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S. Savaglio

Publications and source records attributed to S. Savaglio.

At least 91 records · Page 5Linked to original sources
arXiv2000

Probing the warm Inter-Galactic Medium through absorption against Gamma Ray Bursts X-ray afterglows

Gamma Ray Burst (GRB) afterglows close to their peak intensity are among the brightest X-ray sources in the sky. Despite their fast power-law like decay, when fluxes are integrated from minutes up to hours after the GRB event, the corresponding number counts (logN-logF relation) far exceeds that of any other high redshift (z>0.5) source, the flux of which is integrated over the same time interval. We discuss how to use X-ray afterglows of GRBs as distant beacons to probe the warm (10^5 K =300) would be very well suited to exploit the new diagnostic and study the physical conditions in the Universe at the critical moment when structure is being formed.

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arXiv2000

VLT spectroscopy of GRB 990510 and GRB 990712; probing the faint and bright end of the GRB host galaxy population

We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identification of several absorption lines in the first epoch GRB 990510 spectrum, we determine the redshift for this burst at z >= 1.619. No clear emission lines are detected. The strength of the MgI feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint (V >~ 28), the GRB afterglow allows us to probe its interstellar medium and - in principle - to measure its metallicity. The optical spectrum of GRB 990712 (whose host galaxy is the brightest of the known GRB hosts at cosmological redshifts), shows clear features both in emission and absorption, at a redshift of z = 0.4331 +/- 0.0004. On the basis of several line emission diagnostic diagrams, we conclude that the host galaxy of GRB 990712 is most likely an HII galaxy. We derive a unreddened [OII] star formation rate of 2.7 +/- 0.8 Msun/yr. Correcting for the measured extinction intrinsic to the host galaxy (A(V) = \gpm{3.4}{2.4}{1.7}), this value increases to \gpm{35}{178}{25} Msun/yr. The [OII] equivalent width, compared to that of field galaxies at z <= 1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and Hbeta emission-line intensities to estimate the global oxygen abundance for the host of GRB 990712: log(O/H) = -3.7 +- 0.4, which is slightly below the lowest metallicity one finds in nearby spiral galaxies. For both GRBs we study the time evolution of the absorption lines, whose equivalent width might be expected to change with time if the burst resides in a dense compact medium. We find no evidence for a significant change in the MgII width.

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arXiv2000

High Resolution Observations of X-ray Absorbers/Emitters

We present photoionization and collisional ionization models, and their application to three important fields: (a) the Warm Absorbers/Emitters in type 1 AGN, (b) the Warm Reflectors in Type 2 AGN, and (c) X-ray absorption of background quasars by intergalactic gas. A number of cases are investigated, and the dependences of the main parameters explored.

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arXiv2000

The Metallicity evolution of Damped Lyman-alpha systems

We have collected data for 69 Damped Lyman-alpha (DLA) systems, to investigate the chemical evolution of galaxies in the redshift interval 0.0 < z < 4.4. In doing that, we have adopted the most general approach used so far to correct for dust depletion. The best solution, obtained through chi^2 minimization, gives as output parameters the global DLA metallicity and the dust-to-metals ratio. Clear evolution of the metallicity vs. redshift is found (99.99% significance level), with average values going from ~1/30 solar at z~4.1 to ~3/5 solar at z~0.5. We also find that the majority of DLAs (~60%) shows dust depletion patterns which most closely resemble that of the warm halo clouds in the Milky Way, and have dust-to-metals ratios very close to warm halo clouds.

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arXiv1999

The Hubble Deep Field South - STIS Imaging

We present the imaging observations made with the Space Telescope Imaging Spectrograph of the Hubble Deep Field - South. The field was imaged in 4 bandpasses: a clear CCD bandpass for 156 ksec, a long-pass filter for 22-25 ksec per pixel typical exposure, a near-UV bandpass for 23 ksec, and a far-UV bandpass for 52 ksec. The clear visible image is the deepest observation ever made in the UV-optical wavelength region, reaching a 10 sigma AB magnitude of 29.4 for an object of area 0.2 square arcseconds. The field contains QSO J2233-606, the target of the STIS spectroscopy, and extends 50"x50" for the visible images, and 25"x25" for the ultraviolet images. We present the images, catalog of objects, and galaxy counts obtained in the field.

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arXiv1999

The Lyman-alpha forest of the QSO in the Hubble Deep Field South

The quasar in the Hubble Deep Field South (HDFS), J2233-606 (z=2.23) has been exhaustively observed by ground based telescopes and by the STIS spectrograph on board the Hubble Space Telescope at low, medium and high resolution in the spectral interval from 1120 A to 10000 A. This very large base-line represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDFS campaign. Here we report the main properties of the Lyman-alpha clouds in the intermediate redshift range 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63\pm8 lines with log N_{HI}\geq14.0 are found (including metal systems) at =1.7, to be compared with ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyman-alpha clouds shows a region spanning z=1.383-1.460 (comoving size of 94 h^{-1}_{65} Mpc, Omega_o=1) with a low density of absorption lines; we detect 5 lines in this region, compared with the 16 expected from an average density along the line of sight. The two point correlation function shows a positive signal up to scales of about 3 h^{-1}_{65} Mpc and an amplitude that is larger for larger HI column densities. The average Doppler parameter is about 27 km/s, comparable to the mean value found at z > 3, thus casting doubts on the temperature evolution of the Lyman-alpha clouds.

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arXiv1998

ESO Imagin Survey I. Preliminary Results

This paper presents the preliminary results of the ESO Imaging Survey (EIS), a public survey being carried out by ESO and member states, in preparation for the VLT first-light. The survey goals, organization, strategy and observations are discussed and an overview is given of the survey pipeline developed to handle EIS data and produce object catalogs. A report is presented on moderately deep I-band observations obtained in the first of four patches surveyed, covering a region of 3.2 square degrees centered at R.A. 22h40m and Dec. -40d. The products available to the community, including pixel maps (with astrometric and photometric calibrations) and the corresponding object catalogs, are also described. In order to evaluate the quality of the data, preliminary estimates are presented for the star and galaxy number counts, and for the angular two-point correlation function obtained from the available data. The present work is meant as a preview of the final release of the EIS data that will become available later this year.

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arXiv1998

The ESO Imaging Survey: Status Report and Preliminary Results

The ESO Imaging Survey (EIS) presented in early issues of the Messenger, and with up-to-date information on the ongoing observations available on the Web (http://www.eso.org/eis), is a concerted effort by ESO and the Member State community to provide targets for the first year of operation of the VLT. It consists of two parts: a relatively wide-angle survey (EIS-WIDE) to cover four pre-selected patches of sky, 6 square degrees each, spread in right ascension to search for distant clusters and quasars and a deep, multicolor survey of four optical (SUSI-2) and two infrared (SOFI) covering the HST/Hubble Deep Field South (HDFS) and its flanking fields (EIS-DEEP). From the start, the main challenge has been to carry out a public survey in a limited amount of time requiring observations, software development and data reduction with the goal of distributing the survey data products before the call for proposal for the VLT. To cope with this one-year timetable, a novel type of collaboration between ESO and the community has been established which has allowed EIS to combine the scientific and technical expertise fo the community with in-house know-how and infrastructure. In spite of the adverse weather conditions in some of the earlier runs, EIS has already proved to be successful experiment achieving most of its scientific and technical goals, thereby laying the ground work for future imaging surveys.

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arXiv1997

Clustering of Absorbers

The observed clustering of Lyman-$α$ lines is reviewed and compared with the clustering of CIV systems. We argue that a continuity of properties exists between Lyman-$α$ and metal systems and show that the small-scale clustering of the absorbers is consistent with a scenario of gravitationally induced correlations. At large scales statistically significant over and under-densities (including voids) are found on scales of tens of Mpc.

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arXiv1997

The UV Background and the Lyman alpha Clouds at High Redshift

The estimate of the ionizing UV background through the proximity effect analysis is discussed. Taking into account the correct bending of the column density distribution which appears in high resolution high s/n data, a value of the UV background ~5x10^{-22} cgs is obtained in the redshift interval z=1.7-4 without indication of any appreciable redshift evolution. Inferences on the spectral shape of the UV background between the HI and HeII edges are provided by the study of metal line ratios like SiIV/CIV in optically thin systems. The presence of a jump near the HeII edge seems favoured at very high redshifts z>3. An increasing HeII jump with increasing redshift can be responsible for the decreasing in the minimum temperatures of the Lyman alpha clouds with increasing z. New studies of the cosmic evolution of star-forming galaxies allow the estimate of a galaxy-dominated UV background intensity ~1.3 x10^{-21} f_{esc} cgs at z=0.5 where f_{esc}<20% is the escape fraction of ionizing UV photons from the galaxies. Finally it is also shown that in a UV background dominated by QSOs and/or star-forming galaxies the cosmological baryon density of the Lyman alpha clouds decreases rapidly with cosmic time possibly due to increasing bulk heating in the intergalactic medium with cosmic time.

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arXiv1997

Heavy element enrichment in the IGM at high redshift

We present a detailed analysis of the ionisation state and heavy element abundances in the Intergalactic Medium (IGM). The CIV doublet is shown by 30 % of the 182 selected optically thin \lya clouds in 10 QSO lines of sight. Direct metallicity calculations have been performed on individual systems with detected CIV and SiIV (10% of the sample) varying the UV photoionising source, cloud density and size and silicon relative abundance. The best solutions for carbon content in this subsample (redshift coverage $z=2.6 - 3.8$) span between 1/6 and 1/300 of the solar value with no evidence of redshift evolution in both the metallicity and the ionising source. Global properties of the whole sample indicate that the metallicity in \lya clouds with CIV and SiIV is not typical of the IGM. The redshift evolution of the UVB is one of the possible sources of the observed SiIV/CIV trend presented by Cowie and collaborators during this meeting. Future detection of heavy elements in lower HI column density ($\log N_{HI} < 14.5$) \lya clouds relies on the presence of OVI and NV at $z=1-2.5$.

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arXiv1997

The power spectrum of the Lyman--$α$ clouds

We estimate the 1--D power spectrum and the integral density of neighbors in 14 QSO lines of sight. We discuss their variation with respect to redshift and column density and compare the results with standard CDM models.

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arXiv1997

Impact of SNIa on SED of high redshift galaxies

We present preliminary results on the effects of SNIa explosions on the Spectral Energy Distribution (SED) of distant galaxies and the possible modifications which may occur in integrated spectra, magnitudes and colours of simple galaxy models of different ages and metallicities few days after a SNIa event.

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arXiv1996

Scaling properties of the Lyman--$α$ forest

We present some statistical features of the large number of \lya absorption lines detected in high redshift quasar spectra, obtained by using the multifractal approach. In the analysed sample of 12 QSO sight--lines, 11 show scaling behaviour with a crossover between two distinct regimes: a non-homogeneous regime at small scales and a homogeneous regime at large scales. The correlation length shows a redshift dependence, suggesting that the \lya forest can be an intermediate phenomenon between a strongly inhomogeneous galaxy distribution in the local Universe and a homogeneous initial mass distribution.

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arXiv1996

The clustering properties of the Lyman-alpha clouds

We analyze the clustering properties of a high-resolution (~10 km/s) sample of about 1600 Lyman-alpha lines in the spectra of 15 quasars, obtained in the framework of an ESO key-programme with the addition of literature data. The two-point correlation function in the velocity space shows a significant signal on small velocity scales (Delta v < 300 km/s) with amplitude and significance increasing with increasing column density. The correlation scale at z~3 turns out to be 200 - 300 h_{50}^{-1} kpc. A trend of increasing correlation with decreasing redshift is apparent. The existence of over- and under-densities on scales of a few tens of Megaparsec is confirmed with a high confidence level and a number of possible structures are identified. The present observations are found to be consistent with models of gravitationally induced correlations. A continuity scenario between Lyman-alpha and metal systems emerges, with a suggested physical association between the Lyman-$α$ clouds with log N_{HI} > 14 and the halos of protogalactic systems.

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arXiv1996

Comparison of stars and decaying neutrinos as additional sources of Intergalactic UV background

A numerical calculation of properties of finite absorbers and of intergalactic medium based on photoionization equilibrium is performed to confront alternative UV sources in addition to quasars. It is seen that a spectrum including a large peak around the HI ionization energy due to decaying neutrinos is too soft in the region up to the HeI edge to explain the relatively small observed ratio of neutral He and H densities in Lyman-limit systems if their size is of the kpc order. The recently proposed decrease of the contribution from unstable neutrinos solves this problem but tends to spoil the consistence between IGM and Lyman-$α$ clouds, which requires a large ratio of fluxes for the HI and HeII ionization frequencies, unless there is a very fast decline of quasars above $z = 3$. On the other hand, the addition of stars to quasars may produce a spectrum sufficiently hard between HI and HeI and thereafter soft up to HeII to allow a reasonable agreement of the properties of denser absorbers with those of IGM. This model seems to favour cold dark matter with additional cosmological constant.

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arXiv1996

The Proximity Effect, the UV Background and the Statistics of the Lyman-Alpha Lines at High Resolution

We present results from high resolution (R\simeq 28000) spectra of six high- redshift QSOs taken at the ESO NTT telescope that allow the detailed study of the Ly-alpha population in the redshift interval z=2.8-4.1. The typical Doppler parameters found for the Ly-alpha lines lie in the interval b=20\div30 km/s, corresponding to temperatures T>24000K, with a fraction of the order 15% in the range 10 =13.3) in the redshift interval z=1.7-4.1. A Maximum Likelihood analysis has been applied to estimate SIMULTANEOUSLY the best fit parameters of the Ly-alpha$ statistics AND of the UV background. After correcting for the blanketing of weak lines, we confirm that the column density distribution is best represented by a double power-law with a break at logN_{HI}\simeq14, with a slope beta_s=1.8 for higher column densities and a flatter slope beta_f=1.4 below the break. A value J_{LL}= 5 \pm1 \times10^{-22} erg/cm^2/s/Hz/sr is derived for the UV background in the redshift interval z=1.7-4.1, consistent with the predicted QSO contribution. No evidence is found for redshift evolution of the UVB in the same redshift interval. The comoving volume density distributions of protogalactic Damped and Lyman Lymit systems and Ly-alpha clouds with log N_{HI}>= 14 and radii R\simeq 200 kpc are found to be similar, suggesting a possible common association with galaxies.

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