Breakthrough Listen

Abstract:

Starting from the assumption that there is a large population (≥108) of stellar-mass isolated black holes (IBH) distributed throughout our Galaxy, we consider the detectable signatures of accretion from the interstellar medium (ISM) that may be associated with such a population. We simulate the nearby (radius 250 pc) part of this population, corresponding to the closest ~35 000 black holes, using current best estimates of the mass distribution of stellar-mass black holes combined with two models for the velocity distribution of stellar-mass IBH which bracket likely possibilities. We distribute this population of objects appropriately within the different phases of the ISM and calculate the Bondi-Hoyle accretion rate, modified by a further dimensionless efficiency parameter λ. Assuming a simple prescription for radiatively inefficient accretion at low Eddington ratios, we calculate the X-ray luminosity of these objects, and similarly estimate the radio luminosity from relations found empirically for black holes accreting at low rates. The latter assumption depends crucially on whether or not the IBH accrete from the ISM in a manner which is axisymmetric enough to produce jets. Comparing the predicted X-ray fluxes with limits from hard X-ray surveys, we conclude that either the Bondi-Hoyle efficiency parameter λ is rather small (=0.01), the velocities of the IBH are rather high, or some combination of both. The predicted radio flux densities correspond to a population of objects which, while below current survey limits, should be detectable with the Square Kilometre Array (SKA). Converting the simulated space velocities into proper motions, we further demonstrate that such IBH could be identified as faint high proper motion radio sources in SKA surveys. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

This is the first paper in a series that presents a multiwavelength analysis of the archetype ultraluminous infrared galaxy IRAS F10214+4724, a gravitationally lensed, starburst/active galactic nucleus at z = 2.3. Here we present a new lens model and spatially resolved radio data, as well as a deep Hubble Space Telescope (HST) F160W map. The lens modelling employs a Bayesian Markov chain Monte Carlo algorithm with extended source, forward ray tracing. Using these high-resolution HST, Multi-Element Radio Linked Interferometer Network (MERLIN) and Very Large Array (VLA) maps, the algorithm allows us to constrain the level of distortion to the continuum spectral energy distribution resulting from emission components with differing magnification factors, due to their size and proximity to the caustic. Our lens model finds that the narrow-line region, and by proxy the active nucleus, is preferentially magnified. This supports previous claims that preferential magnification could mask the expected polycyclic aromatic hydrocarbon spectral features in the Spitzer mid-infrared spectrum which roughly trace the star-forming regions. Furthermore, we show that the arc-to-counter-image flux ratio is not a good estimate of the magnification in this system, despite its common use in the IRAS F10214+4724 literature. Our lens modelling suggests magnifications of μ ∼ 15-20 ± 2 for the HST F814W, MERLIN 1.7 GHz and VLA 8 GHz maps, significantly lower than the canonical values of μ = 50-100 often used for this system. Systematic errors such as the dark matter density slope and colocation of stellar and dark matter centroids dominate the uncertainties in the lens model at the 40 per cent level. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We perform a stacking analysis of Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) data in order to obtain isothermal dust temperatures and rest-frame luminosities at 250 μm (L250), for a well-defined sample of 1599 radio sources over the H-ATLAS Phase 1/Galaxy and Mass Assembly (GAMA) area. The radio sample is generated using a combination of NRAO VLA Sky Survey data and K-band United Kingdom Infrared Telescope Deep Sky Survey-Large Area Survey data, over the redshift range 0.01 < z < 0.8. The far-infrared(FIR) properties of the sample are investigated as a function of 1.4-GHz luminosity, redshift, projected radio-source size and radio spectral index. In order to search for stellar mass-dependent relations, we split the parent sample into those sources which are below and above 1.5 L*K.After correcting for stellar mass and redshift, we find no relation between the 250-μm luminosity and the 1.4-GHz radio luminosity of radio active galactic nuclei. This implies thata galaxy's nominal radio luminosity has little or no bearing on the star formation rate (SFR)and/or dust mass content of the host system, although this does not mean that other variables(e.g. radio source size) related to the jets do not have an effect. The L250 of both the radio detected and non-radio-detected galaxies (defined as those sources not detected at 1.4 GHz but detected in the Sloan Digital Sky Survey with r< 22) rises with increasing redshift. Compact radio sources (<30 kpc) are associated with higher 250 μm luminosities and dust temperatures than their more extended (>30 kpc) counterparts. The higher dust temperature suggests that this may be attributed to enhanced SFRs in compact radio galaxies, but whether this is directly or indirectly due to radio activity (e.g. jet-induced or merger-driven star formation) is as yet unknown.For matched samples in LK and g-r, sub-1.5 L*K and super-1.5 L*K radio-detected galaxies have 0.89±0.18 and 0.49±0.12 times the 250μm luminosity of their non-radio-detected counterparts. Thus, while no difference in L250 is observed in sub-1.5 L*K radio-detected galaxies, a strong deficit is observed in super-1.5 L*K radio-detected galaxies. We explain these results in terms of the hotter, denser and richer halo environments massive radio galaxies maintain and are embedded in. These environments are expected to quench the cold gas and dust supply needed for further star formation and therefore dust production. Our results indicate that all massive radio galaxies (>1.5 L*K) may have systematically lower FIR luminosities(~25 per cent) than their colour-matched non-radio-detected counterparts. Finally, no relation between radio spectral index and L250 is found for the subset of 1.4-GHz radio sources with detections at 330 MHz. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

Sub-millimetre observations of the William Herschel Deep Field (WHDF) using the Large Apex Bolometer Camera (LABOCA) revealed possible sub-mm counterparts for two X-rayabsorbed quasars. The primary aim here is to exploit ExpandedVery LargeArray (EVLA) radio continuum imaging at 8.4GHz to establish the absorbed quasars as radio/sub-mm sources. The main challenge in reducing the WHDF EVLA data was the presence of a strong 4C source at the field edge. A new calibration algorithm was applied to the data to model and subtract this source. The resulting thermal noise limited radiomap covers a sky area which includes the 16× 16arcmin2 Extended WHDF. It contains 41 radio sources above the 4σ detection threshold, 17 of which have primary beam corrected flux densities. The radio observations show that the two absorbed active galactic nuclei (AGN) with LABOCA detections are also coincident with radio sources, confirming the tendency for X-ray-absorbed AGN to be sub-mm bright. These two sources also show strong ultraviolet excess (UVX) which suggest that the nuclear sightline is gas absorbed but not dust absorbed. Of the three remaining LABOCA sources within the ≈5arcmin half-power diameter of the EVLA primary beam, one is identified with a faint nuclear X-ray/radio source in a nearby galaxy, one with a faint radio source and the other is unidentified in any other band. More generally, differential radio source counts calculated from the beam-corrected data are in good agreement with previous observations, showing atS < 50μJy a significant excess over a pure AGN model. In the full area, of 10 sources fainter than this limit, six have optical counterparts of which three are UVX (i.e. likely quasars) including the two absorbed quasar LABOCA sources. The other faint radio counterparts are not UVX but are only slightly less blue and likely to be star-forming/merging galaxies, predominantly at lower luminosities and redshifts. The four faint, optically unidentified radio sources may be either dust-obscured quasars or galaxies. These high-redshift obscured AGN and lower redshift star-forming populations are thus the main candidates to explain the observed excess in the faint source counts and hence also the excess radio background found previously by the Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission (ARCADE2) experiment. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We present Jansky Very Large Array observations of the cold (CO (1→0)) molecular gas in IRAS F10214+4724, a lensed ultraluminous infraRed galaxy (ULIRG) at z = 2.3 with an obscured active nucleus. The galaxy is spatially and spectrally well resolved in the CO (1→0) emission line. The total intensity and velocity maps reveal a reasonably ordered system; however, there is some evidence for minor merger activity. A CO (10) counter-image is detected at the 3σ level. Five of the 42 kms-1 channels (with >5σ detections) are mapped back into the source plane and their total magnification posterior probability distribution functions are sampled. This reveals a roughly linear arrangement, tentatively a rotating disc. We derive a molecular gas mass of Mgas = 1.2 ± 0.2 × 1010 M, assuming a ULIRG LCOto- Mgas conversion ratio of α = 0.8 M (K km s-1 pc2)-1 that agrees well with the derived range of α = 0.3-1.3M (K km s-1 pc2)-1 for separate dynamical mass estimates at assumed inclinations of i = 90°-30°. The lens modelling and CO (1→0) spectrum asymmetry suggest that there may be substantial (factor 2) preferential lensing of certain individual channels; however, the CO (1→0) spatially integrated channel flux uncertainties limit the significance of this result. Based on the AGN and CO (1→0) peak emission positions and the lens model, we predict a distortion of the CO spectral line energy distribution where higher order J lines that may be partially excited by AGN heating will be preferentially lensed owing to their smaller solid angles and closer proximity to the AGN and therefore the cusp of the caustic. Comparison with other lensing inversion results shows that the narrow-line region and AGN radio core in IRAS F10214+4724 are preferentially lensed by a factor of >3 and 11, respectively, relative to the molecular gas emission. This distorts the global continuum emission spectral energy distribution and strongly suggests caution in unsophisticated uses of IRAS F10214+4724 as an archetype high-redshift ULIRG. We explore two large velocity gradient models, incorporating spatial CO (10) and CO (32) information and present tentative evidence for an extended, low-excitation, cold gas component that implies that the total molecular gas mass in IRAS F10214+4724 is a factor of 2 greater than that calculated using spatially unresolved CO observations. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.