arXiv subjects

Niranjan Chandra Roy

Publications and source records attributed to Niranjan Chandra Roy.

6 recordsLinked to original sources

arXiv2026

Investigating black hole accretion and feedback self-regulation in Seyfert galaxies using the FIRE-3 cosmological hydrodynamic simulations

Recent observations of local Seyfert galaxies show an intriguing connection between Active Galactic Nuclei (AGN) luminosity and a deficit of molecular gas on ~50pc scales compared to 200pc, the plausible imprint of AGN feedback. Motivated by these findings, we investigate the interplay between supermassive black hole (BH) accretion, AGN feedback, and nuclear gas reservoirs using high-resolution cosmological hydrodynamic simulations implementing FIRE-3 multi-phase interstellar medium (ISM) physics and multi-component BH accretion and feedback models. Focusing on the late-time evolution of four Milky Way-mass galaxies, we find recurrent cycles of increased gas inflow toward the accretion disc, enhanced BH accretion, feedback self-regulation, and suppressed gas inflow rate until the next fueling event. AGN winds interact with the ISM and escape preferentially through low-density polar channels after opening central cavities on ~10-500pc scales, regulating BH growth and producing episodic behaviour on ~10-100Myr timescales. The simulations reproduce the observed diversity of nuclear morphologies, gas concentrations, and AGN luminosities in late-type Seyfert galaxies, but do not exhibit a clear anti-correlation between gas concentration and AGN luminosity. Higher-luminosity AGN ($L_X$~$10^{41.5-43}$ erg s$^{-1}$) powered by the accretion disc reservoir can coexist with feedback-driven cavities, consistent with observations, but they are more common in simulated galaxies with centrally-peaked gas distributions. Although differences in sample selection, tracer choice, spatial resolution, and stochasticity in AGN fueling may impact underlying concentration-luminosity trends, the apparent tension between simulations and observations points to the timing between gas inflow, accretion-disc depletion, and feedback-driven clearing on ~50-200pc scales as a key constraint on AGN self-regulation models.

astro-ph.GA↗

arXiv2026

Little Red Dots on FIRE: Exploring the formation and observational signatures of ultra-compact early galaxies

Little Red Dots (LRDs) are compact sources with broad Balmer lines, Balmer breaks, anomalous UV emission, rising red continuum, and uncertain origin. We use FIRE cosmological simulations, 3D dust radiative transfer, and synthetic emission-line data cubes to test whether ultra-compact early galaxies can reproduce LRD-like observables without invoking AGN. In progenitors of present-day group halos ($M_{\rm halo} > 10^{13.5} M_{\odot}$), we identify transient phases at $z \approx 4-8$ lasting $\sim 150-400$ Myr in which strong dissipative inflows build massive ($M_{\star} \sim 10^{8.5}-10^{10.5} M_{\odot}$), UV-bright ($-23 \lesssim M_{\rm UV} \lesssim -20$), ultra-compact ($R_{\rm eff} < 300$ pc) stellar cores with extreme circular velocity ($V_{\rm circ} > 500$ km s$^{-1}$) and consistent with several LRD properties: strong Balmer breaks ($F_ν(4200{\rm Å})/F_ν(3500{\rm Å}) \sim 2$); blue UV beta slopes ($β_{\rm UV} \approx -1.25$); dust masses; ALMA non-detections; and Balmer-line widths up to $\sim 1500$ km s$^{-1}$ broadened by galaxy-scale dynamics. However, stellar emission and host-galaxy kinematics alone do not reproduce the red rest-optical continuum, more extreme Balmer breaks ($\gtrsim 2.5$) and line widths ($\gtrsim 2000$ km s$^{-1}$), or the broad-Balmer/narrow-forbidden-line signature of broad-line AGN. The same ultra-compact conditions efficiently fuel central BHs, suggesting a hybrid stellar+AGN scenario in which compact stars explain the UV continuum, Balmer break, and intermediate line widths while AGN supply the red optical continuum and more extreme line properties. With halo masses $M_{\rm halo} \sim 10^{11-12.5} M_\odot$ and comoving abundance $\sim 2 \times 10^{-5} {\rm cMpc}^{-3}$ (for $\sim 20\%$ duty-cycle at $z \approx 4-8$), ultra-compact galaxies can contribute to the massive, bright LRD population.

astro-ph.GA↗

arXiv2026

Little Red Dots on FIRE: The Ability of Bursty Galaxies to Host an Abundant Population of High-Redshift AGN

The James Webb Space Telescope has unveiled an abundant population of potential active galactic nuclei (AGN) at high redshift ($z\gtrsim4$) known as little red dots (LRDs), which are likely hosted in relatively low-mass galaxies. However, previous theoretical models have highlighted the difficulty in continuously feeding massive black holes in the central regions of bursty, high-redshift galaxies because of repeated gas evacuation by stellar feedback. We analyze galaxies in high-redshift FIRE-2 simulations to understand whether they are capable of hosting the observed abundant population of high-redshift AGN. We use a gravitational torque-driven accretion (GTDA) model and a simple free-fall accretion model to derive black hole accretion rates and construct predicted AGN bolometric luminosity functions for $z=5-7$. The GTDA model and the free-fall model with black holes accreting $\lesssim 1$ percent of their central gas supply ($<100 \rm \ pc$) per free-fall time predict AGN abundances that are more than sufficient to explain the most recent LRD observations. The fiducial models, in fact, overpredict the number of low-luminosity AGN as compared with observations. We explore possible resolutions of this tension. A plausible, though likely not unique, scenario for alleviating the AGN overpredictions and which also provides a good match to the host-galaxy UV luminosity distribution suggests that LRDs are super Eddington-accreting, Eddington luminosity-limited, $M_{\rm BH}\gtrsim 2\times10^5 \ \rm M_\odot$ black holes residing in $M_\star\gtrsim 2\times10^7 \ \rm M_\odot$ galaxies. We show that, under simple assumptions, mock observations of such sources can reproduce key observed LRD characteristics.

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arXiv2026

Deep and Sparse Denoising Benchmarks for Spectral Data Cubes of High-z Galaxies: From Simulations to ALMA observations

Beyond cosmic noon, galaxies appear as faint whispers amid noise, yet this epoch is key to understanding massive galaxy assembly. ALMA's sensitivity to cold dust and [C II] emission allows us to probe their interstellar medium, but faint signals make robust denoising essential. We evaluate and benchmark denoising strategies including Principal Component Analysis, Independent Component Analysis, sparse unsupervised representations: iterative soft thresholding with 2D-1D wavelets, and supervised deep learning with a 3D U-Net, to identify techniques that suppress noise while preserving flux and morphology across peak SNRs of 2.5-8, applied to (i) synthetic spectral cubes of rotating toy disk galaxies, (ii) synthetic [C II] IFU cubes from FIRE simulations, and (iii) ALMA [C II] observations of CRISTAL galaxies and W2246-0526. Performance is assessed via RMSE, conservation of flux and spectra, noise reduction, and SNR improvement of the central galaxy. For synthetic cubes: PCA and ICA provide marginal improvement; IST reduces noise effectively at moderate SNRs but can suppress emission at low SNRs; and the U-Net outperforms IST, though it can produce quantifiable hallucinations at lower-SNRs. For moderate-SNR observations (ALMA-CRISTAL), U-Net and IST achieve comparable performance, conserving >91% flux and increasing SNR by >6. However, for observations with complex morphologies absent in the training set (W2246), the U-Net underperforms relative to IST, recovering ~80% flux, while IST robustly conserves flux and improves SNR by ~3, highlighting generalisation challenges and the need for physically-motivated training priors. We conclude that IST is a robust unsupervised denoiser for moderate-SNR data, and a synthetically trained U-Net generalises effectively to real data, dependent on training priors. This framework offers a pathway for transferable denoising for ALMA, VLT/MUSE, and JWST.

astro-ph.GA↗

arXiv2025

Probing the Physics of Dusty Outflows through Complex Organic Molecules in the Early Universe

Galaxy-scale outflows are of critical importance for galaxy formation and evolution. Dust grains are the main sites for the formation of molecules needed for star formation but are also important for the acceleration of outflows that can remove the gas reservoir critical for stellar mass growth. Using the MIRI medium-resolution integral field spectrograph aboard the James Webb Space Telescope (JWST), we detect the 3.28 $μ$m aromatic and the 3.4 $μ$m aliphatic hydrocarbon dust features in absorption in a redshift 4.601 hot dust-obscured galaxy, blue-shifted by $Δ$V=$-5250^{+276}_{-339}$ kms$^{-1}$ from the systemic redshift of the galaxy. The extremely high velocity of the dust indicates that the wind was accelerated by radiation pressure from the central quasar. These results pave a novel way for probing the physics of dusty outflows in active galaxies at early cosmic time.

astro-ph.GA↗

arXiv2025

Powerful nuclear outflows and circumgalactic medium shocks driven by the most luminous known obscured quasar in the Universe

We report integral field spectroscopy observations with the Near-Infrared Spectrograph on board JWST targeting the 60 kpc environment surrounding the most luminous obscured quasar known at $z=4.6$. We detect ionized gas filaments on 40 kpc scales connecting a network of merging galaxies likely to form a cluster. We find regions of low ionization consistent with large-scale shock excitation surrounding the central dust-obscured quasar, out to distances nearly eight times the effective stellar radius of the quasar host galaxy. In the nuclear region, we find an ionized outflow driven by the quasar with velocities reaching 13,000 km s$^{-1}$, one of the fastest discovered to date with an outflow rate of 2000 M$_\odot$ yr$^{-1}$ and a kinetic luminosity of 6$\times10^{46}$ erg s$^{-1}$ resulting in coupling efficiency between the bolometric luminosity of the quasar and the outflow of 5%. The kinetic luminosity of the outflow is sufficient to power the turbulent motion of the gas on galactic and circumgalactic scales and is likely the primary driver of the radiative shocks on interstellar medium and circumgalactic medium scales. This provides compelling evidence supporting long-standing theoretical predictions that powerful quasar outflows are a main driver in regulating the heating and accretion rate of gas onto massive central cluster galaxies.

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