arXiv subjects

R. J. Ivison

Publications and source records attributed to R. J. Ivison.

At least 361 records · Page 20Linked to original sources

arXiv2010

Herschel images of NGC 6720: H2 formation on dust grains

Herschel PACS and SPIRE images have been obtained of NGC 6720 (the Ring Nebula). This is an evolved planetary nebula with a central star that is currently on the cooling track, due to which the outer parts of the nebula are recombining. From the PACS and SPIRE images we conclude that there is a striking resemblance between the dust distribution and the H2 emission, which appears to be observational evidence that H2 forms on grain surfaces. We have developed a photoionization model of the nebula with the Cloudy code which we used to determine the physical conditions of the dust and investigate possible formation scenarios for the H2. We conclude that the most plausible scenario is that the H2 resides in high density knots which were formed after the recombination of the gas started when the central star entered the cooling track. Hydrodynamical instabilities due to the unusually low temperature of the recombining gas are proposed as a mechanism for forming the knots. H2 formation in the knots is expected to be substantial after the central star underwent a strong drop in luminosity about one to two thousand years ago, and may still be ongoing at this moment, depending on the density of the knots and the properties of the grains in the knots.

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arXiv2010

Herschel and SCUBA-2 imaging and spectroscopy of a bright, lensed submillimetre galaxy at z = 2.3

We present a detailed analysis of the far-IR properties of the bright, lensed, z = 2.3, SMG, SMM J2135-0102, using new observations with Herschel, SCUBA-2 and the VLA. These data allow us to constrain the galaxy's SED and show that it has an intrinsic rest-frame 8-1000um luminosity, L(bol), of (2.3 +/- 0.2) x 10^12 L(sun) and a likely SFR of ~400 M(sun)/yr. The galaxy sits on the far-IR/radio correlation for far-IR-selected galaxies. At ~>70um, the SED can be described adequately by dust components with T(d) ~ 30 and 60K. Using SPIRE's Fourier Transform Spectrometer we report a detection of the [CII] 158um cooling line. If the [CII], CO and far-IR continuum arise in photo-dissociation regions, we derive a characteristic gas density, n ~ 10^3 cm^-3, and a far-UV radiation field, G_0, 10^3x stronger than the Milky Way. L([CII])/L(bol) is significantly higher than in local ULIRGs but similar to the values found in local star-forming galaxies and starburst nuclei. This is consistent with SMM J2135-0102 being powered by starburst clumps distributed across ~2 kpc, evidence that SMGs are not simply scaled-up ULIRGs. Our results show that SPIRE's FTS has the ability to measure the redshifts of distant, obscured galaxies via the blind detection of atomic cooling lines, but it will not be competitive with ground-based CO-line searches. It will, however, allow detailed study of the integrated properties of high-redshift galaxies, as well as the chemistry of their ISM, once more suitably bright candidates have been found.

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arXiv2010

The far-infrared/radio correlation as probed by Herschel

We set out to determine the ratio, q(IR), of rest-frame 8-1000um flux, S(IR), to monochromatic radio flux, S(1.4GHz), for galaxies selected at far-IR and radio wavelengths, to search for signs that the ratio evolves with redshift, luminosity or dust temperature, and to identify any far-IR-bright outliers - useful laboratories for exploring why the far-IR/radio correlation is generally so tight when the prevailing theory suggests variations are almost inevitable. We use flux-limited 250-um and 1.4-GHz samples, obtained in GOODS-N using Herschel (HerMES; PEP) and the VLA. We determine bolometric IR output using ten bands spanning 24-1250um, exploiting data from PACS and SPIRE, as well as Spitzer, SCUBA, AzTEC and MAMBO. We also explore the properties of an L(IR)-matched sample, designed to reveal evolution of q(IR) with z, spanning log L(IR) = 11-12 L(sun) and z=0-2, by stacking into the radio and far-IR images. For 1.4-GHz-selected galaxies, we see tentative evidence of a break in the flux ratio, q(IR), at L(1.4GHz) ~ 10^22.7 W/Hz, where AGN are starting to dominate the radio power density, and of weaker correlations with z and T(d). From our 250-um-selected sample we identify a small number of far-IR-bright outliers, and see trends of q(IR) with L(1.4GHz), L(IR), T(d) and z, noting that some of these are inter-related. For our L(IR)-matched sample, there is no evidence that q(IR) changes significantly as we move back into the epoch of galaxy formation: we find q(IR) goes as (1+z)^gamma, where gamma = -0.04 +/- 0.03 at z=0-2; however, discounting the least reliable data at z<0.5 we find gamma = -0.26 +/- 0.07, modest evolution which may be related to the radio background seen by ARCADE2, perhaps driven by <10uJy radio activity amongst ordinary star-forming galaxies at z>1.

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arXiv2010

An excess of star-forming galaxies in the fields of high-redshift QSOs

We present submillimetre and mid-infrared imaging observations of five fields centred on quasi-stellar objects (QSOs) at 1.7<z<2.8. All 5 QSOs were detected previously at submillimetre wavelengths. At 850 (450) um we detect 17 (11) submillimetre galaxies (SMGs) in addition to the QSOs. The total area mapped at 850 um is ~28 arcmin^2 down to RMS noise levels of 1-2 mJy/beam, depending on the field. Integral number counts are computed from the 850 um data using the same analytical techniques adopted by `blank-field' submillimetre surveys. We find that the `QSO-field' counts show a clear excess over the blank-field counts at deboosted flux densities of 2-4 mJy; at higher flux densities the counts are consistent with the blank-field counts. Robust mid-infrared counterparts are identified for all four submillimetre detected QSOs and ~60 per cent of the SMGs. The mid-infrared colours of the QSOs are similar to those of the local ULIRG/AGN Mrk 231 if placed at 1<z<3 whilst most of the SMGs have colours very similar to those of the local ULIRG Arp 220 at 1<z<3. Mid-infrared diagnostics therefore find no strong evidence that the SMGs host buried AGN although we cannot rule out such a possibility. Taken together our results suggest that the QSOs sit in regions of the early universe which are undergoing an enhanced level of major star-formation activity, and should evolve to become similarly dense regions containing massive galaxies at the present epoch. Finally, we find evidence that the level of star-formation activity in individual galaxies appears to be lower around the QSOs than it is around more powerful radio-loud AGN at higher redshifts.

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arXiv2010

Mid-infrared spectroscopy of candidate AGN-dominated submillimeter galaxies

Spitzer spectroscopy has revealed that ~80% of submm galaxies (SMGs) are starburst (SB) dominated in the mid-infrared. Here we focus on the remaining ~20% that show signs of harboring powerful active galactic nuclei (AGN). We have obtained Spitzer-IRS spectroscopy of a sample of eight SMGs which are candidates for harboring powerful AGN on the basis of IRAC color-selection (S8/S4.5>2; i.e. likely power-law mid-infrared SEDs). SMGs with an AGN dominating (>50%) their mid-infrared emission could represent `missing link' sources in an evolutionary sequence involving a major merger. First of all, we detect PAH features in all of the SMGs, indicating redshifts from 2.5-3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Secondly, we see signs of both star-formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGN dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGN are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S8/S4.5>1.65 works well at selecting mid-infrared energetically dominant AGN in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.

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arXiv2010

The LABOCA survey of the Extended Chandra Deep Field South: Two modes of star formation in AGN hosts?

We study the co-existence of star formation and AGN activity in X-ray selected AGN by analyzing stacked 870um submm emission from a deep and wide map of the Extended Chandra Deep Field South, obtained with LABOCA at the APEX telescope. The total X-ray sample of 895 sources with median redshift z~1 is detected at a mean submm flux of 0.49+-0.04mJy, corresponding to a typical star formation rate around 30Msun/yr for a T=35K, beta=1.5 greybody far-infrared SED. The good S/N permits stacking analyses for subgroups. We observe a trend of star formation rate increasing with redshift. An increase of star formation rate with AGN luminosity is indicated at the highest L_2-10>~1E44erg/s luminosities only. Increasing trends with X-ray obscuration as expected in some AGN evolutionary scenarios are not observed for the bulk of the X-ray AGN sample but may be present for the highest intrinsic luminosity objects. This suggests a transition between two modes in the coexistence of AGN activity and star formation. For the bulk of the sample, the X-ray luminosity and obscuration of the AGN are not intimately linked to the global star formation rate of their hosts. The hosts are likely massive and forming stars secularly, at rates similar to the pervasive star formation seen in massive galaxies without an AGN at similar redshifts. The change indicated towards more intense star formation, and a more pronounced increase in star formation rates between unobscured and obscured AGN at highest luminosities suggests that luminous AGN follow an evolutionary path on which obscured AGN activity and intense star formation are linked, possibly via merging. Comparison to local hard X-ray selected AGN supports this interpretation. [Abridged]

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arXiv2010

The Effects of an AGN on Host Galaxy Colour and Morphology Measurements

We assess the effects of simulated active galactic nuclei (AGNs) on the colour and morphology measurements of their host galaxies. To test the morphology measurements, we select a sample of galaxies not known to host AGNs and add a series of point sources scaled to represent specified fractions of the observed V band light detected from the resulting systems; we then compare morphology measurements of the simulated systems to measurements of the original galaxies. AGN contributions >20 per cent bias most of the morphology measurements tested, though the extent of the apparent bias depends on the morphological characteristics of the original galaxies. We test colour measurements by adding to non-AGN galaxy spectra a quasar spectrum scaled to contribute specified fractions of the rest-frame B band light detected from the resulting systems. A quasar fraction of 5 per cent can move the NUV-r colour of an elliptical galaxy from the UV-optical red sequence to the green valley, and 20 per cent can move it into the blue cloud. Combining the colour and morphology results, we find that a galaxy/AGN system with an AGN contribution >20 per cent may appear bluer and more bulge-dominated than the underlying galaxy. We conclude that (1) bulge-dominated, E/S0/Sa, and early-type morphology classifications are accurate for red AGN host galaxies and may be accurate for blue host galaxies, unless the AGN manifests itself as a well-defined point source; and (2) although highly unobscured AGNs, such as the quasar used for our experiments, can significantly bias the measured colours of AGN host galaxies, it is possible to identify such systems by examining optical images of the hosts for the presence of a point source and/or measuring the level of nuclear obscuration.

astro-ph.CO↗

arXiv2010

High-resolution CO and radio imaging of z~2 ULIRGs: extended CO structures, and implications for the universal star formation law

We present high spatial resolution (0.4", ~3.5 kpc) PdBI interferometric data on three ultra-luminous infrared galaxies (ULIRGs) at z~2: two submillimetre galaxies and one submillimetre faint star forming radio galaxy. The three galaxies have been ro- bustly detected in CO rotational transitions, either 12CO(J=4-3) or 12CO(J=3-2), allowing their sizes and gas masses to be accurately constrained. These are the highest spatial resolution observations observed to date (by a factor of ~2) for intermediate-excitation CO emission in z~2 ULIRGs. The galaxies appear extended over several resolution elements, having a mean radius of 3.7 kpc. High-resolution (0.3") combined MERLIN-VLA observations of their radio continua allow an analysis of the star formation behaviour of these galaxies, on comparable spatial scales to that of the CO observations. This 'matched beam' approach sheds light on the spatial distribution of both molecular gas and star formation, and we can therefore calculate accurate star formation rates and gas surface densities: this allows us to place the three systems in the context of a Kennicutt-Schmidt (KS)-style star formation law. We find a difference in size between the CO and radio emission regions, and as such we suggest that using the spatial extent of the CO emission region to estimate the surface density of star formation may lead to error. This size difference also causes the star formation efficiencies within systems to vary by up to a factor of 5. We also find, with our new accurate sizes, that SMGs lie significantly above the KS relation, indicating that stars are formed more efficiently in these extreme systems than in other high-z star forming galaxies.

astro-ph.CO↗

arXiv2010

Unveiling the Nature of Submillimeter Galaxy SXDF850.6

We present an 880 micron Submillimeter Array (SMA) detection of the submillimeter galaxy SXDF850.6. SXDF850.6 is a bright source (S(850 micron) = 8 mJy) detected in the SCUBA Half Degree Extragalactic Survey (SHADES), and has multiple possible radio counterparts in its deep radio image obtained at the VLA. Our new SMA detection finds that the submm emission coincides with the brightest radio emission that is found ~8" north of the coordinates determined from SCUBA. Despite the lack of detectable counterparts in deep UV/optical images, we find a source at the SMA position in near-infrared and longer wavelength images. We perform SED model fits to UV-optical-IR photometry (u, B, V, R, i', z', J, H, K, 3.6 micron, 4.5 micron, 5.8 micron, and 8.0 micron) and to submm-radio photometry (850 micron, 880 micron, 1100 micron, and 21 cm) independently, and we find both are well described by starburst templates at a redshift of z ~= 2.2 (+/- 0.3). The best-fit parameters from the UV-optical-IR SED fit are a redshift of z = 1.87 (+0.15/-0.07), a stellar mass of M_star = 2.5 +2.2/-0.3 x 10^11 M_sun, an extinction of A_V = 3.0 (+0.3/-1.0) mag, and an age of 720 (+1880/-210) Myr. The submm-radio SED fit provides a consistent redshift of z ~ 1.8-2.5, an IR luminosity of L_IR = (7-26) x 10^12 L_sun, and a star formation rate of 1300-4500 M_sun/yr. These results suggest that SXDF850.6 is a mature system already having a massive amount of old stellar population constructed before its submm bright phase and is experiencing a dusty starburst, possibly induced by major mergers.

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arXiv2009

Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA: II. The spectral index of submillimetre galaxies

We have employed the GMRT and the VLA to map the Lockman Hole. At 610 and 1,400 MHz, we reach noise levels of 15 and 6 uJy/beam, respectively, with well-matched resolutions (~5"). At this depth we obtained reliable detections for about half of the known submm galaxies (SMGs) in the field. For radio-identified SMGs, which are typically at z ~ 2, we measure a mean radio spectral index of alpha = -0.75 +/- 0.06 and standard deviation of 0.29, between rest-frame ~1.8 and ~4.2 GHz. The slope of their continuum emission is indistinguishable from that of local star-forming galaxies and suggests that extended optically-thin synchrotron emission dominates the radio output of SMGs. Cooling effects by synchrotron emission and Inverse Compton scattering off the CMB do not seem to affect their radio SEDs. For those SMGs judged by Spitzer mid-IR colours and spectroscopy to host obscured AGN, we find a clear deviation from the rest of the sample - they typically have steeper radio spectral indices, alpha ~< -1.0. These findings suggest these mid-IR-/AGN-selected SMGs may have an intrinsically different injection mechanism for relativistic particles, or they might reside in denser environments. This work provides a reliable spectral template for the estimation of far-IR/radio photometric redshifts, and will enable accurate statistical K-corrections for the large samples of SMGs expected with SCUBA-2 and Herschel.

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arXiv2009

BLAST: the far-infrared/radio correlation in distant galaxies

We investigate the correlation between far-infrared (FIR) and radio luminosities in distant galaxies, a lynchpin of modern astronomy. We use data from the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer, the Large Apex BOlometer CamerA (LABOCA), the Very Large Array (VLA) and the Giant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field South (ECDFS). For a catalogue of BLAST 250-micron-selected galaxies, we re-measure the 70--870-micron flux densities at the positions of their most likely 24-micron counterparts, which have a median [interquartile] redshift of 0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density ratio, q_250 = log_10 (S_250micron / S_1400MHz), and the bolometric equivalent, q_IR. At z ~= 0.6, where our 250-micron filter probes rest-frame 160-micron emission, we find no evolution relative to q_160 for local galaxies. We also stack the FIR and submm images at the positions of 24-micron- and radio-selected galaxies. The difference between q_IR seen for 250-micron- and radio-selected galaxies suggests star formation provides most of the IR luminosity in ~< 100-uJy radio galaxies, but rather less for those in the mJy regime. For the 24-micron sample, the radio spectral index is constant across 0 < z < 3, but q_IR exhibits tentative evidence of a steady decline such that q_IR is proportional to (1+z)^(-0.15 +/- 0.03) - significant evolution, spanning the epoch of galaxy formation, with major implications for techniques that rely on the FIR/radio correlation. We compare with model predictions and speculate that we may be seeing the increase in radio activity that gives rise to the radio background.

astro-ph.CO↗

arXiv2009

The LABOCA Survey of the Extended Chandra Deep Field South

We present a sensitive 870 micron survey of the Extended Chandra Deep Field South (ECDFS) using LABOCA on the APEX telescope. The LABOCA ECDFS Submillimetre Survey (LESS) covers the full 30' x 30' field size of the ECDFS and has a uniform noise level of 1.2 mJy/beam. LESS is thus the largest contiguous deep submillimetre survey undertaken to date. The noise properties of our map show clear evidence that we are beginning to be affected by confusion noise. We present a catalog of 126 SMGs detected with a significance level above 3.7 sigma. The ECDFS exhibits a deficit of bright SMGs relative to previously studied blank fields but not of normal star-forming galaxies that dominate the extragalactic background light (EBL). This is in line with the underdensities observed for optically defined high redshift source populations in the ECDFS (BzKs, DRGs,optically bright AGN and massive K-band selected galaxies). The differential source counts in the full field are well described by a power law with a slope of alpha=-3.2, comparable to the results from other fields. We show that the shape of the source counts is not uniform across the field. The integrated 870 micron flux densities of our source-count models account for >65% of the estimated EBL from COBE measurements. We have investigated the clustering of SMGs in the ECDFS by means of a two-point correlation function and find evidence for strong clustering on angular scales <1'. Assuming a power law dependence for the correlation function and a typical redshift distribution for the SMGs we derive a spatial correlation length of r_0=13+/-6 h^-1 Mpc.

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arXiv2009

An AzTEC 1.1-mm Survey for ULIRGs in the field of the Galaxy Cluster MS 0451.6-0305

We have undertaken a deep (sigma~1.1 mJy) 1.1-mm survey of the z=0.54 cluster MS 0451.6-0305 using the AzTEC camera on the James Clerk Maxwell Telescope. We detect 36 sources with S/N>3.5 in the central 0.10 deg^2 and present the AzTEC map, catalogue and number counts. We identify counterparts to 18 sources (50%) using radio, mid-infrared, Spitzer IRAC and Submillimeter Array data. Optical, near- and mid-infrared spectral energy distributions are compiled for the 14 of these galaxies with detectable counterparts, which are expected to contain all likely cluster members. We then use photometric redshifts and colour selection to separate background galaxies from potential cluster members and test the reliability of this technique using archival observations of submillimetre galaxies. We find two potential MS 0451-03 members, which, if they are both cluster galaxies have a total star-formation rate (SFR) of ~100 solar masses per year -- a significant fraction of the combined SFR of all the other galaxies in MS 0451-03. We also examine the stacked rest-frame mid-infrared, millimetre and radio emission of cluster members below our AzTEC detection limit and find that the SFRs of mid-IR selected galaxies in the cluster and redshift-matched field populations are comparable. In contrast, the average SFR of the morphologically classified late-type cluster population is ~3 times less than the corresponding redshift-matched field galaxies. This suggests that these galaxies may be in the process of being transformed on the red-sequence by the cluster environment. Our survey demonstrates that although the environment of MS 0451-03 appears to suppress star-formation in late-type galaxies, it can support active, dust-obscured mid-IR galaxies and potentially millimetre-detected LIRGs.

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arXiv2009

AzTEC Half Square Degree Survey of the SHADES Fields -- I. Maps, Catalogues, and Source Counts

We present the first results from the largest deep extragalactic millimetre-wavelength survey undertaken to date. These results are derived from maps covering over 0.7 deg^2, made at 1.1mm, using the AzTEC continuum camera mounted on the James Clerk Maxwell Telescope. The maps were made in the two fields originally targeted at 0.85mm with SCUBA in the SHADES project, namely the Lockman Hole East (mapped to a depth of 0.9-1.3 mJy rms) and the Subaru XMM Deep Field (1.0-1.7 mJy rms). The wealth of existing and forthcoming deep multi-frequency data in these two fields will allow the bright mm source population revealed by these images to be explored in detail in subsequent papers. Here we present the maps themselves, a catalogue of 114 high-significance sub-millimetre galaxy detections, and a thorough statistical analysis leading to the most robust determination to date of the 1.1mm source number counts. Through careful comparison, we find that both the COSMOS and GOODS North fields, also imaged with AzTEC, contain an excess of mm sources over the new 1.1mm source-count baseline established here. In particular, our new AzTEC/SHADES results indicate that very luminous high-redshift dust enshrouded starbursts (S_{1.1} > 3 mJy) are 25-50% less common than would have been inferred from these smaller surveys, thus highlighting the potential roles of cosmic variance and clustering in such measurements. We compare number count predictions from recent models of the evolving mm/sub-mm source population to these SMG surveys, which provide important constraints for the ongoing refinement of semi-analytic and hydrodynamical models of galaxy formation, and find that all recent models over-predict the number of bright sub-millimetre galaxies found in this survey.

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arXiv2009

First redshift determination of an optically/UV faint submillimeter galaxy using CO emission lines

We report the redshift of a distant, highly obscured submm galaxy (SMG), based entirely on the detection of its CO line emission. We have used the newly commissioned Eight-MIxer Receiver (EMIR) at the IRAM 30m telescope, with its 8 GHz of instantaneous dual-polarization bandwidth, to search the 3-mm atmospheric window for CO emission from SMMJ14009+0252, a bright SMG detected in the SCUBA Lens Survey. A detection of the CO(3--2) line in the 3-mm window was confirmed via observations of CO(5--4) in the 2-mm window. Both lines constrain the redshift of SMMJ14009+0252 to z=2.9344, with high precision (dz=2 10^{-4}). Such observations will become routine in determining redshifts in the era of the Atacama Large Millimeter/submillimeter Array (ALMA).

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arXiv2009

A Search for Neutral Carbon towards two z=4.05 Submillimetre Galaxies, GN20 and GN20.2

Using the IRAM Plateau-de-Bure Interferometer (PdBI) we have searched for the upper fine structure line of neutral carbon (CI(2->1), nu_rest = 809 GHz) and 12CO(J=7->6) (nu_rest=806GHz) towards the submillimetre galaxies (SMGs) GN20 (SMM J123711.9+622212, z = 4.055) and GN20.2 (SMM J123708.8+622202, z = 4.051). The far-infrared (FIR) continuum is detected at 8sigma significance in GN20, with a flux density of S_1.8mm = 1.9+-0.2mJy, while no continuum is detected in GN20.2. Both sources are statistically undetected in both CI and CO(7-6) lines; we derive line luminosity limits for both CI and CO of L'<~2x10^10K km s^-1 pc^2. Assuming carbon excitation temperatures of T_ex = 30K (the galaxies' measured dust temperatures), we infer CI mass limits of M_CI < 5.4x10^6 Msun (GN20) and M_CI < 6.8x10^6 Msun (GN20.2). The derived CI abundance limits are <1.8x10^-5 for GN20 and <3.8x10^-5 for GN20.2 implying that the systems have Milky Way level neutral carbon enrichment (X[CI]/X[H2]) or lower, similar to high-redshift carbon-detected systems (at 5x10^-5) but about 50 times less than the neutral carbon enrichment of local starburst galaxies. Observations of GN20 and GN20.2 in high-resolution MERLIN+VLA radio maps of GOODS-N are used to further constrain the sizes and locations of active regions. We conclude that the physical gas properties of young rapidly evolving systems like GN20 and GN20.2 are likely significantly different than starburst/ULIRG environments in the local Universe yet similar to z~2 SMGs. Unless gravitationally amplified examples can be found, observations of galaxies like GN20 will require the order of magnitude increase in sensitivity of the Atacama Large Millimetre Array (ALMA) to constrain their CI and high-J CO content, despite the fact that they are the brightest systems at z~4.

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arXiv2009

A 100-kpc inverse Compton X-ray halo around 4C60.07 at z=3.79

We analyse a 100-ks Chandra observation of the powerful radio galaxy, 4C60.07 at z=3.79. We identify extended X-ray emission with Lx~10^45 erg/s across a ~90-kpc region around the radio galaxy. The energetics of this X-ray halo and its morphological similarity to the radio emission from the galaxy suggest that it arises from inverse Compton (IC) scattering, by relativistic electrons in the radio jets, of Cosmic Microwave Background photons and potentially far-infrared photons from the dusty starbursts around this galaxy. The X-ray emission has a similar extent and morphology to the Ly-alpha halo around the galaxy, suggesting that it may be ionising this halo. Indeed we find that the GHz-radio and X-ray and Ly-alpha luminosities of the halo around 4C60.07 are identical to those of 4C41.17 (also at z=3.8) implying that these three components are linked by a single physical process. This is only the second example of highly-extended IC emission known at z>3, but it underlines the potential importance of IC emission in the formation of the most massive galaxies at high redshifts. In addition, we detect two X-ray luminous active galactic nuclei (AGN) within ~30kpc of the radio galaxy. These two companion AGN imply that the radio and starburst activity in the radio galaxy is triggered through multiple mergers of massive progenitors on a short timescale, ~100Myrs. These discoveries demonstrate the wealth of information which sensitive X-ray observations can yield into the formation of massive galaxies at high redshifts.

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arXiv2009

Confirming a Population of Hot-Dust Dominated, Star Forming Ultraluminous Galaxies at High-Redshift

We identify eight z>1 radio sources undetected at 850um but robustly detected at 70um, confirming that they represent ultraluminous infrared galaxies (ULIRGs) with hotter dust temperatures (T_dust=52+-10 K) than Submillimetre Galaxies (SMGs) at similar luminosities and redshifts. These galaxies share many properties with SMGs: ultra-violet (UV) spectra consistent with starbursts, high stellar masses and radio luminosities. We can attribute their radio emission to star formation since high-resolution MERLIN radio maps show extended emission regions (with half light radii of 2-3kpc), which are unlikely to be generated by AGN activity. These observations provide the first direct confirmation of hot, dusty ULIRGs which are missed by current submillimetre surveys. They have significant implications for future observations from the Herschel Space Observatory and SCUBA2, which will select high redshift luminous galaxies with less selection biases.

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