T. Dwelly

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.

Dust and metal column densities in gamma-ray burst host galaxies

In this paper we present the results from the analysis of a sample of 28 gamma-ray burst (GRB) afterglow spectral energy distributions, spanning the X-ray through to near-infrared wavelengths. This is the largest sample of GRB afterglow spectral energy distributions thus far studied, providing a strong handle on the optical depth distribution of soft X-ray absorption and dust-extinction systems in GRB host galaxies. We detect an absorption system within the GRB host galaxy in 79 per cent of the sample, and an extinction system in 71 per cent of the sample, and find the Small Magellanic Cloud (SMC) extinction law to provide an acceptable fit to the host galaxy extinction profile for the majority of cases, consistent with previous findings. The range in the soft X-ray absorption to dust-extinction ratio, N-H,N-X/A(V), in GRB host galaxies spans almost two orders of magnitude, and the typical ratios are significantly larger than those of the Magellanic Clouds or Milky Way. Although dust destruction could be a cause, at least in part, for the large N-H,N-X/A(V) ratios, the good fit provided by the SMC extinction law for the majority of our sample suggests that there is an abundance of small dust grains in the GRB environment, which we would expect to have been destroyed if dust destruction were responsible for the large N-H,N-X/A(V) ratios. Instead, our analysis suggests that the distribution of N-H,N-X/A(V) in GRB host galaxies may be mostly intrinsic to these galaxies, and this is further substantiated by evidence for a strong negative correlation between N-H,N-X/A(V) and metallicity for a subsample of GRB hosts with known metallicity.

1H 0707−495 in 2011: an X-ray source within a gravitational radius of the event horizon

The narrow-line Seyfert 1 Galaxy 1H 0707−495 went into a low state from 2010 December to 2011 February, discovered by a monitoring campaign using the X-Ray Telescope on the Swift satellite. We triggered a 100 ks XMM-Newton observation of the source in 2011 January, revealing the source to have dropped by a factor of 10 in the soft band, below 1 keV, and a factor of 2 at 5 keV, compared with a long observation in 2008. The sharp spectral drop in the source usually seen around 7 keV now extends to lower energies, below 6 keV in our frame. The 2011 spectrum is well fitted by a relativistically blurred reflection spectrum similar to that which fits the 2008 data, except that the emission is now concentrated solely to the central part of the accretion disc. The irradiating source must lie within 1 gravitational radius of the event horizon of the black hole, which spins rapidly. Alternative models are briefly considered, but none has any simple physical interpretation.

Long XMM observation of the narrow-line Seyfert 1 galaxy IRAS 13224−3809: rapid variability, high spin and a soft lag

The narrow-line Seyfert 1 galaxy IRAS 13224−3809 has been observed with XMM-Newton for 500 ks. The source is rapidly variable on time-scales down to a few 100 s. The spectrum shows strong broad Fe − K and L emission features which are interpreted as arising from reflection from the inner parts of an accretion disc around a rapidly spinning black hole. Assuming a power law emissivity for the reflected flux and that the innermost radius corresponds to the innermost stable circular orbit, the black hole spin is measured to be 0.989 with a statistical precision better than 1 per cent. Systematic uncertainties are discussed. A soft X-ray lag of 100 s confirms this scenario. The bulk of the power-law continuum source is located at a radius of 2-3 gravitational radii.

The dust extinction curves of gamma-ray burst host galaxies

The composition and amount of interstellar dust within gamma-ray burst (GRB) host galaxies is of key importance when addressing selection effects in the GRB redshift distribution, and when studying the properties of their host galaxies. As well as the implications for GRB research, probing the dust within the high-z hosts of GRBs also contributes to our understanding of the conditions of the interstellar medium and star-formation in the distant Universe. Nevertheless, the physical properties of dust within GRB host galaxies continues to be a highly contended issue. In this paper we explore the mean extinction properties of dust within the host galaxies of a sample of 17 GRBs with total host galaxy visual extinction AV 1), which may be indicative of there being a dependence between dust abundance and the wavelength dependence of dust extinction, as has been previously speculated.

The link between SCUBA and Spitzer: cold galaxies at z ~<1

We show that the far-IR properties of distant Luminous and UltraLuminous InfraRed Galaxies (LIRGs and ULIRGs, respectively) are on average divergent from analogous sources in the local Universe. Our analysis is based on Spitzer Multiband Imaging Photometer (MIPS) and Infrared Array Camera (IRAC) data of LIR > 10 10 L� ,7 0μm selected objects in the 0.1 1 SubMillimetre Galaxies (SMGs) discovered in blank-field submillimetre surveys. The Herschel Space Observatory is well placed to fully characterize the nature of these objects, as its coverage extends over a major part of the far-IR/sub-mm SED for a wide redshift range.

Investigating the far-IR/radio correlation of star-forming Galaxies to z=3

In order to examine the far-infrared (far-IR)/radio correlation at high redshift, we have studied the Spitzer 70 μm /far-IR properties of sub-mJy radio sources from the 13^H XMM–Newton/Chandra Deep Field by redshift and galaxy type: active galactic nucleus (AGN) or star-forming galaxy (SFG). We directly detect 70 μm counterparts (at >3σ significance) for 22.5 per cent (92/408) of the radio sources, while for the rest we perform stacking analysis by redshift and galaxy type. For the sources detected at 70 μm , we find that the median and scatter of the observed flux density ratio, q_(70), are similar to previous results in the literature, but with a slight decrease in q_(70) towards higher redshifts. Of the radio sources detected at 70 μm 8/92 were already classified as AGN, but two of which maybe SFGs. For the stacked sources, we obtain a detection for the SFGs at every redshift bin which implies they have mean flux densities a factor ~5 below the original 70 μm detection limit. For the stacked AGN, we obtain a detection only in our highest redshift bin (1 ≤ z ≤ 5) where we may be sampling hot dust associated with the AGN at rest frame 12−35 μm . The combined observed mean value of q_(70) for the SFGs (detected and non-detected at 70 μm) decreases gradually with redshift, consistent with tracks derived from empirical spectral energy distributions (SEDs) of local SFGs. Upon closer inspection and when comparing with tracks of appropriate luminosity, the values of q_(70) broadly agree at low redshift. However, at z ~ 1, the observed q_(70) [for ultraluminous infrared galaxies (ULIRGs)] is 2 σ below the value seen for local ULIRGs tracks, implying a difference in the SED between local and z ~ 1 ULIRGs. At higher redshifts, the convergence of the tracks and the higher uncertainties in q_(70) do not allow us to determine if such a difference persists.

Correlated X-ray/ultraviolet/optical variability in the very low mass AGN NGC 4395

We report the results of a 1-yr Swift X-ray/ultraviolet (UV)/optical programme monitoring the dwarf Seyfert nucleus in NGC4395 in 2008-2009. The UV/optical flux from the nucleus was found to vary dramatically over the monitoring period, with a similar pattern of variation in each of the observed UV/optical bands (spanning 1900-5500 A). In particular, the luminosity of NGC4395 in the 1900 A band changed by more than a factor of 8 over the monitoring period. The fractional variability was smaller in the UV/optical bands than that seen in the X-rays, with the X-ray/optical ratio increasing with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and each UV/optical band were well correlated, with cross-correlation coefficients of ≥0.7, significant at 99.9per cent confidence. Archival Swift observations from 2006 sample the intra-day X-ray/optical variability on NGC4395. These archival data show a very strong correlation between the X-ray and b bands, with a cross-correlation coefficient of 0.84 (significant at >99per cent confidence). The peak in the cross-correlation function is marginally resolved and asymmetric, suggesting that X-rays lead the b band, but by ≤1h. In response to recent (2011 August) very high X-ray flux levels from NGC4395 we triggered Swift target of opportunity observations, which sample the intra-hour X-ray/UV variability. These observations indicate, albeit with large uncertainties, a lag of the 1900 A band behind the X-ray flux of ∼400s. The tight correlation between the X-ray and UV/optical lightcurves, together with the constraints we place on the lag time-scale, is consistent with the UV/optical variability of NGC4395 being primarily due to reprocessing of X-ray photons by the accretion disc.

A deep INTEGRAL hard X-ray survey of the 3C 273/Coma region

We present an analysis of the deepest hard X-ray survey to date of about 2500 deg2 performed by the IBIS instrument on board INTEGRAL in the 20-60 keV band, with a total exposure time of 4 Ms. We find 34 candidate sources, for which we try to find counterparts at other wavelengths. The ratio of Seyfert 1 to Seyfert 2 is significantly more than the ratio found in the optical. This effect may be explained in the framework of the receding-torus model, but could also be due to absorption columns large enough to affect the 20-60 keV band. None of the predicted Compton-thick objects with 1024

The comoving infrared luminosity density: domination of cold galaxies across 0 < z < 1

In this paper, we examine the contribution of galaxies with different infrared (IR) spectral energy distributions (SEDs) to the comoving IR luminosity density (IRLD), a proxy for the comoving star formation rate (SFR) density. We characterize galaxies as having either a cold or hot IR SED depending on whether the rest-frame wavelength of their peak IR energy output is above or below 90 μ m. Our work is based on a far-IR selected sample both in the local Universe and at high redshift, the former consisting of IRAS 60 μ m-selected galaxies at z 10^(11) L_⊙) , the majority of which are cold, dominate the IRLD. We therefore infer that cold galaxies dominate the IRLD across the whole 0 < z < 1 range, hence appear to be the main driver behind the increase in SFR density up to z∼ 1 whereas local luminous galaxies are not, on the whole, representative of the high-redshift population.