The DSS-14 C-band exciter

The development and implementation of a C-band exciter for use with the Block IV Receiver-Exciter Subsystem at Deep Space Station 14 (DSS-14) has been completed. The exciter supplements the standard capabilities of the Block IV system by providing a drive signal for the C-band transmitter while generating coherent translation frequencies for C-band (5-GHz) to S-band (2.2- to 2.3-GHz) Doppler extraction, C-band to L-band (1.6-GHz) zero delay measurements, and a level calibrated L-band test signal. Exciter functions are described, and a general explanation and description of the C-band uplink controller is presented

HST imaging of hyperluminous infrared galaxies

We present HST WFPC2 I band imaging for a sample of 9 Hyperluminous Infrared Galaxies spanning a redshift range 0.45 < z < 1.34. Three of the sample have morphologies showing evidence for interactions, six are QSOs. Host galaxies in the QSOs are reliably detected out to z ~ 0.8. The detected QSO host galaxies have an elliptical morphology with scalelengths spanning 6.5 < r_{e}(Kpc) < 88 and absolute k corrected magnitudes spanning -24.5 < M_{I} < -25.2. There is no clear correlation between the IR power source and the optical morphology. None of the sources in the sample, including F15307+3252, show any evidence for gravitational lensing. We infer that the IR luminosities are thus real. Based on these results, and previous studies of HLIRGs, we conclude that this class of object is broadly consistent with being a simple extrapolation of the ULIRG population to higher luminosities; ULIRGs being mainly violently interacting systems powered by starbursts and/or AGN. Only a small number of sources whose infrared luminosities exceed 10^{13}Lsun are intrinsically less luminous objects which have been boosted by gravitational lensing.Comment: 16 Pages. Accepted for publication in MNRA

Faint Radio Sources and Star Formation History

Faint extragalactic radio sources provide important information about the global history of star formation. Sensitive radio observations of the Hubble Deep Field and other fields have found that sub-mJy radio sources are predominantly associated with star formation activity rather than AGN. Radio observations of star forming galaxies have the advantage of being independent of extinction by dust. We use the FIR-radio correlation to compare the radio and FIR backgrounds, and make several conclusions about the star forming galaxies producing the FIR background. We then use the redshift distribution of faint radio sources to determine the evolution of the radio luminosity function, and thus estimate the star formation density as a function of redshift.Comment: 12 pages, 9 figures, latex using texas.sty, to appear in the CD-ROM Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and Cosmology, held in Paris, France, Dec. 14-18, 1998. Eds.: J. Paul, T. Montmerle, and E. Aubourg (CEA Saclay). No changes to paper, just updated publication info in this commen

Thermal Emission from HII Galaxies: Discovering the Youngest Systems

We studied the radio properties of very young massive regions of star formation in HII galaxies, with the aim of detecting episodes of recent star formation in an early phase of evolution where the first supernovae start to appear. Our sample consists of 31 HII galaxies, characterized by strong Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm observations were obtained. The radio spectral energy distribution has a range of behaviours; 1) there are galaxies where the SED is characterized by a synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with possible free-free absorption at long wavelengths. The latter SEDs were found in a few galaxies and represent a signature of heavily embedded massive star clusters closely related to the early stages of massive star formation. Based on the comparison of the star formation rates determined from the recombination lines and those determined from the radio emission we find that SFR(Ha) is on average five times higher than SFR(1.4GHz). We confirm this tendency by comparing the ratio between the observed flux at 20 cm and the expected one, calculated based on the Ha star formation rates, both for the galaxies in our sample and for normal ones. This analysis shows that this ratio is a factor of 2 smaller in our galaxies than in normal ones, indicating that they fall below the FIR/radio correlation. These results suggest that the emission of these galaxies is dominated by a recent and massive star formation event in which the first supernovae (SN) just started to explode. We conclude that the systematic lack of synchrotron emission in those systems with the largest equivalent width of Hb can only be explained if those are young starbursts of less than 3.5Myr of age.Comment: Accepted for publication in Ap

New Results from a Near-Infrared Search for Hidden Broad-Line Regions in Ultraluminous Infrared Galaxies

This paper reports the latest results from a near-infrared search for hidden broad-line regions (BLRs: FWHM >~ 2,000 km/s) in ultraluminous infrared galaxies (ULIGs). The new sample contains thirty-nine ULIGs from the 1-Jy sample selected for their lack of BLRs at optical wavelengths. The results from this new study are combined with those from our previous optical and near-infrared surveys to derive the fraction of all ULIGs with optical or near-infrared signs of genuine AGN activity (either a BLR or [Si VI] emission). Comparisons of the dereddened emission-line luminosities of the optical or obscured BLRs detected in the ULIGs of the 1-Jy sample with those of optical quasars indicate that the obscured AGN/quasar in ULIGs is the main source of energy in at least 15 -- 25% of all ULIGs in the 1-Jy sample. This fraction is 30 -- 50% among ULIGs with L_ir > 10^{12.3} L_sun. These results are compatible with those from recent mid-infrared spectroscopic surveys carried out with ISO. (abridged)Comment: 40 pages including 10 figures and 3 tables (Table 3 should be printed in landscape mode

The star-formation history of the universe - an infrared perspective

A simple and versatile parameterized approach to the star formation history allows a quantitative investigation of the constraints from far infrared and submillimetre counts and background intensity measurements. The models include four spectral components: infrared cirrus (emission from interstellar dust), an M82-like starburst, an Arp220-like starburst and an AGN dust torus. The 60 $\mu$m luminosity function is determined for each chosen rate of evolution using the PSCz redshift data for 15000 galaxies. The proportions of each spectral type as a function of 60 $\mu$m luminosity are chosen for consistency with IRAS and SCUBA colour-luminosity relations, and with the fraction of AGN as a function of luminosity found in 12 $\mu$m samples. The luminosity function for each component at any wavelength can then be calculated from the assumed spectral energy distributions. With assumptions about the optical seds corresponding to each component and, for the AGN component, the optical and near infrared counts can be accurately modelled. A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850 $\mu$m can be found with pure luminosity evolution in all 3 cosmological models investigated: $\Omega_o$ = 1, $\Omega_o$ = 0.3 ($\Lambda$ = 0), and $\Omega_o$ = 0.3, $\Lambda$ = 0.7. All 3 models also give an acceptable fit to the integrated background spectrum. Selected predictions of the models, for example redshift distributions for each component at selected wavelengths and fluxes, are shown. The total mass-density of stars generated is consistent with that observed, in all 3 cosmological models.Comment: 20 pages, 25 figures. Accepted for publication in ApJ. Full details of models can be found at http://astro.ic.ac.uk/~mrr/countmodel

Potential mechanical loss mechanisms in bulk materials for future gravitational wave detectors

Low mechanical loss materials are needed to further decrease thermal noise in upcoming gravitational wave detectors. We present an analysis of the contribution of Akhieser and thermoelastic damping on the experimental results of resonant mechanical loss measurements. The combination of both processes allows the fit of the experimental data of quartz in the low temperature region (10 K to 25 K). A fully anisotropic numerical calculation over a wide temperature range (10 K to 300 K) reveals, that thermoelastic damping is not a dominant noise source in bulk silicon samples. The anisotropic numerical calculation is sucessfully applied to the estimate of thermoelastic noise of an advanced LIGO sized silicon test mass.Comment: 7 pages, 3 figures, submitted to Journal of Physics: Conference Series (AMALDI8

Pendulum Mode Thermal Noise in Advanced Interferometers: A comparison of Fused Silica Fibers and Ribbons in the Presence of Surface Loss

The use of fused-silica ribbons as suspensions in gravitational wave interferometers can result in significant improvements in pendulum mode thermal noise. Surface loss sets a lower bound to the level of noise achievable, at what level depends on the dissipation depth and other physical parameters. For LIGO II, the high breaking strength of pristine fused silica filaments, the correct choice of ribbon aspect ratio (to minimize thermoelastic damping), and low dissipation depth combined with the other achievable parameters can reduce the pendulum mode thermal noise in a ribbon suspension well below the radiation pressure noise. Despite producing higher levels of pendulum mode thermal noise, cylindrical fiber suspensions provide an acceptable alternative for LIGO II, should unforeseen problems with ribbon suspensions arise.Comment: Submitted to Physics Letters A (Dec. 14, 1999). Resubmitted to Physics Letters A (Apr. 3, 2000) after internal (LSC) review process. PACS - 04.80.Nn, 95.55.Ym, 05.40.C

57-Fe Mossbauer study of magnetic ordering in superconducting K_0.85Fe_1.83Se_2.09 single crystals

The magnetic ordering of superconducting single crystals of K_0.85Fe_1.83Se_2.09 has been studied between 10K and 550K using 57-Fe Mossbauer spectroscopy. Despite being superconducting below T_sc ~30K, the iron sublattice in K_0.85Fe_1.83Se_2.09 clearly exhibits magnetic order from well below T_sc to its N\'eel temperature of T_N = 532 +/- 2K. The iron moments are ordered perpendicular to the single crystal plates, i.e. parallel to the crystal c-axis. The order collapses rapidly above 500K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.Comment: 6 pages, 4 figures Submitted to Phys.Rev.