HST optical spectral index map of the jet of 3C 273

We present HST images at 622 nm and 300 nm of the jet in 3C273 and determine the run of the optical spectral index at 0.2" along the jet. The smoothness of spectral index changes shows that the physical conditions are varying smoothly across the jet. There is no correlation between the optical flux and spectral index, as would be expected for relativistic electrons suffering strong cooling due to synchrotron emission. We find no evidence for localized acceleration or loss sites. This suggests that the spectral shape is not changing much throughout the jet. We show that relativistic beaming and/or sub-equipartition magnetic fields cannot remove the discrepancy between light-travel time along the jet and the lifetime of electrons emitting optical synchrotron radiation. We consider this further evidence in favour of a distributed electron acceleration process.Comment: Accepted for publication by Astronomy and Astrophysics (13 pages, 8 figures

X-rays from the jet in 3C 273: clues from the radio-optical spectra

Using new deep VLA and HST observations of the large-scale jet in 3C273 matched to 0.3" resolution, we have detected excess near-ultraviolet emission (300 nm) above a synchrotron cutoff spectrum accounting for the emission from radio through optical (3.6 cm - 620 nm). This necessitates a two-component model for the emission. The radio-optical-X-ray spectral energy distributions suggest a common origin for the UV excess and the X-rays from the jet.Comment: Accepted for publication by A&A Letters (4 pages, 3 figures

The radio-ultraviolet spectral energy distribution of the jet in 3C273

We present deep VLA and HST observations of the large-scale jet in 3C 273 matched to 0.3" resolution. The observed spectra show a significant flattening in the infrared-ultraviolet wavelength range. The jet's emission cannot therefore be assumed to arise from a single electron population and requires the presence of an additional emission component. The observed smooth variations of the spectral indices along the jet imply that the physical conditions vary correspondingly smoothly. We determine the maximum particle energy for the optical jet using synchrotron spectral fits. The slow decline of the maximum energy along the jet implies particle reacceleration acting along the entire jet. In addition to the already established global anti-correlation between maximum particle energy and surface brightness, we find a weak positive correlation between small-scale variations in maximum particle energy and surface brightness. The origin of these conflicting global and local correlations is unclear, but they provide tight constraints for reacceleration models.Comment: 28 pages, lots of figures, accepted for publication in A&

Distant galaxy clusters in the COSMOS field found by HIROCS

We present the first high-redshift galaxy cluster candidate sample from the HIROCS survey found in the COSMOS field. It results from a combination of public COSMOS with proprietary H-band data on a 0.66 square degree part of the COSMOS field and comprises 12 candidates in the redshift range 1.23 < z < 1.55. We find an increasing fraction of blue cluster members with increasing redshift. Many of the blue and even some of the reddest member galaxies exhibit disturbed morphologies as well as signs of interaction.Comment: 5 pages, 5 figures, in print format, accepted for publication by A&A Letter

Synchrotron and Inverse Compton Constraints on Lorentz Violations for Electrons

We present a method for constraining Lorentz violation in the electron sector, based on observations of the photons emitted by high-energy astrophysical sources. The most important Lorentz-violating operators at the relevant energies are parameterized by a tensor c^{nu mu) with nine independent components. If c is nonvanishing, then there may be either a maximum electron velocity less than the speed of light or a maximum energy for subluminal electrons; both these quantities will generally depend on the direction of an electron's motion. From synchrotron radiation, we may infer a lower bound on the maximum velocity, and from inverse Compton emission, a lower bound on the maximum subluminal energy. With observational data for both these types of emission from multiple celestial sources, we may then place bounds on all nine of the coefficients that make up c. The most stringent bound, on a certain combination of the coefficients, is at the 6 x 10^(-20) level, and bounds on the coefficients individually range from the 7 x 10^(-15) level to the 2 x 10^(-17) level. For most of the coefficients, these are the most precise bounds available, and with newly available data, we can already improve over previous bounds obtained by the same methods.Comment: 28 page

Radial velocities for the Hipparcos-Gaia Hundred-Thousand-Proper-Motion project

(abridged) The Hundred-Thousand-Proper-Motion (HTPM) project will determine the proper motions of ~113500 stars using a 23-year baseline. The proper motions will use the Hipparcos data, with epoch 1991.25, as first epoch and the first intermediate-release Gaia astrometry, with epoch ~2014.5, as second epoch. The expected HTPM proper-motion standard errors are 30-190 muas/yr, depending on stellar magnitude. Depending on the characteristics of an object, in particular its distance and velocity, its radial velocity can have a significant impact on the determination of its proper motion. The impact of this perspective acceleration is largest for fast-moving, nearby stars. Our goal is to determine, for each star in the Hipparcos catalogue, the radial-velocity standard error that is required to guarantee a negligible contribution of perspective acceleration to the HTPM proper-motion precision. We employ two evaluation criteria, both based on Monte-Carlo simulations, with which we determine which stars need to be spectroscopically (re-)measured. Both criteria take the Hipparcos measurement errors into account. For each star in the Hipparcos catalogue, we determine the confidence level with which the available radial velocity and its standard error, taken from the XHIP compilation catalogue, are acceptable. We find that for 97 stars, the radial velocities available in the literature are insufficiently precise for a 68.27% confidence level. We also identify 109 stars for which radial velocities are currently unknown yet need to be acquired to meet the 68.27% confidence level. To satisfy the radial-velocity requirements coming from our study will be a daunting task consuming a significant amount of spectroscopic telescope time. Fortunately, the follow-up spectroscopy is not time-critical since the HTPM proper motions can be corrected a posteriori once (improved) radial velocities become available.Comment: Accepted in A&

A Strategy for Identifying the Grid Stars for the Space Interferometry Mission (SIM)

We present a strategy to identify several thousand stars that are astrometrically stable at the micro-arcsecond level for use in the SIM (Space Interferometry Mission) astrometric grid. The requirements on the grid stars make this a rather challenging task. Taking a variety of considerations into account we argue for K giants as the best type of stars for the grid, mainly because they can be located at much larger distances than any other type of star due to their intrinsic brightness. We show that it is possible to identify suitable candidate grid K giants from existing astrometric catalogs. However, double stars have to be eliminated from these candidate grid samples, since they generally produce much larger astrometric jitter than tolerable for the grid. The most efficient way to achieve this is probably by means of a radial velocity survey. To demonstrate the feasibility of this approach, we repeatedly measured the radial velocities for a pre-selected sample of 86 nearby Hipparcos K giants with precisions of 5-8 m/s. The distribution of the intrinsic radial velocity variations for the bona-fide single K giants shows a maximum around 20 m/s, which is small enough not to severely affect the identification of stellar companions around other K giants. We use the results of our observations as input parameters for Monte-Carlo simulations on the possible design of a radial velocity survey of all grid stars. Our favored scenario would result in a grid which consists to 68% of true single stars and to 32% of double or multiple stars with periods mostly larger than 200 years, but only 3.6% of all grid stars would display astrometric jitter larger than 1 microarcsecond. This contamination level is probably tolerable.Comment: LaTeX, 21 pages, 8 figures, accepted by PASP (February 2001 issue). Also available at http://beehive.ucsd.edu/ftp/pub/grid/kgiants.htm

The Luminosity Function Of Field Galaxies And Its Evolution Since z=1

We present the B-band luminosity function and comoving space and luminosity densities for a sample of 2779 I-band selected field galaxies based on multi-color data from the CADIS survey. The sample is complete down to I_815 = 22 without correction and with completeness correction extends to I_815=23.0. By means of a new multi-color analysis the objects are classified according to their spectral energy distributions (SEDs) and their redshifts are determined with typical errors of delta z <= 0.03. We have split our sample into four redshift bins between z=0.1 and z=1.04 and into three SED bins E-Sa,Sa-Sc and starbursting (emission line) galaxies. The evolution of the luminosity function is clearly differential with SED. The normalization phi* of luminosity function for the E-Sa galaxies decreases towards higher redshift, and we find evidence that the comoving galaxy space density decreases with redshift as well. In contrast, we find phi* and the comoving space density increasing with redshift for the Sa-Sc galaxies. For the starburst galaxies we find a steepening of the luminosity function at the faint end and their comoving space density increases with redshift.Comment: 15 pages, 14 figures, accepted by Astronomy&Astrophysic