arXiv subjects

Paul Alexander

Publications and source records attributed to Paul Alexander.

69 records · Page 4Linked to original sources

arXiv2008

Deep 610-MHz Giant Metrewave Radio Telescope observations of the Spitzer extragalactic First Look Survey field - III. The radio properties of Infrared-Faint Radio Sources

Infrared-Faint Radio Sources (IFRSs) are a class of source which are bright at radio frequencies, but do not appear in deep infrared images. We report the detection of 14 IFRSs within the Spitzer extragalactic First Look Survey field, eight of which are detected near to the limiting magnitude of a deep R-band image of the region, at R ~ 24.5. Sensitive Spitzer Space Telescope images are stacked in order to place upper limits on their mid-infrared flux densities, and using recent 610-MHz and 1.4-GHz observations we find that they have spectral indices which vary between alpha = 0.05 and 1.38, where we define alpha such that S = S_0 nu^(- alpha), and should not be thought of as a single source population. We place constraints on the luminosity and linear size of these sources, and through comparison with well-studied local objects in the 3CRR catalogue demonstrate that they can be modelled as being compact (< 20 kpc) Fanaroff-Riley Class II (FRII) radio galaxies located at high redshift (z > 4).

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arXiv2008

Magnetic fields in star-forming galaxies at high and low redshift

As part of an ongoing series of deep GMRT surveys we have observed the Spitzer extragalactic First Look Survey field, producing the deepest wide-field 610-MHz survey published to date. We reach an rms noise of 30 microJy/beam before primary beam correction, with a resolution of ~6 arcsec over an area of ~4 square degrees. By combining these observations with the existing 1.4-GHz VLA survey produced by Condon et al. (2003), along with infrared data in up to seven wavebands from the Spitzer Space Telescope, optical photometry from SDSS and a range of spectroscopic redshift surveys, we are able to study the relationship between radio luminosity and star formation rate in star-forming galaxies up to z ~ 1. The large amount of multi-wavelength data available allows accurate k-corrections to be performed in the radio due to the knowledge of the radio spectral index, and in the infrared through the use of a semi-empirical radiative transfer model which models star-forming regions, warm dust surrounding these regions, and diffuse interstellar dust, taking into account the star formation rate, star formation history and hydrogen column density within each galaxy. A strong correlation is seen between radio luminosity and infrared-derived star formation rates, which is best fit by a slightly non-linear power-law. We look for cosmic evolution in the comparative radio brightness of star-forming galaxies by searching for deviations away from the global relationship. Any such deviation would indicate a systematic variation in one or more of the properties controlling synchrotron radiation, in particular an increase in the magnetic field strengths of star-forming galaxies. The data shows no evidence for such an effect, suggesting that there has been little evolution in the magnetic fields of galaxies since z ~ 1.

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arXiv2008

The duty cycle of local radio galaxies

We use a volume- and flux-limited sample of local ($0.03 \leq z \leq 0.1$) radio galaxies with optical counterparts to address the question of how long a typical galaxy spends in radio-active and quiescent states. The length of the active phase has a strong dependence on the stellar mass of the host galaxy. Radio sources in the most massive hosts are also retriggered more frequently. The time spent in the active phase has the same dependence on stellar mass as does the gas cooling rate, suggesting the onset of the quiescent phase is due to fuel depletion. We find radio and emission line AGN activity to be independent, consistent with these corresponding to different accretion states.

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arXiv2008

A Semi-Empirical Model of the Infrared Emission from Galaxies

We present a semi-empirical model for the infrared emission of dust around star-forming sites in galaxies. Our approach combines a simple model of radiative transfer in dust clouds with a state-of-the-art model of the microscopic optical properties of dust grains pioneered by Draine & Li. In combination with the Starburst 99 stellar spectral synthesis package, this framework is able to produce synthetic spectra for galaxies which extend from the Lyman limit through to the far-infrared. We use it to probe how model galaxy spectra depend upon the physical characteristics of their dust grain populations, and upon the energy sources which heat that dust. We compare the predictions of our model with the 8- and 24-micron luminosities of sources in the Spitzer First Look Survey, and conclude by using the models to analyse the relative merits of various colour diagnostics in distinguishing systems out to a redshift of 2 with ongoing star formation from those with only old stellar populations.

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arXiv2008

A 610-MHz survey of the Lockman Hole with the Giant Metrewave Radio Telescope - I. Observations, data reduction and source catalogue for the central 5 square degrees

We present observations of the Lockman Hole with the Giant Metrewave Radio Telescope (GMRT). Twelve pointings were observed, covering a total area of ~ five square degrees with a resolution of 6 x 5 arcsec, position angle +45deg. The majority of the pointings have an rms noise of 60 microJy/beam before correction for the attenuation of the GMRT primary beam. Techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic is presented, along with a catalogue of 2845 sources detected above 6 sigma. Radio source counts are calculated at 610 MHz and combined with existing 1.4-GHz source counts, in order to show that pure luminosity evolution of the local radio luminosity functions for active galactic nuclei and starburst galaxies is sufficient to account for the two source counts simultaneously.

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arXiv2007

A 610-MHz survey of the ELAIS-N1 field with the Giant Metrewave Radio Telescope - Observations, data analysis and source catalogue

Observations of the ELAIS-N1 field taken at 610 MHz with the Giant Metrewave Radio Telescope are presented. Nineteen pointings were observed, covering a total area of 9 square degrees with a resolution of 6" x 5", PA +45 deg. Four of the pointings were deep observations with an rms of 40 microJy before primary beam correction, with the remaining fifteen pointings having an rms of 70 microJy. The techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic is presented, along with a catalogue of 2500 sources detected above 6 sigma. This work complements the large amount of optical and infrared data already available on the region. We calculate 610-MHz source counts down to 270 microJy, and find further evidence for the turnover in differential number counts below 1 mJy, previously seen at both 610 MHz and 1.4 GHz.

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arXiv2007

Deep 610-MHz GMRT observations of the Spitzer extragalactic First Look Survey field - I. Observations, data analysis and source catalogue

Observations of the Spitzer extragalactic First Look Survey field taken at 610 MHz with the Giant Metrewave Radio Telescope are presented. Seven individual pointings were observed, covering an area of 4 square degrees with a resolution of 5.8'' x 4.7'', PA 60 deg. The r.m.s. noise at the centre of the pointings is between 27 and 30 microJy before correction for the GMRT primary beam. The techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic, along with a catalogue of 3944 sources detected above 5 sigma, are presented. The survey complements existing radio and infrared data available for this region.

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arXiv2006

Chemical enrichment of the intracluster medium by FR II radio sources

We present 2D axisymmetric hydrodynamic simulations investigating the long term effect of FR II radio galaxies on the metal distribution of the surrounding intra-cluster medium (ICM). A light jet is injected into a cooling flow atmosphere for 10-30 Myr. We then follow the subsequent evolution for 3 Gyr on a spherical grid spanning 3 Mpc in radius. A series of passive tracer particles were placed in an annulus about the cluster core to simulate metal carrying clouds in order to calculate the metallicity (Z) as a function of time and radial distance from the cluster centre. The jet has a significant effect on the ICM over the entire 3 Gyr period. By the end of the simulations, the jets produced metallicities of ~10% of the initial metallicity of the cluster core throughout much of the cluster. The jets transport the metals not only in mixing regions, but also through upwelling ICM behind the jet, enriching the cluster over both long and short distances.

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arXiv2006

Simulations of multi-phase turbulence in jet cocoons

The interaction of optically emitting clouds with warm X-ray gas and hot, tenuous radio plasma in radio jet cocoons is modelled by 2D compressible hydrodynamic simulations. The initial setup is the Kelvin-Helmholtz instability at a contact surface of density contrast 10,000. The denser medium contains clouds of higher density. Optically thin radiation is realised via a cooling source term. The cool phase effectively extracts energy from the other gas which is both, radiated away and used for acceleration of the cold phase. This increases the system's cooling rate substantially and leads to a massively amplified cold mass dropout. We show that it is feasible, given small seed clouds of order 100 solar masses, that all of the optically emitting gas in a radio jet cocoon may be produced by this mechanism on the propagation timescale of the jet. The mass is generally distributed as T^-1/2 with temperature, with a prominent peak at 14,000 K. This peak is likely to be related to the counteracting effects of shock heating and a strong rise in the cooling function. The volume filling factor of cold gas in this peak is of the order 10^-5 to 10^-3 and generally increases during the simulation time. The simulations tend towards an isotropic scale free Kolmogorov-type energy spectrum over the simulation timescale. We find the same Mach-number density relation as Kritsuk & Norman (2004) and show that this relation may explain the velocity widths of emission lines associated with high redshift radio galaxies, if the environmental temperature is lower, or the jet-ambient density ratio is less extreme than in their low redshift counterparts.

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arXiv2006

A population of high-redshift type-2 quasars-II. Radio Properties

We present multi-frequency radio observations of a sample of z~2 obscured (type-2) quasars in the Spitzer extragalactic First Look Survey area. We combine the public data at 1.4 GHz, used in the selection of these sources, with new observations at 610 MHz (GMRT) and at 4.9 GHz (VLA). We find the sample includes sources with steep, flat and gigahertz-peaked spectra. There are no strong correlations between the presence or absence of emission lines in the optical spectra and the radio spectral properties of the sample. However, there are no secure flat-spectrum type-2 quasars with narrow emission lines which would be problematic for unified schemes. Most of the population have straight radio spectra with spectral index alpha~1 as is expected for developed, potentially FRI-like, jets in which continous injection of relativistic electrons is accompanied by inverse-Compton losses against the cosmic microwave background.

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arXiv2003

Probing the Physics of Interacting Galaxies

Both Arp 140 and Arp 104 reveal extended tidal tails in the HI. The Halpha and FIR fluxes of Arp 140 yield similar SFR of ~0.8 solar masses per year. In contrast the Halpha flux of Arp 104 yields a SFR of ~0.05 solar masses per year, ~ 20 times smaller than that obtained from the FIR flux. Spectra were used to examine the changing velocity of atomic and molecular gas in NGC 5218 (Arp 104). The atomic and molecular gas were found to be dynamically similar with comparable velocities and velocity widths across the galaxy; consistent with the two phases responding similarly to the interaction, or enhanced HI to CO conversion in the centre of the galaxy.

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arXiv1999

Heating of the IGM by FRII radio sources

We present results of a numerical integration of the hydrodynamical equations governing the self-similar, two-dimensional gas flow behind the bow shock of an FRII radio source embedded in an IGM with a power law density profile. The model predicts pressure gradients within the cocoons consistent with modest backflow. For very steep external density profiles sources may well not expand in a self-similar fashion and in this case the model is not self-consistent. The assumption of ram pressure confinement of the cocoons perpendicular to the jet axis is found to overestimate the ratio of the pressure in front of the radio hot spots and that in the cocoons. Based on the properties of the gas between bow shock and cocoon we calculate the X-ray surface brightness of the flow. This emission is found to be a good tracer of the density distribution within the flow and varies significantly with the properties of the unshocked IGM. The cooling-time of the shocked IGM is found to be comparable to, or greater than, the Hubble time. The influence of a radio source on the evolution of its gaseous surroundings therefore extends well beyond the limited life time of the source itself. We compare our results with the X-ray map of Cygnus A and find some evidence for cold, dense gas clumps in the surroundings of this object. The extended X-ray emission observed around 3C 356 may also be caused by the bow shock of this radio source. We also present an empirical model for the X-ray emission of the shocked IGM due to thermal bremsstrahlung.

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arXiv1998

On the cosmological evolution of the FRII radio source population

We present an analytical model for the cosmological evolution of the FRII source population. Based on an earlier model for the intrinsic radio luminosity - linear size evolution of these objects, we construct theoretical source samples. The source distributions in the radio power - linear size plane of these samples are then compared with that of an observed flux-limited sample. We find that the source parameters determining the radio luminosity of FRII objects can not be independent of each other. The best-fitting models predict the jet power to be correlated either with the life time of the source or with the shape of the density distribution of the source environment. The latter case is consistent with the observed tendency of the most luminous radio sources at high redshift to be located in richer and more extended environments than their low redshift counterparts. We also find evidence for a class of FRII sources distinctly different from the main population. These sources are extremely old and/or are located in very underdense environments. The luminosity function of FRII sources resulting from the model is in good agreement with previous results for high luminosity sources. The apparent luminosity evolution of the radio luminosity function is not reproduced because of the high flux limit of the used comparison sample. The cosmological evolution of the median linear size of FRII sources is found to be mild.

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arXiv1998

The mutual interaction of powerful radio galaxies and their environments

Using a self-similar model for the expansion of cocoons surrounding the jets in powerful extragalactic radio sources (type FRII), we investigate the influence of the properties of the gas surrounding these objects on their evolution. The variation of their radio luminosity as a function of linear size for individual sources is determined in dependence of the density distribution of the IGM the source is embedded in. Based on these results, the cosmological evolution of the FRII population and hence that of the environments of these sources can be constrained. The environments are found to be denser and/or more extended at high redshifts. The bow shock surrounding the cocoon of FRII sources compresses and heats the IGM. A numerical integration of the hydrodynamic equations governing the gas flow between the bow shock and the cocoon boundary is presented. From this we determine the appearance of the large scale structure of radio galaxies in X-rays and the cooling times for the IGM heated by the passage of the bow shock. This has important implications for the cosmological evolution of the IGM.

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arXiv1997

Evolutionary tracks of FRII sources through the P-D diagram

This is the second in a series of papers presenting an analytical model for the evolution of FRII radio sources. In this paper we evaluate the expected radio emission from a radio source incorporating energy loss processes for the relativistic electrons. By combining these results with our earlier dynamical model we calculate evolutionary tracks through the Power-Linear size diagram. These tracks are in good agreement with the observed distribution of sources in this diagram. The effects of different forms for the evolution of the magnetic field in the cocoon, the redshift of the source, the environment of the source and the defining parameters of the jet are investigated. The evolutionary tracks are found to be insensitive to the assumed form of the magnetic field evolution. Some evidence against protons as a major constituent of the jet material is also found.

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