arXiv subjects

J. Vernet

Publications and source records attributed to J. Vernet.

66 records · Page 4Linked to original sources

arXiv2004

GRAPES, Grism Spectroscopy of the Hubble Ultra Deep Field: Description and Data Reduction

We present deep unbiased spectroscopy of the Hubble Ultra Deep Field (UDF) carried out using the slitless grism spectroscopy mode of the Advance Camera for Surveys on the Hubble Space Telescope (HST). The GRIsm ACS Program for Extragalactic Science (GRAPES) achieves continuum detection as faint as $z_{AB}=27.2$ using 40 orbits ($9.2 \times 10^4$ seconds) on HST. The data were taken at four orientation angles to correct for the overlap of spectra. GRAPES data provide a unique, uninterrupted, low resolution (R=100) spectral coverage for $5500Å< λ< 10500Å$, and allow us to detect high redshift galaxies at $4 < z < 7$ whether they have $\lya$ lines or just show the Lyman Break, as well as find low luminosity AGNs in an unbiased fashion. This paper describes in detail the observations and the data reduction, and examines the quality of the extracted spectra. Subsequent papers will deal with the analysis of the data. The extracted and calibrated GRAPES spectra will be available from MAST at STScI.

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arXiv2003

The manifold spectra and morphologies of EROs

Deep VLT optical spectroscopy, HST+ACS (GOODS) imaging and VLA observations are used to unveil the nature of a complete sample of 47 EROs with R-Ks>5 and Ks<20. The spectroscopic redshift completeness is 62%. Morphological classification was derived for each ERO through visual inspection and surface brightness profile fitting. Three main ERO morphological types are found: E/S0 galaxies (30-37%), spiral-like (24-46%) and irregular systems (17-39%). The only ERO detected in the radio is likely to host an obscured AGN. The average radio luminosity of the star-forming EROs undetected in the radio implies star formation rates of the order of 33 Msun yr^-1. The colors, redshifts and masses of the E/S0 galaxy subsample imply a minimum formation redshift z_f \sim 2. With this z_f there is enough time to have old and massive stellar spheroids already assembled at z \sim 1. We verify that the R-Ks vs. J-Ks color diagram is efficient in segregating old and dusty-star-forming EROs.

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arXiv2003

Kinematically quiet halos around z~2.5 radio galaxies. Keck spectroscopy

We present the results of the kinematic study of the extended gas in a sample of 10 high redshift radio galaxies (z~2.5) based on high signal to noise Keck II and VLT long slit spectroscopy. In addition to the typical high surface brightness kinematically perturbed regions (FWHM and velocity shifts >1000 km/s), we find in all objects giant low surface brightness halos which show quieter kinematics with typical emission line FWHM and velocity shifts of several hundred km/s. The giant halos often extend for more than 100 kpc and sometimes beyond the radio structures. Their emission line specta are typical of high redshift active galaxies. Continuum is also often detected. The halos are enriched with heavy elements at tens of kpc from the active nucleus. The quasar continuum is the dominant source of ionization of the quiescent halos along the radio axis. The detection of giant quiescent halos in all objects suggests that they could be a common ingredient of high redshift radio galaxies. The radio galaxies seem to be embedded within the halos. The nature, origin of the halos and the cosmological implications are also discussed.

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arXiv2003

Intrinsic and dust-induced polarization in gamma-ray burst afterglows: the case of GRB 021004

Polarization measurements for the optical counterpart to GRB 021004 are presented and discussed. Our observations were performed with the TNG and the VLT-UT3 (Melipal) during the first and fourth night after the gamma-ray burst discovery. We find robust evidence of temporal evolution of the polarization, which is therefore, at least partially, intrinsic to the optical transient. We do not find convincing evidence of wavelength dependence for the intrinsic polarization of the transient, in agreement with current polarization models for optical afterglows. We discuss the role of dust, both in our galaxy and in the host, in modifying the transmitted polarization vector, showing how a sizable fraction of the observed polarized flux is due to Galactic selective extinction, while it is not possible to single out any clear contribution from dust in the host galaxy. We discuss how our data compare to those obtained by different groups showing that a two-component model is required to describe the complete dataset. This is not surprising given the complex lightcurve of GRB 021004.

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arXiv2003

Near-IR bright galaxies at z~2. Entering the spheroid formation epoch ?

Spectroscopic redshifts have been measured for 9 K-band luminous galaxies at 1.7 < z < 2.3, selected with Ks < 20 in the "K20 survey" region of the Great Observatories Origins Deep Survey area. Star formation rates (SFRs) of ~100-500 Msun/yr are derived when dust extinction is taken into account. The fitting of their multi-color spectral energy distributions indicates stellar masses M ~ 10^11 Msun for most of the galaxies. Their rest-frame UV morphology is highly irregular, suggesting that merging-driven starbursts are going on in these galaxies. Morphologies tend to be more compact in the near-IR, a hint for the possible presence of older stellar populations. Such galaxies are strongly clustered, with 7 out of 9 belonging to redshift spikes, which indicates a correlation length r_0 ~ 9-17 h^-1 Mpc (1 sigma range). Current semianalytical models of galaxy formation appear to underpredict by a large factor (about 30) the number density of such a population of massive and powerful starburst galaxies at z ~ 2. The high masses and SFRs together with the strong clustering suggest that at z ~ 2 we may have started to explore the major formation epoch of massive early-type galaxies.

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arXiv2002

Giant low surface brightness halos in distant radio galaxies: USS0828+193

We present results on the spectroscopic study of the ionized gas in the high redshift radio galaxy USS0828+193 at z=2.57. Thanks to the high S/N of the emission lines in the Keck spectrum, we have been able to perform a detailed kinematic study by means of the spectral decomposition of the emission line profiles. This study reveals the existence of two types of material in this object: a) a low surface brightness component with apparent quiescent kinematics consistent with gravitational motions and b) a perturbed component with rather extreme kinematics. The quiescent halo extends across the entire object for ~80 kpc. It is enriched with heavy elements and apparently ionized by the continuum from the active nucleus. The properties of the quiescent halo and its origin are discussed in this paper. We propose that it could be part of a structure that surrounds the entire object, although its nature is not clear (a rotating disc? low surface brightness satellites? a cooling flow nebula? material ejected in galactic winds? other?).

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arXiv2000

Radio galaxies at z~2.5: results from Keck spectropolarimetry

In classifying the ensemble of powerful extragalactic radio sources, considerable evidence has accumulated that radio galaxies and quasars are orientation-dependent manifestations of the same parent population: massive spheroidal galaxies containing correspondingly massive black holes. [...] In this study, we exploit the opportunity to eliminate the quasar glare by performing sensitive spectropolarimetry with the Keck II telescope of a sample of radio galaxies with redshifts around 2.5. [...] We show that dust-reflected quasar light generally dominates the restframe ultraviolet continuum of these sources and that a highly clumped scattering medium results in almost grey scattering of the active galactic nucleus photons. The observations, however, do not exclude a substantial star formation rate averaged over a Gyr of evolution. The sub-mm reradiation from the scattering dust is likely to represent only a small fraction (~10%) of the total far infrared luminosity. An analysis of the emission lines excited in the interstellar medium of the host galaxy by the hard quasar radiation field reveals evidence of a dramatic chemical evolution within the spheroid during this epoch. Secondary nitrogen production in intermediate mass stars produces a characteristic signature in the NV/CIV and NV/HeII line ratios which has been seen previously in the broad line region of quasars at similar redshifts. We find intriguing correlations between the strengths of the Ly alpha and NV emission lines and the degree of ultraviolet continuum polarization which may represent the dispersal of dust associated with the chemical enrichment of the spheroid.

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arXiv2000

Interpreting the kinematics of the extended gas in distant radiogalaxies from 8-10m telescope spectra

The nature of the extreme kinematics in the extended gas of distant radio galaxies (z>0.7) is still an open question. With the advent of the 8-10 m telescope generation and the development of NIR arrays we are in the position for the first time to develop a more detailed study by using lines other than Lya and [OII]3727 depending on redshift. In this paper we review the main sources of uncertainty in the interpretation of the emission line kinematics: the presence of several kinematic components, Lya absorption by neutral gas/dust and the contribution of scattered light to some of the lines. As an example, several kinematic components can produce apparent, false rotation curves. We propose methods to solve these uncertainties. We propose to extend the methods applied to low redshift radio galaxies to investigate the nature of the kinematics in distant radio galaxies: by means of the spectral decomposition of the strong optical emission lines (redshifted into the NIR) we can isolate the different kinematic components and study the emission line ratios for the individual components. If shocks are responsible for the extreme kinematics, we should be able to isolate a kinematic component (the shocked gas) with large FWHM (>900 km/s), low ionization level [OIII]5007/Hb~2-4 and weak HeII4686/Hb<0.07, together with a narrow component (~few hundred km/s) with higher ionization level and strong HeII emission (HeII/Hb~0.5)

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arXiv2000

Gas metallicities and early evolution of distant radiogalaxies

By modeling the rich emission line spectra of a sample of high redshift (HzRG, z\sim 2.5) radio galaxies we find that solar and supersolar metallicities are common in the extended gas of these objects. Our models and the comparison with high redshift quasars suggest that HzRG at z\sim 2.5 are associated with intense star formation activity. This is consistent with chemical evolution models for giant ellipticals and it supports the idea that distant powerful radio galaxies are progenitors of giant ellipticals. We might be witnessing different evolutive status in different objects.

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arXiv1999

The NV/CIV ratio in high redshift radio galaxies

Deep Keck spectropolarimetry has been obtained of a sample of powerful radio galaxies with $z \sim 2.5 \pm 1$. In addition to a clear 2200Ådust scattering/extinction signature and an inverse correlation between the degree of continuum polarization and the Lyman-αemission line strength (relative to C IV), we find an intriguing positive correlation between the N~V/C IV ratio and the polarization. The line ratio, which varies by almost a dex amongst objects having a very similar ionization state, is most likely to indicate an abundance ratio. We speculate that the correlation with polarization represents a connection between nitrogen enhancement and dust production following a major starburst event connected with the birth of the quasar.

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arXiv1998

Optical continuum structure of Cygnus A

As a prerequisite for interpreting new observations of the most distant radio galaxies, we make an optical study of the closest powerful radio source, Cygnus A. Using Keck imaging- and spectro-polarimetry in conjunction with HST, WFPC2 broad and narrow band imaging, we are able to identify specific geometrical structures in the galaxy with optical continuum components distinguished by both colour and polarimetric properties. A 4 kpc diameter dusty ring of young stars forms the equator of a double ionization cone which is co-axial with the radio jets.

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arXiv1997

An Anomalous UV Extension in NGC6251

Deep U-band FOC images of the nuclear region of NGC6251 have revealed a region of extended emission which is most probably radiation scattered from a continuum source in the nucleus. This radiation lies interior to a dust ring, is nearly perpendicular to the radio jet axis, and is seen primarily in the FOC U and b filters. The extension has a low observed polarization($\le 10%$), and is unlikely to arise from line emission. We know of no other examples similar to what we have found in NGC 6251, and we offer some tentative explanations. The nuclear morphology shows clear similarities to that seen in the nucleus of NGC 4261 except for the extended U-band radiation.

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