arXiv subjects

Rhys Taylor

Publications and source records attributed to Rhys Taylor.

At least 19 recordsLinked to original sources

arXiv2026

A machine learning approach to estimating HI deficiency in galaxies

Measurements of the HI content of galaxies serve as an important tracer for probing the impact of environment on galaxy evolution. More specifically, the HI deficiency (defined as the difference between expected unaltered and observed HI content of a galaxy) is closely related with environmental effects, which are most significant in large groups and clusters. In this work, we aim to estimate the HI deficiency of ALFALFA galaxies and investigate its relation with galactic environment. Using a random forest machine learning algorithm, we developed a predictive model capable of estimating the original HI content of a galaxy based solely on its optical properties. The model was trained on a subsample of 6 982 isolated ALFALFA galaxies with optical photometric data from the Sloan Digital Sky Survey (SDSS). Our predictive model outperforms the traditional approach, in which HI mass is linearly related to optical size (both on a logarithmic scale). The model achieves RMSE $\approx$ 0.22 dex and $R^2 \approx$ 0.80, compared with RMSE $\approx$ 0.26 dex and $R^2 \approx$ 0.70 for the traditional method. We applied this model to predict the expected HI content for non-isolated ALFALFA galaxies, enabling the calculation of HI deficiency. Controlling for the effects of internal factors, like stellar mass and presence of AGN, we find an increase in binned median HI deficiency of 0.15 dex attributable to environmental effects. In addition, we evaluate the temporal evolution of the predicted HI mass, and associated HI deficiency, due to the evolving stellar populations, following a gas removal event.

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arXiv2026

Declarative bespoke modelling: A new approach

Modern numerical models are increasingly complex, opaque, and computationally expensive, yet frequently fail to predict even qualitative features of observed phenomena. We propose a new paradigm, Declarative Bespoke Modelling, in which the modeller explicitly declares the relationship between model inputs and outputs. We demonstrate that this approach achieves perfect predictive accuracy, unconditional numerical stability, and complete interpretability. It represents a natural endpoint of contemporary modelling practice and near-zero CO2 emission.

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arXiv2026

High resolution observations of 'dark' neutral hydrogen clouds in the Virgo cluster with the Very Large Array

We have observed six `dark' neutral hydrogen (HI) clouds discovered in the Virgo cluster by the Arecibo Galaxy Environment Survey (AGES) with the Karl G. Jansky Very Large Array (VLA), giving higher angular and velocity resolution than the original AGES observations. We detected compact HI emission in two of the sources, AGESVC1 231 and AGESVC1 274, allowing us to firmly associate them with faint ($m_g > 18.5$), blue ($g-i < 0.1$) optical counterparts with high $M_{HI}/L_g$ ratios. In a further two sources, we detected low column-density extended HI emission, consistent with these being dispersing clouds from ram-pressure stripping or tidal interactions. The final two sources were not detected with the VLA, allowing us to set low column-density limits on the HI detected by AGES that are consistent with these clouds also being formed from HI that is dispersing into the intra-cluster medium. The four HI sources not associated with optical counterparts thus appear likely to be relatively short-lived objects. No evidence was found for either pressure-supported turbulent spheres or stable dark galaxies.

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arXiv2026

Ultra Diffuse Galaxies in clusters : the peculiar gas loss of VCC 1964

Context : Ultra Diffuse Galaxies are low surface brightness systems which have been detected in HI in the field, where their line widths sometimes indicate significant dark matter deficits. They are rarely detected in HI in clusters, making their dynamical properties difficult to assess. The relation between field and cluster populations is unclear. Aims : Detecting UDGs entering a cluster could give important clues to their evolution, both in terms of their dynamics but also as to whether they are structurally similar - i.e. if cluster UDGs are generally the same as field UDGs except with less gas and an older stellar population. Methods : We use data from two deep Arecibo surveys, the Arecibo Galaxy Environment Survey and the Widefield Arecibo Virgo Environment Survey, to measure the gas content of the UDG-candidate VCC 1964. Optical properties are quantified from the Sloan Digital Sky Survey and DESI Legacy Surveys. Results : We find a significant 9 kpc offset between the HI and optical components of VCC 1964, no evidence of asymmetry in the HI, and only a modest deficiency level. This suggests a wholesale displacement of the gas content. The line width is 4-5 sigma deviant from the baryonic and over 6 sigma deviant from the optical forms of the Tully-Fisher relation. The optical component is blue and smooth. Conclusions : VCC 1964 is consistent with a UDG experiencing gas displacement due to ram pressure as it enters the cluster for the first time. Intriguingly, its dynamics imply a significant dark matter deficit, however we cannot rule out that this may be due to the gas being displaced out of equilibrium.

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arXiv2025

On the completeness and reliability of visual source extraction : an examination of eight thousand data cubes by eye

[Edited for arXiv] Source extraction in HI radio surveys is still often performed using visual inspection, but the efficacy of such procedures lacks rigorous quantitative assessment due to their laborious nature. Algorithmic methods are often preferred due to their repeatable results and speed. I here quantitatively assess visual source extraction using a large sample of artificial sources and a comparatively rapid source extraction tool, and compare the results with those from automatic techniques. I injected 4,232 sources into a total of 8,500 emission-free data cubes, with at most one source per cube. Sources covered a wide range of signal-to-noise and velocity widths. I blindly searched all cubes, measuring the completeness and reliability for pairs of signal-to-noise and line width values. Smaller control tests were performed to account for the possible biases in the search, which gave results in good agreement with the main experiment. I also searched cubes injected with artificial sources using algorithmic extractors, and compare these results with a set of catalogues independently reported from real observational data. I find that the results of visual extraction follow a tight relation between integrated signal-to-noise and completeness. Visual extraction compares favourably in efficacy with the algorithmic methods, tending to recover a higher fraction of fainter sources. Visual source extraction can be a surprisingly rapid procedure which gives higher completeness levels than automatic techniques, giving predictable, quantifiable results which are not strongly subject to the whims of the observer. For recovering the faintest features, algorithmic extractors can be competitive with visual inspection but cannot yet out-perform it, though their advantage in speed can be a significant compensating factor.

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arXiv2025

FRELLED Reloaded: Multiple techniques for astronomical data visualisation in Blender

I present version 5.0 of FRELLED, the FITS Realtime Explorer of Low Latency in Every Dimension. This is a 3D data visualisation package for the popular Blender art software, designed to allow inspection of astronomical volumetric data sets (primarily, but not exclusively, radio wavelength data cubes) in real time using a variety of visualisation techniques. The suite of Python scripts that comprise FRELLED have been almost completely recoded and many new ones added, bringing FRELLED's operating environment from Blender version 2.49 to 2.79. Principle new features include: an enormously simplified installation procedure, a more modular graphical appearance that takes advantage of Blender 2.79's improved interface, much faster loading of FITS data, support for larger data sets, options to show the data as height maps in 2D mode or isosurfaces in 3D mode, utilisation of standard astropy and other Python modules to support a greater range of FITS files (with a particular emphasis on higher-frequency radio data such as from ALMA, the Atacama Large Millimetre Array), and the capability of exporting the data to Blender 2.9+ which supports stereoscopic 3D displays in virtual reality headsets. In addition, in-built help files are accessible from each menu panel, as well as direct links to a complete wiki and set of video tutorials. Finally, the code itself is much more modular, allowing easier maintainability and, over the longer term, a far easier prospect of migrating to more recent versions of Blender.

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arXiv2022

The Arecibo Galaxy Environment Survey XII : Optically dark HI clouds in the Leo I Group

Using data from the Arecibo Galaxy Environment Survey, we report the discovery of five HI clouds in the Leo I group without detected optical counterparts. Three of the clouds are found midway between M96 and M95, one is only 10$^{\prime}$ from the south-east side of the well-known Leo Ring, and the fifth is relatively isolated. HI masses range from 2.6$\times$10$^{6}$ - 9.0$\times$10$^{6}$M$_{\odot}$ and velocity widths (W50) from 16 - 42 km/s. Although a tidal origin is the most obvious explanation, this formation mechanism faces several challenges. For the most isolated cloud, the difficulties are its distance from neighbouring galaxies and the lack of any signs of disturbance in the HI discs of those systems. Some of the clouds also appear to follow the baryonic Tully-Fisher relation between mass and velocity width for normal, stable galaxies which is not expected if they are tidal in origin. Three clouds are found between M96 and M95 which have no optical counterparts, but have otherwise similar properties and location to the optically detected galaxy LeG 13. While overall we favour a tidal debris scenario to explain the clouds, we cannot rule out a primordial origin. If the clouds were produced in the same event that gave rise to the Leo Ring, they may provide important constraints on any model attempting to explain that structure

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arXiv2022

The Arecibo Galaxy Environment Survey (AGES) XI: the expanded Abell 1367 field. Data catalogue and HI census over the surveyed volume

Many galaxy properties are known to correlate with the environment in which the galaxies are embedded. Their cold, neutral gas content, usually assessed through 21cm HI observations, is related to many other galaxy properties as it is the underlying fuel for star formation. With its high sensitivity and broad sky coverage the blind Arecibo Galaxy Environment Survey (AGES) survey brings significant improvement to the census of HI properties of galaxies in a wide range of environments, from voids to the core of a massive cluster. Here we present an HI census over a volume of ~44000 Mpc$^{3}$ towards the merging cluster Abell 1367 and the large-scale structure (LSS) surrounding the cluster out to cz = 20000 km/s. The survey is sensitive down to a column density of N$_{HI}$ = 1.5 x 10$^{17}$ cm$^{-2}$ for emission filling the beam and a line width of 10 km/s. As an approximate mass sensitivity limit, a member of A1367 (at a distance of 92 Mpc), containing M$_{HI}$ = 2.7x10$^{8}$ M$_{\odot}$ distributed over a top-hat profile of 50 km/s width would be detected at 4$\sigma$. The results are analysed in combination with optical spectroscopy data from SDSS which we use to estimate the local galaxy density based on the Voronoi-Delaunay method. In total we detect 457 HI sources, 213 of which are detected for the first time by the AGES survey, 134 of which are presented in this article for the first time. 225 of the detections are in the cluster and 232 in the remaining volume surveyed. Here we present the full catalogue of HI detections and their basic properties, including optical ones. We concentrate on the difference between the cluster and the foreground and background LSS, revealing a continuous correlation of HI detected fraction (and HI deficiency) with local galaxy density, independent of global environment.

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arXiv2022

Environmental effects in Herschel observations of the ionized carbon content of star forming dwarf galaxies in the Virgo cluster

We use archival Herschel data to examine the singly ionized carbon ([CII]) content of 14 star forming dwarf galaxies in the Virgo cluster. We use spectral energy distribution (SED) fits to far infrared, mid infrared, near infrared, optical and ultraviolet data to derive the total infrared continuum (TIR) for these galaxies. We compare the [CII]/TIR ratio for dwarf galaxies in the central part of Virgo to those in the southern part of the cluster and to galaxies with similar TIR luminosities and metallicities in the Herschel Dwarf Galaxy Survey (DGS) sample of field dwarf galaxies to look for signs of [CII] formation independent of star formation. Our analysis indicates that the sample of Virgo dwarfs in the central part of the cluster has significantly higher values of [CII]/TIR than the sample from the southern part of the cluster and the sample from the DGS, while the southern sample is consistent with the DGS. This [CII]/TIR excess implies that a significant fraction of the [CII] in the dwarf galaxies in the cluster center has an origin other than star formation and is likely to be due to environmental processes in the central part of the cluster. We also find a surprisingly strong correlation between [CII]/TIR and the local ram pressure felt by the dwarf galaxies in the cluster. In this respect, we claim that the excess [CII] we see in these galaxies is likely to be due to formation in ram pressure shocks.

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arXiv2021

The Widefield Arecibo Virgo Extragalactic Survey: Early Results on Known Dark Sources

The Widefield Arecibo Virgo Extragalactic Survey (WAVES) was an ongoing HI survey of the Virgo Cluster with the Arecibo Observatory's 305m William E. Gordon Telescope at the time of its structural failure. The full 20 square degrees of the southern field and 10 of the planned 35 square degrees of the northern field had been observed to full depth, adding to 25 square degrees observed to the same depth in the cluster by the Arecibo Galaxy Environment Survey. We here review what WAVES reveals about four optically dark HI structures that were previously discovered in the survey area, including two that are not seen despite being well above our detection limit.

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arXiv2020

Deep optical imaging of the dark galaxy candidate AGESVC1 282

The blind HI survey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown. They might be crucial objects since they could be the so-called dark galaxies, that is, the dark matter halos without stellar content that are expected from cosmological simulations. In order to reveal the nature of the dark clouds, we took a deep optical image of one them, AGESVC1 282, with the newly-commissioned 1.4m Milankovi\'c Telescope. After observing it for 10.4h in the $L$-filter, the image reached a surface-brightness limit of about 29.1 mag arcsec$^{-2}$ in $V$. No optical counterpart was detected. We placed an upper limit on the $V$-band luminosity of the object of $1.1\times10^7\,L_\odot$, giving a stellar mass below $1.4\times10^7\,M_\odot$ and a HI-to-stellar mass ratio above 3.1. By inspecting archival HI observations of the surrounding region, we found that none of the standard explanations for optically dark HI clouds fits the available constraints on this object.

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arXiv2020

Mapping the working of environmental effects in A963

We qualitatively assess and map the relative contribution of pre-processing and cluster related processes to the build-up of A963, a massive cluster at z=0.2 showing an unusually high fraction of star forming galaxies in its interior. We use Voronoi binning of positions of cluster members on the plane of the sky in order to map the 2D variations of galaxy properties in the centre and infall region of A963. We map four galaxy parameters (fraction of star forming galaxies, specific star formation rate, HI deficiency and age of the stellar population) based on full SED fitting, 21cm imaging and optical spectroscopy. We find an extended region dominated by passive galaxies along a north-south axis crossing the cluster centre, possibly associated with known filaments of the large-scale structure. There are signs that the passive galaxies in this region were quenched long before their arrival in the vicinity of the cluster. Contrary to that, to the east and west of the cluster centre lie regions of recent accretion dominated by gas rich, actively star forming galaxies not associated with any substructure or filament. The few passive galaxies in this region appear to be recently quenched, and some gas rich galaxies show signs of ongoing ram-pressure stripping. We report the first tentative observations at 21cm of ongoing ram-pressure stripping at z=0.2, as well as observed inflow of low-entropy gas into the cluster along filaments of the large-scale structure. The observed galaxy content of A963 is a result of strongly anisotropic accretion of galaxies with different properties. Gas rich, star forming galaxies are being accreted from the east and west of the cluster and these galaxies are being quenched at r<R200, while the bulk of the accretion, containing multiple groups, happens along the north-south axis and brings mostly passive galaxies.

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arXiv2020

Faint and fading tails : the fate of stripped HI gas in Virgo cluster galaxies

Although many galaxies in the Virgo cluster are known to have lost significant amounts of HI gas, only about a dozen features are known where the HI extends significantly outside its parent galaxy. Previous numerical simulations have predicted that HI removed by ram pressure stripping should have column densities far in excess of the sensitivity limits of observational surveys. We construct a simple model to try and quantify how many streams we might expect to detect. This accounts for the expected random orientation of the streams in position and velocity space as well as the expected stream length and mass of stripped HI. Using archival data from the Arecibo Galaxy Environment Survey, we search for any streams which might previously have been missed in earlier analyses. We report the confident detection of ten streams as well as sixteen other less sure detections. We show that these well-match our analytic predictions for which galaxies should be actively losing gas, however the mass of the streams is typically far below the amount of missing HI in their parent galaxies, implying that a phase change and/or dispersal renders the gas undetectable. By estimating the orbital timescales we estimate that dissolution rates of 1-10 Msolar/yr are able to explain both the presence of a few long, massive streams and the greater number of shorter, less massive features.

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arXiv2019

The Widefield Arecibo Virgo Extragalactic Survey I: New structures in the ALFALFA Virgo 7 cloud complex and an extended tail on NGC 4522

We are carrying out a sensitive blind survey for neutral hydrogen (HI) in the Virgo cluster and report here on the first 5\deg\ x 1\deg\ area covered, which includes two optically-dark gas features: the five-cloud ALFALFA Virgo 7 complex (Kent et al. 2007, 2009) and the stripped tail of NGC 4522 (Kenney et al. 2004). We discover a sixth cloud and low velocity gas that extends the velocity range of the complex to over 450 km/s, find that around half of the total HI flux comes from extended emission rather than compact clouds, and see around 150 percent more gas, raising the total HI mass from 5.1 x 10$^8$ M$_\odot$ to 1.3 x 10$^9$ M$_\odot$. This makes the identification of NGC 4445 and NGC 4424 by Kent et al. (2009) as possible progenitors of the complex less likely, as it would require an unusually high fraction of the gas removed to have been preserved in the complex. We also identify a new component to the gas tail of NGC 4522 extending to ~200 km/s below the velocity range of the gas in the galaxy, pointing towards the eastern end of the complex. We consider the possibility that NGC 4522 may be the parent galaxy of the complex, but the large velocity separation (~1800 km/s) leads us to rule this out. We conclude that, in the absence of any better candidate, NGC 4445 remains the most likely parent galaxy, although this requires it to have been particularly gas-rich prior to the event that removed its gas into the complex.

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arXiv2018

Simulating the evolution of optically dark HI clouds in the Virgo cluster : will no-one rid me of this turbulent sphere ?

Most detected neutral atomic hydrogen (HI) at low redshift is associated with optically bright galaxies. However, a handful of HI clouds are known which appear to be optically dark and have no nearby potential progenitor galaxies, making tidal debris an unlikely explanation. In particular, 6 clouds identified by the Arecibo Galaxy Environment Survey are interesting due to the combination of their small size, isolation, and especially their broad line widths atypical of other such clouds. A recent suggestion is that these clouds exist in pressure equilibrium with the intracluster medium, with the line width arising from turbulent internal motions. Here we explore that possibility by using the FLASH code to perform a series of 3D hydro simulations. Our clouds are modelled using spherical Gaussian density profiles, embedded in a hot, low-density gas representing the intracluster medium. The simulations account for heating and cooling of the gas, and we vary the structure and strength of their internal motions. We create synthetic HI spectra, and find that none of our simulations reproduce the observed cloud parameters for longer than about 100 Myr : the clouds either collapse, disperse, or experience rapid heating which would cause ionisation and render them undetectable to HI surveys. While the turbulent motions required to explain the high line widths generate structures which appear to be inherently unstable, making this an unlikely explanation for the observed clouds, these simulations demonstrate the importance of including the intracluster medium in any model seeking to explain the existence of these objects.

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arXiv2016

Visualising three-dimensional volumetric data with an arbitrary coordinate system

Astronomical data does not always use Cartesian coordinates. Both all-sky observational data and simulations of rotationally symmetric systems, such as accretion and protoplanetary discs, may use spherical polar or other coordinate systems. Standard displays rely on Cartesian coordinates, but converting non-Cartesian data into Cartesian format causes distortion of the data and loss of detail. I here demonstrate a method using standard techniques from computer graphics that avoids these problems with 3D data in arbitrary coordinate systems. The method adds minimum computational cost to the display process and is suitable for both realtime, interactive content and producing fixed rendered images and videos. Proof-of-concept code is provided which works for data in spherical polar coordinates.

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arXiv2016

The Structure of Halo Gas Around M33

Understanding the distribution of gas in and around galaxies is vital for our interpretation of galaxy formation and evolution. As part of the Arecibo Galaxy Environment Survey (AGES) we have observed the neutral hydrogen (HI) gas in and around the nearby Local Group galaxy M33 to a greater depth than previous observations. As part of this project we investigated the absence of optically detected dwarf galaxies in its neighbourhood, which is contrary to predictions of galaxy formation models. We observed 22 discrete clouds, 11 of which were previously undetected and none of which have optically detected counterparts. We find one particularly interesting hydrogen cloud, which has many similar characteristics to hydrogen distributed in the disk of a galaxy. This cloud, if it is at the distance of M33, has a HI mass of around 10^7 Msun and a diameter of 18 kpc, making it larger in size than M33 itself.

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arXiv2015

The Arecibo Galaxy Environment Survey X: The Structure of Halo Gas Around M33

As part of the HI Arecibo Galaxy Environments Survey (AGES) we have observed 5$\times$4 degrees of sky centred on M33, reaching a limiting column density of $\sim 1.5 \times 10^{17}$ cm$^{-2}$ (line width of 10 km s$^{-1}$ and resolution 3.5\arcmin). We particularly investigate the absence of optically detected dwarf galaxies around M33, something that is contrary to galaxy formation models. We identify 22 discrete HI clouds, 11 of which are new detections. The number of objects detected and their internal velocity dispersion distribution is consistent with expectations from standard galaxy formation models. However, the issue remains open as to whether the observed velocity dispersions can be used as a measure of the HI clouds total mass i.e. are the velocities indicative of virialised structures or have they been influenced by tidal interactions with other structures in the Local Group? We identify one particularly interesting HI cloud, AGESM33-31, that has many of the characteristics of HI distributed in the disc of a galaxy, yet there is no known optical counterpart associated with it. This object has a total HI mass of $1.22 \times 10^{7}$ M$_{\odot}$ and a diameter of 18 kpc if at the distance of M33 ($D_{M33}=840$ kpc). However, we also find that there are numerous other HI clouds in this region of sky that have very similar velocities and so it is plausible that all these clouds are actually associated with debris from the Magellanic stream.

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