arXiv subjects

T. H. Jarrett

Publications and source records attributed to T. H. Jarrett.

At least 19 recordsLinked to original sources

arXiv2025

Probing the Infrared/Radio correlation of the full IRAS Revised Bright Galaxy Sample with MeerKAT and the VLA

We study the infrared/radio correlation of galaxies in the IRAS Revised Bright Galaxy Sample using new MeerKAT observations at $\rmν= 1.28\, GHz$, complemented with VLA data. We classify the objects by primary energy source (Active Galactic Nuclei vs. Star-Forming) and take into account their merger status. With this, we aim to explore the effect of galaxy-galaxy interaction on the total-infrared (TIR)/radio correlation ($q_\mathrm{TIR}$) of star-forming galaxies by comparing the $q_\mathrm{TIR}$ distribution between isolated and interacting/merging sources. We found the median $q_\mathrm{TIR}$ to be $2.61 \pm 0.01$ (scatter = 0.16) for isolated galaxies and $2.51 \pm 0.08$ (scatter = 0.26) for interacting/merging galaxies. Our analysis reveals that interacting/merging galaxies exhibit lower $q_\mathrm{TIR}$ and higher dispersion compared to isolated galaxies, and the difference is marginally significant. Interacting/merging galaxies have redder $W2-W3$ colours, higher star formation rates (SFR) and specific SFR compared to isolated objects. We observe a significant decrease in $q_\mathrm{TIR}$ with increasing radio luminosity for isolated galaxies. Additionally, we find the median ratio of TIR ($8 \,μm < λ< 1000\, μm$) to far-infrared (FIR; $40 \,μm < λ< 120\, μm$) luminosities to be $\left \approx2.29$. By examining the relation between $L_\mathrm{TIR}$ and the mid-infrared (MIR) star-formation rate indicator ($L_\mathrm{12\,μm}$) employed for our interacting/merging sample, we note a strong and consistent (similar non-linear behaviour) relationship between the TIR/radio and TIR/MIR ratios. Finally, we show that already at $z<0.1$, $q_\mathrm{TIR}$ exhibits a dependence on stellar mass, with more massive galaxies displaying a lower $q_\mathrm{TIR}$.

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arXiv2025

Evolutionary Map of the Universe: Detection and Analysis of the Shell Surrounding the Runaway Wolf-Rayet Star WR16

We present the first radio--continuum detection of the circumstellar shell around the well-known WN8 type Wolf-Rayet star WR16 at 943.5\,MHz using the \ac{ASKAP} \ac{EMU} survey. At this frequency, the shell has a measured flux density of 72.2$\pm$7.2\,mJy. Using previous \ac{ATCA} measurements at 2.4, 4.8, and 8.64~GHz, as well as the \ac{EMU} observations of the star itself, we determine a spectral index of $α\,=\,+0.74\pm0.02$, indicating thermal emission. We propose that the shell and star both exhibit thermal emission, supported by the its appearance in near-infrared and H$α$ observations. The latest \textit{Gaia} parallax is used to determine a distance of 2.28$\pm$0.09\,kpc. This star is well-known for its surrounding circular nebulosity, and using the distance and an angular diameter of 8\farcm42, we determine the shell size to be 5.57$\pm$0.22~pc. We use the \textit{Gaia} \acp{PM} of WR16 to determine peculiar velocities of the star as $V_α(pec) =$ --45.3$\pm$5.4\,\kms\ and $V_δ(pec) =$ 22.8$\pm$4.7\,\kms, which indicates that the star is moving in a north-west direction, and translates to a peculiar tangential velocity to be 50.7$\pm$6.9\,\kms. We also use these \acp{PM} to determine the shell's origin, estimate an age of $\sim 9500\pm 1300$\,yr, and determine its average expansion velocity to be $280\pm40$\,\kms. This average expansion velocity suggests that the previous transitional phase is a \ac{LBV} phase, rather than a \ac{RSG} phase. We also use the measured flux at 943.5~MHz to determine a mass-loss rate of $1.753\times 10^{-5}~M_\odot~$yr$^{-1}$, and use this to determine a lower-limit on ionising photons of $N_{UV} > 1.406\times 10^{47}~$s$^{-1}$.

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arXiv2025

MeerKAT view of Hickson Compact Groups:I. Data description and release

Context: Hickson Compact Groups (HCGs) are dense gravitationally-bound collections of 4-10 galaxies ideal for studying gas and star formation quenching processes. Aims: We aim to understand the transition of HCGs from possessing complex HI tidal structures (so-called phase 2 groups) to a phase where galaxies have lost most or all their HI (phase 3). We also seek to detect diffuse H i gas that was previously missed by the Very Large Array (VLA). Methods: We observed three phase 2 and three phase 3 HCGs with MeerKAT and reduced the data using the Containerized Automated Radio Astronomy Calibration (CARACal) pipeline. We produced data cubes, moment maps, integrated spectra, and compared our findings with previous VLA and Green Bank Telescope (GBT) observations. Results: Compared with previous VLA observations, MeerKAT reveals much more extended tidal features in phase 2 and some new high surface brightness features in phase 3 groups. However, no diffuse HI component was found in phase 3 groups. We also detected many surrounding galaxies for both phase 2 and phase 3 groups, most of which are normal disk galaxies. Conclusions: The difference between phase 2 and phase 3 groups is still substantial, supporting previous findings that the transition between the two phases must be abrupt.

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arXiv2024

Diprotodon on the sky. The Large Galactic Supernova Remnant (SNR) G278.94+1.35

We present a re-discovery of G278.94+1.35 as possibly one of the largest known Galactic supernova remnants (SNR) - that we name Diprotodon. While previously established as a Galactic SNR, Diprotodon is visible in our new EMU and GLEAM radio continuum images at an angular size of 3.33x3.23 deg, much larger than previously measured. At the previously suggested distance of 2.7 kpc, this implies a diameter of 157x152 pc. This size would qualify Diprotodon as the largest known SNR and pushes our estimates of SNR sizes to the upper limits. We investigate the environment in which the SNR is located and examine various scenarios that might explain such a large and relatively bright SNR appearance. We find that Diprotodon is most likely at a much closer distance of $\sim$1 kpc, implying its diameter is 58x56 pc and it is in the radiative evolutionary phase. We also present a new Fermi-LAT data analysis that confirms the angular extent of the SNR in gamma-rays. The origin of the high-energy emission remains somewhat puzzling, and the scenarios we explore reveal new puzzles, given this unexpected and unique observation of a seemingly evolved SNR having a hard GeV spectrum with no breaks. We explore both leptonic and hadronic scenarios, as well as the possibility that the high-energy emission arises from the leftover particle population of a historic pulsar wind nebula.

astro-ph.HE↗

arXiv2024

A Galactic Scale Magnetized Wind Around a Normal Star-Forming Galaxy

Galaxy formation theory identifies superwinds as a key regulator of star formation rates, galaxy growth, and chemical enrichment. Thermal and radiation pressure are known to drive galactic-scale winds in dusty starbursting galaxies (e.g. M82), but modern numerical simulations have recently highlighted that cosmic-ray (CR) driven winds may be especially important in normal galaxies with modest star formation rate surface densities. However, CR-driven winds have yet to be conclusively observed -- leaving significant uncertainty in their detailed microphysics. We present MeerKAT radio continuum and HI spectral-line observations of one such normal galaxy, NGC 1532; a nearby ($D\sim15\,\mathrm{Mpc}$) and edge-on ($i \gtrsim 80^{\circ}$) spiral galaxy tidally interacting with its smaller elliptical companion, NGC 1531. We find magnetized, highly-ordered radio continuum loops extending $\sim10$ kpc above and below the disk; visibly connecting discrete star-forming regions in the disk with the nucleus. The deep MeerKAT HI observations place an upper limit on the column density of neutral gas coincident with the outflow to $N_\mathrm{HI} \lesssim 3 \times 10^{19}\,\mathrm{cm}^{-2}$. Unlike previously observed outflows -- for which ejected gas and dust can be traced across multiple wavelengths -- the loops in NGC 1532 show no detectable signs of dust or gas coincident with the radio emission far from the disk. We explore multiple possible mechanisms for driving this magnetic wind and favor an explanation where cosmic-ray pressure plays a significant role in launching these outflows.

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arXiv2024

Testing the Bullet Dwarf Collision Scenario in the NGC 1052 Group Through Morphologies and Stellar Populations

NGC 1052-DF2 and -DF4 are two ultra-diffuse galaxies that have been reported as deficient in dark matter and associated with the same galaxy group. Recent findings suggest that DF2 and DF4 are part of a large linear substructure of dwarf galaxies that could have been formed from a high-velocity head-on encounter of two gas-rich galaxies, known as a bullet dwarf collision. Based on new observations from the Hubble Space Telescope, combined with existing imaging from the u band to mid-infrared, we test the bullet dwarf scenario by studying the morphologies and stellar populations of the trail dwarfs. We find no significant morphological differences between the trail dwarfs and other dwarfs in the group, while for both populations, their photometric major axes unexpectedly align parallel with the trail. We find that the trail dwarfs have significantly older ages and higher metallicities than the comparison sample, supporting the distinctiveness of the trail. These observations provide key constraints for any formation model, and we argue that they are currently best explained by the bullet dwarf collision scenario, with additional strong tests anticipated with future observations.

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arXiv2024

A Comprehensive Investigation of Environmental Influences on Galaxies in Group Environments

Environment has long been known to impact the evolution of galaxies, but disentangling its effects from mass evolution requires careful analysis of statistically significant samples. By implementing advanced visualisation methods to test group-finding algorithms, we utilise a mass-complete sample of galaxies to z < 0.1, comprising spectroscopic redshifts from prominent surveys such as the 2dFGRS and GAMA. Our group-finding methods identify 1,413 galaxy groups made up of 8,990 galaxies, corresponding to 36% of galaxies associated with group environments. We also search for close pairs, with separations of $r_{sep}$ < 50 h$^{-1}$ kpc and $v_{sep}$ < 500 km/s, and classify them into major ($M_{sec}/M_{prim} \leq$ 0.25) and minor ($M_{sec}/M_{prim}$ > 0.25) pairs. To examine the impact of environmental factors, we employ bespoke WISE photometry to derive a star-forming main sequence relation that shows star-formation (SF) within galaxies is pre-processed as a function of group membership. Our analysis reveals that SF in galaxies is pre-processed as a function of group membership. We observe an increase in the fraction of quiescent galaxies relative to the field as group membership rises, quantified using the environmental quenching efficiency metric ($ε_{env}$). Within the star-forming population, we detect pre-processing with the relative difference in specific SF rates ($Δ sSFR$), showing a net decrease in SF as group membership increases, particularly at larger stellar masses. Our sample of close pairs at low stellar masses shows enhanced SF efficiencies compared to the field, while at larger masses, deficiencies are evident. Our results indicate that the small-scale environments of galaxies influence SF properties, demonstrating that galaxies do not evolve in isolation over cosmic time but are shaped by complex interactions between internal dynamics and external influences.

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arXiv2024

The S$^4$G-WISE View of Global Star Formation in the Nearby Universe

In this work we present source-tailored WISE mid-infrared photometry (at 3.4$μ$m, 4.6$μ$m, 12$μ$m, and 23$μ$m) of 2812 galaxies in the extended Spitzer Survey of Stellar Structure in Galaxies (S$^4$G) sample, and characterise the mid-infrared colors and dust properties of this legacy nearby galaxy data set. Informed by the relative emission between W3 (12$μ$ m) and W4 (23$μ$ m), we re-derive star formation rate (SFR) scaling relations calibrated to L$_{\rm TIR}$, which results in improved agreement between the two tracers. By inverse-variance weighting the W3 and W4-derived SFRs, we generate a combined mid-infrared SFR that is a broadly robust measure of star formation activity in dusty, star-forming galaxies in the nearby Universe. In addition, we investigate the use of a W3-derived dust density metric, $Σ_{\rm 12μm}$ (L$_\odot$/kpc$^2$), to estimate the SFR deficit of low mass, low dust galaxies. This is achieved by combining WISE with existing GALEX ultraviolet (UV) photometry, which we further use to explore the relationship between dust and UV emission as a function of morphology. Finally, we use our derived SFR prescriptions to examine the location of galaxies in the log SFR - log M$_\textrm{stellar}$ plane, as a function of morphological type, which underscores the complexity of dust-derived properties seen in galaxies of progressively earlier type.

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arXiv2024

WALLABY Pilot Survey: the Tully-Fisher relation in the NGC 4808, Vela and NGC 5044 fields

The Tully-Fisher Relation (TFR) is a well-known empirical relationship between the luminosity of a spiral galaxy and its circular velocity, allowing us to estimate redshift independent distances. Here we use high signal-to-noise HI 21-cm integrated spectra from the second pilot data release (PDR2, 180 deg2) of the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY). In order to prepare for the full WALLABY survey, we have investigated the TFR in phase 2 of the pilot survey with a further three fields. The data were obtained with wide-field Phased Array Feeds on the Australian Square Kilometre Array Pathfinder (ASKAP) and have an angular resolution of 30 arcsec and a velocity resolution of ~4 km/s. Galaxy luminosities have been measured from the Wide-field Infrared Survey Explorer (WISE), and optical galaxy inclinations from the Dark Energy Camera Legacy Survey. We present TFRs for wavelengths from 0.8-3.4μm. We examine sources of galaxy inclination data and investigate magnitudes from the DECam Local Volume Exploration Survey (DELVE) and DENIS catalogues and the 4HS target catalogue based on the VISTA Hemisphere Survey (VHS). We consider the baryonic TFR. These are all of interest for TFR using the full WALLABY survey of 200,000 galaxies. We demonstrate that WALLABY TFR distances can take their place among state of the art studies of the local velocity field.

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arXiv2024

WISE2MBH: A scaling-based algorithm for probing supermassive black hole masses through WISE catalogues

Supermassive Black Holes (SMBHs) are commonly found at the centers of massive galaxies. Estimating their masses ($M_\text{BH}$) is crucial for understanding galaxy-SMBH co-evolution. We present WISE2MBH, an efficient algorithm that uses cataloged Wide-field Infrared Survey Explorer (WISE) magnitudes to estimate total stellar mass ($M_*$) and scale this to bulge mass ($M_\text{Bulge}$), and $M_\text{BH}$, estimating the morphological type ($T_\text{Type}$) and bulge fraction ($B/T$) in the process. WISE2MBH uses scaling relations from the literature or developed in this work, providing a streamlined approach to derive these parameters. It also distinguishes QSOs from galaxies and estimates the galaxy $T_\text{Type}$ using WISE colors with a relation trained with galaxies from the 2MASS Redshift Survey. WISE2MBH performs well up to $z\sim0.5$ thanks to K-corrections in magnitudes and colors. WISE2MBH $M_\text{BH}$ estimates agree very well with those of a selected sample of local galaxies with $M_\text{BH}$ measurements or reliable estimates: a Spearman score of $\sim$0.8 and a RMSE of $\sim$0.63 were obtained. When applied to the ETHER sample at $z\leq0.5$, WISE2MBH provides $\sim$1.9 million $M_\text{BH}$ estimates (78.5\% new) and $\sim$100 thousand upper limits. The derived local black hole mass function (BHMF) is in good agreement with existing literature BHMFs. Galaxy demographic projects, including target selection for the Event Horizon Telescope, can benefit from WISE2MBH for up-to-date galaxy parameters and $M_\text{BH}$ estimates. The WISE2MBH algorithm is publicly available on GitHub.

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arXiv2024

Confirmation of a Substantial Discrepancy between Radio and UV--IR Measures of the Star Formation Rate Density at 0.2 < z < 1.3

We present the initial sample of redshifts for 3,839 galaxies in the MeerKAT DEEP2 field -- the deepest $\sim$1.4\,GHz radio field yet observed. Using a spectrophotometric technique combining coarse optical spectra with broadband photometry, we obtain redshifts with $σ_z \leq 0.01(1+z)$. The resulting radio luminosity functions between $0.2<z<1.3$ from our sample of 3,839 individual galaxies are in remarkable agreement with those inferred from modeling radio source counts, confirming an excess in radio-based SFRD$(z$) measurements at late times compared to those from the UV--IR. Several sources of systematic error are discussed -- with most having the potential of exacerbating the discrepancy -- with the conclusion that significant work remains to have confidence in a full accounting of the star formation budget of the universe.

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arXiv2024

Radio continuum from the most massive early-type galaxies detected with ASKAP RACS

All very massive early-type galaxies contain supermassive blackholes but are these blackholes all sufficiently active to produce detectable radio continuum sources? We have used the 887.5~MHz Rapid ASKAP Continuum Survey DR1 to measure the radio emission from morphological early-type galaxies brighter than $K_S=9.5$ selected from the 2MASS Redshift Survey, HyperLEDA and RC3. In line with previous studies, we find median radio power increases with infrared luminosity, with $P_{1.4} \propto L_K^{2.2}$, although the scatter about this relation spans several orders of magnitude. All 40 of the $M_K<-25.7$ early-type galaxies in our sample have measured radio flux densities that are more than $2σ$ above the background noise, with $1.4~{\rm GHz}$ radio powers spanning $\sim 3 \times 10^{20}$ to $\sim 3\times 10^{25}~{\rm W~Hz^{-1}}$. Cross matching our sample with integral field spectroscopy of early-type galaxies reveals that the most powerful radio sources preferentially reside in galaxies with relatively low angular momentum (i.e. slow rotators). While the infrared colours of most galaxies in our early-type sample are consistent with passive galaxies with negligible star formation and the radio emission produced by active galactic nuclei or AGN remnants, very low levels of star formation could power the weakest radio sources with little effect on many other star formation rate tracers.

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arXiv2023

The AMIGA sample of isolated galaxies -- Effects of Environment on Angular momentum

We investigate the relationship between the baryonic angular momentum and mass for a sample of 36 isolated disc galaxies with resolved HI kinematics and infrared WISE photometry drawn from -- and representative in terms of morphologies, stellar masses and HI-to-star fraction of -- the carefully-constructed AMIGA sample of isolated galaxies. Similarly to previous studies performed on non-isolated galaxies, we find that the relation is well described by a power law $j_{bar} \propto M_{bar}^α$. We also find a slope of $α= 0.54 \pm 0.08$ for the AMIGA galaxies, in line with previous studies in the literature; however, we find that the specific angular momenta of the AMIGA galaxies are on average higher than those of non-isolated galaxies in the literature. This is consistent with theories stipulating that environmental processes involving galaxy-galaxy interaction are able to impact the angular momentum content of galaxies. However, no correlation was found between the angular momentum and the degree of isolation, suggesting that there may exist a threshold local number density beyond which the effects of the environment on the angular momentum become important.

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arXiv2023

Specific star formation rates in the $M_{\rm bh}$-$M_{\rm ,sph}$ diagram and the evolutionary pathways of galaxies across the sSFR-$M_{\rm }$ diagram

It has been suggested that the bulge-to-total stellar mass ratio or feedback from black holes (BHs), traced by the BH-to-(total stellar) mass ratio, might establish a galaxy's specific star formation rate (sSFR). We reveal that a galaxy's morphology -- reflecting its formation history, particularly accretions and mergers -- is a far better determinant of the sSFR. Consequently, we suggest that galaxy formation models which regulate the sSFR primarily through BH feedback prescriptions or bulge-regulated disc fragmentation consider acquisitions and mergers which establish the galaxy morphology. We additionally make several new observations regarding current ($z\sim0$) star-formation rates. (i) Galaxies with little to no star formation have bulges with an extensive range of stellar masses; bulge mass does not dictate presence/absence on the `star-forming main sequence'. (ii) The (wet merger)-built, dust-rich S0 galaxies are the `green valley' bridging population between elliptical galaxies on the `red sequence' and spiral galaxies on the blue star-forming main sequence. (iii) The dust-poor S0 galaxies are not on the star-forming main sequence nor in the `green valley'. Instead, they wait in the field for gas accretion and/or minor mergers to transform them into spiral galaxies. Mid-infrared sample selection can miss these (primordial) low dust-content and low stellar-luminosity S0 galaxies. Finally, the appearance of the quasi-triangular-shaped galaxy-assembly sequence, previously dubbed the Triangal, which tracks the morphological evolution of galaxies, is revealed in the sSFR-(stellar mass) diagram.

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arXiv2023

EMU Detection of a Large and Low Surface Brightness Galactic SNR G288.8-6.3

We present the serendipitous detection of a new Galactic Supernova Remnant (SNR), G288.8-6.3 using data from the Australian Square Kilometre Array Pathfinder (ASKAP)-Evolutionary Map of the Universe (EMU) survey. Using multi-frequency analysis, we confirm this object as an evolved Galactic SNR at high Galactic latitude with low radio surface brightness and typical SNR spectral index of $α= -0.41\pm0.12$. To determine the magnetic field strength in SNR G288.8-6.3, we present the first derivation of the equipartition formulae for SNRs with spectral indices $α>-0.5$. The angular size is $1.\!^\circ 8\times 1.\!^\circ 6$ $(107.\!^\prime 6 \times 98.\!^\prime 4)$ and we estimate that its intrinsic size is $\sim40$pc which implies a distance of $\sim1.3$kpc and a position of $\sim140$pc above the Galactic plane. This is one of the largest angular size and closest Galactic SNRs. Given its low radio surface brightness, we suggest that it is about 13000 years old.

astro-ph.HE↗

arXiv2023

A New WISE Calibration of Stellar Mass

We derive new empirical scaling relations between WISE mid-infrared galaxy photometry and well-determined stellar masses from SED modeling of a suite of optical-infrared photometry provided by the DR4 Catalogue of the GAMA-KiDS-VIKING survey of the southern G23 field. The mid-infrared source extraction and characterization are drawn from the WISE Extended Source Catalogue (WXSC) and the archival ALLWISE catalog, combining both resolved and compact galaxies in the G23 sample to a redshift of 0.15. Three scaling relations are derived: W1 3.4 micron luminosity versus stellar mass, and WISE W1-W2, W1-W3 colors versus mass-to-light ratio (sensitive to a variety of galaxy types from passive to star-forming). For each galaxy in the sample, we then derive the combined stellar mass from these scaling relations, producing Mstellar estimates with better than $\sim$25-30% accuracy for galaxies with $>$10$^{9}$ Msolar and $<$40 - 50% for lower luminosity dwarf galaxies. We also provide simple prescriptions for rest-frame corrections and estimating stellar masses using only the W1 flux and the W1-W2 color, making stellar masses more accessible to users of the WISE data. Given a redshift or distance, these new scaling relations will enable stellar mass estimates for any galaxy in the sky detected by WISE with high fidelity across a range of mass-to-light.

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arXiv2022

WALLABY Pre-Pilot and Pilot Survey: the Tully Fisher Relation in Eridanus, Hydra, Norma and NGC4636 fields

The WALLABY pilot survey has been conducted using the Australian SKA Pathfinder (ASKAP). The integrated 21-cm HI line spectra are formed in a very different manner compared to usual single-dish spectra Tully-Fisher measurements. It is thus extremely important to ensure that slight differences (e.g. biases due to missing flux) are quantified and understood in order to maximise the use of the large amount of data becoming available soon. This article is based on four fields for which the data are scientifically interesting by themselves. The pilot data discussed here consist of 614 galaxy spectra at a rest wavelength of 21cm. Of these spectra, 472 are of high enough quality to be used to potentially derive distances using the Tully-Fisher relation. We further restrict the sample to the 251 galaxies whose inclination is sufficiently close to edge-on. For these, we derive Tully-Fisher distances using the deprojected WALLABY velocity widths combined with infrared (WISE W1) magnitudes. The resulting Tully-Fisher distances for the Eridanus, Hydra, Norma and NGC 4636 clusters are 21.5, 53.5, 69.4 and 23.0 Mpc respectively, with uncertainties of 5--10\%, which are better or equivalent to the ones obtained in studies using data obtained with giant single dish telescopes. The pilot survey data show the benefits of WALLABY over previous giant single-dish telescope surveys. WALLABY is expected to detect around half a million galaxies with a mean redshift of $z = 0.05 (200 Mpc)$. This study suggests that about 200,000 Tully-Fisher distances might result from the survey.

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arXiv2022

NGC 3314a/b and NGC 3312: Ram pressure stripping in Hydra I Cluster substructure

Cluster substructure and ram pressure stripping in individual galaxies are among the primary evidence for the ongoing growth of galaxy clusters as they accrete galaxies and groups from their surroundings. We present a multi-wavelength study of the center of the Hydra I galaxy cluster, including exquisite new MeerKAT HI and DECam Halpha imaging which reveal conclusive evidence for ram pressure stripping in NGC 3312, NGC 3314a and NGC 3314b through compressed HI contours, well-defined HI tails, and ongoing star formation in the stripped gas. In particular, we quantify the stripped material in NGC 3312 and NGC 3314a, which makes up between 8% and 35% of the gas still in the disk, is forming stars at ~0.5 M_Sun yr^-1, and extends ~30-60 kpc from the main disk. The estimated stellar mass in the tails is an order of magnitude less than the HI mass. A fourth "ring" galaxy at the same velocity does not show signs of ram pressure in HI. In addition, we use the HI and stellar morphologies, combined with a Beta model of the hot intracluster medium, to constrain the real distances of the galaxies to the cluster center, and we use the chance alignment of NGC 3314b behind NGC 3314a to break the degeneracy between whether the galaxies are in front or in back of the cluster. The drag seen in the HI tails supports our preferred scenario that NGC 3312 and NGC 3314a are moving towards us as part of a foreground substructure which has already passed its pericenter and is on "out fall" from the cluster. The high surviving HI content of the galaxies may suggest that the substructure/intragroup medium can protect them from the harshest effects of ram pressure, or that in fact the galaxies are on more tangential orbits.

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