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L. Pentericci

Publications and source records attributed to L. Pentericci.

232 records · Page 13Linked to original sources
arXiv2002

The most distant structure of galaxies known: a protocluster at z=4.1

Imaging and spectroscopy with the Very Large Telescope have revealed 20 Lyman-alpha emitters within a projected distance of 1.3 Mpc and 600 km/s of the luminous radio galaxy TN J1338-1942 at z=4.1. Compared to the field density of Lyman-alpha emitters, this implies an overdensity on the order of 15. The structure has a projected size of at least 2.7 Mpc x 1.8 Mpc and a velocity dispersion of 325 km/s, which makes it the most distant structure known. Using the galaxy overdensity and assuming a bias parameter b = 3 - 5, the mass is estimated to be ~10^15 M_sun. The radio galaxy itself is surrounded by an uniquely asymmetric Lyman-alpha halo. Taken together with our previous data on PKS 1138-262 at z ~ 2.16, these results suggest that luminous radio sources are excellent tracers of high density regions in the early Universe, which evolve into present-day clusters. The statistics of bright radio sources and of concentrations in the Lyman break galaxy population are consistent with the picture that each of those concentrations harbours an active or passive luminous radio source.

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arXiv2002

VLT observations of the z=6.28 quasar SDSS 1030+0524

We present new VLT spectroscopic observations of the most distant quasar known, SDSS J1030+0524 at z=6.28 which was recently discovered by the Sloan Digital Sky Survey. We confirm the presence of a complete Gunn-Peterson trough caused by neutral hydrogen in the intergalactic medium. There is no detectable flux over the wavelength range from 8450 to 8710 A. We set an improved limit on the drop of the flux level blueward of the Ly alpha line: a factor of > 200. Below 8450 A the spectrum shows a rise in flux, with a large fraction (> 60 %) of the total emission produced by few narrow features of transmitted flux. We discuss the obvious proximity effect around this quasar, with the presence of transmitted flux with many absorption features in a region of about 23h^{-1} comoving Mpc. If assuming the surrounding medium were completely neutral, the size of this region would imply a quasar lifetime of \~1.3x10^7 years. We also present near-IR spectroscopy of both SDSS J1030+0524 and of SDSS J1306+05, the second most distant quasar known at redshift 6.0. We combine measurements of the CIV line and limits on the HeII emission with the NV line measurements from the optical spectra to derive line ratios, and by implication the abundances of these early quasar environments. The results are indistinguishable from those of lower redshift quasars and indicate little or no evolution in the abundances from z ~ 6 to z ~ 2. The line ratios suggest supersolar metallicities, implying that the first stars around the quasars must have formed at least a few hundreds of Myrs prior to the observation, i.e. at redshift higher than 8.

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arXiv2001

The X-ray -- radio alignment in the z = 2.2 radio galaxy PKS 1138--262

We present high resolution X-ray observations of the narrow line radio galaxy PKS 1138-262 at z = 2.156 with the ACIS-S detector on the Chandra observatory. These observations show that the X-ray emission from 1138-262 is dominated by emission from the AGN with a (rest frame) 2 to 10 keV luminosity of 4x10^{45} erg/s. The relative X-ray and radio properties of the AGN in 1138-262 are similar to those seen for the AGN in the archetype powerful radio galaxy Cygnus A. Between 10% and 25% (depending on energy) of the X-ray emission from 1138--262 is spatially extended on scales of 10'' to 20''. The extended X-ray emission is elongated, with a major axis aligned with that of the radio source. While the X-ray and radio emissions are elongated on similar scales and position angles, there is no one-to-one correspondence between the radio and X-ray features in the source. The most likely origin for the extended X-ray emission in 1138-262 is thermal emission from shocked gas, although we cannot rule-out a contribution from inverse Compton emission. If the emission is from hot gas, the gas density is 0.05 cm^{-3} and the gas mass is 2.5x10^{12} M_solar. The pressure in this hot gas is adequate to confine the radio emitting plasma and the optical line emitting gas. We set an upper limit of 1.5x10^{44} erg/s to the 2 to 10 keV luminosity of any normal cluster atmosphere associated with 1138-262. No emission was detected from any of the Ly alpha emitting galaxies in the (proto-) cluster around 1138-262, outside of the Ly alpha halo of 1138-262 itself, to a 2 to 10keV luminosity limit of 1.2x10^{43}erg/s.

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arXiv2001

The population of galaxies in the forming cluster around the radio galaxy MRC 1138-262 at z=2.2

We have recently discovered a forming cluster around the radio galaxy MRC 1138-262 at redshift 2.2. Besides the population of Ly alpha emitting galaxies that have been confirmed spectroscopically, we have detected many candidate H alpha emitters that seem to have a different spatial distribution from the other galaxies: they are more clustered towards the center of the cluster and seem to be distributed along the same direction as the radio source. We present here the characteristics of the Ly alpha and H alpha emitters and study the nature of these populations.

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arXiv2001

A search for high redshift clusters associated with radio galaxies at 2 < z < 4

High redshift radio galaxies are amongst the most massive galaxies in the early Universe and have properties expected from central galaxies in forming clusters. We are carrying out an observational programme on the VLT to find and study galaxy proto clusters around radio galaxies at redshifts 2 < z < 4. First, we use narrow band imaging to select candidate galaxies which show excess Lyman alpha emission at redshifts similar to the central radio galaxy. Then, we use multi object spectroscopy to confirm the redshifts of these candidates and measure the velocity dispersion of the cluster members. Our goal is to observe a sample of about 10 targets and investigate galaxy overdensities as a function of redshift. Here, we report on the current progress of the programme and show some preliminary results which include the discovery of a structure of galaxies at redshift 4.1.

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arXiv2001

Observations of radio galaxy MRC 1138-262: merging galaxies embedded in a giant Lyman alpha halo

The radio galaxy MRC 1138-262 at z=2.16 is most likely a brightest cluster galaxy in an early stage of evolution. Here we present observations of the luminous emission line halo and the stellar components of this radio galaxy. Optical narrow band imaging shows a very extended (about 160 kpc) and luminous Lyman alpha halo. Infrared narrow band imaging reveals a much smaller H alpha halo with a very different morphology. We advocate a model in which the inner part of the halo is photoionized by direct AGN illumination or by UV photons from young stars. Far from the nucleus (about 25 kpc), there is a region of greatly enhanced Lyman alpha emission. At this location, it is likely that shock ionization is important as indicated by a bend in the radio jet. Spectroscopy of several continuum clumps in the halo shows that, although there are striking differences between the emission and absorption features of the spectra of various regions, they have properties similar to those of Lyman-break galaxies. This is further evidence for a scenario in which massive galaxies form hierarchically from smaller building blocks.

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arXiv2001

NICMOS observations of high redshift radio galaxies: witnessing the formation of bright elliptical galaxies?

We present the results of a near infrared imaging program of a sample of 19 radio galaxies with redshift between 1.7 and 3.2, using the NICMOS Camera 2 on the Hubble Space Telescope. The galaxies were observed in H-band which, for 18 of the 19 targets, samples the rest-frame optical emission longwards of the 4000 A break. The images show a wide range of morphologies, from unresolved to compact systems, to systems with substructures such as multiple emission peaks, to systems comprised of several components spread over large areas. The morphologies of 5 of the lowest redshift targets are well represented by a de Vaucouleurs profile, consistent with them being elliptical galaxies. Their average effective radius is a factor of 2 smaller than that of z=1 3CR radio galaxies at similar restframe wavelengths. The near infrared continuum light is generally well aligned with the radio axis and the aligned light has typical V-H colors of around 3.5-4. For several galaxies where WFPC2 images were available, we computed a high resolution map of the optical-to-infrared spectral indices: all multi-component systems present net color gradients between the various clumps. We argue that in these galaxies the continuum emission has probably a stellar origin. Finally in many cases we observe nearby faint objects close to the radio sources. The number density of these faint objects is slightly higher than what is found in the deep NICMOS parallel observations of random fields. These objects also tend to be aligned with the radio sources axis, suggesting that they may be related to the presence of the AGN. (shortened astro-ph version)

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arXiv2000

A search for clusters at high redshift - II. A proto cluster around a radio galaxy at z=2.16

VLT spectroscopic observations of Ly alpha--excess objects in the field of the clumpy radio galaxy 1138-262 at z=2.16 have led to the discovery of 14 galaxies and one QSO at approximately the same distance as the radio galaxy. All galaxies have redshifts in the range 2.16 \pm 0.02, centered around the redshift of the radio galaxy, and are within a projected physical distance of 1.5 Mpc from it. The velocity distribution suggests that there are two galaxy subgroups having velocity dispersions of ~500 km/s and ~300 km/s and a relative velocity of 1800 km/s. If these are virialized structures, the estimated dynamical masses for the subgroups are ~9 and ~3x10E13 solar masses respectively, implying a total mass for the structure of more than 10E14 solar masses. The new observations, together with previous results, suggest that the structure of galaxies around 1138-262 is likely to be a forming cluster.

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arXiv2000

VLA radio continuum observations of a new sample of high redshift radio galaxies

We present new deep multi-frequency radio-polarimetric images of a sample of high redshift radio galaxies (HzRGs), having redshift between 1.7 and 4.1. The radio data at 4.7 and 8.2 GHz were taken with the Very Large Array in the A configuration and provide a highest angular resolution of 0.2''. Maps of total intensity, radio spectral index, radio polarization and internal magnetic field are presented for each source. The morphology of most objects is that of standard FRII double radio sources, but several contain multiple hot-spots in one or both lobes. Compared to similar samples of HzRGs previously imaged, there is a higher fraction (29%) of compact steep spectrum sources (i.e. sources with a projected linear size less than 20 kpc). Radio cores are identified in about half of the sample and tend to have relatively steep spectra (alpha < -1). Polarization is detected in all but 4 sources, with typical polarization at 8.2 GHz of around 10-20%. The Faraday rotation can be measured in most of the radio galaxies: the observed rotation measure (RM) of 8 radio sources exceeds 100 rad m^{-2} in at least one of the lobes, with large gradients between the two lobes. We find no dependence of Faraday rotation with other properties of the radio sources. If the origin of the Faraday rotation is local to the sources, as we believe, then the intrinsic RM is more than a 1000 rad m^{-2}. Because low redshift radio galaxies residing at the center of clusters usually show extreme RMs, we suggest that the high-z large RM sources also lie in very dense environments. Finally, we find that the fraction of powerful radio galaxies with extreme Faraday rotation increases with redshift, as would be expected if their average environment tends to become denser with decreasing cosmic epoch.

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arXiv2000

Search for clusters at high redshift - I. Candidate Lya emitters near 1138-262 at z=2.2

Radio, optical and X-ray observations of the powerful radio galaxy PKS 1138-262 at z=2.156 have suggested that this galaxy is a massive galaxy in the center of a forming cluster. We have imaged 1138-262 and the surrounding 38 square arcminute field with the Very Large Telescope in a broad band and a narrow band encompassing the redshifted Lya emission. We detect 50 objects with rest equivalent width larger than 20 A and a luminous, highly extended Lya halo around 1138-262. If the radio galaxy is at the center of a forming cluster, these objects are candidate Lya emitting cluster galaxies.

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arXiv1999

Deep Lyman alpha imaging of radio galaxy 1138-262 at redshift 2.2

Observations of the powerful radio galaxy 1138-262 at z=2.2 suggest that this galaxy is a massive galaxy in the center of a forming cluster. We have imaged the field of 1138-262 with the Very Large Telescope in a narrow band encompassing the redshifted Lyman alpha emission. We detect 34 probable line emitters in the 6'x6' field. These are candidate Lyman alpha emitting galaxies in the supposed cluster. The observations also reveal that the radio galaxy is enclosed by a very extended Lyman alpha halo (160 kpc).

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arXiv1998

Dust in 3C324

The results of a deep submillimetre observation using SCUBA of the powerful radio galaxy 3C324, at redshift z=1.206, are presented. At 850 microns, emission from the location of the host radio galaxy is marginally detected at the 4.2 sigma level, 3.01 +/- 0.72 mJy, but there is no detection of emission at 450 microns to a 3 sigma limit of 21 mJy. A new 32 GHz radio observation using the Effelsberg 100m telescope confirms that the sub-millimetre signal is not associated with synchrotron emission. These observations indicate that both the mass of warm dust within 3C324, and the star formation rate, lie up to an order of magnitude below the values recently determined for radio galaxies at z = 3 to 4. The results are compared with dust masses and star formation rates derived in other ways for 3C324.

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arXiv1998

HST images and properties of the most distant radio galaxies

We present Hubble Space Telescope images of 11 high redshift radio galaxies (between $z=2.3$ and $z=3.6$). The galaxies were observed with the WFPC2 camera in a broad band filter (F606W or F707W, roughly equivalent to V or R-band), for 2 orbits each. We find that on the scale of the HST observations there is a wide variety of morphological structures of the hosting galaxies: most objects have a clumpy, irregular appearance, consisting of a bright nucleus and a number of smaller components, suggestive of merging systems. Some observed structures could be due (at least partly) to the presence of dust distributed through the galaxies. The UV continuum emission is generally elongated and aligned with the axis of the radio sources, however the characteristics of the ``alignment effect'' differ from case to case, suggesting that the phenomenon cannot be explained by a single physical mechanism. We compare the properties of our radio galaxies with those of the UV dropout galaxies and conclude that (i) the most massive radio galaxies may well evolve from an aggregate of UV dropout galaxies and (ii) high redshift radio galaxies probably evolve into present day brightest cluster galaxies.

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arXiv1998

Erratum for: An X-ray Cluster at Redshift 2.156?

A third possible explanation for the X-ray emission from the z=2.156 radio galaxy 1138-262 is thermal emission from a very dense, sub-cluster `halo', perhaps associated with the (forming) cD galaxy on a scale < 100 kpc. For instance, increasing the gas density from 0.01 cm^{-3} to 0.1 cm^{-3} would decrease the required hot gas mass by the same factor, and could possibly alleviate constraints on cosmological structure formation models. The pressure in this gas would be very high (10^{-9} dynes cm^{-2}), comparable to the pressure in the optical line emitting nebulosity and to the minimum pressure in the radio source, and the cooling time would be short (< fewx10^{8} years). Circumstantial evidence for such very dense, hot gas enveloping some high z radio sources has been reviewed by Fabian et al. (1986). Fabian (1991) suggests that in some cases the `inferred pressures are close to the maximum that can be obtained by gas cooling in a potential well of a galaxy' (ie. the cooling time = gravitational free-fall time), and he designates such systems as `maximal cooling flows', with implied cooling flow rates up to 2000 M_solar year^{-1}. High resolution X-ray imaging with AXAF should be able to test whether 1138-262 has a normal cluster atmosphere, a `maximal cooling flow', or an unusually X-ray loud AGN.

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arXiv1997

An X-ray Cluster at Redshift 2.156?

We present X-ray observations of the narrow line radio galaxy 1138-262 at z = 2.156 with the High Resolution Imager (HRI) on ROSAT. Observations at other wave-bands, and in particular extremely high values of Faraday rotation of the polarized radio emission, suggest that the 1138-262 radio source is in a dense environment, perhaps a hot, cluster-type atmosphere. We detect X-ray emission from the vicinity of 1138-262, and we discuss possible origins for this emission. The X-ray, optical, and radio data all favor thermal emission from a hot cluster atmosphere as the mechanism responsible for the X-rays, although we cannot rule out a contribution from the active nucleus. If this interpretation is correct, then 1138-262 becomes the most distant, by far, of known X-ray emitting clusters. The X-ray luminosity for 1138-262 is 6.7+/-1.3 x 10**44 ergs/sec for emitted energies between 2 keV and 10 keV.

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