arXiv subjects

Sandra Savaglio

Publications and source records attributed to Sandra Savaglio.

45 records · Page 3Linked to original sources

arXiv1999

Hubble Space Telescope Observations of the Associated Absorption Systems in Q0122+0338

We have studied a spectrum of Q0122+0338 (z = 1.202) obtained by the FOS on board HST. We present the analysis of three associated absorption systems at z = 1.207, 1.199 and 1.166. The most complex of these at z = 1.207 shows strong absorption from the highly ionized transitions of Lyman alpha, Lyman beta, N V, O VI, Si III, Si IV, and possibly P V. We derive minimal ionic column densities and compare them with those predicted from numerical photoionization models. We find that conditions in the absorbing gas are consistent with an absorber with metallicity twice solar and a total absorbing column density of N(H) = 2*10^19 cm^2. The kinematics of the absorption lines in the z = 1.207 system suggest that a correlation exists between the relative velocity and the creation ionization potential energy for each transition. This is evidence that a complex, multi-component absorber exists. Althought the location of the absorber is uncertain, we consider the origin of this absorption system using the available data and discuss how the high-ionization, and high-metallicity indicate that the absorber may be intrinsic to Q0122+0338.

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arXiv1999

The power spectrum of the Lyman-alpha clouds

We investigate the clustering properties of 13 QSO lines of sight in flat space, with average redshifts from z~2 to 4. We estimate the 1-D power spectrum and the integral density of neighbours, and discuss their variation with respect to redshift and column density. We compare the results with standard CDM models, and estimate the power spectrum of Lyman-alpha clustering as a function both of redshift and column density. We find that a) there is no significant periodicity or characteristic scale; b) the clustering depends both on column density and redshift; c) the clustering increases linearly only if at the same time the HI column density decreases strongly with redshift. The results remain qualitatively the same assuming an open cosmological model.

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arXiv1998

The metal absorption systems of the Hubble Deep Field South QSO

The Hubble Deep Field South (HDFS) has been recently selected and the observations are planned for October 1998. We present a high resolution (FWHM $\simeq 14$ \kms) spectrum of the quasar J2233--606 ($z_{em}\simeq2.22$) which is located 5.1 arcmin East of the HDFS. The spectrum obtained with the New Technology Telescope redward of the Lyman--$α$ emission line covers the spectral range 4386--8270 Å. This range corresponds to redshift intervals for CIV and MgII intervening systems of $z=1.83-2.25$ and $z=0.57-1.95$ respectively. The data reveal the presence of two complex intervening CIV systems at redshift $z=1.869$ and $z=1.943$ and two complex associated ($z_{abs} \approx z_{em}$) systems. Other two CIV systems at $z=1.7865$ and $z=2.077$, suggested by the presence of strong Lyman--$α$ lines in low resolution ground based and Hubble Space Telescope (HST) STIS observations (Sealey et al. 1998) have been identified. The system at $z=1.943$ is also responsible for the Lyman limit absorption seen in the HST/STIS spectrum. The main goal of the present work is to provide astronomers interested in the Hubble Deep Field South program with information related to absorbing structures at high redshift, which are distributed along the nearby QSO line of sight. For this purpose, the reduced spectrum, obtained from three hours of integration time, has been released to the astronomical community.

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arXiv1997

Damped Lyman-alpha Absorption Associated with an Early-Type Galaxy at Redshift z = 0.16377

We report new HST and ground-based observations of a damped Lyman-alpha absorption system toward the QSO 0850+4400. The redshift of the absorption system is z = 0.163770 and the neutral hydrogen column density of the absorption system is log N = 19.81 cm**-2. The absorption system is by far the lowest redshift confirmed damped Lyman-alpha absorption system yet identified, which provides an unprecedented opportunity to examine the nature, impact geometry, and kinematics of the absorbing galaxy in great detail. The observations indicate that the absorption system is remarkable in three respects: First, the absorption system is characterized by weak metal absorption lines and a low metal abundance, possibly less than 4% of the solar metal abundance. This cannot be explained as a consequence of dust, because the neutral hydrogen column density of the absorption system is far too low for obscuration by dust to introduce any significant selection effects. Second, the absorption system is associated with a moderate-luminosity early-type S0 galaxy, although the absorption may actually arise in one of several very faint galaxies detected very close to the QSO line of sight. Third, the absorbing material moves counter to the rotating galaxy disk, which rules out the possibility that the absorption arises in a thin or thick co-rotating gaseous disk. These results run contrary to the expectation that low-redshift damped Lyman-alpha absorption systems generally arise in the gas- and metal-rich inner parts of late-type spiral galaxies. We suggest instead that mounting evidence indicates that low-redshift galaxies of a variety of morphological types may contain significant quantities of low metal abundance gas at large galactocentric distances.

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arXiv1996

Multifractal structure of Ly$α$ clouds: An example with the spectrum of QSO 0055--26

Ly$α$ forests are usually associated with intergalactic gas clouds intercepting the quasar sight--line. Using as an example the Ly$α$ forest of QSO 0055--26 ($z_{em} = 3.66$), we show that the probability of observing a line at the velocity difference $Δv$ has an intermittent structure in the redshift space. On small scales (70 km~s$^{-1} \leq Δv \leq 1580$ km~s$^{-1}$) the signature of intermittency appears as a self--similar structure of spikes systematically visible at the same redshift on all scales. This behaviour can be interpreted as due to the presence of clustering of the Ly$α$ lines, which appear as singularities of the density of the probability measure. From a direct measurement of the generalized dimensions, we show that the intermittency can be described by a multifractal structure which is due to a Fourier phase correlation of the signal. The multifractal structure disappears at scales larger than $Δv \simeq 1580$ km~s$^{-1}$.

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arXiv1996

The Optical Identification of a Primeval Galaxy at z >~ 4.4

We have obtained with the SUSI CCD camera on the ESO 3.5m NTT deep images in the BVRI bands of the field centered on the QSO BRI 1202-0725 ($z_{\it em}=4.694$). In the final combined frames the stellar images have FWHM of 1,1,0.6 and 0.65 arcsec respectively. The R and I images show clearly a galaxy $2.2''$ from the QSO, corresponding to $13h^{-1}_{50}$ kpc at $z\sim 4.5$. Possible identification with the metal absorption systems seen in the line of sight to the QSO, including the highest redshift damped system known to date at $z=4.383$, are discussed. We conclude that its colours can be reconciled only with the spectrum of a primeval galaxy at z >~ 4.4, making it the most distant galaxy detected so far. From its magnitudes and models of young galaxy evolution we deduce that it is forming stars at a rate $\sim 30 M_{\odot}$ yr$^{-1}$ and has an estimated age of the order of $10^8$ yr or less, implying that the bulk of the stellar population formed at $z < 6$.

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arXiv1995

Abundances in the Lyman--alpha clouds

We have re--examined the chemical composition of \lya clouds using the composite--cloud technique, in which each \lya line in a spectrum is shifted to its rest frame wavelength and all rest frame spectra are co--added to form an `averaged' \lya cloud spectrum. We illustrate how various estimates of the spectrum and redshift evolution of the background ionizing UV flux lead to very different predictions for the relative strengths of heavy element lines in the forest clouds. We also show how the potential abundance limits depend on the various observational quantities and how different procedures may be required for OVI and CIV. Preliminary results are presented from an analysis of two high redshift QSO echelle spectra.

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arXiv1994

Influence of Strings with Axionic Content on the Polarization of Extragalactic Sources

It is shown that the axion concentrated in electroweak strings which are consequently stabilized may produce a rotation of the polarization angle of radiation from extragalactic sources larger than that caused by a background field, and that axionic walls attached to global strings have an effect which depends on the frequency. We discuss the validity of the classical and quantum treatments of radiation and indicate that the latter produces conversion of linear into circular polarization. We describe possible anomalies in the observations which might suggest these influences of the axion.

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arXiv1993

The metal systems in Q0000--2619 at high resolution

We have obtained high, 11 and 14 \kms, and medium, 40 and 53 \kms, resolution spectra of the $z_{em} = 4.11$ quasar Q0000--2619 covering the range 4400 Å to 9265 Å. We identify nine metal absorption systems, of which four were previously known. A fifth previously suggested system at $z_{abs} \approx 3.409$ (Turnshek et al~ 1991) is ruled out by our data. Two of the eight systems for which the \lya~ line is in the observable range have a damped \lya~ line. Six of the nine systems show evidence for complex sub--component structure. At our resolution and S/N we identify a total of 21 sub--components in the nine systems. Five of the nine systems (11 of the 21 components) fall within the $\pm 5000$ \kms~ range of the emission redshift, and are hence classified as \zae~ absorbers. For the two damped systems we find metal abundances of $\leq 1$% and $\leq 8$% of solar values at redshifts of 3.0541 and 3.3901 respectively. These upper limits are consistent with what would be expected from previous determinations at lower redshifts, and our data are hence compatible with earlier conclusions that no evidence is yet found for chemical evolution of intervening damped and Lyman limit absorbers. For the \zae~ systems we found indications of metallicities comparable to, and even in excess of solar values. These much higher values compared to the damped systems, are in favour of the intrinsic hypothesis for these systems.

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