Discerning the exact nature of the sub-mJy radio population has been historically difficult due to the low luminosity of these sources at most wavelengths. Using deep ground based optical follow-up and observations from the Spitzer Space Telescope we are able to disentangle the radio-selected active galactic nuclei (AGN) and star-forming galaxy (SFG) populations for the first time in a deep multifrequency VLA/MERLIN Survey of the 13 H XMM-Newton/Chandra Deep Field. The discrimination diagnostics include radio morphology, radio spectral index, radio/near-infrared (near-IR) and mid-IR/radio flux density ratios. We are now able to calculate the extragalactic Euclidean normalized source counts separately for AGN and SFGs. We find that while SFGs dominate at the faintest flux densities and account for the majority of the upturn in the counts, AGN still make up around one quarter of the counts at ∼50 μJy (1.4 GHz). Using radio luminosity as an unobscured star formation rate (SFR) measure we are then able to examine the comoving SFR density of the Universe up to z = 3 which agrees well with measures at other wavelengths. We find a rough correlation of SFR with stellar mass for both the sample presented here and a sample of local radio-selected SFGs from the 6df-NVSS survey. This work also confirms the existence of, and provides alternative evidence for, the evolution of distribution of star formation by galaxy mass: 'downsizing'. As both these samples are SFR-selected, this result suggests that there is a maximum SFR for a given galaxy that depends linearly on its stellar mass. The low 'characteristic times' (inverse specific SFR) of the SFGs in our sample are similar to those of the 6dF-NVSS sample, implying that most of these sources are in a current phase of enhanced star formation.

We present the results of a deep X-ray survey conducted with XMM-Newton, centred on the UK ROSAT 13 H deep field area. This region covers 0.18 deg 2 , and is the first of the two areas covered with XMM-Newton as part of an extensive multiwavelength survey designed to study the nature and evolution of the faint X-ray source population. We have produced detailed Monte Carlo simulations to obtain a quantitative characterization of the source detection procedure and to assess the reliability of the resultant sourcelist. We use the simulations to establish a likelihood threshold, above which we expect less than seven (3 per cent) of our sources to be spurious. We present the final catalogue of 225 sources. Within the central 9 arcmin, 68 per cent of source positions are accurate to 2 arcsec, making optical follow-up relatively straightforward. We construct the N (>S) relation in four energy bands: 0.2-0.5, 0.5-2, 2-5 and 5-10 keV. In all but our highest energy band we find that the source counts can be represented by a double power law with a bright-end slope consistent with the Euclidean case and a break around 10 −14y erg cm −2 s −1 . Below this flux, the counts exhibit a flattening. Our source counts reach densities of 700, 1300, 900 and 300 deg −2 at fluxes of 4.1 × 10 −16 , 4.5 × 10 −16 , 1.1 × 10 −15 and 5.3 × 10 −15 erg cm −2 s −1 in the 0.2-0.5, 0.5-2, 2-5 and 5-10 keV energy bands, respectively. We have compared our source counts with those in the two Chandra deep fields and Lockman hole, and found our source counts to be amongst the highest of these fields in all energy bands. We resolve >51 per cent (>50 per cent) of the X-ray background emission in the 1-2 keV (2-5 keV) energy bands.

We present the X‐ray spectra of 86 optically identified sources in the 13HXMM–Newton/Chandra deep field which have >70 X‐ray counts. The majority of these sources have 2–10 keV fluxes between 10−15 and 5 × 10−14 erg cm−2 s−1. The sample consists of 50 broad‐line active galactic nuclei (BLAGN), 25 narrow emission‐line galaxies (NELGs), six absorption‐line galaxies and five Galactic stars. The majority (42/50) of the BLAGN have X‐ray spectra which are consistent with a power‐law shape. They have a mean photon index 〈Γ〉= 2.0 ± 0.1 and an intrinsic dispersion σΓ= 0.4 ± 0.1. Three of the BLAGN show curved spectra, with more emission near the high‐ and low‐energy ends of the spectrum relative to the emission in the 1–2 keV range than can be reproduced by the power‐law model. Five BLAGN show a deficit of soft X‐rays, indicating absorption. We consider a source to be significantly absorbed if a power‐law model fit is rejected with >99 per cent confidence and an absorbed power‐law model produces an acceptable fit, or if the best‐fitting power law is abnormally hard (Γ < 1). Significant absorption is more common in the NELGs (13/25) and absorption‐line galaxies (2/6) than in the BLAGN (5/50), but is not universal in any of these classes of object. The majority of the 20 absorbed sources have X‐ray spectra consistent with a simple cold photoelectric absorber, but a significant minority (6/20) require more complex models with either an additional component of soft X‐ray emitting plasma, or an ionized absorber. Of the 16 narrow emission‐ and absorption‐line galaxies which do not show evidence for X‐ray absorption, only two objects are likely to be powered by star formation, and both have 2–10 keV X‐ray luminosities of ≤ 1040 erg s−1. The X‐ray emission in the other 14 unabsorbed NELGs and galaxies is most likely powered by AGN, which are not detected in the optical because they are outshone by their luminous host galaxies. The Galactic stars show multitemperature thermal spectra which peak between 0.5 and 1 keV. Star/AGN discrimination is possible for four of the five stars solely from their X‐ray spectra.

We present intensive quasi-simultaneous X-ray and radio monitoring of the narrow line Seyfert 1 galaxy NGC 4051, over a 16 month period in 2000-2001. Observations were made with the Rossi Timing X-ray Explorer (RXTE) and the Very Large Array (VLA) at 8.4 and 4.8 GHz. In the X-ray band NGC 4051 behaves much like a Galactic black hole binary (GBH) system in a `soft-state'. In such systems, there has so far been no firm evidence for an active, radio-emitting jet like those found in `hard state' GBHs. VLBI observations of NGC 4051 show three co-linear compact components. This structure resembles the core and outer hot spots seen in powerful, jet-dominated, extragalactic radio sources and suggests the existence of a weak jet. Radio monitoring of the core of NGC 4051 is complicated by the presence of surrounding extended emission and by the changing array configurations of the VLA. Only in the A configuration is the core reasonably resolved. We have carefully removed the contaminations of the core by extended emission in the various arrays. The resulting lightcurve shows no sign of large amplitude variability (i.e. factor 50 %) over the 16 month period. Within the most sensitive configuration (A array) we see marginal evidence for radio core variability of ~25% (~0.12 mJy at 8.4GHz) on a 2-week timescale, correlated with X-ray variations. Even if the radio variations in NGC 4051 are real, the percentage variability is much less than in the X-ray band. Within the B configuration observations, where sensitivity is reduced, there is no sign of correlated X-ray/radio variability. The lack of radio variability in NGC 4051, which we commonly see in `hard state' GBHs, may be explained by orientation effects. Another possibility is that the radio emission arises from the X-ray corona, although the linear structure of the compact radio components here is hard to explain.Comment: 13 pages, 15 figures, 3 tables Accepted to MNRAS November 23. Received 2010 November 23; in original form 2010 August 2

We present the results of a deep 610-MHz survey of the 1 H XMM-Newton/Chandra survey area with the Giant Metre-wave Radio Telescope. The resulting maps have a resolution of ∼7 arcsec and an rms noise limit of 60 µJy. To a 5σ detection limit of 300 µJy, we detect 223 sources within a survey area of 64 arcmin in diameter. We compute the 610-MHz source counts and compare them to those measured at other radio wavelengths. The well-known flattening of the Euclidean-normalized 1.4-GHz source counts below ∼2 mJy, usually explained by a population of starburst galaxies undergoing luminosity evolution, is seen at 610 MHz. The 610-MHz source counts can be modelled by the same populations that explain the 1.4-GHz source counts, assuming a spectral index of −0.7 for the starburst galaxies and the steep spectrum active galactic nucleus (AGN) population. We find a similar dependence of luminosity evolution on redshift for the starburst galaxies at 610 MHz as is found at 1.4 GHz (i.e. 'Q' = 2.45 +0.3 −0.4 ).