arXiv subjects

Sugata Kaviraj

Publications and source records attributed to Sugata Kaviraj.

At least 37 records · Page 2Linked to original sources

arXiv2023

Emergence and cosmic evolution of the Kennicutt-Schmidt relation driven by interstellar turbulence

The scaling relations between the gas content and star formation rate of galaxies provide useful insights into processes governing their formation and evolution. We investigate the emergence and the physical drivers of the global Kennicutt-Schmidt (KS) relation at $0.25 \leq z \leq 4$ in the cosmological hydrodynamic simulation NewHorizon capturing the evolution of a few hundred galaxies with a resolution of $\sim$ 40 pc. The details of this relation vary strongly with the stellar mass of galaxies and the redshift. A power-law relation $Σ_{\rm SFR} \propto Σ_{\rm gas}^{a}$ with $a \approx 1.4$, like that found empirically, emerges at $z \approx 2 - 3$ for the most massive half of the galaxy population. However, no such convergence is found in the lower-mass galaxies, for which the relation gets shallower with decreasing redshift. At the galactic scale, the star formation activity correlates with the level of turbulence of the interstellar medium, quantified by the Mach number, rather than with the gas fraction (neutral or molecular), confirming previous works. With decreasing redshift, the number of outliers with short depletion times diminishes, reducing the scatter of the KS relation, while the overall population of galaxies shifts toward low densities. Using pc-scale star formation models calibrated with local Universe physics, our results demonstrate that the cosmological evolution of the environmental and intrinsic conditions conspire to converge towards a significant and detectable imprint in galactic-scale observables, in their scaling relations, and in their reduced scatter.

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arXiv2023

Investigating the Dominant Environmental Quenching Process in UVCANDELS/COSMOS Groups

We explore how the fraction of quenched galaxies changes in groups of galaxies with respect to the distance to the center of the group, redshift, and stellar mass to determine the dominant process of environmental quenching in $0.2 < z < 0.8$ groups. We use new UV data from the UVCANDELS project in addition to existing multiband photometry to derive new galaxy physical properties of the group galaxies from the zCOSMOS 20k Group Catalog. Limiting our analysis to a complete sample of log$(M_*/M_{\odot})>10.56$ group galaxies we find that the probability of being quenched increases slowly with decreasing redshift, diverging from the stagnant field galaxy population. A corresponding analysis on how the probability of being quenched increases with time within groups suggests that the dominant environmental quenching process is characterized by slow ($\sim$Gyr) timescales. We find a quenching time of approximately $4.91^{+0.91}_{-1.47} $Gyrs, consistent with the slow processes of strangulation (Larson et al. 1980) and delayed-then-rapid quenching (Wetzel et al. 2013 arXiv:1206.3571v2 [astro-ph.CO]), although more data are needed to confirm this result.

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arXiv2023

Optimizing Roman's High Latitude Wide Area Survey for Low Surface Brightness Astronomy

One of the last remaining frontiers in optical/near-infrared observational astronomy is the low surface brightness regime (LSB, V-band surface brightness, $μ_V>$ 27 AB mag/arcsec$^2$). These are the structures at very low stellar surface densities, largely unseen by even current wide-field surveys such as the Legacy Survey. Studying this domain promises to be transformative for our understanding of star formation in low-mass galaxies, the hierarchical assembly of galaxies and galaxy clusters, and the nature of dark matter. It is thus essential to reach depths beyond $μ_V$ = 30 AB mag/arcsec$^2$ to detect the faintest extragalactic sources, such as dwarf galaxies and the stellar halos around galaxies and within galaxy clusters. The High Latitude Wide Area Survey offers a unique opportunity to statistically study the LSB universe at unprecedented depths in the IR over an area of $\sim$2000 square degrees. The high spatial resolution will minimize source confusion, allowing an unbiased characterization of LSB structures, including the identification of stars in nearby LSB galaxies and globular clusters. In addition, the combination of Roman with other upcoming deep imaging observatories (such as Rubin) will provide multi-wavelength coverage to derive photometric redshifts and infer the stellar populations of LSB objects.

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arXiv2023

Translators of galaxy morphology indicators between observation and simulation

Based on the recent advancements in the numerical simulations of galaxy formation, we anticipate the achievement of realistic models of galaxies in the near future. Morphology is the most basic and fundamental property of galaxies, yet observations and simulations still use different methods to determine galaxy morphology, making it difficult to compare them. We hereby perform a test on the recent NewHorizon simulation which has spatial and mass resolutions that are remarkably high for a large-volume simulation, to resolve the situation. We generate mock images for the simulated galaxies using SKIRT that calculates complex radiative transfer processes in each galaxy. We measure morphological indicators using photometric and spectroscopic methods following observer's techniques. We also measure the kinematic disk-to-total ratios using the Gaussian mixture model and assume that they represent the true structural composition of galaxies. We found that spectroscopic indicators such as $V/σ$ and $λ_{R}$ closely trace the kinematic disk-to-total ratios. In contrast, photometric disk-to-total ratios based on the radial profile fitting method often fail to recover the true kinematic structure of galaxies, especially for small galaxies. We provide translating equations between various morphological indicators.

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arXiv2023

Relaxed blue ellipticals: accretion-driven stellar growth is a key evolutionary channel for low mass elliptical galaxies

How elliptical galaxies form is a key question in observational cosmology. While the formation of massive ellipticals is strongly linked to mergers, the low mass (Mstar < 10^9.5 MSun) regime remains less well explored. In particular, studying elliptical populations when they are blue, and therefore rapidly building stellar mass, offers strong constraints on their formation. Here, we study 108 blue, low-mass ellipticals (which have a median stellar mass of 10^8.7 MSun) at z < 0.3 in the COSMOS field. Visual inspection of extremely deep optical HSC images indicates that less than 3 per cent of these systems have visible tidal features, a factor of 2 less than the incidence of tidal features in a control sample of galaxies with the same distribution of stellar mass and redshift. This suggests that the star formation activity in these objects is not driven by mergers or interactions but by secular gas accretion. We combine accurate physical parameters from the COSMOS2020 catalog, with measurements of local density and the locations of galaxies in the cosmic web, to show that our blue ellipticals reside in low-density environments, further away from nodes and large-scale filaments than other galaxies. At similar stellar masses and environments, blue ellipticals outnumber their normal (red) counterparts by a factor of 2. Thus, these systems are likely progenitors of not only normal ellipticals at similar stellar mass but, given their high star formation rates, also of ellipticals at higher stellar masses. Secular gas accretion, therefore, likely plays a significant (and possibly dominant) role in the stellar assembly of elliptical galaxies in the low mass regime.

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arXiv2022

Central star formation in an early-type galaxy I~Zw~81 in the Bootes void

The origin of star-formation in customarily passively evolving early-type massive galaxies is poorly understood. We present a case study of a massive galaxy, I~Zw~81, inside the Bootes void. The void galaxy is known to host an active galactic nuclei (AGN). Our detailed 2D decomposition of the surface brightness distribution in the $Canada$ $France$ $Hawaii$ $Telescope$ ($CFHT$) g- and r-bands revealed multiple structural components such as a nuclear point source, a bar, a ring, and an inner exponential disk followed by an outer low surface brightness (LSB) disk. I~Zw~81 turns out to be a disk-dominated galaxy with lenticular morphology. The modelling of the multi-wavelength spectral energy distribution (SED) shows that the galaxy is star-forming (SF), and belongs to the blue cloud. We find that the optical (g$-$r) color of the bar is bluer than the disks, and the far- and near-ultraviolet emission inside the galaxy observed with Ultraviolet Imaging Telescope (UVIT) onboard {\em AstroSat} is concentrated in the central few kpc region enclosing the bar. The strong bar might be playing a pivotal role in driving the gas inflow and causing SF activity in tandem with the minor merger-like interactions as evident from the deep $CFHT$ data. The low-luminosity AGN is insufficient to quench the central SF. The results are peculiar from the standpoint of a massive barred lenticular galaxy.

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arXiv2022

How the spectral energy distribution and galaxy morphology constrain each other, with application to morphological selection using galaxy colours

We introduce an empirical methodology to study how the spectral energy distribution (SED) and galaxy morphology constrain each other and implement this on 8000 galaxies from the HST CANDELS survey in the GOODS-South field. We show that the SED does constrain morphology and present a method that quantifies the strength of the link between these two quantities. Two galaxies with very similar SEDs are around three times more likely to also be morphologically similar, with SED constraining morphology most strongly for relatively massive red ellipticals. We apply our methodology to explore likely upper bounds on the efficacy of morphological selection using colour. We show that, under reasonable assumptions, colour selection is relatively ineffective at separating homogeneous morphologies. Even with the use of up to six colours for morphological selection, the average purity in the resultant morphological classes is only around 60 per cent. While the results can be improved by using the whole SED, the gains are not significant, with purity values remaining around 70 per cent or below.

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arXiv2021

Optimization of the Observing Cadence for the Rubin Observatory Legacy Survey of Space and Time: a pioneering process of community-focused experimental design

Vera C. Rubin Observatory is a ground-based astronomical facility under construction, a joint project of the National Science Foundation and the U.S. Department of Energy, designed to conduct a multi-purpose 10-year optical survey of the southern hemisphere sky: the Legacy Survey of Space and Time. Significant flexibility in survey strategy remains within the constraints imposed by the core science goals of probing dark energy and dark matter, cataloging the Solar System, exploring the transient optical sky, and mapping the Milky Way. The survey's massive data throughput will be transformational for many other astrophysics domains and Rubin's data access policy sets the stage for a huge potential users' community. To ensure that the survey science potential is maximized while serving as broad a community as possible, Rubin Observatory has involved the scientific community at large in the process of setting and refining the details of the observing strategy. The motivation, history, and decision-making process of this strategy optimization are detailed in this paper, giving context to the science-driven proposals and recommendations for the survey strategy included in this Focus Issue.

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arXiv2021

Introducing the NewHorizon simulation: Galaxy properties with resolved internal dynamics across cosmic time

Hydrodynamical cosmological simulations are increasing their level of realism by considering more physical processes and having greater resolution or larger statistics. However, usually either the statistical power of such simulations or the resolution reached within galaxies are sacrificed. Here, we introduce the NewHorizon project in which we simulate at high resolution a zoom-in region of $\sim(16\,\rm Mpc)^3$ that is larger than a standard zoom-in region around a single halo and is embedded in a larger box. A resolution of up to 34 pc is reached within galaxies; this allows the simulation to capture the multi-phase nature of the interstellar medium and the clumpy nature of the star formation process in galaxies. In this introductory paper, we present several key fundamental properties of galaxies and their black holes, including the galaxy mass function, cosmic star formation rate, galactic metallicities, the Kennicutt-Schmidt relation, the stellar-to-halo mass relation, galaxy sizes, stellar kinematics and morphology, gas content within galaxies and its kinematics, and the black hole mass and spin properties over time. The various scaling relations are broadly reproduced by NewHorizon with some differences with the standard observables. Owing to its exquisite spatial resolution, NewHorizon captures the inefficient process of star formation in galaxies, which evolve over time from being more turbulent, gas rich, and star bursting at high redshift. These high-redshift galaxies are also more compact, and they are more elliptical and clumpier until the level of internal gas turbulence decays enough to allow for the formation of discs. The NewHorizon simulation gives access to a broad range of galaxy formation and evolution physics at low-to-intermediate stellar masses, which is a regime that will become accessible in the near future through surveys such as the LSST.

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arXiv2021

The Star Formation Rates of Elliptical Galaxies from Core-Collapse Supernovae

The level of star formation in elliptical galaxies is poorly constrained, due to difficulties in quantifying the contamination of flux-based estimates of star formation from unrelated phenomena, such as AGN and old stellar populations. We here utilise core-collapse supernovae (CCSNe) as unambiguous tracers of recent star formation in ellipticals within a cosmic volume. We firstly isolate a sample of 421 z < 0.2, r < 21.8 mag CCSNe from the SDSS-II Supernova Survey. We then introduce a Bayesian method of identifying ellipticals via their colours and morphologies in a manner unbiased by redshift and yet consistent with manual classification from Galaxy Zoo 1. We find ~ 25 % of z < 0.2 r < 20 mag galaxies in the Stripe 82 region are ellipticals (~ 28000 galaxies). In total, 36 CCSNe are found to reside in ellipticals. We demonstrate that such early-types contribute a non-negligible fraction of star formation to the present-day cosmic budget, at 11.2 $\pm$ 3.1 (stat) $^{+3.0}_{-4.2}$ (sys) %. Coupling this result with the galaxy stellar mass function of ellipticals, the mean specific star formation rate (SSFR; $\overline{S}$) of these systems is derived. The best-fit slope is given by log ($\overline{S}(M)$/yr) = - (0.80 $\pm$ 0.59) log ($M/10^{10.5}\rm{M}_{\odot}$) - 10.83 $\pm$ 0.18. The mean SSFR for all log ($M/\rm{M}_{\odot}$) > 10.0 ellipticals is found to be $\overline{S} = 9.2 \pm 2.4$ (stat) $^{+2.7}_{-2.3}$ (sys) $\times 10^{-12}$ yr$^{-1}$, which is consistent with recent estimates via SED-fitting, and is 11.8 $\pm$ 3.7 (stat) $^{+3.5}_{-2.9}$ (sys) % of the mean SSFR level on the main sequence as also derived from CCSNe. We find the median optical spectrum of elliptical CCSN hosts is statistically consistent with that of a control sample of ellipticals that do not host CCSNe, implying that these SN-derived results are well-representative of the total low-z elliptical population.

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arXiv2021

Exploring the origin of thick disks using the NewHorizon and Galactica simulations

Ever since a thick disk was proposed to explain the vertical distribution of the Milky Way disk stars, its origin has been a recurrent question. We aim to answer this question by inspecting 19 disk galaxies with stellar mass greater than $10^{10}\,\rm M_\odot$ in recent cosmological high-resolution zoom-in simulations: Galactica and NewHorizon. The thin and thick disks are reasonably reproduced by the simulations with scale heights and luminosity ratios as observed. We then spatially classify the thin and thick disks and find that the thick disk stars are older, metal-poorer, kinematically-hotter, and higher in accreted star fraction, while both disks are dominated by the stars formed in situ. Half of the in-situ stars in the thick disks are formed before the galaxies develop their disks, and the rest are formed in spatially and kinematically thinner disks and then thickened with time by heating. However, the 19 galaxies have various properties and evolutionary routes, highlighting the need for statistically-large samples to draw general conclusions. We conclude from our simulations that the thin and thick disk components are not entirely distinct in terms of formation processes, but rather markers of the evolution of galactic disks. Moreover, as the combined result of the thickening of the existing disk stars and the continued formation of young thin-disk stars, the vertical distribution of stars does not change much after the disks settle, pointing to the modulation of both orbital diffusion and star formation by the same confounding factor: the proximity of galaxies to marginal stability.

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arXiv2020

Eigengalaxies: describing galaxy morphology using principal components in image space

We demonstrate how galaxy morphologies can be represented by weighted sums of "eigengalaxies" and how eigengalaxies can be used in a probabilistic framework to enable principled and simplified approaches in a variety of applications. Eigengalaxies can be derived from a Principal Component Analysis (PCA) of sets of single- or multi-band images. They encode the image space equivalent of basis vectors that can be combined to describe the structural properties of large samples of galaxies in a massively reduced manner. As an illustration, we show how a sample of 10,243 galaxies in the Hubble Space Telescope CANDELS survey can be represented by just 12 eigengalaxies. We show in some detail how this image space may be derived and tested. We also describe a probabilistic extension to PCA (PPCA) which enables the eigengalaxy framework to assign probabilities to galaxies. We present four practical applications of the probabilistic eigengalaxy framework that are particularly relevant for the next generation of large imaging surveys: we (i) show how low likelihood galaxies make for natural candidates for outlier detection (ii) demonstrate how missing data can be predicted (iii) show how a similarity search can be performed on exemplars (iv) demonstrate how unsupervised clustering of objects can be implemented.

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arXiv2020

Why do extremely massive disc galaxies exist today?

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios. Since mergers are the principal drivers of disc-to-spheroid morphological transformation, and the most massive galaxies typically have the richest merger histories, it is surprising that discs exist at all at the highest stellar masses (e.g. beyond the knee of the mass function). Using Horizon-AGN, a cosmological hydro-dynamical simulation, we show that extremely massive (M*> 10^11.4 MSun) discs are created via two channels. In the primary channel (accounting for ~70% of these systems and ~8% of massive galaxies) the most recent, significant merger (stellar mass ratio > 1:10) between a massive spheroid and a gas-rich satellite `spins up' the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for ~30% of these systems and ~3% of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. Not unexpectedly, the fraction of massive discs is larger at higher redshift, due to the Universe being more gas-rich. The morphological mix of galaxies at the highest stellar masses is, therefore, a strong function of the gas fraction of the Universe. Finally, these massive discs have similar black-hole masses and accretion rates to massive spheroids, providing a natural explanation for why a minority of powerful AGN are surprisingly found in disc galaxies.

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arXiv2020

The low-surface-brightness Universe: a new frontier in the study of galaxy evolution

New and forthcoming deep-wide surveys, from instruments like the HSC, LSST and EUCLID, are poised to revolutionize our understanding of galaxy evolution, by revealing aspects of galaxies that are largely invisible in past wide-area datasets. These surveys will open up the realm of low-surface-brightness (LSB) and dwarf galaxies -- which dominate the galaxy number density -- for the first time at cosmological distances. They will also reveal key, unexplored LSB structures which strongly constrain our structure-formation paradigm, such as merger-induced tidal features and intra-cluster light. However, exploitation of these revolutionary new datasets will require us to address several data-analysis challenges. Data-processing pipelines will have to preserve LSB structures, which are susceptible to sky over-subtraction. Analysis of the prodigious data volumes will require machine-learning (in particular unsupervised techniques), to augment or even replace traditional methods. Cosmological simulations, which are essential for a statistical understanding of the physics of galaxy evolution, will require mass and spatial resolutions that are high enough to resolve LSB/dwarf galaxies and LSB structures. And finally, estimation of physical properties (e.g. stellar masses and star formation rates) will require reliable redshift information. Since it is unlikely that even next-generation spectrographs will provide complete spectral coverage in the LSB/dwarf regime outside the nearby Universe, photometric redshifts may drive the science from these datasets. It is necessary, therefore, that the accuracy of these redshifts is good enough (e.g. < 10 per cent) to enable statistical studies in the LSB/dwarf regime. I outline the tremendous discovery potential of new/forthcoming deep-wide surveys and describe techniques which will enable us to solve the data-analysis challenges outlined above. (Abridged)

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arXiv2019

Galaxy morphological classification in deep-wide surveys via unsupervised machine learning

Galaxy morphology is a fundamental quantity, that is essential not only for the full spectrum of galaxy-evolution studies, but also for a plethora of science in observational cosmology. While a rich literature exists on morphological-classification techniques, the unprecedented data volumes, coupled, in some cases, with the short cadences of forthcoming 'Big-Data' surveys (e.g. from the LSST), present novel challenges for this field. Large data volumes make such datasets intractable for visual inspection (even via massively-distributed platforms like Galaxy Zoo), while short cadences make it difficult to employ techniques like supervised machine-learning, since it may be impractical to repeatedly produce training sets on short timescales. Unsupervised machine learning, which does not require training sets, is ideally suited to the morphological analysis of new and forthcoming surveys. Here, we employ an algorithm that performs clustering of graph representations, in order to group image patches with similar visual properties and objects constructed from those patches, like galaxies. We implement the algorithm on the Hyper-Suprime-Cam Subaru-Strategic-Program Ultra-Deep survey, to autonomously reduce the galaxy population to a small number (160) of 'morphological clusters', populated by galaxies with similar morphologies, which are then benchmarked using visual inspection. The morphological classifications (which we release publicly) exhibit a high level of purity, and reproduce known trends in key galaxy properties as a function of morphological type at z<1 (e.g. stellar-mass functions, rest-frame colours and the position of galaxies on the star-formation main sequence). Our study demonstrates the power of unsupervised machine learning in performing accurate morphological analysis, which will become indispensable in this new era of deep-wide surveys.

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arXiv2019

AGN in dwarf galaxies: frequency, triggering processes and the plausibility of AGN feedback

While AGN are considered to be key drivers of the evolution of massive galaxies, their potentially significant role in the dwarf-galaxy regime (M*< 10^9 MSun) remains largely unexplored. We combine optical and infrared data, from the Hyper Suprime-Cam (HSC) and the Wide-field Infrared Explorer (WISE) respectively, to explore the properties of ~800 AGN in dwarfs at low redshift (z<0.3). Infrared-selected AGN fractions are ~10-30 per cent in dwarfs, which, for reasonable duty cycles, indicates a high BH-occupation fraction. Visual inspection of the deep HSC images indicates that the merger fraction in dwarf AGN (~6 per cent) shows no excess compared to a control sample of non-AGN, suggesting that the AGN-triggering processes are secular in nature. Energetic arguments indicate that, in both dwarfs and massive galaxies, bolometric AGN luminosities (L_AGN) are significantly greater than supernova luminosities (L_SN). L_AGN/L_SN is, in fact, higher in dwarfs, with predictions from simulations suggesting that this ratio only increases with redshift. Together with the potentially high BH-occupation fraction, this suggests that, if AGN feedback is an important driver of massive-galaxy evolution, the same is likely to be true in the dwarf regime, contrary to our classical thinking.

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arXiv2019

New Horizon: On the origin of the stellar disk and spheroid of field galaxies at $z=0.7$

The origin of the disk and spheroid of galaxies has been a key open question in understanding their morphology. Using the high-resolution cosmological simulation, New Horizon, we explore kinematically decomposed disk and spheroidal components of 144 field galaxies with masses greater than $\rm 10^9\,M_{\odot}$ at $z=0.7$. The origins of stellar particles are classified according to their birthplace (in situ or ex situ) and their orbits at birth. Before disk settling, stars form mainly through chaotic mergers between proto-galaxies and become part of the spheroidal component. When disk settling starts, we find that more massive galaxies begin to form disk stars from earlier epochs; massive galaxies commence to develop their disks at $z\sim1-2$, while low-mass galaxies do after $z\sim1$. The formation of disks is affected by accretion as well, as mergers can trigger gas turbulence or induce misaligned gas infall that prevents galaxies from forming co-rotating disk stars. The importance of accreted stars is greater in more massive galaxies, especially in developing massive spheroids. A significant fraction of the spheroids comes from the disk stars that are perturbed, which becomes more important at lower redshifts. Some ($\sim12.5\%$) of our massive galaxies develop counter-rotating disks from the gas infall misaligned with the existing disk plane, which can last for more than a Gyr until they become the dominant component, and flip the angular momentum of the galaxy in the opposite direction. The final disk-to-total ratio of a galaxy needs to be understood in relation to its stellar mass and accretion history. We quantify the significance of the stars with different origins and provide them as guiding values.

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arXiv2019

Next Generation LSST Science

The Large Synoptic Survey Telescope (LSST) can advance scientific frontiers beyond its groundbreaking 10-year survey. Here we explore opportunities for extended operations with proposal-based observing strategies, new filters, or transformed instrumentation. We recommend the development of a mid-decade community- and science-driven process to define next-generation LSST capabilities.

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