arXiv subjects

Steve Rawlings

Publications and source records attributed to Steve Rawlings.

At least 19 recordsLinked to original sources

arXiv2014

Black-hole masses, accretion rates and hot- and cold-mode accretion in radio galaxies at z~1

Understanding the evolution of accretion activity is fundamental to our understanding of how galaxies form and evolve over the history of the Universe. We analyse a complete sample of 27 radio galaxies which includes both high-excitation (HEGs) and low excitation galaxies (LEGs), spanning a narrow redshift range of 0.9 < z < 1.1 and covering a factor of ~1000 in radio luminosity. Using data from the Spitzer Space Telescope combined with ground-based optical and near-infrared imaging, we show that the host galaxies have masses in the range of 10.7 < log (M /M_sun) < 12.0 with HEGs and LEGs exhibiting no difference in their mass distributions. We also find that HEGs accrete at significantly higher rates than LEGs, with the HEG/LEG division lying at an Eddington ratio of ~0.04, which is in excellent agreement with theoretical predictions of where the accretion rate becomes radiatively inefficient, thus supporting the idea of HEGs and LEGs being powered by different modes of accretion. Our study also shows that at least up to L_151MHz ~3x10^27 W /Hz /sr, HEGs and LEGs are indistinguishable in terms of their radio properties. From this result we infer that, at least for the lower radio luminosity range, another factor besides accretion rate must play an important role in the process of triggering jet activity.

astro-ph.GA↗

arXiv2013

Evolution of the Milky Way in Semi-Analytic Models: Detecting Cold Gas at z=3 with ALMA and SKA

We forecast the abilities of the Atacama Large Millimeter/submillimeter Array (ALMA) and the Square Kilometer Array (SKA) to detect CO and HI emission lines in galaxies at redshift z=3. A particular focus is set on Milky Way (MW) progenitors at z=3 for their detection within 24 h constitutes a key science goal of ALMA. The analysis relies on a semi-analytic model, which permits the construction of a MW progenitor sample by backtracking the cosmic history of all simulated present-day galaxies similar to the real MW. Results: (i) ALMA can best observe a MW at z=3 by looking at CO(3-2) emission. The probability of detecting a random model MW at 3-sigma in 24 h using 75 km/s channels is roughly 50%, and these odds can be increased by co-adding the CO(3-2) and CO(4-3) lines. These lines fall into ALMA band 3, which therefore represents the optimal choice towards MW detections at z=3. (ii) Higher CO transitions contained in the ALMA bands geq6 will be invisible, unless the considered MW progenitor coincidentally hosts a major starburst or an active black hole. (iii) The high-frequency array of SKA, fitted with 28.8 GHz receivers, would be a powerful instrument for observing CO(1-0) at z=3, able to detect nearly all simulated MWs in 24 h. (iv) HI detections in MWs at z=3 using the low-frequency array of SKA will be impossible in any reasonable observing time. (v) SKA will nonetheless be a supreme ha survey instrument through its enormous instantaneous field-of-view (FoV). A one year pointed HI survey with an assumed FoV of 410 sqdeg would reveal at least 10^5 galaxies at z=2.95-3.05. (vi) If the positions and redshifts of those galaxies are known from an optical/infrared spectroscopic survey, stacking allows the detection of HI at z=3 in less than 24 h.

astro-ph.CO↗

arXiv2013

Ground-state 12CO emission and a resolved jet at 115 GHz (rest-frame) in the radio loud quasar 3C318

An analysis of 44 GHz VLA observations of the z = 1.574 radio-loud quasar 3C318 has revealed emission from the redshifted J = 1 - 0 transition of the CO molecule and spatially resolved the 6.3 kpc radio jet associated with the quasar at 115 GHz rest-frame. The continuum-subtracted line emitter is spatially offset from the quasar nucleus by 0.33" (2.82 kpc in projection). This spatial offset has a significance of >8-sigma and, together with a previously published -400 km/s velocity offset measured in the J = 2 - 1 CO line relative to the systemic redshift of the quasar, rules out a circumnuclear starburst or molecular gas ring and suggests that the quasar host galaxy is either undergoing a major merger with a gas-rich galaxy or is otherwise a highly disrupted system. If the merger scenario is correct then the event may be in its early stages, acting as the trigger for both the young radio jets in the quasar and a starburst in the merging galaxy. The total molecular gas mass in the spatially offset line emitter as measured from the ground-state CO line M_H2 = 3.7 (+/-0.4) x 10^10 (alpha_CO/0.8) M_solar. Assuming that the line-emitter can be modelled as a rotating disk, an inclination-dependent upper limit is derived for its dynamical mass M_dyn sin^2(i) < 3.2 x 10^9 M_solar, suggesting that for M_H2 to remain less than M_dyn the inclination angle must be i < 16 degrees. The far infrared and CO luminosities of 246 extragalactic systems are collated from the literature for comparison. The high molecular gas content of 3C318 is consistent with that of the general population of high redshift quasars and sub-millimetre galaxies.

astro-ph.CO↗

arXiv2012

Gas and dust in a z=2.8 obscured quasar

We present new detections of the CO(5-4), CO(7-6), [CI](1-0) and [CI](2-1) molecular and atomic line transitions towards the unlensed, obscured quasar AMS12 (z=2.7672), observed with the IRAM PdBI. This is the first unlensed, high redshift source to have both [CI] transitions detected. Continuum measurements between 70 $μ$m and 3 mm are used to constrain the FIR SED, and we find a best fit FIR luminosity of log[Lfir/Lsol] = 13.5+/-0.1, dust temperature T_d = 88+/-8 K and emissivity index β = 0.6+/-0.1. The highly-excited molecular gas probed by CO(3-2), (5-4) and (7-6), is modelled with large velocity gradient (LVG) models. The gas kinetic temperature T_g, density n(H2), and the characteristic size r0, are determined using the dust temperature from the FIR SED as a prior for the gas temperature. The best fitting parameters are T_g = 90+/-8 K, n(H2) = 10^(3.9+/-0.1) cm^(-3) and r0 = 0.8+/-0.04 kpc. The ratio of the [CI] lines gives a [CI] excitation temperature of 43+/-10 K, indicating the [CI] and the high-excitation CO are not in thermal equilibrium. The [CI] excitation temperature is below that of T_d and T_g of the high-excitation CO, perhaps because [CI] lies at a larger radius where there may also be a large reservoir of CO at a cooler temperature, perhaps detectable through the CO(1-0). Using the [CI](1-0) line we can estimate the strength of the CO(1-0) line and hence the gas mass. This suggests that a significant fraction (~30%) of the molecular gas is missed from the high-excitation line analysis. The Eddington limited black hole mass is found from the bolometric luminosity to be Mbh >~ 1.5x10^9 Msol. Along with the stellar mass of 3x10^11 Msol, these give a black hole - bulge mass ratio of Mbh/Mbulge >~ 0.005. This is in agreement with studies on the evolution of the Mbh/Mbulge relationship at high redshifts, which find a departure from the local value ~0.002.

astro-ph.CO↗

arXiv2012

The cosmic spin of the most massive black holes

Under the assumption that jets in active galactic nuclei are powered by accretion and the spin of the central supermassive black hole, we are able to reproduce the radio luminosity functions of high- and low-excitation galaxies. High-excitation galaxies are explained as high-accretion rate but very low spin objects, while low-excitation galaxies have low accretion rates and bimodal spin distributions, with approximately half of the population having maximal spins. At higher redshifts (z~1), the prevalence of high accretion rate objects means the typical spin was lower, while in the present day Universe is dominated by low accretion rate objects, with bimodal spin distributions.

astro-ph.HE↗

arXiv2012

Radio imaging of the Subaru/XMM-Newton Deep Field - III. Evolution of the radio luminosity function beyond z=1

We present spectroscopic and eleven-band photometric redshifts for galaxies in the 100-uJy Subaru/XMM-Newton Deep Field radio source sample. We find good agreement between our redshift distribution and that predicted by the SKA Simulated Skies project. We find no correlation between K-band magnitude and radio flux, but show that sources with 1.4-GHz flux densities below ~1mJy are fainter in the near-infrared than brighter radio sources at the same redshift, and we discuss the implications of this result for spectroscopically-incomplete samples where the K-z relation has been used to estimate redshifts. We use the infrared--radio correlation to separate our sample into radio-loud and radio-quiet objects and show that only radio-loud hosts have spectral energy distributions consistent with predominantly old stellar populations, although the fraction of objects displaying such properties is a decreasing function of radio luminosity. We calculate the 1.4-GHz radio luminosity function (RLF) in redshift bins to z=4 and find that the space density of radio sources increases with lookback time to z~2, with a more rapid increase for more powerful sources. We demonstrate that radio-loud and radio-quiet sources of the same radio luminosity evolve very differently. Radio-quiet sources display strong evolution to z~2 while radio-loud AGNs below the break in the radio luminosity function evolve more modestly and show hints of a decline in their space density at z>1, with this decline occurring later for lower-luminosity objects. If the radio luminosities of these sources are a function of their black hole spins then slowly-rotating black holes must have a plentiful fuel supply for longer, perhaps because they have yet to encounter the major merger that will spin them up and use the remaining gas in a major burst of star formation.

astro-ph.CO↗

arXiv2011

Evidence for cosmic evolution in the spin of the most massive black holes

We use results from simulations of the production of magnetohydrodynamic jets around black holes to derive the cosmic spin history of the most massive black holes. We assume that the efficiency of jet production is a monotonic function of spin a, as given by the simulations, and that the accretion flow geometry is similarly thick for quasars accreting close to the Eddington ratio and for low-excitation radio galaxies accreting at very small Eddington rates. We use the ratio of the comoving densities of the jet power and the radiated accretion power associated with supermassive black holes with Mbh>~10^8 Msol to estimate the cosmic history of the characteristic spin a. The evolution of this ratio, which increases with decreasing z, is consistent with a picture where the z~0 active galactic nuclei have typically higher spins than those at z~2 (with typical values a~0.35-0.95 and a~0.0-0.25 respectively). We discuss the implications in terms of the relative importance of accretion and mergers in the growth of supermassive black holes with Mbh>~10^8 Msol.

astro-ph.HE↗

arXiv2011

Cosmology with the Square Kilometre Array

We review how the Square Kilometre Array (SKA) will address fundamental questions in cosmology, focussing on its use for neutral Hydrogen (HI) surveys. A key enabler of its unique capabilities will be large (but smart) receptors in the form of aperture arrays. We outline the likely contributions of Phase-1 of the SKA (SKA1), Phase-2 SKA (SKA2) and pathfinding activities (SKA0). We emphasise the important role of cross-correlation between SKA HI results and those at other wavebands such as: surveys for objects in the EoR with VISTA and the SKA itself; and huge optical and near-infrared redshift surveys, such as those with HETDEX and Euclid. We note that the SKA will contribute in other ways to cosmology, e.g. through gravitational lensing and $H_{0}$ studies.

astro-ph.CO↗

arXiv2011

The Square Kilometre Array

We review the current status of the Square Kilometre Array (SKA) by outlining the science drivers for its Phase-1 (SKA1) and setting out the timeline for the key decisions and milestones on the way to the planned start of its construction in 2016. We explain how Phase-2 SKA (SKA2) will transform the research scope of the SKA infrastructure, placing it amongst the great astronomical observatories and survey instruments of the future, and opening up new areas of discovery, many beyond the confines of conventional astronomy.

astro-ph.IM↗

arXiv2011

Observational constraints on the spin of the most massive black holes from radio observations

We use recent progress in simulating the production of magnetohydrodynamic jets around black holes to derive the cosmic spin history of the most massive black holes, with masses >~10^8 Msol. Assuming the jet efficiency depends on spin a, we can approximately reproduce the observed `radio loudness' of quasars and the local radio luminosity function. Using the X-ray luminosity function and the local mass function of supermassive black holes, SMBHs we can reproduce the individual radio luminosity functions of radio sources showing high- and low-excitation narrow emission lines. The data favour spin distributions that are bimodal, with one component around spin zero and the other close to maximal spin. In the low-excitation galaxies, the two components have similar amplitudes. For the high-excitation galaxies, the amplitude of the high-spin peak is typically much smaller than that of the low-spin peak. A bimodality should be seen in the radio loudness of quasars. We predict that the low-excitation galaxies are dominated by SMBHs with masses >~10^8 Msol, down to radio luminosity densities ~10^21 W Hz-1 sr-1 at 1.4~GHz. Our model is also able to predict the radio luminosity function at z=1, and predicts it to be dominated by high-excitation galaxies above luminosity densities >~10^26 W Hz-1 sr-1, in full agreement with the observations. From our parametrisation and using the best fitting jet efficiencies there is marginal evidence for evolution in spin: the mean spin increases slightly from ~0.25 at z=1 to ~0.35 at z=0, and the fraction of SMBHs with a>=0.5 increases from 0.16+-0.03 at z=1 to 0.24+-0.09 at z=0. Our results are in excellent agreement with the mean radiative efficiency of quasars, as well as recent cosmological simulations. We discuss the implications in terms of accretion and SMBH mergers, and galactic black holes (Abridged).

astro-ph.HE↗

arXiv2010

Evidence for a maximum jet efficiency for the most powerful radio galaxies

We use new mid-infrared (mid-IR) photometry from the Spitzer Space Telescope to study the relations between low-frequency radio luminosity density L_151MHz, mid-IR (12um rest-frame) luminosity L_12um, and optical-emission-line ([OII]) luminosity L_[OII], for a complete sample of z~1 radio galaxies from the 3CRR, 6CE, 6C*, 7CRS and TOOT00 surveys. The narrow redshift span of our sample (0.9<z<1.1) means that it is unbiased to evolutionary effects. We find evidence that these three quantities are positively correlated. The scaling between L_12um and L_[OII] is similar to that seen in other AGN samples, consistent with both L_12um and L_[OII] tracing accretion rate. We show that the positive correlation between L_12um and L_151MHz implies that there is a genuine lack of objects with low values of L_12um at high values of L_151MHz. Given that L_12um traces accretion rate, while L_151MHz traces jet power, this can be understood in terms of a minimum accretion rate being necessary to produce a given jet power. This implies that there is a maximum efficiency with which accreted energy can be chanelled into jet power and that this efficiency is of order unity.

astro-ph.HE↗

arXiv2010

A doubled double hotspot in J0816+5003 and the logarithmic slope of the lensing potential

We present an analysis of observations of the doubly-lensed double hotspot in the giant radio galaxy J0816+5003 from MERLIN, MDM, WIYN, WHT, UKIRT and the VLA. The images of the two hotspot components span a factor of two in radius on one side of the lensing galaxy at impact parameters of less than 500pc. Hence we measure the slope of the lensing potential over a large range in radius, made possible by significant improvement in the accuracy of registration of the radio and optical frame and higher resolution imaging data than previously available. We also infer the lens and source redshifts to be 0.332 and > 1 respectively. Purely on the basis of lens modelling, and independently of stellar velocity dispersion measurements, we find the potential to be very close to isothermal.

astro-ph.CO↗

arXiv2010

The evolution of the Fundamental Plane of radio galaxies from z~0.5 to the present day

We present deep spectroscopic data for a 24-object subsample of our full 41-object z~0.5 radio galaxy sample in order to investigate the evolution of the Fundamental Plane of radio galaxies. We find that the low-luminosity, FRI-type, radio galaxies in our sample are consistent with the local Fundamental Plane of radio galaxies defined by Bettoni et al. when corrected for simple passive evolution of their stellar populations. However, we find that the higher luminosity, FRII-type radio galaxies are inconsistent with the local Fundamental Plane if only passive evolution is considered, and find evidence for a rotation in the Fundamental Plane at z~0.5 when compared with the local relation. We show that neither passive evolution, nor a mass-dependent evolution in the mass-to-light ratio, nor an evolution in the size of the host galaxies can, by themselves, plausibly explain the observed tilt. However, we suggest that some combination of all three effects, with size evolution as the dominant factor, may be sufficient to explain the difference between the planes. We also find evidence for a correlation between host galaxy velocity dispersion and radio luminosity at the 97% significance level within our sub-sample, although further observations are required in order to determine whether this is different for the FRI and FRII radio sources. Assuming that the M_BH - sigma relation still holds at z~0.5, this implies that radio luminosity scales with black hole mass, in agreement with previous studies.

astro-ph.CO↗

arXiv2010

Evidence of different star formation histories for high- and low-luminosity radio galaxies

We present the results of our investigation into the stellar populations of 24 radio galaxies at z~0.5 drawn from four complete, low-frequency selected radio surveys. We use the strength of the 4000A break as an indicator of recent star formation, and compare this with radio luminosity, optical spectral classification and morphological classification. We find evidence of different star formation histories for high- and low-luminosity radio sources; our group of low radio luminosity sources (typically FRI-type sources) has systematically older stellar populations than the higher radio luminosity group. Our sample is also fairly well divided by optical spectral classification. We find that galaxies classified as having low excitation spectra (LEGs) possess older stellar populations than high excitation line objects (HEGs), with the HEGs showing evidence for recent star formation. We also investigate the link between radio morphology, as used by Owen & Laing (1989), and the stellar populations. We find that there is a preference for the "fat-double" sources to have older stellar populations than the "classical double" sources, although this is also linked to these sources lying predominantly in the LEG and HEG categories respectively. These results are consistent with the hypothesis that HEGs are powered by accretion of cold gas, which could be supplied, for example, by recent mergers, secular instabilities, or filamentary cold flows. These processes could also trigger star formation in the host galaxy. The host galaxies of the LEGs do not show evidence for recent star formation and an influx of cold gas, and are consistent with being powered by the accretion of the hot phase of the inter-stellar medium.

astro-ph.CO↗

arXiv2010

A Virtual Sky with Extragalactic HI and CO Lines for the SKA and ALMA

We present a sky simulation of the atomic HI emission line and the first ten CO rotational emission lines of molecular gas in galaxies beyond the Milky Way. The simulated sky field has a comoving diameter of 500/h Mpc, hence the actual field-of-view depends on the (user-defined) maximal redshift zmax; e.g. for zmax=10, the field of view yields ~4x4 sqdeg. For all galaxies, we estimate the line fluxes, line profiles, and angular sizes of the HI and CO emission lines. The galaxy sample is complete for galaxies with cold hydrogen masses above 10^8 Msun. This sky simulation builds on a semi-analytic model of the cosmic evolution of galaxies in a Lambda-cold dark matter (LCDM) cosmology. The evolving CDM-distribution was adopted from the Millennium Simulation, an N-body CDM-simulation in a cubic box with a side length of 500/h Mpc. This side length limits the coherence scale of our sky simulation: it is long enough to allow the extraction of the baryon acoustic oscillations (BAOs) in the galaxy power spectrum, yet the position and amplitude of the first acoustic peak will be imperfectly defined. This sky simulation is a tangible aid to the design and operation of future telescopes, such the SKA, the LMT, and ALMA. The results presented in this paper have been restricted to a graphical representation of the simulated sky and fundamental dN/dz-analyzes for peak flux density limited and total flux limited surveys of HI and CO. A key prediction is that HI will be harder to detect at redshifts z>2 than predicted by a no-evolution model. The future verification or falsification of this prediction will allow us to qualify the semi-analytic models.

astro-ph.CO↗

arXiv2009

Millimetre observations of a sample of high-redshift obscured quasars

We present observations at 1.2 mm with MAMBO-II of a sample of z>~2 radio-intermediate obscured quasars, as well as CO observations of two sources with the Plateau de Bure Interferometer. Five out of 21 sources (24%) are detected at a significance of >=3sigma. Stacking all sources leads to a statistical detection of = 0.96+-0.11 mJy and stacking only the non-detections also yields a statistical detection, with = 0.51+-0.13 mJy. This corresponds to a typical far-infrared luminosity L_FIR~4x10^12 Lsol. If the far-infrared luminosity is powered entirely by star-formation, and not by AGN-heated dust, then the characteristic inferred star-formation rate is ~700 Msol yr-1. This far-infrared luminosity implies a dust mass of M_dust~3x10^8 Msol. We estimate that such large dust masses on kpc scales can plausibly cause the obscuration of the quasars. We present dust SEDs for our sample and derive a mean SED for our sample. This mean SED is not well fitted by clumpy torus models, unless additional extinction and far-infrared re-emission due to cool dust are included. There is a hint that the host galaxies of obscured quasars must have higher far-infrared luminosities and cool-dust masses and are therefore often found at an earlier evolutionary phase than those of unobscured quasars. For one source at z=2.767, we detect the CO(3-2) transition, with S_CO Delta nu=630+-50 mJy km s-1, corresponding to L_CO(3-2)= 3.2x10^7 Lsol, or L'_CO(3-2)=2.4x10^10 K km s-1 pc2. For another source at z=4.17, the lack of detection of the CO(4-3) line yields a limit of L'_CO(4-3)<1x10^10 K km s-1 pc2. Molecular gas masses, gas depletion timescales and gas-to-dust ratios are estimated (Abridged).

astro-ph.GA↗

arXiv2009

The TexOx-1000 redshift survey of radio sources I: the TOOT00 region

We present optical spectroscopy, near-infrared (mostly K-band) and radio (151-MHz and 1.4-GHz) imaging of the first complete region (TOOT00) of the TexOx-1000 (TOOT) redshift survey of radio sources. The 0.0015-sr (~ 5 deg^2) TOOT00 region is selected from pointed observations of the Cambridge Low-Frequency Survey Telescope at 151 MHz at a flux density limit of ~= 100 mJy, ~ 5-times fainter than the 7C Redshift Survey (7CRS), and contains 47 radio sources. We have obtained 40 spectroscopic redshifts (~ 85% completeness). Adding redshifts estimated for the 7 other cases yields a median redshift z_med ~ 1.25. We find a significant population of objects with FRI-like radio structures at radio luminosities above both the low-redshift FRI/II break and the break in the radio luminosity function. The redshift distribution and sub-populations of TOOT00 are broadly consistent with extrapolations from the 7CRS/6CE/3CRR datasets underlying the SKADS Simulated Skies Semi-Empirical Extragalactic Database, S^3-SEX.

astro-ph.CO↗

arXiv2009

Forecasts for Dark Energy Measurements with Future HI Surveys

We use two independent methods to forecast the dark energy measurements achievable by combining future galaxy redshift surveys based on the radio HI emission line with Cosmic Microwave Background (CMB) data from the {\sl Planck} satellite. In the first method we focus on the `standard ruler' provided by the baryon acoustic oscillation (BAO) length scale. In the second method we utilize additional information encoded in the galaxy power spectrum including galaxy bias from velocity-space distortions and the growth of cosmic structure. We find that a radio synthesis array with about 10 per cent of the collecting area of the Square Kilometre Array (SKA), equipped with a wide ($10-100 ~ {\rm deg}^2$) field-of-view, would have the capacity to perform a $20{,}000 ~ {\rm deg}^2$ redshift survey to a maximum redshift $z_{\rm max} \sim 0.8$ and thereby produce dark energy measurements that are competitive with surveys likely to be undertaken by optical telescopes around 2015. There would then be powerful arguments for adding collecting area to such a `Phase-1' SKA because of the square-law scaling of survey speed with telescope sensitivity for HI surveys, compared to the linear scaling for optical redshift surveys. The full SKA telescope should, by performing a $20{,}000 ~ {\rm deg}^2$ HI redshift survey to $z_{\rm max} \sim 2$ around 2020, yield an accurate measurement of cosmological parameters independent of CMB datasets. Combining CMB ({\sl Planck}) and galaxy power spectrum (SKA) measurements will drive errors in the dark energy equation-of-state parameter $w$ well below the 1 per cent level. The major systematic uncertainty in these forecasts is the lack of direct information about the mass function of high-redshift HI-emitting galaxies.

astro-ph.CO↗