arXiv subjects

Shyam Menon

Publications and source records attributed to Shyam Menon.

5 recordsLinked to original sources

arXiv2025

Density modulated star formation efficiency: implications for the observed abundance of ultra-violet luminous galaxies at z>10

The number density of UV luminous galaxies discovered by the James Webb Space Telescope at ultra high redshift ($z \gtrsim 10$) is higher, and declines much more slowly with increasing redshift, than expected from extrapolations of lower redshift observations or pre-launch physics-based models. Most of these models assume star formation efficiencies (SFE) of only a few percent, motivated by observations of nearby galaxies. In this work, we incorporate a scaling of SFE with gas surface density (which we refer to as Density Modulated SFE; DMSFE), motivated by cloud-scale simulations and theory, into a semi-analytic cosmological model (SAM) of galaxy formation which is calibrated to match the observed rest-UV sizes of high redshift galaxies. We also model the impact of dust and bursty star formation on the SAM-predicted properties of observed galaxies. We show that with plausible values of the main parameters, such as the fraction of gas in dense clouds $f_{\rm dense}$, our new models easily reproduce or even exceed the observed galaxy number densities at $z\sim 6$-17. While no single value of $f_{\rm dense}$ is able to reproduce the very shallow observed decline of the galaxy number density at $z\gtrsim 12$, it is plausible and even expected for $f_{\rm dense}$ to have some effective dependence on cosmic time, which could bring these models into closer agreement with the data. We show that the combined effects of DMSFE, decreasing dust attenuation, and increasingly bursty star formation at earlier cosmic epochs could conspire to reproduce the observed evolution.

astro-ph.GA↗

arXiv2025

Planets and planetesimals at cosmic dawn: Vortices as planetary nurseries

Low-mass, metal-enriched stars were likely present as early as cosmic dawn. In this work, we investigate whether these stars could have hosted planets in their protoplanetary disks. If so, these would have been the first planets to form in the Universe, emerging in systems with metallicities much lower than solar. In the core accretion model, planetesimals serve as the building blocks of planets, meaning that planetesimal formation is a prerequisite for planet formation. In a non-structured disk, planetesimal formation typically requires near-solar metallicities according to our current understanding. However, mechanisms that concentrate solid material can significantly lower this metallicity threshold. Here, we explore whether vortices can facilitate the formation of the first planets and planetesimals at cosmic dawn. Vortices are prime sites for planetesimal formation due to their ability to efficiently trap and concentrate dust. We conduct simulations spanning a range of metallicities, and identify a metallicity threshold at Z >=~ 0.04 Zsun for planetesimal formation. If these planetesimals remain inside the vortex long enough to accrete the remaining trapped solids, Mercury-mass planets can form. The formation of Mars-mass planets or larger requires a metallicity of Z >=~ 0.08 Zsun. These results assume a low level of disk turbulence, with higher turbulence levels leading to higher metallicity thresholds.

astro-ph.EP↗

arXiv2025

A Tentative Detection of Molecular Hydrogen (H$_2$) Emission Lines at Cosmic Dawn

We present a theoretical framework for interpreting far-ultraviolet (FUV) fluorescent emission from molecular hydrogen (H$_2$) in high-redshift galaxies, motivated by the unique capabilities of the James Webb Space Telescope (JWST) to probe the rest frame FUV at cosmic dawn. Using the Meudon photodissociation region (PDR) code, we model the H$_2$ fluorescence spectrum under extreme interstellar medium (ISM) conditions in terms of high pressure ($10^{11}~\mathrm{K~ cm^{-3}}$), high radiation field ($10^6$ $G_0$) combined with low metallicity ($Z = 0.1~Z_\odot$) and high cosmic ionization rate ($\zeta = 10^{-14}~\mathrm{s}^{-1}$), characteristic of early galaxies. As a case study, we apply this framework to stacked NIRSpec spectra from the JWST Advanced Deep Extragalactic Survey (JADES) for galaxies at redshifts $z\geq7$. The stacked spectrum exhibits emission features consistent in profile and wavelength with the predicted H$_2$ fluorescence lines, including a blue shift suggestive of an outflow of molecular gas. Although individual features remain below robust detection thresholds, this demonstration illustrates the feasibility of using FUV fluorescence modeling to guide and interpret JWST spectroscopy of the molecular ISM at high redshift. Our framework provides a foundation for future searches for molecular hydrogen emission and the study of galactic feedback processes in the early universe.

astro-ph.GA↗

arXiv2025

Linking stellar populations to HII regions across nearby galaxies. II. Infrared Reprocessed and UV Direct Radiation Pressure in HII Regions

Radiation pressure is a key mechanism by which stellar feedback disrupts molecular clouds and drives HII region expansion. This includes direct radiation pressure exerted by UV photons on dust grains, pressure associated with photoionization, and infrared (IR) radiation pressure on grains due to dust-reprocessed IR photons. We present a new method that combines high resolution mid-IR luminosities from JWST-MIRI, optical attenuation and nebular line measurements from VLT-MUSE, and HST H$\alpha$-based region sizes to estimate the strength of radiation pressure in $\approx 18,000$ HII regions across 19 nearby star-forming galaxies. This is the most extensive and direct estimate of these terms beyond the Local Group to date. In the disks of galaxies, we find that the total reprocessed IR pressure is on average 5% of the direct UV radiation pressure. This fraction rises to 10% in galaxy centers. We expect reprocessed IR radiation pressure to dominate over UV radiation pressure in regions where $L_{\rm F2100W}/L_{\rm H\alpha}^{\rm corr} \gtrsim 75$. Radiation pressure due to H ionizations is lower than pressure on dust in our sample, but appears likely to dominate the radiation pressure budget in dwarf galaxies similar to the Small Magellanic Cloud. The contribution from all radiation pressure terms appears to be subdominant compared to thermal pressure from ionized gas, reinforcing the view that radiation pressure is most important in compact, heavily embedded, and young regions.

astro-ph.GA↗

arXiv2022

Single and two-phase fluid droplet breakup in impulsively generated high-speed flow

Aerobreakup of fluid droplets under the influence of impulsively generated high-speed gas flow using an open-ended shock tube is studied using experiments and numerical simulations. Breakup of mm-sized droplets at high Weber number was analyzed for water and two-phase nanofluids consisting of dispersions of Al2O3 and TiO2 nanoparticles in water with high loading of 20 and 40 weight % respectively. Droplet breakup is visualized using high-speed imaging in the experimental setup, where an open ended shock tube generates impulsive high-speed flow impinging on a droplet held stationary using an acoustic levitator. Axisymmetric simulations using the Volume of Fluid technique are conducted to capture the gas dynamics of the flowfield and droplet deformation at the initial stages. Fluid droplets are subject to a transient flow field generated by the open ended shock tube, characterized by a propagating incident shock wave, a recirculating vortex ring, and standing shock cells. Droplet breakup for all fluids proceeds through an initial flattening of the droplet followed by generation of a liquid sheet at the periphery in the presence of a curved, detached shock front at the leading edge. The breakup appears to follow a sheet stripping process whereby stretched ligaments undergo secondary atomization through viscous shear. Mist generated in the wake of the droplet appears to expand laterally due to the unconstrained expansion of the high-speed gas jet. The breakup morphology of droplets for all fluids appears consistent with previous observations using conventional shock tubes. Lateral deformation of the coherent droplet mass is observed to be higher for nanofluids as compared to water. This is attributed to higher viscosity and Ohnesorge number of nanofluid droplets, which results in delayed breakup and increased lateral stretching.

physics.flu-dyn↗