arXiv subjects

James Nianias

Publications and source records attributed to James Nianias.

7 recordsLinked to original sources

arXiv2026

Extragalactic microlensing through Ultra Diffuse Galaxies

Stellar microlensing is a powerful method to constrain compact dark matter models, uncover binary stars, and exoplanets during caustic crossing events. At cosmological distances, {\it James-Webb Space Telescope} ({\it JWST}) is routinely detecting microlensed giant stars in highly magnified galaxies behind massive lensing clusters. Here, we explore for the first time microlensing in modest redshift galaxies commonly seen through local Ultra Diffuse Galaxies (UDGs). Using the UDG NGC1052-DF2 as a proof-of-concept, we find that detecting microlensing events through UDGs is possible. Euclid is ideal for identifying samples of low-redshift star-forming galaxies seen through local galaxies for deeper cadenced follow-up, where our zeroth-order calculation estimates that $\mathcal{O}(1-10)$ events per year are expected over the whole sky under the monitoring of LSST, assuming all UDGs share similar distance and stellar content as NGC1052-DF2. Such measurements can help settle the DM issue for UDG galaxies by providing a stellar mass estimate weighted more by the low end of the stellar initial mass function (IMF) than standard stellar synthesis estimates. Note, lower efficiency is estimated for {\it JWST} of $\sim 5.6\times10^{-2}\,\textrm{yr}^{-1}$ over its five background galaxies is expected for typical {\it JWST}$\sim29$\,mag visits, and a low Vera Rubin Legacy Survey of Space and Time (LSST) detection rate of$\sim 0.03\,\textrm{yr}^{-1}$ such that NGC1052-DF2 might not be a prime target given its lack of low-redshift background galaxies.

astro-ph.GA↗

arXiv2026

Delayed Feedback in High-$z$ Starbursts Revealed by Lyman-$α$ Profiles and Metal Line Diagnostics

Lyman-$ α$ emission, which couples strongly to gas kinematics, is a key observable for probing outflows from star-forming galaxies in the early universe. Inferences of outflow properties from Lyman-$ α$, however, often lack contextual comparisons with more direct outflow diagnostics from down-the-barrel metal absorption lines and driving-source properties from metal emission lines. Here, we make such checks by taking advantage of the lensing magnification provided by galaxy clusters for 573 Lyman-$ α$ sources observed with the Multi-Unit Spectroscopic Explorer (MUSE). Using metal emission lines to measure systemic redshifts, we confirm that the Lyman-$ α$ profiles are consistent with outflowing gas: single peaks redshifted relative to, or double peaks straddling, the systemic redshift. In cases where metal absorption lines are detected, their blueshifted velocities indicate outflows, while their line ratios point to absorption by a clumpy medium. We demonstrate a significant positive correlation between absorption-line outflow velocity and Lyman-$ α$ line dispersion, confirming that Lyman-$ α$ profile morphology encodes useful kinematic information about outflowing gas. We further find that the equivalent width of high-ionization metal emission lines are anticorrelated with measured outflow velocity, suggesting younger stellar populations produce slower outflows, and providing a critical empirical constraint on feedback timescales in high-$ z $ starbursts. Fitting model Lyman-$ α$ profiles based on simple expanding shell geometry to those observed, we find that such models successfully reproduce some aspects of the data, yet often yield unphysical parameters -- calling for caution when inferring outflow

astro-ph.GA↗

arXiv2025

Transient star B/R ratio and star formation in $z\gtrsim 1$ lensed galaxies

The extreme magnification from galaxy clusters and microlenses therein allows the detection of individual, luminous stars in lensed galaxies as transient events, and hence provides a valuable window into the high mass stellar population in $z>1$ galaxies. As these bright stars can only be formed at specific ages, the relative abundance of transient events at blue (B) and red (R) optical wavelengths ($B/R$ ratio) can provide insights into the recent star formation history of galaxies that are not well constrained by their spectral energy distributions (SEDs). Here, we forward model the transient detection rates in an idealized mock scenario to find that the $B/R$ ratio of strongly lensed $z>1$ galaxies decreases quickly with increasing age. This ratio has moderate sensitivity to metallicity and comparatively low sensitivity to dust attenuation, with no significant dependency on the stellar initial mass function. Fitting model stellar populations to either the SED or $B/R$ ratio alone of ``Warhol'' arc ($z = 0.94$), we find that neither a simple single starburst nor a more complex star formation can simultaneously reproduce both constraints. We then demonstrate that a best-fit model constrained by both the B/R ratio and SED requires a star-formation rate that has varied quite dramatically over the past $\sim$50 Myr, for which the total stellar mass formed over this time is a factor of 10 (with $2-3σ$ significance) different from the best-fit models to the SED alone. Our work shows that the transient $B/R$ ratio can be used as an additional powerful constraint on the recent star formation history of higher-redshift galaxies in future works that are strongly lensed by galaxy clusters.

astro-ph.CO↗

arXiv2025

Constraining the z $\sim$ 1 Initial Mass Function with {\it HST} and {\it JWST} Lensed Stars in MACS J0416.1-2403

Our understanding of galaxy properties and evolution is contingent on knowing the initial mass function (IMF), and yet to date, the IMF is constrained only to local galaxies. Individual stars are now becoming routinely detected at cosmological distances, where luminous stars such as supergiants in background galaxies strongly lensed by galaxy clusters are temporarily further magnified by huge factors (up to $10^{4}$) by intracluster stars, thus being detected as transients. The detection rate of these events depends on the abundance of luminous stars in the background galaxy and is thus sensitive to the IMF and the star-formation history (SFH), especially for the blue supergiants detected as transients in the rest-frame ultraviolet/optical filters. As a proof of concept, we use simple SFH and IMF models constrained by spectral energy distributions (SEDs) to see how well we can predict the {\it HST} and {\it JWST} transient detection rate in a lensed arc dubbed ``Spock'' ($z = 1.0054$). We find that demanding a simultaneous fit of the SED and the transient detection rate places constraints on the IMF, independent of the assumed simple SFH model. We conclude our likelihood analysis indicates that the data definitively prefers the ``Spock'' galaxy to have a Salpeter IMF ($α= 2.35$) rather than a Top-heavy IMF ($α= 1$) -- which is thought to be the case in the early universe -- with no clear excess of supergiants above the standard IMF.

astro-ph.CO↗

arXiv2024

Tracing Outflows from Stellar Feedback in the Early Universe with Lyman-$α$

Blind spectroscopy of massive lensing galaxy clusters with MUSE has revealed large numbers of gravitationally-lensed Lyman-$ α$ emitters exhibiting asymmetric profiles at $ 2.9 \leq z \leq 6.7 $, suggesting abundant outflows from low-mass star-forming galaxies in the early universe. Are these primaeval galaxies experiencing their first bursts of star formation, or established galaxies experiencing rejuvenation? With JWST rest-frame optical/NIR continuum imaging now available for many of these objects, we can search for older stellar populations. Here, we search for spectroscopic confirmation of outflows from these galaxies, finding a few high-signal-to-noise cases in which blueshifted interstellar absorption lines are detected. Next, we analyse the star formation histories with combined HST + JWST photometry. We find most them to be well characterised by very young, low metallicity stellar populations. However, despite the rest-frame optical/NIR coverage of JWST, we cannot place strict upper bounds on the mass in old stars (age $ > 100\,\text{Myr} $).

astro-ph.GA↗

arXiv2024

On the Evidence for Molecular Outflows in High-redshift Dusty Star-forming Galaxies

Galactic-scale outflows of molecular gas from star-forming galaxies constitute the most direct evidence for regulation of star formation. In the early universe ($ z > 4 $), such outflows have recently been inferred from gravitationally-lensed dusty star-forming galaxies (DSFGs) based on ubiquitous detections of OH absorption extending to more blueshifted velocities than [CII] or CO emission in spatially-integrated spectra. Because these lines are redshifted to sub-mm wavelengths, such measurements require careful corrections for atmospheric absorption lines, and a proper accounting of sometimes large variations in measurement uncertainties over these lines. Taking these factors into consideration, we re-analyze OH and [CII] data taken with ALMA for the five sources where such data is available, of which four were categorised as exhibiting outflows. Based on their spatially-integrated spectra alone, we find statistically significant ($ \geq 3 σ$) OH absorption more blueshifted than [CII] emission in only one source. By contrast, searching channel maps for signals diluted below the detection threshold in spatially-integrated spectra, we find evidence for a separate kinematic component in OH absorption in all five sources in the form of: (i) more blueshifted OH absorption than [CII] emission and/or (ii) a component in OH absorption exhibiting a different spatio-kinematic pattern than [CII] emission, the latter presumably tracing gas in a rotating disc. Providing a more complete and accurate assessment of molecular outflows in gravitationally-lensed DSFGs, we suggest methods to better assess the precision of corrections for atmospheric absorption and to more accurately measure the source continuum in future observations.

astro-ph.GA↗

arXiv2023

A Lens Finder Map to check claimed High-z Galaxies behind SMACS J0723.3-7327

The first science image released by the JWST reveals numerous galaxies in the distant background of the galaxy cluster SMACS J0723.3-7327. Some have claimed redshifts of up to $z \simeq 20$, challenging standard cosmological models for structure formation. Here, we present a lens model for SMACS J0723.3-7327 anchored on five spectroscopically-confirmed systems at $1.38 \leq z \leq 2.21$ that are multiply lensed, along with twelve other systems with proposed image counterparts sharing common colours, spectral energy distributions, and morphological features, but having unknown redshifts. Constrained only by their image positions and, where available, redshifts, our lens model correctly reproduces the positions and correctly predicts the morphologies and relative brightnesses of all these image counterparts, as well as providing geometrically-determined redshifts spanning $1.4 \lesssim z \lesssim 6.7$ for the twelve candidate multiply-lensed galaxies lacking spectroscopic measurements. From this lens model, we create a lens finder map that defines regions over which galaxies beyond a certain redshift are predicted to be multiply lensed. Applying this map to three galaxies claimed to be at $10 \lesssim z \lesssim 20$, we find no image counterparts at locations (with an uncertainty of $\sim$$0.^{\prime\prime}5$) where they ought to be sufficiently magnified to be detectable - suggesting instead that these galaxies lie at $z \lesssim 1.7-3.2$. In lieu of spectroscopy, the creation of reliable lens finder maps for cluster fields are urgently needed to test and constrain redshifts inferred from photometry for a rapidly increasing number of candidate high-$z$ galaxies found with the JWST.

astro-ph.GA↗