A. J. Benson

Carnegie Institution for Science, Washington, West Virginia, United States

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Publications (151)559.19 Total impact

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    ABSTRACT: We propose a common terminology for use in describing both temporal merger trees and spatial structure trees for dark-matter halos. We specify a unified data format in HDF5 and provide example I/O routines in C, FORTRAN and PYTHON.
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    ABSTRACT: We present a comparison of 14 galaxy formation models: 12 different semi-analytical models and 2 halo-occupation distribution models for galaxy formation based upon the same cosmological simulation and merger tree information derived from it. The participating codes have proven to be very successful in their own right but they have all been calibrated independently using various observational data sets, stellar models, and merger trees. In this paper we apply them without recalibration and this leads to a wide variety of predictions for the stellar mass function, specific star formation rates, stellar-to- halo mass ratios, and the abundance of orphan galaxies. The scatter is much larger than seen in previous comparison studies primarily because the codes have been used outside of their native environment within which they are well tested and calibrated. The purpose of the `nIFTy comparison of galaxy formation models' is to bring together as many different galaxy formation modellers as possible and to investigate a common approach to model calibration. This paper provides a unified description for all participating models and presents the initial, uncalibrated comparison as a baseline for our future studies where we will develop a common calibration framework and address the extent to which that reduces the scatter in the model predictions seen here.
    Monthly Notices of the Royal Astronomical Society 05/2015; 451(4). DOI:10.1093/mnras/stv1149 · 5.23 Impact Factor
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    ABSTRACT: We analyze the stellar kinematics of 39 dwarf early-type galaxies (dEs) in the Virgo Cluster. Based on the specific stellar angular momentum λRe and the ellipticity, we find 11 slow rotators and 28 fast rotators. The fast rotators in the outer parts of the Virgo Cluster rotate significantly faster than fast rotators in the inner parts of the cluster. Moreover, 10 out of the 11 slow rotators are located in the inner 3° (D < 1 Mpc) of the cluster. The fast rotators contain subtle disk-like structures that are visible in high-pass filtered optical images, while the slow rotators do not exhibit these structures. In addition, two of the dEs have kinematically decoupled cores and four more have emission partially filling in the Balmer absorption lines. These properties suggest that Virgo Cluster dEs may have originated from late-type star-forming galaxies that were transformed by the environment after their infall into the cluster. The correlation between λRe and the clustercentric distance can be explained by a scenario where low luminosity star-forming galaxies fall into the cluster, their gas is rapidly removed by ram-pressure stripping, although some of it can be retained in their core, their star formation is quenched but their stellar kinematics are preserved. After a long time in the cluster and several passes through its center, the galaxies are heated up and transformed into slow rotating dEs.
    The Astrophysical Journal 02/2015; 799(2):172. DOI:10.1088/0004-637X/799/2/172 · 6.28 Impact Factor
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    A. J. Benson · E. Toloba · L. Mayer · J. D. Simon · P. Guhathakurta
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    ABSTRACT: We model the dynamics of dwarf early-type galaxies in the Virgo cluster when subject to a variety of environmental processes. We focus on how these processes imprint trends in dynamical state (rotational vs. pressure support as measured by the $\lambda^*_{\rm Re/2}$ statistic) with projected distance from the cluster center, and compare these results to observational estimates. We find a large scatter in the gradient of $\lambda^*_{\rm Re/2}$ with projected radius. A statistical analysis shows that models with no environmental effects produce gradients as steep as those observed in none of the 100 cluster realizations we consider, while in a model incorporating tidal stirring by the cluster potential 34% of realizations produce gradients as steep as that observed. Our results suggest that tidal stirring may be the cause of the observed radial dependence of dwarf early-type dynamics in galaxy clusters.
    The Astrophysical Journal 10/2014; 799(2). DOI:10.1088/0004-637X/799/2/171 · 6.28 Impact Factor
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    ABSTRACT: We analyze the stellar kinematics of 39 dwarf early-type galaxies (dEs) in the Virgo cluster. Based on the specific stellar angular momentum lambda_e and the ellipticity, we find 11 slow rotators and 28 fast rotators. The fast rotators in the outer parts of the Virgo cluster rotate significantly faster than fast rotators in the inner parts of the cluster. Moreover, 10 out of the 11 slow rotators are located in the inner 3 degrees (D < 1 Mpc) of the cluster. The fast rotators contain subtle disky structures that are visible in high-pass filtered optical images, while the slow rotators do not exhibit these structures. In addition, two of the dEs have kinematically decoupled cores and four more have emission partially filling in the Balmer absorption lines. These properties suggest that Virgo cluster dEs may have originated from late-type star-forming galaxies that were transformed by the environment after their infall into the cluster. The correlation between lambda_e and the clustercentric distance can be explained by a scenario where low luminosity star-forming galaxies fall into the cluster, their gas is rapidly removed by ram pressure stripping, although some of it can be retained in their core, their star-formation is quenched but their stellar kinematics are preserved. After a long time in the cluster and several passes through its center, the galaxies are heated up and transformed into slow rotating dEs.
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    Anthony R. Pullen · Andrew J. Benson · Leonidas A. Moustakas
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    ABSTRACT: We describe the methodology to include nonlinear evolution, including tidal effects, in the computation of subhalo distribution properties in both cold (CDM) and warm (WDM) dark matter universes. Using semi-analytic modeling, we include effects from dynamical friction, tidal stripping, and tidal heating, allowing us to dynamically evolve the subhalo distribution. We calibrate our nonlinear evolution scheme to the CDM subhalo mass function in the Aquarius N-body simulation, producing a subhalo mass function within the range of simulations. We find tidal effects to be the dominant mechanism of nonlinear evolution in the subhalo population. Finally, we compute the subhalo mass function for $m_\chi=1.5$ keV WDM including the effects of nonlinear evolution, and compare radial number densities and mass density profiles of subhalos in CDM and WDM models. We show that all three signatures differ between the two dark matter models, suggesting that probes of substructure may be able to differentiate between them.
    The Astrophysical Journal 07/2014; 792(1). DOI:10.1088/0004-637X/792/1/24 · 6.28 Impact Factor
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    ABSTRACT: As galaxy formation and evolution over long cosmic time-scales depends to a large degree on the structure of the universe, the assembly history of galaxies is potentially a powerful approach for learning about the universe itself. In this paper we examine the merger history of dark matter halos based on the Extended Press-Schechter formalism as a function of cosmological parameters, redshift and halo mass. We calculate how major halo mergers are influenced by changes in the cosmological values of $\Omega_{\rm m}$, $\Omega_{\Lambda}$, $\sigma_{8}$, the dark matter particle temperature (warm vs. cold dark matter), and the value of a constant and evolving equation of state parameter $w(z)$. We find that the merger fraction at a given halo mass varies by up to a factor of three for halos forming under the assumption of Cold Dark Matter, within different underling cosmological parameters. We find that the current measurements of the merger history, as measured through observed galaxy pairs as well as through structure, are in agreement with the concordance cosmology with the current best fit giving $1 - \Omega_{\rm m} = \Omega_{\rm \Lambda} = 0.84^{+0.16}_{-0.17}$. To obtain a more accurate constraint competitive with recently measured cosmological parameters from Planck and WMAP requires a measured merger accuracy of $\delta f_{\rm m} \sim 0.01$, implying surveys with an accurately measured merger history over 2 - 20 deg$^{2}$, which will be feasible with the next generation of imaging and spectroscopic surveys such as Euclid and LSST.
    Monthly Notices of the Royal Astronomical Society 07/2014; 444(2). DOI:10.1093/mnras/stu1385 · 5.23 Impact Factor
  • Rachel Kennedy · Carlos Frenk · Shaun Cole · Andrew Benson
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    ABSTRACT: Particle physics theories predict the existence of particles (such as keV mass sterile neutrinos) which could behave as warm dark matter (WDM), producing a cutoff in the linear density power spectrum on the scale of dwarf galaxies. Thus, the abundance of Milky Way satellite galaxies depends on the mass of the warm particle and also scales with the mass of the host galactic halo. We use the galform semi-analytic model of galaxy formation to compare predicted satellite luminosity functions to Milky Way data and determine a lower bound on the thermally produced WDM particle mass. This depends strongly on the Milky Way halo mass and, to some extent, on the baryonic physics assumed. For our fiducial model, we find that for a particle mass of 3.3 keV (the 2σ lower limit from an analysis of the Lyman α forest by Viel et al.) the Milky Way halo mass is required to be >1.4 × 1012 M⊙. For this same fiducial model, we also find that all WDM particle masses are ruled out (at 95 per cent confidence) if the Milky Way halo mass is smaller than 1.1 × 1012 M⊙, while if the mass of the Galactic halo is greater than 1.8 × 1012 M⊙, only WDM particle masses larger than 2 keV are allowed.
    Monthly Notices of the Royal Astronomical Society 06/2014; 442(3):2487-2495. DOI:10.1093/mnras/stu719 · 5.23 Impact Factor
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    Andrew J. Benson
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    ABSTRACT: We constrain a highly simplified semi-analytic model of galaxy formation using the z ≈ 0 stellar mass function of galaxies. Particular attention is paid to assessing the role of random and systematic errors in the determination of stellar masses, to systematic uncertainties in the model, and to correlations between bins in the measured and modelled stellar mass functions, in order to construct a realistic likelihood function. We derive constraints on model parameters and explore which aspects of the observational data constrain particular parameter combinations. We find that our model, once constrained, provides a remarkable match to the measured evolution of the stellar mass function to z = 1, although fails dramatically to match the local galaxy H i mass function. Several ‘nuisance parameters’ contribute significantly to uncertainties in model predictions. In particular, systematic errors in stellar mass estimate are the dominant source of uncertainty in model predictions at z ≈ 1, with additional, non-negligble contributions arising from systematic uncertainties in halo mass functions and the residual uncertainties in cosmological parameters. Ignoring any of these sources of uncertainties could lead to viable models being erroneously ruled out. Additionally, we demonstrate that ignoring the significant covariance between bins the observed stellar mass function leads to significant biases in the constraints derived on model parameters. Careful treatment of systematic and random errors in the constraining data, and in the model being constrained, is crucial if this methodology is to be used to test hypotheses relating to the physics of galaxy formation.
    Monthly Notices of the Royal Astronomical Society 05/2014; 444(3). DOI:10.1093/mnras/stu1630 · 5.23 Impact Factor
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    ABSTRACT: We compare predictions of cooled masses and cooling rates from three stripped-down Semi-Analytic Models (SAMs) of galaxy formation with the results of N-body+SPH simulations with gas particle mass of 3.9x10^6 Msun, where radiative cooling of a gas of primordial composition is implemented. We also run a simulation where cooling is switched on at redshift ~2, in order to test cooling models in a regime in which their approximations are expected to be valid. We confirm that cooling models implemented in SAMs are able to predict the amount of cooled mass at z=0 to within ~20 per cent. However, some relevant discrepancies are found. (i) When the contribution from poorly resolved halos is subtracted out, SAMs tend to under-predict by ~30 per cent the mass that cools in the infall-dominated regime. (ii) At large halo masses SAMs tend to over-predict cooling rates, though the numerical result may be affected by the use of SPH. (iii) As found in our previous work, cooling rates are found to be significantly affected by model details: simulations disfavour models with large cores and with quenching of cooling at major mergers. (iv) When cooling is switched on at z~2, cold gas accumulates very quickly in the simulated halos. This accumulation is reproduced by SAMs with varying degrees of accuracy.
    Monthly Notices of the Royal Astronomical Society 04/2014; 441(3). DOI:10.1093/mnras/stu655 · 5.23 Impact Factor
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    Rachel Kennedy · Carlos Frenk · Shaun Cole · Andrew Benson
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    ABSTRACT: Well-motivated particle physics theories predict the existence of particles (such as sterile neutrinos) which acquire non-negligible thermal velocities in the early universe. These particles could behave as warm dark matter (WDM) and generate a small-scale cutoff in the linear density power spectrum which scales approximately inversely with the particle mass. If this mass is of order a keV, the cutoff occurs on the scale of dwarf galaxies. Thus, in WDM models the abundance of small galaxies, such as the satellites that orbit in the halo of the Milky Way, depends on the mass of the warm particle. The abundance also scales with the mass of the host galactic halo. We use the \galform semi-analytic model of galaxy formation to calculate the properties of galaxies in universes in which the dark matter is warm. Using this method, we can compare the predicted satellite luminosity functions to the observed data for the Milky Way dwarf spheroidals, and determine a lower bound on the thermally produced WDM particle mass. This depends strongly on the value of the Milky Way halo mass and, to some extent, on the baryonic physics assumed; we examine both of these dependencies. For our fiducial model we find that for a particle mass of 3.3 keV (the 2$\sigma$ lower limit found by Viel et al. from a recent analysis of the Lyman-$\alpha$ forest) the Milky Way halo mass is required to be $> 1.4 \times 10^{12}$ \msun. For this same fiducial model, we also find that all WDM particle masses are ruled out (at 95% confidence) if the halo of the Milky Way has a mass smaller than $1.1 \times 10^{12}$ \msun, while if the mass of the Galactic halo is greater than 1.8 $\times 10^{12}$ \msun, only WDM particle masses larger than 2 keV are allowed.
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    ABSTRACT: A robust prediction of LCDM cosmology is the halo circular velocity function (CVF), a dynamical cousin of the halo mass function. However, the correspondence between theoretical and observed CVFs is uncertain: cluster galaxies are reported to exhibit a power-law CVF consistent with N-body simulations, but that of the field is distinctly Schechter-like, flattened relative to LCDM expectations at circular velocities v_c < 200 km/s. Groups offer a powerful probe of the role of environment in this discrepancy as they bridge the field and clusters. Here, we construct the CVF for a large, mass- and multiplicity-complete sample of group galaxies from the Sloan Digital Sky Survey. Using independent photometric v_c estimators, we find no transition from field- to LCDM-shaped CVF above v_c = 50 km/s as a function of group halo mass. All groups with 12.4 < log(M_halo/M_sun) < 15.1 (Local Group analogs to rich clusters) display similar Schechter-like CVFs that are marginally suppressed at low-v_c compared to that of the field. Conversely, moderate agreement with N-body results emerges for samples saturated with late-type galaxies. Indeed, isolated late-types have a CVF remarkably similar to LCDM predictions. We conclude that the flattening of the low-v_c slope in groups is due to their depressed late-type fractions and that environment affects the CVF only to the extent that it correlates with this quantity. Previous cluster analyses may thus suffer from significant interloper contamination. These results represent an important benchmark for cosmological models of galaxy formation.
    The Astrophysical Journal 07/2013; 793(1). DOI:10.1088/0004-637X/793/1/49 · 6.28 Impact Factor
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    ABSTRACT: We present a clustering analysis of X-ray selected AGN by compiling X-ray samples from the literature and re-estimating the dark matter (DM) halo masses of AGN in a uniform manner. We find that moderate luminosity AGN (Lx(2-10 keV)=10^42-10^44 erg/sec) in the z=0-1.3 Universe are typically found in DM haloes with masses of ~10^13 Msun. We then compare our findings to the theoretical predictions of the coupled galaxy and black hole formation model GALFORM. We find good agreement when our calculation includes the hot-halo mode of accretion onto the central black hole. This type of accretion, which is additional to the common cold accretion during disk instabilities and galaxy mergers, is tightly coupled to the AGN feedback in the model. The hot-halo mode becomes prominent in DM haloes with masses greater than ~10^12.5 Msun, where AGN feedback typically operates, giving rise to a distinct class of moderate luminosity AGN that inhabit rich clusters and superclusters. Cold gas fuelling of the black hole cannot produce the observationally inferred DM halo masses of X-ray AGN. Switching off AGN feedback in the model results in a large population of luminous quasars (Lx(2-10 keV) > 10^44 erg/sec) in DM haloes with masses up to ~10^14 Msun, which is inconsistent with the observed clustering of quasars. The abundance of hot-halo AGN decreases significantly in the z~3-4 universe. At such high redshifts, the cold accretion mode is solely responsible for shaping the environment of moderate luminosity AGN. Our analysis supports two accretion modes (cold and hot) for the fuelling of supermassive black holes and strongly underlines the importance of AGN feedback in cosmological models both of galaxy formation and black hole growth.
    Monthly Notices of the Royal Astronomical Society 05/2013; 435(1). DOI:10.1093/mnras/stt1327 · 5.23 Impact Factor
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    ABSTRACT: We study the oscillations and stability of self-gravitating cylindrically symmetric fluid systems and collisionless systems. This is done by studying small perturbations to the equilibrium system and finding the normal modes, using methods similar to those used in astroseismology. We find that there is a single sequence of purely radial modes that become unstable if the adiabatic exponent is less than 1. Nonradial modes can be divided into p modes, which are stable and pressure-driven, and g modes, which are are gravity driven. The g modes become unstable if the adiabatic exponent is greater than the polytrope index. These modes are analogous to the modes of a spherical star, but their behavior is somewhat different because a cylindrical geometry has less symmetry than a spherical geometry. This implies that perturbations are classified by a radial quantum number, an azimuthal quantum number, and wavelength in the z direction, which can become arbitrarily large. We find that decreasing this wavelength increases the frequency of stable modes and increases the growth rate of unstable modes. We use use variational arguments to demonstrate that filaments of collisionless matter with ergodic distribution functions are stable to purely radial perturbations, and that filaments with ergodic power-law distribution functions are stable to all perturbations.
    Monthly Notices of the Royal Astronomical Society 05/2013; 437(3). DOI:10.1093/mnras/stt2077 · 5.23 Impact Factor
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    Arya Farahi · Andrew J. Benson
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    ABSTRACT: We present a new method to compute the first crossing distribution in excursion set theory for the case of correlated random walks. We use a combination of the path integral formalism of Maggiore & Riotto, and the integral equation solution of Zhang & Hui, and Benson et al. to find a numerically robust and convenient algorithm to derive the first crossing distribution in terms of a perturbative expansion around the limit of an uncorrelated random walk. We apply this methodology to the specific case of a Gaussian random density field filtered with a Gaussian smoothing function. By comparing our solutions to results from Monte Carlo calculations of the first crossing distribution we demonstrate that our method accurate for power spectra $P(k)\propto k^n$ for $n=1$, becoming less accurate for smaller values of $n$. It is therefore complementary to the method of Musso & Sheth, which will therefore be more useful for standard $\Lambda$CDM power spectra. Our approach is quite general, and can be adapted to other smoothing functions, and also to non-Gaussian density fields.
    Monthly Notices of the Royal Astronomical Society 03/2013; 433(4). DOI:10.1093/mnras/stt987 · 5.23 Impact Factor
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    ABSTRACT: We study the impact of star formation and stellar feedback prescriptions on galaxy properties predicted by means of "stripped-down" versions of independently developed semi-analytic models (SAMs). These include cooling, star formation, feedback from supernovae (SNe) and simplified prescriptions for galaxy merging, but no chemical evolution, disc instabilities or AGN feedback. We run these versions on identical samples of dark matter (DM) haloes extracted from high-resolution N-body simulations in order to perform both statistical analysis and object-by-object comparisons. We compare our results with previous work based on stripped-down versions of the same SAMs including only gas cooling, and show that all feedback models provide coherent modifications in the distribution of baryons between the various gas phases. In particular, we find that the predicted hot gas fractions are considerably increased by up to a factor of three, while the corresponding cold gas fractions are correspondingly decreased, and a significant amount of mass is ejected from the DM halo. Nonetheless, we also find relevant differences in the predicted properties of model galaxies among the three SAMs: these deviations are more relevant at mass scales comparable to that of our own Galaxy, and are reduced at larger masses, confirming the varying impact of stellar feedback at different mass scales. We also check the effect of enhanced star formation events (i.e. starbursts modes), defined in connection with galaxy mergers. We find that, in general, these episodes have a limited impact in the overall star formation histories of model galaxies, even in massive DM halos where merger-driven star formation has often been considered very important.
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    Andrew Benson · Aparna Venkatesan · J. Michael Shull
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    ABSTRACT: The escape of ionizing radiation from galaxies plays a critical role in the evolution of gas in galaxies, and the heating and ionization history of the intergalactic medium. We present semi-analytic calculations of the escape fraction of ionizing radiation for both hydrogen and helium from galaxies ranging from primordial systems to disk-type galaxies that are not heavily dust-obscured. We consider variations in the galaxy density profile, source type, location, and spectrum, and gas overdensity/distribution factors. For sufficiently hard first-light sources, the helium ionization fronts closely track or advance beyond that of hydrogen. Key new results in this work include calculations of the escape fractions for He I and He II ionizing radiation, and the impact of partial ionization from X-rays from early AGN or stellar clusters on the escape fractions from galaxy halos. When factoring in frequency-dependent effects, we find that X-rays play an important role in boosting the escape fractions for both hydrogen and helium, but especially for He II. We briefly discuss the implications of these results for recent observations of the He II reionization epoch at low redshifts, as well as the UV data and emission-line signatures from early galaxies anticipated from future satellite missions.
    The Astrophysical Journal 12/2012; 770(1). DOI:10.1088/0004-637X/770/1/76 · 6.28 Impact Factor
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    ABSTRACT: Testing model predictions of the cold dark matter cosmology for the
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    ABSTRACT: Models of disk galaxy formation commonly predict the existence of an extended reservoir of accreted hot gas surrounding massive spirals at low redshift. As a test of these models, we use X-ray and Hα data of the two massive, quiescent edge-on spirals NGC 5746 and NGC 5170 to investigate the amount and origin of any hot gas in their halos. Contrary to our earlier claim, the Chandra analysis of NGC 5746, employing more recent calibration data, does not reveal any significant evidence for diffuse X-ray emission outside the optical disk, with a 3σ upper limit to the halo X-ray luminosity of 4×10 39 erg s −1. An identical study of the less massive NGC 5170 also fails to detect any extraplanar X-ray emission. By extracting hot halo properties of disk galaxies formed in cosmological hydrodynamical simulations, we compare these results to expectations for cosmological accretion of hot gas by spirals. For Milky Way–sized galaxies, these high-resolution simulations predict hot halo X-ray luminosities which are lower by a factor of ∼ 2 compared to our earlier results reported by Toft et al. (2002). We find the new simulation predictions to be consistent with our observational constraints for both NGC 5746 and NGC 5170, while also confirming that the hot gas detected so far around more actively star-forming spirals is in general probably associated with stellar activity in the disk. Observational results on quiescent disk galaxies at the high-mass end are nevertheless providing powerful constraints on theoretical predictions, and hence on the assumed input physics in numerical studies of disk galaxy formation and evolution.
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    ABSTRACT: We present a spectro-photometric survey of 2522 extragalactic globular clusters (GCs) around twelve early-type galaxies, nine of which have not been published previously. Combining space-based and multi-colour wide field ground-based imaging, with spectra from the Keck DEIMOS instrument, we obtain an average of 160 GC radial velocities per galaxy, with a high velocity precision of 15 km/s per GC. After studying the photometric properties of the GC systems, such as their spatial and colour distributions, we focus on the kinematics of metal-poor (blue) and metal-rich (red) GC subpopulations to an average distance of ~8 effective radii from the galaxy centre. Our results show that for some systems the bimodality in GC colour is also present in GC kinematics. The kinematics of the red GC subpopulations are strongly coupled with the host galaxy stellar kinematics. The blue GC subpopulations are more dominated by random motions, especially in the outer regions, and decoupled from the red GCs. Peculiar GC kinematic profiles are seen in some galaxies: the blue GCs in NGC 821 rotate along the galaxy minor axis, whereas the GC system of the lenticular galaxy NGC 7457 appears to be strongly rotation supported in the outer region. We supplement our galaxy sample with data from the literature and carry out a number of tests to study the kinematic differences between the two GC subpopulations. We confirm that the GC kinematics are coupled with the host galaxy properties and find that the velocity kurtosis and the slope of their velocity dispersion profiles is different between the two GC subpopulations in more massive galaxies.
    Monthly Notices of the Royal Astronomical Society 09/2012; 428(1). DOI:10.1093/mnras/sts029 · 5.23 Impact Factor

Publication Stats

7k Citations
559.19 Total Impact Points

Institutions

  • 2013–2014
    • Carnegie Institution for Science
      • Department of Terrestrial Magnetism
      Washington, West Virginia, United States
  • 2001–2011
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States
    • Università degli Studi di Trieste
      • Department of Physics
      Trst, Friuli Venezia Giulia, Italy
  • 1998–2009
    • Durham University
      • Department of Physics
      Durham, England, United Kingdom
  • 2004–2008
    • University of Oxford
      • Department of Physics
      Oxford, ENG, United Kingdom
  • 2006
    • University of Victoria
      • Department of Physics and Astronomy
      Victoria, British Columbia, Canada
  • 2002
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, IL, United States