arXiv subjects

Yihu Fang

Publications and source records attributed to Yihu Fang.

5 recordsLinked to original sources

arXiv1998

Reobservation of Close QSO Groups: The Size Evolution and Shape of Lyman Alpha Forest Absorbers

Probing the size and shape of the absorbers that lie in front of the QSOs, in particular the Ly alpha forest, we analyze 785 absorption lines in spectra of five QSOs in close groupings: a pair and a triplet. Both QSO groups have been observed before, but these data are a drastic improvement over earlier data, and provide a qualitatively different view of the nature of the absorbers. The pair samples a scale critical in determining the size upper bound of Ly alpha absorbers, with significant leverage in redshift compared to previous studies. The triplet data represents the spatially densest sample of Ly alpha forest ever studied, and an almost ideally-suited probe of the shape of absorbers. We observe a significant number of Ly alpha lines in common between the triplet sightlines, for lines stronger than rest equivalent width W > 0.4A (and no detected metal lines) and velocity differences up to 200 km/s, corresponding to a two-point correlation function xi = 1.88 +0.78/-0.50 on scales 0.5 to 0.8 Mpc/h with < z > = 2.14, and inconsistent at the 99.999% level with the absence of any clustering. These data also show that a significant fraction of the W > 0.4A Ly alpha forest absorbers span all three sightlines to the triplet, indicating that the strong-lined absorbers are consistent with nearly round shapes, chosen from a range of possible cylinders of different elongations. This may be inconsistent with results from hydrodynamic/gravitational simulations of H I in the early Universe indicating that the theoretical counterparts of Ly alpha forest clouds are long and filamentary. We conclude, tentatively, that the W > 0.4A Ly alpha forest objects are sheetlike. This is supported by the uniformity of linestrengths between the three sightlines, for W > 0.4A. (Abstract continues.)

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arXiv1995

The Size and Nature of Lyman alpha Forest Clouds Probed by QSO Pairs and Groups

We describe a robust Bayesian statistical method for determining Lyman alpha forest cloud sizes in spherical and in thin disk geometries, using absorption in adjacent sightlines toward closely separated QSO pairs and groups, apply this method to the available data, and discuss implications of our results for models of Ly alpha clouds. Under the assumption of a population of uniform- size and unclustered clouds, the data from Q1343+2640A/B give a 99% confidence lower and upper bounds 61 0.4 A threshold on all linelists, we find a trend of larger inferred cloud radius with larger proper separation of QSO pairs, significant at the 3.4 sigma level. This indicates that the idealization of unclustered, uniform-sized clouds does not accurately describe the Ly alpha cloud population.

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arXiv1994

A New Method for Measuring the Hi Gunn-Peterson Effect at High Redshift

In order to carry out a systematic and thorough measurement of the HI Gunn-Peterson effect at high redshift, a quantitatively testable and repeatable procedure, in particular, a robust statistical weighting technique, is developed. It is applied to an echelle spectrum of resolution 15 km s$^{-1}$ of PKS 1937-101 with z=3.787. A weighted intensity distribution which is derived overwhelmingly from pixels close to the continuum level in the Ly alpha forest region is constructed by the evaluation of how closely correlated each pixel is with its neighboring pixels. The merit of the distribution is its stronger and narrower peak compared to the unweighted, as well as its smaller dependence on uncertainty of strong absorption lines and noise spikes. By comparison to the weighted intensity distribution of synthetic Ly alpha forest spectrum with various chosen diffuse HI opacities, a chi square fit is performed. In addition to a weak line population with power law N_H distribution $β=1.7$ extrapolated down to 10$^{12}$ cm$^{-2}$, a best chi square fit requires a GP opacity of $0.115 \pm 0.025$ at average z=3.4 with estimation of the contribution from the variance of $β$. Although no evidence of more than 1-2\% error is seen in the continuum extrapolation, the possible systematic overestimation due to the slope can be as high as the level of the $χ^2$ fit, which is investigated by splitting the \Lya forest region into subsamples to check the continuum drop's dependence on absorber redshift.

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arXiv1994

Spectroscopy of the Double Quasars Q1343+264 Ab: II. the Relationship between Galaxies Ans QSO C~IV Absorption Lines -

We present spectra of QSOs 1343+266 A B, covering Ly-beta to C~IV emission at 2Å~resolution, and find further evidence that this system is not gravitationally lensed. We see, however, a large number of Ly-alpha and some C~IV absorption features common to both spectra. The frequency of C~IV lines in common indicates that these absorbers have a radius close to the sightline separation, $39~h^{-1}$kpc ($q_o=0.5$), implying a spatial number density of these absorbers the same as current day $L>0.3L_*$ galaxies. Furthermore, the velocity mismatch for C~IV systems indicates absorber masses in the range $10^{11}~M_{\odot} < M < 10^{12}~M_{\odot}$. These results confirm that typical galaxies cause QSO C~IV-line absorption. A complete line-list of metal-lines and the Ly-alpha forest is presented

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arXiv1994

Spectroscopy of the Double Quasars Q1343+266AB: I. a New Determination of the Size of Lyman-Alpha Forest Absorbers

We have obtained spectroscopy of Q 1343+266 AB, a pair of quasars at redshift z = 2.03 with a projected separation of 9.5 arcseconds. This system is well-suited for probing the Ly-alpha forest, since the two component spectra show several Ly-alpha lines in common and several others not. Using Bayesian statistics, under the idealization of uniform-radius spherical absorbers, we find that the Ly-alpha cloud radius at $z \approx 1.8$ lies in the range $40\hkpc < R < 280 \hkpc$ with $98\%$ confidence in a $(Ω_o, Λ_o) = (1, 0)$ model universe, where $h =(H_o / 100 km s^{-1} Mpc^{-1})$. The median value of R is 90 h^-1 kpc. \ These numbers scale up by a factor 1.44 for $(Ω_o, Λ_o) =(0.1, 0)$; and by a factor 1.85 for (0.1, 0.9). Pressure-confined and freely-expanding Ly-alpha cloud models in which the absorbers formed in a significantly more compressed state at $z \gg 2$ are contradicted by these new data, as are models involving stably-confined gas concentrations in photoionization equilibrium within minihalos of cold dark matter. The comoving density of Ly-alpha forest objects at $z\sim 2$ is $\sim 0.3 h (R/100 kpc)^{-2} Mpc^{-3}$ in a (1,0) model universe. This suggests a possible identification of Ly-alpha clouds with the unvirialized, collapsing progenitors of the faint, blue galaxies at $z\sim 1$.

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