arXiv subjects

Edward J. Shaya

Publications and source records attributed to Edward J. Shaya.

31 records · Page 2Linked to original sources

arXiv2015

Two Planes of Satellites in the Centaurus A Group

Tip of the red giant branch measurements based on Hubble Space Telescope and ground-based imaging have resulted in accurate distances to 29 galaxies in the nearby Centaurus A Group. All but two of the 29 galaxies lie in either of two thin planes roughly parallel with the supergalactic equator. The planes are only slightly tilted from the line-of-sight, leaving little ambiguity regarding the morphology of the structure. The planes have characteristic r.m.s. long axis dimensions of ~300 kpc and short axis dimensions of ~60 kpc, hence axial ratios ~0.2, and are separated in the short axis direction by 303 kpc.

astro-ph.GA↗

arXiv2014

Infall of nearby galaxies into the Virgo cluster as traced with HST

We measured the Tip of the Red Giant Branch distances to nine galaxies in the direction to the Virgo cluster using the Advanced Camera for Surveys on the Hubble Space Telescope. These distances put seven galaxies: GR 34, UGC 7512, NGC 4517, IC 3583, NGC 4600, VCC 2037 and KDG 215 in front of the Virgo, and two galaxies: IC 3023, KDG 177 likely inside the cluster. Distances and radial velocities of the galaxies situated between us and the Virgo core clearly exhibit the infall phenomenon toward the cluster. In the case of spherically symmetric radial infall we estimate the radius of the "zero-velocity surface" to be (7.2+-0.7) Mpc that yields the total mass of the Virgo cluster to be (8.0+-2.3) X 10^{14} M_sun in good agreement with its virial mass estimates. We conclude that the Virgo outskirts does not contain significant amounts of dark matter beyond its virial radius.

astro-ph.GA↗

arXiv2013

Cosmicflows-2: The Data

Cosmicflows-2 is a compilation of distances and peculiar velocities for over 8000 galaxies. Numerically the largest contributions come from the luminosity-linewidth correlation for spirals, the TFR, and the related Fundamental Plane relation for E/S0 systems, but over 1000 distances are contributed by methods that provide more accurate individual distances: Cepheid, Tip of the Red Giant Branch, Surface Brightness Fluctuation, SNIa, and several miscellaneous but accurate procedures. Our collaboration is making important contributions to two of these inputs: Tip of the Red Giant Branch and TFR. A large body of new distance material is presented. In addition, an effort is made to assure that all the contributions, our own and those from the literature, are on the same scale. Overall, the distances are found to be compatible with a Hubble Constant H_0 = 74.4 +-3.0 km/s/Mpc. The great interest going forward with this data set will be with velocity field studies. Cosmicflows-2 is characterized by a great density and high accuracy of distance measures locally, falling to sparse and coarse sampling extending to z=0.1.

astro-ph.CO↗

arXiv2013

Discrete clouds of neutral gas between the galaxies M31 and M33

Spiral galaxies must acquire gas to maintain their observed level of star formation beyond the next few billion years (Leroy et al. 2008). A source of this material may be the gas that resides between galaxies, but our understanding of the state and distribution of this gas is incomplete (Shull et al. 2012). Radio observations (Braun & Thilker 2004) of the Local Group of galaxies have revealed hydrogen gas extending from the disk of the galaxy M31 at least halfway to M33. This feature has been interpreted to be the neutral component of a condensing intergalactic filament (Davé et al. 2001) which would be able to fuel star formation in M31 and M33, but simulations suggest that such a feature could also result from an interaction between both galaxies within the past few billion years (Bekki 2008). Here we report radio observations showing that about 50 per cent percent of this gas is composed of clouds, while the rest is distributed in an extended, diffuse component. The clouds have velocities comparable to those of M31 and M33, and have properties suggesting they are unrelated to other Local Group objects. We conclude that the clouds are likely to be transient condensations of gas embedded in an intergalactic filament and are therefore a potential source of fuel for future star formation in M31 and M33.

astro-ph.CO↗

arXiv2011

A Dynamical Model of the Local Group

This dynamical model for the 28 galaxies with distances less than 1.5 Mpc, and not apparently tight satellites, is constrained by the initial condition that peculiar velocities at high redshift are small and growing in accordance with the standard cosmology. The solution is a satisfactory fit to most of the measured redshifts, distances, and proper motions, with some interesting exceptions that call for further investigation. The model predicts Milky Way rotation speed 256 km/s, consistent with Reid et al. (2009a). Ten Local Group galaxies emanate from low supergalactic latitude and supergalactic longitude ~ 70 degrees, perhaps as remnants from failed assembly of a larger galaxy. NGC 6822 passes close to the Milky Way at redshift ~0.27, in an orbit similar to the Magellanic Clouds. Leo I has heliocentric angular velocity 0.33 mas/yr, perhaps measurable by the mean stellar motion, and 15 galaxies have proper motions greater than 0.05 mas/yr, measurable for any with masers.

astro-ph.CO↗

arXiv2007

Our Peculiar Motion Away from the Local Void

The peculiar velocity of the Local Group of galaxies manifested in the Cosmic Microwave Background dipole is found to decompose into three dominant components. The three components are clearly separated because they arise on distinct spatial scales and are fortuitously almost orthogonal in their influences. The nearest, which is distinguished by a velocity discontinuity at ~7 Mpc, arises from the evacuation of the Local Void. We lie in the Local Sheet that bounds the void. Random motions within the Local Sheet are small. Our Galaxy participates in the bulk motion of the Local Sheet away from the Local Void. The component of our motion on an intermediate scale is attributed to the Virgo Cluster and its surroundings, 17 Mpc away. The third and largest component is an attraction on scales larger than 3000 km/s and centered near the direction of the Centaurus Cluster. The amplitudes of the three components are 259, 185, and 455 km/s, respectively, adding collectively to 631 km/s in the reference frame of the Local Sheet. Taking the nearby influences into account causes the residual attributed to large scales to align with observed concentrations of distant galaxies and reduces somewhat the amplitude of motion attributed to their pull. On small scales, in addition to the motion of our Local Sheet away from the Local Void, the nearest adjacent filament, the Leo Spur, is seen to be moving in a direction that will lead to convergence with our filament. Finally, a good distance to an isolated galaxy within the Local Void reveals that this dwarf system has a motion of at least 230 km/s away from the void center. Given the velocities expected from gravitational instability theory in the standard cosmological paradigm, the distance to the center of the Local Void must be at least 23 Mpc from our position. The Local Void is large!

astro-ph↗

arXiv2007

Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest

Precision astrometry at microarcsecond accuracy has application to a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument that delivers parallaxes at about 4 microarcsec on targets as faint as V = 20, differential accuracy of 0.6 microarcsec on bright targets, and with flexible scheduling. The science topics are drawn primarily from the Team Key Projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics. SIM PlanetQuest is currently in the detailed design phase, having completed all of the enabling technologies needed for the flight instrument in 2005. It will be the first space-based long baseline Michelson interferometer designed for precision astrometry. SIM will contribute strongly to many astronomical fields including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. SIM will search for planets with masses as small as an Earth orbiting in the `habitable zone' around the nearest stars using differential astrometry, and could discover many dozen if Earth-like planets are common. It will be the most capable instrument for detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. SIM will observe significant numbers of very high- and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper motion measurements, SIM will probe the galactic mass distribution and the formation and evolution of the Galactic halo. (abridged)

astro-ph↗

arXiv2007

Tip of the Red Giant Branch Distances. II. Zero-Point Calibration

The luminosity of the Tip of the Red Giant Branch (TRGB) provides an excellent measure of galaxy distances and is easily determined in the resolved images of nearby galaxies observed with Hubble Space Telescope. There is now a large amount of archival data relevant to the TRGB methodology and which offers comparisons with other distance estimators. Zero-point issues related to the TRGB distance scale are reviewed in this paper. Consideration is given to the metallicity dependence of the TRGB, the transformations between HST flight systems and Johnson-Cousins photometry, the absolute magnitude scale based on Horizontal Branch measurements, and the effects of reddening. The zero-point of the TRGB is established with a statistical accuracy of 1%, modulo the uncertainty in the magnitude of the Horizontal Branch, with a typical rms uncertainty of 3% in individual galaxy distances at high Galactic latitude. The zero-point is consistent with the Cepheids period-luminosity relation scale but invites reconsideration of the claimed metallicity dependence with that method. The maser distance to NGC 4258 is consistent with TRGB but presently has lower accuracy.

astro-ph↗

arXiv2006

Tip of the Red Giant Branch Distances. I. Optimization of a Maximum Likelihood Algorithm

Accurate distances to galaxies can be determined from the luminosities of stars at the Tip of the Red Giant Branch (TRGB). We use a Maximum Likelihood algorithm to locate the TRGB in galaxy color-magnitude diagrams. The algorithm is optimized by introducing reliable photometric errors and a completeness characterization determined with artificial star experiments. The program is extensively tested using Monte-Carlo simulations, artificial galaxies, and a sample of nearby dwarf galaxies observed with HST/WFPC2 and ACS. Our procedure is shown to be reliable, to have good accuracy, and to not introduce any systematic errors. The methodology is especially useful in cases where the TRGB approaches the photometric limit and/or the RGB is poorly populated.

astro-ph↗

arXiv2005

Numerical action reconstruction of the dynamical history of dark matter haloes in N-body simulations

We test the ability of the numerical action method (NAM) to recover the individual orbit histories of mass tracers in an expanding universe in a region of radius 26Mpc/h, given the masses and redshift-space coordinates at the present epoch. The mass tracers are represented by dark matter haloes identified in a high resolution N-body simulation of the standard LCDM cosmology. Since previous tests of NAM at this scale have traced the underlying distribution of dark matter particles rather than extended haloes, our study offers an assessment of the accuracy of NAM in a scenario which more closely approximates the complex dynamics of actual galaxy haloes. We show that NAM can recover present-day halo distances with typical errors of less than 3 per cent, compared to 5 per cent errors assuming Hubble flow distances. The total halo mass and the linear bias were both found to be constained at the 50 per cent level. The accuracy of individual orbit reconstructions was limited by the inability of NAM, in some instances, to correctly model the positions of haloes at early times solely on the basis of the redshifts, angular positions, and masses of the haloes at the present epoch. Improvements in the quality of NAM reconstructions may be possible using the present-day three-dimensional halo velocities and distances to further constrain the dynamics. This velocity data is expected to become available for nearby galaxies in the coming generations of observations by SIM and GAIA.

astro-ph↗

arXiv1998

Antibiasing: High Mass-to-Light Ratios in Dense Clusters

Modeling of the velocity field of the Local Supercluster leads to the conclusion that there must be a substantial differential between the mass-to-light ratios of most galaxies and those in a small number of special places. Specifically, M/L=1000 is indicated for the full Virgo Cluster while M/L=150 is appropriate for most environments. It is argued that a higher M/L value is characteristic of E/S0 knots, regions where the galaxy density is high and collapse time scales are short. Regions larger than galaxies where collapse is ocurring only on the order of the age of the universe, the environment where spirals and irregulars predominate, are characterized by a low M/L. Overall, the mean density of the universe is well below closure. There could be comparable total mass in the relatively rare E/S0 knots as there is in the environments dominated by spirals and irregulars.

astro-ph↗

arXiv1996

Hubble Space Telescope Planetary Camera Images of NGC 1316

We present HST Planetary Camera V and I~band images of the central region of the peculiar giant elliptical galaxy NGC 1316. The inner profile is well fit by a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an assumed distance of 16.9 Mpc, the deprojected luminosity density reaches $\sim 2.0 \times 10^3 L_{\sun}$ pc$^{-3}$. Outside the inner two or three arcseconds, a constant mass-to-light ratio of $\sim 2.2 \pm 0.2$ is found to fit the observed line width measurements. The line width measurements of the center indicate the existence of either a central dark object of mass $2 \times 10^9 M_{\sun}$, an increase in the stellar mass-to-light ratio by at least a factor of two for the inner few arcseconds, or perhaps increasing radial orbit anisotropy towards the center. The mass-to-light ratio run in the center of NGC 1316 resembles that of many other giant ellipticals, some of which are known from other evidence to harbor central massive dark objects (MDO's). We also examine twenty globular clusters associated with NGC 1316 and report their brightnesses, colors, and limits on tidal radii. The brightest cluster has a luminosity of $9.9 \times 10^6 L_{\sun}$ ($M_V = -12.7$), and the faintest detectable cluster has a luminosity of $2.4 \times 10^5 L_{\sun}$ ($M_V = -8.6$). The globular clusters are just barely resolved, but their core radii are too small to be measured. The tidal radii in this region appear to be $\le$ 35 pc. Although this galaxy seems to have undergone a substantial merger in the recent past, young globular clusters are not detected.

astro-ph↗

arXiv1995

Action Principle Solutions For Galaxy Motions Within 3000 Km/s

The numerical action variational principle is used to find fully nonlinear solutions for the orbits of the mass tracers given their present redshifts and angular positions and the cosmological boundary condition that the peculiar velocities are small at high redshift. A solution predicts the distances of the mass tracers, and is tested by a comparison with measured distances. The current numerical results use 289 luminosity-linewidth distance measurements designed to be close to unbiased. A catalog of 1138 tracers approximates the luminosity distribution of galaxies in the vicinity of the Local Supercluster, at redshifts cz < 3000 km/s. These mass tracers include groups with crossing times less than the Hubble time and isolated galaxies. The measure of merit of a solution is the sum of the mean square differences between the predicted and observed distance moduli. In the 3000 km/s sample, this reduced $χ^2$ statistic has a well-defined minimum value at M/L=175 and $t_0 = 10.0$ Gyr, and $χ^2$ at the minimum is about 1.29 times the value expected from just the standard deviation of the distance measurements. We have tested for the effect of the mass at greater distance by using the positions of Abell clusters as a model for the large-scale mass distribution. This external mass model reduces the minimum value of $χ^2$ by about 10\% ($\sim 1 σ$). The value of the cosmological density parameter $Ω_0$ is determined by the global mean mass-to-light ratio. Our preliminary analysis yields $Ω_0= 0.17 \pm 0.10$ at one standard deviation. A tighter bound is expected to come out of a larger sample of measured distances now available.

astro-ph↗