The ultraviolet absorption spectrum. of the z=2.72 QSO HS 1700+6416 II. Sulfur and neon in heavy-element absorbing systems.

Details The ultraviolet absorption spectrum. of the z=2.72 QSO HS 1700+6416 II. Sulfur and neon in heavy-element absorbing systems. The ultraviolet absorption spectrum. of the z=2.72 QSO HS 1700+6416 II. Sulfur and neon in heavy-element absorbing systems.

Aug 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...312...33k&link_type=abstract

Astronomy and Astrophysics, v.312, p.33-48

Astronomy and Astrophysics

Astrophysics

10

Cosmology: Observations, Quasars: Absorption Lines, Quasars: Individual Hs 1700+6416, Galaxies: Abundances

Scientific paper

We present ultraviolet spectra of the bright high-redshift QSO HS 1700+6416 obtained with the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope. These observations cover the wavelength range 1160-1680A at a resolution R=~2000 with a maximum signal-to-noise per pixel of 8 in the raw data and 15 in smoothed data. The presence of 14 out of 15 heavy-element absorbing systems identified in Faint Object Spectrograph (FOS) data of HS 1700 is confirmed by the GHRS data. Furthermore, we find evidence for two metal-rich absorbers at redshifts z=0.214 and 0.5525. We report the first detection of QSO absorption lines of NeIII-NeVII and SIII-SV in addition to strong absorption by OIV, OV, HeI, CIII, NIII and NIV known already from the FOS observations. The detection of strong absorption by OV and NIV in absorber systems at low redshift hints at a highly ionized gaseous component although the intensity of a metagalactic radiation field should decrease when QSOs with z>=2 are the dominant ionizing sources. The blending problem as a result of the high absorption line density and the low spectral resolution severely affects the quantitative analysis of the data. To allow for unresolved velocity structure we inferred column densities by adopting alternatively a velocity dispersion parameter b derived from neutral hydrogen or b=100km/s. For 3 Lyman Limit systems (LLS) at z=1.8465, 2.315, 2.16795 of similar neutral hydrogen column density we find neon column densities in the range logNeIII=14.7-15.5, logNeIV=14.7-15.5, logNeV<=15.5, logNeVI=14.6-14.85 and logNeVII=14.2-15.0. The best fit to observed CNO ionization in z=2 absorbers towards HS 1700 had been found for a hard ionizing radiation field fnu_{prop.to}ν^-α^ with α=0.6 when calculating photoionization models (Vogel & Reimers 1995). Adopting the same parameters these models can account for the observed Neiv, Nev, Nevi column density ratios and yield abundances of [Ne/H]=-0.4 for the z=1.8465, 2.315 systems and upper limits [Ne/H]<-0.57,-0.66 for systems at z=2.16795, 2.433, respectively. However, in order to explain the large spread in ionization stages observed for neon in the frame of photoionization models one needs at least two gaseous phases of different density. Sulphur column densities lie in the range logSIII=13.6-13.7 and logSIV=13.6-14.1 for the three systems at z=0.722, 0.8642, 1.1573. Assuming two regions of different density photoionized by a single metagalactic radiation field we find abundances [C/H]<-1.0, [N/H]=-1.25, [O/H]=-1.2, [Si/H]<-0.84, [S/H]=-1.04, and finally [Ne/H]<-0.8 for the LLS at z=1.1573. Calculating a similar model for the LLS at z=0.8642 we find abundances [C/H]=-1.2, [N/H]=-0.66, [O/H]=-0.57, [Si/H]<-0.37 and [S/H]=-0.5. The oxygen and sulphur abundances behave as expected for nucleosynthesis in SNII. These preliminary results combined with results for the high-redshift systems (Vogel & Reimers 1995) confirm the trend of increasing heavy-element abundances with decreasing redshift as indicated by the results e.g. for the z=0.7913 system towards PKS 2145+06. From ultraviolet data alone we cannot constrain models when a mixture of different ionization mechanism and cloud conditions is present. Only the combination of high-resolution, high signal-to-noise optical data with ultraviolet observations will help to clarify the ionization structure of metal line systems, i.e. to identify the ionizing mechanisms and to derive the physical properties of the absorbers.

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