Final Merger Research Articles

The gravitational wave 'probability event horizon' for double neutron star mergers

Gravitational waves generated by the final merger of double neutron star (DNS) binary systems are a key target for the gravitational wave (GW) interferometric detectors, such as LIGO (Laser Interferometer Gravitational-wave Observatory), and the next generation detectors, Advanced LIGO. The cumulative GW signal from DNS mergers in interferometric data will manifest as ‘geometrical noise’: a non-continuous stochastic background with a unique statistical signature dominated by the spatial and temporal distribution of the sources. Because geometrical noise is highly non-Gaussian, it could potentially be used to identify the presence of a stochastic GW background from DNS mergers. We demonstrate this by fitting to a simulated distribution of transients using a model for the DNS merger rate and idealized Gaussian detector noise. Using the cosmological ‘probability event horizon’ concept and recent bounds for the Galactic DNS merger rate, we calculate the evolution of the detectability of DNS mergers with observation time. For Advanced LIGO sensitivities and a detection threshold assuming optimal filtering, there is a 95 per cent probability that a minimum of one DNS merger signal will be detectable from the ensemble of events comprising the stochastic background during 12–211 d of observation. For initial LIGO sensitivities, we identify an interesting regime where there is a 95 per cent probability that at least one DNS merger with signal-to-noise ratio greater than unity will occur during 4–68 d of observation. We propose that there exists an intermediate detection regime with pre-filtered signal-to-noise ratio less than unity, where the DNS merger rate is high enough that the geometrical signature could be identified in interferometer data.

A Compact Binary Merger Model for the Short, Hard GRB 050509b

The first X-ray afterglow for a short ({approx}30ms), hard {gamma}-ray burst was detected by Swift on 9 May 2005 (GRB 050509b). No optical or radio counterpart was identified in follow-up observations. The tentative association of the GRB with a nearby giant elliptical galaxy at redshift z = 0.2248 would imply the progenitor had traveled several tens of kpc from its point of origin, in agreement with expectations linking these events to the final merger of compact binaries driven by gravitational wave emission. We model the dynamical merger of such a system and the time-dependent evolution of the accretion tori thus created. The resulting energetics, variability, and expected durations are consistent with GRB 050509b originating from the tidal disruption of a neutron star by a stellar mass black hole, or of the merger of two neutron stars followed by prompt gravitational collapse of the massive remnant. We discuss how the available {gamma}-ray and X-ray data provides a probe for the nature of the relativistic ejecta and the surrounding medium.

The Afterglow of Massive Black Hole Coalescence

The final merger of a pair of massive black holes in a galactic nucleus is compelled by gravitational radiation. Gravitational waves from the mergers of black holes of masses 105-107(1 + z)-1 M☉ at redshifts of 1-20 will be readily detectable by the Laser Interferometer Space Antenna, but an electromagnetic afterglow would be helpful in pinpointing the source and its redshift. Long before the merger, the binary hollows out any surrounding gas and shrinks slowly compared to the viscous timescale of a circumbinary disk. The inner gas disk is truncated at the radius where gravitational torque from the binary balances the viscous torque, and accretion onto the black holes is diminished. Initially, the inner truncation radius is able to follow the shrinking binary inward. But eventually the gravitational radiation timescale becomes shorter than the viscous timescale in the disk, leading to a merged black hole surrounded by a hollow disk of gas. We show that the subsequent viscous evolution of the hollow, radiation pressure-dominated disk will create an ~1043.5(M/106 M☉) ergs s-1 X-ray source on a timescale ~7(1 + z)(M/106 M☉)1.32 yr. This justifies follow-up monitoring of gravitational wave events with next-generation X-ray observatories. Analysis of the detailed light curve of these afterglows will yield new insights into the subtle physics of accretion onto massive black holes.

Influence of the Choice of Core-Envelope Transition Point on the Binary Merger of Two Main-sequence Components

We have studied the influence of different choices of core-envelope transition point on the final merger of contact binaries with two main-sequence components. A binary of 1.00 + 0.90 M-circle dot with an initial orbital period of 0.35 d is examined. The mass fraction of the primary mixed with the matter of the secondary, q(mix), determined by the chosen core-envelope transition point, ranges from 0.04 to 1.00 in our analysis. If as qmix 0.71, since no hydrogen-rich matter of the secondary is mixed into the core of the mergers; for q(mix) > 0.71, the larger qmix is, the greater the mixing, hence the longer the blue straggler lifetime, tBS, and also the greater the luminosity. For qmix = 1.00, tBS similar to 8.5 x 10(8) yr. Estimation by del(r) - del(a) = 0.0 shows that the point at which tBS begins to increase is about q(mix) = 0.68. In comparison with the homogeneously mixed models, the merger with a helium profile similar to that of the primary is less luminous and has a shorter tBS.

Dynamics and star formation activity of CG J1720-67.8 unveiled through integral field spectroscopy and radio observations

CG J1720-67.8 is an ultra compact group of several galaxies with a low-velocity dispersion, and displaying the hallmarks of mutual interaction and possible tidal dwarf galaxy formation. In hierarchical models, the system is a possible precursor to a massive elliptical galaxy. In this paper, we use new optical integral field spectroscopic and radio observations to investigate the evolutionary status of the group in more detail: global star formation rates are estimated using Ha and 1.4-GHz radio continuum measurements; H I observations provide an upper limit to the global neutral gas content; optical broadband colours and spectra provide ages and stellar mass estimates for the tidal dwarf candidates; the bi-dimensional Ha velocity field is used to trace the kinematics of the group and its members, which are compared with numerical simulations of galaxy encounters. The observations suggest a model in which multiple interactions have occurred, with the latest strong encounter involving at least two major components within the last 200 Myr. Debris from the encounter fuels ongoing star formation at the global level of ∼20 M ○. yr -1 , with self-gravity within the tidal tail possibly providing a mechanism to enhance the star formation rate of the tidal dwarf candidates, with bursts of star formation in clumps of mass ∼2 x 10 7 M ○. appearing within the last 10 Myr. The amount of time required for final merging of all group components remains uncertain.

Gravitational Waves from Merging Intermediate‐Mass Black Holes

The discovery of an intermediate-mass black hole (IMBH) supports a runaway path of supermassive black hole (SMBH) formation in galactic nuclei. No concrete model to explain all the steps of this bottom-up scenario for SMBHs is yet known, but here we propose to use gravitational radiation to probe the merging history of IMBHs. Collisions of black holes of mass 103-106 M☉ will produce gravitational radiation of 10-1 to 102 Hz in their final merging phase. We assume that a thousand 103 M☉ IMBHs form a 106 M☉ black hole in each galaxy via two different merging histories—hierarchical growth and monopolistic growth—using a theoretical model of quasar formation having a peak at z ≃ 2.5. We find that there would be 22-67 IMBH merging events per year in the universe and that the event numbers of the two models apparently differ in the frequency of gravitational radiation. Most of the bursts by these events will be detectable by currently proposed space gravitational wave antennas, such as LISA or DECIGO. We conclude that the statistics of the signals would provide both a galaxy distribution and a formation model of SMBHs.

Generating Hot Gas in Simulations of Disk-Galaxy Major Mergers

We report on the merger-induced generation of a shock-heated gas wind and the formation of a remnant gas halo in simulations of colliding disk galaxies. The simulations use cosmologically motivated initial conditions and include the effects of radiative cooling, star formation, stellar feedback, and the nonadiabatic heating of gas. The nonadiabatic heating, i.e., shocks generated in the final merger forces gas out of the central region of the merger remnant and into the dark matter halo. We demonstrate that the amount of heating depends on the size of the progenitor disk galaxy as well as the initial orbit on which the galaxies are placed. On the basis of these dependencies, we motivate a possible recipe for including this effect in semianalytic models of galaxy formation.

Public Sector Unions Shaping Hospital Privatization: The Creation of Boston Medical Center

In this paper the author examines, through a case study of a hospital merger in Boston, MA, the ways in which a locally scaled union strategy can shape privatization. She first examines recent economic changes in the US hospital industry, which have resulted in a record number of mergers, closures, and conversions. A case study of the merger between a public hospital, Boston City Hospital, and a private hospital, Boston University Hospital, into the private, nonprofit, Boston Medical Center is presented to demonstrate the ways in which unions can participate in setting the terms of such mergers. The strategy of one labor union local in particular, the Service Employees International Union (SEIU) Local 285, is examined in regard to protecting both the provision of public healthcare and the conditions of employment during and after the process of privatization. By utilizing a local strategy and waging a highly public campaign, leaders of SEIU Local 285 were able to build coalitions with other labor unions and public healthcare advocates and played an instrumental role in setting the terms of the merger. As a result, the final merger agreement contained significant protection for quality public healthcare and labor practices favorable for workers. Unions that previously represented public sector workers now represent private sector workers, and wages for all employees moved upward as part of the agreement. In addition, the new private sector entity was charged with what had formerly been a public sector mission. In effect, although the public hospital was ultimately privatized, the private sector was restructured by local level, public sector union activism.

Gravitational Waves from Merging Intermediate-Mass Black-Holes

The discovery of an intermediate-mass black-hole (IMBH) supports a runaway path of a super-massive black-hole (SMBH) formation in a galactic nuclei. A concrete model to explain all the steps to this bottom-up scenario forSMBH is not yet known, but we here propose to use gravitational radiation to probe the merging history of IMBHs. Collisions of black-holes of mass 10 3 ∼ 10 6 M will produce gravitational radiation of 10 - 1 ∼10 2 Hz in their final merging phase. We simply model a thousand IMBHs with a mass of 10 3 M in a galactic system, and assume their accumulating history in two extreme cases; hierarchical growth and monopolistic growth. We find that there would be 40 ∼ 100 IMBH merging events a year in the Universe, and that the event numbers of two models apparently differ in the frequency of gravitational radiation. Most of the bursts of gravitational wave by these events are well detectable by currently proposed space gravitational wave antenna, such as LISA or DECIGO. We also show that each burst signal is distinguishable, and conclude that the statistics of the signals would tell us both a galaxy distribution and a formation model of SMBHs. A detailed discussion is presented in Matsubayashi et al. [Astrophys. J. 396 (2004). L61].

The Decihertz Laser Interferometer Can Determine the Position of the Coalescing Binary Neutron Stars within an Arcminute a Week before the Final Merging Event to the Black Hole

It may be possible to construct a laser interferometer gravitational wave antenna in space with hrms ~ 10-23 at f ~ 0.1 Hz in ~2020. This decihertz antenna may be called the Decihertz Interferometer Gravitational Wave Observatory and Big Bang Observer (DECIGO/BBO). The analysis of 1-10 yr of observational data of the coalescing binary neutron stars or black holes at the distance of ~300 Mpc will give us the spatial position within approximately an arcminute and the time of the coalescence within ~0.1 s beforehand. With the knowledge of the accurate position and the time of the final merging event, the follow-up simultaneous observation using high-frequency (f ~ 100 Hz) gravitational wave antennae as well as electromagnetic wave antennae from the radio frequency to the ultra-high-energy gamma ray will reveal the physics in the enigmatic event of the coalescence and the formation of the black hole.

The Mass Function of Supergiant Molecular Complexes and Implications for Forming Young Massive Star Clusters in the Antennae (NGC 4038/4039)

We have used previously published observations of the CO emission from the Antennae (NGC 4038/4039) to study the detailed properties of the supergiant molecular complexes with the goal of understanding the formation of young massive star clusters. Over a mass range from 5 × 106 to 9 × 108 M☉ the molecular complexes follow a power-law mass function with a slope of -1.4 ± 0.1, which is very similar to the slope seen at lower masses in molecular clouds and cloud cores in the Galaxy. Compared with the spiral galaxy M51, which has a similar surface density and total mass of molecular gas, the Antennae contain clouds that are an order of magnitude more massive. Many of the youngest star clusters lie in the gas-rich overlap region, where extinctions as high as AV ~ 100 mag imply that the clusters must lie in front of the gas. Young clusters found in other regions of the galaxies can be as far as 2 kpc from the nearest massive cloud, which suggests that either young clusters can form occasionally in clouds less massive than 5 × 106 M☉ or that these young clusters have already destroyed their parent molecular clouds. Combining data on the young clusters, thermal and nonthermal radio sources, and the molecular gas suggests that young massive clusters could have formed at a constant rate in the Antennae over the last 160 Myr and that sufficient gas exists to sustain this cluster formation rate well into the future. However, this conclusion requires that a very high fraction of the massive clusters that form initially in the Antennae do not survive as long as 100 Myr. Furthermore, if most young massive clusters do survive for long periods, the Antennae must be experiencing a relatively short burst of cluster formation to prevent the final merger remnant from exceeding the observed specific frequency of star clusters in elliptical galaxies by a wide margin. Finally, we compare our data with two models for massive star cluster formation and conclude that the model in which young massive star clusters form from dense cores within the observed supergiant molecular complexes is most consistent with our current understanding of this merging system.

Tuning gravitational-wave detector networks to measure compact binary mergers

Gravitational waves generated by the final merger of compact binary systems depend on the structure of the binary's members. If the binary contains neutron stars, measuring such waves can teach us about the properties of matter at extreme densities. Unfortunately, these waves are typically at high frequency where the sensitivity of broad-band detectors is not good. Learning about dense matter from these waves will require networks of broad-band detectors combined with narrow-band detectors that have good sensitivity at high frequencies. This paper presents an algorithm by which a network can be ``tuned'', in accordance with the best available information, in order to most effectively measure merger waves. The algorithm is presented in the context of a toy model that captures the qualitative features of narrow-band detectors and of certain binary neutron star merger wave models. By using what is learned from a sequence of merger measurements, the network can be gradually tuned in order to accurately measure the waves. The number of measurements needed to reach this stage depends upon the waves' signal strength, the number of narrow-band detectors available for the measurement, and the detailed characteristics of the waves that carry the merger information. Future studies will go beyond this toy model, encompassing a more realistic description of both the detectors and the gravitational waves.

Accretion during the Merger of Supermassive Black Holes

We study the evolution of disk accretion during the merger of supermassive black hole binaries in galactic nuclei. In hierarchical galaxy formation models, the most common binaries are likely to arise from minor galactic mergers, and have unequal mass black holes. Once such a binary becomes embedded in an accretion disk at a separation of the order of 0.1 pc, the merger proceeds in two distinct phases. During the first phase, the loss of orbital angular momentum to the gaseous disk shrinks the binary on a timescale of ~10 Myr. The accretion rate onto the primary black hole is not increased, and can be substantially reduced, during this disk-driven migration. At smaller separations, gravitational radiation becomes the dominant angular momentum loss process, and any gas trapped inside the orbit of the secondary is driven inwards by the inspiralling black hole. The implied accretion rate just prior to coalescence exceeds the Eddington limit, so the final merger is likely to occur within a common envelope formed from the disrupted inner disk, and be accompanied by high velocity (~10,000 km/s) outflows.

Effect of the environment on the fundamental plane of elliptical galaxies

We present an analysis of the location of interacting E/S0 galaxies on the Fundamental Plane (FP). Using the NEMO package, we performed simulations of close encounters and mergers of two self-gravitating spherical galaxies. Two models for encounting galaxies – Plummer's and Hernquist's – were used. The number of particles ranged from 20 000 to 50 000 per galaxy. The changes of central density, half-mass radius and central velocity dispersion were analysed. It was found that close encounters between galaxies alter noticeably the above parameters within a very short time interval (107–108 years) just before the final merger. The amplitudes of parameter changes strongly depend on the initial mass concentration of a model. In some experiments we considered an encounter of two spherical galaxies with dark matter components. The effect of parameter changes was less pronounced than for the experiments without a dark halo. The results of the simulations were used to discuss the FP for interacting early-type galaxies.

William Godwin and the "Ars Rhetorica"

THERE IS A RECIPROCAL RELATIONSHIP BETWEEN A NATION'S POLITICAL institutions and value it places on oratory. Some form of this maxim would have appeared a commonplace remark (and a locus of argument) throughout eighteenth century, and within context of political debate attempt to divorce oratory from a political or religious institution would have seemed unusual. The classical education of Englishmen who were prepared for the senate, bar, or encouraged them to view great oratory, among other arts, as indivisible from institutions; among ancient Romans, they judged, demise of oratory resulted from rise of luxury and degradation of senatorial virtue. (1) In eighteenth-century England, where democratic element of Commons supposedly balanced oligarchic and monarchic elements of Lords and King, connection between oratory and free institutions was played upon for sake of ideological support and liberally exploited by Whigs for purpose of political criticism. At same time, however, and often in reaction to this exploitation, eloquence was viewed as dangerous to free institutions because of its ability to rouse passions and put them under control of a talented speaker. (2) Fear of rhetoric went hand in hand with fear of complete subjection to democratic element and of tyranny that would inevitably result. From one point of view, therefore, it should seem strange (and eventually did seem strange) that William Godwin, long a partisan of parliamentary reform, would not seek it through democratic venue of public speaking, or at least approve of others' doing so. (3) From another point of view, it makes sense, however, that when in Enquiry concerning Political Justice (1793) he ended up proposing viability of a society without formal government, he should also discard public speaking with institutions which it would serve, in order to relocate scene of significant communication in private conversation. (4) Godwin's view of ars rhetorica harbors a further complication, moreover. (5) For while in 1793 he declared conversation medium of gradual reform, he also assiduously pursued his plan of enlightenment in public sphere of print journalism, venue open to private citizens who could use it to influence public decisions. This is an important choice, since he had earlier treated press virtually as a constitutional institution, as an extension of House of Commons' function of representing public opinion. To Godwin, not only press, but jury too had earlier constituted an important venue of public opinion, and it was not only not private, it was both traditionally a venue of authoritative public speaking and an institution deciding upon imposition of force. To be sure, value Godwin placed on press and jury in 1780S reflects his involvement with Foxite Whigs. The society without government he envisions in Political Justice, however, would integrate press into its new plan of education and would long preserve jury, to hear cases of violence against neighbors and property, before disappearing in final triumphant merging of private judgment and public good. This continuation of earlier views into later formulations is symptomatic of very rhetorical problem Godwin had continually to address: how to convince his audience that reform, even radical change, was possible. For, granted that in 1793 Godwin envisioned ultimate demise of parliamentary government and of entire legal edifice, yet if he wanted to effect this end gradually (and gradualness is a major component of his thought) what would he allow to carry over from old institutions, and how could old, rather than simply disappear, actually become something new? This question is intimately related also to his view of art of rhetoric, because it was originally in and for institutions of legislative assemblies and law courts that such an art was declared and described and (along with pulpit oratory) formed into stock of education in eighteenth century. …

Lipids in viral fusion.

Enveloped animal viruses infect host cells by fusion of viral and target membranes. This crucial fusion event occurs either with the plasma membrane of the host cells or with the endosomal membranes (1). Fusion is triggered by specific glycoproteins in the virus membrane (2) and involves a range of steps before the final merging of membranes occurs. These steps include molecular processes, such as envelope protein conformational changes; aggregation of envelope protein; lipid–envelope protein interactions; and fusion pore formation and pore widening (see Fig. 1) (3,4). The reader is referred to a number of reviews on viral glycoprotein-mediated membrane fusion (5–9). Because lipids are integral elements in the membrane fusion process, a number of key issues concerning the role of lipids deserve thorough examination.

Nuclear Gasdynamics in Arp 220: Subkiloparsec‐Scale Atomic Hydrogen Disks

We present new, high angular resolution (~022) MERLIN observations of neutral hydrogen (H I) absorption and λ21 cm radio continuum emission across the central ~900 pc of the ultraluminous infrared galaxy Arp 220. Spatially resolved H I absorption is detected against the morphologically complex and extended λ21 cm radio continuum emission, consistent with two counterrotating disks of neutral hydrogen, with a small bridge of gas connecting the two. Column densities across the two nuclei are high, lying in the range 8 × 1019Ts(K) NH 2.4 × 1020Ts(K) cm-2 (Ts is spin temperature) and corresponding to optical extinctions of 0.052Ts(K) AV 0.155Ts(K) mag, with the higher column densities in the eastern nucleus. Velocity gradients are clearly visible across each nucleus, reaching 1010 ± 20 km s-1 kpc-1 in P.A. ~ 55° and 830 ± 20 km s-1 kpc-1 in P.A. ~ 270° for eastern and western nuclei, respectively. These gradients imply dynamical masses MD = 1.1 × 109( i) (E) and 1.7 × 108( i) M☉ (W), assuming that the neutral gas is distributed in two thin circularly rotating disks. We propose a merger model in which the two nuclei represent the galaxy cores that have survived the initial encounter and are now in the final stages of merging, similar to conclusions drawn from previous CO studies. However, we suggest that instead of being coplanar with the main CO disk (in which the eastern nucleus is embedded), the western nucleus lies above it and, as suggested by the bridge of H I connecting the two nuclei, will soon complete its final merger with the main disk. We suggest that the collection of radio supernovae (RSNe) detected in VLBA studies in the more compact western nucleus represents the second burst of star formation associated with this final merger stage and that free-free absorption due to ionized gas in the bulgelike component can account for the observed RSN distribution.

Nature of Widely Separated Ultraluminous Infrared Galaxies

In the complete sample of ultraluminous infrared galaxies (ULIRGs) compiled by Kim (1995) about 5% consists of widely separated galaxies which are presumably in the early phase of interaction. This fact is contrary to the conventional view that ULIRGs are in the final stages of the merger of two gas-rich disk galaxies. We have undertaken high resolution CO(J=1-0) observations for the ultraluminous infrared galaxies that have nuclear separations larger than 20 kpc. We have detected the CO emission in 5 out of 6 systems, but only in one component of the ULIRG pairs. 4 of them have LINER spectral type and 1 galaxy has Seyfert II spectral type. In K'-band images these components are also brighter than the other components which have either HII-region spectra or no detectable emission lines. Using the standard conversion factor, the molecular gas content is estimated to be a few times 10 $^{10}$ M$_\odot$, similar to that of the other ultraluminous galaxies. The result indicates that the galaxy containing the molecular gas is also the source of most, if not all, of the huge far-infrared luminosity of the system. The optical and K'-band imaging observations and optical spectra suggest multiple merger scenario for 1 system. If the remaining systems are in an early stage of a binary tidal interaction, the commonly accepted interpretation of the ULIRG phenomenon as the final merger stage of two disk galaxies may need to be re-examined.

Statistical Properties of Ultraluminous [ITAL]IRAS[/ITAL] Galaxies from an [ITAL]HST[/ITAL] Imaging Survey

We perform photometric measurements on a large HST snapshot imaging survey sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative nuclei from bright clumps in all the sample targets, mainly based on a quantitative criterion of I-band luminosity as well as the global and local morphological information. All the sources are then classified into three categories with multiple, double, and single nucleus/nuclei. The resultant fractions of multiple, double, and single nucleus/nuclei ULIRGs are 18%, 39%, and 43%, respectively. This supports the multiple merger scenario as a possible origin of ULIRGs, in addition to the commonly accepted pair merger model. Further statistical studies indicate that the AGN fraction increases from multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULIRGs. For the single nucleus category, there is a high-luminosity tail in the luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This indicates that active galactic nuclei tend to appear at final merging stage. For multiple/double nuclei galaxies, we also find a high fraction of very close nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This strengthens the conclusion that systems at late merging phase preferentially host ULIRGs.

Post-Newtonian smoothed particle hydrodynamics calculations of binary neutron star coalescence. II. Binary mass ratio, equation of state, and spin dependence

Using our new Post-Newtonian SPH (smoothed particle hydrodynamics) code, we study the final coalescence and merging of neutron star (NS) binaries. We vary the stiffness of the equation of state (EOS) as well as the initial binary mass ratio and stellar spins. Results are compared to those of Newtonian calculations, with and without the inclusion of the gravitational radiation reaction. We find a much steeper decrease in the gravity wave peak strain and luminosity with decreasing mass ratio than would be predicted by simple point-mass formulae. For NS with softer EOS (which we model as simple $\Gamma=2$ polytropes) we find a stronger gravity wave emission, with a different morphology than for stiffer EOS (modeled as $\Gamma=3$ polytropes as in our previous work). We also calculate the coalescence of NS binaries with an irrotational initial condition, and find that the gravity wave signal is relatively suppressed compared to the synchronized case, but shows a very significant second peak of emission. Mass shedding is also greatly reduced, and occurs via a different mechanism than in the synchronized case. We discuss the implications of our results for gravity wave astronomy with laser interferometers such as LIGO, and for theoretical models of gamma-ray bursts (GRBs) based on NS mergers.