Publications


Discovery of a nearby 1700 km/s star ejected from the Milky Way by Sgr A*

Monthly Notices of the Royal Astronomical Society Oxford University Press 491 (2019) 2465-2480

D Boubert, TS Li, D Erkal, GS Da Costa, DB Zucker, AP Ji, K Kuehn, GF Lewis, D Mackey, JD Simpson, N Shipp, Z Wan, V Belokurov, J Bland-Hawthorn, SL Martell, T Nordlander, D De Silva, M-Y Wang

<jats:title>Abstract</jats:title> <jats:p>We present the serendipitous discovery of the fastest Main Sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a ∼2.35 M⊙ A-type star located at a distance of ∼9 kpc from the Sun and has a heliocentric radial velocity of 1017 ± 2.7  km s−1 without any signature of velocity variability. The current 3-D velocity of the star in the Galactic frame is 1755 ± 50  km s−1. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A* with a velocity of ∼1800  km s−1 and travelled for 4.8 Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion Vy, ⊙ = 246.1 ± 5.3  km s−1 or position R0 = 8.12 ± 0.23 kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disk of young stars at the Galactic centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.</jats:p>


Trapped orbits and solar-neighbourhood kinematics

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 495 (2020) 895-904

J Binney


Angle-action variables for orbits trapped at a Lindblad resonance

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 495 (2020) 886-894

J Binney


Intermediate-mass Black Holes' Effects on Compact Object Binaries

ASTROPHYSICAL JOURNAL American Astronomical Society 892 (2020) ARTN 130

B Deme, Y Meiron, B Kocsis

Although their existence is not yet confirmed observationally, intermediate mass black holes (IMBHs) may play a key role in the dynamics of galactic nuclei. In this paper, we neglect the effect of the nuclear star cluster itself and investigate only how a small reservoir of IMBHs influences the secular dynamics of stellar-mass black hole binaries, using N-body simulations. We show that our simplifications are valid and that the IMBHs significantly enhance binary evaporation by pushing the binaries into the Hill-unstable region of parameter space, where they are separated by the SMBH's tidal field. For binaries in the S-cluster region of the Milky Way, IMBHs drive the binaries to merge in up to 1-6% of cases, assuming five IMBHs within 5 pc of mass 10,000 solar masses each. Observations of binaries in the Galactic center may strongly constrain the population of IMBHs therein.


Making a Supermassive Star by Stellar Bombardment

ASTROPHYSICAL JOURNAL American Astronomical Society 892 (2020) ARTN 36

H Tagawa, Z Haiman, B Kocsis

Approximately two hundred supermassive black holes (SMBHs) have been discovered within the first $\sim$Gyr after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation, gas accretion, dynamical friction from stars and gas, stellar collisions, and gas ejection. We find that when the collapsing gas fragments at a density of $\lesssim 3\times 10^{10}\,\mathrm{cm^{-3}}$, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a $\lesssim 10^4~{\rm M_\odot}$ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with $Z\lesssim10^{-5}\,{\rm Z_\odot}$ with suppressed ${\rm H_2}$ abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as $\sim 10^5-10^6\,{\rm M_\odot}$. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.


Cosmic Evolution of Stellar-mass Black Hole Merger Rate in Active Galactic Nuclei

ASTROPHYSICAL JOURNAL American Astronomical Society 896 (2020) ARTN 138

Y Yang, I Bartos, Z Haiman, B Kocsis, S Marka, H Tagawa


GW170817A as a Hierarchical Black Hole Merger

ASTROPHYSICAL JOURNAL LETTERS American Astronomical Society 890 (2020) ARTN L20

V Gayathri, I Bartos, Z Haiman, S Klimenko, B Kocsis, S Marka, Y Yang

Despite the rapidly growing number of stellar-mass binary black hole mergers discovered through gravitational waves, the origin of these binaries is still not known. In galactic centers, black holes can be brought to each others' proximity by dynamical processes, resulting in mergers. It is also possible that black holes formed in previous mergers encounter new black holes, resulting in so-called hierarchical mergers. Hierarchical events carry signatures such as higher-than usual black hole mass and spin. Here we show that the recently reported gravitational-wave candidate, GW170817A, could be the result of such a hierarchical merger. In particular, its chirp mass $\sim40$ M$_\odot$ and effective spin of $\chi_{\rm eff}\sim0.5$ are the typically expected values from hierarchical mergers within the disks of active galactic nuclei. We find that the reconstructed parameters of GW170817A strongly favor a hierarchical merger origin over having been produced by an isolated binary origin (with an Odds ratio of $&gt;10^3$, after accounting for differences between the expected rates of hierarchical versus isolated mergers)


Spin Evolution of Stellar-mass Black Hole Binaries in Active Galactic Nuclei

The Astrophysical Journal American Astronomical Society 899 (2020) 26-26

H Tagawa, Z Haiman, I Bartos, B Kocsis


Formation and Evolution of Compact-object Binaries in AGN Disks

ASTROPHYSICAL JOURNAL American Astronomical Society 898 (2020) ARTN 25

H Tagawa, Z Haiman, B Kocsis


Effective spin distribution of black hole mergers in triples

Monthly Notices of the Royal Astronomical Society 493 (2020) 3920-3931

G Fragione, B Kocsis

&#xA9; 2020 The Author(s). Many astrophysical scenarios have been proposed to explain the several black hole (BH) and neutron star binary mergers observed via gravitational waves (GWs) by the LIGO-Virgo collaboration. Contributions from various channels can be statistically disentangled by mass, spin, eccentricity, and redshift distributions of merging binaries. In this paper,we investigate the signatures of BH-BH binary mergers induced by a third companion through the Lidov-Kozai mechanism in triple systems. We adopt different prescriptions for the supernovae natal kicks and consider different progenitor metallicities and initial orbital parameters. We show that the typical eccentricity in the LIGO band is 0.01-0.1 and that the merger rate is in the range 0.008-9Gpc-3 yr-1, depending on the natal kick prescriptions and progenitor metallicity. Furthermore, we find that the typical distribution of effective projected spin is peaked at Xeff ~ 0 with significant tails. We show that the triple scenario could reproduce the distribution of Xeff. We find that the triple channel may be strongly constrained by the misalignment angle between the binary component spins in future detections with spin precession.


K-CLASH: spatially-resolving star-forming galaxies in field and cluster environments at z ≈ 0.2-0.6

Monthly Notices of the Royal Astronomical Society Oxford University Press (2020)

AL Tiley, JP Stott, R Davies, LJ Prichard, A Bunker, M Bureau, M Cappellari, M Jarvis, A Robotham, L Cortese, S Bellstedt, B Ansarinejad

We present the KMOS-CLASH (K-CLASH) survey, a K-band Multi-Object Spectrograph (KMOS) survey of the spatially-resolved gas properties and kinematics of 191 (predominantly blue) Hα-detected galaxies at 0.2 ≲ z ≲ 0.6 in field and cluster environments. K-CLASH targets galaxies in four Cluster Lensing And Supernova survey with Hubble (CLASH) fields in the KMOS IZ-band, over 7′ radius (≈2–3 Mpc) fields-of-view. K-CLASH aims to study the transition of star-forming galaxies from turbulent, highly star-forming disc-like and peculiar systems at z ≈ 1–3, to the comparatively quiescent, ordered late-type galaxies at z ≈ 0, and to examine the role of clusters in the build-up of the red sequence since z ≈ 1. In this paper, we describe the K-CLASH survey, present the sample, and provide an overview of the K-CLASH galaxy properties. We demonstrate that our sample comprises star-forming galaxies typical of their stellar masses and epochs, residing both in field and cluster environments. We conclude K-CLASH provides an ideal sample to bridge the gap between existing large integral-field spectroscopy surveys at higher and lower redshifts. We find that star-forming K-CLASH cluster galaxies at intermediate redshifts have systematically lower stellar masses than their star-forming counterparts in the field, hinting at possible “downsizing” scenarios of galaxy growth in clusters at these epochs. We measure no difference between the star-formation rates of Hα-detected, star-forming galaxies in either environment after accounting for stellar mass, suggesting that cluster quenching occurs very rapidly during the epochs probed by K-CLASH, or that star-forming K-CLASH galaxies in clusters have only recently arrived there, with insufficient time elapsed for quenching to have occured.


Electromagnetic transients and gravitational waves from white dwarf disruptions by stellar black holes in triple systems

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2020)

G Fragione, BD Metzger, R Perna, NWC Leigh, B Kocsis

Mergers of binaries comprised of compact objects can give rise to explosive transient events, heralding the birth of exotic objects which cannot be formed through single star evolution. Using a large number of direct N-body simulations, we explore the possibility that a white dwarf (WD) is dynamically driven to tidal disruption by a stellar-mass black hole (BH) as a consequence of the joint effects of gravitational wave (GW) emission and Lidov-Kozai oscillations imposed by the tidal field of a outer tertiary companion orbiting the inner BH-WD binary. We explore the sensitivity of our results to the distributions of natal kick velocities imparted to the BH and WD upon formation, adiabatic mass loss, semi-major axes and eccentricities of the triples, and stellar mass ratios. We find rates of WD-TDEs in the range $1.2\times 10^{-3}-1.4$ Gpc$^{-3}$ yr$^{-1}$ for $z\leq 0.1$, rarer than stellar TDEs in triples by a factor of $\sim 3$--$30$. The uncertainty in the TDE rates may be greatly reduced in the future using gravitational wave (GW) observations of Galactic binaries and triples with LISA. WD-TDEs may give rise to high energy X-ray or gamma-ray transients of duration similar to long gamma-ray bursts but lacking the signatures of a core-collapse supernova, while being accompanied by a supernova-like optical transient which lasts for only days. WD--BH and WD--NS binaries will also emit GWs in the LISA band before the TDE. The discovery and identification of triple-induced WD-TDE events by future time domain surveys and/or GWs could enable the study of the demographics of BHs in nearby galaxies.


Binary Intermediate-mass Black Hole Mergers in Globular Clusters

The Astrophysical Journal American Astronomical Society 899 (2020) 149-149

A Rasskazov, G Fragione, B Kocsis


Formation channels of slowly rotating early-type galaxies

Astronomy and Astrophysics EDP Sciences 635 (2020) A129

D Krajnovic, U Ural, H Kuntschner, P Goudfrooij, M Wolfe, M Cappellari, R Davies, TP de Zeeuw, P-A Duc, E Emsellem, A Karick, RM McDermid, S Mei, T Naab

We study the evidence for a diversity of formation processes in early-type galaxies by presenting the first complete volume-limited sample of slow rotators with both integral-field kinematics from the ATLAS3D Project and high spatial resolution photometry from the Hubble Space Telescope. Analysing the nuclear surface brightness profiles of 12 newly imaged slow rotators, we classify their light profiles as core-less, and place an upper limit to the core size of about 10 pc. Considering the full magnitude and volume-limited ATLAS3D sample, we correlate the presence or lack of cores with stellar kinematics, including the proxy for the stellar angular momentum (λRe) and the velocity dispersion within one half-light radius (σe), stellar mass, stellar age, α-element abundance, and age and metallicity gradients. More than half of the slow rotators have core-less light profiles, and they are all less massive than 1011 M⊙. Core-less slow rotators show evidence for counter-rotating flattened structures, have steeper metallicity gradients, and a larger dispersion of gradient values (Δ[Z/H]¯ = −0.42 ± 0.18) than core slow rotators (Δ[Z/H]¯ = −0.23 ± 0.07). Our results suggest that core and core-less slow rotators have different assembly processes, where the former, as previously discussed, are the relics of massive dissipation-less merging in the presence of central supermassive black holes. Formation processes of core-less slow rotators are consistent with accretion of counter-rotating gas or gas-rich mergers of special orbital configurations, which lower the final net angular momentum of stars, but support star formation. We also highlight core fast rotators as galaxies that share properties of core slow rotators (i.e. cores, ages, σe, and population gradients) and core-less slow rotators (i.e. kinematics, λRe, mass, and larger spread in population gradients). Formation processes similar to those for core-less slow rotators can be invoked to explain the assembly of core fast rotators, with the distinction that these processes form or preserve cores.


Relaxation of spherical stellar systems

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 490 (2019) 478-490

JY Lau, J Binney


Anisotropic Mass Segregation in Rotating Globular Clusters

ASTROPHYSICAL JOURNAL American Astronomical Society 887 (2019) ARTN 123

A Szolgyen, Y Meiron, B Kocsis

We investigate the internal dynamics of anisotropic, rotating globular clusters with a multimass stellar population by performing new direct N-body simulations. In addition to the well-known radial mass segregation effect, where heavy stars and stellar remnants sink toward the center of the cluster, we find a mass segregation in the distribution of orbital inclinations as well. This newly discovered anisotropic mass segregation leads to the formation of a disk-like structure of massive objects near the equatorial plane of a rotating cluster. This result has important implications on the expected spatial distribution of black holes in globular clusters.


Spectroscopy of the Young Stellar Association Price-Whelan 1: origin in the magellanic leading arm and constraints on the Milky Way Hot Halo

Astrophysical Journal American Astronomical Society 887 (2019) 115

DL Nidever, AM Price-Whelan, Y Choi, RL Beaton, TT Hansen, D Boubert, D Aguado, R Ezzeddine, S Oh, NW Evans

We report spectroscopic measurements of stars in the recently discovered young stellar association Price-Whelan 1 (PW 1), which was found in the vicinity of the Leading Arm (LA) of the Magellanic Stream (MS). We obtained Magellan+MIKE high-resolution spectra of the 28 brightest stars in PW 1 and used The Cannon to determine their stellar parameters. We find that the mean metallicity of PW 1 is [Fe/H] = −1.23 with a small scatter of 0.06 dex and the mean RV is ${V}_{\mathrm{hel}}$ = 276.7 $\mathrm{km}\,{{\rm{s}}}^{-1}\,$ with a dispersion of $11.0$ $\mathrm{km}\,{{\rm{s}}}^{-1}$. Our results are consistent in ${\text{}}{T}_{\mathrm{eff}}$, $\mathrm{log}g$, and [Fe/H] with the young and metal-poor characteristics (116 Myr and [Fe/H] = −1.1) determined for PW 1 from our discovery paper. We find a strong correlation between the spatial pattern of the PW 1 stars and the LA II gas with an offset of −10fdg15 in ${L}_{\mathrm{MS}}$ and +1fdg55 in ${B}_{\mathrm{MS}}$. The similarity in metallicity, velocity, and spatial patterns indicates that PW 1 likely originated in LA II. We find that the spatial and kinematic separation between LA II and PW 1 can be explained by ram pressure from Milky Way (MW) gas. Using orbit integrations that account for the LMC and MW halo and outer disk gas, we constrain the halo gas density at the orbital pericenter of PW 1 to be ${{\boldsymbol{n}}}_{\mathrm{halo}}(17\,\mathrm{kpc})={2.7}_{-2.0}^{+3.4}\times {10}^{-3}\,\mathrm{atoms}\,{\mathrm{cm}}^{-3}$ and the disk gas density at the midplane at $20\,\mathrm{kpc}$ to be ${{\boldsymbol{n}}}_{\mathrm{disk}}(20\,\mathrm{kpc},0)={6.0}_{-2.0}^{+1.5}\times {10}^{-2}\,\mathrm{atoms}\,{\mathrm{cm}}^{-3}$. We, therefore, conclude that PW 1 formed from the LA II of the MS, making it a powerful constraint on the MW–Magellanic interaction.


Hierarchical Black Hole Mergers in Active Galactic Nuclei.

Physical review letters American Physical Society (APS) 123 (2019) ARTN 181101

Y Yang, I Bartos, V Gayathri, K Ford, Z Haiman, S Klimenko, B Kocsis, S Márka, Z Márka, B McKernan, R O'Shaughnessy

The origins of the stellar-mass black hole mergers discovered by LIGO/Virgo are still unknown. Here we show that if migration traps develop in the accretion disks of active galactic nuclei (AGNs) and promote the mergers of their captive black holes, the majority of black holes within disks will undergo hierarchical mergers-with one of the black holes being the remnant of a previous merger. 40% of AGN-assisted mergers detected by LIGO/Virgo will include a black hole with mass &#x2273;50M_{&#x2299;}, the mass limit from stellar core collapse. Hierarchical mergers at traps in AGNs will exhibit black hole spins (anti)aligned with the binary's orbital axis, a distinct property from other hierarchical channels. Our results suggest, although not definitively (with odds ratio of &#x223C;1), that LIGO's heaviest merger so far, GW170729, could have originated from this channel.


SIGNALS: I. Survey description

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 489 (2019) 5530-5546

L Rousseau-Nepton, RP Martin, C Robert, L Drissen, P Amram, S Prunet, T Martin, I Moumen, A Adamo, A Alarie, P Barmby, A Boselli, F Bresolin, M Bureau, L Chemin, RC Fernandes, F Combes, C Crowder, L Della Bruna, S Duarte Puertas, F Egusa, B Epinat, VF Ksoll, M Girard, V Gómez Llanos, D Gouliermis, K Grasha, C Higgs, J Hlavacek-Larrondo, I-T Ho, J Iglesias-Páramo, G Joncas, ZS Kam, P Karera, RC Kennicutt, RS Klessen, S Lianou, L Liu, Q Liu, AL de Amorim, JD Lyman, H Martel, B Mazzilli-Ciraulo, AF McLeod, A-L Melchior, I Millan, M Mollá, R Momose, C Morisset, H-A Pan, AK Pati, A Pellerin, E Pellegrini, I Pérez, A Petric, H Plana, D Rahner, T Ruiz Lara, L Sánchez-Menguiano, K Spekkens, G Stasińska, M Takamiya, N Vale Asari, JM Vílchez

<jats:title>ABSTRACT</jats:title> <jats:p>SIGNALS, the Star formation, Ionized Gas, and Nebular Abundances Legacy Survey, is a large observing programme designed to investigate massive star formation and H ii regions in a sample of local extended galaxies. The programme will use the imaging Fourier transform spectrograph SITELLE at the Canada–France–Hawaii Telescope. Over 355 h (54.7 nights) have been allocated beginning in fall 2018 for eight consecutive semesters. Once completed, SIGNALS will provide a statistically reliable laboratory to investigate massive star formation, including over 50 000 resolved H ii regions: the largest, most complete, and homogeneous data base of spectroscopically and spatially resolved extragalactic H ii regions ever assembled. For each field observed, three datacubes covering the spectral bands of the filters SN1 (363–386 nm), SN2 (482–513 nm), and SN3 (647–685 nm) are gathered. The spectral resolution selected for each spectral band is 1000, 1000, and 5000, respectively. As defined, the project sample will facilitate the study of small-scale nebular physics and many other phenomena linked to star formation at a mean spatial resolution of ∼20 pc. This survey also has considerable legacy value for additional topics, including planetary nebulae, diffuse ionized gas, and supernova remnants. The purpose of this paper is to present a general outlook of the survey, notably the observing strategy, galaxy sample, and science requirements.</jats:p>


WISDOM project – V. Resolving molecular gas in Keplerian rotation around the supermassive black hole in NGC 0383

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 490 (2019) 319-330

EV North, TA Davis, M Bureau, M Cappellari, S Iguchi, L Liu, K Onishi, M Sarzi, MD Smith, TG Williams

<jats:title>ABSTRACT</jats:title> <jats:p>As part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM), we present a measurement of the mass of the supermassive black hole (SMBH) in the nearby early-type galaxy NGC 0383 (radio source 3C 031). This measurement is based on Atacama Large Millimeter/sub-millimeter Array (ALMA) cycle 4 and 5 observations of the 12CO(2–1) emission line with a spatial resolution of 58 × 32 pc2 (0.18 arcsec × 0.1 arcsec). This resolution, combined with a channel width of 10 km s−1, allows us to well resolve the radius of the black hole sphere of influence (measured as RSOI = 316 pc  =  0.98 arcsec), where we detect a clear Keplerian increase of the rotation velocities. NGC 0383 has a kinematically relaxed, smooth nuclear molecular gas disc with weak ring/spiral features. We forward model the ALMA data cube with the Kinematic Molecular Simulation (KinMS) tool and a Bayesian Markov Chain Monte Carlo method to measure an SMBH mass of (4.2 ± 0.7) × 109 M⊙, a F160W-band stellar mass-to-light ratio that varies from 2.8 ± 0.6 M⊙/L$_{\odot ,\, \mathrm{F160W}}$ in the centre to 2.4 ± 0.3 M⊙$/\rm L_{\odot ,\, \mathrm{F160W}}$ at the outer edge of the disc and a molecular gas velocity dispersion of 8.3 ± 2.1 km s−1(all 3σ uncertainties). We also detect unresolved continuum emission across the full bandwidth, consistent with synchrotron emission from an active galactic nucleus. This work demonstrates that low-J CO emission can resolve gas very close to the SMBH ($\approx 140\, 000$ Schwarzschild radii) and hence that the molecular gas method is highly complimentary to megamaser observations, as it can probe the same emitting material.</jats:p>

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