arXiv subjects

R. J. Ivison

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

At least 451 records · Page 25Linked to original sources

arXiv2004

Fact: Many SCUBA galaxies harbour AGNs

Deep SCUBA surveys have uncovered a large population of ultra-luminous galaxies at z>1. These sources are often assumed to be starburst galaxies, but there is growing evidence that a substantial fraction host an AGN (i.e., an accreting super-massive black hole). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultra-deep X-ray observations (the 2 Ms Chandra Deep Field-North) and deep optical spectroscopic data. We argue that upward of 38% of bright (f850um>=5mJy) SCUBA galaxies host an AGN, a fraction of which are obscured QSOs (i.e., L_X>3x10^{44} erg/s). However, using evidence from a variety of analyses, we argue that in almost all cases the AGNs are not bolometrically important (i.e., <20%). Thus, star formation appears to dominate their bolometric output. A substantial fraction of bright SCUBA galaxies show evidence for binary AGN activity. Since these systems appear to be interacting and merging at optical/near-IR wavelengths, their super-massive black holes will eventually coalesce.

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arXiv2003

A 1200 micron MAMBO Survey of the ELAIS N2 and Lockman Hole East Fields

Using the MPIfR Max Planck Millimeter Bolometer array (MAMBO) on the IRAM 30m Telescope we have mapped the ELAIS N2 and Lockman Hole East Fields at 1200 micron to a rms noise level of 0.8-1.0 mJy per 11" beam. The areas surveyed are 326 arcmin^2 in the ELAIS N2 field and 212 arcmin^2 in the Lockman Hole,and cover the 260 arcmin^2 previously observed by SCUBA [5].

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arXiv2003

Interferometric Observations of Powerful CO Emission from the three Submillimeter Galaxies at z=2.30, 2.51 and 3.35

We report IRAM Plateau de Bure, millimeter interferometry of three z=~2.4 to 3.4, SCUBA deep field galaxies. Our CO line observations confirm the rest-frame UV/optical redshifts, thus more than doubling the number of confirmed, published redshifts of the faint submillimeter population and proving their high-z nature. In all three sources our measurements of the intrinsic gas and dynamical mass are large (1e10 to 1e11 Msun). In at least two cases the data show that the submillimeter sources are part of an interacting system. Together with recent information gathered in the X-ray, optical and radio bands our observations support the interpretation that the submm-population consists of gas rich (gas to dynamical mass ratio ~0.5) and massive, composite starburst/AGN systems, which are undergoing a major burst of star formation and are evolving into m*-galaxies.

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arXiv2003

Further multiwavelength observations of the SSA22 Ly_alpha emitting `blob'

We present new follow-up observations of the sub-mm luminous Ly_alpha-emitting object in the SSA22 z=3.09 galaxy overdensity, referred to as `Blob 1' by Steidel et al.(2000). In particular we discuss high resolution Hubble Space Telescope optical imaging, Owens Valley Radio Observatory spectral imaging, Keck spectroscopy, VLA 20cm radio continuum imaging, and Chandra X-ray observations. We also present a more complete analysis of the existing James Clerk Maxwell Telescope sub-mm data. We detect several optical continuum components which may be associated with the core of the submillimeter emitting region. A radio source at the position of one of the HST components (22:17:25.94, +00:12:38.9) identifies it as the likely counterpart to the submillimeter source. We also tentatively detect the CO(4-3) molecular line, centered on the radio position. We use the CO(4-3) intensity to estimate a limit on the gas mass for the system. The optical morphology of sources within the Ly_alpha cloud appears to be filamentary, while the optical source identified with the radio source has a dense knot which may be an AGN or compact starburst. We obtain a Keck-LRIS spectrum of this object, despite its faintness (R=26.8). The spectrum reveals weak Ly_alpha emission, but no other obvious features, suggesting that the source is not an energetic AGN (or that it is extremely obscured). We use non-detections in deep Chandra X-ray images to constrain the nature of the `Blob'. Although conclusive evidence regarding the nature of the object remains hard to obtain at this redshift, the evidence presented here is at least consistent with a dust-obscured AGN surrounded by a starburst situated at the heart of this giant Ly_alpha cloud.

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arXiv2003

The formation of cluster elliptical galaxies as revealed by extensive star formation

The most massive galaxies in the present-day Universe are found to lie in the centres of rich clusters. They have old, coeval stellar populations suggesting that the bulk of their stars must have formed at early epochs in spectacular starbursts - luminous phenomena at submillimetre wavelengths. The most popular model of galaxy formation predicts that these galaxies form in proto-clusters at high-density peaks in the early Universe. Such peaks are signposted by massive high-redshift radio galaxies. Here we report deep submillimetre mapping of seven high-redshift radio galaxies and their environments. These data confirm not only the presence of spatially extended massive star-formation activity in the radio galaxies themselves, but also in companion objects previously undetected at any wavelength. The prevalence, orientation, and inferred masses of these submillimetre companion galaxies suggest that we are witnessing the synchronous formation of the most luminous elliptical galaxies found today at the centres of rich galaxy clusters.

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arXiv2003

Gas and Dust in the Extremely Red Object ERO J164502+4626.4

We report the first detection of the lowest CO transition in a sub-millimetre bright galaxy and extremely red object (ERO) at z = 1.44 using the Very Large Array. The total J = 1 - 0 line luminosity of ERO J164502+4626.4 is (7+-1) x 10^{10} K km s^{-1} pc^2, which yields a total molecular gas mass of ~6 x 10^{10} Msun. We also present a map of the 850-um continuum emission obtained using SCUBA, from which we infer a far-IR luminosity and dust mass of L_FIR ~ 9 x 10^{12} Lsun and M_d ~ 9 x 10^{8} Msun. We find tentative evidence that the CO and sub-mm dust emission is extended over several tens of kpc. If confirmed by high-resolution imaging, this implies that ERO J164502+4626.4 is not simply a high redshift counterpart of a typical Ultra Luminous Infrared Galaxy (ULIRG).

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arXiv2003

An HST Morphological Survey of Faint EROs

We present the results from a survey for Extremely Red Objects (EROs) in deep, high resolution optical images taken from the Hubble Space Telescope (HST) Medium Deep Survey. We have surveyed 35 deep F814W HST/WFPC2 fields in the near-infrared to a typical depth of K~20. From a total area of 206 arcmin^2 and to a limit of K=20.0 we identify 224 EROs ((1.14+/-0.08) arcmin^-2) with (I_{814}-K)=>4.0 and 83 ((0.41+/-0.05) arcmin^-2) with (I_{814}-K)=>5.0. We find that the slope of the number counts of the (I_{814}-K)=>4.0 EROs flattens beyond K~19, in line with results from previous surveys, and the typical colours of the EROs become redder beyond the break magnitude. We morphologically classify our ERO sample using visual and quantitative schemes and find that 35% of our sample exhibit clear disk components, 15% are disturbed or irregular, a further 30% are either spheroidal or compact and the remaining 20% are unclassifiable. Using a quantitative measure of morphology, we find that the ERO morphological distribution evolves across the break in their counts, such that low concentration (disk-like) galaxies decline. We relate the morphological and colour information for our EROs and conclude that those EROs morphologically classified as bulges do indeed possess SEDs consistent with passive stellar populations; while EROs with dusty star-forming SEDs are mostly associated with disk-like and peculiar galaxies. However, ~30% of disk EROs reside in the passive region of I/J/K colour-colour space. These could be either genuinely passive systems, lower redshift contaminants to the high-z ERO population, or systems with composite star-forming and passive SEDs.

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arXiv2003

Chandra Detections of SCUBA Galaxies around High-z Radio Sources

The most massive galaxies in the present day universe are the giant ellipticals found in the centers of rich clusters. These have old, coeval stellar populations, suggesting they formed at high redshift, and are expected to host supermassive black holes (SMBHs). The recent detection of several high-redshift radio galaxies (HzRGs) at submm wavelengths confirms that indeed some massive galaxies may have formed the bulk of their stellar populations in spectacular dust-enshrouded starbursts at high-z. In this letter we compare sensitive Chandra X-ray images - which identify actively-fueled SMBHs - and submm observations - capable of detecting obscured activity in luminous galaxies at high redshift - of the environments of three HzRGs. These observations exhibit overdensities of X-ray sources in all 3 fields and a close correspondence between the Chandra and SCUBA populations. This suggests that both substantial star formation and nuclear activity may be occuring in these regions. We identify possible pairs of Chandra sources with each of 2 SCUBA sources, suggesting that their ultraluminous activity may be triggered by the interaction of two massive galaxies, each of which hosts an accreting SMBH. The presence of two SMBHs in the resulting remanent is predicted to produce a flattened stellar core in the galaxy, a morphological signature frequently seen in luminous cluster ellipticals. Hence the confirmation of pairs of Chandra sources within individual, luminous SCUBA galaxies would provide additional evidence that these galaxies at z~2-4 are the progenitors of the giant elliptical galaxies found in clusters at the present-day.

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arXiv2003

Sub-millimetre observations of the Hubble Deep Field and Flanking Fields

We present an extended analysis of the SCUBA observations of the Hubble Deep Field (HDF), expanding the areal coverage of the Hughes et al. 1998 study by a factor of ~1.8 and containing at least three further sources in addition to the five in that study. We also announce the public release of the reduced data products. The map is the deepest ever made in the sub-millimetre, obtained in excellent conditions (median 850um optical depth of 0.16). Two independent reductions were made, one with SURF and the other with a wholly algorithmic IDL analysis which we present in detail here. Of the three new sources, all appear to be at z>~0.9 and one is provisionally associated with an Extremely Red Object (I-K>5). There appears to be no significant cross-correlation signal between the 850um fluctuations and sources detected by ISOCAM, the VLA or Chandra, nor with Very Red Objects (I-K>4), nor quasars and quasar candidates in the HDF (notwithstanding a small number of individual weak candidate detections). This is consistent with interpretations where the 850um-selected galaxies are at higher redshifts than those currently probed by ISOCAM/VLA, and predominantly not Compton-thin AGN. There are only one or two compelling cases for the radio source being the sub-mm source. Nevertheless, most SCUBA-HDF point sources have a nearby radio source apparently well-separated from the sub-mm centroid.

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arXiv2003

The spectroscopic redshifts of SCUBA galaxies: implications for spheroid formation

We present spectroscopic identifications for a sample of 55 submillimeter(submm)-selected `SCUBA' galaxies, lying at redshifts z=0.7 to 3.7, that were pinpointed in deep 1.4-GHz VLA radio maps. We describe their properties, especially the presence of active galactic nuclei (AGN) in the sample, and discuss the connection of the SCUBA galaxies and the formation of spheroidal components of galaxies, which requires knowledge of their masses and the timescales of their very luminous activity. For a subset of the galaxies, we show their disturbed and diverse Hubble Space Telescope (HST) optical morphologies.

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arXiv2003

A median redshift of 2.4 for galaxies bright at submillimetre wavelengths

A significant fraction of the energy emitted in the early Universe came from very luminous galaxies that are largely hidden at optical wavelengths (because of interstellar dust grains); this energy now forms part of the cosmic background radiation at wavelengths near 1mm. These submillimetre (submm) galaxies were resolved from the background in 1997 but have been difficult to identify and study due to the poor spatial resolution of submm instruments. This has impeded the determination of their distances (or redshifts, z), a crucial element in understanding their nature and evolution. Here we report spectroscopic redshifts for ten representative submm galaxies that we identified reliably using high resolution radio observations. The median redshift for our sample is 2.4, with a quartile range of z = 1.9-2.8. The submm population therefore coexists with the peak activity of quasars, which are thought to be massive black holes in the process of accreting matter, suggesting a close relationship between the growth of massive black holes and luminous dusty galaxies. The space density of submm galaxies at z>2 is about 1000 times greater than that of similarly luminous galaxies in the present-day Universe, so they represent an important component of star formation at high redshifts.

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arXiv2003

The z=2.51 Extremely Red Submillimeter Galaxy SMMJ04431+0210

We report the redshift measurement for the submillimeter selected galaxy SMMJ04431+0210 (N4) using the Near Infrared Spectrograph on the Keck-II telescope. The data show H-alpha, [NII]6583,6548, and [OIII]5007 lines at a redshift of z=2.51. The high nuclear [NII]/H-alpha line ratio is consistent with a LINER or Type-II AGN. The H-alpha emission is spatially resolved, suggesting the presence of significant star-forming activity outside the nucleus. From imaging with the Near Infrared Camera on the Keck-I telescope, we find an extremely red near-infrared color of J-K=3.2 for N4. Follow-up redshifted CO(3-2) observations with the Owens Valley Millimeter Array constrain the mass of molecular gas to be less than 4x10^{10} Msun, after correcting for lensing. The CO to sub-mm flux limit, the spectroscopic line ratios, and the spectral energy distribution for N4 are all within the range of properties found in other high-redshift sub-mm sources and local ultraluminous infrared galaxies. After the correction for lensing, N4 is the weakest intrinsic sub-mm selected source with a known redshift and represents the first redshift for the <2mJy 850um sources which are responsible for the bulk of the emission from the sub-mm population as a whole. We argue that N4 contains either an AGN or LINER nucleus surrounded by an extended region of active star-formation.

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arXiv2003

A Vigorous Starburst in the SCUBA Galaxy N2 850.4

We present optical and near-IR spectroscopy of a z=2.38 hyperluminous IR galaxy, covering the restframe wavelength range from 1000-5000A. It appears to comprise two components separated by less than 1" on the sky (<8kpc); one component (B) is blue, the other (P) is red in restframe UV-optical colours. The combined system has an optical luminosity of ~8L* and its restframe optical spectrum is characteristic of a Seyfert AGN. However, its restframe UV spectrum exhibits striking features associated with young stars, including P-Cygni lines from stellar winds and blue-shifted interstellar absorption lines indicative of a galactic outflow. Redshifts are derived from stellar photospheric lines in the UV and from narrow emission lines in the restframe optical, and these are compared to that measured for the molecular gas recently detected with the IRAM interferometer. The offsets indicate that the far-IR emission is most likely associated with the near-IR source P, which hosts the Seyfert nucleus, while the UV-bright component B is blueshifted by 400km/s. This suggests that the two components are probably merging and the resulting gravitational interactions have triggered the hyperluminous activity. Modelling of the UV spectral features implies that the starburst within the UV component of this system has been going on for at least ~10Myrs. Assuming that the bolometrically-dominant obscured component has a similar lifetime, we estimate that it has so far formed a total stellar mass of ~10^11 Mo. If this star formation continues at its present level for substantially longer, or if this activity is repeated, then the present-day descendant of N2 850.4 will be a very luminous galaxy.

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arXiv2003

The nature of X-ray selected EROs

We report on the X-ray, optical, near-infrared, submillimetre and radio properties of five Extremely Red Objects (EROs) selected at X-ray wavelengths by XMM-Newton in the Lockman Hole field. They all have enough counts in the X-ray band to allow spectral fitting: four are most probably obscured, Compton-thin AGN with redshift dependent absorbing column densities of 10^22 - 10^24 cm-2, whilst the fifth is best fitted by a thermal spectrum and is likely to be a massive elliptical galaxy in a deep gravitational potential. Their optical/near-infrared colours and sizes suggest that X-ray selected EROs comprise a mixture of dusty 'starburst' galaxies and non-dusty galaxies that are dominated by either star-light or light from an active nucleus. The colour diagnostics are supported by the submillimetre and radio data: the two AGN with 'starburst' colours have submillimetre or radio flux densities that imply large star-formation rates, whilst those with 'elliptical' colours do not. The one source detected in the submillimetre waveband has narrow emission lines at a redshift of 1.45. Although the bulk of its radio emission originates from processes other than star formation, it is most probably a radio-quiet ultraluminous infrared galaxy.

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arXiv2002

Mm/submm observations of symbiotic binary stars

We present and discuss mm/submm observations of quiescent S-type symbiotic systems, and compare them with popular models proposed to account for their radio emission. We find that the M giant mass-loss rates derived from our observations are systematically higher than those reported for single M giants.

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arXiv2002

The Diversity of Extremely Red Objects

We present a multi-wavelength study of Extremely Red Objects (EROs) employing deep RIzJHK photometry of a 8.5'x8.5' region to identify 68 EROs with R-K>5.3 and K<20.5 (5-sigma). This is combined with an extremely deep 1.4-GHz radio map (sigma=3.5uJy), sensitive enough to detect an active galaxy with L_1.4>10^23 W/Hz at z>1 or a SFR of >25Mo/yr. We detect radio emission from 21 EROs at >12.6uJy and resolve a third of these with our 1.6'' FWHM beam. The SEDs of most of these radio EROs are consistent with dust-reddened starbursts at z~1. At z~1 the radio luminosities of these EROs indicate far-infrared luminosities of L_FIR>10^12 Lo, meaning half are ultraluminous infrared galaxies. We conclude that >16+/-5% of EROs with K<20.5 are luminous infrared galaxies at z>1. We also photometrically classify the EROs to investigate the mix of dusty/active and evolved/passive systems in the radio-undetected EROs. We suggest that at least 30%, and perhaps up to ~60%, of all EROs with R-K>5.3 and K<20.5 are dusty, star-forming systems at z>1. The SFD in this optically faint (R>26) population is rho^* (0.1-100Mo)=0.11+/-0.03 Mo/yr/Mpc^3, comparable to that in H-alpha emitting galaxies at z~1, and greater than that in UV-selected samples at these epochs. This support the claim of a strong increase in obscured star formation at high redshifts. Using the observed counts of the radio-detected EROs we model the break in the K-band number counts of all EROs at K~19-20 and propose that the passive ERO class dominates the total population in a narrow range around K~20, with dusty EROs dominating at fainter magnitudes. [Abridged]

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arXiv2002

Obscured AGN from the ELAIS Deep X-ray Survey

The sources discovered in deep hard X-ray surveys with 2-8 keV fluxes of 10^-14 erg cm^-2 s^-1 make up the bulk of the X-ray background at these energies. We present here detailed multi-wavelength observations of three such sources from the ELAIS Deep X-ray Survey. The observations include sensitive near-infrared spectroscopy with the Subaru Telescope and X-ray spectral information from the Chandra X-ray Observatory. The sources observed all have optical-to-near-IR colours redder than an unobscured quasar and comprise a reddened quasar, a radio galaxy and an optically-obscured AGN. The reddened quasar is at a redshift z=2.61 and shows a very large X-ray absorbing column of N_H approx 3.10^23 cm^-2. This contrasts with the relatively small amount of dust reddening, implying a gas-to-dust ratio along the line-of-sight a hundred times greater than that of the Milky Way. The radio galaxy at z=1.57 shows only narrow emission lines, but has a surprisingly soft X-ray spectrum. The softness of this spectrum either indicates an unusually low gas-to-dust ratio for the absorbing medium or X-ray emission related to the young radio source. The host galaxy is extremely red (R-K=6.4) and its optical/near-IR spectrum is best fit by a strongly reddened (A_V~2) starburst. The third X-ray source discussed is also extremely red (R-K=6.1) and lies in a close grouping of three other R-K>6 galaxies. No emission or absorption lines were detected from this object, but its redshift (and that of one of the nearby galaxies) are constrained by SED-fitting to be just greater than z=1. The extremely red colours of these two galaxies can be accounted for by old stellar populations. These observations illustrate the diverse properties of hard X-ray selected AGN.

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arXiv2002

A SCUBA galaxy in the Protocluster around 53W002 at z=2.4

We analyse an 850-um SCUBA map of the environment of the z=2.39 radio galaxy 53W002, which has been shown to reside in an over-density of Ly-alpha detected galaxies. We identify four luminous submillimeter (submm) sources within a 2.3' (1.2Mpc at z=2.39) diameter area around the radio galaxy (which itself is a weak submm source). We employ a 1.4-GHz map to accurately locate the counterpart of one of these sources, SMMJ17142+5016, and identify this source with a narrow-line AGN with an extended Ly-alpha halo at z=2.390 which is member of the structure around 53W002. Hence SMMJ17142+5016 is the first spectroscopically-confirmed, submm-selected companion to a high-redshift radio galaxy. We discuss the OHS JHK spectrum of this galaxy and in addition present five new constraints on its spectral energy distribution longward of 1um, using these to estimate its bolometric luminosity as ~8x10^12 Lo, or a star formation rate of ~1000Mo/yr if young stars provide the bulk of the luminosity. This result provides direct support for the statistical detection of over-densities of SCUBA galaxies around high-redshift radio galaxies and confirms theoretical predictions that SCUBA galaxies, as the progenitors of massive ellipticals, should be strongly clustered in the highest density regions of the distant Universe.

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