arXiv subjects

Arunima Banerjee

Publications and source records attributed to Arunima Banerjee.

At least 19 recordsLinked to original sources

arXiv2026

Co-evolution of bar and spiral arms in TNG50 simulations using Information Theory

Using Information Theory, we investigate the co-evolution of bars and spiral arms in barred-spiral galaxies from the cosmological magneto-hydrodynamic Illustris TNG50 simulations. We first calculate Mutual Information (MI) between a structural or kinematic parameter of the bar (bar strength $A_{2bar}$, bar length $r_{bar}$, bar pattern speed $\Omega$) and that of a spiral arm (spiral strength $A_{2spiral}$, spiral arm pitch angle $\Psi$). We calculate MI in two different galaxy samples: (i) one forming bars before spirals (ii) other forming spirals before bars. We note, spirals form immediately after bars in the first sample, whereas bars form 1.7 Gyrs after spirals in the second. We find a high mean MI value in each of these samples (0.4 - 0.5), and in the combined sample (0.4-0.8), confirming a fair degree of association of the bar and the spiral arm. To identify whether the bar or the spiral arm effectively drives their co-evolution, we calculate the Transfer Entropy (TE) (bar-to-spiral TE, spiral-to-bar TE), from the time series data of each of the above bar-spiral parameter pairs. We find that the median bar-to-spiral TE and spiral-to-bar TE values vary between $ 0.33$ and $ 0.42$ for each galaxy sample, comparable to those of the combined sample. A similar trend was observed in our calculated Liang information flow rates. Our novel approach may possibly indicate that the bar and the spiral arm regulate their co-evolution on an equal footing.

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arXiv2026

Identifying Warped Galaxies in Pan-STARRS and Euclid using Deep Convolutional Neural Network

Warped galactic discs are common, yet their detection remains challenging, as the outskirts of galaxies are typically faint. Advances in deep imaging surveys improve the detectability of such features, while machine learning enables efficient analysis of large datasets. Using the Pan-STARRS EGIPS catalogue, we develop a deep learning framework by fine-tuning the Zoobot convNext-nano model on 1000 edge-on galaxy FITS images to distinguish warped and non-warped edge-on galaxies with 83\% accuracy. The trained model is then applied to a larger sample, identifying 2088 warped and 1398 non-warped galaxies with a high prediction probability threshold ($\geq 0.85$). Additionally, we use the model to predict on 3226 edge-on galaxies from the Euclid Q1 survey, demonstrating the model's ability to generalise across datasets with differing resolutions. To analyse the model predictions, we employ LayerCAM to identify the regions of galaxy images that contribute to the classification. We find that warped galaxies differ primarily in their structural properties, exhibiting lower axis ratios and higher asymmetry. Warped galaxies were found to be bluer, with younger stellar populations and enhanced star formation. These results highlight the effectiveness of deep learning methods in identifying subtle morphological features, such as warps, and demonstrate their potential for studying structural properties of galaxies in current and upcoming large imaging surveys.

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arXiv2025

$\mbox{H}$ $\mbox{I}$ 21-cm Absorption Spectra Classification using Machine Learning

$\mbox{H}$ $\mbox{I}$ 21-cm absorption, an extremely useful tool to study the cold atomic hydrogen gas, can arise either from the intervening galaxies along the line-of-sight towards the background radio source or from the radio source itself. Determining whether $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines detected as part of large, blind surveys are `intervening' or `associated' using optical spectroscopy would be unfeasible. We therefore investigate a more efficient, machine learning (ML)-based method to classify $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines. Using a sample of 118 known $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines from the literature, we train six ML models (Gaussian naive Bayes, logistic regression, decision tree, random forest, SVM and XGBoost) on the spectral parameters obtained by fitting the Busy function to the absorption spectra. We found that a random forest model trained on these spectral parameters gives the most reliable classification results, with an accuracy of 89%, a $F_1$-score of 0.9 and an AUC score of 0.94. We note that the linewidth parameter $w_{20}$ is the most significant spectral parameter that regulates the classification performance of this model. Retraining this random forest model only with this linewidth and the integrated optical depth parameters yields an accuracy of 88%, a $F_1$-score of 0.88 and an AUC score of 0.91. We have applied this retrained random forest model to predict the type of 30 new $\mbox{H}$ $\mbox{I}$ 21-cm absorption lines detected in recent blind surveys, viz. FLASH, illustrating the potential of the techniques developed in this work for future large $\mbox{H}$ $\mbox{I}$ surveys with the Square Kilometre Array.

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arXiv2025

Evolution of low surface brightness ultra-thin galaxies: The role of dark matter halo and bar formation on disk thickness

We investigate how stellar disks sustain their ultrathin structure throughout their evolution. We follow the evolution of ultrathin stellar disks with varying dark matter (DM) halo concentration ($c$) using collisionless $N$-body simulations with \texttt{AREPO}. We test models embedded in steep ($c = 12$), shallow ($c = 2$), and intermediate ($c = 6$) DM concentrations. Our models match the observed structural properties of the stellar disk in the low surface brightness (LSB) ultrathin galaxy FGC~2366, specifically its surface brightness, disk scalelength, and vertical thinness ($h_{z}/R_{D} = 0.1$), while excluding gas, allowing us to isolate the effects of DM. The internal disk heating mechanism driven by bars is suppressed in the LSB ultrathin stellar disks regardless of the DM concentration. The ratio of disk thickness ($h_z$) to scalelength ($R_D$) remains constant at $\leq 0.1$ throughout their evolution. To clearly establish that the LSB nature of stellar disks is the key to preventing disk thickening, we construct the initial conditions by increasing the stellar mass fraction from $f_{s} \sim 0.01$ to $0.02$ and $0.04$, respectively, while keeping the total mass equal to $10^{11} M_\odot$ and $h_z/R_D \leq 0.1$ unchanged. We find that models with a higher stellar mass fraction embedded in a shallow DM potential ($c = 2$) form bars and undergo significant disk thickening ($h_{z}/R_{D} \gg 0.1$) concurrent with the bar growth. We conclude that if the LSB disks are thin to begin with, they remain so throughout their evolution in isolation, regardless of the concentration of the DM halo.

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arXiv2025

Can fractal dimension distinguish between grand-design and flocculent spiral arms?

About two-thirds of disk galaxies host spiral arms, ranging from well-delineated grand-design spirals to fragmented flocculent spiral galaxies. We introduce fractal dimension $D_B$ as a non-parametric measure to distinguish between grand-designs and flocculents. We calculate the $D_B$ of 197 grand-designs and 322 flocculents from SDSS DR18, using the samples of \citet{Buta..2015} and \citet{Sarkar..2023}. Our calculated median values of $D_B$ are $1.29^{+0.06}_{-0.04}$ and $1.38^{+0.05}_{-0.06}$ for the grand-designs and flocculents, respectively. In addition, a Kolmogorov-Smirnov (K-S) test rejects null hypothesis that these distributions are drawn from the same population. Finally, using a Random Forest (RF) model, we compare the effectiveness of $D_B$ in classifying spiral arm morphology, as compared to five other parameters viz. total atomic hydrogen HI mass $M_{HI}$, ratio of atomic hydrogen mass-to-blue luminosity $M_{HI} /L_B$, concentration index $C_i$, clumpiness $S$ and arm-contrast $C$. Our results indicate that $D_B$ has the highest feature index (30.8\%), followed by $C_i$ (26.0\%) and $M_{HI}$ (21.0\%). In fact, C, the metric routinely used to distinguish between the spiral morphologies has a feature importance of 8.3\%. Further, $D_B$ for grand-designs is found to anti-correlate with the central velocity dispersion with a correlation coefficient of -0.3 and $p \ll 0.05$. A high value of central velocity dispersion indicates a central Q-barrier, which favors the formation of grand designs according to the density wave theory. Thus, fractal dimension serves as a robust metric to distinguish between spiral morphologies and also links to the formation mechanism of spiral features.

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arXiv2025

The dynamical lineage of ultra-diffuse galaxies from TNG50-1

The formation and evolution of the ultra-diffuse galaxies (UDGs) continues to remain a puzzle. Similarities and differences in the morphological and the kinematical properties of the UDGs with their possible precursors, namely low-surface brightness (LSBs), L*-type high-surface brightness (HSBs) and dwarf galaxies, may provide crucial constraints on their origin and evolution. We selected samples of UDGs, LSBs, HSBs and dwarfs from TNG50-1. We first obtained a few possible scaling relations involving some mass properties to analyse if the regression fits for UDGs are in compliance with those of the other samples. Then, we studied individual galaxy cutouts to evaluate the intrinsic shapes of their dark-matter (DM) and stellar components, orbital and kinematical properties related to their stellar velocity dispersion. Finally, we constructed the mock IFU data using the SimSpin code to extract the stellar kinematic moment maps. We observe that the UDGs and the dwarf galaxies have nearly similar regression fits in a. stellar-to-gas mass ratio vs gas mass, b. stellar-to-gas mass ratio vs total dynamical mass, c. stellar central surface density vs ratio of stellar-to-total dynamical mass, and d. total baryonic mass vs total dynamical mass parameter spaces. Next, we find that the isolated UDGs are prolate rotators similar to the dwarf population, while the tidally-bound UDGs can exhibit both prolate and oblate-rotating shapes. The DM and stellar velocity anisotropy properties of the UDGs suggest that they reside in a cored, dwarf-like halo and may be classified by early-type galaxies. Finally, the stellar kinematic properties suggest that both the UDGs and the dwarfs are slow-rotators having low to nearly no-rotations in contrast to the late-type, disc-dominated, fast-rotating LSBs and HSBs. Therefore, we may conclude that the UDGs and the dwarfs possibly have a common dynamical lineage.

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arXiv2025

Identifying lopsidedness in spiral galaxies using a Deep Convolutional Neural Network

About 30\% of disk galaxies show lopsidedness in their stellar disk. Although such a large-scale asymmetry in the disk can be primarily looked upon as a long-lived mode ($m=1$), the physical origin of the lopsidedness in the disk continues to be a puzzle. In this work, we employ a transfer-learning approach for the automated identification of lopsided galaxies using SDSS DR18 imaging by fine-tuning a Zoobot model, a deep convolutional neural network package pre-trained on the Galaxy Zoo dataset. We obtain 7,042 well-resolved, nearly face-on spiral galaxies from SDSS DR18 over the redshift range 0.01 $\leq z \leq 0.1$, with extinction-corrected g-band model magnitude < 16 and Petrosian radius (enclosing 90 \% of the flux) $\geq$ 3 arcsec. Out of these, we visually identify 490 lopsided and 444 symmetric galaxy samples suitable for training. The trained model achieves a testing accuracy of $(87 \pm 0.02)$ \%, averaged over 10 independent trials. Using the best-performing model, we identify 3,679 lopsided and 2,429 symmetric galaxies from the remaining sample. Of these, 2,658 lopsided and 1,455 symmetric galaxies are predicted with are predicted with high prediction probability $P_{pred} \geq 0.85$. Lopsided galaxies in our predicted samples are relatively high star-forming, bluer, low-concentration (late-type), low-mass galaxies compared to the symmetric galaxies. Our study produces an usable catalogue of lopsided and symmetric galaxies, which will offer new insights into the formation of lopsidedness in disk galaxies. The dataset and the best-performing model are made publicly available through GitHub at https://github.com/bijusaha-astro/CNN_lopsided

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arXiv2025

Are purely gaseous bars in dwarf-irregulars a myth?

About two-thirds of the galactic disks exhibit a central ellipsoidal stellar component called the bar, with or without a gaseous counterpart. However, there are a few dwarf galaxies with purely gaseous bars: NGC3741, NGC2915 and DDO168. This is a puzzle as gas is a collisional medium, and a gaseous bar is expected to be ripped off by shock waves. We study the formation of gaseous bars in these galaxies by constructing dynamical models constrained by stellar photometry and HI observations already available. We first analytically study the dynamical stability of the galactic disks against global $m=2$ perturbations. Our results indicate that the stellar and the gas disks are moderately unstable against these bar modes. Using N-body + hydrodynamical simulations employing $RAMSES$, we next find that a purely gaseous bar is formed in an oblate dark matter halo of vertical-to-planar axes ratio $c/a = 0.6 - 0.8$, with a relatively high-spin parameter $\Lambda = 0.04 - 0.07$, which survives for more than ten dynamical times. Further, the low values of our calculated Mach numbers $M=2-6$ of the gaseous medium comply with the survival of the gaseous bars, unaffected by shock waves. Interestingly, our simulations show the formation of a tiny stellar bar in each case. However, the temporal evolution of the change in angular momentum $L_z$ of the different disk components indicates the exchange of $L_z$ between the gas disk and the dark matter halo only; the $L_z$ of the stellar disk remained unchanged, indicating a weak stellar bar.

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arXiv2024

How does a low surface brightness galaxy form spiral arms?

The formation and evolution of spiral arms in low surface brightness galaxies (LSBs) are not well-understood. We study the dynamics of spiral arms in two prototypical LSBs, F568-VI and F568-01, using both analytical models and N-body + hydrodynamical simulations. We first consider the disk as a 2-component system of gravitationally-coupled stars and gas in the force field of a \emph{spherical} dark matter halo, subjected to local, non-axisymmetric perturbations. However, no local spirals are formed. We next assume the disk to be a 1-component system of stars in the net gravitational potential of a galaxy with a \emph{spherical} dark matter halo perturbed by a global $m=2$ instability. In this case, the growth time for spiral formation was low, equal to 0.78 and 0.96 Gyrs, respectively, corresponding to a few dynamical times of the galaxies. Finally, we simulate the LSBs using the N-body + hydrodynamical simulation code RAMSES. \emph{Our results show that a quadrupolar field associated with an oblate halo with an axial ratio of 0.7} is necessary to drive a long-lived global spiral in the LSB disks. Further, feedback corresponding to a supernova mass fraction of $\sim$ 0.05 is essential to comply with the observed stellar surface density. The simulated spirals survives for about ten dynamical times and the average pattern speed lies between 10 - 15 $\rm{kms^{-1}{kpc}^{-1}}$. The spiral arm thus formed is therefore a transient global pattern driven by the tidal field of the oblate dark matter halo.

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arXiv2023

The dynamical lineage of field ultra-diffuse galaxies

Ultra-diffuse galaxies (UDGs) exhibit morphological similarities with other low-luminosity galaxies indicating a possible evolutionary connection. We investigate for common dynamical characteristics of isolated, HI-rich UDGs with other low luminosity field galaxies, namely the low surface brightness galaxies (LSBs) and the dwarf irregulars (dIrrs). From the galaxy scaling relation studies, we note that UDGs and LSBs constitute statistically different populations. However, for UDGs and dIrrs, the null hypotheses of these statistical tests cannot be rejected : stellar mass versus atomic hydrogen mass; stellar mass versus dynamical mass; and dark matter core density versus core radius mass scaling. Interestingly, the dynamical models suggest that UDGs, LSBs and dIrrs constitute different galaxy populations as reflected by their radial-to-vertical velocity dispersion, and the rotational velocity-to-total stellar velocity dispersion. Finally, we observe that the total HI and stellar mass mostly regulate the variance in the structural and kinematical data both for the UDGs and dIrrs, while the ratio of radial-to-vertical velocity dispersion, and the total HI mass dominate the same in LSBs. UDGs and LSBs represent statistically different galaxy populations with respect to their mass and structural properties. But the fact that their structural parameters follow the same distributions is not ruled out. However, UDGs, dIrrs and LSBs constitute very different populations as far as their kinematical parameters are concerned. Finally, we note that the variation in the structural and kinematical data of both the UDGs and the dIrrs is mostly accounted for by their stellar mass and HI mass, whereas for the LSBs, the same is explained by the ratio of the radial-to-vertical stellar dispersion followed by the HI mass. Thus we conclude that the UDGs and dIrrs to share common dynamical lineage.

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arXiv2023

\HI{} 21cm observations and dynamical modelling of the thinnest galaxy: FGC 2366

Superthin galaxies are bulgeless low surface brightness galaxies with unusually high major-to-minor axes ratio of the stellar disc, i.e.,$10<a/b<20$. We present Giant Metrewave Radio Telescope (GMRT) \HI{} 21cm radio-synthesis observations of FGC 2366, the thinnest galaxy known with $a/b=21.6$. Employing the 3-D tilted-ring modelling using Fully Automated TiRiFiC (FAT), we determine the structure and kinematics of the \HI{} gas disc, obtaining an asymptotic rotational velocity equal to 100 \kms and a total \HI{} mass equal to 10$^9 M_{\odot}$. Using $z$-band stellar photometry, we obtain a central surface brightness of 22.8 mag ${\rm{arcsec}}^{-2}$, a disc scale length of 2.6 kpc, and a scaleheight of 260 pc. Next, we determine the dark matter density profile by constructing a mass model and find that an NFW dark matter halo best fits the steeply-rising rotation curve. With the above mass inventory in place, we finally construct the dynamical model of the stellar disc of FGC 2366 using the stellar dynamical code "AGAMA". To identify the key physical mechanisms responsible for the superthin vertical structure, we carry out a Principal Component Analysis of the data corresponding to all the relevant dynamical parameters and $a/b$ for a sample of superthin and extremely thin galaxies studied so far. We note that the first two principal components explain 80$\%$ of the variation in the data, and the significant contribution is from the compactness of the mass distribution, which is fundamentally responsible for the existence of superthin stellar discs.

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arXiv2023

Analyzing the cosmic web environment in the vicinity of grand-design and flocculent spirals with local geometric index

We explore the environment of a combined set of $367$ grand-design and $619$ flocculent spiral galaxies. We introduce a novel estimator called the \textit{local geometric index} to quantify the morphology of the local environment of these $986$ spirals. The local geometric index allows us to classify the environment of galaxies into voids, sheets, filaments, and clusters. We find that grand-designs are mostly located in dense environments like clusters and filaments ($\sim 78\%$), whereas the fraction of the flocculents lying in sparse environments like voids and sheets is significantly higher ($ > 10\%$) than that of the grand-designs. A $p$-value $<$ $10 ^{-3}$ from a Kolmogorov-Smirnov test indicates that our results are statistically significant at $99.9\%$ confidence level. Further, we note that dense environments with large tidal flows are dominated by the grand-designs. On the other hand, low-density environments such as sheets and voids favor the growth of flocculents.

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arXiv2023

Analyzing the cosmic architecture in the vicinity of Grand-design and Flocculent spiral galaxies with Local Geometric Index

We explore the environment of 351 grand-design and 541 flocculent spiral galaxies recently identified employing convolutional neural networks from the $17^{th}$ data release of Sloan Digital Sky Survey. We introduce a novel estimator called the Local Geometric Index to quantify the morphology of the local environment of these 892 spirals. Based on the local geometric index of the galaxies, we identify their local environments to be voids, sheets, filaments or clusters. We find that grand-designs are mostly located in dense environments like clusters and filaments ($\sim 76\%$), whereas a reasonable fraction of the flocculents lie in sparse environments like voids and sheets ($\sim 45\%$). A $p$-value $<$ $10 ^{-10}$ from a Kolmogorov-Smirnov test indicates that our results are statistically significant at $99.9\%$ confidence level. Further, we note that dense environments with large tidal flows are dominated by the grand-designs. Metal-poor environments, such as sheets and voids, with a high abundance of gas clouds, on the other hand, are mostly populated by the flocculents.

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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.

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arXiv2022

Are superthin galaxies low surface brightness galaxies seen edge-on? The star formation probe

Superthin galaxies (STs) are edge-on disc galaxies with strikingly high planar-to-vertical axes ratios of $\sim 10 - 20$ with no bulge component, and central surface brightness in $B$-band $>$ 23 mag arcsec$^{-2}$ comparable to low surface brightness galaxies (LSBs). Although STs and LSBs have similar dynamical, stellar and atomic hydrogen (HI) masses on an average, it is tricky to conclude if they constitute the same galaxy population, given the edge-on and face-on orientations of the STs and the LSBs respectively. We systematically study star formation rate (SFR) in a sample of STs and LSBs using SED fitting of photometric data in ten bands including GALEX: FUV, NUV, SDSS: u,g,r,i,z \& 2MASS: J, H, Ks using stellar population synthesis models employing the publicly-available software MAGPHYS (Multi-Wavelength Analysis of Galaxy Physical Properties). The estimated median SFRs for LSBs and STs are $0.4^{+2.2}_{-0.3} $ $M_{\odot}yr^{-1}$ \& $0.2^{+0.9}_{-0.2}$ $M_{\odot}yr^{-1}$ respectively. Our calculations indicate that this deficit in the SFR of an ST can be attributed to inclination and opacity effects. Therefore, we conclude that STs and LSBs have equal intrinsic SFR over and above other physical properties, which possibly implies that STs are just LSBs seen in edge-on.

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arXiv2021

HI 21 cm observation and mass models of the extremely thin galaxy FGC 1440

We present observations and models of the kinematics and distribution of neutral hydrogen (HI) in the superthin galaxy FGC 1440 with an optical axial ratio $a/b = 20.4$. Using the Giant Meterwave Radio telescope (GMRT), we imaged the galaxy with a spectral resolution of 1.7 $\rm kms^{-1}$ and a spatial resolution of $15" \times 13.5"$. We find that FGC 1440 has an asymptotic rotational velocity of 141.8 $\rm kms^{-1}$ . The structure of the HI disc in FGC 1440 is that of a typical thin disc warped along the line of sight, but we can not rule out the presence of a central thick HI disc. We find that the dark matter halo in FGC 1440 could be modeled by a pseudo-isothermal (PIS) profile with $\rm R_{c}/ R_{d} <2$, where $R_{c}$ is the core radius of the PIS halo and $R_{d}$ the exponential stellar disc scale length. We note that in spite of the unusually large axial ratio of FGC 1440, the ratio of the rotational velocity to stellar vertical velocity dispersion, $\frac{V_{Rot}}{σ_{z}} \sim 5 - 8$, which is comparable to other superthins. Interestingly, unlike previously studied superthin galaxies which are outliers in the $log_{10}(j_{*}) - log_{10}(M_{*})$ relation for ordinary bulgeless disc galaxies, FGC 1440 is found to comply with the same. The values of $j$ for the stars, gas and the baryons in FGC 1440 are consistent with those of normal spiral galaxies with similar mass.

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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.

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arXiv2021

How "cold" are the stellar discs of superthin galaxies?

Superthin galaxies are a class of bulgeless, low surface brightness galaxies with strikingly high values of planar-to-vertical axes ratio $\rm(b/a> 10 - 20)$, possibly indicating the presence of an ultra-cold stellar disc. Using the multi-component galactic disc model of gravitationally-coupled stars and gas in the force field of the dark matter halo as well as the stellar dynamical code AGAMA (Action-based Galaxy Modelling Architecture), we determine the vertical velocity dispersion of stars and gas as a function of galacto-centric radius for five superthin galaxies (UGC 7321, IC 5249, FGC 1540, IC2233 and UGC00711) using observed stellar and atomic hydrogen (HI) scale heights as constraints, using a Markov Chain Monte Carlo Method. We find that the central vertical velocity dispersion for the stellar disc in the optical band varies between $σ_{0s}$ $\sim$ $10.2 - 18.4$ $\rm{kms}^{-1}$ and falls off with an exponential scale length of $2.6$ to $3.2$ $R_{d}$ where $R_{d}$ is the exponential stellar disc scale length. Interestingly, in the 3.6 $μ$m, the same, averaged over the two components of the stellar disc, varies between $5.9$ to $11.8$ $\rm{kms}^{-1}$, both of which confirm the presence of "ultra-cold" stellar discs in superthin galaxies. Interestingly, the global median of the multi-component disc dynamical stability parameter $Q_N$ of our sample superthins is found to be 5 $\pm$ 1.5, which higher than the global median value of 2.2 $\pm$ 0.6 for a sample of spiral galaxies.

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