arXiv subjects

C. Robertson

Publications and source records attributed to C. Robertson.

8 recordsLinked to original sources

arXiv2026

Overlap Zoo Beta: A Catalogue of ~800 Occulting Pairs in the DESI Legacy Survey using Citizen Science

Overlapping galaxies, in which a foreground galaxy partially overlaps a background galaxy, offer a unique opportunity to measure dust attenuation, a key nuisance parameter in galaxy studies, empirically and in great detail by modelling the light of both the foreground and background galaxy and inferring the missing light in the overlapping region. However, the current catalogue of overlapping pairs is relatively limited in number compared to catalogues dedicated to individual galaxies. Expanding this catalogue is not only a necessity to facilitate further detailed dust studies beyond the few limited studies conducted thus far, but also to improve pair-to-pair variance and support automated identification through machine learning techniques. To achieve this, we utilise galaxies classified as "overlapping" from Galaxy Zoo DECaLS (GZD-1, -2, and -5), along with images from Data Release 10 (DR10) of the DESI Legacy Survey, in our individual citizen science project to classify these pairs directly using volunteers. This new catalogue will not only provide a wealth of targets for future dust studies but will also contribute to a deeper understanding of these pairs and dust as a whole.

astro-ph.GA↗

arXiv2025

Radiochromic Film Dosimetry for VHEE and UHDR: Considerations for the CLEAR Facility and Comparisons with Alanine, RPL and Dosimetry Phantoms

Radiochromic films (RCFs) offer valuable two-dimensional dosimetry capabilities with a conceptually simple operating principle that requires minimal investment, making them particularly suitable for very high energy electron (VHEE) FLASH dosimetry where dosimetry standards are currently lacking. However, achieving high-accuracy measurements with RCFs presents significant practical challenges. Without the standardised protocols that ensure reliable outcomes across applications and facilities, knowledge of RCF behaviour becomes essential. This paper identifies common sources of error in RCF preparation, scanning, and processing while proposing specific mitigation strategies to improve accuracy and efficiency. Using our optimised RCF protocol at the CLEAR facility, we demonstrate relative agreement within 5% compared to alanine dosimeters when measuring Gaussian VHEE beams, establishing a foundation for reliable dosimetry in these advanced radiotherapy applications.

physics.med-ph↗

arXiv2025

Galaxy And Mass Assembly: A new approach to quantifying dust in galaxies

We introduce a new approach to quantifying dust in galaxies by combining information from the Balmer decrement (BD) and the dust mass ($M_d$). While there is no explicit correlation between these two properties, they jointly probe different aspects of the dust present in galaxies. We explore two new parameters that link BD with $M_d$ by using star formation rate sensitive luminosities at several wavelengths (ultraviolet, H$\alpha$, and far-infrared). This analysis shows that combining the BD and $M_d$ in these ways provides new metrics that are sensitive to the degree of optically thick dust affecting the short wavelength emission. We show how these new ''dust geometry'' parameters vary as a function of galaxy mass, star formation rate, and specific star formation rate. We demonstrate that they are sensitive probes of the dust geometry in galaxies, and that they support the ''maximal foreground screen'' model for dust in starburst galaxies.

astro-ph.GA↗

arXiv2024

Physical properties of strong 1 < z < 3 Balmer and Paschen lines emitters observed with JWST

The ultraviolet continuum traces young stars while the near-infrared unveils older stellar populations and dust-obscured regions. Balmer emission lines provide insights on gas properties and young stellar objects but are highly affected by dust attenuation. The near-infrared Paschen lines suffer less dust attenuation and can be used to measure star formation rates (SFRs) in star-forming regions obscured by dust clouds. We select 13 sources between redshifts 1 and 3 observed with HST, JWST/NIRCam and NIRSpec based on the availability of at least one Balmer and one Paschen line with S/N > 5. With a newly-developed version of CIGALE, we fit their hydrogen line equivalent widths (EWs) and photometric data. We assess the impacts of the removal of spectroscopic data by comparing the quality of the fits of the spectro-photometric data to those with photometric data only. We compare the single (BC03) vs binary (BPASS) stellar populations models in the fitting process of spectro-photometric data. We derive the differential attenuation and explore different attenuation recipes by fitting spectro-photometric data with BC03. For each stellar model and for each input dataset (with and without EWs), we quantify the deviation on the SFRs and stellar masses from the "standard" choice. On average, the SFRs are overestimated and the stellar masses are underestimated when EWs are not included as input data. We find a major contribution of the H${\alpha}$ emission line to the broadband photometric measurements of our sources, and a trend of increasing contribution with specific SFR. Using the BPASS models has a significant impact on the derived SFRs and stellar masses. We show that a flexible attenuation recipe provides more accurate estimates of the dust attenuation parameters, especially the differential attenuation which agrees with the original value of Charlot & Fall (2000).

astro-ph.GA↗

arXiv2024

Ground- and Space-Based Dust Observations of VV 191 Overlapping Galaxy Pair

The Balmer decrement (H$\alpha$/H$\beta$) provides a constraint on attenuation, the cumulative effects of dust grains in the ISM. The ratio is a reliable spectroscopic tool for deriving the dust properties of galaxies that determine many different quantities such as star formation rate, metallicity, and SED models. Here we measure independently both the attenuation and H$\alpha$/H$\beta$ of an occulting galaxy pair: VV 191. Attenuation measurements in the visible spectrum (A$_{V,stars}$) from dust maps derived from the F606W filter of HST and the F090W filter of JWST are matched with spaxel-by-spaxel H$\alpha$/H$\beta$ observations from the George and Cynthia Mitchell Spectrograph (GCMS) of the McDonald Observatory. The 0.5 to 0.7 micron bandpass covers the Balmer lines for VV 191. The dust maps of JWST and HST provide the high sensitivity necessary for comparisons and tracking trends of the geometrically favorable galaxy. We present maps and plots of the Balmer lines for the VV 191 galaxy pair and for a specific region highlighting dust lanes for VV 191b in the overlap region. We compute A$_{V, HII}$ from H$\alpha$/H$\beta$ and plot both quantities against A$_{V, stars}$. Our results show that regions with higher dust content, residing closer to the spiral center, dominate ionized gas attenuation, leading to an overestimation of A$_{V, HII}$ by a factor or 2. Further out in the spiral arms, the lower dust content leads to more agreement between the attenuations, indicating lower SFR and larger contribution from older stars to the stellar continuum outside the Petrosian radius.

astro-ph.GA↗

arXiv2023

Galaxy And Mass Assembly: The xSAGA Galaxy Complement in Nearby Galaxy Groups

Groups of galaxies are the intermediate density environment in which much of the evolution of galaxies is thought to take place. In spectroscopic redshift surveys, one can identify these as close spatial redshift associations. However, spectroscopic surveys will always be more limited in luminosity and completeness than imaging ones. Here we combine the Galaxy And Mass Assembly group catalogue with the extended Satellites Around Galactic Analogues (xSAGA) catalogue of Machine Learning identified low-redshift satellite galaxies. We find 1825 xSAGA galaxies within the bounds of the GAMA equatorial fields (m < 21), 1562 of which could have a counterpart in the GAMA spectroscopic catalogue (m < 19.8). Of these, 1326 do have a GAMA counterpart with 974 below z=0.03 (true positives) and 352 above (false positives). By crosscorrelating the GAMA group catalogue with the xSAGA catalogue, we can extend and characterize the satellite content of GAMA galaxy groups. We find that most groups have <5 xSAGA galaxies associated with them but richer groups may have more. Each additional xSAGA galaxy contributes only a small fraction of the group's total stellar mass (<<10%). Selecting GAMA groups that resemble the Milky Way halo, with a few (<4) bright galaxies, we find xSAGA can add a magnitude fainter sources to a group and that the Local Group does not stand out in the number of bright satellites. We explore the quiescent fraction of xSAGA galaxies in GAMA groups and find a good agreement with the literature.

astro-ph.GA↗

arXiv2014

Defect fusing rules in affine Toda field theory

The concept of fusing rules for defects in a^(1)_r affine Toda field theories is introduced in the classical and quantum contexts. This idea is employed in finding a new transmission matrix for the a^(1)_3 theory which obeys the triangle relations and the crossing-unitarity relations.

hep-th↗

arXiv2014

Folding defect affine Toda field theories

A folding process is applied to fused a^(1)_r defects to construct defects for the non-simply laced affine Toda field theories of c^(1)_n, d^(2)_n and a^(2)_2n at the classical level. Support for the hypothesis that these defects are integrable in the folded theories is provided by the observation that transmitted solitons retain their form. Further support is given by the demonstration that energy and momentum are conserved.

hep-th↗