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The Committee on Radio Astronomy Frequencies (CRAF) is an Expert Committee of the European Science Foundation. It aims to provide a cost-effective single voice on frequency protection issues for European radio astronomy observatories and research institutes, achieving a significantly greater impact than that achievable by individual national institutions. By working together, European observatories and institutes can profit from synergy effects, cover many more topics, and learn from each other. CRAF was founded in 1988 and has since then been engaged with the International Telecommunication Union (ITU), in particular its Radiocommunication Sector (ITU-R), and the European Conference of Postal and Telecommunications Administrations (CEPT) and its European Communications Committee (ECC). This is the self-evaluation report prepared by CRAF for its periodic review of the years 2011-2021., Comment: 75 pages

We study the evolutionary path of the Fornax cluster galaxy NGC$~$1436, which is known to be currently transitioning from a spiral into a lenticular morphology. This galaxy hosts an inner star-forming disc and an outer quiescent disc, and we analyse data from the MeerKAT Fornax Survey, ALMA, and the Fornax3D survey to study the interstellar medium and the stellar populations of both disc components. Thanks to the combination of high resolution and sensitivity of the MeerKAT data, we find that the $\textrm{H}\scriptstyle\mathrm{I}$ is entirely confined within the inner star-forming disc, and that its kinematics is coincident with that of the CO. The cold gas disc is now well settled, which suggests that the galaxy has not been affected by any environmental interactions in the last $\sim1~$Gyr. The star formation history derived from the Fornax3D data shows that both the inner and outer disc experienced a burst of star formation $\sim5$ Gyr ago, followed by rapid quenching in the outer disc and by slow quenching in the inner disc, which continues forming stars to this day. We claim that NGC$~$1436 has begun to effectively interact with the cluster environment 5$~$Gyr ago, when a combination of gravitational and hydrodynamical interactions caused the temporary enhancement of the star-formation rate. Furthermore, due to the weaker gravitational binding $\textrm{H}\scriptstyle\mathrm{I}$ was stripped from the outer disc, causing its rapid quenching. At the same time, accretion of gas onto the inner disc stopped, causing slow quenching in this region., Comment: Monthly Notices of the Royal Astronomical Society (MNRAS), accepted for publication. Data available at the MeerKAT Fornax Survey website https://sites.google.com/inaf.it/meerkatfornaxsurvey

We present the serendipitous discovery of (1) a large double radio relic associated with the galaxy cluster PSZ2 G277.93+12.34 and (2) a new odd radio circle, ORC J1027-4422, both found in the same deep MeerKAT 1.3 GHz wide-band radio continuum image. The angular separation of the two arc-shaped cluster relics is ~16 arcmin or ~2.6 Mpc for a cluster redshift of z ~ 0.158. The thin southern relic, which shows several ridges/shocks including one possibly moving inwards, has a linear extent of ~1.64 Mpc. In contrast, the northern relic is about twice as wide, twice as bright, but only has a largest linear size of ~0.66 Mpc. Complementary SRG/eROSITA X-ray images reveal extended emission from hot intracluster gas between the two relics and around the narrow-angle tail (NAT) radio galaxy PMN J1033-4335 (z ~ 0.153) located just east of the northern relic. The radio morphologies of the NAT galaxy and the northern relic, which are also detected with the Australian Square Kilometer Array Pathfinder (ASKAP) at 888 MHz, suggest both are moving in the same outward direction. The discovery of ORC J1027-4422 in a different part of the same MeerKAT image makes it the 4th known single ORC. It has a diameter of ~90 arcsec corresponding to 400 kpc at a tentative redshift of z ~ 0.3 and remains undetected in X-ray emission. Supported by simulations, we discuss similarities between outward moving galaxy and cluster merger shocks as the formation mechanisms for ORCs and radio relics, respectively., Comment: 16 pages, 11 figures, MNRAS, in press

We present deep optical imaging and photometry of four objects classified as "Almost-Dark" galaxies in the ALFALFA survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have HI masses of $10^7$-$10^9$ $M_{\odot}$ and distances of $\sim$9-100 Mpc. Observations with the WIYN 3.5m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of $\sim$24-25 $\mathrm{mag}\,\mathrm{arcsec}^{-2}$ in the g-band. We also present the results of HI synthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high velocity fly-by. Blind HI surveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle., Comment: 38 pages, 26 figures. Accepted by AJ

The transformation and evolution of a galaxy is strongly influenced by interactions with its environment. Neutral hydrogen (\HI) is an excellent way to trace these interactions. Here, we present \HI\ observations of the spiral galaxy NGC~895, which was previously thought to be isolated. High-sensitivity \HI\ observations from the MeerKAT large survey project MIGHTEE reveal possible interaction features, such as extended spiral arms, and the two newly discovered \HI\ companions, that drive us to change the narrative that it is an isolated galaxy. We combine these observations with deep optical images from the Hyper Suprime Camera to show an absence of tidal debris between NGC 895 and its companions. We do find an excess of light in the outer parts of the companion galaxy MGTH$\_$J022138.1-052631 which could be an indication of external perturbation and thus possible sign of interactions. Our analysis shows that NGC~895 is an actively star-forming galaxy with a SFR of $\mathrm{1.75 \pm 0.09 [M_{\odot}/yr]}$, a value typical for high stellar mass galaxies on the star forming main sequence. It is reasonable to state that different mechanisms may have contributed to the observed features in NGC~895 and this emphasizes the need to revisit the target with more detailed observations. Our work shows the high potential and synergy of using state-of-the-art data in both \HI\ and optical to reveal a more complete picture of galaxy environments., Comment: 14 pages, 10 figures. Accepted for publication in MNRAS

We report the detection of stimulated hydrogen radio recombination line (RRL) emission from ionized gas in a $z=0.89$ galaxy using 580--1670 MHz observations from the MeerKAT Absorption Line Survey (MALS). The RRL emission originates in a galaxy that intercepts and strongly lenses the radio blazar PKS 1830-211 ($z=2.5$). This is the second detection of RRLs outside of the local universe and the first clearly associated with hydrogen. We detect effective H144$\alpha$ (and H163$\alpha$) transitions at observed frequencies of 1156 (798) MHz by stacking 17 (27) RRLs with 21$\sigma$ (14$\sigma$) significance. The RRL emission contains two main velocity components and is coincident in velocity with HI 21 cm and OH 18 cm absorption. We use the RRL spectral line energy distribution and a Bayesian analysis to constrain the density ($n_e$) and the volume-averaged pathlength ($\ell$) of the ionized gas. We determine $\log( n_e ) = 2.0_{-0.7}^{+1.0}$ cm$^{-3}$ and $\log( \ell ) = -0.7_{-1.1}^{+1.1}$ pc towards the north east (NE) lensed image, likely tracing the diffuse thermal phase of the ionized ISM in a thin disk. Towards the south west (SW) lensed image, we determine $\log( n_e ) = 3.2_{-1.0}^{+0.4}$ cm$^{-3}$ and $\log( \ell ) = -2.7_{-0.2}^{+1.8}$ pc, tracing gas that is more reminiscent of H II regions. We estimate a star formation (surface density) rate of $\Sigma_{\mathrm{SFR}} \sim 0.6$ M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$ or SFR $\sim 50$ M$_{\odot}$ yr$^{-1}$, consistent with a star-forming main sequence galaxy of $M_{\star} \sim 10^{11}$ M$_{\odot}$. The discovery presented here opens up the possibility of studying ionized gas at high redshifts using RRL observations from current and future (e.g., SKA and ngVLA) radio facilities., Comment: 17 pages, 10 figures, 2 tables, accepted in ApJ

Around 400 million years after the Big Bang, the ultraviolet emission from star-forming galaxies reionized the Universe. Ionizing radiation (Lyman Continuum, LyC) is absorbed by cold neutral hydrogen gas (HI) within galaxies, hindering the escape of LyC photons. Since the HI reservoir of LyC emitters has never been mapped, major uncertainties remain on how LyC photons escape galaxies and ionize the intergalactic medium. We have directly imaged the neutral gas in the nearby reionization-era analog galaxy Haro 11 with the 21cm line to identify the mechanism enabling ionizing radiation escape. We find that merger-driven interactions have caused a bulk offset of the neutral gas by about 6 kpc from the center of the galaxy, where LyC emission production sites are located. This could facilitate the escape of ionizing radiation into our line of sight. Galaxy interactions can cause both elevated LyC production and large-scale displacement of HI from the regions where these photons are produced. They could contribute to the anisotropic escape of LyC radiation from galaxies and the reionization of the Universe. We argue for a systematic assessment of the effect of environment on LyC production and escape., Comment: 15 pages, 9 figures, accepted for publication in MNRAS

Axions provide a natural and well-motivated dark matter candidate, with the capability to convert directly to photons in the presence of an electromagnetic field. A particularly compelling observational target is the conversion of dark matter axions into photons in the magnetospheres of highly magnetised neutron stars, which is expected to produce a narrow spectral peak centred at the frequency of the axion mass. We point the MeerKAT radio telescope towards the isolated neutron star J0806.4$-$4123 for $10$-hours of observation and obtain the radio spectra in the frequency range $769$-$1051$ MHz. By modelling the conversion process of infalling axion dark matter (DM), we then compare these spectra to theoretical expectations for a given choice of axion parameters. Whilst finding no signal above $5\sigma$ in the data, we provide a unique constraint on the Primakoff coupling of axion DM, $g_{{\rm a}\gamma\gamma}\lesssim 9.3 \times 10^{-12}\,{\rm GeV}^{-1}$ at the $95\%$ confidence level, in the mass range $3.18$-$4.35\,\mu$eV. This result serves the strongest constraint in the axion mass range $4.20$-$4.35\,\mu$eV., Comment: 6 pages, 3 figures, accepted by Physical Review D

The identification of AGN in large surveys has been hampered by seemingly discordant classifications arising from differing diagnostic methods, usually tracing distinct processes specific to a particular wavelength regime. However, as shown in Yao et al. (2020), the combination of optical emission line measurements and mid-infrared photometry can be used to optimise the discrimination capability between AGN and star formation activity. In this paper we test our new classification scheme by combining the existing GAMA-WISE data with high-quality MeerKAT radio continuum data covering 8 deg$^2$ of the GAMA G23 region. Using this sample of 1 841 galaxies (z < 0.25), we investigate the total infrared (derived from 12$\mu$m) to radio luminosity ratio, q(TIR), and its relationship to optical-infrared AGN and star-forming (SF) classifications. We find that while q(TIR) is efficient at detecting AGN activity in massive galaxies generally appearing quiescent in the infrared, it becomes less reliable for cases where the emission from star formation in the host galaxy is dominant. However, we find that the q(TIR) can identify up to 70 % more AGNs not discernible at optical and/or infrared wavelengths. The median q(TIR) of our SF sample is 2.57 $\pm$ 0.23 consistent with previous local universe estimates.

We report the emergence of a new HI 21-cm absorption at z_abs = 1.172635 in the damped Lyman-alpha absorber (DLA) towards the gamma-ray blazar PKS 2355-106 (z_em~1.639) using science verification observations (June 2020) from the MeerKAT Absorption Line Survey (MALS). Since 2006, this DLA is known to show a narrow HI 21-cm absorption at z_abs = 1.173019 coinciding with a distinct metal absorption line component. We do not detect significant HI 21-cm optical depth variations from this known HI component. A high resolution optical spectrum (August 2010) shows a distinct Mg I absorption at the redshift of the new HI 21-cm absorber. However, this component is not evident in the profiles of singly ionized species. We measure the metallicity ([Zn/H] = -(0.77\pm0.11) and [Si/H]= -(0.96\pm0.11)) and depletion ([Fe/Zn] = -(0.63\pm0.16)) for the full system. Using the apparent column density profiles of Si II, Fe II and Mg I we show that the depletion and the N(Mg I)/N(Si II) column density ratio systematically vary across the velocity range. The region with high depletion tends to have slightly larger N(Mg I)/N(Si II) ratio. The two HI 21-cm absorbers belong to this velocity range. The emergence of z_abs = 1.172635 can be understood if there is a large optical depth gradient over a length scale of ~0.35 pc. However, the gas producing the z_abs = 1.173019 component must be nearly uniform over the same scale. Systematic uncertainties introduced by the absorption line variability has to be accounted for in experiments measuring the variations of fundamental constants and cosmic acceleration even when the radio emission is apparently compact as in PKS 2355-106., Comment: 8 pages, 7 figures and accepted for publication in MNRAS

Dwarf galaxies are affected by all the evolutionary processes normally at work in galaxies of any mass. As fainter and less massive galaxies, however, dwarf galaxies are particularly susceptible to environmental mechanisms that can more easily perturb these systems. Importantly, the presence of nearby large galaxies are expected to have a profound effect on dwarf galaxies. Gravitational (especially tidally-induced) effects from the large galaxy can cause mass to be lost from the dwarf, and the passage of the dwarf through the gaseous medium surrounding the large galaxy can additionally cause the dwarf to lose its own gas through a process called ram pressure stripping. Such effects are considered to be the main sources of difference between "satellite" and "field" dwarf galaxy populations. Here, we report on new observations of the gaseous content of Wolf-Lundmark-Melotte (WLM), an archetype of isolated, gas-rich field dwarf galaxies in the Local Universe, which shows a much more complex situation. Previous studies of its gaseous disk suggest it has perturbed kinematics; here, we identify four trailing, extended gas clouds in the opposite direction to WLM's spatial motion, as well as a spatial offset between the WLM gas and stars. Overall, the morphology and kinematics of this gas shows that WLM is undergoing ram pressure stripping, despite being 930 and 830 kpc from the Milky Way and M31, respectively. This finding indicates the presence of an inter-galactic, gaseous reservoir far from large galaxies whose evolutionary role on galaxies, both large and small, may not be fully appreciated., Comment: 7 pages, 7 Figures, accepted as a Letter by Astronomy Astrophysics, March 28 2022, including proofs from the Journal

Observations of the neutral atomic hydrogen (HI) in the nuclear starburst galaxy NGC 4945 with MeerKAT are presented. We find a large amount of halo gas, previously missed by HI observations, accounting for 6.8% of the total HI mass. This is most likely gas blown into the halo by star formation. Our maps go down to a $3\sigma$ column density level of $5\times10^{18} cm^{-2}$ . We model the HI distribution using tilted-ring fitting techniques and find a warp on the galaxy's approaching and receding sides. The HI in the northern side of the galaxy appears to be suppressed. This may be the result of ionisation by the starburst activity in the galaxy, as suggested by a previous study. The origin of the warp is unclear but could be due to past interactions or ram pressure stripping. Broad, asymmetric HI absorption lines extending beyond the HI emission velocity channels are present towards the nuclear region of NGC 4945. Such broad lines suggest the existence of a nuclear ring moving at a high circular velocity. This is supported by the clear rotation patterns in the HI absorption velocity field. The asymmetry of the absorption spectra can be caused by outflows or inflows of gas in the nuclear region of NGC 4945. The continuum map shows small extensions on both sides of the galaxy's major axis that might be signs of outflows resulting from the starburst activity., Comment: 20 pages, 15 figures, 3 tables, accepted for publication in MNRAS

We present the discovery of neutral gas detected in both damped Ly$\alpha$ absorption (DLA) and HI 21-cm emission outside of the stellar body of a galaxy, the first such detection in the literature. A joint analysis between the Cosmic Ultraviolet Baryon Survey and the MeerKAT Absorption Line Survey reveals an HI bridge connecting two interacting dwarf galaxies (log$(M_{\text{star}}/\text{M}_{\odot}) = 8.5 \pm 0.2$) that host a $z = 0.026$ DLA with log[$N$(HI)/cm$^{-2}$]$ = 20.60 \pm 0.05$ toward the QSO J2339-5523 ($z_{\text{QSO}} = 1.35$). At impact parameters of $d = 6$ and $33$ kpc, the dwarf galaxies have no companions more luminous than $\approx 0.05L_{*}$ within at least $\Delta v = \pm 300$ km s$^{-1}$ and $d \approx 350$ kpc. HI 21-cm emission is spatially coincident with the DLA at the 2-3$\sigma$ level per spectral channel over several adjacent beams. However, HI 21-cm absorption is not detected against the radio-bright QSO; if the background UV and radio sources are spatially aligned, the gas is either warm or clumpy (with spin temperature to covering factor ratio $T_{s}/f_{c} > 1880$ K). VLT-MUSE observations demonstrate that the $\alpha$-element abundance of the ionized ISM is consistent with the DLA ($\approx 10$% solar), suggesting that the neutral gas envelope is perturbed ISM gas. This study showcases the impact of dwarf-dwarf interactions on the physical and chemical state of neutral gas outside of star-forming regions. In the SKA era, joint UV and HI 21-cm analyses will be critical for connecting the cosmic neutral gas content to galaxy environments., Comment: Accepted for publication in ApJL. 16 pages, 4 figures, 2 tables

We report on the detection of a large, extended HI cloud complex in the GAMA G23 field, located at a redshift of $z\,\sim\,0.03$, observed as part of the MeerHOGS campaign (a pilot survey to explore the mosaicing capabilities of MeerKAT). The cloud complex, with a total mass of $10^{10.0}\,M_\odot$, lies in proximity to a large galaxy group with $M_\mathrm{dyn}\sim10^{13.5}\,M_\odot$. We identify seven HI peak concentrations, interconnected as a tenuous 'chain' structure, extending $\sim 400\,\mathrm{kpc}$ from east-to-west, with the largest (central) concentration containing $10{^{9.7}}\,M_\odot$ in HI gas distributed across $50\,\mathrm{kpc}$. The main source is not detected in ultra-violet, optical or infrared imaging. The implied gas mass-to-light ($M_\mathrm{HI}$/$L_\mathrm{r}$) is extreme ($>$1000) even in comparison to other 'dark clouds'. The complex has very little kinematic structure ($110\,\mathrm{km}\,\mathrm{s}^{-1}$), making it difficult to identify cloud rotation. Assuming pressure support, the total mass of the central concentration is $>10^{10.2}\,M_\odot$, while a lower limit to the dynamical mass in the case of full rotational support is $10^{10.4}\,M_\odot$. If the central concentration is a stable structure, it has to contain some amount of unseen matter, but potentially less than is observed for a typical galaxy. It is, however, not clear whether the structure has any gravitationally stable concentrations. We report a faint UV--optical--infrared source in proximity to one of the smaller concentrations in the gas complex, leading to a possible stellar association. The system nature and origins is enigmatic, potentially being the result of an interaction with or within the galaxy group it appears to be associated with., Comment: 25 pages, 10 figures, accepted for publication in ApJ, minor corrections

We present results from deep HI and optical imaging of AGC 229101, an unusual HI source detected at v$_{\rm helio}$ = 7116 km/s in the ALFALFA survey. Initially classified as a candidate "dark" source because it lacks a clear optical counterpart in SDSS or DSS2 imaging, AGC 229101 has $10^{9.31\pm0.05}$ solar masses of HI, but an HI line width of only 43$\pm$9 km/s. Low resolution WSRT imaging and higher resolution VLA B-array imaging show that the source is significantly elongated, stretching over a projected length of ~80 kpc. The HI imaging resolves the source into two parts of roughly equal mass. WIYN pODI optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the HI. The peak surface brightness of the optical source is only $\mu_{g}$ = 26.6 mag arcsec$^{-2}$, well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only $10^{7.32\pm0.33}$ solar masses, yielding an overall neutral gas-to-stellar mass ratio of M$_{\rm HI}$/M$_*=$~98$_{+111}\atop^{-52}$. We demonstrate the extreme nature of this object by comparing its properties to those of other HI-rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC~229101, including a tidal encounter with neighboring objects and a merger of two dark HI clouds., Comment: 18 pages, 8 figures, accepted by AJ

We report the discovery of large amounts of previously undetected cold neutral atomic hydrogen (\HI) around the core triplet galaxies in the nearby NGC~7232 galaxy group with MeerKAT. With a physical resolution of $\sim$1 kpc, we detect a complex web of low surface brightness \HI\ emission down to a 4$\sigma$ column density level of $\sim$1 $\times$ 10$^{19}$ cm$^{-2}$ (over 44 \kms ). The newly discovered H\,{\sc i} streams extend over $\sim$20 arcmin corresponding to 140~kpc in projection. This is $\sim$3 times the \HI\ extent of the galaxy triplet (52 kpc). The \HI\ debris has an \HI\ mass of $\sim$6.6 $\times 10^9$~M$_{\odot}$, more than 50\% of the total \HI\ mass of the triplet. Within the galaxy triplet, NGC~7233 and NGC~7232 have lost a significant amount of \HI\ while NGC~7232B appears to have an excess of \HI. The \HI\ deficiency in NGC~7232 and NGC~7233 indicates that galaxy-galaxy interaction in the group concentrates on this galaxy pair while the other disc galaxies have visited them over time. In comparison to the AMIGA sample of isolated galaxies we find that with regards to its total \HI\ mass the NGC~7232/3 galaxy triplet is not \HI\ deficient. Despite the many interactions associated to the triplet galaxies, no \HI\ seems to have been lost from the group (yet)., Comment: 17 pages, 11 figures, accepted for publication in MNRAS

ESO 149-G003 is a close-by, isolated dwarf irregular galaxy. Previous observations with the ATCA indicated the presence of anomalous neutral hydrogen (HI) deviating from the kinematics of a regularly rotating disc. We conducted follow-up observations with the MeerKAT radio telescope during the 16-dish Early Science programme as well as with the MeerLICHT optical telescope. Our more sensitive radio observations confirm the presence of anomalous gas in ESO 149-G003, and further confirm the formerly tentative detection of an extraplanar HI component in the galaxy. Employing a simple tilted-ring model, in which the kinematics is determined with only four parameters but including morphological asymmetries, we reproduce the galaxy's morphology, which shows a high degree of asymmetry. By comparing our model with the observed HI, we find that in our model we cannot account for a significant (but not dominant) fraction of the gas. From the differences between our model and the observed data cube we estimate that at least 7%-8% of the HI in the galaxy exhibits anomalous kinematics, while we estimate a minimum mass fraction of less than 1% for the morphologically confirmed extraplanar component. We investigate a number of global scaling relations and find that, besides being gas-dominated with a neutral gas-to-stellar mass ratio of 1.7, the galaxy does not show any obvious global peculiarities. Given its isolation, as confirmed by optical observations, we conclude that the galaxy is likely currently acquiring neutral gas. It is either re-accreting gas expelled from the galaxy or accreting pristine intergalactic material., Comment: 16 pages, 14 figures, MNRAS, Accepted 2020 December 2, in original form 2019 September 18

We present observations and models of the kinematics and the distribution of the neutral hydrogen (HI) in the isolated dwarf irregular galaxy, Wolf-Lundmark-Melotte (WLM). We observed WLM with the Green Bank Telescope (GBT) and as part of the MeerKAT Early Science Programme, where 16 dishes were available. The HI disc of WLM extends out to a major axis diameter of 30 arcmin (8.5 kpc), and a minor axis diameter of 20 arcmin (5.6 kpc) as measured by the GBT. We use the MeerKAT data to model WLM using the TiRiFiC software suite, allowing us to fit different tilted-ring models and select the one that best matches the observation. Our final best-fitting model is a flat disc with a vertical thickness, a constant inclination and dispersion, and a radially-varying surface brightness with harmonic distortions. To simulate bar-like motions, we include second-order harmonic distortions in velocity in the tangential and the vertical directions. We present a model with only circular motions included and a model with non-circular motions. The latter describes the data better. Overall, the models reproduce the global distribution and the kinematics of the gas, except for some faint emission at the 2-sigma level. We model the mass distribution of WLM with a pseudo-isothermal (ISO) and a Navarro-Frenk-White (NFW) dark matter halo models. The NFW and the ISO models fit the derived rotation curves within the formal errors, but with the ISO model giving better reduced chi-square values. The mass distribution in WLM is dominated by dark matter at all radii., Comment: Accepted for publication in MNRAS, 25 pages, 21 figures, 5 tables

MeerKATHI is the current development name for a radio-interferometric data reduction pipeline, assembled by an international collaboration. We create a publicly available end-to-end continuum- and line imaging pipeline for MeerKAT and other radio telescopes. We implement advanced techniques that are suitable for producing high-dynamic-range continuum images and spectroscopic data cubes. Using containerization, our pipeline is platform-independent. Furthermore, we are applying a standardized approach for using a number of different of advanced software suites, partly developed within our group. We aim to use distributed computing approaches throughout our pipeline to enable the user to reduce larger data sets like those provided by radio telescopes such as MeerKAT. The pipeline also delivers a set of imaging quality metrics that give the user the opportunity to efficiently assess the data quality., Comment: 4 pages, 1 figure, in conference proceedings of the ADASS XXIX, to appear in ASPC. CARACal (formerly MeerKATHI) can be accessed at: https://caracal.readthedocs.io

We present new high-resolution HI spectral line imaging of Coma P, the brightest HI source in the system HI 1232$+$20. This extremely low surface brightness galaxy was first identified in the ALFALFA survey as an "(Almost) Dark" object: a clearly extragalactic HI source with no obvious optical counterpart in existing optical survey data (although faint ultraviolet emission was detected in archival GALEX imaging). Using a combination of data from the Westerbork Synthesis Radio Telescope and the Karl G. Jansky Very Large Array, we investigate the HI morphology and kinematics at a variety of physical scales. The HI morphology is irregular, reaching only moderate maxima in mass surface density (peak $\sigma_{\rm HI}\sim 10$ $M_{\odot}$ pc$^{-2}$). Gas of lower surface brightness extends to large radial distances, with the HI diameter measured at 4.0$\pm$0.2 kpc inside the 1 $M_{\odot}$ pc$^{-2}$ level. We quantify the relationships between HI gas mass surface density and various types of star formation by considering GALEX far ultraviolet observations and H$\alpha$ nondetections. We describe Coma P's complex HI kinematics using spatially resolved position-velocity analysis and three-dimensional modeling. Both methods of analysis suggest that Coma P's kinematics show signatures of either the collision of two HI disks or a significant infall event. Coma P is just consistent (within 3$\sigma$) with the known M$_{\rm HI}$ -- D$_{\rm HI}$ scaling relation. It is either too large for its HI mass, has too low an HI mass for its HI size, or the two HI components artificially extend its HI size. Coma P lies within the empirical scatter at the faint end of the baryonic Tully--Fisher relation, although the complexity of the HI dynamics complicates the interpretation. The collective HI characteristics of Coma P make it unusual among known galaxies in the nearby universe. [Abridged], Comment: 26 pages, 17 figures. Published in the Astronomical Journal 155, 65

We discuss the ground-breaking science that will be possible with a wide area survey, using the MeerKAT telescope, known as MeerKLASS (MeerKAT Large Area Synoptic Survey). The current specifications of MeerKAT make it a great fit for science applications that require large survey speeds but not necessarily high angular resolutions. In particular, for cosmology, a large survey over $\sim 4,000 \, {\rm deg}^2$ for $\sim 4,000$ hours will potentially provide the first ever measurements of the baryon acoustic oscillations using the 21cm intensity mapping technique, with enough accuracy to impose constraints on the nature of dark energy. The combination with multi-wavelength data will give unique additional information, such as exquisite constraints on primordial non-Gaussianity using the multi-tracer technique, as well as a better handle on foregrounds and systematics. Such a wide survey with MeerKAT is also a great match for HI galaxy studies, providing unrivalled statistics in the pre-SKA era for galaxies resolved in the HI emission line beyond local structures at z > 0.01. It will also produce a large continuum galaxy sample down to a depth of about 5\,$\mu$Jy in L-band, which is quite unique over such large areas and will allow studies of the large-scale structure of the Universe out to high redshifts, complementing the galaxy HI survey to form a transformational multi-wavelength approach to study galaxy dynamics and evolution. Finally, the same survey will supply unique information for a range of other science applications, including a large statistical investigation of galaxy clusters as well as produce a rotation measure map across a huge swathe of the sky. The MeerKLASS survey will be a crucial step on the road to using SKA1-MID for cosmological applications and other commensal surveys, as described in the top priority SKA key science projects (abridged)., Comment: Larger version of the paper submitted to the Proceedings of Science, "MeerKAT Science: On the Pathway to the SKA", Stellenbosch, 25-27 May 2016

We use new deep 21 cm HI observations of the moderately inclined galaxy NGC 4559 in the HALOGAS survey to investigate the properties of extra-planar gas. We use TiRiFiC to construct simulated data cubes to match the HI observations. We find that a thick disk component of scale height $\sim\,2\,\mathrm{kpc}$, characterized by a negative vertical gradient in its rotation velocity (lag) of $\sim13 \pm 5$ km s$^{-1}$ kpc$^{-1}$ is an adequate fit to extra-planar gas features. The tilted ring models also present evidence for a decrease in the magnitude of the lag outside of $R_{25}$, and a radial inflow of $\sim 10$ km s$^{-1}$. We extracted lagging extra-planar gas through Gaussian velocity profile fitting. From both the 3D models and and extraction analyses we conclude that $\sim10-20\%$ of the total {\HI} mass is extra-planar. Most of the extra-planar gas is spatially coincident with regions of star formation in spiral arms, as traced by H$\alpha$ and GALEX FUV images, so it is likely due to star formation processes driving a galactic fountain. We also find the signature of a filament of a kinematically "forbidden" HI, containing $\sim 1.4\times 10^{6}$ M$_{\odot}$ of HI, and discuss its potential relationship to a nearby HI hole. We discover a previously undetected dwarf galaxy in HI located $\sim 0.4^{\circ}$ ($\sim 58$ kpc) from the center of NGC 4559, containing $\sim 4\times10^{5}$ M$_{\odot}$. This dwarf has counterpart sources in SDSS with spectra typical of HII regions, and we conclude it is two merging blue compact dwarf galaxies., Comment: Accepted for Publication in ApJ, 18 pages, 11 figures

We analyze the relationships between atomic, neutral hydrogen (HI) and star formation (SF) in the 12 low-mass SHIELD galaxies. We compare high spectral (~0.82 km/s/channel) and spatial resolution (physical resolutions of 170 pc - 700 pc) HI imaging from the VLA with H\alpha and far-ultraviolet imaging. We quantify the degree of co-spatiality between star forming regions and regions of high HI column densities. We calculate the global star formation efficiencies (SFE, $\Sigma_{\rm SFR}$ / $\Sigma_{\rm HI}$), and examine the relationships among the SFE and HI mass, HI column density, and star formation rate (SFR). The systems are consuming their cold neutral gas on timescales of order a few Gyr. While we derive an index for the Kennicutt-Schmidt relation of N ~ 0.68 $\pm$ 0.04 for the SHIELD sample as a whole, the values of N vary considerably from system to system. By supplementing SHIELD results with those from other surveys, we find that HI mass and UV-based SFR are strongly correlated over five orders of magnitude. Identification of patterns within the SHIELD sample allows us to bin the galaxies into three general categories: 1) mainly co-spatial HI and SF regions, found in systems with highest peak HI column densities and highest total HI masses, 2) moderately correlated HI and SF regions, found in systems with moderate HI column densities, and 3) obvious offsets between HI and SF peaks, found in systems with the lowest total HI masses. SF in these galaxies is dominated by stochasticity and random fluctuations in their ISM., Comment: Accepted for publication by the Astrophysical Journal, high-resolution version and survey data available at https://www.macalester.edu/~jcannon/shield/

We present kinematic analyses of the 12 galaxies in the "Survey of HI in Extremely Low-mass Dwarfs" (SHIELD). We use multi-configuration interferometric observations of the HI 21cm emission line from the Karl G. Jansky Very Large Array (VLA) to produce image cubes at a variety of spatial and spectral resolutions. Both two- and three-dimensional fitting techniques are employed in an attempt to derive inclination-corrected rotation curves for each galaxy. In most cases, the comparable magnitudes of velocity dispersion and projected rotation result in degeneracies that prohibit unambiguous circular velocity solutions. We thus make spatially resolved position-velocity cuts, corrected for inclination using the stellar components, to estimate the circular rotation velocities. We find circular velocities <30 km/s for the entire survey population. Baryonic masses are calculated using single-dish HI fluxes from Arecibo and stellar masses derived from HST and Spitzer imaging. Comparison is made with total dynamical masses estimated from the position-velocity analysis. The SHIELD galaxies are then placed on the baryonic Tully-Fisher relation. There exists an empirical threshold rotational velocity <15 km/s, below which current observations cannot differentiate coherent rotation from pressure support. The SHIELD galaxies are representative of an important population of galaxies whose properties cannot be described by current models of rotationally-dominated galaxy dynamics., Comment: Accepted for publication by the Astrophysical Journal, high-resolution version and survey data available at https://www.macalester.edu/~jcannon/shield/

We report a tight linear relation between the HI circular velocity measured at 6 $R_{\rm e}$ and the stellar velocity dispersion measured within 1 $R_{\rm e}$ for a sample of 16 early-type galaxies with stellar mass between $10^{10}$ and $10^{11}$ $\mathrm{M}_\odot$. The key difference from previous studies is that we only use spatially resolved $v_\mathrm{circ}$(HI) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that $v_\mathrm{circ}$(HI) = 1.33 $\sigma_\mathrm{e}$ with an observed scatter of just 12 percent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The $v_\mathrm{circ}$(HI)-$\sigma_\mathrm{e}$ relation is shallower than those based on $v_\mathrm{circ}$ measurements obtained from stellar kinematics and modelling at smaller radius, implying that \vcirc\ declines with radius -- as in bulge-dominated spirals. Indeed, the value of $v_\mathrm{circ}$(HI) is typically 25 percent lower than the maximum $v_\mathrm{circ}$ derived at $\sim0.2\ R_\mathrm{e}$ from dynamical models. Under the assumption of power-law total density profiles $\rho \propto r^{-\gamma}$, our data imply an average logarithmic slope $\langle\gamma\rangle=2.18\pm0.03$ across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies., Comment: MNRAS, accepted

We study the HI K-band Tully-Fisher relation and the baryonic Tully-Fisher relation for a sample of 16 early-type galaxies, taken from the ATLAS3D sample, which all have very regular HI disks extending well beyond the optical body (> 5 R_eff). We use the kinematics of these disks to estimate the circular velocity at large radii for these galaxies. We find that the Tully-Fisher relation for our early-type galaxies is offset by about 0.5-0.7 magnitudes from the relation for spiral galaxies. The residuals with respect to the spiral Tully-Fisher relation correlate with estimates of the stellar mass-to-light ratio, suggesting that the offset between the relations is mainly driven by differences in stellar populations. We also observe a small offset between our Tully-Fisher relation with the relation derived for the ATLAS3D sample based on CO data representing the galaxies' inner regions (< 1 R_eff). This indicates that the circular velocities at large radii are systematically 10% lower than those near 0.5-1 R_eff, in line with recent determinations of the shape of the mass profile of early-type galaxies. The baryonic Tully-Fisher relation of our sample is distinctly tighter than the standard one, in particular when using mass-to-light ratios based on dynamical models of the stellar kinematics. We find that the early-type galaxies fall on the spiral baryonic Tully-Fisher relation if one assumes M/L_K = 0.54 M_sun/L_sun for the stellar populations of the spirals, a value similar to that found by recent studies of the dynamics of spiral galaxies. Such a mass-to-light ratio for spiral galaxies would imply that their disks are 60-70% of maximal. Our analysis increases the range of galaxy morphologies for which the baryonic Tully-Fisher relations holds, strengthening previous claims that it is a more fundamental scaling relation than the classical Tully-Fisher relation., Comment: Accepted for publication in Astronomy & Astrophysics

Using data taken as part of the Bluedisk project we study the connection between neutral hydrogen (HI) in the environment of spiral galaxies and that in the galaxies themselves. We measure the total HI mass present in the environment in a statistical way by studying the distribution of noise peaks in the HI data cubes obtained for 40 galaxies observed with WSRT. We find that galaxies whose HI mass fraction is high relative to standard scaling relations have an excess HI mass in the surrounding environment as well. Gas in the environment consists of gas clumps which are individually below the detection limit of our HI data. These clumps may be hosted by small satellite galaxies and\or be the high-density peaks of a more diffuse gas distribution in the inter-galactic medium. We interpret this result as an indication for a picture in which the HI-rich central galaxies accrete gas from an extended gas reservoir present in their environment., Comment: 15 pages, 13 figures. Accepted for publication in MNRAS

As part of the Bluedisk survey we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies We compare the metallicity profiles of a sample of HI-rich galaxies against a control sample of HI-'normal' galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its HI mass fraction (M(HI) / Mstar). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the HI-rich and control samples. This contradicts a previous indication that these outer drops are exclusive to HI-rich galaxies. These effects are not driven by bars, although we do find some indication that barred galaxies have flatter metallicity profiles. By applying a simple analytical model we are able to account for the variety of metallicity profiles that the two samples present. The success of this model implies that the metallicity in these isolated galaxies may be in a local equilibrium, regulated by star formation. This insight could provide an explanation of the observed local mass-metallicity relation., Comment: Accepted for publication in MNRAS

Our work is based on the "Bluedisk" project, a program to map the neutral gas in a sample of 25 HI-rich spirals and a similar number of control galaxies with the Westerbork Synthesis Radio Telescope (WSRT). In this paper we focus on the HI properties of the galaxies in the environment of our targeted galaxies. In total, we extract 65 galaxies from the WSRT cubes with stellar masses between $10^8M_{\odot}$ and $10^{11}M_{\odot}$. Most of these galaxies are located on the same HI mass-size relation and "HI-plane" as normal spiral galaxies. We find that companions around HI-rich galaxies tend to be HI-rich as well and to have larger R90,HI/R50,HI. This suggests a scenario of "HI conformity", similar to the colour conformity found by Weinmann et al. (2006): galaxies tend to adopt the HI properties of their neighbours. We visually inspect the outliers from the HI mass-size relation and galaxies which are offset from the HI plane and find that they show morphological and kinematical signatures of recent interactions with their environment. We speculate that these outliers have been disturbed by tidal or ram-pressure stripping processes, or in a few cases, by accretion events., Comment: 16 pages, 12 figures; accepted for publication in MNRAS

We present VLA HI spectral line imaging of 5 sources discovered by ALFALFA. These targets are drawn from a larger sample of systems that were not uniquely identified with optical counterparts during ALFALFA processing, and as such have unusually high HI mass to light ratios. These candidate "Almost Dark" objects fall into 4 categories: 1) objects with nearby HI neighbors that are likely of tidal origin; 2) objects that appear to be part of a system of multiple HI sources, but which may not be tidal in origin; 3) objects isolated from nearby ALFALFA HI detections, but located near a gas-poor early-type galaxy; 4) apparently isolated sources, with no object of coincident redshift within ~400 kpc. Roughly 75% of the 200 objects without identified counterparts in the $\alpha$.40 database (Haynes et al. 2011) fall into category 1. This pilot sample contains the first five sources observed as part of a larger effort to characterize HI sources with no readily identifiable optical counterpart at single dish resolution. These objects span a range of HI mass [7.41 < log(M$_{\rm HI}$) < 9.51] and HI mass to B-band luminosity ratios (3 < M$_{\rm HI}$/L$_{\rm B}$ < 9). We compare the HI total intensity and velocity fields to SDSS optical imaging and to archival GALEX UV imaging. Four of the sources with uncertain or no optical counterpart in the ALFALFA data are identified with low surface brightness optical counterparts in SDSS imaging when compared with VLA HI intensity maps, and appear to be galaxies with clear signs of ordered rotation. One source (AGC 208602) is likely tidal in nature. We find no "dark galaxies" in this limited sample. The present observations reveal complex sources with suppressed star formation, highlighting both the observational difficulties and the necessity of synthesis follow-up observations to understand these extreme objects. (abridged), Comment: Astronomical Journal, in press

We analyze the radial distribution of HI gas for 23 disk galaxies with unusually high HI content from the Bluedisk sample, along with a similar-sized sample of "normal" galaxies. We propose an empirical model to fit the radial profile of the HI surface density, an exponential function with a depression near the center. The radial HI surface density profiles are very homogeneous in the outer regions of the galaxy; the exponentially declining part of the profile has a scale-length of $\sim 0.18$ R1, where R1 is the radius where the column density of the HI is 1 M$_{\odot}$ pc$^{-2}$. This holds for all galaxies, independent of their stellar or HI mass. The homogenous outer profiles, combined with the limited range in HI surface density in the non-exponential inner disk, results in the well-known tight relation between HI size and HI mass. By comparing the radial profiles of the HI-rich galaxies with those of the control systems, we deduce that in about half the galaxies, most of the excess gas lies outside the stellar disk, in the exponentially declining outer regions of the HI disk. In the other half, the excess is more centrally peaked. We compare our results with existing smoothed-particle hydrodynamical simulations and semi-analytic models of disk galaxy formation in a $\Lambda$ Cold Dark Matter universe. Both the hydro simulations and the semi-analytic models reproduce the HI surface density profiles and the HI size-mass relation without further tuning of the simulation and model inputs. In the semi-analytic models, the universal shape of the outer HI radial profiles is a consequence of the {\em assumption} that infalling gas is always distributed exponentially. The conversion of atomic gas to molecular form explains the limited range of HI surface densities in the inner disk. These two factors produce the tight HI mass-size relation., Comment: 15 pages, 14 figures, submitted to MNRAS

We have examined star formation in two very luminous (M_V=-22 to -23) Sc-type spiral galaxies, NGC 801 and UGC 2885, using ultra-deep Halpha images. We combine these with UBV and 2MASS JHK images and HI maps to explore the star formation characteristics of disk galaxies at high luminosity. Halpha traces star formation in these galaxies to 4-6 disk scale lengths, but the lack of detection of Halpha further out is likely due to loss of Lyman continuum photons. Considering gravitational instabilities alone, we find that the gas and stars in the outer regions are marginally stable in an average sense, but considering dissipative gas and radial and azimuthal forcing, the outer regions are marginally unstable to form spiral arms. Star formation is taking place in spiral arms, which are regions of locally higher gas densities. Furthermore, we have traced smooth exponential stellar disks over 3-orders of magnitude and 4-6 disk scale lengths, in spite of a highly variable gravitational instability parameter. Thus, gravitational instability thresholds do not seem relevant to the stellar disk. One possibility for creating an exponential disk is that the molecular cloud densities and star formation rates have exponential profiles and this forces the stellar disk to build up such a profile. Another possibility is that the stellar disk is continuously adjusted to an exponential shape regardless of the star formation profile, for example through global dynamical process that scatter stars. However, such scattering processes are only known to operate in spiral systems, in which case they cannot explain the same dilemma of smooth exponential disks observed in dwarf irregular galaxies., Comment: To be published in AJ

We introduce the "Bluedisk" project, a large program at the Westerbork Synthesis Radio Telescope (WSRT) that has mapped the HI in a sample of 23 nearby galaxies with unusually high HI mass fractions, along with a similar-sized sample of control galaxies matched in stellar mass, size, inclination and redshift. This paper presents the sample selection, observational set-up, data reduction strategy, and a first analysis of the sizes and structural properties of the HI disks. We find that the HI-rich galaxies lie on the same HI mass versus HI size relation as normal spiral galaxies, extending it to total HI masses of $2 \times 10^{10} M_{\odot}$ and radii R1 of $\sim 100$ kpc (where R1 is defined as the radius where the HI column density reaches 1 $M_{\odot}$ pc$^{-2}$). HI-rich galaxies have significantly larger values of HI-to-optical size ratio at fixed stellar mass, concentration index, stellar and star formation rate surface density compared to the control sample. The disks of HI-rich galaxies are also significantly more clumpy (i.e. have higher HI Gini and $\Delta$Area coefficient) than those of normal spirals. There is no evidence that the disks of HI-rich galaxies are more disturbed: HI-rich galaxies exhibit no difference with respect to control samples in their distributions of HI asymmetry indices or optical/HI disk position angle differences. In fact, the center of the HI distribution corresponds more closely with the center of the optical light in the HI-rich galaxies than in the controls. All these results argue against a scenario in which new gas has been brought in by mergers. It is possible that they may be more consistent with cooling from a surrounding quasi-static halo of warm/hot gas., Comment: 26 pages, 14 figures. Submitted to MNRAS

Galaxy formation is significantly modulated by energy output from supermassive black holes at the centers of galaxies which grow in highly efficient luminous quasar phases. The timescale on which black holes transition into and out of such phases is, however, unknown. We present the first measurement of the shutdown timescale for an individual quasar using X-ray observations of the nearby galaxy IC 2497, which hosted a luminous quasar no more than 70,000 years ago that is still seen as a light echo in `Hanny's Voorwerp', but whose present-day radiative output is lower by at least 2 and more likely by over 4 orders of magnitude. This extremely rapid shutdown provides new insights into the physics of accretion in supermassive black holes, and may signal a transition of the accretion disk to a radiatively inefficient state., Comment: 4 pages, 2 figures. Astrophysical Journal Letters, in press

This is the first of a series of papers in which the kinematics of disk galaxies over a range of scales is scrutinised employing spectroscopy. A fundamental aspect of these studies is presented here: the new publicly available software tool TiRiFiC (http://www.astro.uni-bonn.de/~gjozsa/tirific.html) enables a direct fit of a ``tilted-ring model'' to spectroscopic data cubes. The algorithm generates model data cubes from the tilted-ring parametrisation of a rotating disk, which are automatically adjusted to reach an optimum fit via a chi-squared minimisation method to an observed data cube. The structure of the new software, the shortcomings of the previously available programs to produce a tilted-ring model, and the performance of TiRiFiC are discussed. Our method is less affected by the well-known problem of beam smearing that occurs when fitting to the velocity field. Since with our method we fit many data points in a data cube simultaneously, TiRiFiC is sensitive to very faint structures and can hence be used to derive tilted-ring models significantly extending in radius beyond those derived from a velocity field. The software is able to parametrise HI disks of galaxies that are intersected by the line-of-sight twice or more, i.e. if the disks are heavily warped, and/or with a significant shift of the projected centre of rotation, and/or if seen edge-on. Furthermore, our method delivers the surface-brightness profile of the examined galaxy in addition to the orientational parameters and the rotation curve. In order to derive kinematic and morphological models of disk galaxies, especially reliable rotation curves, a direct-fit method as implemented in our code should be the tool of choice., Comment: 37 pages, 24 figures. Accepted for publication in Astronomy and Astrophysics