arXiv subjects

Huub Röttgering

Publications and source records attributed to Huub Röttgering.

52 records · Page 3Linked to original sources

arXiv2013

A diversity of dusty AGN tori: Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies

The AGN-heated dust distribution (the "torus") is increasingly recognized not only as the absorber required in unifying models, but as a tracer for the reservoir that feeds the nuclear Super-Massive Black Hole. Yet, even its most basic structural properties (such as its extent, geometry and elongation) are unknown for all but a few archetypal objects. Since most AGNs are unresolved in the mid-infrared, we utilize the MID-infrared interferometric Instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) that is sensitive to structures as small as a few milli-arcseconds (mas). We present here an extensive amount of new interferometric observations from the MIDI AGN Large Program (2009 - 2011) and add data from the archive to give a complete view of the existing MIDI observations of AGNs. Additionally, we have obtained high-quality mid-infrared spectra from VLT/VISIR. We present correlated and total flux spectra for 23 AGNs and derive flux and size estimates at 12 micron using simple axisymmetric geometrical models. Perhaps the most surprising result is the relatively high level of unresolved flux and its large scatter: The median "point source fraction" is 70 % for type 1 and 47 % for type 2 AGNs meaning that a large part of the flux is concentrated on scales smaller than about 5 mas (0.1 - 10 pc). Among sources observed with similar spatial resolution, it varies from 20 % - 100 %. For 18 of the sources, two nuclear components can be distinguished in the radial fits. While these models provide good fits to all but the brightest sources, significant elongations are detected in eight sources. The half-light radii of the fainter sources are smaller than expected from the size ~ L^0.5 scaling of the bright sources and show a large scatter, especially when compared to the relatively tight size--luminosity relation in the near-infrared.

astro-ph.CO↗

arXiv2013

Probing the nature of compact ultra-steep spectrum radio sources with the e-EVN and e-MERLIN

We present first results from electronic Multi-Element Remotely Linked Interferometer Network (e-MERLIN) and electronic European VLBI Network (e-EVN) observations of a small sample of ultra-steep spectrum radio sources, defined as those sources with a spectral index alpha < -1.4 between 74 MHz and 325 MHz, which are unresolved on arcsecond scales. Such sources are currently poorly understood and a number of theories as to their origin have been proposed in the literature. The new observations described here have resulted in the first detection of two of these sources at milliarcsecond scales and show that a significant fraction of ultra-steep spectrum sources may have compact structures which can only be studied at the high resolution available with very long baseline interferometry (VLBI).

astro-ph.CO↗

arXiv2012

XMM-Newton observations of the merging galaxy cluster CIZA J2242.8+5301

We studied the intracluster medium of the galaxy cluster CIZA J2242.8+5301 using deep XMM-Newton observations. The cluster hosts a remarkable 2-Mpc long, ~50-kpc wide radio relic that has been nicknamed the "Sausage". A smaller, more irregular counter-relic is also present, along with a faint giant radio halo. We analysed the distribution of the ICM physical properties, and searched for shocks by trying to identify density and temperature discontinuities. East of the southern relic, we find evidence of shock compression corresponding to a Mach number of 1.3, and speculate that the shock extends beyond the length of the radio structure. The ICM temperature increases at the northern relic. More puzzling, we find a "wall" of hot gas east of the cluster centre. A partial elliptical ring of hot plasma appears to be present around the merger. While radio observations and numerical simulations predict a simple merger geometry, the X-ray results point towards a more complex merger scenario.

astro-ph.CO↗

arXiv2012

Impact of Redshift Information on Cosmological Applications with Next-Generation Radio Surveys

In this paper, we explore how the forthcoming generation of large-scale radio continuum surveys, with the inclusion of some degree of redshift information, can constrain cosmological parameters. By cross-matching these radio surveys with shallow optical to near-infrared surveys, we can essentially separate the source distribution into a low- and a high-redshift sample, thus providing a constraint on the evolution of cosmological parameters such as those related to dark energy. We examine two radio surveys, the Evolutionary Map of the Universe (EMU) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN). A crucial advantage is their combined potential to provide a deep, full-sky survey. The surveys used for the cross-identifications are SkyMapper and SDSS, for the southern and northern skies, respectively. We concentrate on the galaxy clustering angular power spectrum as our benchmark observable, and find that the possibility of including such low redshift information yields major improvements in the determination of cosmological parameters. With this approach, and provided a good knowledge of the galaxy bias evolution, we are able to put strict constraints on the dark energy parameters, i.e. w_0=-0.9+/-0.041 and w_a=-0.24+/-0.13, with type Ia supernovae and CMB priors (with a one-parameter bias in this case); this corresponds to a Figure of Merit (FoM) > 600, which is twice better than what is obtained by using only the cross-identified sources and greater than four time better than the case without any redshift information at all.

astro-ph.CO↗

arXiv2012

Near-Infrared Imaging of a z=6.42 Quasar Host Galaxy With the Hubble Space Telescope Wide Field Camera 3

We report on deep near-infrared F125W (J) and F160W (H) Hubble Space Telescope Wide Field Camera 3 images of the z=6.42 quasar J1148+5251 to attempt to detect rest-frame near-ultraviolet emission from the host galaxy. These observations included contemporaneous observations of a nearby star of similar near-infrared colors to measure temporal variations in the telescope and instrument point spread function (PSF). We subtract the quasar point source using both this direct PSF and a model PSF. Using direct subtraction, we measure an upper limit for the quasar host galaxy of m_J>22.8, m_H>23.0 AB mag (2 sigma). After subtracting our best model PSF, we measure a limiting surface brightness from 0.3"-0.5" radius of mu_J > 23.5, mu_H > 23.7 AB magarc (2 sigma). We test the ability of the model subtraction method to recover the host galaxy flux by simulating host galaxies with varying integrated magnitude, effective radius, and Sérsic index, and conducting the same analysis. These models indicate that the surface brightness limit (mu_J > 23.5 AB magarc) corresponds to an integrated upper limit of m_J > 22 - 23 AB mag, consistent with the direct subtraction method. Combined with existing far-infrared observations, this gives an infrared excess log(IRX) > 1.0 and corresponding ultraviolet spectral slope beta > -1.2\pm0.2. These values match those of most local luminous infrared galaxies, but are redder than those of almost all local star-forming galaxies and z~6 Lyman break galaxies.

astro-ph.CO↗

arXiv2011

LOFAR: Recent imaging results & future prospects

The Low Frequency Array (LOFAR) is under construction in the Netherlands and in several surrounding European countries. In this contribution, we describe the layout and design of the telescope, with a particular emphasis on the imaging characteristics of the array when used in its "standard imaging" mode. After briefly reviewing the calibration and imaging software used for LOFAR image processing, we show some recent results from the ongoing imaging commissioning efforts. We conclude by summarizing future prospects for the use of LOFAR in observing the little-explored low frequency Universe.

astro-ph.CO↗

arXiv2011

The Curious Case of Abell 2256

Abell 2256 is a rich, nearby (z=0.0594) galaxy cluster that has significant evidence of merger activity. We present new radio and X-ray observations of this system. The low-frequency radio images trace the diffuse synchrotron emission of the Mpc-scale radio halo and relics as well as a number of recently discovered, more compact, steep spectrum sources. The spectral index across the relics steepens from the north-west toward the south-east. Analysis of the spectral index gradients between low and and high-frequencies shows spectral differences away from the north-west relic edge such that the low-frequency index is significantly flatter than the high frequency spectral index near the cluster core. This trend would be consistent with an outgoing merger shock as the origin of the relic emission. New X-ray data from XMM-Newton reveal interesting structures in the intracluster medium pressure, entropy and temperature maps. The pressure maps show an overall low pressure core co-incident with the radio halo emission, while the temperature maps reveal multiple regions of cool emission within the central regions of Abell 2256. The two cold fronts in Abell 2256 both appear to have motion in similar directions.

astro-ph.CO↗

arXiv2010

Co-evolution of atmospheres, life, and climate

After Earth's origin, our host star, the Sun, was shining 20 to 25 percent less brightly than today. Without greenhouse-like conditions to warm the atmosphere, our early planet would have been an ice ball and life may never have evolved. But life did evolve, which indicates that greenhouse gases must have been present on early Earth to warm the planet. Evidence from the geologic record indicates an abundance of the greenhouse gas CO2. CH4 was probably present as well, and in this regard methanogenic bacteria, which belong to a diverse group of anaerobic procaryotes that ferment CO 2 plus H2 to CH4, may have contributed to modification of the early atmosphere. Molecular oxygen was not present, as is indicated by the study of rocks from that era, which contain iron carbonate rather than iron oxide. Multicellular organisms originated as cells within colonies that became increasingly specialized. The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant oxygen accumulated in the atmosphere and formed the ozone layer in the upper atmosphere. Aided by the absorption of harmful UV radiation in the ozone layer, life colonized Earth's surface. Our own planet is a very good example of how life forms modified the atmosphere over the planets' life time. We show that these facts have to be taken into account when we discover and characterize atmospheres of Earth-like exoplanets. If life has originated and evolved on a planet, then it should be expected that a strong co-evolution occurred between life and the atmosphere, the result of which is the planets' climate.

astro-ph.EP↗

arXiv2009

Dust emission from a parsec-scale structure in the Seyfert 1 nucleus of NGC 4151

We report mid-IR interferometric measurements with \sim 10 mas resolution, which resolve the warm (T = 285 +25 / -50 K) thermal emission at the center of NGC 4151. Using pairs of VLT 8.2 m telescopes with MIDI and by comparing the data to a Gaussian model, we determined the diameter of the dust emission region, albeit only along one position angle, to be 2.0 +/- 0.4 pc (FWHM). This is the first size and temperature estimate for the nuclear warm dust distribution in a Seyfert 1 galaxy. The parameters found are comparable to those in Seyfert 2 galaxies, thus providing direct support for the unified model. Using simple analytic temperature distributions, we find that the mid-infrared emission is probably not the smooth continuation of the hot nuclear source that is marginally resolved with K band interferometry. We also detected weak excess emission around 10.5 micron in our shorter baseline observation, possibly indicating that silicate emission is extended to the parsec scale.

astro-ph.CO↗

arXiv2009

Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey

The emission of warm dust dominates the mid-infrared spectra of active galactic nuclei (AGN). Only interferometric observations provide the necessary angular resolution to resolve the nuclear dust and to study its distribution and properties. The investigation of dust in AGN cores is hence one of the main science goals for the MID-infrared Interferometric instrument MIDI at the VLTI. As the first step, the feasibility of AGN observations was verified and the most promising sources for detailed studies were identified. This was carried out in a "snapshot survey" with MIDI using Guaranteed Time Observations. In the survey, observations were attempted for 13 of the brightest AGN in the mid-infrared which are visible from Paranal. The results of the three brightest, best studied sources have been published in separate papers. Here we present the interferometric observations for the remaining 10, fainter AGN. For 8 of these, interferometric measurements could be carried out. Size estimates or limits on the spatial extent of the AGN-heated dust were derived from the interferometric data of 7 AGN. These indicate that the dust distributions are compact, with sizes on the order of a few parsec. The derived sizes roughly scale with the square root of the luminosity in the mid-infrared, s ~ sqrt(L), with no clear distinction between type 1 and type 2 objects. This is in agreement with a model of nearly optically thick dust structures heated to T ~ 300 K. For three sources, the 10 micron feature due to silicates is tentatively detected either in emission or in absorption. Based on the results for all AGN studied with MIDI so far, we conclude that in the mid-infrared the differences between individual galactic nuclei are greater than the generic differences between type 1 and type 2 objects.

astro-ph.CO↗

arXiv2009

Resolving the obscuring torus in NGC 1068 with the power of infrared interferometry: Revealing the inner funnel of dust

We present new interferometric data obtained with MIDI (MID infrared Interferometric instrument) for the Seyfert II galaxy NGC 1068, with an extensive coverage of sixteen uv points. These observations resolve the nuclear mid-infrared emission from NGC 1068 in unprecedented detail with a maximum resolution of 7 mas. For the first time, sufficient uv points have been obtained, allowing us to generate an image of the source using maximum entropy image reconstruction. The features of the image are similar to those obtained by modelling. We find that the mid-infrared emission can be represented by two components, each with a Gaussian brightness distribution. The first, identified as the inner funnel of the obscuring torus, is hot (800K), 1.35 parsec long, and 0.45 parsec thick in FWHM at a PA=-42 degrees (from north to east). It has an absorption profile different than standard interstellar dust and with evidence for clumpiness. The second component is 3 by 4 pc in FWHM with T=300K, and we identify it with the cooler body of the torus. The compact component is tilted by 45 degrees with respect to the radio jet and has similar size and orientation to the observed water maser distribution. We show how the dust distribution relates to other observables within a few parsecs of the core of the galaxy such as the nuclear masers, the radio jet, and the ionization cone. We compare our findings to a similar study of the Circinus galaxy and other relevant studies. Our findings shed new light on the relation between the different parsec-scale components in NGC 1068 and the obscuring torus.

astro-ph.GA↗

arXiv2008

Radio-loud high-redshift protogalaxy candidates in Bootes

We used the Near Infrared Camera on Keck I to obtain Ks-band images of four candidate high-redshift radio galaxies selected using optical and radio data in the NOAO Deep Wide-Field Survey in Bootes. Our targets have 1.4 GHz radio flux densities greater than 1 mJy, but are undetected in the optical to fainter than 24 Vega mag. Spectral energy distribution fitting suggests that three of these objects are at z > 3, with radio luminosities near the FR-I / FR-II break. The other has photometric redshift 1.2, but may in fact be at higher redshift. Two of the four objects exhibit diffuse morphologies in Ks -band, suggesting that they are still in the process of forming.

astro-ph↗

arXiv2005

The CFHT Legacy Survey: The Morphology-Density Relation of Galaxies out to Z~1

We study the relationships between galaxy total luminosity (M_g), morphology, color and environment as a function of redshift. We use a magnitude-limited sample of 65,624 galaxies in the redshift range 0 2 number fractions at z=0.1 agree well with those based on SDSS galaxies. We find that the n>2 galaxy number fraction is constant with redshift in the field. However, for overdense environments this fraction is larger and increases towards lower redshifts, higher densities and higher luminosities. Rest-frame color-magnitude diagrams show that the color distribution is bimodal out to our redshift limit of z~1 with a prominent red-sequence of galaxies at 0.2 1). For all environments, this fraction increases towards lower redshifts and higher luminosities. The red fraction within cluster-like regions changes 60% faster with redshift as compared to the field for M_g<-19.5. Using, for the first time, observations across many cluster-field interfaces distributed over a single, large volume, we trace the large-scale morphology-density relation and the Butcher-Oemler effect over a period of almost 8 Gyr.

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arXiv1999

Ultra-Steep Spectrum Radio Galaxies at Hy Redshifts

Radio sources have traditionally provided convenient beacons for probing the early Universe. Hy Spinrad was among the first of the tenacious breed of observers who would attempt to obtain optical identifications and spectra of the faintest possible `radio galaxies' to investigate the formation and evolution of galaxies at hy redshift. Modern telescopes and instruments have made these tasks much simpler, although not easy, and here we summarize the current status of our hunts for hy redshift radio galaxies (HyZRGs) using radio spectral and near-IR selection.

astro-ph↗

arXiv1998

Extremely Red Galaxies: Old or Dusty ?

The nature of the extremely red galaxies is poorly known, the critical issue being whether they are old ellipticals at $z>1.5$ or distant star-forming galaxies strongly reddened by dust extinction. Here we review the little we know about them, and we present recent and preliminary results from our ongoing submillimeter -- millimeter observations.

astro-ph↗

arXiv1998

Vigorous star formation hidden by dust in a galaxy at $z=1.4$

Near-infrared surveys have revealed a substantial population of enigmatic faint galaxies with extremely red optical-to-near-infrared colours and with a sky surface density comparable to that of faint quasars. There are two scenarios for these extreme colours: (i) these distant galaxies have formed virtually all their stars at very high redshifts and, due to the absence of recently formed stars, the colours are extremely red and (ii) these distant galaxies contain large amounts of dust, severely reddening the rest-frame UV--optical spectrum. HR10 ($z = 1.44$) is considered the archetype of the extremely red galaxies. Here we report the detection of the continuum emission from HR10 at 850$μ$m and at 1250$μ$m, demonstrating that HR10 is a very dusty galaxy undergoing a major episode of star formation. Our result provides a clear example of a high-redshift galaxy where the star formation rate inferred from the ultraviolet luminosity would be underestimated by a factor up to 1000, and shows that great caution should be used to infer the global star formation history of the Universe from optical observations only.

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