arXiv subjects
Publications and source records attributed to S. D'Odorico.
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.
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.
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.
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.
Spectra of the \ze~$=4.12$ quasar Q$0000-2619$ have been obtained in the range $λλ= 4880-8115$ Å~with a resolution of 13 \kms~and signal--to--noise ratio of $s/n=15-60$ per resolution element. The mode of the distribution of the Doppler parameters for the \lya~lines is $\simeq 25$ \kms. The fraction of lines with $10 3$.
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$^{-3}. This provides a lower limit to the average star formation rate of the order of 10^{-2} Mo/yr/Mpc^{-3} at z~ 4.25.
We present results from high resolution (R\simeq 28000) spectra of six high- redshift QSOs taken at the ESO NTT telescope that allow the detailed study of the Ly-alpha population in the redshift interval z=2.8-4.1. The typical Doppler parameters found for the Ly-alpha lines lie in the interval b=20\div30 km/s, corresponding to temperatures T>24000K, with a fraction of the order 15% in the range 10 =13.3) in the redshift interval z=1.7-4.1. A Maximum Likelihood analysis has been applied to estimate SIMULTANEOUSLY the best fit parameters of the Ly-alpha$ 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.
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 $2\times 10^{-4}$.
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.
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 $2\times 10^{-4}$.
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.
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 $z\sim 5$.