We present a multiwavelength study of the double radio relic cluster A1240 at z=0.195. Our Subaru-based weak lensing analysis detects three mass clumps forming a ~4 Mpc filamentary structure elongated in the north-south orientation. The northern ($M_
{200}=2.61_{-0.60}^{+0.51}times10^{14} M_{odot}$) and middle ($M_{200}=1.09_{-0.43}^{+0.34}times10^{14} M_{odot}$) mass clumps separated by ~1.3 Mpc are associated with A1240 and co-located with the X-ray peaks and cluster galaxy overdensities revealed by Chandra and MMT/Hectospec observations, respectively. The southern mass clump ($M_{200}=1.78_{-0.55}^{+0.44}times10^{14} M_{odot}$), ~1.5 Mpc to the south of the middle clump, coincides with the galaxy overdensity in A1237, the A1240 companion cluster at z=0.194. Considering the positions, orientations, and polarization fractions of the double radio relics measured by the LOFAR study, we suggest that A1240 is a post-merger binary system in the returning phase with the time-since-collision ~1.7 Gyr. With the SDSS DR16 data analysis, we also find that A1240 is embedded in the much larger-scale (~80 Mpc) filamentary structure whose orientation is in remarkable agreement with the hypothesized merger axis of A1240.
The detection of an intermediate-mass black hole population ($10^2-10^6 M_odot$) will provide clues to their formation environments (e.g., disks of active galactic nuclei, globular clusters) and illuminate a potential pathway to produce supermassive
black holes. Ground-based gravitational-wave detectors are sensitive to a subset of such mergers and have been used to detect one $142^{+28}_{-16} M_odot$ intermediate-mass black hole formation event. However, ground-based detector data contain numerous incoherent short duration noise transients that can mimic the gravitational-wave signals from merging intermediate-mass black holes, limiting the sensitivity of searches. Here we search for binary black hole mergers using a Bayesian-inspired ranking statistic which measures the coherence or incoherence of triggers in multiple-detector data. We use this statistic to identify candidate events with lab-frame total masses $gtrsim55 M_odot$ using data from LIGOs second observing run. Our analysis does not yield evidence for new intermediate-mass black holes. However, we find support for some stellar-mass binary black holes not reported in the first LIGO--Virgo gravitational-wave transient catalog, GWTC-1.
In our companion paper (Brought to Light I: Michea et al. 2021), we reveal spectacular spiral galaxy-like features in deep optical imaging of nine Virgo early-type dwarf galaxies, hidden beneath a dominating smooth stellar disk. Using a new combinati
on of approaches, we find that bar- and spiral-like features contribute 2.2-6.4% of the total flux within 2 R$_{rm{eff}}$. In this study, we conduct high resolution simulations of cluster harassment of passive dwarf galaxies. Following close pericenter passages of the cluster core, tidal triggering generates features in our model disks that bear a striking resemblance to the observed features. However, we find the disks must be highly rotationally supported (V$_{rm{peak}}/sigma_0 sim 3$), much higher than typically observed. We propose that some early-type dwarfs may contain a few percent of their mass in a cold, thin disk which is buried in the light of a hot, diffuse disk, and only revealed when they undergo tidal triggering. The red optical colors of our sample do not indicate any recent significant star formation, and our simulations show that very plunging pericenter passages (r$_{rm{peri}}<0.25$r$_{rm{vir}}$) are required for tidal triggering. Thus, many cluster early-type dwarfs with less plunging orbits may host a yet undetected cold stellar disk component. We discuss possible origin scenarios and consider why similar mass star-forming galaxies in the field are significantly more thin disk dominated than in our cluster sample.
Third-generation (3G) gravitational-wave detectors will observe thousands of coalescing neutron star binaries with unprecedented fidelity. Extracting the highest precision science from these signals is expected to be challenging owing to both high si
gnal-to-noise ratios and long-duration signals. We demonstrate that current Bayesian inference paradigms can be extended to the analysis of binary neutron star signals without breaking the computational bank. We construct reduced order models for $sim 90,mathrm{minute}$ long gravitational-wave signals, covering the observing band ($5-2048,mathrm{Hz}$), speeding up inference by a factor of $sim 1.3times 10^4$ compared to the calculation times without reduced order models. The reduced order models incorporate key physics including the effects of tidal deformability, amplitude modulation due to the Earths rotation, and spin-induced orbital precession. We show how reduced order modeling can accelerate inference on data containing multiple, overlapping gravitational-wave signals, and determine the speedup as a function of the number of overlapping signals. Thus, we conclude that Bayesian inference is computationally tractable for the long-lived, overlapping, high signal-to-noise-ratio events present in 3G observatories.
We study the stellar and gas kinematics of the brightest group galaxies (BGGs) in dynamically relaxed and unrelaxed galaxy groups for a sample of 154 galaxies in the SAMI galaxy survey. We characterize the dynamical state of the groups using the lumi
nosity gap between the two most luminous galaxies and the BGG offset from the luminosity centroid of the group. We find that the misalignment between the rotation axis of gas and stellar components is more frequent in the BGGs in unrelaxed groups, although with quite low statistical significance. Meanwhile galaxies whose stellar dynamics would be classified as `regular rotators based on their kinemetry are more common in relaxed groups. We confirm that this dependency on group dynamical state remains valid at fixed stellar mass and Sersic index. The observed trend could potentially originate from a differing BGG accretion history in virialised and evolving groups. Amongst the halo relaxation probes, the group BGG offset appears to play a stronger role than the luminosity gap on the stellar kinematic differences of the BGGs. However, both the group BGG offset and luminosity gap appear to roughly equally drive the misalignment between the gas and stellar component of the BGGs in one direction. This study offers the first evidence that the dynamical state of galaxy groups may influence the BGGs stellar and gas kinematics and calls for further studies using a larger sample with higher signal-to-noise.
We present the first study of the effect of ram-pressure unwinding the spiral arms of cluster galaxies. We study 11 ram-pressure stripped galaxies from GASP (GAs Stripping Phenomena in galaxies) in which, in addition to more commonly observed jellyfi
sh features, dislodged material also appears to retain the original structure of the spiral arms. Gravitational influence from neighbours is ruled out and we compare the sample with a control group of undisturbed spiral galaxies and simulated stripped galaxies. We first confirm the unwinding nature, finding the spiral arm pitch angle increases radially in 10 stripped galaxies and also simulated face-on and edge-on stripped galaxies. We find only younger stars in the unwound component, while older stars in the disc remain undisturbed. We compare the morphology and kinematics with simulated ram-pressure stripping galaxies, taking into account the estimated inclination with respect to the intracluster medium and find that in edge-on stripping, unwinding can occur due to differential ram-pressure caused by the disc rotation, causing stripped material to slow and pile-up. In face-on cases, gas removed from the outer edges falls to higher orbits, appearing to unwind. The pattern is fairly short-lived (<0.5Gyr) in the stripping process, occurring during first infall and eventually washed out by the ICM wind into the tail of the jellyfish galaxy. By comparing simulations with the observed sample, we find a combination of face-on and edge-on unwinding effects are likely to be occurring in our galaxies as they experience stripping with different inclinations with respect to the ICM.
Dimensionality reduction and clustering are often used as preliminary steps for many complex machine learning tasks. The presence of noise and outliers can deteriorate the performance of such preprocessing and therefore impair the subsequent analysis
tremendously. In manifold learning, several studies indicate solutions for removing background noise or noise close to the structure when the density is substantially higher than that exhibited by the noise. However, in many applications, including astronomical datasets, the density varies alongside manifolds that are buried in a noisy background. We propose a novel method to extract manifolds in the presence of noise based on the idea of Ant colony optimization. In contrast to the existing random walk solutions, our technique captures points which are locally aligned with major directions of the manifold. Moreover, we empirically show that the biologically inspired formulation of ant pheromone reinforces this behavior enabling it to recover multiple manifolds embedded in extremely noisy data clouds. The algorithms performance is demonstrated in comparison to the state-of-the-art approaches, such as Markov Chain, LLPD, and Disperse, on several synthetic and real astronomical datasets stemming from an N-body simulation of a cosmological volume.
Gravitational-wave observations of binary neutron star coalescences constrain the neutron-star equation of state by enabling measurement of the tidal deformation of each neutron star. This deformation is determined by the tidal deformability paramete
r $Lambda$, which was constrained using the first binary neutron star gravitational-wave observation, GW170817. Now, with the measurement of the second binary neutron star, GW190425, we can combine different gravitational-wave measurements to obtain tighter constraints on the neutron-star equation of state. In this paper, we combine data from GW170817 and GW190425 to place constraints on the neutron-star equation of state. To facilitate this calculation, we derive interpolated marginalized likelihoods for each event using a machine learning algorithm. These likelihoods, which we make publicly available, allow for results from multiple gravitational-wave signals to be easily combined. Using these new data products, we find that the radius of a fiducial 1.4 $M_odot$ neutron star is constrained to $11.6^{+1.6}_{-0.9}$ km at 90% confidence and the pressure at twice the nuclear saturation density is constrained to $3.1^{+3.1}_{-1.3}times10^{34}$ dyne/cm$^2$ at 90% confidence. This result is dominated by GW170817 and is consistent with findings from other works.
A clear link between a dwarf-dwarf merger event and enhanced star formation (SF) in the recent past was recently identified in the gas-dominated merger remnant VCC 848, offering by far the clearest view of a gas-rich late-stage dwarf-dwarf merger. We
present a joint analysis of JVLA HI emission-line mapping, optical imaging and numerical simulations of VCC 848, in order to examine the impact of the merger on the stellar and gaseous distributions. VCC 848 has less than 30% of its HI gas concentrated within the central high-surface-brightness star-forming region, while the remaining HI is entrained in outlying tidal features. Particularly, a well-defined tidal arm reaches N(HI) comparable to the galaxy center but lacks SF. The molecular gas mass inferred from the current SF rate (SFR) dominates over the atomic gas mass in the central ~ 1.5 kpc. VCC 848 is consistent with being a main-sequence star-forming galaxy for its current stellar mass and SFR. The HII region luminosity distribution largely agrees with that of normal dwarf irregulars with similar luminosities, except that the brightest HII region is extraordinarily luminous. Our N-body/hydrodynamical simulations imply that VCC 848 is a merger between a gas-dominated primary progenitor and a gas-bearing star-dominated secondary. The progenitors had their first passage on a near-radial non-coplanar orbit more than 1 Gyr ago. The merger did not build up a core as compact as typical compact dwarfs with centralized starburst, which may be partly ascribed to the star-dominated nature of the secondary, and in a general sense, a negative stellar feedback following intense starbursts triggered at early stages of the merger.
It has long been speculated that many starburst or compact dwarf galaxies are resulted from dwarf-dwarf galaxy merging, but unequivocal evidence for this possibility has rarely been reported in the literature. We present the first study of deep optic
al broadband images of a gas-dominated blue compact dwarf galaxy (BCD) VCC848 (Mstar=2e8Msun) which hosts extended stellar shells and thus is confirmed to be a dwarf-dwarf merger. VCC848 is located in the outskirts of the Virgo Cluster. By analyzing the stellar light distribution, we found that VCC848 is the result of a merging between two dwarf galaxies with a primary-to-secondary mass ratio < ~ 5 for the stellar components and < ~ 2 for the presumed dark matter halos. The secondary progenitor galaxy has been almost entirely disrupted. The age-mass distribution of photometrically selected star cluster candidates in VCC848 implies that the cluster formation rate (CFR, proportional to star formation rate) was enhanced by a factor of ~ 7 - 10 during the past 1 Gyr. The merging-induced enhancement of CFR peaked near the galactic center a few hundred Myr ago and has started declining in the last few tens of Myr. The current star formation activities, as traced by the youngest clusters, mainly occur at large galactocentric distances (> ~ 1 kpc). The fact that VCC848 is still (atomic) gas-dominated after the period of most violent collision suggests that gas-rich dwarf galaxy merging can result in BCD-like remnants with extended atomic gas distribution surrounding a blue compact center, in general agreement with previous numerical simulations.
