D. Moss

The star formation history of the Universe as revealed by deep radio observations

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 nowable 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.

X-ray spectra of sources in the 13HXMM–Newton/Chandra deep field

We present the X-ray spectra of 86 optically identified sources in the 13^H XMM–Newton/Chandra deep field which have >70 X-ray counts. The majority of these sources have 2–10 keV fluxes between 10^(−1)5 and 5 × 10^(−1)4 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 σ_Γ= 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 ≤ 10^(40) 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.

XMM–Newton 13H deep field – I. X-ray sources

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. Ke yw ords: surveys ‐ galaxies: active ‐ quasars: general ‐ X-rays: galaxies.

Measuring the cosmic star-formation rate density using deep radio surveys

There is now good agreement between the various methods of estimating the space density of the star-formation rate (SFRD) at low redshifts ( z z ~ 3, and so potentially offer an excellent way to measure the SFRD. Indeed, modelling of the sub-mJy source counts requires an additional population of faint steep spectrum objects, that are very likely to be starburst galaxies.

Disentangling the AGN and Star-forming Contribution to the Sub-mJy Radio Counts

The true nature of the faint radio population remains elusive despite the many observations of the “sub-mJy” bump over the last two decades. This lack of information is largely due to the faint magnitudes of the optical counterparts to the radio sources. There are strong theoretical reasons (and a few observational ones) to believe that this rise in the counts is due to the emergence of a rapidly evolving star-forming population. Now, for the first time, we are able to separate the AGN and star-forming populations below 1mJy using a combination of multi-wavelength data from Spitzer , GMRT, MERLIN, CFHT, Keck, UKIRT, Subaru, Chandra and XMM-Newton . The many discriminators between these emission mechanisms include MIR colours, MIR/radio flux ratios, X-ray luminosities/spectra, optical spectra, radio morphologies and radio spectra. We can now derive the source counts separately for AGN and star-forming galaxies confirming that the latter population rise sharply at faint flux densities.

AGN in deep radio/X-ray surveys: Hunting the earliest massive galaxies

Despite the plethora of deep (sub-mJy) radio surveys there remains considerable doubt as to the exact nature of the galaxies contributing to the source counts. Current evidence suggests that star formation in moderately luminous normal galaxies is responsible for the bulk of the emission below 1 mJy. However given the sensitivities of these surveys we would expect a fraction of these sources to be distant radio galaxies. Using deep VLA and GMRT data we have found ~20 high-z candidate radio galaxies in two fields using the classical ultra-steep radio spectrum technique (De Breuck et al. 2000) and selecting galaxies with faint (i’ > 25) optical counterparts. Several of these sources have X-ray detections in our deep XMM/Chandra observations and have fluxes high enough to put them in the quasar regime if they lie above redshift 3. Recently performed Spitzer GTO observations and upcoming near-infrared observations will help reveal the nature of these sources.