B3 0003+387: AGN Marked Large-Scale Structure at z=1.47?

We present evidence for a significant overdensity of red galaxies, as much as a factor of 14 over comparable field samples, in the field of the z=1.47 radio galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies are consistent with their being at z>0.8. The radio galaxy and one of the red galaxies are separated by 5" and show some evidence of a possible interaction. However, the red galaxies do not show any strong clustering around the radio galaxy nor around any of the brighter red galaxies. The data suggest that we are looking at a wall or sheet of galaxies, possibly associated with the radio galaxy at z=1.47. Spectroscopic redshifts of these red galaxies will be necessary to confirm this large-scale structure.Comment: 19 pages, 7 figures, LaTeX2e/AASTeX v5.0.2. The full photometric catalog is included as a separate deluxetable file. To appear in the Astronomical Journal (~Nov 00

The Variability of Sagittarius A* at Centimeter Wavelengths

We present the results of a 3.3-year project to monitor the flux density of Sagittarius A* at 2.0, 1.3, and 0.7 cm with the VLA. The fully calibrated light curves for Sgr A* at all three wavelengths are presented. Typical errors in the flux density are 6.1%, 6.2%, and 9.2% at 2.0, 1.3, and 0.7 cm, respectively. There is preliminary evidence for a bimodal distribution of flux densities, which may indicate the existence of two distinct states of accretion onto the supermassive black hole. At 1.3 and 0.7 cm, there is a tail in the distribution towards high flux densities. Significant variability is detected at all three wavelengths, with the largest amplitude variations occurring at 0.7 cm. The rms deviation of the flux density of Sgr A* is 0.13, 0.16, and 0.21 Jy at 2.0, 1.3, and 0.7 cm, respectively. During much of this monitoring campaign, Sgr A* appeared to be relatively quiescent compared to results from previous campaigns. At no point during the monitoring campaign did the flux density of Sgr A* more than double its mean value. The mean spectral index of Sgr A* is alpha=0.20+/-0.01, with a standard deviation of 0.14. The spectral index appears to depend linearly on the observed flux density at 0.7 cm with a steeper index observed during outbursts. This correlation is consistent with the expectation for outbursts that are self-absorbed at wavelengths of 0.7 cm or longer and inconsistent with the effects of simple models for interstellar scintillation. Much of the variability of Sgr A*, including possible time lags between flux density changes at the different wavelengths, appears to occur on time scales less than the time resolution of our observations (8 days). Future observations should focus on the evolution of the flux density on these time scales.Comment: 16 pages, 10 figures, accepted for publication in A

Chandra Observations of low velocity dispersion groups

Deviations of galaxy groups from cluster scaling relations can be understood in terms of an excess of entropy in groups. The main effect of this excess is to reduce the density and thus luminosity of the intragroup gas. Given this, groups should also should show a steep relationship between X-ray luminosity and velocity dispersion. However, previous work suggests that this is not the case with many measuring slopes flatter than the cluster relation. Examining the group L_X:\sigma relation shows that much of the flattening is caused by a small subset of groups which show very high X-ray luminosities for their velocity dispersions (or vice versa). Detailed Chandra study of two such groups shows that earlier ROSAT results were subject to significant (~30-40%) point source contamination, but confirm that a significant hot IGM is present in these groups, although these are two of the coolest systems in which intergalactic X-ray emission has been detected. Their X-ray properties are shown to be broadly consistent with those of other galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its X-ray luminosity correspondingly high for its temperature, compared to most groups. This leads us to suggest that the velocity dispersion in these systems has been reduced in some way, and we consider how this might have come about.Comment: Accepted for publication in Ap

Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz

We measured the linear polarization of Sagittarius A* to be 7.2 +/- 0.6 % at 230 GHzusing the BIMA array with a resolution of 3.6 x 0.9 arcsec. This confirms the previously reported detection with the JCMT 14-m antenna. Our high resolution observations demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We find a position angle 139 +/- 4 degrees that differs substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 2 x 10^6 rad m^-2. These new observations when combined with the JCMT observations at 150, 375 and 400 GHz suggest RM =-4.3 +/- 0.1 x 10^5 rad m^-2. This RM may be caused by an external Faraday screen. Barring a special geometry or a high number of field reversals, this RM rules out accretion rates greater than ~ 10^-7 M_sun y^-1. This measurement is inconsistent with high accretion rates necessary in standard advection dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low accretion rates as a major factor in the overall faintness of Sgr A*.Comment: accepted for publication in ApJ, 18 pages, 4 figure

Scaling laws for the response of nonlinear elastic media with implications for cell mechanics

We show how strain stiffening affects the elastic response to internal forces, caused either by material defects and inhomogeneities or by active forces that molecular motors generate in living cells. For a spherical force dipole in a material with a strongly nonlinear strain energy density, strains change sign with distance, indicating that even around a contractile inclusion or molecular motor there is radial compression; it is only at long distance that one recovers the linear response in which the medium is radially stretched. Scaling laws with irrational exponents relate the far-field renormalized strain to the near-field strain applied by the inclusion or active force.Comment: 4 pages, 3 figures, accepted to Physical Review Letter

Constraining dark energy models using the lookback time to galaxy clusters and the age of the universe

An impressive amount of different astrophysical data converges towards the picture of a spatially flat universe undergoing a today phase of accelerated expansion. The nature of the dark energy dominating the energy content of the universe is still unknown and a lot of different scenarios are viable candidates to explain cosmic acceleration. Most of the methods employed to test these cosmological models are essentially based on distance measurements to a particular class of objects. A different method, based on the lookback time to galaxy clusters and the age of the universe, is used here. In particular, we constrain the characterizing parameters of three classes of dark energy cosmological models to see whether they are in agreement with this kind of data, based on time measurements rather than distance observations.Comment: 13 pages, 8 figures, accepted for publication on Physical Review

Simulating the Hot X-ray Emitting Gas in Elliptical Galaxies

We study the chemo-dynamical evolution of elliptical galaxies and their hot X-ray emitting gas using high-resolution cosmological simulations. Our Tree N-body/SPH code includes a self-consistent treatment of radiative cooling, star formation, supernovae feedback, and chemical enrichment. We present a series of LCDM cosmological simulations which trace the spatial and temporal evolution of heavy element abundance patterns in both the stellar and gas components of galaxies. X-ray spectra of the hot gas are constructed via the use of the vmekal plasma model, and analysed using XSPEC with the XMM EPN response function. Simulation end-products are quantitatively compared with the observational data in both the X-ray and optical regime. We find that radiative cooling is important to interpret the observed X-ray luminosity, temperature, and metallicity of the interstellar medium of elliptical galaxies. However, this cooled gas also leads to excessive star formation at low redshift, and therefore results in underlying galactic stellar populations which are too blue with respect to observations.Comment: 6 pages, 3 figures, to appear in the proceedings of "The IGM/Galaxy Connection - The Distribution of Baryons at z=0", ed. M. Putman & J. Rosenberg; High resolution version is available at http://astronomy.swin.edu.au/staff/dkawata/research/papers.htm

The Self-Regulated Growth of Supermassive Black Holes

We present a series of simulations of the self--regulated growth of supermassive black holes (SMBHs) in galaxies via three different fueling mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in all three scenarios follow the same black hole fundamental plane (BHFP) and correlation with bulge binding energy seen in simulations of major mergers, and observed locally. Furthermore, provided that the total gas supply is significantly larger than the mass of the SMBH, its limiting mass is not influenced by the amount of gas available or the efficiency of black hole growth. This supports the assertion that SMBHs accrete until they reach a critical mass at which feedback is sufficient to unbind the gas locally, terminating the inflow and stalling further growth. At the same time, while minor and major mergers follow the same projected correlations (e.g., the $M_{BH}-\sigma$ and Magorrian relations), SMBHs grown via disk instabilities do not, owing to structural differences between the host bulges. This finding is supported by recent observations of SMBHs in pseudobulges and bulges in barred systems, as compared to those hosted by classical bulges. Taken together, this provides support for the BHFP and binding energy correlations as being more "fundamental" than other proposed correlations in that they reflect the physical mechanism driving the co-evolution of SMBHs and spheroids.Comment: 15 pages, 16 figures, accepted for publication in Ap