arXiv subjects

S Amrutha

Publications and source records attributed to S Amrutha.

5 recordsLinked to original sources

arXiv2026

Spatial Correlations of PAH, UV, H$\alpha$ Emission and IMF - PAH Variations in the Star-Forming Complexes of NGC 628

We examine the spatial correlation of emission from polycyclic aromatic hydrocarbons (PAH) (3.3 $\mu$m, 7.7 $\mu$m, and 11.3 $\mu$m) with the far-ultraviolet (FUV), near-ultraviolet (NUV), and H$\alpha$ emission from star-forming complexes (SFCs) in different regions of NGC 628. We use the James Webb Space Telescope to detect PAH emission, along with the Ultraviolet Imaging Telescope and Multi Unit Spectroscopic Explorer observations to sample the UV and H$\alpha$ emission, respectively. We investigate the correlation of PAH luminosities with FUV, NUV, and H$\alpha$ luminosities for the extracted SFCs. We find that the arm and spur SFCs show bright PAH emission, except for those with only NUV emission, located primarily in bubbles and superbubbles. We also examine these correlations in the phantom void, the largest superbubble in NGC 628. We investigate the trend for FUV-NUV and the Initial mass function (IMF) index $\alpha_2$ derived from the ratio of B stars with PAH band ratios (F335M/F1130W, F770W/F1130W, and F335M/F770W). We find that PAH band ratios increase as the IMF becomes more top-heavy and the SFCs become bluer, consistent with enhanced PAH excitation and ionisation in stronger UV radiation fields. However, $\alpha_2$ of spur SFCs shows a flat trend with PAH band ratios compared to arms. Upon closer examination, two SFCs in the shell region of the phantom void showed a flatter IMF compared to the SFC located near the elongated bubble. This is likely due to gas accumulation, possibly through feedback mechanisms in the shell region, while the SFC near the elongated bubble may have formed in a region affected by shear.

astro-ph.GA↗

arXiv2026

Morphological complexity of NGC 628 - a multiwavelength multiscale analysis using the ordinal pattern framework

As statistical systems, galaxies exhibit a rich interplay between organized structure and stochastic fluctuations across a broad range of spatial scales. This duality motivates the need for quantitative frameworks capable of capturing their morphological complexity. The ordinal patterns framework, along with its associated statistical measures: permutation entropy ($H$), disequilibrium ($D_E$), statistical complexity ($C$), and ordinal network node entropy, has recently emerged as a powerful tool for analyzing such complexity in physical systems. We apply this framework in a multiwavelength, multiscale analysis of the galaxy NGC 628, utilizing observations in the near-ultraviolet, near-infrared, mid-infrared, and millimeter bands. Our results reveal a characteristic spatial scale of approximately 200 parsecs, marking the transition from small-scale structures influenced by star formation and stellar feedback to larger-scale morphology governed by the galaxy's dynamics. Furthermore, we find that the $C$ vs. $H$ trajectories for all wavelengths converge toward a common attractor curve, consistent with the behavior of isotropic Gaussian random fields. This convergence suggests a universal statistical behavior in galactic structure at large scales, despite the differing physical processes traced by each wavelength.

astro-ph.GA↗

arXiv2026

Star Formation Beyond the Optical Disk : The Low-Density Outskirts of NGC2090

We present a far-ultraviolet (FUV) analysis of the star-forming complexes (SFCs) in the nearby spiral galaxy NGC\,2090, based on observations from the Ultraviolet Imaging Telescope (UVIT), and compare it with emission from the optical and infrared bands. NGC\,2090 exhibits prominent star formation in its extended outer disk, with FUV emission traced out to $\sim$30 kpc, far beyond the truncation of the old stellar disk at $\sim$5 kpc. It is classified as an extended UV (XUV) disk galaxy. We identify and characterize the SFCs both within and beyond the optical radius (R$_{25}$), estimating their physical sizes and star formation rates (SFRs). The outer-disk SFCs are generally smaller in area and show a narrower distribution of SFR surface density ($\Sigma_{\mathrm{SFR}}$) compared to the inner-disk SFCs. We investigate the properties of the inner disk using mid-infrared data from the James Webb Space Telescope (JWST), and find that the polycyclic aromatic hydrocarbon (PAH) emission is strongly correlated with regions of active star formation. The specific SFR (sSFR) increases with radius, consistent with a scenario of inside-out disk growth. The observed number of SFCs and their H$\alpha$-to-FUV flux ratios in the outer disk of NGC\,2090 indicate ongoing massive star formation and are consistent with a top-heavy IMF, implying that the upper end of the IMF is not truncated in the low-density, metal-poor outskirts. These results suggest that XUV disks can host significant massive star formation despite their low stellar density and metallicity.

astro-ph.GA↗

arXiv2025

A pilot method to determine the high mass end of the Stellar Initial Mass Function in galaxies using UVIT, H$\alpha$-MUSE observations and applied to NGC628

We present a pilot method to estimate the high-mass initial mass function (IMF) across the arm, interarm, and spur regions in galaxies and apply it to NGC 628. We extracted star-forming complexes (SFCs) from H$\alpha$ VLT/MUSE and UVIT (FUV and NUV) observations of NGC 628 and used ALMA observations to define the molecular gas distribution. We find that the extinction-corrected H$\alpha$ and FUV luminosities correlate well. Using the fact that O stars have a shorter lifetime (10$^7$ yr) compared to B stars (10$^8$ yr), we estimated the approximate number of O stars from H$\alpha$ emission, and the number of B0 ($M_{*} > 10 M_{\odot}$), and B1 ($10 M_{\odot} \geq M_{*} \geq 3 M_{\odot}$) stars using FUV, NUV observations. We derived the IMF index ($\alpha$) for different regions using O to B0 ($\alpha_{1}$) and B0 to B1 ($\alpha_{2}$) stellar ratios. Our findings indicate that if we assume H$\alpha$ arises only from O8-type stars, the resulting $\alpha_{1}$ value is consistent with the canonical IMF index. It steepens when we assume O stars with masses up to 100 $M_{\odot}$ with mean $\alpha_{1}= 3.16 \pm 0.62$. However, the $\alpha_{2}$ does not change for large variations in the O-star population, and the mean $\alpha= 2.64 \pm 0.14$. When we include only blue SFCs ($ FUV-NUV\leq0.3$), mean $\alpha_{2}$ is $2.43 \pm 0.06$. The IMF variation for SFCs in arms and spurs is insignificant. We also find that $\alpha_{2}$ correlates with different properties of the SFCs, the most prominent being the extinction-corrected UV color (FUV-NUV).

astro-ph.GA↗

arXiv2024

A Comparative Study of Star Forming Dwarf Galaxies using the UVIT

We present a Far-Ultraviolet (FUV) study of sixteen star-forming dwarf galaxies (SFDGs) using the Ultraviolet Imaging Telescope (UVIT). Morphologically, SFDGs are classified as dwarf spirals, dwarf irregulars, and blue compact dwarfs (BCDs). We extracted the star-forming complexes (SFCs) from the sample galaxies, derived their sizes, and estimated the FUV+24$μm$ star formation rates (SFRs). We also determined the approximate stellar disk mass associated with the SFCs using IRAC 3.6-micron images. We derived the specific SFRs (sSFRs), as well as the SFR densities ($Σ(SFR)$) for the SFCs. We find that the lower $Σ(SFR)$ for each type is different, with the dwarf irregulars having the lowest $Σ(SFR)$ compared to others. However, the median size of the SFCs in the dwarf irregulars is the largest compared to the other two types when compared at roughly the same distance. We have derived the star-forming main sequence (SFMS) on the scale of SFCs for all three classes of SFDGs. We find that although all SFDGs approximately follow the global SFMS relation, i.e. $SFR\propto {M_{*}}^α$ (where globally $α\approx1$ for low surface brightness galaxies and $0.9$ for SFDGs), on the scale of SFCs the $α$ value for each type is different. The $α$ values for dwarf spirals, dwarf irregulars, and BCDs are found to be 0.74$\pm$0.13, 0.8$\pm$0.16, and 0.8$\pm$0.19, respectively. However, the age of all SFCs approximately corresponds to 1 Gyr. Finally, we find that the outer SFCs in most galaxies except BCDs have a high sSFR, supporting the inside-out model of galaxy growth.

astro-ph.GA↗