1,721,171 research outputs found
The Rising Stellar Velocity Dispersion Of M87 From Integrated Starlight
Full text linkWe have measured the line-of-sight velocity distribution from integrated stellar light at two points in the outer halo of M87 (NGC 4486), the second-rank galaxy in the Virgo Cluster. The data were taken at R = 480 '' (similar to 41.5 kpc) and R = 526 '' (similar to 45.5 kpc) along the SE major axis. The second moment for a non-parametric estimate of the full velocity distribution is 420 +/- 23 km s(-1) and 577 +/- 35 km s(-1), respectively. There is intriguing evidence in the velocity profiles for two kinematically distinct stellar components at the position of our pointing. Under this assumption, we employ a two-Gaussian decomposition and find the primary Gaussian having rest velocities equal to M87 (consistent with zero rotation) and second moments of 383 +/- 32 km s(-1) and 446 +/- 43 km s(-1), respectively. The asymmetry seen in the velocity profiles suggests that the stellar halo of M87 is not in a relaxed Stateand confuses a clean dynamical interpretation. That said, either measurement (full or two component model) shows a rising velocity dispersion at large radii, consistent with previous integrated light measurements, yet significantly higher than globular cluster measurements at comparable radial positions. These integrated light measurements at large radii, and the stark contrast they make to the measurements of other kinematic tracers, highlight the rich kinematic complexity of environments like the center of the Virgo Cluster and the need for caution when interpreting kinematic measurements from various dynamical tracers.NSF Astronomy and Astrophysics Postdoctoral Fellowship AST-1203057UT Continuing University FellowshipPhysics Department at Southwestern UniversityNational Aeronautics and Space AdministrationNSF-0908639AstronomyMcDonald Observator
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Hobby-Eberly Telescope Observations Of The Dark Halo In NGC 821
Full text linkWe present stellar line-of-sight velocity distributions (LOSVDs) of elliptical galaxy NGC 821 obtained to approximately 100 '' (over two effective radii) with long-slit spectroscopy from the Hobby-Eberly Telescope. Our measured stellar LOSVDs are larger than the planetary nebulae measurements at similar radii. We fit axisymmetric orbit-superposition models with a range of dark halo density profiles, including two-dimensional kinematics at smaller radii from SAURON data. Within our assumptions, the best-fitted model gives a total enclosed mass of 2.0 x 10(11) M(circle dot) within 100 '', with an accuracy of 2%; this mass is equally divided between halo and stars. At 1 R(e), the best-fitted dark matter halo accounts for 13% of the total mass in the galaxy. This dark halo is inconsistent with previous claims of little to no dark matter halo in this galaxy from planetary nebula measurements. We find that a power-law dark halo with a slope 0.1 is the best-fitted model; both the no dark halo and Navarro-Frenk-White models are worse fits at a greater than 99% confidence level. NGC 821 does not appear to have the expected dark halo density profile. The internal moments of the stellar velocity distribution show that the model with no dark halo is radially anisotropic at small radii and tangentially isotropic at large radii, while the best-fitted halo models are slightly radially anisotropic at all radii. We test the potential effects of model smoothing and find that there are no effects on our results within the errors. Finally, we run models using the planetary nebula kinematics and assuming our best-fitted halos and find that the planetary nebulae require radial orbits throughout the galaxy.Texas Advanced Research Program 003658-0243-2001NSF-CAREER AST03-49095Pennsylvania State UniversityStanford UniversityLudwig-Maximilians-Universitat MunchenGeorg-August-Universitat GottingenUniversity of Texas at AustinAstronom
The Dark Matter Density Profile Of The Fornax Dwarf
Full text linkWe construct axisymmetric Schwarzschild models to measure the mass profile of the Local Group dwarf galaxy Fornax. These models require no assumptions to be made about the orbital anisotropy of the stars, as is the case for commonly used Jeans models. We test a variety of parameterizations of dark matter density profiles and find cored models with uniform density rho c = (1.6 +/- 0.1) x 10(-2) M-circle dot pc(-3) fit significantly better than the cuspy halos predicted by cold dark matter simulations. We also construct models with an intermediate-mass black hole, but are unable to make a detection. We place a 1 sigma upper limit on the mass of a potential intermediate-mass black hole at M. <= 3.2 x 10(4) M-circle dot.NSF-0908639Astronom
Variations in A Universal Dark Matter Profile for Dwarf Spheroidals
Full text linkUsing a newly developed modeling technique, we present orbit-based dynamical models of the Carina, Draco, Fornax, Sculptor, and Sextans dwarf spheroidal (dSph) galaxies. These models calculate the dark matter profiles non-parametrically without requiring any assumptions to be made about their profile shapes. By lifting this restriction, we discover a host of dark matter profiles in the dSphs that are different from the typical profiles suggested by both theorists and observers. However, when we scale these profiles appropriately and plot them on a common axis, they appear to follow an approximate r (1) power law with considerable scatter.NSF-0908639Astronom
EFFECT OF A DARK MATTER HALO ON THE DETERMINATION OF BLACK HOLE MASSES
Full text linkStellar dynamical modeling is a powerful method to determine the mass of black holes in quiescent galaxies. However, in previous work the presence of a dark matter halo has been ignored in the modeling. Gebhardt & Thomas in 2009 showed that accounting for a dark matter halo increased the black hole mass of the massive galaxy M87 by a factor of two. We used a sample of 12 galaxies to investigate the effect of accounting for a dark matter halo in the dynamical modeling in more detail, and also updated the masses using improved modeling. The sample of galaxies possesses Hubble Space Telescope and ground-based observations of stellar kinematics. Their black hole masses have been presented before, but without including a dark matter halo in the models. Without a dark halo, we find a mean increase in the estimated mass of 1.5 for the whole sample compared to previous results. We attribute this change to using a more complete orbit library. When we include a dark matter halo, along with the updated models, we find an additional increase in black hole mass by a factor of 1.2 in the mean, much less than for M87. We attribute the smaller discrepancy in black hole mass to using data that better resolve the black hole's sphere of influence. We redetermined the M-center dot-sigma(*) and M-center dot-L-V relationships using our updated black hole masses and found a slight increase in both normalization and intrinsic scatter.DAADDeutsche Forschungsgemeinschaft SPP1177, Wi 1369/23-2NSF 0908639Astronom
The black hole mass, stellar mass-to-light ratio and dark halo in M87
Full text linkWe model the dynamical structure of M87 (NGC4486) using high spatial resolution long-slit observations of stellar light in the central regions, two-dimensional stellar light kinematics out to half of the effective radius, and globular cluster velocities out to eight effective radii. We simultaneously fit for four parameters: black hole mass, dark halo core radius, dark halo circular velocity, and stellar mass-to-light (M/L) ratio. We find a black hole mass of 6.4 (+/- 0.5) x 109 M(circle dot) (the uncertainty is 68% confidence marginalized over the other parameters). The stellar M/L(V) = 6.3 +/- 0.8. The best-fit dark halo core radius is 14 +/- 2 kpc, assuming a cored logarithmic potential. The best-fit dark halo circular velocity is 715 +/- 15 km s(-1). Our black hole mass is over a factor of 2 larger than previous stellar dynamical measures, and our derived stellar M/L ratio is two times lower than previous dynamical measures. When we do not include a dark halo, we measure a black hole mass and stellar M/L ratio that is consistent with previous measures, implying that the major difference is in the model assumptions. The stellar M/L ratio from our models is very similar to that derived from stellar population models of M87. The reason for the difference in the black hole mass is because we allow the M/L ratio to change with radius. The dark halo is degenerate with the stellar M/L ratio, which is subsequently degenerate with the black hole mass. We argue that dynamical models of galaxies that do not include the contribution from a dark halo may produce a biased result for the black hole mass. This bias is especially large for a galaxy with a shallow light profile such as M87, and may not be as severe in galaxies with steeper light profiles unless they have a large stellar population change with radius.NSF-CAREER AST03-49095Astronom
Ancient Light From Young Cosmic Cities: Physical And Observational Signatures Of Galaxy Proto-Clusters
Full text linkA growing number of galaxy clusters at z = 1-2 is being discovered as part of deep optical, IR, X-ray, and Sunyaev-Zel'dovich effect surveys. For a complete picture of cluster formation, however, it is important that we also start probing the much earlier epoch, between redshifts of about 2 and 7, during which these clusters and their galaxies first began to form. Because the study of these so-called proto-clusters is currently quite limited by small number statistics, widely varying selection techniques, and many assumptions, we have performed a large systematic study of cluster formation utilizing cosmological simulations. We use the Millennium Simulations to track the evolution of dark matter and galaxies in about 3000 clusters from the earliest times to z = 0. We define an effective radius R-e for proto-clusters and characterize their growth in size and mass with cosmic time. We show that the progenitor regions of galaxy clusters (ranging in mass from similar to 10(14) to a few times 10(15) M-circle dot) can already be identified in galaxy surveys at very early times (at least up to z similar to 5), provided that the galaxy overdensities are measured on a sufficiently large scale (R-e similar to 5-10 Mpc comoving) and with sufficient statistics. We present the overdensities in matter, dark matter halos, and galaxies as functions of present-day cluster mass, redshift, bias, and window size that can be used to interpret the wide range of structures found in real surveys. We also derive the probability that a structure having a galaxy overdensity delta(gal), defined by a set of observational selection criteria, is indeed a proto-cluster, and we show how their z = 0 masses can already be estimated long before virialization. We present overdensity profiles as a function of radius, and we further show how the projected surface overdensities of proto-clusters decrease as the uncertainties in redshift measurements increase. We provide a table of proto-cluster candidates selected from the literature and discuss their properties in light of our simulation predictions. This paper provides the general framework that will allow us to extend the study of cluster formation out to much higher redshifts using the large number of proto-clusters that are expected to be discovered in, e. g., the upcoming HETDEX and Hyper Suprime-Cam surveys.NASAJPL/CaltechAstronom
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Discovery of a Large Number of Candidate Protoclusters Traced By Similar to 15 Mpc-Scale Galaxy Overdensities in COSMOS
Full text linkTo demonstrate the feasibility of studying the epoch of massive galaxy cluster Formation in a more systematic manner using current and future galaxy surveys, we report the discovery of a large sample of protocluster candidates in the 1.62 deg(2) COSMOS/UltraVISTA field traced by optical/infrared selected galaxies using photometric redshifts. By comparing properly smoothed three-dimensional galaxy density maps of the observations and a set of matched simulations incorporating the dominant observational effects (galaxy selection and photometric redshift uncertainties), we first confirm that the observed similar to 15 comoving Mpc-scale galaxy clustering is consistent with Lambda CDM models. Using further the relation between high-z overdensity and the present day cluster mass calibrated in these matched simulations, we found 36 candidate structures at 1.6 < z < 3.1, showing overdensities consistent with the progenitors of M-z=0 similar to 10(15) M-circle dot clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M-z=0 similar to 10(14) M-circle dot are similar to 70%. For each structure, about 15%-40% of photometric galaxy candidates are expected to be true protocluster members that will merge into a cluster-scale halo by z = 0. With solely photometric redshifts, we successfully rediscover two spectroscopically confirmed structures in this field, suggesting that our algorithm is robust. This work generates a large sample of uniformly selected protocluster candidates, providing rich targets for spectroscopic follow-up and subsequent studies of cluster Formation. Meanwhile, it demonstrates the potential for probing early cluster Formation with upcoming redshift surveys such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime Focus Spectrograph survey.Astronom
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The Black Hole Mass In The Brightest Cluster Galaxy NGC 6086
Full text linkWe present the first direct measurement of the central black hole mass, M-center dot, in NGC 6086, the Brightest Cluster Galaxy (BCG) in A2162. Our investigation demonstrates for the first time that stellar-dynamical measurements of M-center dot in BCGs are possible beyond the nearest few galaxy clusters. We observed NGC 6086 with laser guide star adaptive optics and the integral-field spectrograph (IFS) OSIRIS at the W. M. Keck Observatory and with the seeing-limited IFS GMOS-N at Gemini Observatory North. We combined the IFS data sets with existing major-axis kinematics and used axisymmetric stellar orbit models to determine M-center dot and the R-band stellar mass-to-light ratio, M-*/LR. We find M-center dot = 3.6(-1.1)(+1.7) x 10(9) M-circle dot and M-*/L-R = 4.6(-0.7)(+0.3) M-circle dot L-circle dot(-1) (68% confidence) from models using the most massive dark matter halo allowed within the gravitational potential of the host cluster. Models fitting only IFS data confirm M-center dot similar to 3x10(9) M circle dot and M-*/L-R similar to 4M(circle dot) L-circle dot(-1), with weak dependence on the assumed dark matter halo structure. When data out to 19 kpc are included, the unrealistic omission of dark matter causes the best-fit black hole mass to decrease dramatically, to 0.6 x 10(9) M-circle dot, and the best-fit stellar mass-to-light ratio to increase to 6.7 M circle dot L-(c),R(-1) . The latter value is at further odds with stellar population studies favoring M-*/L-R similar to 2M(circle dot) L-circle dot(-1) . Biases from dark matter omission could extend to dynamical models of other galaxies with stellar cores, and revised measurements of M-center dot could steepen the empirical scaling relationships between black holes and their host galaxies.NSF AST-1009663, 0908639NSF Center for Adaptive Optics AST 98-76783Miller Institute for Basic Research in Science, the University of California, BerkeleyW. M. Keck FoundationAstronom
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Measuring Dark Matter Profiles Non-Parametrically In Dwarf Spheroidals: An Application To Draco
Full text linkWe introduce a novel implementation of orbit-based (or Schwarzschild) modeling that allows dark matter density profiles to be calculated non-parametrically in nearby galaxies. Our models require no assumptions to be made about velocity anisotropy or the dark matter profile. The technique can be applied to any dispersion-supported stellar system, and we demonstrate its use by studying the Local Group dwarf spheroidal galaxy (dSph) Draco. We use existing kinematic data at larger radii and also present 12 new radial velocities within the central 13 pc obtained with the VIRUS-W integral field spectrograph on the 2.7 m telescope at McDonald Observatory. Our non-parametric Schwarzschild models find strong evidence that the dark matter profile in Draco is cuspy for 20 <= r <= 700 pc. The profile for r >= 20 pc is well fit by a power law with slope alpha = -1.0 +/- 0.2, consistent with predictions from cold dark matter simulations. Our models confirm that, despite its low baryon content relative to other dSphs, Draco lives in a massive halo.NSF-0908639Astronom
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