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E. Giallongo

Publications and source records attributed to E. Giallongo.

140 records · Page 8Linked to original sources
arXiv1997

The absorption spectrum of the QSO PKS 2126-158 (z_em = 3.27) at high resolution

Spectra of the z_em = 3.268 quasar PKS 2126-158 have been obtained in the range lambda lambda 4300-6620 A with a resolution R = 27000 and an average signal-to-noise ratio s/n = 25 per resolution element. The list of the identified absorption lines is given together with their fitted column densities and Doppler widths. The modal value of the Doppler parameter distribution for the Ly-alpha lines is = 25 km/s. The column density distribution can be described by a power-law dn/dN \propto N^{-beta} with beta = 1.5. 12 metal systems have been identified, two of which were previously unknown. In order to make the column densities of the intervening systems compatible with realistic assumptions about the cloud sizes and the silicon to carbon overabundance, it is necessary to assume a jump beyond the He II edge in the spectrum of the UV ionizing background at z = 3 a factor 10 larger than the standard predictions for the integrated quasar contribution. An enlarged sample of C IV absorptions (71 doublets) has been used to analyze the statistical properties of this class of absorbers strictly related to galaxies. The column density distribution is well described by a single power-law, with beta=1.64 and the Doppler parameter distribution shows a modal value b = 14 km/s. The two point correlation function has been computed in the velocity space for the individual components of C IV features. A significant signal is obtained for scales smaller than 200-300 km/s, xi(30< Delta v < 90 km/s) = 32.71 +- 2.89. A trend of decreasing clustering amplitude with decreasing column density is apparent, analogously to what has been observed for Ly-alpha lines.

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arXiv1997

The Stellar UV Background at z<1.5 and the Baryon Density of Photoionized Gas

We use new studies of the cosmic evolution of star-forming galaxies to estimate the production rate of ionizing photons from hot, massive stars at low and intermediate redshifts. The luminosity function of blue galaxies in the Canada-France Redshift Survey shows appreciable evolution in the redshift interval z=0-1.3, and generates a background intensity at 1 ryd of J_L~ 1.3 x 10^{-21} f_{esc} ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1} at z~0.5, where f_esc is the unknown fraction of stellar Lyman-continuum photons which can escape into the intergalactic space, and we have assumed that the absorption is picket fence-type. We argue that recent upper limits on the H-alpha surface brightness of nearby intergalactic clouds constrain this fraction to be <~ 20%. The background ionizing flux from galaxies can exceed the QSO contribution at z~ 0.5 if f_{esc}>~ 6%. We show that, in the general framework of a diffuse background dominated by QSOs and/or star-forming galaxies, the cosmological baryon density associated with photoionized, optically thin gas decreases rapidly with cosmic time. The results of a recent Hubble Space Telescope survey of OVI absorption lines in QSO spectra suggest that most of this evolution may be due to the bulk heating and collisional ionization of the intergalactic medium by supernova events in young galaxy halos.

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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

Properties of the Lyman Alpha Clouds from non-equilibrium Photoionization Models

We investigate the thermal and ionization history of Lyman alpha clouds photoionized by a time--dependent UV background, including non equilibrium effects. The results show that it is possible to obtain temperatures as low as T~15000 K (or, equivalently, Doppler parameters b~15 km/s) at z=3 for cloud total densities n~10^-4 cm^-3, if (i) the reionization epoch occurred at z_i~10, and (ii) the UV background has a factor 70-100 decrease at the HeII edge. A trend towards smaller b with increasing redshift is present in the redshift interval z=1-5. Higher densities lead to higher values of b and smaller hydrogen correction factors, n_{HII}/n_{HI}. The correction factors for helium are also given. For a hydrogen column density N_{HI}=3 10^{14} cm^{-2}, cloud sizes are larger than 100 kpc, consistent with recent observations of quasar pairs. Pressure confined models, instead, yield implausibly low cloud densities at low redshift, and too small sizes at intermediate redshift. The implications of the model are confronted with the available observational data.

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arXiv1996

Search for Galaxies at z>4 from a Deep Multicolor Survey

We present deep BVrI multicolor photometry in the field of the quasar BR1202-07 (z_{em}=4.694) aimed at selecting field galaxies at z>4. We compare the observed colors of the galaxies in the field with those predicted by spectral synthesis models including UV absorption by the intergalactic medium and we define a robust multicolor selection of galaxies at z>4. We provide spectroscopic confirmation of the high redshift QSO-companion galaxy (z=4.702) selected by our method. The first estimate of the surface density of galaxies in the redshift interval 4<z<4.5 is obtained for the same field, corresponding to a comoving volume density of ~ 10^{-3}$ Mpc$^{-3}. This provides a lower limit to the average star formation rate of the order of 10^{-2} Mo/yr/Mpc^{-3} at z~ 4.25.

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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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arXiv1995

The Variability of Quasars. II. Frequency Dependence

The variability of 30 PG quasars has been observed in the red band during three years. A rest-frame structure function analysis shows an increase of variability with the time interval up to 0.2 mag r.m.s. after two years. A comparison with the IUE data available for PG quasars shows that the variability increases with the rest-frame frequency at each time interval. Evidence for this effect is supported by the analysis of the average variability of other published quasar samples. The effect can completely account for the increase of variability with redshift found in other studies.

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arXiv1995

CLUSTERING PROPERTIES OF THE LY$α$ CLOUDS

The spectra of the QSOs PKS2126-158 ($z_{em}=3.27$) and Q0055-269 ($z_{em}=3.66$) have been observed at high resolution ($2.2 - 2.7 \times 10^4$). Significant clustering, with $ξ\simeq 1$ at $Δv=100$ km s$^{-1}$, is detected in both cases for lines with $\log N_{HI} \geq 13.8$. A similar result is obtained analysing the data available in the literature for Q0014+81. An anticlustering signal is also detected on scales $Δv \sim 600$ km s$^{-1}$. Two voids of size $\sim 2000$ km s$^{-1}$ are found in the spectrum of Q0055-269 with a probability of $2\times 10^{-4}$.

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arXiv1995

New results on the Gunn-Peterson Effect at High Redshift

Attempts to measure the optical depth due to a diffuse intergalactic medium (IGM) in the spectra of high redshift quasars (GP test) within the ESO key program on the intergalactic medium are reviewed. It is shown that there is no evidence for any Gunn-Peterson effect up to the highest redshifts observable in quasar spectra. The Ly$α$ line statistics consistent with this limit is not able to explain the strong absorption observed at the HeII forest in one QSO, leaving room for a true HeII GP effect. The HI/HeII GP ratio implies a steep ionizing UV background at $z\sim 5$ and ionizing sources of stellar origin.

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arXiv1995

Clustering of the Lyman Alpha Clouds in the Line of Sight to the Z=3.66 QSO 0055-269

The spectrum of the Q0055-269 ($z=3.66$) has been observed at the resolution of 14 km s$^{-1}$ in the wavelength interval 4750--6300 Å. The statistical distribution of the Doppler parameter for the Ly$_α$ lines is peaked at $b\simeq 23$ km s$^{-1}$. The column density distribution is described by a power-law with a break at $\log N_{HI} \simeq 14.5$. Significant clustering, with $ξ\simeq 1$ at $Δv=100$ km s$^{-1}$, is detected for lines with $\log N_{HI} \geq 13.8$. Two voids of size $\sim 2000$ km s$^{-1}$ are found in the spectrum with a probability of $2\times 10^{-4}$.

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arXiv1994

The Space Distribution of the Lyman Alpha Clouds in the Line of Sight to the Z=3.66 QSO 0055-269

The spectrum of the quasar Q0055-269 ($z=3.66$) has been observed at the resolution of 14 km s$^{-1}$ in the wavelength interval 4750--6300 Å. We give a list of Lyman$-α$ lines and metal-line systems for which column densities and Doppler widths have been derived by a fitting procedure. The statistical distribution of the Doppler parameter for the Lyman$-α$ lines is peaked at $b\simeq 23$ km s$^{-1}$, with 13\% of lines with $15\leq b\leq 20$ km s$^{-1}$. The column density distribution of the Lyman$-α$ lines is described by a power-law with a break or cutoff at $\log N_{HI} \simeq 14.5$. A featureless power-law distribution is rejected with a probability of $99.94$\%. Significant clustering, with $ξ\simeq 1$ at $Δv=100$ km s$^{-1}$, is detected only for lines with $\log N_{HI}\magcir 13.8$. In addition, two voids of size $\sim 2000$ km s$^{-1}$ are found in the spectrum with a random probability of $2\times 10^{-4}$. {}From the proximity effect for lines in the interval $\log N_{HI}=13.3-14.2$ it is possible to infer an UV ionizing background $J=5\times 10^{-22}$ ergs s$^{-1}$ cm$^{-2}$ Hz$^{-1}$ sr$^{-1}$, within a factor of two from the integrated quasar contribution.

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arXiv1994

The Gunn-Peterson Effect in the Spectrum of the Z=4.7 QSO 1202-0725: The Intergalactic Medium at Very High Redshift

A measure of the average depression between Lyman absorption lines in the spectrum of the faint quasar BR1202-0725 ($z_{em}=4.695$) is presented. The relatively high resolution of the spectrum ($\sim 40$ km s$^{-1}$) allows the selection of regions free of strong absorption lines in the Lyman alpha forest. A reliable evaluation of the continuum shape is based on the careful flux calibration and on the large wavelength interval covered (5000--9300 ~Å). A best estimate of $τ_{_{GP}}\leq 0.02\pm 0.03$ has been found for the Gunn-Peterson optical depth at the highest absorption redshift observed at this resolution, $z \simeq 4.3$. The derived baryon density of the intergalactic medium is $Ω_{IGM}\mincir 0.01$ if the observed quasars are the major contributor to the ionizing UV background flux. This limit, when compared with the total baryon density deduced from the nucleosynthesis, could imply that most of the baryons are already in bound systems at $z\sim 5$.

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arXiv1992

Coordinated UV-optical observations of quasars: the evolution of the Lyman absorption

The average flux decrement shortward the Ly$_α$ emission, due to the well-known ``forest'' of absorptions, has been measured in the spectra of 8 quasars. Quasi-simultaneous optical and IUE observations of the two low redshift quasars PKS 0637--75 (z=0.654) and MC 1104+16 (z=0.632) have been carried out, obtaining relatively high S/N, spectrophotometrically calibrated data on their energy distribution from the rest frame H$_β$ to the Lyman continuum. Six more quasars in the redshift range 2.5-3.4 have been observed in the optical domain. For all the quasars the ``intrinsic'' continuum slope and normalization have been estimated longward the Ly$_α$ emission and extrapolated towards the Lyman continuum to measure the average depressions, which have been compared with the model statistics of the Ly$_α$ clouds. When all the known classes of absorbers are taken into account with plausible values for their equivalent width distribution and evolution, a good agreement is obtained with the observations. The results for the observed continuum decrement at $z \sim 0.65$ are identical to those predicted by the evolution with redshift of the number of Ly$_α$ forest systems including the HST data and within $2σ$ of the predicted value using the ``standard'' Ly$_α$ evolution (as determined only at high z).

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