We introduce Nessie, a galaxy group finder implemented in Rust and distributed as both a Python and R package. Nessie employs the friends-of-friends (FoF) algorithm and requires only on-sky position and redshift as input, making it immediately applicable to surveys that lack a well-defined luminosity function. We implement several algorithmic optimizations-including binary search and k-d tree pre-selection-that significantly improve performance by reducing unnecessary galaxy pair checks. To validate the accuracy of Nessie, we tune its parameters using a suite of GALFORM mock lightcones and achieve a strong Figure of Merit. We further demonstrate its reliability by applying it to both the GAMA and SDSS surveys, where it produces group catalogues consistent with those in the literature. Additional functionality is included for comparison with simulations and mock catalogues. Benchmarking on a standard MacBook Pro (M3 chip with 11 cores) shows that version 1 of Nessie can process similar to 1 million galaxies in similar to 10 s, highlighting its speed and suitability for next-generation redshift surveys.

We investigate how galaxy evolution varies with environment in the nearby Universe by comparing an “average” reference volume in the Southern Galactic Pole (SGP) dataset from to the Nexus region, a dynamically assembling superstructure centred on the Abell 4038 galaxy cluster. We quantify environmental effects using the quenched fraction (f_Q) and the specific star formation rate (sSFR) for the star-forming population, measured as functions of stellar mass and group-scale halo mass from . We decouple the stellar–halo mass dependence, demonstrating that f_Q increases with stellar mass in both field and group environments, while group galaxies show an additional dependence on halo mass. The Nexus exhibits systematic differences relative to the SGP baseline, consistent with increased heterogeneity in accretion histories and pre-processing within a forming superstructure. For star-forming galaxies, the mean log(sSFR) declines strongly with stellar mass and shows additional environment-linked suppression in group-scale halos. Within the Nexus, splitting the sample into three projected radial zones around Abell 4038 shows that environmental regulation is not spatially uniform, driven largely by variations in the sampled halo mass function. Finally, a projected phase-space (PPS) analysis of Abell 4038 links quenching to orbital history within the cluster, though this trend is strongly mass-dependent: low-mass galaxies (log(M_stellar) < 10) show no significant change in f_Q. These results demonstrate that the drivers of galaxy evolution depend jointly on stellar mass, local group halo mass, and location within the surrounding large-scale structure, motivating future large-scale, multi-wavelength cosmic web surveys.

Accurate estimation of dark matter halo masses for galaxy groups is central to studies of galaxy evolution and for leveraging group catalogues as cosmological probes. In this work, we present a comprehensive evaluation and calibration of two complementary halo mass estimators: a dynamical estimator based on a modified virial theorem (MVT) and an empirical summed stellar mass to halo mass relation (sSHMR), which uses the summed mass of the three most massive group galaxies as a proxy for halo mass. Using a suite of state-of-the-art semi-analytic models (SAMs; Shark, SAGE, and GAEA) to produce observationally motivated mock light-cone catalogues, we rigorously quantify the accuracy, uncertainty, and model dependence of each method. The MVT halo mass estimator achieves negligible systematic bias (mean Delta = -0.01 dex) and low scatter (mean sigma = 0.20 dex) as a function of the predicted halo mass, with no sensitivity to the SAM baryonic physics. The calibrated sSHMR yields the highest precision, with mean Delta = 0.02 dex and mean sigma= 0.14 dex as a function of the predicted halo mass but exhibits greater model dependence due to its sensitivity to varying baryonic physics and physical prescriptions across the SAMs. We demonstrate the application of these estimators to observational group catalogues, including the construction of the empirical halo mass function and the mapping of quenched fractions in the stellar mass-halo mass plane. We provide clear guidance on the optimal application of each method: the MVT is recommended for GAMA-like surveys (i < 19.2) calibrated to z < 0.1 and should be used for studies that require minimal model dependence, while the sSHMR is optimal for high-precision halo mass estimation across diverse catalogues with magnitude limits of Z < 21.2 or brighter and to redshifts of z <= 0.3. These calibrated estimators will be of particular value for upcoming wide-area spectroscopic surveys, enabling robust and precise analyses between the galaxy-halo connection and the underlying dark matter distribution.

Large-scale galaxy redshift surveys conducted over the last couple of decades have proven crucial in deepening our understanding of structure growth in the Universe and galaxy evolution. While there have been several such surveys, until now those that achieve the high completeness and precision necessary to probe the low-mass end of galaxy groups have been limited to relatively low redshifts (z≲0.3), with surveys exploring the more distant Universe being constrained by small sample sizes and/or low redshift completeness. The recent Deep Extragalactic VIsible Legacy Survey (DEVILS) aims to explore galaxy environment over the last ∼6 Gyr with a completeness level comparable to the most complete local Universe surveys (>85%). In this work, we present the galaxy group catalogue for the D10-COSMOS field from DEVILS, which achieves a redshift completeness of 90% for galaxies with Y<21.2 mag. We showcase the science potential by exploring the impact of environment on the fraction and power of active galactic nuclei (AGN), finding that satellites in galaxy groups show no evidence of altered AGN properties, while satellites in clusters exhibit increased AGN fractions but decreased AGN luminosities.

The ALMA [CII] Resolved Ism in STar-forming gALaxies (CRISTAL) survey is a Cycle 8 ALMA Large Programme that studies the cold gas component of high-redshift galaxies. Its sub-arcsecond resolution observations are key to disentangling physical mechanisms that shape galaxies during cosmic dawn. In this paper, we explore the morphology and kinematics of the cold gas, star-forming, and stellar components in the star-forming main-sequence galaxy CRISTAL-05/HZ3, at z = 5.54. Our analysis includes 0.3" spatial resolution ( 2 kpc) ALMA observations of the [CII] line. While CRISTAL-05 was previously classified as a single source, our observations reveal that the system is a close interacting pair surrounded by an extended component of carbon-enriched gas. This is imprinted in the disturbed elongated [CII] morphology and the separation of the two components in the position-velocity diagram ( 100 km/s). The central region is composed of two components, named C05-NW and C05-SE, with the former being the dominant one. A significant fraction of the [CII] arises beyond the close pair up to 10 kpc, while the regions forming new massive stars and the stellar component seem compact (r_[CII] 4 r_UV), as traced by rest-frame UV and optical imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope. Our kinematic model, using the DYSMALpy software, yields a minor contribution of dark matter of C05-NW within a radius of 2x Reff. Finally, we explore the resolved [CII]/FIR ratios as a proxy for shock-heating produced by this merger. We argue that the extended [CII] emission is mainly caused by the merger, which could not be discerned with lower-resolution observations. Our work emphasizes the need for high-resolution observations to fully characterize the dynamic stages of infant galaxies and the physical mechanisms that drive the metal enrichment of the circumgalactic medium.

High-redshift quasars are thought to live in the densest regions of space, which should be made evident by an overdensity of galaxies around them. However, campaigns to identify these overdensities by searching for Lyman-break galaxies (LBGs) and Lyman alpha emitters (LAEs) have had mixed results. These may be explained by either the small field of view of some of the experiments, the broad redshift ranges targeted by LBG searches, and the inherently high uncertainty of quasar redshifts estimated from ultraviolet emission lines, which makes it difficult to place the Ly-alpha emission line within a narrowband filter. Here, we present a 3 square degree search (similar to 1000 pMpc(2)) for LAEs around the z = 6.9 quasar VIK J2348-3054 using the Dark Energy CAMera (DECam) housed on the 4m Blanco telescope, finding 38 LAEs. The systemic redshift of VIK J2348-3054 is known from ALMA [CII] observations and places the Ly-alpha emission line of companions within the NB964 narrowband of DECam. This is the largest field-of-view LAE search around a z > 6 quasar conducted to date. We find that this field is similar to ten times more overdense than Chandra Deep-Field South, observed previously with the same instrumental setup as well as several combined blank fields. This is strong evidence that VIK J2348-3054 resides in an overdensity of LAEs over several Mpc. Surprisingly, we find a lack of LAEs within 5 physical Mpc of the quasar and take this to most likely be evidence of quasar-suppressing star formation in its immediate vicinity. This result highlights the importance of performing overdensity searches over large areas to properly assess the density of those regions of the Universe.

Observations with the Hubble Space Telescope unexpectedly revealed that the dwarf galaxy ESO 006-001 is a near neighbor to the Local Group at a distance of 2.70 +- 0.11 Mpc. The stellar population in the galaxy is well resolved into individual stars to a limit of M I -0.5 mag. The dominant population is older than 12 Gyr yet displays a significant range in metallicity of -2 < [Fe/H] < -1, as evidenced by a Red Giant Branch with substantial width. Superimposed on the dominant population are stars on the Main Sequence with ages less than 100 Myr and Helium burning Blue Loop stars with ages of several hundred Myr. ESO 006-001 is an example of a transition dwarf; a galaxy dominated by old stars but one that has experienced limited recent star formation in a swath near the center. No H i gas is detected at the location of the optical galaxy in spite of the evidence for young stars. Intriguingly, an H i cloud with a similar redshift is detected 9 kpc away in projection. Otherwise, ESO 006-001 is a galaxy in isolation with its nearest known neighbor IC 3104, itself a dwarf, at a distance of 500 kpc.

High-redshift observations are often biased towards massive and bright galaxies that are not necessarily representative of the full population. In order to accurately study galaxy evolution and mass assembly at these redshifts, observations of ``normal'' main sequence galaxies are required. Here we present Atacama Large Millimeter/Submillimeter Array (ALMA) 0.3" resolution observations of the [CII] emission line at 158$\mu$m of HZ7, a main sequence galaxy at $z=5.25$. Comparing to archival rest-frame UV observations taken by the Hubble Space Telescope (HST), we find strong evidence of the existence of extended [CII] emission, which we estimate to be twice the size of the rest-frame UV emission, yielding one of the first high-redshift objects where a clear signature of a [CII] ``Halo'' has been detected to date. For a matched S\'ersic profile with n = 1, we measured a [CII] effective radius of $0.50\pm 0.04$" (3.07$\pm 0.25$ kpc) and an average rest-frame UV effective radius of $0.2\pm0.04$" ($1.48\pm0.16$ kpc). The [CII] morphology and kinematics of the system suggest a merging event resulting in a non rotating disk system. This event could be responsible for the extended [CII] emission. Alternatively, some potential obscured emission could also explain the [CII] to UV size ratio. These results contribute to the growing consensus with respect to the existence of extended [CII] emission around galaxies.

Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium can provide a unique view of the kinematics associated with the assembly of galaxies. In this paper, we analyzed the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158um line emission obtained with the Atacama Large Millimeter/submillimeter Array. These observations allowed us to compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using 3DBarolo, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the north-south direction suggesting ordered motion, as commonly found in rotating-disk galaxies. But higher-resolution observations would be needed to rule out a compact merger scenario. Our model indicates a low average velocity dispersion, σ < 30 km s^-1. This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will be possibly perturbed at later times by accreting sources.

We present the galaxy group catalogue for the recently-completed 2MASS Redshift Survey (2MRS, Macri2019) which consists of 44572 redshifts, including 1041 new measurements for galaxies mostly located within the Zone of Avoidance. The galaxy group catalogue is generated by using a novel, graph-theory based, modified version of the Friends-of-Friends algorithm. Several graph-theory examples are presented throughout this paper, including a new method for identifying substructures within groups. The results and graph-theory methods have been thoroughly interrogated against previous 2MRS group catalogues and a Theoretical Astrophysical Observatory (TAO) mock by making use of cutting-edge visualization techniques including immersive facilities, a digital planetarium, and virtual reality. This has resulted in a stable and robust catalogue with on-sky positions and line-of-sight distances within 0.5 Mpc and 2 Mpc, respectively, and has recovered all major groups and clusters. The final catalogue consists of 3022 groups, resulting in the most complete "whole-sky" galaxy group catalogue to date. We determine the 3D positions of these groups, as well as their luminosity and comoving distances, observed and corrected number of members, richness metric, velocity dispersion, and estimates of $R_{200}$ and $M_{200}$. We present three additional data products, i.e. the 2MRS galaxies found in groups, a catalogue of subgroups, and a catalogue of 687 new group candidates with no counterparts in previous 2MRS-based analyses.

The Two Micron All-Sky Survey (2MASS) Redshift Survey was started two decades ago with the goal of mapping the three-dimensional distribution of an all-sky flux-limited (K-s < 11.75 mag) sample of ?45,000galaxies. Our first data release presented an unprecedented uniform coverage for most of the celestial sphere, with redshifts for ?98% of our sample. However, we were missing redshifts for ?18% of the catalog entries that were located within the ?Zone of Avoidance? (<CDATA<i)?an important region of the sky for studies of the large-scale structure and cosmic flows. In this second and final data release, we present redshifts for all 1041 2MRS galaxies that previously lacked this information, as well as updated measurements for 27 others.

We present the results of a multisite photometric observing campaign on the rapidly oscillating Ap (roAp) star 2MASS 16400299-0737293 (J1640; V = 12.7). We analyse photometric B data to show the star pulsates at a frequency of 151.93 d(-1) (1758.45 mu Hz; P = 9.5 min) with a peak-to-peak amplitude of 20.68 mmag, making it one of the highest amplitude roAp stars. No further pulsation modes are detected. The stellar rotation period is measured at 3.674 7 +/- 0.000 5 d, and we show that rotational modulation due to spots is in antiphase between broad-band and B observations. Analysis and modelling of the pulsation reveals this star to be pulsating in a distorted quadrupole mode, but with a strong spherically symmetric component. The pulsational phase variation in this star is suppressed, leading to the conclusion that the contribution of l > 2 components dictate the shape of phase variations in roAp stars that pulsate in quadrupole modes. This is only the fourth time such a strong pulsation phase suppression has been observed, leading us to question the mechanisms at work in these stars. We classify J1640 as an A7 Vp SrEu(Cr) star through analysis of classification resolution spectra.