arXiv subjects

Kanak Saha

Publications and source records attributed to Kanak Saha.

At least 37 records · Page 2Linked to original sources

arXiv2026

Escaping ionizing photons from massive spiral galaxies at $z\sim 1$

We report the detection of Lyman continuum (LyC) photons from three massive ($\text{M}_{*}>10^{10}\:\text{M}_{\odot}$) spiral galaxies at a redshift of nearly 1 in the AstroSat UV Deep Field South. Notably, all three systems are viewed at low inclination (i.e., nearly face-on), prompting an investigation into the role of galaxy orientation in the detectability of LyC emission from disk systems. Two of the three galaxies, however, host active galactic nuclei (AGNs), adding complexity to the interpretation of the LyC signal. We present a detailed analysis of the likely star-forming case, and report tentative evidence that a face-on viewing angle may enhance the likelihood of LyC detection in disk galaxies. This represents the first detection of LyC emission from well-characterized spiral galaxies at high redshift, offering a new window into LyC escape mechanisms in such systems. Our findings highlight the need to consider geometric factors and anisotropic escape pathways facilitated by feedback processes alongside more traditional density-bounded scenarios that imply isotropic escape.

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arXiv2025

Chemically primitive dwarf accretion reignites the inner disk assembly of Malin 1

We present a detailed kinematic and stellar population analysis of the inner disk of Malin 1, a giant low surface brightness (GLSB) galaxy with a prominent SB0-type central morphology. AstroSat far-UV imaging reveals clumpy emission features indicating recent star formation. Using MUSE integral field spectroscopy, we identify four star-forming complexes (SFCs) within the inner 10 kpc, each associated with localized ionized gas emission in distinct H$α$ velocity channels. Two of the SFCs, including a far-UV clump, appear on the blue-shifted side ($V_{Hα}=-230~\mathrm{kms^{-1}}$), while the other two are redshifted. The far-UV clump shows a strong velocity offset ($\sim150~\mathrm{kms^{-1}}$) and high gas dispersion ($\sim250~\mathrm{kms^{-1}}$), indicating that it is kinematically decoupled from the rotating disk. The spatial and velocity isolation of these features in the channel map confirms they do not follow regular disk rotation. The far-UV clump hosts young (250-500 Myr), extremely metal-poor ([M/H]$\simeq$ -1.69) and $α$-enriched ($[α/Fe] \sim 0.5$) stellar populations, sharply contrasting with the surrounding super-solar gas-phase metallicity. The young stellar populations in each SFC are chemically distinct (similar to the far-UV clump) from the enriched central ISM, indicating rapid, local star formation from primitive gas before efficient mixing with the enriched ISM. Their spatial and velocity segregation, age synchronicity, and chemical homogeneity suggest an origin of gas delivered by a disrupted, gas-rich dwarf on a high-inclination (off-plane) orbit. These results suggest that the central HSB within $\rm \sim 9^{\prime\prime} (14\ kpc)$ radius component of Malin 1 has grown through discrete, externally driven accretion, contributing to its complex, hybrid disk morphology.

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arXiv2025

The AstroSat UV Deep Field South-V: Constraints on the average escape of ionizing photons in the cosmic dusk

We investigate the escape of ionizing (Lyman-continuum; LyC) photons from 49 star-forming galaxies at redshifts $\sim 1-1.5$, using far-ultraviolet (FUV) imaging from the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat. The sample spans a wide range of stellar masses and UV luminosities. LyC emission is undetected in most galaxies (42/49), and stacking these galaxies yields only an upper limit on the observed LyC-to-nonionizing UV flux density ratio ($({F}_{λ,\rm{LyC}}/{F}_{λ,\rm{UV}})_{\rm{obs}}<0.12$). Including all galaxies (with 7 LyC-leaker candidates) produces a marginal $2.4σ$ detection, suggesting that the average LyC signal is driven by a small number of sources. To identify the conditions favorable for LyC escape, we perform stacking analyses in bins of stellar mass, UV slope, compactness, inclination, and star formation rate surface density. A stacked LyC signal is detected at a significance of $\sim3σ$ from a subset of galaxies that are characterized as being compact, have high star formation rate surface densities, and blue UV continuum slopes, despite each of these being individually undetected in LyC. This provides the first systematic evidence at $z\sim 1-1.5$ linking these properties to LyC escape, consistent with trends observed in the lower redshift universe. Additionally, LyC leakage appears more efficient in low-mass galaxies ($\log_{10}(M_{*}/M_{\odot})<9.5$), with their average absolute escape fraction ranging from $\langle f_{\text{esc,abs}} \rangle\sim0.1-0.2$ depending on stellar population assumptions. These results support the scenario that compact, low-mass starbursts were key contributors to the ionizing photon budget during cosmic reionization.

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arXiv2025

GNHeII J1236+6215: A He II $λ$1640 emitting and potentially LyC leaking galaxy at $z$ = 2.9803 unveiled through JWST & Keck observations

He II $λ$1640 emission in galaxies indicates the presence of sources that produce extreme ionizing photons. Here, we report the discovery of a He II $λ$1640 emitting galaxy, GNHeII J1236+6215, at $z=$ 2.9803 in the GOODS-north field. We use photometry in 17 wavebands from near-UV to infrared to characterize the galaxy SED and combine Keck LRIS and JWST NIRSpec spectra to identify 15 emission lines including He II $λ$1640. We infer that the He$^+$ ionization in the galaxy could be driven by small pockets of young Population III stars or low-metallicity Very Massive Stars (VMSs) rather than AGN or metal-rich Wolf-Rayet stars. The galaxy has a highly ionized ISM ([OIII]5007/[OII]3727 = 7.28$\pm$0.11, [SIII]/[SII] = 1.97$\pm$0.48 and detected Ly$α$, H$α$, H$β$, H$γ$ lines), little reddening by dust (E(B$-$V) = 0.04$\pm$0.12), low metallicity (12 + log(O/H) = 7.85$\pm$0.22), and high star formation rate (SFR$_{\rm SED}$ = 12.2$\pm$2.0 M$_{\odot}$ yr$^{-1}$). In addition to these ISM conditions, we also notice a significant [SII] deficiency ([SII]6718,6732/H$α$ = 0.08$\pm$0.02, $Δ$[SII] = $-$0.12) which may indicate the presence of density-bounded optically thin H~II regions that combined with the low dust extinction favor leaking of ionizing Lyman continuum (LyC) photons. Our best-fit SED model also infers a high nebular ionization (log U = $-2.0$) and a low stellar mass M = 7.8$\pm3.1\times$10$^8$M$_{\odot}$. This discovery not only adds one important object to the known sample of high-redshift He~II emitters but also highlights a potential connection between He$^+$ ionization and favorable ISM conditions for the leakage of ionizing photons from galaxies.

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arXiv2025

Finding massive double-exponential disk galaxies with extended low surface brightness stellar disk -- an IllustrisTNG exploration

We study massive disk galaxies (stellar mass $>=10^{11}\mathrm{M_{\odot}}$) at z=0 from IllustrisTNG simulation to detect galaxies that contain two exponential stellar disks - a central high surface brightness (HSB) disk surrounded by an extended low surface brightness (LSB) envelope. This is motivated by the observation of several giant LSB galaxies (Malin 1, UGC 1378, UGC 1382 etc), reported in the literature, showing such complex morphology. Studying such systems can help us to understand the mass assembly process and growth of stellar disks in massive galaxies and thus can shed light on massive galaxy formation and evolution scenarios. We use the high-resolution IllustrisTNG50 data and perform Sersic plus exponential profile modeling on the idealized, synthetic SDSS g, r-band images of the massive disk galaxies using GALFIT. We identify 7 disk galaxies (12% of the parent sample of disk galaxies) that are best represented by a central Sérsic plus a central HSB disk surrounded by an extended LSB disk. The radial scale lengths of the LSB disk lie in the range of $\sim$ 9.7-31.7 kpc, in agreement with that found in the literature. We study the star formation properties of these simulated double-disk galaxies to understand the distribution of these from blue star-forming to red-quenched region. Some of these double-disk galaxies display a characteristic minima in their (g-r) color radial profiles. The double-disk galaxies are found to lie within $\sim$ 1.5-$\mathrmσ$ region of the Baryonic Tully-Fisher relation from observation.

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arXiv2025

Clump-fed black hole growth in the first billion years of the universe

Understanding how supermassive black holes (SMBHs) form in the early universe is one of the most challenging problems in astrophysics. Their high abundance in the first billion years, as observed by the James Webb Space Telescope, hints towards black hole seeds that accrete mass rapidly. The origin of this accreted mass is not known. Here, we consider a billion solar mass clumpy galaxy at z=5.48 with a 30 million solar mass black hole in the center. We show that the clumps should migrate to the central region because of torques from dynamical friction with the halo, funneling in at least 14 solar masses per year. This is fast enough to grow the observed SMBH, with only 1% of the accreted mass getting in and the rest going to a bulge. Clump-fed accretion could explain most young SMBHs because young galaxies are highly irregular with massive star-forming clumps.

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arXiv2025

The AstroSat UV Deep Field South. III. Evolution of the UV Luminosity Function and Luminosity Density from z~0.8-0.4

We characterise the rest-frame 1500 Å UV luminosity Function (UVLF) from deep AstroSat/UVIT F154W and N242W imaging in the Great Observatories Origins Survey South (GOODS-S) deep field. The UVLFs are constructed and subsequently characterised with fitted Schechter function parameters from FUV observations at z$<0.13$ and NUV observations in seven redshift bins in z~0.8-0.4. The UVLF slope ($α$) and characteristic magnitude ($M^*$) are consistent with previous determinations for this redshift range based on AstroSat/UVIT GOODS-North observations, as well as with those from Galaxy evolution Explorer and Hubble Space Telescope observations. However, differences in the normalisation factor ($ϕ_{*}$) are present for UVLFs for some redshift bins. We compute the UV luminosity density, $ρ_{\rm UV}$, combining our determined UVLF parameters with literature determinations out to z$\sim10$. The $ρ_{\rm UV}$ trend with redshift implies the rapid increase in cosmic star formation till its peak at z$\sim3$ (cosmic noon) followed by a slow decline till present day. Both the initial increase in cosmic star formation and subsequent decline are found to be more rapid than previous determinations.

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arXiv2025

Unveiling galaxy chemical enrichment mechanisms out to z~8 from direct determination of O & Ar abundances from JWST/NIRSPEC spectroscopy

Galaxy chemical enrichment mechanisms have primarily been constrained by [$α$/Fe] and [Fe/H] measurements of individual stars and integrated light from stellar populations. However such measurements are limited at higher redshifts (z>1). Recently, we proposed an analogous diagram of the oxygen-to-argon abundance ratio, log(O/Ar), vs Ar abundance, 12+log(Ar/H), as a new diagnostic window for emission nebulae. In this Letter, using robust line flux measurements including temperature sensitive auroral lines, we present direct determination of O and Ar abundances in nine SFGs from JWST/NIRSPEC spectra at z$\sim$1.3-7.7, and two more with Keck/MOSFIRE spectra at z$\sim$2.2. Utilising their positions on the log(O/Ar) vs 12+log(Ar/H) plane, we present the first inference of galaxy chemical enrichment mechanisms from an ensemble of galaxies. The SFGs at z$\sim$1.3-3.4 are consistent with the solar neighbourhood galactic chemical enrichment models of the Milky Way Galaxy that are driven by core-collapse and Type Ia supernovae. Such enrichment mechanisms thus occur at least out to z$\sim$3.4. However, the highest-redshift SFGs (z$\sim$3.6-7.7) have very low log(O/Ar) values, revealing a different enrichment process at z>3.6. Such low log(O/Ar) values may be caused by a rapid but intermittent star-formation and/or additional sources. The new diagnostic window for SFGs enables us to reveal the unique fingerprints of galaxy chemical enrichment out to cosmic dawn.

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arXiv2025

The AstroSat UV Deep Field South II: A diverse set of Lyman-continuum leakers at $z\sim1$

We report the detection of 5 newly identifed Lyman-continuum (LyC) leaker candidates at redshifts 0.99-1.42 in the AstroSat UV Deep Field South F154W image. We derive physical properties of these galaxies using a combination of spectral-energy distribution fitting and information from publicly available spectra. The estimated escape fraction of these objects vary from 14-85\% after accounting for the IGM attenuation. With only about a dozen known leakers at these redshifts, these detections significantly raise the fraction of LyC leakers in this redshift range. High-resolution HST UV imaging reveals that a subset of the galaxies in our sample have blue star-forming structures that are likely associated with harder ionizing sources. We find tentative evidence that the LyC emission is spatially offset from the non-ionizing UV continuum centers of these galaxies. The integrated properties of these galaxies, such as the UV continuum slope, dust attenuation, stellar mass, and $[\text{O III}] λ5007/ [\text{O II}] λ3727$ ratios, make them atypical compared to known LyC leakers. The leakage of LyC photons from these systems presents a compelling challenge.

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arXiv2024

A geometric approach to estimate background in astronomical images

Estimating the true background in an astronomical image is fundamental to detecting faint sources. In a typical low-photon count astronomical image, such as in the far and near-ultraviolet wavelength range, conventional methods relying on the 3-sigma clipping and median or mode estimation often fail to capture the true background level accurately. As a consequence, differentiating true sources from noise peaks remains a challenging task. Additionally, in such images, effectively identifying and excluding faint sources during the background estimation process remains crucial, as undetected faint sources could contaminate the background. This results in overestimating the true background and obscuring the detection of very faint sources. To tackle this problem, we introduce a geometric approach based on the method of steepest descent to identify local minima in an astronomical image. The proposed algorithm based on the minima statistics effectively reduces the confusion between sources and background in the image; thereby ensuring a better background estimation and enhancing the reliability of faint source detection. Our algorithm performs well compared to conventional methods in estimating the background even in crowded field images. In low-photon count, less crowded images, our algorithm recovers the background within 10\%, while traditional methods drastically underestimate it by a few orders of magnitude. In crowded fields, the conventional methods overestimates the background by $\sim 200\%$ whereas our algorithm recovers the true background within $\sim 14\%$. We provide a simple prescription to create a background map using our algorithm and discuss its application in large astronomical surveys.

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arXiv2024

AstroSat UV Deep Field South -- I. Far and Near-ultraviolet Source Catalog of the GOODS South region

We present the AstroSat UV Deep Field South (AUDFs), an imaging survey using the wide-field Ultraviolet Imaging Telescope on board AstroSat. AUDFs covers $\sim 236$ arcmin$^{2}$ of the sky area, including the Great Observatories Origins Deep Survey (GOODS) South field in F154W and N242W filters. The deep and shallow parts of AUDFs have exposure time $\sim 62000$ and $\sim31000$ sec respectively, in the F154W filter, while in the N242W filter, they are $\sim 64000$ and $\sim34000$ sec. These observations reached a $3σ$ depth of 27.2 and 27.7 AB mag with a $50\%$ completeness limit of 27 and 27.6 AB mag in the F154W and N242W filters, respectively. With the acquired depth, AUDFs is the deepest far and near-UV imaging data covering the largest area known to date at 1.2" - 1.6" spatial resolution. Two primary catalogs were constructed for the F154W and N242W filters, each containing 13495 and 19374 sources brighter than the 3$σ$ detection limit, respectively. Our galaxy counts power-law slope $\sim0.43$~dex~mag$^{-1}$ in the N242W filter matches well with HST/WFC3/UVIS observations. A wide range of extra-galactic science can be achieved with this unique data, such as providing a sample of galaxies emitting ionizing photons in the redshift range $z \sim 1 - 3$ and beyond; constraining the UV luminosity function, investigating the extended-UV (XUV) emission around star-forming galaxies and UV morphologies for $z < 1$. The UV catalog will enhance the legacy value of the existing optical/IR imaging and spectroscopic observations from ground and space-based telescopes on the GOODS South field.

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arXiv2024

Exploring filament galaxies using {\em AstroSat}/ UVIT

We present results from our deep Far-ultraviolet (FUV) survey using {\em AstroSat}/UVIT of a filamentary structure at $z$ $\sim$ $0.072$. A total of four filaments comprising 58 galaxies were probed in our study. We detect 18 filament galaxies in our FUV observation. All filament galaxies are further classified based on their photometric color, nuclear activity, and morphology. The filaments contain galaxies with mixed stellar population types and structures. We do not detect galaxies in our UVIT survey up to a distance of 0.4~Mpc $h^{-1}$ from the filament axis, implying a lack of recent star formation in the inner region of filaments. The FUV star formation rate (SFR) for star-forming galaxies agrees well with the SFR$_{\rm 144 MHz}$ calculated using Low-Frequency Array (LOFAR) radio-continuum observations. We witness an increase in FUV specific-SFR (sSFR) of filament galaxies with increasing distance from the filament spine (D$_{\rm fil}$). The intermediate-to-high stellar mass filament galaxies were more star-forming than cluster galaxies in a fixed stellar mass bin. The FUV morphology of some filament galaxies detected in the filament outskirts (D$_{\rm fil}$ $\gtrsim$ 0.7~Mpc $h^{-1}$) is comparable to or slightly extended than their optical counterpart. The mass assembly of galaxies examined by estimating $(FUV-r)$ color gradients show that more `red-cored' galaxies reside in the outer region of the filaments. Our results prove that the likelihood of merger interaction and gas starvation increases when approaching the filament spine. We report a definitive and in-homogeneous impact of filaments on the galaxies residing inside them.

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arXiv2024

The leakage of Lyman-continuum photons from a major merger at $z\sim1$

We report the detection of Lyman-continuum (LyC) photons from a massive interacting system at $z=1.097$ in the Hubble Ultra Deep Field. The LyC detection is made in the far-ultraviolet F154W band of the UVIT telescope onboard AstroSat. Both JWST and HST imaging of the system reveal signs that it is a likely merger. In particular, high-resolution imaging in the JWST bands reveals an infrared luminous object within the system that is faint in the bluer HST bands. The ionized-gas kinematics from the MUSE-UDF data supports the merger hypothesis. We estimate that the entire system is leaking more than $8 \%$ of its ionizing photons to the intergalactic medium. The SED-derived stellar masses of the two components indicate that this is a major merger with a mass ratio of ${1.13 \pm 0.37}$. This detection hints at the potential contribution of massive interacting systems at higher redshifts, when major mergers were more frequent, to the ionizing budget of the universe.

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arXiv2024

The AstroSat UV Deep Field North: Direct determination of the UV Luminosity Function and its evolution from z~0.8-0.4

We characterize the evolution of the rest-frame 1500 $\unicode{xC5}$ UV luminosity Function (UVLF) from AstroSat/UVIT F154W and N242W imaging in the Great Observatories Origins Survey North (GOODS-N) field. With deep FUV observations, we construct the UVLF for galaxies at z$<0.13$ and subsequently characterise it with a Schechter function fit. The fitted parameters are consistent with previous determinations. With deep NUV observations, we are able to construct the UVLF in seven redshift bins in the range z $\sim$ 0.8 - 0.4, with galaxies identified till $\sim$2 mag fainter than previous surveys, owing to the high angular-resolution of UVIT. The fitted Schechter function parameters are obtained for these UVLFs. At z $\sim$ 0.8 - 0.7, we also utilize Hubble Space Telescope (HST) F275W observations in the GOODS-N field to construct the UVLF in 2 redshift bins, whose fitted Schechter function parameters are then found to be consistent with that determined from UVIT at z $\sim$ 0.75. We thus probe the variation of the fitted UVLF parameters over z $\sim$ 0.8 - 0.4, a span of $\sim$2.7 Gyr in age. We find that the slope of the Schechter function, $α$, is at its steepest at z $\sim$ 0.65, implying highest star-formation at this instant with galaxies being relatively more passive before and after this time. We infer that this may be a short-lived instance of increased cosmic star-formation even though cosmic star-formation may be winding-down over longer timespan at this redshift range.

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arXiv2024

The AstroSat UV Deep Field North: The IRX-$β$ relation for the UV-selected galaxies at $z\sim$ 0.5-0.7

The relation between the observed UV continuum slope ($β$) and the infrared excess (IRX) is used as a powerful probe to understand the nature of dust attenuation law in high-redshift galaxies. We present a study of 83 UV-selected galaxies between redshift 0.5 and 0.7 from the AstroSat UV Deep Field north (AUDFn) that encloses the GOODS-north field. Using empirical relation, we estimate the observed IRX of 52 galaxies that are detected in either one or both of the Herschel PACS 100$μ$m and 160$μ$m bands. We further utilize the multi-band photometric data in 14 - 18 filters from the UVIT, KPNO, HST, Spitzer, and Herschel telescopes to perform spectral energy distribution (SED) modeling. Both the observed and model-derived IRX - $β$ values show a large scatter within the span of previously known relations, signifying diversity in dust attenuation. We found a distinct relation between the best-fit power law slope of the modified Calzetti relation ($δ$) in the IRX - $β$ plane: where the steeper SMC-like attenuation law prefers lower $δ$ values. Our SED model based IRX - $β$ relation shows a preference for steeper SMC-like attenuation which we further confirm from the agreement between extinction-corrected star formation rates derived using H$α$ emission line and the observed FUV plus reprocessed far-IR fluxes. The current study reveals a strong positive correlation between IRX and the galaxy stellar mass between $10^{9.5}$ and $10^{11.0}$ M$_{\odot}$, signifying increased dust production in more massive star-forming galaxies.

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arXiv2024

Star formation exists in all early-type galaxies -- evidence from ubiquitous structure in UV images

Recent surveys have demonstrated the widespread presence of UV emission in early-type galaxies (ETGs), suggesting the existence of star formation in many of these systems. However, potential UV contributions from old and young stars, together with model uncertainties, makes it challenging to confirm the presence of young stars using integrated photometry alone. This is particularly true in ETGs that are fainter in the UV and have red UV-optical colours. An unambiguous way of disentangling the source of the UV is to look for structure in UV images. Optical images of ETGs, which are dominated by old stars, are smooth and devoid of structure. If the UV is also produced by these old stars, then the UV images will share this smoothness, while, if driven by young stars, they will exhibit significant structure. We compare the UV and optical morphologies of 32 ETGs (93 per cent of which are at $z<0.03$) using quantitative parameters (concentration, asymmetry, clumpiness and the Sérsic index), calculated via deep UV and optical images with similar resolution. Regardless of stellar mass, UV-optical colour or the presence of interactions, the asymmetry and clumpiness of ETGs is significantly larger (often by several orders of magnitudes) in the UV than in the optical, while the UV Sérsic indices are typically lower than their optical counterparts. The ubiquitous presence of structure demonstrates that the UV flux across our entire ETG sample is dominated by young stars and indicates that star formation exists in all ETGs in the nearby Universe.

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arXiv2024

LyC Leakers in the AstroSat UV Deep Field: Extreme UV emitters at the Cosmic Noon

We report the direct detection of Lyman Continuum (LyC) emission from 9 galaxies and 1 Active Galactic Nuclei (AGN) at $z$ $\sim$ 1.1-1.6 in the GOODS-North field using deep observations from the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat. The absolute escape fraction of the sources estimated from the far-ultraviolet (FUV) and H$α$ line luminosities using Monte Carlo (MC) analysis of two Inter-Galactic Medium (IGM) models span a range $\sim$ 10 - 55 $\%$. The restframe UV wavelength of the sources falls in the extreme-ultraviolet (EUV) regime $\sim$ 550-700 Å, the shortest LyC wavelength range probed so far. This redshift range remains devoid of direct detections of LyC emission due to the instrumental limitations of previously available facilities. With UVIT having a very low detector noise, each of these sources are detected with an individual signal-to-noise ratio (SNR) $>$ 3 while for the stack of six sources, we achieve an SNR $\sim$ 7.4. The LyC emission is seen to be offset from the optical centroids and extended beyond the UVIT PSF of 1.$^{\prime\prime}6$ in most of the sources. This sample fills an important niche between GALEX and Cosmic Origins Spectrograph (COS) at low-$z$, and HST WFC3 at high-$z$ and is crucial in understanding the evolution of LyC leakers.

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arXiv2023

Bars and boxy/peanut bulges in thin and thick discs III. Boxy/peanut bulge formation and evolution in presence of thick discs

Boxy/peanut (b/p) bulges, the vertically extended inner parts of bars, are ubiquitous in barred galaxies in the local Universe, including our own Milky Way. At the same time, a majority of external galaxies and the Milky Way also possess a thick-disc. However, the dynamical effect of thick-discs in the b/p formation and evolution is not fully understood. Here, we investigate the effect of thick-discs in the formation and evolution of b/ps by using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. Within the suite of models, we systematically vary the mass fraction of the thick disc, and the thin-to-thick disc scale length ratio. The b/ps form in almost all our models via a vertical buckling instability, even in the presence of a massive thick disc. The thin disc b/p is much stronger than the thick disc b/p. With increasing thick disc mass fraction, the final b/p structure gets progressively weaker in strength and larger in extent. Furthermore, the time-interval between the bar formation and the onset of buckling instability gets progressively shorter with increasing thick-disc mass fraction. The breaking and restoration of the vertical symmetry (during and after the b/p formation) show a spatial variation -- the inner bar region restores vertical symmetry rather quickly (after the buckling) while in the outer bar region, the vertical asymmetry persists long after the buckling happens. Our findings also predict that at higher redshifts, when discs are thought to be thicker, b/ps would have more 'boxy-shaped' appearance than more 'X-shaped' appearance. This remains to be tested from future observations at higher redshifts.

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