxies poses a challenge to the hierarchical structure formation scenario advocated by modern cosmology. We determine the geometrical environment of a sample of $315$ flat galaxies from the Revised Flat Galaxy Catalog (RFGC) using `local dimension' $D$, which, on a given length scale, quantifies the dimension of the cosmic structure in which a galaxy is embedded. For galaxies residing in filaments, nodes and sheets, $D \sim 1$, $D \sim 1.5$ and $D \sim 2$ respectively; $D \sim 3$ represents field galaxies. We also determine the local dimensions of a sample of 15,622 non-flat galaxies identified in the Galaxy Zoo project from the Sloan Digital Sky Survey (SDSS). We find that the median values of $D$ for the flat and the non-flat galaxies are $2.2$ and $1.8$ respectively, implying that flat galaxies are located in a relatively sparser environment compared to non-flat galaxies; a Kolmogorov-Smirnov (KS) test indicates that their geometrical environments are different at $>99$\% confidence level. Further, using a group finding algorithm, we study the local environment of a subset of $779$ flat galaxies with major-to-minor axes ratio $a/b >10$ identified as superthin galaxies. We find that the median clusterization index $k_{\rm{min}}$ for superthin flat galaxies $\sim$ $2.3$ while $\sim$ $1.7$ for other flat galaxies, confirming that the superthins reside in an under-dense environment compared to other flat galaxies at $> 98 \%$ confidence level. Our results may therefore have important implications for the formation and evolution models of flat galaxies in the universe.

astro-ph.GA↗

arXiv2021

Galaxy interactions in different environments: An analysis of galaxy pairs from the SDSS

We analyze the galaxy pairs in a volume limited sample ($M_r \leq -21$) from the SDSS to study the effects of galaxy interactions on the star formation rate and colour of galaxies in different environments. We study the star formation rate and colour of the paired galaxies as a function of projected separation and compare the results with their control samples matched in stellar mass, redshift and local density. We find that the major interactions significantly enhance the star formation rate in paired galaxies and turn them bluer with decreasing pair separation within $30$ kpc. The impact of tidal interactions on star formation rate and colour are more significant in the heavier members of the major pairs. The star formation enhancement in major pairs is significantly higher at the low-density environments, where the influence can extend up to $\sim 100$ kpc. Contrarily, the major pairs at high-density environments show suppression in their star formation. Depending on the embedding environments, the major interactions in the intrinsically brighter galaxy pairs can thus enhance or quench star formation. We find that the minor pairs at both low-density and high-density environments are significantly less star-forming and redder than their control galaxies. It indicates that the minor interactions in intrinsically brighter galaxy pairs always suppress the star formation irrespective of their environment. The lighter members in these minor pairs show a greater susceptibility to suppressed star formation. Our results imply that both the major and minor interactions can contribute to the observed bimodality. We conclude that the galaxy evolution is determined by a complex interplay between the galaxy properties, galaxy interactions, and environment.

astro-ph.GA↗

arXiv2021

Testing homogeneity of the galaxy distribution in the SDSS using Renyi entropy

We analyze a set of volume limited sample of galaxies from the SDSS to study the issue of cosmic homogeneity. We use the Renyi entropy of different order to probe the inhomogeneties present in the galaxy distributions. We also calculate the Renyi diveregence to quantify the deviations of the galaxy distribution from a homogeneous Poisson distribution on different length scales. We separately carry out the analysis using the overlapping spheres and the independent voxels. Our analysis suggests that the scale of homogeneity is underestimated in the smaller galaxy samples due to the suppression of inhomogeneities by the overlapping of the measuring speheres. We find that an analysis with the independent voxels and/or use of a significantly larger galaxy sample can help to circumvent or mitigate this problem. Combining the results from these analyses, we find that the galaxy distribution in the SDSS becomes homogeneous on a length scale beyond $140 \, h^{-1}\, {\rm Mpc}$.

astro-ph.CO↗

arXiv2021

Green valley galaxies in the cosmic web: internal versus environmental quenching

We analyze the SDSS data to classify the galaxies based on their colour using a fuzzy set-theoretic method and quantify their environments using the local dimension. We find that the fraction of the green galaxies does not depend on the environment and $10\%-20\%$ of the galaxies at each environment are in the green valley depending on the stellar mass range chosen. Approximately $10\%$ of the green galaxies at each environment host an AGN. Combining data from the Galaxy Zoo, we find that $\sim 95\%$ of the green galaxies are spirals and $\sim 5\%$ are ellipticals at each environment. Only $\sim 8\%$ of green galaxies exhibit signs of interactions and mergers, $\sim 1\%$ have dominant bulge, and $\sim 6\%$ host a bar. We show that the stellar mass distributions for the red and green galaxies are quite similar at each environment. Our analysis suggests that the majority of the green galaxies must curtail their star formation using physical mechanism(s) other than interactions, mergers, and those driven by bulge, bar and AGN activity. We speculate that these are the massive galaxies that have grown only via smooth accretion and suppressed the star formation primarily through mass driven quenching. Using a Kolmogorov-Smirnov test, we do not find any statistically significant difference between the properties of green galaxies in different environments. We conclude that the environmental factors play a minor role and the internal processes play the dominant role in quenching star formation in the green valley galaxies.

astro-ph.GA↗

arXiv2020

Do galactic bars depend on environment?: An information theoretic analysis of Galaxy Zoo 2

We use an information theoretic framework to analyze data from the Galaxy Zoo 2 project and study if there are any statistically significant correlations between the presence of bars in spiral galaxies and their environment. We measure the mutual information between the barredness of galaxies and their environments in a volume limited sample ($M_r \leq -21$) and compare it with the same in datasets where (i) the bar/unbar classifications are randomized and (ii) the spatial distribution of galaxies are shuffled on different length scales. We assess the statistical significance of the differences in the mutual information using a t-test and find that both randomization of morphological classifications and shuffling of spatial distribution do not alter the mutual information in a statistically significant way. The non-zero mutual information between barredness and environment arises due to the finite and discrete nature of the dataset which can be entirely explained by mock Poisson distributions. We also separately compare the cumulative distribution functions of the barred and unbarred galaxies as a function of their local density. Using a Kolmogorov-Smirnov test, we find that the null hypothesis can not be rejected even at $75\%$ confidence level. Our analysis indicates that environments do not play a significant role in the formation of a bar, which is largely determined by the internal processes of the host galaxy.

astro-ph.GA↗

arXiv2020

Effect of microstructure on the electronic transport properties of epitaxial CaRuO$_3$ thin films

We have carried out extensive comparative studies of the structural and transport properties of CaRuO$_3$ thin films grown under various oxygen pressure. We find that the preferred orientation and surface roughness of the films are strongly affected by the oxygen partial pressure during growth. This in turn affects the electrical and magnetic properties of the films. Films grown under high oxygen pressure have the least surface roughness and show transport characteristics of a good metal down to the lowest temperature measured. On the other hand, films grown under low oxygen pressures have high degree of surface roughness and show signatures of ferromagnetism. We could verify that the low frequency resistance fluctuations (noise) in these films arise due to thermally activated fluctuations of local defects and that the defect density matches with the level of disorder seen in the films through structural characterizations.

cond-mat.str-el↗

arXiv2020

Anharmonicity in Raman-active phonon modes in atomically thin MoS$_2$

Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of $\mathrm{E_{2g}^1}$ and $\mathrm{A_{1g}}$ modes for single-layer and natural bilayer MoS$_2$ over a broad range of temperatures ($8 < $T$ < 300$ K). Both the Raman shift and FWHM of these modes show linear temperature dependence for $T>100$ K, whereas they become independent of temperature for $T<100$ K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperature-dependence of the line-width of both the modes. It also plays an important role in determining the temperature-dependence of the frequency of the Raman modes. The observed evolution of the line-width of the A$_{1g}$ mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS$_2$ on two different substrates -- SiO$_2$ and hexagonal boron nitride, we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS$_2$ irrespective of substrate and layer number.

cond-mat.mes-hall↗

arXiv2020

Exploring galaxy colour in different environments of the cosmic web with SDSS

We analyze a set of volume limited samples from the SDSS to study the dependence of galaxy colour on different environments of the cosmic web. We measure the local dimension of galaxies to determine the geometry of their embedding environments and find that filaments host a higher fraction of red galaxies than sheets at each luminosity. We repeat the analysis at a fixed density and recover the same trend which shows that galaxy colours depend on geometry of environments besides local density. At a fixed luminosity, the fraction of red galaxies in filaments and sheets increases with the extent of these environments. This suggests that the bigger structures have a larger baryon reservoir favouring higher accretion and larger stellar mass. We find that the mean colour of the red and blue populations are systematically higher in the environments with smaller local dimension and increases monotonically in all the environments with luminosity. We observe that the bimodal nature of the galaxy colour distribution persists in all environments and all luminosities, which suggests that the transformation from blue to red galaxy can occur in all environments.

astro-ph.GA↗

arXiv2020

Can a conditioning on stellar mass explain the mutual information between morphology and environment?

Recent studies with SDSS have shown that a statistically significant non-zero mutual information between morphology and environment persists up to several tens of Mpc, which awaits an explanation. Galaxies in different environments acquire their stellar mass through accretion and merger and the stellar mass function of galaxies is known to depend on both environment and morphology. Naturally, stellar mass can be an important link between morphology and environment which may explain the non-zero mutual information between the two. Measuring the mutual information between morphology and environment by conditioning the stellar mass would allow us to test this possibility. We employ here a volume and stellar mass limited sample from the $16^{th}$ data release (DR16) of the SDSS and find a non-zero conditional mutual information throughout the entire length scales probed. We compare the results with three different semi-analytic models implemented on the Millennium simulation and find their predictions to be in fairly good agreement with SDSS on smaller length scales ( $\lesssim 30 h^{-1}$ Mpc ), with a clear discrepancy observed at larger length scales ( $\gtrsim 30 h^{-1}$ Mpc ) where the models predict significantly lower conditional mutual information than the SDSS. Our analysis therefore suggests that only environmental and morphology dependence of stellar mass are inadequate in explaining the observed mutual information between morphology and environment and that physical processes which alters morphology may not necessarily have an impact on the stellar mass of galaxies and vice versa.

astro-ph.GA↗

arXiv2020

A study on the statistical significance of mutual information between morphology of a galaxy and its large-scale environment

A non-zero mutual information between morphology of a galaxy and its large-scale environment is known to exist in SDSS upto a few tens of Mpc. It is important to test the statistical significance of these mutual information if any. We propose three different methods to test the statistical significance of these non-zero mutual information and apply them to SDSS and Millennium Run simulation. We randomize the morphological information of SDSS galaxies without affecting their spatial distribution and compare the mutual information in the original and randomized datasets. We also divide the galaxy distribution into smaller subcubes and randomly shuffle them many times keeping the morphological information of galaxies intact. We compare the mutual information in the original SDSS data and its shuffled realizations for different shuffling lengths. Using a t-test, we find that a small but statistically significant (at 99.9% confidence level) mutual information between morphology and environment exists upto the entire length scale probed. We also conduct another experiment using mock datasets from a semi-analytic galaxy catalogue where we assign morphology to galaxies in a controlled manner based on the density at their locations. The experiment clearly demonstrate that mutual information can effectively capture the physical correlations between morphology and environment. Our analysis suggests that physical association between morphology and environment may extend to much larger length scales than currently believed and the information theoretic framework presented here, can serve as a sensitive and useful probe of the assembly bias and large-scale environmental dependence of galaxy properties.

astro-ph.GA↗

arXiv2020

Liquid-phase reinforced Metal matrix (LMM) composite with non-intuitive properties

Over the ages, efforts have been made to use composite design to reinforce metals and alloys in order to increase their strength and modulus. On the other hand, nature herself improves the strength, ductility, stiffness and toughness of materials by strengthening them with liquids having zero strength/modulus. Here, emulating nature, efforts have been made to develop a new class of tin based alloy/composite with liquid metal reinforcement (LMM). Based on thermodynamic calculations, a composition has been designed such that on melting and casting it forms a solid metal (tin solid solution) and the eutectic mixture remains in liquid form at room temperature. The composite structure named as LMM shows multifold improvement in hardness, strength, ductility, toughness and wear resistance as compared to conventional solder alloys. A Finite Element Method (FEM) based simulation shows strain distribution in the composite which results in the unique behavior. The LMM also shows a negative coefficient of thermal expansion which is further verified using in-situ microscopy and thermodynamic calculations.

physics.app-ph↗

arXiv2019

Probing defect states in few-layer MoS$_{2}$ by conductance fluctuation spectroscopy

Despite the concerted effort of several research groups, a detailed experimental account of defect dynamics in high-quality single- and few-layer transition metal dichalcogenides remain elusive. In this paper we report an experimental study of the temperature dependence of conductance and conductance-fluctuations on several few-layer MoS$_{2}$ exfoliated on hexagonal boron nitride and covered by a capping layer of high-$κ$ dielectric HfO$_{2}$. The presence of the high-$κ$ dielectric made the device extremely stable against environmental degradation as well as resistant to changes in device characteristics upon repeated thermal cycling enabling us to obtain reproducible data on the same device over a time-scale of more than one year. Our device architecture helped bring down the conductance fluctuations of the MoS$_2$ channel by orders of magnitude compared to previous reports. The extremely low noise levels in our devices made in possible to detect the generation-recombination noise arising from charge fluctuation between the sulphur-vacancy levels in the band gap and energy-levels at the conductance band-edge. Our work establishes conduction fluctuation spectroscopy as a viable route to quantitatively probe in-gap defect levels in low-dimensional semiconductors.

cond-mat.mes-hall↗

arXiv2019

Unravelling the Cosmic Web: An analysis of the SDSS DR14 with the Local Dimension

We analyze a volume limited galaxy sample from the SDSS to study the environments of galaxies on different length scales in the local Universe. We measure the local dimension of the SDSS galaxies on different length scales and find that the sheets or sheetlike structures are the most prevalent pattern in the cosmic web throughout the entire length scales. The abundance of sheets peaks at $30 \, h^{-1}\, {\rm Mpc}$ and they can extend upto a length scales of $90 \, h^{-1}\, {\rm Mpc}$ . Analyzing mock catalogues, we find that the sheets are non-existent beyond $30 \, h^{-1}\, {\rm Mpc}$ in the Poisson distributions. We find that the straight filaments in the SDSS galaxy distribution can extend only upto a length scale of $30 \, h^{-1}\, {\rm Mpc}$. Our results indicate that the environment of a galaxy exhibits a gradual transition towards higher local dimension with increasing length scales finally approaching a nearly homogeneous network on large scales. We compare our findings with a semi analytic galaxy catalogue from the Millennium Run simulation which are in fairly good agreement with the observations. We also test the effects of the number density of the sample and the cut-off in the goodness of fit which shows that the results are nearly independent of these factors. Finally we apply the method to a set of simulations of the segment Cox process and find that it can characterize such distributions.

astro-ph.CO↗

arXiv2018

Testing isotropy in the Universe using photometric and spectroscopic data from the SDSS

We analyze two volume limited galaxy samples from the SDSS photometric and spectroscopic data to test the isotropy in the local Universe. We use information entropy to quantify the global anisotropy in the galaxy distribution at different length scales and find that the galaxy distribution is highly anisotropic on small scales. The observed anisotropy diminishes with increasing length scales and nearly plateaus out beyond a length scale of 200 Mpc/h in both the datasets. We compare these anisotropies with those predicted by the mock catalogues from the N-body simulations of the Lambda CDM model and find a fairly good agreement with the observations. We find a small residual anisotropy on large scales which decays in a way that is consistent with the linear perturbation theory. The slopes of the observed anisotropy converge to the slopes predicted by the linear theory beyond a length scale of ~ 200 Mpc/h indicating a transition to isotropy. We separately compare the anisotropies observed across the different parts of the sky and find no evidence for a preferred direction in the galaxy distribution.

astro-ph.CO↗

arXiv2016

An information theory based search for homogeneity on the largest accessible scale

We analyze the SDSS DR12 quasar catalogue to test the large-scale smoothness in the quasar distribution. We quantify the degree of inhomogeneity in the quasar distribution using information theory based measures and find that the degree of inhomogeneity diminishes with increasing length scales which finally reach a plateau at $\sim 250 \, h^{-1}\, {\rm Mpc}$. The residual inhomogeneity at the plateau is consistent with that expected for a Poisson point process. Our results indicate that the quasar distribution is homogeneous beyond length scales of $250 \, h^{-1}\, {\rm Mpc}$.

astro-ph.CO↗