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.
Here we report the discovery with the Giant Metrewave Radio Telescope of an extremely large (\(\sim\)115 kpc in diameter) HI ring off-centered from a massive quenched galaxy, AGC 203001. This ring ...does not have any bright extended optical counterpart, unlike several other known ring galaxies. Our deep \(g\), \(r\), and \(i\) optical imaging of the HI ring, using the MegaCam instrument on the Canada-France-Hawaii Telescope, however, shows several regions with faint optical emission at a surface brightness level of \(\sim\)28 mag/arcsec\(^2\). Such an extended HI structure is very rare with only one other case known so far -- the Leo ring. Conventionally, off-centered rings have been explained by a collision with an "intruder" galaxy leading to expanding density waves of gas and stars in the form of a ring. However, in such a scenario the impact also leads to large amounts of star formation in the ring which is not observed in the ring presented in this paper. We discuss possible scenarios for the formation of such HI dominated rings.
We present a novel algorithm based on a Bayesian method for 2D tilted-ring analysis of disk galaxy velocity fields. Compared to the conventional algorithms based on a chi-squared minimisation ...procedure, this new Bayesian-based algorithm suffers less from local minima of the model parameters even with highly multi-modal posterior distributions. Moreover, the Bayesian analysis, implemented via Markov Chain Monte Carlo (MCMC) sampling, only requires broad ranges of posterior distributions of the parameters, which makes the fitting procedure fully automated. This feature will be essential when performing kinematic analysis on the large number of resolved galaxies expected to be detected in neutral hydrogen (HI) surveys with the Square Kilometre Array (SKA) and its pathfinders. The so-called '2D Bayesian Automated Tilted-ring fitter' (2DBAT) implements Bayesian fits of 2D tilted-ring models in order to derive rotation curves of galaxies. We explore 2DBAT performance on (a) artificial HI data cubes built based on representative rotation curves of intermediate-mass and massive spiral galaxies, and (b) Australia Telescope Compact Array (ATCA) HI data from the Local Volume HI Survey (LVHIS). We find that 2DBAT works best for well-resolved galaxies with intermediate inclinations (20 deg < i < 70 deg), complementing three-dimensional techniques better suited to modelling inclined galaxies.
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.
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.
We present the full panchromatic afterglow light curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all ...archival data, and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups, and attempted to mitigate these differences to provide a more uniform dataset. Simple power-law fits to the uniform afterglow light curve indicate a \(t^{0.86\pm0.04}\) rise, a \(t^{-1.92\pm0.12}\) decline, and a peak occurring at \(155\pm4\) days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index (\(-0.584\pm0.002\)) across all epochs. This gives a precise and updated estimate of the electron power-law index, \(p=2.168\pm0.004\). By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized ISM density, \(<\)0.01 cm\(^{-3}\), consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having magnetic field strength between 10\(^{10.4}\) G and 10\(^{16}\) G. Our fits to the afterglow data using an analytical model that includes VLBI proper motion from Mooley et al. (2018), and a structured jet model that ignores the proper motion, indicates that the proper motion measurement needs to be considered while seeking an accurate estimate of the viewing angle.
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).
The `Local Volume HI Survey' (LVHIS) comprises deep HI spectral line and 20-cm radio continuum observations of 82 nearby, gas-rich galaxies, supplemented by multi-wavelength images. Our sample ...consists of all galaxies with Local Group velocities v_LG < 550 km/s or distances D < 10 Mpc that are detected in the HI Parkes All Sky Survey (HIPASS). Using full synthesis observations in at least three configurations of the Australia Telescope Compact Array (ATCA), we obtain detailed HI maps for a complete sample of gas-rich galaxies with Dec < -30 deg. Here we present a comprehensive LVHIS Galaxy Atlas, including the overall gas distribution, mean velocity field, velocity dispersion, and position-velocity diagrams, together with a homogeneous set of measured and derived galaxy properties. Our primary goal is to investigate the HI morphologies, kinematics, and environment at high resolution and sensitivity. LVHIS galaxies represent a wide range of morphologies and sizes; our measured HI masses range from ~10^7 to 10^10 Msun, based on independent distance estimates. The LVHIS Galaxy Atlas (including FITS files) is available on-line.
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.
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.
