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Monica Colpi

Publications and source records attributed to Monica Colpi.

At least 91 records · Page 5Linked to original sources
arXiv2007

Rapid Formation of Supermassive Black Hole Binaries in Galaxy Mergers with Gas

Supermassive black holes (SMBHs) are a ubiquitous component of the nuclei of galaxies. It is normally assumed that, following the merger of two massive galaxies, a SMBH binary will form, shrink due to stellar or gas dynamical processes and ultimately coalesce by emitting a burst of gravitational waves. However, so far it has not been possible to show how two SMBHs bind during a galaxy merger with gas due to the difficulty of modeling a wide range of spatial scales. Here we report hydrodynamical simulations that track the formation of a SMBH binary down to scales of a few light years following the collision between two spiral galaxies. A massive, turbulent nuclear gaseous disk arises as a result of the galaxy merger. The black holes form an eccentric binary in the disk in less than a million years as a result of the gravitational drag from the gas rather than from the stars.

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arXiv2006

Multi-scale simulations of merging galaxies with supermassive black holes

We present the results of the first multi-scale N-Body+SPH simulations of merging galaxies containing central supermassive black holes (SMBHs) and having a spatial resolution of only a few parsecs. Strong gas inflows associated with equal-mass mergers produce non-axisymmetric nuclear disks with masses of order $10^9 M_{\odot}$, resolved by about $10^6$ SPH particles. Such disks have sizes of several hundred parsecs but most of their mass is concentrated within less than $50$ pc. We find that a close SMBH pair forms after the merger. The separation of the two SMBHs then shrinks further owing to dynamical friction against the predominantly gaseous background. The orbits of the SMBHs decay down to the minimum resolvable scale in a few million years for an ambient gas temperature and density typical of a region undergoing a starburst. These results suggest the initial conditions necessary for the eventual coalescence of the two holes arise naturally from the merging of two equal-mass galaxies whose structure and orbits are consistent with the predictions of the $Λ$CDM model. Our findings have important implications for planned gravitational wave detection experiments such as {\it LISA}.

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arXiv2005

The trail of discrete X-ray sources in the early-type galaxy NGC4261: anisotropy in the globular cluster distribution?

Chandra images of the elliptical galaxy NGC4261 have revealed an anisotropy in the spatial distribution of the off-nuclear X-ray sources, interpreted by Zezas et al. as evidence of an association with a young stellar population. Our independent analysis of archive X-ray (Chandra) and optical (INT and HST) observations confirms the anisotropy of the X-ray sources but conducts to a different interpretation for their origin. We find that nearly half of the X-ray sources are associated to a globular cluster (optical counterpart) suggesting that they are accreting low-mass X-ray binaries. Where color index information is available, the X-ray sources are found to reside in red (metal-rich) systems. The luminosity function of the X-ray sources is also consistent with the one drawn from a population of Low Mass X-ray Binaries. We further investigate the properties of the sample of point-like sources obtained from archival optical images that we suggest are good globular cluster candidates and for which we find that the projected spatial distribution is non-homogeneous. In addition, we show that the distributions of the optical and X-ray populations are very similar, which leads us to conclude that the spatial anisotropy of the X-ray sources in NGC4261 is mostly a reflection of the anisotropy of the globular cluster population.

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arXiv2005

Fate of Supermassive Black Holes and Evolution of the M(BH)-sigma Relation in Merging Galaxies: The Effect of Gaseous Dissipation

We analyze the effect of dissipation on the orbital evolution of supermassive black holes (SMBHs) using high-resolution self-consistent gasdynamical simulations of binary equal- and unequal-mass mergers of disk galaxies. The galaxy models are consistent with the LCDM paradigm of structure formation and the simulations include the effects of radiative cooling and star formation. We find that equal-mass mergers always lead to the formation of a close SMBH pair at the center of the remnant with separations limited solely by the adopted force resolution of ~ 100 pc. Instead, the final SMBH separation in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the satellite galaxy can be entirely disrupted before the merger is completed leaving its SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, we show that gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. Moreover, we demonstrate that merging disk galaxies constructed to obey the M(BH)-sigma relation, move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. Collisionless simulations reveal that remnants tend to move away from the mean relation highlighting the role of gas-poor mergers as a possible source of scatter. In dissipational mergers, the interplay between strong gas inflows associated with the formation of massive nuclear disks and the consumption of gas by star formation provides the necessary fuel to the SMBHs and allows the merger remnants to satisfy the relation.

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arXiv2004

Has the black hole in XTE J1118+480 experienced an asymmetric natal kick?

We explore the origin of the Galactic high latitude black hole X-ray binary XTE J1118+480, and in particular its birth location and the magnitude of the kick received by the black hole upon formation in the supernova explosion. We constrain the age of the companion to the black hole using stellar evolution calculations between 2 Gyr and 5 Gyr, making an origin in a globular cluster unlikely. We therefore argue that the system was born in the Galactic disk and the supernova propelled it in its current high latitude orbit. Given the current estimates on its distance, proper motion and radial velocity, we back-trace the orbit of XTE J1118+480 in the Galactic potential to infer the peculiar velocity of the system at different disk crossings over the last 5 Gyr. Taking into account the uncertainties on the velocity components, we infer an average peculiar velocity of 183 \pm 31 km/s. The maximum velocity which the binary can acquire by symmetric supernova mass loss is about 100 km/s, which is 2.7 sigma away from the mean of the peculiar velocity distribution. We therefore argue that an additional asymmetric kick velocity is required. By considering the orientation of the system relative to the plane of the sky, we derive a 95% probability for a non null component of the kick perpendicular to the orbital plane of the binary. The distribution of perpendicular velocities is skewed to lower velocities with an average of 93^{+55}_{-60} km/s.

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arXiv2003

Probing the presence of a single or binary black hole in the globular cluster NGC 6752 with pulsar dynamics

The five millisecond pulsars that inhabit NGC 6752 display locations or accelerations that are quite unusual compared to all other pulsars known in globular clusters. In particular PSR-A, a binary pulsar, lives in the cluster halo, while PSR-B and PSR-E, located in the core, show remarkably high negative spin derivatives. This is suggestive that some uncommon dynamical process is at play in the cluster core that we attribute to the presence of a massive perturber. We here investigate whether a single intermediate-mass black hole, lying on the extrapolation of the Mass versus Sigma relation observed in galaxy spheroids, or a less massive binary consisting of two black holes could play the requested role. To this purpose we simulated binary-binary encounters involving PSR-A, its companion star, and the black hole(s). Various scenarios are discussed in detail. In our close 4-body encounters, a black hole-black hole binary may attract on a long-term stable orbit a millisecond pulsar. Timing measurements on the captured satellite-pulsar, member of a hierarchical triplet, could unambiguously unveil the presence of a black hole(s) in the core of a globular cluster.

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arXiv2003

Is NGC 6752 Hosting a Single or a Binary Black Hole?

The five millisecond pulsars that inhabit NGC 6752 display locations or accelerations remarkably different with respect to all other pulsars known in globular clusters. This may reflect the occurrence of an uncommon dynamics in the cluster core that could be attributed to the presence of a massive perturber. We here investigate whether a single intermediate-mass black hole, lying on the extrapolation of the mass versus sigma relation observed in galaxy spheroids, or, a less massive black hole binary could play the requested role.

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arXiv2003

Spin-perpendicular kicks from evanescent binaries formed in the aftermath of rotational core-collapse and the nature of the observed bimodal distribution of pulsar peculiar velocities

If rotating core collapse leads to the formation of a proto-neutron star binary in super-close orbit, then the lighter star, propelled toward the minimum stable mass, explodes. The neutron star (or black hole) that remains acquires a spin-perpendicular kick of very large amplitude. Kicks of the type are required to explain the geodesic precession in double neutron star binaries such as B1913+16. On the contrary, spin-kick alignment has been claimed for the Vela and Crab pulsars whose kicks are relatively small. We propose that the larger kick component, when present in a pulsar, results from the disruption of an evanescent proto-neutron star binary and is spin perpendicular; the smaller kick component is associated to some other mechanism that leads to less vigorous kicks, predominantly parallel to the spin, because of phase averaging. This gives rise to a "bimodal distribution" in the peculiar velocities of neutron stars, as observed in the pulsar sample. The kick velocity is found to scale with the mass of the star that remains as M^(-2/3). This scenario can explain the run-away black hole GRO J1655-40, the first to show evidence for a natal kick.

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arXiv2003

On the Life and Death of Satellite Haloes

We study the evolution of dark matter satellites orbiting inside more massive haloes using semi-analytical tools coupled with high-resolution N-Body simulations. We select initial satellite sizes, masses, orbital energies, and eccentricities as predicted by hierarchical models of structure formation. Both the satellite and the main halo are described by a Navarro, Frenk & White density profile with various concentrations.

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arXiv2002

Is NGC6752 hiding a double black hole binary in its core ?

NGC6752 hosts in its halo PSR J1911-5958A, a newly discovered binary millisecond pulsar which is the most distant pulsar ever known from the core of a globular cluster. Interestingly, its recycling history seems in conflict with a scenario of ejection resulting from ordinary stellar dynamical encounters. A scattering event off a binary system of two black holes with masses in the range of 3-50 solar masses that propelled PSR J1911-5958A into its current peripheral orbit seems more likely. It is still an observational challenge to unveil the imprint(s) left from such a dark massive binary on cluster's stars: PSR J1911-5958A may be the first case.

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arXiv2002

Formation of an evanescent proto-neutron star binary and the origin of pulsar kicks

If core collapse leads to the formation of a rapidly rotating bar-unstable proto-neutron star surrounded by fall-back material, then we might expect it to cool and fragment to form a double (proto)-neutron star binary into a super-close orbit. The lighter star should survive for awhile, until tidal mass loss propels it toward the minimum stable mass of a (proto)-neutron star, whereupon it explodes. Imshennik and Popov have shown that the explosion of the unstable, cold star can result in a large recoil velocity of the remaining neutron star. Here, we consider several factors that mitigate the effect and broaden the range of final recoil speeds, in particular the finite velocity and gravitational deflection of the ejecta, a range of original masses for the low mass companion and its cooling history, rotational phase averaging of the momentum impulse from non-instantaneous mass loss, and the possibility of a common envelope phase. In spite of these mitigating factors, we argue that this mechanism can still lead to substantial neutron star recoil speeds, close to, or even above, 1000 km/s.

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arXiv2002

The case of PSR J1911-5958A in the outskirts of NGC 6752: signature of a black hole binary in the cluster core?

We have investigated different scenarios for the origin of the binary millisecond pulsar PSR J1911-5958A in NGC 6752, the most distant pulsar discovered from the core of a globular cluster to date. The hypothesis that it results from a truly primordial binary born in the halo calls for accretion-induced collapse and negligible recoil speed at the moment of neutron star formation. Scattering or exchange interactions off cluster stars are not consistent with both the observed orbital period and its offset position. We show that a binary system of two black holes with (unequal) masses in the range of 3-100 solar masses can live in NGC 6752 until present time and can have propelled PSR J1911-5958A into an eccentric peripheral orbit during the last ~1 Gyr.

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arXiv2002

Re-examining the X-Ray versus Spin-Down Luminosity Correlation of Rotation Powered Pulsars

The empirical relation between the X-ray luminosity (in the 2-10 keV band) and the rate of spin-down energy loss of a sample of 37 pulsars is re-examined considering recent data from ASCA, RXTE, BeppoSAX, Chandra, and XMM-Newton. The data show a significant scatter around an average correlation computed including statistical and systematic errors. All the X-ray luminosities are found to lie below a critical line corresponding to a maximum efficiency of conversion of rotational energy into X-rays varying between 0.1 and 80%. The large dispersion of the X-ray luminosity below the critical line indicates that other physical parameters uncorrelated with the period and period derivative need to be included to account for the observed emission at X-ray energies. We indicate a few possibilities that all conspire to lower the luminosity.

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arXiv2002

Radiation from the first forming stars

The evolution of radiation emitted during the dynamical collapse of metal-free protostellar clouds is investigated within a spherically symmetric hydrodynamical scheme that includes the transfer of radiation and the chemistry of the primordial gas. The cloud centre collapses on a time scale of about 10^5-10^6 years, thanks to line cooling from molecular hydrogen (H2). For most of the collapse time, when the evolution proceeds self-similarly, the luminosity slowly rises up to about 10^36 erg/s and is essentially due to H2 IR line emission. Later, continuum IR radiation provides an additional contribution, which is mostly due to the accretion of an infalling envelope upon a small hydrostatic protostellar core which develops in the centre. We follow the beginning of the accretion phase, when the enormous accretion rate (~ 0.1 Msun/yr) produces a very high continuum luminosity of about 10^36 erg/s. Despite the high luminosities, the radiation field is unable to affect the gas dynamics during the collapse and the first phases of accretion, because the opacity of the infalling gas is too small; this is quite different from present-day star formation. We also find that the protostellar evolution is similar among clouds with different initial configurations, including those resulting from 3D cosmological simulations of primordial objects; in particular, the shape of the molecular spectra is quite universal. Finally, we briefly discuss the detectability of this pristine cosmic star formation activity.

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arXiv2001

Disruption of satellites in cosmological haloes

We investigate how the survival of dark matter satellites inside virialized halos depends on tidal stripping and dynamical friction. We use an analytic approach and then compare the results with N-Body simulations. Both the satellites and the primary halos are similar to cosmological haloes and have NFW density profiles. Satellites can either merge with the primary halo or continue to move on barely perturbed orbits, eventually being disrupted, depending on the relative strength of friction and tidal forces. We propose that their actual fate depends simply on their mass ratio relative to the primary halo.

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arXiv2001

The Metamorphosis of Tidally Stirred Dwarf Galaxies

We present results from high-resolution N-Body/SPH simulations of rotationally supported dwarf irregular galaxies moving on bound orbits in the massive dark matter halo of the Milky Way.The dwarf models span a range in disk surface density and the masses and sizes of their dark halos are consistent with the predictions of cold dark matter cosmogonies. We show that the strong tidal field of the Milky Way determines severe mass loss in their halos and disks and induces bar and bending instabilities that transform low surface brightness dwarfs (LSBs) into dwarf spheroidals (dSphs) and high surface brightness dwarfs (HSBs) into dwarf ellipticals (dEs) in less than 10 Gyr. The final central velocity dispersions of the remnants are in the range 8-30 km/s and their final $v/σ$ falls to values $< 0.5$, matching well the kinematics of early-type dwarfs. The transformation requires the orbital time of the dwarf to be $\simlt 3-4$ Gyr, which implies a halo as massive and extended as predicted by hierarchical models of galaxy formation to explain the origin of even the farthest dSph satellites of the Milky Way, Leo I and Leo II. Only dwarfs with central dark matter densities as high as those of Draco and Ursa Minor can survive for 10 Gyr in the proximity of the Milky Way: this is naturally achieved within hierarchical models, where the densest objects should have small orbital times due to their early formation epochs. Part of the gas is stripped and part is funneled to the center due to the bar, generating one strong burst of star formation in HSBs and smaller, multiple bursts in LSBs. Extended low-surface brightness stellar and gaseous streams originate from LSBs and, when projected along the line of sight, can lead to overestimate the mass-to-light ratio of the bound remnant by a factor $\simlt 2$,

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arXiv2001

Neutron star cooling in transiently accreting low mass binaries: a new tool for probing nuclear matter

We explore, using an exact cooling code, the thermal evolution of a neutron star undergoing episodes of intense accretion, alternated by long periods of quiescence (e.g. Soft X-Ray Transients). We find that the soft component of the quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be understood as thermal emission from a cooling neutron star with negligible neutrino emission. In the case of Cen X-4 strong neutrino emission from the inner core is necessary to explain the observation: this may indicate that the neutron star of Cen X-4 is heavier than 1.4 Msun. This study opens the possibility of using the quiescent emission of Soft X-Ray Transients as a tool for probing the core superfluidity in relation to the mass of the neutron star.

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