arXiv subjects

Piero Madau

Publications and source records attributed to Piero Madau.

At least 109 records · Page 6Linked to original sources

arXiv2013

Inside Out and Upside Down: Tracing the Assembly of a Simulated Disk Galaxy Using Mono-Age Stellar Populations

We analyze the present-day structure and assembly history of a high resolution hydrodynamic simulation of the formation of a Milky Way (MW)-like disk galaxy, from the "Eris" simulation suite, dissecting it into cohorts of stars formed at different epochs of cosmic history. At z=0, stars with t_form < 2 Gyr mainly occupy the stellar spheroid, with the oldest (earliest forming) stars having more centrally concentrated profiles. The younger age cohorts populate disks of progressively longer radial scale length and shorter vertical scale height. At a given radius, the vertical density profiles and velocity dispersions of stars vary smoothly as a function of age, and the superposition of old, vertically-extended and young, vertically-compact cohorts gives rise to a double-exponential profile like that observed in the MW. Turning to formation history, we find that the trends of spatial structure and kinematics with stellar age are largely imprinted at birth, or immediately thereafter. Stars that form during the active merger phase at z>3 are quickly scattered into rounded, kinematically hot configurations. The oldest disk cohorts form in structures that are radially compact and relatively thick, while subsequent cohorts form in progressively larger, thinner, colder configurations from gas with increasing levels of rotational support. The disk thus forms "inside-out" in a radial sense and "upside-down" in a vertical sense. Secular heating and radial migration influence the final state of each age cohort, but the changes they produce are small compared to the trends established at formation. The predicted correlations of stellar age with spatial and kinematic structure are in good qualitative agreement with the correlations observed for mono-abundance stellar populations in the MW.

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arXiv2013

The HST/ACS+WFC3 Survey for Lyman Limit Systems II: Science

We present the first science results from our Hubble Space Telescope Survey for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through an analysis of 71 quasars, we determine the incidence frequency of LLS per unit redshift and per unit path length, l(z) and l(x) respectively, over the redshift range 1 < z< 2.6, and find a weighted mean of l(x)=0.29 +/-0.05 for 2.0 < z < 2.5 through a joint analysis of our sample and that of Ribaudo et al. (2011). Through stacked spectrum analysis, we determine a median (mean) value of the mean free path to ionizing radiation at z=2.4 of lambda_mfp = 243(252)h^(-1) Mpc, with an error on the mean value of +/- 43h^(-1) Mpc. We also re-evaluate the estimates of lambda_mfp from Prochaska et al. (2009) and place constraints on the evolution of lambda_mfp with redshift, including an estimate of the "breakthrough" redshift of z = 1.6. Consistent with results at higher z, we find that a significant fraction of the opacity for absorption of ionizing photons comes from systems with N_HI <= 10^{17.5} cm^(-2) with a value for the total Lyman opacity of tau_lyman = 0.40 +/- 0.15. Finally, we determine that at minimum, a 5-parameter (4 power-law) model is needed to describe the column density distribution function f(N_HI, X) at z \sim 2.4, find that f(N_HI,X) undergoes no significant change in shape between z \sim 2.4 and z \sim 3.7, and provide our best fit model for f(N_HI,X).

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arXiv2013

The circumgalactic medium of massive galaxies at z~3: a test for stellar feedback, galactic outflows, and cold streams

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift 3, using the "Eris" suite of cosmological "zoom-in" simulations. The reference run adopts a blastwave scheme for supernova feedback that produces galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. Synthetic spectra through the multiphase CGM produce interstellar absorption line strengths of Lya, CII, CIV, SiII, and SiIV as a function of galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high-redshift by Steidel et al. (2010). Only about one third of all the gas within R_vir is outflowing. The fraction of sightlines within one virial radius that intercept optically thick material is 27%, in agreement with recent observations by Rudie et al. (2012). Such optically thick absorption is shown to trace inflowing "cold" streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar, give origin to strong (log N > 13) CII absorption with a covering factor of 22% within R_vir and 10% within 2 R_vir. Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large OVI halo, with a typical column density log N>14 and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. (2011). Our zoom-in simulations of this single system appear to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings. (Abridged)

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arXiv2013

An Off-center Density Peak in the Milky Way's Dark Matter Halo?

We show that the position of the central dark matter density peak may be expected to differ from the dynamical center of the Galaxy by several hundred parsec. In Eris, a high resolution cosmological hydrodynamics simulation of a realistic Milky-Way-analog disk galaxy, this offset is 300 - 400 pc (~3 gravitational softening lengths) after z=1. In its dissipationless dark-matter-only twin simulation ErisDark, as well as in the Via Lactea II and GHalo simulations, the offset remains below one softening length for most of its evolution. The growth of the DM offset coincides with a flattening of the central DM density profile in Eris inwards of ~1 kpc, and the direction from the dynamical center to the point of maximum DM density is correlated with the orientation of the stellar bar, suggesting a bar-halo interaction as a possible explanation. A dark matter density offset of several hundred parsec greatly affects expectations of the dark matter annihilation signals from the Galactic Center. It may also support a dark matter annihilation interpretation of recent reports by Weniger (2012) and Su & Finkbeiner (2012) of highly significant 130 GeV gamma-ray line emission from a region 1.5 degrees (~200 parsec projected) away from Sgr A* in the Galactic plane.

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arXiv2013

Direct Gravitational Imaging of Intermediate Mass Black Holes in Extragalactic Halos

A galaxy halo may contain a large number of intermediate mass black holes (IMBHs) with masses in the range of 10^{2-6} solar mass. We propose to directly detect these IMBHs by observing multiply imaged QSO-galaxy or galaxy-galaxy strong lens systems in the submillimeter bands with high angular resolution. The silhouette of an IMBH in the lensing galaxy halo would appear as either a monopole-like or a dipole-like variation at the scale of the Einstein radius against the Einstein ring of the dust-emitting region surrounding the QSO. We use a particle tagging technique to dynamically populate a Milky Way-sized dark matter halo with black holes, and show that the surface mass density and number density of IMBHs have power-law dependences on the distance from the center of the host halo if smoothed on a scale of ~ 1 kpc. Most of the black holes orbiting close to the center are freely roaming as they have lost their dark matter hosts during infall due to tidal stripping. Next generation submillimeter telescopes with high angular resolution (< 0.3 mas) will be capable of directly mapping such off-nuclear freely roaming IMBHs with a mass of ~ 10^6 solar mass in a lensing galaxy that harbours a O(10^9) solar mass supermassive black hole in its nucleus.

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arXiv2012

Pseudobulge Formation as a Dynamical Rather than a Secular Process

We investigate the formation and evolution of the pseudobulge in "Eris", a high-resolution N-body + smoothed particle hydrodynamics (SPH) cosmological simulation that successfully reproduces a Milky Way-like massive late-type spiral in a cold dark matter (LCDM) universe. At the present epoch, Eris has a virial mass Mvir=8x10^11 Msun, a photometric stellar mass M*=3.2x10^10 Msun, a bulge-to-total ratio B/T = 0.26, and a weak nuclear bar. We find that the bulk of the pseudobulge forms quickly at high redshift via a combination of non-axisymmetric disk instabilities and tidal interactions or mergers both occurring on dynamical timescales, not through slow secular processes at lower redshift. Its subsequent evolution is not strictly secular either, and is closely intertwined with the evolution of the stellar bar. In fact, the structure that we recognize as a pseudobulge today evolves from a stellar bar that formed at high redshift, was destroyed by minor mergers at z~3, reformed shortly after, and weakened again following a steady gas inflow at z<1. The gradual dissolution of the bar ensues at z=1 and continues until the present without increasing the stellar velocity dispersion in the inner regions. In this scenario the pseudobulge is not a separate component from the inner disk in terms of formation path, rather it is the first step in the inside-out formation of the baryonic disk, in agreement with the fact that pseudobulges of massive spiral galaxies have typically a dominant old stellar population. If our simulations do indeed reproduce the formation mechanisms of massive spirals, then the progenitors of late-type galaxies should have strong bars and small photometric pseudobulges at high redshift.

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arXiv2012

The origin of metals in the circum-galactic medium of massive galaxies at z=3

We present a detailed study of the metal-enriched circum-galactic medium of a massive galaxy at z=3 using "ErisMC", a new cosmological hydrodynamic "zoom-in" simulations of a disk galaxy with mass comparable to the Milky Way. The run adopts a blastwave scheme for supernova feedback that generates galactic outflows without explicit wind particles, a star formation recipe based on a high gas density threshold, and high temperature metal cooling. ErisMC's main progenitor at z=3 resembles a "Lyman break" galaxy of mass M_vir=2.4e11 M_sun, virial radius R_vir=48 kpc, and star formation rate 18 M_sun/yr, and its metal-enriched CGM extends to 200 (physical) kpc from its center. Approximately 41, 9, and 50 percent of all gas-phase metals at z=3 are locked in a hot (T> 3e5 K), warm (3e5 >T> 3e4 K), and cold (T< 3e4 K) medium, respectively. We identify three sources of heavy elements: 1) the main host, responsible for 60% of all the metals found within 3R_vir; 2) its satellite progenitors, responsible for 28% of all the metals within 3R_vir, and for only 5% of those beyond 3R_vir; and nearby dwarfs, which give origin to 12% of all the metals within 3R_vir and 95% of those beyond 3R_vir. Late (z<5) galactic "superwinds" account for only 9% of all the metals observed beyond 2R_vir, the bulk having been released at redshifts 5< z < 8 by early star formation and outflows. In the CGM, lower overdensities are typically enriched by `older', colder metals. Heavy elements are accreted onto Eris along filaments via low-metallicity cold inflows, and are ejected hot via galactic outflows at a few hundred km/s. The outflow mass-loading factor is of order unity for the main halo, but can exceed 10 for nearby dwarfs. We stress that our "zoom-in" simulation focuses on the CGM of a single massive system and cannot describe the enrichment history of the IGM as a whole. (abridged)

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arXiv2012

Insight Into the Formation of the Milky Way Through Cold Halo Substructure. III. Statistical Chemical Tagging in the Smooth Halo

We find that the relative contribution of satellite galaxies accreted at high redshift to the stellar population of the Milky Way's smooth halo increases with distance, becoming observable relative to the classical smooth halo about 15 kpc from the Galactic center. In particular, we determine line-of-sight-averaged [Fe/H] and [alpha/Fe] in the metal-poor main-sequence turnoff (MPMSTO) population along every Sloan Extension for Galactic Understanding and Exploration (SEGUE) spectroscopic line of sight. Restricting our sample to those lines of sight along which we do not detect elements of cold halo substructure (ECHOS), we compile the largest spectroscopic sample of stars in the smooth component of the halo ever observed in situ beyond 10 kpc. We find significant spatial autocorrelation in [Fe/H] in the MPMSTO population in the distant half of our sample beyond about 15 kpc from the Galactic center. Inside of 15 kpc however, we find no significant spatial autocorrelation in [Fe/H]. At the same time, we perform SEGUE-like observations of N-body simulations of Milky Way analog formation. While we find that halos formed entirely by accreted satellite galaxies provide a poor match to our observations of the halo within 15 kpc of the Galactic center, we do observe spatial autocorrelation in [Fe/H] in the simulations at larger distances. This observation is an example of statistical chemical tagging and indicates that spatial autocorrelation in metallicity is a generic feature of stellar halos formed from accreted satellite galaxies.

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arXiv2011

Radiative transfer in a clumpy universe: IV. New synthesis models of the cosmic UV/X-ray background

We present improved synthesis models of the evolving spectrum of the UV/X-ray diffuse background, updating and extending our previous results. Five new main components are added to our radiative transfer code CUBA: (1) the sawtooth modulation of the background intensity from resonant line absorption in the Lyman series of cosmic hydrogen and helium; (2) the X-ray emission from obscured and unobscured quasars; (3) a piecewise parameterization of the distribution in redshift and column density of intergalactic absorbers that fits recent measurements of the mean free path of 1 ryd photons; (4) an accurate treatment of the photoionization structure of absorbers; and (5) the UV emission from star-forming galaxies at all redshifts. We provide tables of the predicted HI and HeII photoionization and photoheating rates for use, e.g., in cosmological hydrodynamics simulations of the Lya forest, and a new metallicity-dependent calibration to the UV luminosity density-star formation rate density relation. A "minimal cosmic reionization model" is also presented in which the galaxy UV emissivity traces recent determinations of the cosmic history of star formation, the luminosity-weighted escape fraction of hydrogen-ionizing radiation increases rapidly with lookback time, the clumping factor of the high-redshift intergalactic medium evolves following the results of recent hydrodynamic simulations, and Population III stars and miniquasars make a negligible contribution to the metagalactic flux. The model provides a good fit to the hydrogen-ionization rates inferred from flux decrement and proximity effect measurements, predicts that cosmological HII (HeIII) regions overlap at redshift 6.7 (2.8), and yields an optical depth to Thomson scattering that is in agreement with WMAP results. (Abridged)

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arXiv2011

On the assembly of the Milky Way dwarf satellites and their common mass scale

We use a particle tagging technique to dynamically populate the N-body Via Lactea II high-resolution simulation with stars. The method is calibrated using the observed luminosity function of Milky Way satellites and the concentration of their stellar populations, and self-consistently follows the accretion and disruption of progenitor dwarfs and the build-up of the stellar halo in a cosmological "live host". Simple prescriptions for assigning stellar populations to collisionless particles are able to reproduce many properties of the observed Milky Way halo and its surviving dwarf satellites, like velocity dispersions, sizes, brightness profiles, metallicities, and spatial distribution. Our model predicts the existence of approximately 1,850 subhalos harboring "extremely faint" satellites (with mass-to-light ratios >5,000) lying beyond the Sloan Digital Sky Survey detection threshold. Of these, about 20 are "first galaxies", i.e. satellites that formed a stellar mass above 10 Msun before redshift 9. The ten most luminous satellites (L> 1e6 Lsun) in the simulation are hosted by subhalos with peak circular velocities today in the range V_max=10-40 km/s that have shed between 80% and 99% of their dark mass after being accreted at redshifts 1.7< z <4.6. The satellite maximum circular velocity and stellar line-of-sight velocity dispersion today follow the relation V_max=2.2 sigma_los. We apply a standard mass estimation algorithm based on Jeans modelling of the line-of-sight velocity dispersion profiles to the simulated dwarf spheroidals, and test the accuracy of this technique. The inner (within 300 pc) mass-luminosity relation for currently detectable satellites is nearly flat in our model, in qualitative agreement with the "common mass scale" found in Milky Way dwarfs. We do, however, predict a weak, but significant positive correlation for these objects: M_300 ~L^{0.088 \pm 0.024}.

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arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey - The Hubble Space Telescope Observations, Imaging Data Products and Mosaics

This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at $z\sim1.5-8$, and to study Type Ia SNe beyond $z>1.5$. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers \sim125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of \sim800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

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arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8--1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z>1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10^9 M_\odot to z \approx 2, reaching the knee of the ultraviolet luminosity function (UVLF) of galaxies to z \approx 8. The survey covers approximately 800 arcmin^2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H=27.7 mag) covers \sim 125 arcmin^2 within GOODS-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (EGS, COSMOS, and UDS) and covers the full area to a 5σ point-source limit of H \gtrsim 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

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arXiv2011

Forming Realistic Late-Type Spirals in a LCDM Universe: The Eris Simulation

Simulations of the formation of late-type spiral galaxies in a cold dark matter LCDM universe have traditionally failed to yield realistic candidates. Here we report a new cosmological N-body/SPH simulation of extreme dynamic range in which a close analog of a Milky Way disk galaxy arises naturally. Termed Eris, the simulation follows the assembly of a galaxy halo of mass Mvir=7.9x10^11 Msun with a total of N=18.6 million particles (gas + dark matter + stars) within the final virial radius, and a force resolution of 120 pc. It includes radiative cooling, heating from a cosmic UV field and supernova explosions, a star formation recipe based on a high gas density threshold (nSF=5 atoms cm^-3 rather than the canonical nSF=0.1 atoms cm^-3), and neglects AGN feedback. At the present epoch, the simulated galaxy has an extended rotationally-supported disk with a radial scale length Rd=2.5 kpc, a gently falling rotation curve with circular velocity at 2.2 disk scale lenghts of V2.2=214 km/s, a bulge-to-disk ratio B/D=0.35, and a baryonic mass fraction that is 30% below the cosmic value. The disk is thin, is forming stars in the region of the Sigma_SFR - Sigma_HI plane occupied by spiral galaxies, and falls on the photometric Tully-Fisher and the stellar mass-halo virial mass relations. Hot (T>3x10^5 K), X-ray luminous halo gas makes only 26% of the universal baryon fraction and follows a flattened density profile proportional to r^-1.13 out to r=100 kpc. Eris appears then to be the first cosmological hydrodynamic simulation in which the galaxy structural properties, the mass budget in the various components, and the scaling relations between mass and luminosity are all consistent with a host of observational constraints. (Abridged)

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arXiv2011

A galaxy as the source of a Civ absorption system close to the epoch of reionization

We find a bright (L_{UV}=2.5 L*_{z=6}) Lyman alpha emitter at redshift z=5.719 at a projected distance of 79 physical kpc from a strong triply ionized carbon (Civ) absorption system at redshift z=5.7238 previously reported in the spectrum of the z_{em} = 6.309 QSO SDSS J1030+0524. This is the highest redshift galaxy-absorber pair detected to-date, supporting the idea that galaxy-wide outflows were already in place at the end of the epoch of reionization. The proximity of this object makes it the most likely source of metals, consistent with models of outflows at lower redshift where significant observational evidence relates metal absorption systems with galaxies hosting outflows. In a typical outflow scenario, a wind of 200 km/s, active since the universe was only 0.6 Gyr old (z ~8.4), could eject metals out to 79 kpc at z=5.719. Although the origin of metals in the intergalactic medium (IGM) is still under debate, our results are consistent with predictions from cosmological simulations which reproduce the evolution of the cosmic density of Civ, from z ~ 6 to the present day based on outflow-driven enrichment of the IGM. We also report two more Lyman alpha emitters in this field, at z=5.973\pm 0.002 and z=5.676\pm 0.002 respectively, the former confirming the original identification by Stiavelli et al. Our results suggest that the colour cut typically used to identify i-dropouts (i_{775}-z_{850}>1.3) misses a non-negligible fraction of blue galaxies with faint UV continuum at z \geq 5.7.

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arXiv2011

New Population Synthesis Models of the Cosmic UV Background

We present improved synthesis models of the evolving spectrum of the UV/X-ray diffuse background, updating and extending our previous results. A "minimal cosmic reionization model" is presented in which the galaxy UV emissivity traces recent determinations of the cosmic history of star formation, the luminosity-weighted escape fraction of hydrogen-ionizing radiation increases rapidly with redshift, from 1% at z=3 to 50% at z=8.5, the clumping factor of the intergalactic medium is C=3, and Population III stars make a negligible contribution to the metagalactic flux. The model provides a good fit to the hydrogen-ionization rates inferred from flux decrement measurements, and yields an optical depth to Thomson scattering, tau_e=0.085, that is in agreement with WMAP results.

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arXiv2011

Delayed Enrichment by Unseen Galaxies: Explaining the Rapid Rise in IGM CIV Absorption from z = 6-5

In the near future, measurements of metal absorption features in the intergalactic medium (IGM) will become an important constraint on models of the formation and evolution of the earliest galaxies, the properties of the first stars, and the reionization and enrichment of the IGM. The first measurement of a metal abundance in the IGM at a redshift approaching the epoch of reionization already offers intriguing hints. Between z=5.8 and 4.7 (a 0.3 Gyr interval only 1 Gyr after the big bang), the measured density of CIV absorbers in the IGM increased by a factor of ~ 3.5 (Ryan-Weber et al. 2009; Becker, Rauch & Sargent 2009). If these values prove to be accurate, they pose two puzzles: (1) The total amount of CIV at z=5.8 implies too little star formation to reionize the IGM by z=6 or to match the WMAP electron scattering optical depth (tau). (2) The rapid growth from z = 6-5 is faster than the buildup of stellar mass or the increase in the star formation rate density over the same interval. We show that a delay of ~ 0.4-0.7 Gyr between the instantaneous production of ionizing photons and the later production of metal absorption features (added to the delay due to stellar lifetimes) can provide the full explanation for both puzzles. We calculate the delay in metal production due to finite stellar lifetimes alone and find that it is too short to explain the rapid CIV density increase. The additional delay could naturally be explained as the result of ~ 200 km/s outflows carrying carbon to distances of ~ 100 kpc, the typical distance between galaxies and CIV absorbers in enrichment simulations, and the typical outflow or absorption region scale observed at z ~ 2-3.

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arXiv2010

Tidal stellar disruptions by massive black hole pairs: II. Decaying binaries

Tidal stellar disruptions have traditionally been discussed as a probe of the single, massive black holes (MBHs) that are dormant in the nuclei of galaxies. In Chen et al. (2009), we used numerical scattering experiments to show that three-body interactions between bound stars in a stellar cusp and a non-evolving "hard" MBH binary will also produce a burst of tidal disruptions, caused by a combination of the secular "Kozai effect" and by close resonant encounters with the secondary hole. Here we derive basic analytical scalings of the stellar disruption rates with the system parameters, assess the relative importance of the Kozai and resonant encounter mechanisms as a function of time, discuss the impact of general relativistic (GR) and extended stellar cusp effects, and develop a hybrid model to self-consistently follow the shrinking of an MBH binary in a stellar background, including slingshot ejections and tidal disruptions. In the case of a fiducial binary with primary hole mass M_1=10^7\msun and mass ratio q=M_2/M_1=1/81, embedded in an isothermal cusp, we derive a stellar disruption rate \dot{N_*} ~ 0.2/yr lasting ~ 3X10^5 yr. This rate is 3 orders of magnitude larger than the corresponding value for a single MBH fed by two-body relaxation, confirming our previous findings. For q<<0.01, the Kozai/chaotic effect could be quenched due to GR/cusp effects by an order of magnitude, but even in this case the stellar-disruption rate is still two orders of magnitude larger than that given by standard relaxation processes around a single MBH. Our results suggest that >~10% of the tidal-disruption events may originate in MBH binaries.

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arXiv2010

Recoiling Massive Black Holes in Gas-Rich Galaxy Mergers

The asymmetric emission of gravitational waves produced during the coalescence of a massive black hole (MBH) binary imparts a velocity "kick" to the system that can displace the hole from the center of its host. Here we study the trajectories and observability of MBHs recoiling in three (one major, two minor) gas-rich galaxy merger remnants that were previously simulated at high resolution, and in which the pairing of the MBHs had been shown to be successful. We run new simulations of MBHs recoiling in the major merger remnant with Mach numbers in the range 1<M<6, and use simulation data to construct a semi-analytical model for the orbital evolution of MBHs in gas-rich systems. We show that: 1) in major merger remnants the energy deposited by the moving hole into the rotationally supported, turbulent medium makes a negligible contribution to the thermodynamics of the gas. This contribution becomes significant in minor merger remnants, potentially allowing for an electromagnetic signature of MBH recoil; 2) in major merger remnants, the combination of both deeper central potential well and drag from high-density gas confines even MBHs with kick velocities as high as 1200 km/s within 1 kpc from the host's center; 3) kinematically offset nuclei may be observable for timescales of a few Myr in major merger remnants in the case of recoil velocities in the range 700-1,000 km/s; 4) in minor mergers remnants the effect of gas drag is weaker, and MBHs with recoil speeds in the range 300-600 km/s will wander through the host halo for longer timescales. When accounting for the probability distribution of kick velocities, however, we find that the likelihood of observing recoiling MBHs in gas-rich galaxy mergers is very low, typically below 10^-5 - 10^-6.

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