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The metal absorption systems of the QSO 0103-260 and the galaxy redshift distribution in the FORS Deep Field

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 Added by Otmar Stahl
 Publication date 2003
  fields Physics
and research's language is English




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Using the Uves echelle spectrograph at the ESO VLT, we observed the absorption line spectrum of the QSO 0103-260 in the Fors Deep Field. In addition to the expected Ly-alpha forest lines, we detected at least 16 metal absorption systems with highly different ionization levels in the observed spectral range. The redshifts of the metal absorption systems are strongly correlated with the redshift distribution of the high-z galaxies in the Fors Deep Field and with the strength (but not the number density) of the Ly-alpha forest lines. Both the metal systems and the galaxies show clustering at least up to the QSO emission line redshift of 3.365, but only few of these galaxy accumulations seem to form bound systems.



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Dedicating a major fraction of its guaranteed time, the FORS consortium established a FORS Deep Field which contains a known QSO at z = 3.36. It was imaged in UBgRIz with FORS at the VLT as well as in J and Ks with the NTT. Covering an area 6-8 times larger as the HDFs but with similar depth in the optical it is one of the largest deep fields up to date to investigate i) galaxy evolution in the field from present up to z $sim$ 5, ii) the galaxy distribution in the line of sight to the QSO, iii) the high-z QSO environment and iv) the galaxy-galaxy lensing signal in such a large field. In this presentation a status report of the FORS Deep Field project is given. In particular, the field selection, the imaging results (number counts, photometric redshifts etc.) and the first spectroscopic results are presented.
We present a catalogue and atlas of low-resolution spectra of a well defined sample of 341 objects in the FORS Deep Field. All spectra were obtained with the FORS instruments at the ESO VLT with essentially the same spectroscopic set-up. The observed extragalactic objects cover the redshift range 0.1 to 5.0. 98 objects are starburst galaxies and QSOs at z > 2. Using this data set we investigated the evolution of the characteristic spectral properties of bright starburst galaxies and their mutual relations as a function of the redshift. Significant evolutionary effects were found for redshifts 2 < z < 4. Most conspicuous are the increase of the average C IV absorption strength, of the dust reddening, and of the intrinsic UV luminosity, and the decrease of the average Ly alpha emission strength with decreasing redshift. In part the observed evolutionary effects can be attributed to an increase of the metallicity of the galaxies with cosmic age. Moreover, the increase of the total star-formation rates and the stronger obscuration of the starburst cores by dusty gas clouds suggest the occurrence of more massive starbursts at later cosmic epochs.
Results of a careful analysis of the highly ionized absorption systems, observed over the redshift range 2.198--2.2215 in the zem=2.24 HDFS-QSO J2233-606, are presented. Strong OVI and NeVIII absorptions are detected. Most of the lines show signature of partial coverage which varies from species to species. This can be understood if the clouds cover the continuum emission region completely and only a fraction of the broad emission line region. Using photo-ionization models we analyze in more detail the component at zabs = 2.198. Absolute abundances are close to solar but the [N/C] abundance ratio is larger than solar. This result, which is consistent with the analysis of high-z QSO broad emission-lines, confirms the physical association of the absorbing gas with the AGN. The observed column densities of NIV, NV and NeVIII favor a two-zone model for the absorbing region where NeVIII is predominantly produced in the highly ionized zone. It is most likely that in QSO J2233-606, the region producing the NeVIII absorption can not be a warm absorber. One of the Lyalpha absorption lines at zabs = 2.2215 has a flat bottom typical of saturated lines and non-zero residual intensity in the core, consistent with partial coverage. There is no metal-line from this Lyalpha cloud detectable in the spectrum which suggests either large chemical inhomogeneities in the gas or that the gas is very highly ionized. If the latter is true the cloud could have a total hydrogen column density consistent with that of X-ray absorbers. It is therefore of first importance to check whether or not there is an X-ray warm-absorber in front of this QSO.
We explore the build-up of stellar mass in galaxies over a wide redshift range 0.4 < z < 5.0 by studying the evolution of the specific star formation rate (SSFR), defined as the star formation rate per unit stellar mass, as a function of stellar mass and age. Our work is based on a combined sample of ~ 9000 galaxies from the FORS Deep Field and the GOODS-S field, providing high statistical accuracy and relative insensitivity against cosmic variance. As at lower redshifts, we find that lower-mass galaxies show higher SSFRs than higher mass galaxies, although highly obscured galaxies remain undetected in our sample. Furthermore, the highest mass galaxies contain the oldest stellar populations at all redshifts, in principle agreement with the existence of evolved, massive galaxies at 1 < z < 3. It is remarkable, however, that this trend continues to very high redshifts of z ~ 4. We also show that with increasing redshift the SSFR for massive galaxies increases by a factor of ~ 10, reaching the era of their formation at z ~ 2 and beyond. These findings can be interpreted as evidence for an early epoch of star formation in the most massive galaxies, and ongoing star-formation activity in lower mass galaxies.
We present the results of a search for Lyman-alpha emission galaxies at z~ 5.7 in the FORS Deep Field. The objective of this study is to improve the faint end of the luminosity function of high-redshift Lyman-alpha emitting galaxies and to derive properties of intrinsically faint Lyman-alpha emission galaxies in the young universe. Using FORS2 at the ESO VLT and a set of special interference filters, we identified candidates for high-redshift Lyman-alpha galaxies. We then used FORS2 in spectroscopic mode to verify the identifications and to study their spectral properties. The narrow-band photometry resulted in the detection of 15 likely Lyman-alpha emission galaxies. Spectra with an adequate exposure time could be obtained for eight galaxies. In all these cases the presence of Lyman-alpha emission at z = 5.7 was confirmed spectroscopically. The line fluxes of the 15 candidates range between 3 and 16 * 10^-21 Wm^-2, which corresponds to star-formation rates not corrected for dust between 1 and 5 Msun/yr. The luminosity function derived for our photometrically identified objects extends the published luminosity functions of intrinsically brighter Lyman-alpha galaxies. With this technique the study of high-redshift Lyman-alpha emission galaxies can be extended to low intrinsic luminosities.
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