arXiv subjects

Clayton Robertson

Publications and source records attributed to Clayton Robertson.

8 recordsLinked to original sources

arXiv2025

Investigating the need for a robust ultraviolet filter set aboard the Habitable Worlds Observatory

High resolution, ultraviolet imaging is often unavailable across the sky, even in heavily studied fields such as the Chandra Deep Field - South. The Habitable Worlds Observatory is one of two upcoming missions with the possibility of significant UV capabilities, and the only one early enough in development to consider suggestions to its design. In this paper, we conduct an initial study of how current common UV filter sets affect the results of spectral energy distribution fitting for the estimation of galaxy parameter. This initial look is intended to motivate the need for future, more robust, SED fitting of mock galaxies. We compare the broad near UV and far UV filters used by the GALEX mission to the three more narrow Swift UVOT filters. We find that the GALEX filters result in larger errors when calculating the UV beta parameter compared to UVOT, and provide little constraint on the star formation age of a galaxy. We further note the ability of the UVOT filters to investigate the 2175Å attenuation bump; GALEX has a reduced capacity to trace this same feature. Ultimately, we recommend that in order to optimize the effectiveness of HWO's ultraviolet capacity for transformative astrophysics, a minimum of a FUV filter with three medium band NUV filters should be adopted. This will combine the power of GALEX's wavelength range with the finer sampling of UVOT around an important dust feature.

astro-ph.GA↗

arXiv2025

Dissecting Reionisation with the Cosmic Star Formation and Active Galactic Nuclei Luminosity History

The combination of the $z=0-13.5$ cosmic star formation history and active galactic nuclei (AGN) luminosity history as inferred by the James Webb Space Telescope is connected to the cosmic spectral energy distribution (CSED) to explore the sources of reionisation. We compute the redshift evolution of the corresponding cosmic ionising photon emissivity, the neutral fraction and the cosmic microwave background optical depth. We use the generative SED modelling code ProSpect to bracket the ionising emissivity between escape fractions of $f_{\mathrm{esc}} = 1 - 100\%$ for both the stars and AGN. Stars alone could have achieved reionisation by $z\approx 6$ with $f_{\mathrm{esc}} \gtrsim 30\%$ for solar metallicity ($Z=0.02$) stars or $f_{\mathrm{esc}} \gtrsim 10\%$ for metal-poor ($Z=10^{-4}$) stars. On the other hand, AGN by themselves would have struggled to produce sufficiently many ionising photons even with $f_{\mathrm{esc}} = 100\%$. A hybrid model containing both stars and AGN is explored where we find best fit (median$\pm 1\sigma$) $f_{\mathrm{esc}}=$ $12\%$ ($14^{+9}_{-7}\%$) for the stars and $f_{\mathrm{esc}}=$ $63\%$ ($60^{+28}_{-32}\%$) for the AGN, maintained at all redshifts. In essence, the joint growth of stellar mass and supermassive black holes produces neither more nor fewer ionising photons than needed to reionise $\gtrsim 99\%$ of the intergalactic medium by $z\approx 6$.

astro-ph.GA↗

arXiv2025

Self-Consistent JWST Census of Star Formation and AGN activity at z=5.5-13.5

The cosmic star formation history (CSFH) and cosmic active galactic nuclei (AGN) luminosity history (CAGNH) are self consistently measured at $z = 5.5-13.5$. This is achieved by analyzing galaxies detected by the James Webb Space Telescope from $\approx 400 \, \mathrm{arcmin^{2}}$ fields from the PEARLS, CEERS, NGDEEP, JADES and PRIMER surveys. In particular, the combination of spectral energy distribution fitting codes, EAZY and \textsc{ProSpect}, are employed to estimate the photometric redshifts and astrophysical quantities of $3751$ distant galaxies, from which we compute the stellar mass, star formation rate and AGN luminosity distribution functions in four redshift bins. Integrating the distribution functions, we find that the CSFH rises by $\approx 1$~dex over $z = 13.5 - 5.5$ and the CAGNH rises by $\approx 1$~dex over $z = 10.5 - 5.5$. We connect our results of the CSFH and CAGNH at $z=13.5-5.5$ to that from $z= 5-0$ to determine the summary of $\gtrsim 13$ Gyr of star formation and AGN activity, from the very onset of galaxy formation to the present day.

astro-ph.GA↗

arXiv2025

A comparison of dust content and properties in GAMA/G10-COSMOS/3D-HST and SIMBA cosmological simulations

The abundance of dust within galaxies directly influences their evolution. Contemporary models attempt to match this abundance by simulating the processes of dust creation, growth, and destruction. While these models are accurate, they require refinement, especially at earlier epochs. This study aims to compare simulated and observed datasets and identify discrepancies between the two, providing a basis for future improvements. We utilise simulation data from the SIMBA cosmological simulation suite and observed data from the Galaxy and Mass Assembly (GAMA), a subset of the Cosmic Evolution Survey (G10-COSMOS), and the Hubble Space Telescope (3D-HST). We selected galaxies in the observed and simulated data in a stellar mass range of $(10^{8.59} < M_\odot < 10^{11.5})$ and at redshift bins centering around $(z = 0.0)$, $(z = 0.1)$, $(z = 0.5)$, $(z = 1.0)$, and $(z = 1.5)$ in a homogeneous dust mass range $((10^{6} < M_D [M_\odot] < 10^{9}))$. Our results show notable deviations between SIMBA and observed data for dust-poor and rich galaxies, with strong indications that differences in galaxy populations and SIMBA limitations are the underlying cause rather than the dust physics implemented in SIMBA itself.

astro-ph.GA↗

arXiv2024

The Galaxy Zoo Catalogs for the Galaxy And Mass Assembly (GAMA) Survey

Galaxy Zoo is an online project to classify morphological features in extra-galactic imaging surveys with public voting. In this paper, we compare the classifications made for two different surveys, the Dark Energy Spectroscopic Instrument (DESI) imaging survey and a part of the Kilo-Degree Survey (KiDS), in the equatorial fields of the Galaxy And Mass Assembly (GAMA) survey. Our aim is to cross-validate and compare the classifications based on different imaging quality and depth. We find that generally the voting agrees globally but with substantial scatter i.e. substantial differences for individual galaxies. There is a notable higher voting fraction in favor of ``smooth'' galaxies in the DESI+\rev{\sc zoobot} classifications, most likely due to the difference between imaging depth. DESI imaging is shallower and slightly lower resolution than KiDS and the Galaxy Zoo images do not reveal details such as disk features \rev{and thus are missed in the {\sc zoobot} training sample}. \rev{We check against expert visual classifications and find good agreement with KiDS-based Galaxy Zoo voting.} We reproduce the results from Porter-Temple+ (2022), on the dependence of stellar mass, star-formation, and specific star-formation on the number of spiral arms. This shows that once corrected for redshift, the DESI Galaxy Zoo and KiDS Galaxy Zoo classifications agree well on population properties. The zoobot cross-validation increases confidence in its ability to compliment Galaxy Zoo classifications and its ability for transfer learning across surveys.

astro-ph.GA↗

arXiv2024

EPOCHS Paper X: Environmental effects on Galaxy Formation and Protocluster Galaxy candidates at $4.5<z<10$ from JWST observations

In this paper we describe our search for galaxy protocluster candidates at $4.5< z < 10$ and explore the environmental and physical properties of their member galaxies identified through JWST wide-field surveys within the CEERS, JADES, and PEARLS NEP-TDF fields. Combining with HST data, we identify 2948 robust $z>4.5$ candidates within an area of 185.4 arcmin$^2$. We determine nearest neighbour statistics and galaxy environments. We find that high-$z$ galaxies in overdense environments exhibit higher star formation activity compared to those in underdense regions. Galaxies in dense environments have a slightly increased SFR at a given mass compared with galaxies in the lower density environments. At the high mass end we also find a gradual flattening of the $M_{\star}$-SFR slope. We find that galaxies in high-density regions often have redder UV slopes than those in low-density regions, suggesting more dust extinction, weaker Lyman-alpha emission and / or a higher damped Lyman-alpha absorption. We also find that the mass-size relation remains consistent and statistically similar across all environments. Furthermore, we quantitatively assess the probability of a galaxy belonging to a protocluster candidate. In total, we identified 26 overdensities at $z=5-7$ and estimate their dark matter halo masses. We find that all protocluster candidates could evolve into clusters with $M_{\rm halo} > 10^{14}M_{\odot}$ at $z = 0$, thereby supporting the theoretical and simulation predictions of cluster formation. Notably, this marks an early search for protocluster candidates in JWST wide field based on photometric data, providing valuable candidates to study cosmic structure formation at the early stages.

astro-ph.GA↗

arXiv2024

JWST NIRCam Photometry: A Study of Globular Clusters Surrounding Bright Elliptical Galaxy VV 191a at z=0.0513

James Webb Space Telescope NIRCam images have revealed 154 reliable globular cluster (GC) candidates around the $z = 0.0513$ elliptical galaxy VV~191a after subtracting 34 likely interlopers from background galaxies inside our search area. NIRCam broadband observations are made at 0.9-4.5 $μ$m using the F090W, F150W, F356W, and F444W filters. Using PSF-matched photometry, the data are analyzed to present color-magnitude diagrams (CMDs) and color distributions that suggest a relatively uniform population of GCs, except for small fractions of reddest (5-8%) and bluest (2-4%) outliers. GC models in the F090W vs. (F090-F150W) diagram fit the NIRCam data well and show that the majority of GCs detected have a mass of approximately $\sim$$10^{6.5}$$M_{\odot}$, with metallicities [Fe/H] spanning the typical range expected for GCs (-2.5$\le$ [Fe/H]$\le$ 0.5). However, the models predict $\sim$0.3-0.4 mag bluer (F356W-F444W) colors than the NIRCam data for a reasonable range of GC ages, metallicities, and reddening. Although our data does not quite reach the luminosity function turnover, the measured luminosity function is consistent with previous measurements, suggesting an estimated peak at $m_{\rm AB}$$\sim$-9.4 mag, $\pm$0.2 mag in the F090W filter.

astro-ph.GA↗

arXiv2023

Cosmic Evolution Early Release Science Survey (CEERS): Multi-classing Galactic Dwarf Stars in the deep JWST/NIRCam

Low mass (sub)stellar objects represent the low end of the initial mass function, the transition to free-floating planets and a prominent interloper population in the search for high-redshift galaxies. Without proper motions or spectroscopy, can one identify these objects photometrically? JWST/NIRCam has several advantages over HST/WFC3 NIR: more filters, a greater wavelength range, and greater spatial resolution. Here, we present a catalogue of (sub)stellar dwarfs identified in the Cosmic Evolution Early Release Science Survey (CEERS). We identify 518 stellar objects down to $m_F200W \sim 28$ using half-light radius, a full three magnitudes deeper than typical HST/WFC3 images. A kNN nearest neighbour algorithm identifies and types these sources, using four HST/WFC3 and four NIRCam filters, trained on SpeX spectra of nearby brown dwarfs. The kNN with four neighbors classifies well within two subtypes: e.g M2$\pm$2 or T4$\pm$2, achieving $\sim$95% precision and recall. More granular typing results in worse metrics. In CEERS, we find 9 M8$\pm$2, 2 L6$\pm$2, 1 T4$\pm$2, and 15 T8$\pm$2. We compare the observed long wavelength NIRCam colours -- not used in the kNN -- to those expected for brown dwarf atmospheric models. The NIRCam F356W-F444W and F410M-F444W colours are redder by a magnitude for the type assigned by the kNN, hinting at a wider variety of atmospheres for these objects. We find a 300-350pc scale-height for M6$\pm$2 dwarfs plus a second structural component and a 150-200pc scale-height for T6$\pm$2 type dwarfs, consistent with literature values.

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