arXiv subjects

Ramya Sethuram

Publications and source records attributed to Ramya Sethuram.

9 recordsLinked to original sources

arXiv2026

Tracing gas outflows in molecular gas-rich galaxies with the SKA

Active galactic nuclei with powerful radio jets play a key role in galaxy evolution through their ability to regulate the cold gas reservoirs that fuel star formation. Jet-driven feedback can heat, compress, or expel atomic and molecular gas, thereby reshaping the interstellar medium and altering star formation efficiency. However, the physical coupling between AGN activity and the multi-phase interstellar medium remains poorly constrained, particularly in radio-loud systems where mechanical feedback and multiphase outflows are expected to dominate. SKA will provide major advances in the study of cold gas in AGN host galaxies through sensitive observations of H~{\sc i} emission and absorption, together with access to selected low-frequency molecular transitions within the SKA~1 frequency range, including OH, H$_2$CO, CH$_3$OH, and, at high redshift, low-$J$ transitions of CO, HCN, and HCO$^{+}$ in rare bright systems. Combined with radio continuum measurements, these tracers will provide direct constraints on gas mass, kinematics, turbulence, and inflow/outflow signatures, enabling detailed studies of feedback-regulated cold gas reservoirs in AGN environments. In this chapter, we examine how SKA1 observations of neutral hydrogen, complemented by molecular-line and radio continuum studies, can be used to quantify multiphase gas flows and feedback energetics in molecular-gas-rich radio galaxies. SKA surveys will enable population-level studies of AGN-driven feedback, providing a new framework for understanding how radio jets regulate the cold interstellar medium and star formation across cosmic time.

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arXiv2026

DESI as sparse Integral Field Spectrograph I: Spatially resolved chemical enrichment in star-forming galaxies at $z\leq0.1$

We present a spatially resolved chemical abundance analysis of 2291 star-forming galaxies at $z \leq 0.1$, spanning nearly four orders of magnitude in stellar mass ($8 \le \rm log (M_{\star}/M_{\odot}) \le 11.5$), by exploiting the multi-fibre spectra from the Dark Energy Spectroscopic Instrument (DESI) as a sparse integral field spectrograph. In the inner regions ($<2R_e$), the radial gas-phase metallicity profiles show an outward-declining trend for massive galaxies, with the steepest gradient ($\nabla_{log(O/H)}$) $\sim-0.08$ dex/R$_{e}$, whereas low-mass dwarf galaxies exhibit nearly flat profiles ($\nabla_{log(O/H)}\sim-0.02$ dex/R$_{e}$). The large galactocentric radii ($\sim$5 R$_{e}$) probed in this study, reveal flat metallicity profiles near the disk-halo interface. Strikingly, these flat metallicity values are consistent across a wide stellar mass range, likely reflecting the influence of low SFR and metal poor inflows in the outer regions. The metallicity gradient - stellar mass relation exhibits a turnover at $\log(M_\star/M_\odot) \sim 10.5$, beyond which gradients become shallower, possibly driven by the chemical equilibrium in the inner disk of massive galaxies and/or dilution from cosmic gas accretion. At fixed stellar mass, a strong size dependence is observed, where compact galaxies show flatter gradients and higher central enrichment than their extended counterparts. The abundance gradients are further linked with the stellar age distribution within the galactic disk, where galaxies with younger outskirts show steeper gradients than the ones with older outskirts, consistent with ongoing inside-out disc growth sustaining centrally concentrated chemical enrichment. These results underscore the interplay of star formation efficiency, stellar feedback, and metal-poor gas accretion in governing the radial chemical structure in galaxies.

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arXiv2025

Design and Fabrication of a lightweight three-lens corrector system for the 2.34-m Vainu Bappu Telescope

The Vainu Bappu Telescope (VBT) is a 2.34-m reflector, primarily supported on-axis field of view, offering high-resolution and low-to-medium resolution spectroscopic observations in its prime and Cassegrain configurations. This study presents the design and fabrication of a compact, lightweight, three-element wide-field corrector (WFC) utilizing three spherical lenses to cover a polychromatic wavelength range over a 30$'$ FoV at prime focus. The WFC design was optimized using ZEMAX, ensuring precision in aberrations, tolerances, and atmospheric dispersion. The fabricated lenses met stringent tolerances, with a $\pm$1 mm deviation in radius of curvature and $\pm$2 mm deviation in center thickness. A mechanical mount was developed to integrate all the WFC lenses, and wavefront error testing for the WFC system was performed using ZYGO interferometry, yielding a Wavefront Error of 0.05 $λ$. Laboratory performance tests were designed and conducted using a dedicated setup with achromatic lenses and 100 $μm$ fiber-coupled polychromatic light source showed a deviation of 0.1 pixel on-axis and 0.5 pixel at the extreme off-axis field compared to the ZEMAX design, demonstrating that the optical performance of WFC is with minimal aberrations across the entire FoV. The successful integration of the WFC at the VBT prime focus will increase the FoV, enabling the multi-fiber, multi-spectrograph setup in 30$'$ field that will facilitate both OMR and Echelle spectrograph to be used on the same night along with the addition of new multi-object spectrograph and an integral field unit instrument. This will mark a significant upgrade for the VBT, broadening its research potential, and expanding its observational versatility.

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arXiv2024

Exploring neutral hydrogen in the radio MOlecular Hydrogen Emission Galaxies (MOHEGs) and prospects with the SKA

The empirical studies of cold gas content serve as an essential aspect in comprehending the star formation activities and evolution in galaxies. However, it is not straightforward to understand these processes because they depend on various physical properties of the Interstellar Medium. Massive FRI/II type radio galaxies rich in molecular hydrogen with less star formation activities are known as radio Molecular Hydrogen Emission Galaxies (MOHEGs). We present a study of neutral hydrogen gas-associated radio MOHEGs at redshifts <0.2 probed via the HI 21-cm absorption line. Neutral hydrogen is detected in 70% of these galaxies, which are located at a distance of 8 - 120 kiloparsec from the neighboring galaxies. These galaxies show a scarcity of HI gas as compared to merging galaxies at similar redshifts. We found no strong correlation between N(HI), N(H), and galaxy properties, independent of whether the HI is assumed to be cold or warm, indicating that the atomic gas is probably playing no important role in star formation. The relationship between total hydrogen gas surface density and star formation surface density deviates from the standard Kennicutt-Schmidt law. Our study highlights the importance of HI studies and offers insights into the role of atomic and molecular hydrogen gas in explaining the properties of these galaxies. In the upcoming HI 21-cm absorption surveys with next-generation radio telescopes such as the Square Kilometre Array (SKA) and pathfinder instruments, it may be possible to provide better constraints to such correlations.

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arXiv2023

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES): driving science cases and expected outcomes

The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES) is a $2-5~μ$m, high-contrast integral field spectrograph (IFS) currently being built for Keck Observatory. With both low ($R\lesssim250$) and medium ($R\sim3500-7000$) spectral resolution IFS modes, SCALES will detect and characterize significantly colder exoplanets than those accessible with near-infrared ($\sim1-2~μ$m) high-contrast spectrographs. This will lead to new progress in exoplanet atmospheric studies, including detailed characterization of benchmark systems that will advance the state of the art of atmospheric modeling. SCALES' unique modes, while designed specifically for direct exoplanet characterization, will enable a broader range of novel (exo)planetary observations as well as galactic and extragalactic studies. Here we present the science cases that drive the design of SCALES. We describe an end-to-end instrument simulator that we use to track requirements, and show simulations of expected science yields for each driving science case. We conclude with a discussion of preparations for early science when the instrument sees first light in $\sim2025$.

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arXiv2022

Design of an IR Imaging Channel for the Keck Observatory SCALES Instrument

A next-generation instrument named, Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES), is being planned for the W. M. Keck Observatory. SCALES will have an integral field spectrograph (IFS) and a diffraction-limited imaging channel to discover and spectrally characterize the directly imaged exoplanets. Operating at thermal infrared wavelengths (1-5 micron, and a goal of 0.6-5 micron), the imaging channel of the SCALES is designed to cover a 12"x 12" field of view with low distortions and high throughput. Apart from expanding the mid-infrared science cases and providing a potential upgrade/alternative for the NIRC2, the H2RG detector of the imaging channel can take high-resolution images of the pupil to aid the alignment process.Further, the imaging camera would also assist in small field acquisition for the IFS arm. In this work, we present the optomechanical design of the imager and evaluate its capabilities and performances.

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arXiv2021

The Sharpest Ultraviolet view of the star formation in an extreme environment of the nearest Jellyfish Galaxy IC 3418

We present the far ultraviolet (FUV) imaging of the nearest Jellyfish or Fireball galaxy IC3418/VCC 1217, in the Virgo cluster of galaxies, using Ultraviolet Imaging Telescope (UVIT) onboard the ASTROSAT satellite. The young star formation observed here in the 17 kpc long turbulent wake of IC3418, due to ram pressure stripping of cold gas surrounded by hot intra-cluster medium, is a unique laboratory that is unavailable in the Milkyway. We have tried to resolve star forming clumps, seen compact to GALEX UV images, using better resolution available with the UVIT and incorporated UV-optical images from Hubble Space Telescope archive. For the first time, we resolve the compact star forming clumps (fireballs) into sub-clumps and subsequently into a possibly dozen isolated stars. We speculate that many of them could be blue supergiant stars which are cousins of SDSS J122952.66+112227.8, the farthest star (~17 Mpc) we had found earlier surrounding one of these compact clumps. We found evidence of star formation rate (4 - 7.4 x 10^-4 M_sun per yr ) in these fireballs, estimated from UVIT flux densities, to be increasing with the distance from the parent galaxy. We propose a new dynamical model in which the stripped gas may be developing vortex street where the vortices grow to compact star forming clumps due to self-gravity. Gravity winning over turbulent force with time or length along the trail can explain the puzzling trend of higher star formation rate and bluer/younger stars observed in fireballs farther away from the parent galaxy.

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arXiv2020

Interacting system NGC 7805/6 (Arp 112) and its tidal dwarf galaxy candidate

We present results from our Giant Metrewave Radio Telescope (GMRT) HI, Himalayan Chandra Telescope (HCT) Halpha, 1m Sampurnanand Telescope (ST) and 1.3m Devasthal Fast Optical Telescope (DFOT) deep optical observations of NGC 7805/6 (Arp 112) system to test KUG 2359+311's tidal dwarf galaxy (TDG) candidacy and explore the properties of the interacting system. Our GMRT HI map shows no HI detection associated with KUG 2359+311, nor any HI tail or bridge-like structure connecting KUG 2359+311 to the NGC 7805/6 system. Our HCT Halpha image on the other hand, shows strong detections in KUG 2359+311, with net SFR ~ 0.035$\pm 0.009 {\rm M}_{\odot}\,{\rm yr}^{-1}$. The Halpha data constrains the redshift of KUG 2359+311 to $0.00 \le z \le 0.043$, compared to the redshift of NGC 7806 of ~ 0.015. TDGs detected to date have all been HI rich, and displayed HI, ionised gas and stellar tidal debris trails (bridges or tails) linking them to their parent systems. But neither our HI data nor our optical images, while three magnitudes deeper than SDSS, reveal tidal trail connecting KUG 2359+311 to NGC 7805/6. Lack of HI , presence of an old stellar population, ongoing star formation, reasonably high SFR compared to normal dwarf galaxies suggest that KUG 2359+311 may not be an Arp 112 TDG. It is most likely a case of a regular gas-rich dwarf galaxy undergoing a morphological transformation after having lost its entire gas content to an interaction with the Arp 112 system. Redshift and metallicity from future spectroscopic observations of KUG 2359+311 would help clarify the nature of this enigmatic structure.

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arXiv2018

JINGLE, a JCMT legacy survey of dust and gas for galaxy evolution studies: I. Survey overview and first results

JINGLE is a new JCMT legacy survey designed to systematically study the cold interstellar medium of galaxies in the local Universe. As part of the survey we perform 850um continuum measurements with SCUBA-2 for a representative sample of 193 Herschel-selected galaxies with M*>10^9Msun, as well as integrated CO(2-1) line fluxes with RxA3m for a subset of 90 of these galaxies. The sample is selected from fields covered by the Herschel-ATLAS survey that are also targeted by the MaNGA optical integral-field spectroscopic survey. The new JCMT observations combined with the multi-wavelength ancillary data will allow for the robust characterization of the properties of dust in the nearby Universe, and the benchmarking of scaling relations between dust, gas, and global galaxy properties. In this paper we give an overview of the survey objectives and details about the sample selection and JCMT observations, present a consistent 30 band UV-to-FIR photometric catalog with derived properties, and introduce the JINGLE Main Data Release (MDR). Science highlights include the non-linearity of the relation between 850um luminosity and CO line luminosity, and the serendipitous discovery of candidate z>6 galaxies.

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