arXiv subjects

Ian D. Roberts

Publications and source records attributed to Ian D. Roberts.

At least 19 recordsLinked to original sources

arXiv2026

Exploring the physics of ram pressure stripping with radio continuum observations in the SKA era

Satellite galaxies in clusters are significantly more likely to be red and passive than similar mass galaxies in the field. This fact is known as the environmental quenching of galaxy star formation, which is believed to be driven by ram pressure stripping (RPS). The large velocity differences between the infalling galaxies and the intracluster medium (ICM) result in a strong ram pressure on their interstellar medium (ISM), which can strip it from the stellar disk. The stripped ISM can be studied at various wavelengths, including the radio band, thanks to the synchrotron emission produced by the magnetic fields and relativistic electrons embedded in them. This emission is typically steep-spectrum and thus best observed at low frequencies. Thus, continuum studies of the RPS effect are currently mostly carried out with LOFAR, limiting them to the northern hemisphere. SKA-Low will permit us to extend them to the southern sky, where they will synergize with the southern observatories and the upcoming ELT. Lastly, the sub-arcsecond resolution provided by SKA-Mid will facilitate the exploration of the polarization and filamentary structure of RPS radio tails and allow us to detect them up to $z\simeq0.5$, advancing our understanding of the impact of RPS on satellite galaxies in clusters and groups.

astro-ph.GA↗

arXiv2026

Early Results from the Coma Legacy IFU Survey (CLIFS): Ram Pressure Induced Shocks and Ionization in Jellyfish Tails

Jellyfish galaxies, which exhibit tails of gas opposite to their direction of motion, are a galaxy population showcasing the most extreme effects of ram pressure stripping (RPS). We present the emission line properties of a preliminary sample of five jellyfish galaxies in the Coma cluster, observed with the WEAVE Large-IFU as part of the Coma Legacy IFU Survey (CLIFS). When complete, CLIFS will form a sample of 29 jellyfish galaxies in Coma, selected based on the presence of one-sided tails in the radio continuum, enabling a comprehensive picture of the effects of ram pressure on galaxies in the Coma cluster. We extract emission line properties and confirm consistency between disk fluxes measured from WEAVE and MaNGA for galaxies with overlapping disk coverage between surveys. Comparing resolved radio and H$\alpha$-based star formation rates, we find that, in contrast to the disk, the dominant source of tail emission is not star formation. We find evidence for diffuse ionized gas excited by RPS-driven shocks in the tails, as indicated by: (1) LINER-like tail emission with the [OI]/H$\alpha$ BPT diagnostic; (2) enhanced [OII]/H$\alpha$ ratios in the tails relative to the disks; and (3) similarly elevated emission line velocities and velocity dispersions in the tails with respect to the disks. These results demonstrate that ram-pressure-driven shocks dominate the ionized emission in jellyfish galaxy tails.

astro-ph.GA↗

arXiv2025

The Competing Influence of Ram Pressure and Tidal Interaction in NGC 2276

The evolution of galaxies in groups is profoundly influenced by a variety of physical processes, with ram pressure and tidal interactions playing pivotal roles in shaping their structural and evolutionary pathways. The relative influence of these two processes is still debated in groups compared to clusters, as ram pressure is less understood there. We study NGC 2276, a nearby galaxy (z$\sim$0.0079) where the dominant process is still an open question. We examine the distribution of stellar populations in NGC 2276 using multiwavelength data to assess potential evidence of tidal interactions and ram-pressure stripping. We present the first HST WFC3/UVIS images of NGC 2276, and use them to investigate the distribution of stellar populations across the disk of NGC 2276, where we assume that the bluer broadband filters mainly trace younger stellar populations, while the redder filters trace mainly older stellar populations. Furthermore, by comparing HST images with maps of H$\alpha$ emission from Calar Alto's PMAS/PPAK integral field unit (IFU) and near-IR maps from Spitzer's IRAC, we identify arm-like overdensity features that trace the spiral structure of this galaxy and tracked the variation of their pitch angle with radius. Our results indicate that the distribution of the stellar populations is asymmetrical. The youngest stellar populations (up to $\sim$100 Myr) show higher concentration on the leading side of the galaxy and are more diffuse on the trailing side, consistent with gas compression due to ram-pressure. This asymmetry is visible in the red filters as well. We also show that the average pitch angles of the overdensity features increase with galactocentric distance. Our findings are consistent with the fact that ram pressure is the leading mechanism for the peculiar morphology of NGC 2276, but do not exclude the possibility that tidal interactions could have played a role.

astro-ph.GA↗

arXiv2025

VERTICO IX: Signatures of environmental processing of the gas in Virgo cluster spiral galaxies through mapping of CO isotopologues

In this work we study CO isotopologue emission in the largest cluster galaxy sample to date: 48 VERTICO spiral galaxies in Virgo. We show for the first time in a significant sample that the physical conditions within the molecular gas appear to change as a galaxy's ISM is affected by environmental processes. 13CO is detected across the sample, both directly and via stacking, while C18O is detected in a smaller number of systems. We use these data to study trends with global and radial galaxy properties. We show that the CO/13CO line ratio changes systematically with a variety of galaxy properties, including mean gas surface density, HI-deficiency and galaxy morphology. 13CO/C18O line ratios vary significantly, both radially and between galaxies, suggesting real variations in abundances are present. Such abundance changes may be due to star formation history differences, or speculatively even stellar initial mass function variations. We present a model where the optical depth of the molecular gas appears to change as a galaxy's ISM is affected by environmental processes. The molecular gas appears to become more transparent as the molecular medium is stripped, and then more opaque as the tightly bound remnant gas settles deep in the galaxy core. This explains the variations we see, and also helps explain similar observations in cluster early-type galaxies. Next generation simulations and dedicated observations of additional isotopologues could thus provide a powerful tool to help us understand the impact of environment on the ISM, and thus the quenching of galaxies.

astro-ph.GA↗

arXiv2025

JWST Reveals a Candidate Jellyfish Galaxy at z=1.156

We report the discovery of COSMOS2020-635829 as a candidate jellyfish galaxy undergoing ram pressure stripping in a (proto)cluster at $z > 1$. High-resolution imaging from the James Webb Space Telescope reveals a symmetric stellar disk coupled to a unilateral tail of star-forming knots to the south. Using Gemini GMOS IFU observations, we show that these extra-planar continuum sources are embedded within an ionized gas tail that is kinematically connected to the disk of COSMOS2020-635829. If confirmed, this represents the highest-redshift discovery of a ram pressure stripped ionized gas tail. The tail sources are characterized by extremely young stellar populations ($\lesssim 100\,\mathrm{Myr}$), have stellar masses of ${\sim}10^8\,\mathrm{M_\odot}$, and star formation rates of $0.1\text{--}1\,\mathrm{M_\odot\,yr^{-1}}$. This work shows that ram pressure stripping can potentially perturb group and cluster galaxies at $z > 1$ and may contribute to environmental quenching even near Cosmic Noon.

astro-ph.GA↗

arXiv2025

The phase-space of tailed radio galaxies in massive clusters

The radio jets of radio galaxies in galaxy clusters are often bent due to the ram pressure of the intracluster medium. In this paper we start with a well-defined sample of galaxy clusters and subsequently identifying tailed radio sources in these known environments. Our sample consists of 81 galaxy clusters from the Planck ESZ cluster sample. We present a catalogue of 127 extended cluster radio sources, including brightest cluster galaxies, obtained by visually inspecting Karl G. Jansky Very Large Array (1-2 GHz) observations. We have determined the bending angle of 109 well-structured sources, and classified them accordingly: 84 narrow-angle tailed sources (NATs), 16 wide-angle tailed sources (WATs), and 9 non-bent radio sources. We find a negative correlation between the bending angle and the distance to the cluster centre (impact radius), and we observe that NATs generally have smaller impact radii than the regular galaxy population and WATs. We present a phase-space diagram of tailed radio galaxy velocities and impact radii and find that NATs have a significant excess in the high-velocity and low-impact radius region of phase space, indicating they undergo the largest amount of ram pressure bending. We compared the results from our sample with those for jellyfish galaxies, and suggest that the mechanism responsible for bending the radio tails is similar to the stripping of gas in jellyfish galaxies, although tailed radio galaxies are more concentrated in the centre of the phase space. Finally, we find that NATs and WATs have the same occurrence ratio in merging and relaxed clusters. However, their distribution in the phase-space is significantly different. We report an excess of NATs in the high-velocity and low-impact-radius phase-space region in merging clusters, and an excess of relaxed clusters in the low-velocity and low-impact-radius region.

astro-ph.CO↗

arXiv2025

Deep galaxy stellar mass functions as a function of star formation rate in the Virgo cluster environment

We analyze deep ($M_*\gtrsim10^7~{M}_{\odot}$) galaxy stellar mass functions (SMFs) of the Virgo cluster using stellar masses derived as part of the Next Generation Virgo Survey (NGVS). The total SMF has a slope of $α=-1.35^{+0.02}_{-0.02}$ which is similar to or steeper than typical field values. Using deep \ha{} data from the Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) we separate out star-forming galaxies, quiescent galaxies with no ongoing star formation, and low-SFR galaxies that are intermediate between these two populations. For each of these populations, the shape of the SMF is found to be universal throughout the cluster, from the core to the outskirts. The star-forming and quiescent SMFs show stark differences with values seen in field galaxies. The relative fraction of quiescent galaxies is highest in the core of the cluster, with low-SFR and star-forming galaxies more significant in the outer regions of the cluster. At low stellar masses ($M_*\lesssim10^9~{M}_{\odot}$), the quiescent fraction in the main cluster is significantly higher than that of the field and even satellites of massive groups. At high stellar masses, the quiescent fraction is similar to other studies of cluster galaxies. We model the quiescent population in the infall region of the cluster as a combination of backsplash and field quiescent galaxies, and find that the backsplash fractions needed to explain the observed population are unrealistically high. This suggests the existence of a third population of low-mass galaxies that are pre-processed outside the virial radius of the cluster, possibly in groups prior to infall.

astro-ph.GA↗

arXiv2025

Truncated star formation and ram pressure stripping in the Coma Cluster

We use 45 galaxies from the Mapping Nearby Galaxies at Apache Point Observatory survey to study the physical drivers of star formation quenching in the Coma cluster. We measure specific star formation rate (sSFR) radial profiles for the Coma sample as well as a control sample of non-cluster field galaxies. We find that compared to the control sample, galaxies within the Coma Cluster have sSFR profiles that fall off more steeply with galactocentric radius. We then apply a toy model based on slow-then-rapid quenching via ram pressure stripping. We find that this model is able to reproduce the difference in sSFR profiles between field and Coma galaxies. These results demonstrate that ram pressure stripping plays a significant role in quenching star formation in the nearest massive galaxy cluster.

astro-ph.GA↗

arXiv2025

The Merging Galaxy Cluster Environment Affects the Morphology of Radio-AGN

It has previously been found that the galaxy cluster environment can affect the fueling and evolution of Active Galactic Nuclei (AGN). This work examines the effect of the merging cluster environment on the properties of radio-AGN by comparing the radio morphology of cluster members in a sample of four merging and eight relaxed galaxy clusters at low redshift (z<0.2). Using 144-MHz data from the LOFAR Two-metre Sky Survey (LoTSS) and Zooniverse, we classify the radio morphology of the radio-detected cluster members using the following morphology classes: compact, compact extended, extended, jetted, and disturbed. We find that the merging cluster environment has a statistically significant, higher population proportion of disturbed (bent and head tail) sources, indicating that the merging environment can affect the morphology of cluster radio-AGN. We also investigate the number of AGN that are detected in the radio data only, and the number that are detected in both the radio and optical data in mergers and non-mergers. We find that the merging cluster environment has a higher population proportion of AGN that are identified only as radio-AGN compared to AGN that are identified as both radio and optical AGN. Overall, we find that the merging environment affects certain radio-AGN (disturbed and only radio identified AGN), but not all.

astro-ph.GA↗

arXiv2025

No Evidence of Asymmetrically Enhanced Star Formation in Infalling Galaxies in UNIONS

Ram pressure stripping is a well-known environmental quenching mechanism that removes gas from galaxies infalling into groups and clusters. In some extreme examples of ram pressure stripping, galaxies with extended gas tails show evidence of enhanced star formation prior to quenching. In this work we use a sample of 5277 local satellite galaxies in which a stripped tail of gas has not necessarily been observed, to quantify the strength of ram pressure-enhanced star formation and compare these results to a control sample of 8360 field galaxies. We use u-band imaging from the Ultraviolet-Near Infrared Northern Survey (UNIONS) as a star formation tracer and several metrics to quantify star formation asymmetry. We compare these results to environmental properties of the galaxy, such as their time since infall and host halo mass, to constrain the degree of ram pressure enhanced star formation as a function of environment. We find no significant differences between the satellite and the field samples. We further restrict our sample to galaxies which we most expect to be experiencing significant ram pressure but find no strong evidence of these galaxies having systematically enhanced star formation. Finally, we investigate the properties of the most asymmetric galaxies in our sample and again find no strong evidence of ram pressure-induced star formation enhancement. We conclude that any star formation enhancement must be small for infalling galaxies, suggesting that this effect is either uncommon or short-lived.

astro-ph.GA↗

arXiv2025

Does the HCN/CO ratio trace the star-forming fraction of gas? II. Variations in CO and HCN Emissivity

We model emissivities of the HCN and CO $J=1-0$ transitions using measured properties of clouds found in normal star forming galaxies and more extreme systems. These models are compared with observations of HCN and CO $J=1-0$ transitions. We combine these model emissivities with predictions of gravoturbulent models of star formation, explore the impact of excitation and optical depth on CO and HCN emission, and assess if observed HCN/CO ratios track the fraction of gravitationally-bound dense gas, $f_\mathrm{grav}$, in molecular clouds. Our modeled HCN/CO ratios and emissivities are consistent with measurements from observations. CO emission shows a range of optical depths across different environments, from optically thick in normal galaxies to moderately optically thin in extreme systems. HCN is only moderately optically thick, with significant subthermal excitation in both normal and extreme galaxies. We find an anticorrelation between HCN/CO and $f_\mathrm{grav}$ as predicted by gravoturbulent models of star formation. Instead this ratio tracks gas at moderate densities ($n>10^{3.5}\ \mathrm{cm}^{-3}$), which is below the standard dense gas threshold of $n>10^{4.5}\ \mathrm{cm}^{-3}$. Variations in CO emissivity depend strongly on optical depth, due to variations in the dynamics of the cloud gas. HCN emissivity depends more strongly on excitation, and thus does not directly track variations in CO emissivity. We conclude that a single line ratio, such as HCN/CO, will not consistently track the fraction of gravitationally-bound, star-forming gas if the critical density for star formation varies in molecular clouds. This work highlights important uncertainties that need to be considered when observationally applying an HCN conversion factor in order to estimate the dense (i.e. $n>10^{4.5}\ \mathrm{cm}^{-3}$) gas content in nearby galaxies.

astro-ph.GA↗

arXiv2024

Merger Shocks Enhance Quenching in Local Galaxy Clusters

We report evidence for enhanced quenching in low-redshift galaxy clusters hosting radio relics. This effect is strongest for low-mass galaxies and is consistent with a rapid quenching of star formation. These results imply that merger shocks in the intracluster medium play a role in driving environmental quenching, which we argue is due to the elevated ram pressure experienced by satellite galaxies in these disturbed systems.

astro-ph.GA↗

arXiv2023

Ram-pressure stripped radio tail and two ULXs in the spiral galaxy HCG 97b

We report LOFAR and VLA detections of extended radio emission in the spiral galaxy HCG 97b, hosted by an X-ray bright galaxy group. The extended radio emission detected at 144 MHz, 1.4 GHz and 4.86 GHz is elongated along the optical disk and has a tail that extends 27 kpc in projection towards the centre of the group at GHz frequencies or 60 kpc at 144 MHz. Chandra X-ray data show two off-nuclear ultra-luminous X-ray sources (ULXs), with the farther one being a plausible candidate for an accreting intermediate-mass black hole (IMBH). The asymmetry observed in both CO emission morphology and kinematics indicates that HCG 97b is undergoing ram-pressure stripping, with the leading side at the southeastern edge of the disk. Moreover, the VLA 4.86 GHz image reveals two bright radio blobs near one ULX, aligning with the disk and tail, respectively. The spectral indices in the disk and tail are comparable and flat ($α> -1$), suggesting the presence of recent outflows potentially linked to ULX feedback. This hypothesis gains support from estimates showing that the bulk velocity of the relativistic electrons needed for transport from the disk to the tail is approximately $\sim 1300$ $\rm km~s^{-1}$. This velocity is much higher than those observed in ram-pressure stripped galaxies ($100-600$ $\rm km~s^{-1}$), implying an alternative mechanism aiding the stripping process. Therefore, we conclude that HCG 97b is subject to ram pressure, with the formation of its stripped radio tail likely influenced by the putative IMBH activities.

astro-ph.GA↗

arXiv2023

VERTICO and IllustrisTNG: The spatially resolved effects of environment on galactic gas

It has been shown in previous publications that the TNG100 simulation quantitatively reproduces the observed reduction in each of the total atomic and total molecular hydrogen gas for galaxies within massive halos, i.e.~dense environments. In this Letter, we study how well TNG50 reproduces the resolved effects of a Virgo-like cluster environment on the gas surface densities of satellite galaxies with $m_* > \! 10^9\,{\rm M}_\odot$ and ${\rm SFR} \! > 0.05\,{\rm M}_\odot\,{\rm yr}^{-1}$. We select galaxies in the simulation that are analogous to those in the HERACLES and VERTICO surveys, and mock-observe them to the common specifications of the data. Although TNG50 does not quantitatively match the observed gas surface densities in the centers of galaxies, the simulation does qualitatively reproduce the trends of gas truncation and central density suppression seen in VERTICO in both HI and H$_2$. This result promises that modern cosmological hydrodynamic simulations can be used to reliably model the post-infall histories of cluster satellite galaxies.

astro-ph.GA↗

arXiv2023

On the encounter between the GASP galaxy JO36 and the radio plume of GIN 049

We report on the serendipitous discovery of an unprecedented interaction between the radio lobe of a radio galaxy and a spiral galaxy. The discovery was made thanks to LOFAR observations at 144 MHz of the galaxy cluster Abell 160 ($z=0.04317$) provided by the LOFAR Two-metre Sky Survey. The new low-frequency observations revealed that one of the radio plumes of the central galaxy GIN 049 overlaps with the spiral galaxy JO36. Previous studies carried out with MUSE revealed that the warm ionized gas in the disk of JO36, traced by the H$α$ emission, is severely truncated with respect to the stellar disk. We further explore this unique system by including new uGMRT observations at 675 MHz to map the spectral index. The emerging scenario is that JO36 has interacted with the radio plume in the past 200-500 Myr. The encounter resulted in a positive feedback event for JO36 in the form of a star formation rate burst of $\sim14$ $M_\odot$ yr$^{-1}$. In turn, the galaxy passage left a trace in the radio-old plasma by re-shaping the old relativistic plasma via magnetic draping.

astro-ph.GA↗

arXiv2023

VERTICO VII: Environmental quenching caused by suppression of molecular gas content and star formation efficiency in Virgo Cluster galaxies

We study how environment regulates the star formation cycle of 33 Virgo Cluster satellite galaxies on 720 parsec scales. We present the first resolved star-forming main sequence for cluster galaxies, dividing the sample based on their global HI properties and comparing to a control sample of field galaxies. HI-poor cluster galaxies have reduced star formation rate (SFR) surface densities with respect to both HI-normal cluster and field galaxies (0.5 dex), suggesting that mechanisms regulating the global HI content are responsible for quenching local star formation. We demonstrate that the observed quenching in HI-poor galaxies is caused by environmental processes such as ram pressure stripping (RPS) simultaneously reducing molecular gas surface density and star formation efficiency (SFE), compared to regions in HI-normal systems (by 0.38 and 0.22 dex, respectively). We observe systematically elevated SFRs that are driven by increased molecular gas surface densities at fixed stellar mass surface density in the outskirts of early-stage RPS galaxies, while SFE remains unchanged with respect to the field sample. We quantify how RPS and starvation affect the star formation cycle of inner and outer galaxy discs as they are processed by the cluster. We show both are effective quenching mechanisms with the key difference being that RPS acts upon the galaxy outskirts while starvation regulates the star formation cycle throughout disc, including within the truncation radius. For both processes, the quenching is caused by a simultaneous reduction in molecular gas surface densities and SFE at fixed stellar mass surface density.

astro-ph.GA↗

arXiv2023

VERTICO VI: Cold-gas asymmetries in Virgo cluster galaxies

We analyze cold-gas distributions in Virgo cluster galaxies using resolved CO(2-1) (tracing molecular hydrogen, H2) and HI observations from the Virgo Environment Traced In CO (VERTICO) and the VLA Imaging of Virgo in Atomic Gas (VIVA) surveys. From a theoretical perspective, it is expected that environmental processes in clusters will have a stronger influence on diffuse atomic gas compared to the relatively dense molecular gas component, and that these environmental perturbations can compress the cold interstellar medium in cluster galaxies leading to elevated star formation. In this work we observationally test these predictions for star-forming satellite galaxies within the Virgo cluster. We divide our Virgo galaxy sample into HI-normal, HI-tailed, and HI-truncated classes and show, unsurprisingly, that the HI-tailed galaxies have the largest quantitative HI asymmetries. We also compare to a control sample of non-cluster galaxies and find that Virgo galaxies, on average, have HI asymmetries that are 40 +/- 10 per cent larger than the control. There is less separation between control, HI-normal, HI-tailed, and HI-truncated galaxies in terms of H2 asymmetries, and on average, Virgo galaxies have H2 asymmetries that are only marginally (20 +/- 10 per cent) larger than the control sample. We find a weak correlation between HI and H2 asymmetries over our entire sample, but a stronger correlation for those specific galaxies being strongly impacted by environmental perturbations. Finally, we divide the discs of the HI-tailed Virgo galaxies into a leading half and trailing half according to the observed tail direction. We find evidence for excess molecular gas mass on the leading halves of the disc. This excess molecular gas on the leading half is accompanied by an excess in star formation rate such that the depletion time is, on average, unchanged.

astro-ph.GA↗

arXiv2023

VERTICO V: The environmentally driven evolution of the inner cold gas discs of Virgo cluster galaxies

The quenching of cluster satellite galaxies is inextricably linked to the suppression of their cold interstellar medium (ISM) by environmental mechanisms. While the removal of neutral atomic hydrogen (HI) at large radii is well studied, how the environment impacts the remaining gas in the centres of galaxies, which are dominated by molecular gas, is less clear. Using new observations from the Virgo Environment traced in CO survey (VERTICO) and archival HI data, we study the HI and molecular gas within the optical discs of Virgo cluster galaxies on 1.2-kpc scales with spatially resolved scaling relations between stellar ($Σ_{\star}$), HI ($Σ_\mathrm{HI}$), and molecular gas ($Σ_\mathrm{mol}$) surface densities. Adopting HI deficiency as a measure of environmental impact, we find evidence that, in addition to removing the HI at large radii, the cluster processes also lower the average $Σ_\mathrm{HI}$ of the remaining gas even in the central 1.2 kpc. The impact on molecular gas is comparatively weaker than on the HI, and we show that the lower $Σ_\mathrm{mol}$ gas is removed first. In the most HI-deficient galaxies, however, we find evidence that environmental processes reduce the typical $Σ_\mathrm{mol}$ of the remaining gas by nearly a factor of 3. We find no evidence for environment-driven elevation of $Σ_\mathrm{HI}$ or $Σ_\mathrm{mol}$ in HI-deficient galaxies. Using the ratio of $Σ_\mathrm{mol}$-to-$Σ_\mathrm{HI}$ in individual regions, we show that changes in the ISM physical conditions, estimated using the total gas surface density and midplane hydrostatic pressure, cannot explain the observed reduction in molecular gas content. Instead, we suggest that direct stripping of the molecular gas is required to explain our results.

astro-ph.GA↗