Major Mergers Research Articles

The Star-forming Main Sequence of the Host Galaxies of Low-redshift Quasars

We investigate the star-forming main sequence of the host galaxies of a large, well-defined sample of 453 redshift ∼0.3 quasars with previously available star formation rates by deriving stellar masses from modeling their broadband (grizy) spectral energy distribution. We perform two-dimensional, simultaneous, multi-filter decomposition of Pan-STARRS1 3π Steradian Survey images to disentangle the active galactic nucleus (AGN) from its host galaxy, by explicitly considering, for the first time, the wavelength variation of galaxy structures. We quantify the Sérsic profiles and sizes of the host galaxies from mock AGNs generated from both real and idealized galaxies. Detailed morphological classifications of the calibration galaxy sample using Hubble Space Telescope images enable us to estimate crude morphological types of the quasars. Although the majority (∼60%) of the quasars are hosted by bulge-dominated, early-type galaxies, a substantial fraction (∼40%) reside in disk-dominated, late-type galaxies, suggesting that at least in these systems major mergers have not played a significant role in regulating their AGN activity, in agreement with recent simulations and observations of nearby quasars. The vast majority (∼90%) of the quasars have star formation rates that place them on or above the galaxy star-forming main sequence, with more rapidly accreting AGNs displaced further above the main sequence. Quasar host galaxies generally follow the stellar mass–size relation defined by inactive galaxies, both for late-type and early-type systems, but roughly 1/3 of the population has smaller sizes at a given stellar mass, reminiscent of compact star-forming galaxies at higher redshift.

Seeing the forest and the trees: A radio investigation of the ULIRG Mrk 273

Galaxy mergers have been observed to trigger nuclear activity by feeding gas to the central supermassive black hole. One such class of objects are Ultra Luminous InfraRed Galaxies (ULIRGs), which are mostly late stage major mergers of gas-rich galaxies. Recently, large-scale (∼100 kpc) radio continuum emission has been detected in a select number of ULIRGs, all of which also harbour powerful Active Galactic Nuclei (AGN). This hints at the presence of large-scale radio emission being evidence for nuclear activity. Exploring the origin of this radio emission and its link to nuclear activity requires high sensitivity multi-frequency data. We present such an analysis of the ULIRG Mrk 273. Using the International LOFAR telescope (ILT), we detected spectacular large-scale arcs in this system. This detection includes, for the first time, a giant ∼190 kpc arc in the north. We propose these arcs are fuelled by a low power radio AGN triggered by the merger. We also identified a bright ∼45 kpc radio ridge, which is likely related to the ionised gas nebula in that region. We combined this with high sensitivity data from APERture Tile In Focus (Apertif) and archival data from the Very Large Array (VLA) to explore the spectral properties. The ILT simultaneously allowed us to probe the nucleus at a resolution of ∼0.3″, where we detected three components, and, for the first time, diffuse emission around these components. Combining this with archival high frequency VLA images of the nucleus allowed us to detect absorption in one component, and a steep spectrum radio AGN in another. We then extrapolate from this case study to the importance of investigating the presence of radio emission in more ULIRGs and what it can tell us about the link between mergers and the presence of radio activity.

Reconstructing the Disrupted Dwarf Galaxy Gaia-Sausage/Enceladus Using Its Stars and Globular Clusters

We combine spectroscopic, photometric, and astrometric information from APOGEE data release 17 and Gaia early data release 3 to perform a self-consistent characterization of Gaia-Sausage/Enceladus (GSE), the remnant of the last major merger experienced by the Milky Way, considering stars and globular clusters (GCs) altogether. Our novel set of chemodynamical criteria to select genuine stars of GSE yields a metallicity distribution function with a median [Fe/H] of −1.22 and 0.23 dex dispersion. Stars from GSE present an excess of [Al/Fe] and [Mg/Mn] (also [Mg/Fe]) in comparison to surviving Milky Way dwarf satellites, which can be explained by differences in star formation efficiencies and timescales between these systems. However, stars from Sequoia, another proposed accreted halo substructure, essentially overlap the GSE footprint in all analyzed chemical-abundance spaces, but present lower metallicities. Among the probable GCs of GSE with APOGEE observations available, we find no evidence for atypical [Fe/H] spreads with the exception of ω Centauri (ωCen). Under the assumption that ωCen is a stripped nuclear star cluster, we estimate the stellar mass of its progenitor to be M ⋆ ≈ 1.3 × 109 M ⊙, well within literature expectations for GSE. This leads us to envision GSE as the best available candidate for the original host galaxy of ωCen. We also take advantage of Gaia's photometry and APOGEE metallicities as priors to determine fundamental parameters for eight high-probability (>70%) GC members of GSE via statistical isochrone fitting. Finally, the newly determined ages and APOGEE [Fe/H] values are utilized to model the age–metallicity relation of GSE.

PROFITABILITY ANALYSIS OF PUBLIC SECTOR BANKS IN INDIA

Economic development of any nation depends on its efficient financial operations of the banking networks. Indian banking sector facing severe NPA issues. In order to overcome this problem, Government of India started merging activities. Some major mergers took place in recent years which made the Indian Banking Sector effective, innovative and competitive in the global level. Public sector banks which left as an independent public sector bank after merging process conducted by government of India in the last few years are taken as a sample for the study. The purpose of this article is to determine the financial factors affecting the performance of public sector banks in India. This article analyzed the financial data of six independent public sector banks for the period of four years from 2017-18 to 2020-21. For analysis purpose, used secondary data from various financial websites and only quantitative data is considered and ratio analysis and descriptive statistics are used for analysis purpose. The present study has examined the financial performance in the public sector banks which has a significant result in terms of Net Profit Margin, Return on Net Worth, Operating Expense / Total Income and Capital Adequacy Ratios Bank of India performed better. Central Bank of India functioned well in getting significant long term returns and all selected sample banks shown negative outcome in the relation of Net Profit / Total Funds. Bank of Maharashtra revealed its inefficiency in maintaining negative loan turnover ratio, while Central Bank of India more efficient bank which has Positive Loan turnover ratio. Outperformed Punjab and Sind Bank in terms of Total Assets Turnover Ratio, Advances / Loans Funds and Earnings Per Share. All selected six sample Banks has less than one (<1) Current ratio in shown inefficient in making its dues in a time. Punjab and Sind bank and UCO Bank used its huge amount of interest earned income for making payment of Interest expenses. From the study it is clear that there is a need to improve the overall profitable performance of selected sample of public sector banks by the government which is in need for its survival. KEYWORDS: Profitable Performance, Public sector banks, ratio analysis.

Spatially resolved gas-phase metallicity in FIRE-2 dwarfs: late-time evolution of metallicity relations in simulations with feedback and mergers

ABSTRACT We present an analysis of spatially resolved gas-phase metallicity relations in five dwarf galaxies ($\rm \mathit{M}_{halo} \approx 10^{11}\, {\rm M}_\odot$, $\rm \mathit{M}_\star \approx 10^{8.8}{-}10^{9.6}\, {\rm M}_\odot$) from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, which include an explicit model for sub-grid turbulent mixing of metals in gas, near z ≈ 0, over a period of 1.4 Gyr, and compare our findings with observations. While these dwarf galaxies represent a diverse sample, we find that all simulated galaxies match the observed mass–metallicity (MZR) and mass–metallicity gradient (MZGR) relations. We note that in all five galaxies, the metallicities are effectively identical between phases of the interstellar medium (ISM), with 95 ${{\ \rm per\ cent}}$ of the gas being within ±0.1 dex between the cold and dense gas (T < 500 K and nH > 1 cm−3), ionized gas (near the H αT ≈ 104 K ridge-line), and nebular regions (ionized gas where the 10 Myr-averaged star formation rate is non-zero). We find that most of the scatter in relative metallicity between cold dense gas and ionized gas/nebular regions can be attributed to either local starburst events or metal-poor inflows. We also note the presence of a major merger in one of our galaxies, m11e, with a substantial impact on the metallicity distribution in the spatially resolved map, showing two strong metallicity peaks and triggering a starburst in the main galaxy.

Influence of the Gaia–Sausage–Enceladus on the Density Shape of the Galactic Stellar Halo Revealed by Halo K Giants from the LAMOST Survey

We present a study of the influence of the Gaia–Sausage–Enceladus (GSE) on the density shape of the Galactic stellar halo using 11,624 K giants from the LAMOST survey. Every star is assigned a probability of being a member of the GSE based on its spherical velocities and metallicity by a Gaussian mixture model. We divide the stellar halo into two parts by the obtained probabilities, of which one is composed of the GSE members and defined as the GSE-related halo, and the other one is referred to as the GSE-removed halo. Using a nonparametric method, the radial number density profiles of the two stellar halos can be well described by a single power law with a variable flattening q (). The index α is 4.92 ± 0.12 for the GSE-related halo and 4.25 ± 0.14 for the GSE-removed halo. Both of the stellar halos are vertically flattened at smaller radii but become more spherical at larger radii. We find that the GSE-related halo is less vertically flattened than the GSE-removed halo, and the difference of q between the two stellar halos ranges from 0.07 to 0.15. However, after the consideration of the bias, it is thought to be within 0.08 at most of the radii. Finally, we compare our results with two Milky Way analogs that experience a significant major merger in the TNG50 simulation. The study of the two analogs also shows that the major merger–related stellar halo has a smaller ellipticity than the major merger–removed stellar halo.

The velocity structure of the intracluster medium during a major merger: Simulated microcalorimeter observations

Major mergers between galaxy clusters can produce large turbulent and bulk flow velocities in the intracluster medium (ICM) and thus imprint useful diagnostic features in X-ray spectral emission lines from heavy ions. As successfully achieved by Hitomi in observations of the Perseus cluster, measurements of gas velocities in clusters from high-resolution X-ray spectra will be achievable with upcoming X-ray calorimeters such as those on board XRISM, Athena, or a Lynx like mission. An interesting application to clusters involves detecting multiple velocity components or velocity gradients from diagnostic observations of specific interesting locations across the cluster. To explore this possibility in the case of a major head-on cluster merger, we performed velocity analyzes of a cluster-cluster merger from a hydrodynamical simulation by means of X-ray synthetic spectra with a spectral resolution on the order of a few eV. We observed the system along two extreme line-of-sight directions: (1) perpendicular to the plane of the merger and (2) along the merger axis. In these geometrical configurations, we found that clear non-Gaussian shapes of the iron He-like Kα line at 6.7 keV are expected. While the velocity dispersion predicted from the simulations can be retrieved for the brightest 100 ks pointings with XRISM Resolve, some discrepancy with respect to the expected value is noted and can be attributed to the complex non-Gaussian line shapes. Measurements in low surface brightness regions, especially when multiple velocity components are present along the line of sight, require high signal-to-noise ratio and the larger collecting area of the Athena X-IFU calorimeter is therefore required. With the latter, we also investigated the ICM temperature and velocity gradient across the merger bow shock edge, from 20″-wide annuli extracted from a single 1 Ms X-IFU observation. For both temperature and velocity dispersion, we found best-fit values that are consistent with predictions from the simulations within 1-σ. The uncertainties on the inferred velocity dispersion are, however, too large to place any stringent constraints on the shallow gradient downstream of the shock. Additionally, we present simulated images of the thermal and kinetic Sunyaev–Zeldovich effects from this merging system, using the above viewing configurations and compare the results at angular resolutions appropriate for future observatories such as CMB-S4 and the Atacama Large Aperture Submillimeter Telescope (AtLAST).

The Size–Mass Relation of Post-starburst Galaxies in the Local Universe

We present a study of the size–mass relation for local post-starburst (PSB) galaxies at z ≲ 0.33 selected from the Sloan Digital Sky Survey Data Release 8. We find that PSB galaxies with stellar mass (M *) at 109 M ☉ < M * < 1012 M ☉ have a galaxy size smaller than or comparable to those of quiescent galaxies (QGs). After controlling redshift and stellar mass, the sizes of PSBs are ∼13% smaller on average than those of QGs; such differences become larger and significant toward the low-M * end, especially at 109.5 M ☉ ≲ M * ≲ 1010.5 M ☉ where PSBs can be on average ∼19% smaller than QGs. By comparing predictions of possible PSB evolutionary pathways from cosmological simulations, we suggest that a fast quenching of star formation following a short-lived starburst event (which might be induced by a major merger) should be the dominant pathway of our PSB sample. Furthermore, by cross-matching with group catalogs, we confirm that local PSBs at M * ≲ 1010 M ☉ are more clustered than more massive ones. PSBs residing in groups are found to be slightly larger in galaxy size and more disk-like compared to field PSBs, which is qualitatively consistent with and thus hints at the environment-driven fast quenching pathway for group PSBs. Taken together, our results support multiple evolutionary pathways for local PSB galaxies: while massive PSBs are thought of as products of fast quenching following a major merger-induced starburst, environment-induced fast quenching should play a role in the evolution of less massive PSBs, especially at M * ≲ 1010 M ☉.

Particle re-acceleration and diffuse radio sources in the galaxy cluster Abell 1550

Context. Radio observations of galaxy clusters reveal a plethora of diffuse, steep-spectrum sources related to the re-acceleration of cosmic-ray electrons, such as halos, relics, and phoenices. In this context, the LOw Frequency ARray Low-Band Antenna (LOFAR-LBA) Sky Survey (LoLSS) provides the most sensitive images of the sky at 54 MHz to date, allowing us to investigate re-acceleration processes in a poorly explored frequency regime. Aims. We study diffuse radio emission in the galaxy cluster Abell 1550, with the aim of constraining particle re-acceleration in the intra-cluster medium. Methods. We exploited observations at four different radio frequencies: 54, 144, 400, and 1400 MHz. To complement our analysis, we made use of archival Chandra X-ray data. Results. At all frequencies we detect an ultra-steep spectrum radio halo (Sν ∝ ν−1.6) with an extent of ∼1.2 Mpc at 54 MHz. Its morphology follows the distribution of the thermal intra-cluster medium inferred from the Chandra observation. West of the centrally located head-tail radio galaxy, we detect a radio relic with a projected extent of ∼500 kpc. From the relic, a ∼600 kpc long bridge departs and connects with the halo. Between the relic and the radio galaxy, we observe what is most likely a radio phoenix, given its curved spectrum. The phoenix is connected to the tail of the radio galaxy through two arms, which show a nearly constant spectral index for ∼300 kpc. Conclusions. The halo could be produced by turbulence induced by a major merger, with the merger axis lying in the NE-SW direction. This is supported by the position of the relic, whose origin could be attributed to a shock propagating along the merger axis. It is possible that the same shock has also produced the phoenix through adiabatic compression, while we propose that the bridge could be generated by electrons which were pre-accelerated by the shock, and then re-accelerated by turbulence. Finally, we detect hints of gentle re-energisation in the two arms that depart from the tail of the radio galaxy.

High resolution spectral imaging of CO(7–6), [CI](2–1), and continuum of three high-z lensed dusty star-forming galaxies using ALMA

High-redshift dusty star-forming galaxies with very high star formation rates (500−3000 M⊙ yr−1) are key to understanding the formation of the most extreme galaxies in the early Universe. Characterising the gas reservoir of these systems can reveal the driving factor behind the high star formation. Using molecular gas tracers such as, high-J CO lines, neutral carbon lines, and the dust continuum, we can estimate the gas density and radiation field intensity in their interstellar media. In this paper, we present high resolution (∼0.4″) observations of CO(7−6), [CI](2−1), and dust continuum of three lensed galaxies from the South pole telescope – sub-millimetre galaxies (SPT-SMG) sample at z ∼ 3 with the Atacama Large Millimetre/submillimetre Array. Our sources have high intrinsic star formation rates (&gt; 850 M⊙ yr−1) and rather short depletion timescales (&lt; 100 Myr). Based on the L[CI](2−1)/LCO(7 − 6) and L[CI](2−1)/LIR ratios, our galaxy sample has similar radiation field intensities and gas densities compared to other submillimetre galaxies. We performed visibility-based lens modelling on these objects to reconstruct the kinematics in the source plane. We find that the cold gas masses of the sources are compatible with simple dynamical mass estimates using ULIRG-like values of the CO-H2 conversion factor αCO, but not Milky Way-like values. We find diverse source kinematics in our sample: SPT0103−45 and SPT2147−50 are likely rotating disks, while SPT2357−51 is possibly a major merger. The analysis presented in the paper could be extended to a larger sample to determine better statistics of morphologies and interstellar medium properties of high-z dusty star-forming galaxies.

A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). VII. Redshift Evolution of Radio Galaxy Environments at z = 0.3–1.4

We examine the redshift evolution of density environments around 2163 radio galaxies with the stellar masses of ∼109–1012 M ⊙ between redshifts of z = 0.3–1.4, based on the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and Faint Images of the Radio Sky at Twenty cm. We use the k-nearest neighbor method to measure the local galaxy number density around our radio galaxy sample. We find that the overdensities of the radio galaxies are weakly but significantly anticorrelated with redshift. This is consistent with the known result that the relative abundance of less-massive radio galaxies increases with redshift, because less-massive radio galaxies reside in relatively low-density regions. Massive radio galaxies with stellar masses of M * > 1011 M ⊙ are found in high density environments compared with the control sample galaxies with radio nondetection and matched stellar mass. Less-massive radio galaxies with M * < 1011 M ⊙ reside in average density environments. The fraction of the radio galaxies associated with the neighbors within a typical major merger scale, <70 kpc, is higher than (comparable to) that of the control galaxies at M * > 1011 M ⊙ (M * < 1011 M ⊙). We also find that the local densities around the radio galaxies are anticorrelated with the radio luminosities and black hole mass accretion rates at a fixed stellar mass. These findings suggest that massive radio galaxies have matured through galaxy mergers in the past, and have supermassive black holes whose mass accretion almost ceased at z > 1.4, while less-massive radio galaxies undergo active accretion just at this epoch, as they have avoided such merger events.

The formation of the first quasars: the black hole seeds, accretion, and feedback models

ABSTRACT Supermassive black holes (SMBHs) of $\sim 10^9\, \rm M_{\odot }$ are generally believed to be the central engines of the luminous quasars observed at z ≳ 6, but their astrophysical origin remains elusive. To investigate the formation of these distant quasars, we perform a suite of zoom-in simulations on a favourable halo, with a mass of $\sim 10^{13}\, \rm M_{\odot }$ at z = 6 and a history of multiple major mergers, ideal for BH growth. We test BH seeds of $10 {\!-\!} 10^6\, \rm M_{\odot }$, and various accretion and feedback models, including thin-disc and slim-disc accretion. We find that abundant gas supply and efficient angular momentum transport by gravitational torques are critical to BH accretion, and that the final BH mass depends strongly on the seed mass and radiative efficiency which in turn depends on feedback model. In our simulations, heavy seeds of $\gtrsim 10^4\, \rm M_{\odot }$ can grow rapidly to SMBHs of $10^8 {\!-\!} 10^9\, \rm M_{\odot }$ by z ∼6 in both thin and slim disc accretion models, provided that the duty cycle of near-Eddington accretion with an Eddington ratio λEdd ≳ 0.6 is maintained at $\gtrsim 40{{\ \rm per\ cent}}$. In particular, for a $10^5\, \rm M_{\odot }$ seed, both our fiducial model and the supercritical accretion model with no spin produce bright quasars with SMBHs of $\sim 10^9\, \rm M_{\odot }$ at z ∼ 7.5. On the other hand, the light seeds of $\lesssim 10^3\, \rm M_{\odot }$ failed to grow to $10^8\, \rm M_{\odot }$ by z ∼ 6 in our simulations. We caution that the results are inconclusive due to limitations of our numerical methods and physical models, and we stress the need for further investigations on the growth of BH seeds with more advanced methods.

Characterization of low surface brightness structures in annotated deep images

Context. The identification and characterization of low surface brightness (LSB) stellar structures around galaxies such as tidal debris of ongoing or past collisions is essential to constrain models of galactic evolution. So far most efforts have focused on the numerical census of samples of varying sizes, either through visual inspection or more recently with deep learning. Detailed analyses including photometry have been carried out for a small number of objects, essentially because of the lack of convenient tools able to precisely characterize tidal structures around large samples of galaxies. Aims. Our goal is to characterize in detail, and in particular obtain quantitative measurements, of LSB structures identified in deep images of samples consisting of hundreds of galaxies. Methods. We developed an online annotation tool that enables contributors to delineate the shapes of diffuse extended stellar structures with precision, as well as artifacts or foreground structures. All parameters are automatically stored in a database which may be queried to retrieve quantitative measurements. We annotated LSB structures around 352 nearby massive galaxies with deep images obtained with the Canada-France-Hawaii Telescope as part of two large programs: Mass Assembly of early-Type GaLAxies with their fine Structures and Ultraviolet Near Infrared Optical Northern Survey/Canada-France Imaging Survey. Each LSB structure was delineated and labeled according to its likely nature: stellar shells, streams associated with a disrupted satellite, tails that formed in major mergers, ghost reflections, or cirrus. Results. From our database containing 8441 annotations, the area, size, median surface brightness, and distance to the host of 228 structures were computed. The results confirm the fact that tidal structures defined as streams are thinner than tails, as expected by numerical simulations. In addition, tidal tails appear to exhibit a higher surface brightness than streams (by about 1 mag), which may be related to different survival times for the two types of collisional debris. We did not detect any tidal feature fainter than 27.5 magarcsec−2, while the nominal surface brightness limits of our surveys range between 28.3 and 29 magarcsec−2, a difference that needs to be taken into account when estimating the sensitivity of future surveys to identify LSB structures. Conclusions. We compiled an annotation database of observed LSB structures around nearby massive galaxies including tidal features that may be used for quantitative analysis and as a training set for machine learning algorithms.

The Host Galaxy of the Recoiling Black Hole Candidate in 3C 186: An Old Major Merger Remnant at the Center of a z = 1 Cluster

3C 186, a radio-loud quasar at z = 1.0685, was previously reported to have both velocity and spatial offsets from its host galaxy, and has been considered as a promising candidate for a gravitational wave recoiling black hole triggered by a black hole merger. Another possible scenario is that 3C 186 is in an ongoing galaxy merger, exhibiting a temporary displacement. In this study, we present analyses of new deep images from the Hubble Space Telescope WFC3-IR and Advanced Camera for Surveys, aiming to characterize the host galaxy and test this alternative scenario. We carefully measure the light-weighted center of the host and reveal a significant spatial offset from the quasar core (11.1 ± 0.1 kpc). The direction of the confirmed offset aligns almost perpendicularly to the radio jet. We do not find evidence of a recent merger, such as a young starburst in disturbed outskirts, but only marginal light concentration in F160W at ∼30 kpc. The host consists of mature (≳200 Myr) stellar populations and one compact star-forming region. We compare with hydrodynamical simulations and find that those observed features are consistently seen in late-stage merger remnants. Taken together, those pieces of evidence indicate that the system is not an ongoing/young merger remnant, suggesting that the recoiling black hole scenario is still a plausible explanation for the puzzling nature of 3C 186.

Early-type galaxy density profiles from IllustrisTNG – III. Effects on outer kinematic structure

ABSTRACT Early-type galaxies (ETGs) possess total density profiles close to isothermal, which can lead to non-Gaussian line-of-sight velocity dispersion (LOSVD) under anisotropic stellar orbits. However, recent observations of local ETGs in the MASSIVE Survey reveal outer kinematic structures at 1.5Reff (effective radius) that are inconsistent with fixed isothermal density profiles; the authors proposed varying density profiles as an explanation. We aim to verify this conjecture and understand the influence of stellar assembly on these kinematic features through mock ETGs in IllustrisTNG. We create mock Integral-Field-Unit observations to extract projected stellar kinematic features for 207 ETGs with stellar mass $M_{\ast }\geqslant 10^{11} \, \mathrm{M_{\odot}}$ in TNG100-1. The mock observations reproduce the key outer (1.5Reff) kinematic structures in the MASSIVE ETGs, including the puzzling positive correlation between velocity dispersion profile outer slope γouter and the kurtosis h4’s gradient. We find that h4 is uncorrelated with stellar orbital anisotropy beyond Reff; instead, we find that the variations in γouter and outer h4 (a good proxy for h4 gradient) are both driven by variations of the density profile at the outskirts across different ETGs. These findings corroborate the proposed conjecture and rule out velocity anisotropy as the origin of non-Gaussian outer kinematic structure in ETGs. We also find that the outer kurtosis and anisotropy correlate with different stellar assembly components, with the former related to minor mergers or flyby interactions while the latter is mainly driven by major mergers, suggesting distinct stellar assembly origins that decorrelates the two quantities.

Detecting the Memory Effect from a Massive Black Hole Merger at the Galactic Center through Lunar Ranging

A gravitational wave pulse from a major merger of massive black holes at the Galactic center induces a permanent increase in the Earth–Moon separation. For black holes of mass ∼106 M ⊙, the shift in the local gravitational potential is comparable to the Earth–Moon potential, leading to the Moon being perturbed relative to the Earth during the passage of the pulse. The permanent increase in the Earth–Moon separation is a fraction of a millimeter, measurable by lunar ranging for future merger events.

Timing the last major merger of galaxy clusters with large halo sparsity

ABSTRACT Numerical simulations have shown that massive dark matter haloes, which today host galaxy clusters, assemble their mass over time alternating periods of quiescent accretion and phases of rapid growth associated with major merger episodes. Observations of such events in clusters can provide insights on the astrophysical processes that characterize the properties of the intra-cluster medium, as well as the gravitational processes that contribute to their assembly. It is therefore of prime interest to devise a fast and reliable way of detecting such perturbed systems. We present a novel approach to identifying and timing major mergers in clusters characterized by large values of halo sparsity. Using halo catalogues from the MultiDark-Planck2 simulation, we show that major merger events disrupt the radial mass distribution of haloes, thus leaving a distinct universal imprint on the evolution of halo sparsity over a period not exceeding two dynamical times. We exploit this feature using numerically calibrated distributions to test whether an observed galaxy cluster with given sparsity measurements has undergone a recent major merger and to eventually estimate when such an event occurred. We implement these statistical tools in a specifically developed public python library lammas, which we apply to the analysis of Abell 383 and Abell 2345 as test cases. Finding that, for example, Abell 2345 had a major merger about 2.1 ± 0.2 Gyr ago. This work opens the way to detecting and timing major mergers in galaxy clusters solely through measurements of their mass at different radii.

NIHAO – XXVII. Crossing the green valley

ABSTRACT The transition of high-mass galaxies from being blue and star-forming to being red and dead is a crucial step in galaxy evolution, yet not fully understood. In this work, we use the NIHAO (Numerical Investigation of a Hundred Astrophysical Objects) suite of galaxy simulations to investigate the relation between the transition time through the green valley and other galaxy properties. The typical green valley crossing time of our galaxies is approximately 400 Myr, somewhat shorter than observational estimates. The crossing of the green valley is triggered by the onset of active galactic nucleus (AGN) feedback and the subsequent shutdown of star formation. Interestingly, the time spent in the green valley is not related to any other galaxy properties, such as stellar age or metallicity, or the time at which the star formation quenching takes place. The crossing time is set by two main contributions: the ageing of the current stellar population and the residual star formation in the green valley. These effects are of comparable magnitude, while major and minor mergers have a negligible contribution. Most interestingly, we find the time that a galaxy spends to travel through the green valley is twice the e-folding time of the star formation quenching. This result is stable against galaxy properties and the exact numerical implementation of AGN feedback in the simulation. Assuming a typical crossing time of about 1 Gyr inferred from observations, our results imply that any mechanism or process aiming to quench star formation must do it on a typical time-scale of 500 Myr.

Detection of a Superluminous Spiral Galaxy in the Heart of a Massive Galaxy Cluster

It is well established that brightest cluster galaxies (BCGs), residing in the centers of galaxy clusters, are typically massive and quenched galaxies with cD or elliptical morphology. An optical survey suggested that an exotic galaxy population, superluminous spiral and lenticular galaxies, could be the BCGs of some galaxy clusters. Because the cluster membership and the centroid of a cluster cannot be accurately determined based solely on optical data, we followed up a sample of superluminous disk galaxies and their environments using XMM-Newton X-ray observations. Specifically, we explored seven superluminous spiral and lenticular galaxies that are candidate BCGs. We detected massive galaxy clusters around five superluminous disk galaxies and established that one superluminous spiral, 2MASX J16273931+3002239, is the central BCG of a galaxy cluster. The temperature and total mass of the cluster are keV and M 500 = (2.39 ± 0.19) × 1014 M ⊙. We identified the central galaxies of the four clusters that do not host superluminous disk galaxies at their cores, and established that the centrals are massive elliptical galaxies. However, for two of the clusters, the offset superluminous spirals are brighter than the central galaxies, implying that the superluminous disk galaxies are the brightest cluster galaxies. Our results demonstrate that superluminous disk galaxies are rarely the central systems of galaxy clusters. This is likely because galactic disks are destroyed by major mergers, which are more frequent in high-density environments. We speculate that the disks of superluminous disk galaxies in cluster cores may have been reformed due to mergers with gas-rich satellites.

Toward Astrometric Constraints on a Supermassive Black Hole Binary in the Early-type Galaxy NGC 4472

The merger of two galaxies, each hosting a supermassive black hole (SMBH) of mass 106 M ⊙ or more, could yield a bound SMBH binary. For the early-type galaxy NGC 4472, we study how astrometry with a next-generation Very Large Array could be used to monitor the reflex motion of the primary SMBH of mass M pri, as it is tugged on by the secondary SMBH of mass . Casting the orbit of the putative SMBH binary in terms of its period P, semimajor axis a bin, and mass ratio , we find the following: (1) Orbits with fiducial periods of P = 4 yr and 40 yr could be spatially resolved and monitored. (2) For a 95% accuracy of 2 μas per monitoring epoch, subparsec values of a bin could be accessed over a range of mass ratios notionally encompassing major and minor galaxy mergers. (3) If no reflex motion is detected for M pri after 1 (10) yr of monitoring, an SMBH binary with period P = 4 (40) yr and mass ratio q > 0.01 (0.003) could be excluded. This would suggest no present-day evidence for a past major merger like that recently simulated, where scouring by a q ∼ 1 SMBH binary formed a stellar core with kinematic traits like those of NGC 4472. (4) Astrometric monitoring could independently check the upper limits on q from searches for continuous gravitational waves from NGC 4472.