Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.246..545p&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 246, NO.4/OCT15, P. 545, 1990
Astronomy and Astrophysics
Astronomy
117
Scientific paper
We have used the new UCL coude echelle spectrograph at the Anglo-Australian Telescope to record a 629-a-wide portion of the Ly α forest in the spectrum of the QSO 2206 - 199N with a velocity resolution of 6.5 km s^-1^ (0.08-0.09 A), a factor of ~2 better than the highest achieved previously and ~20 times higher than that of most QSO absorption line surveys. At this resolution practically all of the 101 absorption lines we detect are resolved. We have selected a subset of 40 Ly {alpah} lines with unblended and unsaturated absorption profiles and deduced values of the velocity dispersion parameter b and neutral hydrogen column density N(H I) by fitting theoretical Voigt profiles. The lines extend over the redshift interval Z_abs_ = 2.103-2.587. We report two major findings. First, most Ly α lines are narrower than b = 22 km s^-1^, implying cloud temperatures below T = 30000 K, the value generally assumed for Ly α clouds. Secondly, there is a well- defined correlation between b and N (H I) over the column-density range spanned by our data, log N (H I) ~12.7- 14.0. We interpret the trend as an indication that the line broadening is mainly due to largescale, rather than thermal, motions and that, consequently, all Ly α clouds may be at temperatures lower than 5000-10 000 K, the upper limit implied by the narrowest lines we observe. These results, while confirming tentative indications of the existence of narrow Ly α lines from earlier work, contrast sharply with the commonly held view that the Ly α clouds are tenuous, highly ionized structures of approximately galactic dimensions. Rather, the observations favour an interpretation of the clouds as dense filaments or sheets, which are mostly neutral and are therefore several orders of magnitude smaller and less massive than considered up to now. The two-point correlation function for the 79 Ly α lines measured in the echelle spectrum is flat on all scales {DELTA}v = 50-20 000 km s^-1^, but the size of the present sample is insufficient to verify existing claims of weak clustering on scales {DELTA}v = <= 300 km s^-1^. We discuss the implications of these conclusions for models of the origin, confinement and evolution of the Ly α clouds and consider their bearing on several related issues of current interest.
Hunstead Richard W.
Mar D. P.
Pettini Marco
Smith Lleweilun J.
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