arXiv subjects

K. Aditya

Publications and source records attributed to K. Aditya.

13 recordsLinked to original sources

arXiv2026

A Massive Galaxy at the Edge of Feedback-Free Efficiency

The efficiency with which galaxies convert their available baryonic reservoir into stars sets a fundamental ceiling on stellar mass assembly in the early Universe and encodes the cumulative effect of stellar feedback. in this paper, we report the measurement of star-formation efficiency (SFE) of a photometrically and spectroscopically vetted reference sample at $z=9-10$, out of which 31 are spectroscopically confirmed. Among them, we highlight a spectroscopically confirmed galaxy, \texttt{UNCOVER 3686} at $z = 9.31$, which has a stellar mass $M_\star = 10^{9.55}$ M$_\odot$ and physical properties comparable to the predictions of the feedback-free starburst (FFB) scenario. We use this galaxy as an anchor for the first direct observational test of whether galaxy at cosmic dawn with physical properties predicted in the FFB theory reach the maximum baryon conversion efficiencies predicted by the feedback-free starburst scenario. We find the SFE for this galaxy to lie between $\approx 20\%$ and $60\%$, a factor of 2 to 6 above empirical model predictions. The compact morphology ($R_e = 0.45$ kpc), young stellar age ($\sim 160$ Myr), low metallicity $(Z_\star/Z_\odot \approx 0.2)$, and inferred gas density $(n_{\rm gas} \sim 3\times 10^{3}$ cm$^{-3}$) of this galaxy are consistent with feedback-free (FFB) galaxy formation conditions. We conclude that UNCOVER 3686 is an excellent candidate for an FFB galaxy in which the global star formation efficiency approaches the theoretical limits due to weak stellar feedback.

astro-ph.GA↗

arXiv2026

Feeding and Feedback in Dwarf Galaxies (FeeD) -- I. Evidence of nuclear ultra-fast and galaxy-scale outflows in the dwarf galaxy Arp 151

Feeding and feedback regulated by supermassive black holes (SMBHs) play a central role in galaxy growth and evolution, yet these processes remain poorly understood in low-mass galaxies. In particular, the presence, properties, and role of ultra-fast nuclear outflows (UFOs) in low-mass galaxy systems are largely unexplored. We analyze available NuSTAR X-ray observations of Arp 151 and find a possible evidence ($\sim 2\sigma$ confidence) for a fast outflow with a velocity of $\sim0.18c$ from the central BH. Furthermore, we have also detected an optical galaxy-scale outflow in MaNGA Integral Field Unit data. The estimated nuclear and galaxy-scale mass outflow rates are $\sim0.015$ $M_\odot$/yr from {\it NuSTAR} and $\sim0.43$ $M_\odot$/yr from MaNGA, respectively. Our estimates suggest that such outflows may significantly regulate the feedback process in the galaxy. Comparing the kinetics of the UFO and the galaxy-scale outflow indicates that they are in the momentum-conserving phase. This tentative detection implies that dwarf galaxies are also able to generate UFOs, which so far have been detected in massive galaxies. Thus, the AGN feedback may also be important for the evolution of the dwarf galaxies.

astro-ph.GA↗

arXiv2025

Can dwarf spheroidal galaxies host a central black hole ?

We construct mass models of Milky Way dwarf spheroidal galaxies to place constraints on the central black hole (BH) masses they can host. We model the galaxies as a three-component system consisting of the stars, dark matter halo, and a central black hole, using the Osipkov--Merritt--Cuddeford class of anisotropic distribution function. The posterior distribution of black hole mass remains flat toward the low-mass end, indicating that the kinematic data places an upper limit on the black hole mass. Our analysis yields a 95% credible upper limit of $\log(M_{\bullet}/M_{\odot}) < 6$. We combine our results with black hole mass measurements and upper limits from the literature to construct a unified $M_{\bullet}$--$\sigma_{}$ relation spanning $\sigma_{} \sim 10$--$300,\mathrm{km,s^{-1}}$, described by $\log(M_{\bullet}) = 8.32 + 4.08,\log\left(\sigma_{}/200,\mathrm{km,s^{-1}}\right)$, with an intrinsic scatter of $\sigma_{\rm int} = 0.55$. We compare the inferred limits to models of black hole growth via momentum-driven accretion and stellar capture, which predict black hole masses in the range $10^{3}$--$10^{4},M_{\odot}$ for the range $\sigma_{} \sim 6$--$12,\mathrm{km,s^{-1}}$, in close agreement with the $M_{\bullet}$--$\sigma_{*}$ relation within the 95% credible upper limits on the black hole masses derived in this work.

astro-ph.GA↗

arXiv2025

Is Milky Way gravitationally stable? A TNG50 view from cosmic noon to the present day

We investigate the stability of Milky Way analogs (MWAs) in the \texttt{TNG50} simulation against the growth of local axisymmetric instabilities, tracing their evolution from cosmic noon ($z=2.5$) to the present day ($z=0$). Using a two-component stability criterion that accounts for stars, gas, and the force field of the dark matter halo, we compute the net stability parameter ($Q_{T}$), the critical gas surface density ($\Sigma_{c}$), and the instability timescale ($\tau$) for 10 barred and 10 unbarred MWAs. We find that these galaxies remain stable to axisymmetric instabilities at all epochs, with $Q_{T}^{\min}>2$. The stability levels increase toward higher redshift, where enhanced gas velocity dispersion counterbalances the destabilizing effect of larger gas fractions. Further, the barred MWAs consistently show lower $Q_{T}^{\min}$ than unbarred ones. The gas density remains subcritical ($\Sigma_{g}<\Sigma_{c}$) across radii and epochs, implying that local axisymmetric instabilities are not the primary channel for star formation. Growth timescales are short (a few Myr) in central regions but increase exponentially to several Gyr in the outer disc, naturally explaining the concentration of star formation toward galactic centers. We study the effect of gas dissipation and turbulence in ISM and find that while MWAs are stable against axisymmetric instabilities $(Q_{T}>1)$, a combination of gas dissipation and turbulence in ISM can destabilize the disc at small scales even when $Q_{T}>1$.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2024

How does dark matter stabilize disc galaxies?

The study presents a theoretical framework for understanding the role of dark matter on the stability of the galactic disc. We model the galaxy as a two-component system consisting of stars and gas in equilibrium with an external dark matter halo. We derive the equations governing the growth of perturbations and obtain a stability criterion that connects the potential of the dark matter halo and the gas fraction with the stability levels of the galaxy. We find that a two-component disc is more susceptible to the growth of gravitational instabilities than individual components, particularly as gas fractions increase. However, the external field, due to the dark matter halo, acts as a stabilizing agent and increases the net stability levels even in the presence of a cold gas component. We apply the stability criterion to models of the Milky Way, low surface brightness galaxies, and baryon-dominated cold rotating disc galaxies observed in the early universe. Our results show that the potential due to the dark matter halo plays a significant role in stabilizing nearby galaxies, such as the Milky Way, and low surface brightness galaxies, which would otherwise be prone to local gravitational instabilities. However, we find that the baryon-dominated cold disc galaxies observed in the early universe remain susceptible to the growth of local gravitational instabilities despite the stabilizing effect of the dark matter halo.

astro-ph.GA↗

arXiv2024

Challenges in modeling the dark matter halo of NGC 1052-DF2: Cored versus cuspy halo models

The discovery of NGC 1052-DF2 and subsequent modeling have shown that NGC 1052-DF2 is deficient in dark matter and is in conflict with the standard stellar-to-halo mass ratio. In this work, we aim to resolve the degeneracy between the dynamical models on the mass estimate of the NGC 1052-DF2.We constructed mass models of NGC 1052-DF2 using an anisotropic distribution function with a radially varying anisotropy parameter and studied the effect of the various model parameters on the dark matter estimates. We used the observed stellar photometry as an input parameter to construct the distribution function and employed a Markov Chain Monte Carlo (MCMC) method to estimate the dark matter model parameters.We find that mass models with a cuspy dark matter halo have comparable $\chi^{2}$ to models with zero dark matter. Moreover, the cuspy dark matter halo fails to consistently account for the observed velocity dispersion in the inner and outer regions of the galaxy. Consequently, we rule out the possibility of a cuspy dark matter halo for describing the mass models of NGC 1052 - DF2. Our study shows that the cored dark matter halo model with a total mass of $\log(M_{DM}/M_{\odot})=10.5$ explains the observed kinematics but requires an extraordinarily large scale length (20kpc) and an outer cutoff radius (26kpc). While the cored mass model provides a comparatively better fit, our findings emphasize that the mass models are largely unconstrained by the available kinematic data. Our results suggest that NGC 1052 - DF2 may not only have an ultra-diffuse stellar distribution but that it can, within uncertainties in the available kinematic data, potentially host an ultra-diffuse dark matter distribution compatible with the standard stellar-to-halo mass relation (SHMR) predicted by galaxy formation and evolution models

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2023

Stability of galaxies across morphological sequence

We investigate the stability of nearby disc galaxies and galaxies at redshift ($z$) equal to 4.5. We explore the connection between the stability parameter $(Q_{RW})$, star formation rate ($SFR$), gas fraction $(f^{Gas})$, and the time scale for growth of gravitational instabilities $(\tau)$. We find that, despite differences in morphology $91$ $\%$ of the nearby galaxies have a minimum value of stability parameter ($Q^{Min}_{RW}$) greater than $1$ indicating stability against the growth of axisymmetric instabilities. The spirals in our sample have higher median star formation rate, lower median $Q_{RW}$, a lower $f^{Gas}$ and small time scale for growth of gravitational instabilities than irregular galaxies. We find that the gravitational instabilities in spirals convert a large fraction of gas into stars quickly, depleting the gas reservoirs. On the other hand, star formation occurs more gradually over longer timescales in irregulars with a higher gas fraction. We then compare the stability of the nearby galaxies with galaxies at $z\,=\,4.5$. We find that net stability levels in the nearby galaxies and the galaxies at $z\,=\,4.5$ are primarily driven by the stellar disc suggesting the presence of an inherent mechanism that self-regulates the stability. Finally, upon removing the contribution of the dark matter to the total potential, the median $Q_{RW}$ for the nearby galaxies and galaxies at $z \,= \,4.5$ remains unchanged indicating that the baryons can self-regulate the stability levels, at least in a statistical sense.

astro-ph.GA↗

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, $\frac{V_{Rot}}{\sigma_{z}} \sim 5 - 8$, 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.

astro-ph.GA↗

arXiv2020

Dynamical modelling of disc vertical structure in superthin galaxy `UGC 7321' in braneworld gravity: An MCMC study

Low surface brightness (LSBs) superthins constitute classic examples of very late-type galaxies, with their disc dynamics strongly regulated by their dark matter halos. In this work we consider a gravitational origin of dark matter in the brane world scenario, where the higher dimensional Weyl stress term projected onto the 3-brane acts as the source of dark matter. In the context of the braneworld model, this dark matter is referred to as the \emph{`dark mass'}.This model has been successful in reproducing the rotation curves of several low surface brightness and high surface brightness galaxies. Therefore it is interesting to study the prospect of this model in explaining the vertical structure of galaxies which has not been explored in the literature so far. Using our 2-component model of gravitationally-coupled stars and gas in the external force field of this \emph{dark mass}, we fit the observed scale heights of stellar and atomic hydrogen (HI) gas of superthin galaxy `UGC7321' using the Markov Chain Monte Carlo approach. We find that the observed scaleheights of `UGC7321' can be successfully modelled in the context of the braneworld scenario. In addition, the model predicted rotation curve also matches the observed one. The implications on the model parameters are discussed.

gr-qc↗

arXiv2020

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 $\sigma_{0s}$ $\sim$ $10.2 - 18.4$ $\rm{kms}^{-1}$ 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 $\mu$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.

astro-ph.GA↗

arXiv2019

Stability of two-fluid galactic disc under the influence of an external tidal field

We consider the dynamics of rotationally supported thin galactic disc composed of stars and gas under the influence of external tidal field and derive the coupled differential equations governing the evolution of instabilities. Further linearising the governing equation a modified dispersion relation and stability criterion for appraising the stability of the two fluid galactic disc under the influence of external tidal field is obtained. Possible applications and method for the same are discussed.

astro-ph.GA↗